Advanced Course - AUCSC · PDF fileADVANCED COURSE CHAPTER 1 ... COATED CROSS-COUNTRY PIPELINE...

Advanced Course

Appalachian Underground Corrosion Short CourseWest Virginia UniversityMorgantown, West Virginia

Copyright © 2013

APPALACHIAN UNDERGROUND CORROSION SHORT COURSEADVANCED COURSE

CHAPTER 1 - PIPE-TO-SOIL POTENTIAL SURVEYS AND ANALYSIS . . . . . . . . . . 1-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

CORROSION MECHANISMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

PIPE-TO-SOIL POTENTIAL MEASUREMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2

TYPES OF POTENTIAL SURVEYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2

SINGLE ELECTRODE METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3

TWO ELECTRODE METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3

SIDE-DRAIN MEASUREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4

ANALYZING PIPE-TO-SOIL POTENTIAL DATA . . . . . . . . . . . . . . . . . . . . . . . . . 1-4

PIPE-TO-SOIL POTENTIAL SURVEYS AND ANALYSIS . . . . . . . . . . . . . . . . . . . 1-5

INTERPRETATION OF POTENTIALS UNDER NON-STRAY CURRENTCONDITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-6

COATED CROSS-COUNTRY PIPELINE WITHOUT CATHODICPROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7

GAS SERVICE LINE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7

GROUNDING SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8

STEEL GAS AND WATER LINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8

CRITERIA FOR CATHODIC PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-9

WHICH CRITERION? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-10

POTENTIAL MEASUREMENT OF -850 mV TO A Cu/CuSO4 REFERENCEELECTRODE WITH CATHODIC PROTECTION APPLIED . . . . . . . . . . . . . . . . 1-10

POLARIZED POTENTIAL OF -850 mV MEASURED TO A Cu/CuSO4 REFERENCE ELECTRODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-11

100 MILLIVOLT POLARIZATION CRITERION . . . . . . . . . . . . . . . . . . . . . . . . . . 1-12

CRITERIA FOR SPECIAL CONDITIONS - NET PROTECTIVE CURRENTCRITERION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-13

OTHER CRITERIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-14

E LOG I CURVE CRITERION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-14

300 MILLIVOLT POTENTIAL SHIFT CRITERION . . . . . . . . . . . . . . . . . . . . . . . 1-15

USE OF CRITERIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-16

CHAPTER 2 - EVALUATION OF UNDERGROUND COATINGS USINGABOVEGROUND TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

SURVEY SEQUENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

REFERENCED PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

SAFETY CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3

PIPELINE LOCATING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3

DIRECT CURRENT VOLTAGE GRADIENT SURVEYS (DCVG) . . . . . . . . . . . . . 2-4

ALTERNATING CURRENT VOLTAGE GRADIENT SURVEYS (ACVG) . . . . . . . 2-7

CLOSE-INTERVAL SURVEYS (CIS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

AC CURRENT ATTENUATION SURVEYS (ELECTROMAGNETIC) . . . . . . . . . 2-10

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11

CHAPTER 3 - MATERIALS FOR CATHODIC PROTECTION . . . . . . . . . . . . . . . . . . 3-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

CONSIDERATIONS FOR ALL MATERIALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

GALVANIC ANODES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

Prepared Backfill and Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2

Wire and Cable Attachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Magnesium Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Zinc Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

IMPRESSED CURRENT CATHODIC PROTECTION SYSTEMS . . . . . . . . . . . . . 3-5

Types of Impressed Current Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

High Silicon Cast Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

Graphite Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

Mixed Metal Oxide Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

Platinum Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

Aluminum Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Mixed Metal Oxide Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Lead Silver Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Magnetite Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Polymer Conductive Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

Scrap Steel Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

Backfill for Impressed Current Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

WIRE AND CABLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

CABLE TO STRUCTURE CONNECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

CABLE SPLICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

SPLICE ENCAPSULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

POWER SUPPLIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12

JUNCTION BOXES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13

PERMANENTLY INSTALLED REFERENCE ELECTRODES . . . . . . . . . . . . . . . 3-14

TEST STATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

FLANGE ISOLATORS AND DIELECTRIC UNIONS . . . . . . . . . . . . . . . . . . . . . . 3-15

MONOLITHIC WELD IN ISOLATORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16

CASING ISOLATORS AND END SEALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16

CHAPTER 4 - DYNAMIC STRAY CURRENT ANALYSIS . . . . . . . . . . . . . . . . . . . . . 4-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

STRAY CURRENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

THE EARTH AS A CONDUCTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

POTENTIAL GRADIENTS IN THE EARTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2

DETECTION OF DYNAMIC STRAY CURRENTS . . . . . . . . . . . . . . . . . . . . . . . . 4-2

Interpreting Beta Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3

Determining the Point of Maximum Exposure . . . . . . . . . . . . . . . . . . . . . . . . . 4-4

METHODS OF MITIGATING THE EFFECTS OF DYNAMIC STRAY CURRENT 4-4

Controlling Stray Currents at the Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4

Mitigation (Drainage) Bonds and Reverse Current Switches . . . . . . . . . . . . . . 4-4

Sizing the Mitigation (Drain) Bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5

Use of Sacrificial Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

Use of Impressed Current Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

STRAY CURRENT FROM ALTERNATING CURRENT (AC) SOURCES . . . . . . 4-8

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9

CHAPTER 5 - DESIGN OF IMPRESSED CURRENT CATHODIC PROTECTION . . 5-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

REVIEW OF IMPRESSED CURRENT SYSTEM FUNDAMENTALS . . . . . . . . . . 5-1

Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2

Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2

INFORMATION USEFUL FOR DESIGN OF AN IMPRESSED CURRENTCATHODIC PROTECTION SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2

DESIGN OF AN IMPRESSED CURRENT ANODE BED . . . . . . . . . . . . . . . . . . . 5-3

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3

Selecting an Anode Bed Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3

Selecting Anode Bed Type Based on Site Selection . . . . . . . . . . . . . . . . . . . . 5-4

Distributed Anode Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4

Remote Anode Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4

Deep Anode Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4

Hybrid Anode Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5

Tests Required for Anode Bed Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5

DESIGN CALCULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7

Existing Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7

Current Requirement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7

Determining the Number of Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9

Calculating the Anode Resistance-to-Earth . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10

Calculating Total Circuit Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12

Sizing the Rectifier or DC Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13

DESIGN OF DISTRIBUTED ANODE BED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-14

DESIGN OF DEEP ANODE BED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-16

CHAPTER 6 - DESIGN OF GALVANIC ANODE CATHODIC PROTECTION . . . . . . 6-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

GALVANIC ANODE APPLICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

General Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

Specific Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

GALVANIC ANODE CATHODIC PROTECTION DESIGN PARAMETERS . . . . . 6-1

Galvanic Anode Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

Anode Current Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Current Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Electrolyte Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Total Circuit Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Anode Bed Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Resistance of Interconnecting Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

Cathode-to-Earth Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

Anode Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

GALVANIC ANODE DESIGN CALCULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4

Simplified Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

SPECIFICATION AND MAINTENANCE OF GALVANIC ANODEINSTALLATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8

CHAPTER 7 - MIC INSPECTION AND TESTING

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

SITE SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

COATING INSPECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2

MIC AND DEPOSIT SAMPLING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2

MIC SAMPLING AND TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2

SUPPORTING ANALYSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3

METALLURGICAL INSPECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4

April 29, 2013 Revision

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CHAPTER 1

PIPE-TO-SOIL POTENTIAL SURVEYS AND ANALYSIS

INTRODUCTION

The objective of this chapter is to present themost important corrosion control measurement,the pipe-to-soil potential measurement, and thevarious methods that can be used to make thismeasurement.

This chapter will also show how pipe-to-soilpotential data can be used to identify andevaluate corrosion problems as well as assist indetermining the effectiveness of a cathodicprotection system.

Also included in this chapter is a discussion ofthe criteria for cathodic protection and theapplications for each.

All pipe-to-soil potential values given in thischapter will be with respect to a saturatedcopper/copper sulfate reference electrode(CSE). Unless otherwise noted, or in anexample where lateral measurements are beingtaken, all potential values given will be with thereference electrode placed at grade directlyover the structure being tested. Keep in mindthat the pipeline potential is referenced to theelectrode location not to the structureconnection point. See Figure 1-1 for a typicalpipe-to-soil potential measurement test setup.

Because the normal pipe-to-soil potential ofsteel measured with respect to a CSE is almostalways negative, this chapter will presentexamples using potentials with negative values.Therefore, negative potentials with greaterabsolute values are “more negative”. Forexample, a potential measurement of -600 mVis “more negative” than a measurement of-400 mV.

CORROSION MECHANISMS

The corrosion of an underground or submergedmetallic structure is electrochemical in nature.There are basically two different mechanismswhich are responsible for this corrosion andthese are termed electrolytic corrosion andgalvanic corrosion.

Electrolytic corrosion, often called stray currentcorrosion, results from currents which areintroduced into the ground from neighboringsources of direct current (DC) such as electricrailways, DC powered machinery, and foreigncathodic protection systems.

Galvanic corrosion is the result of the naturalelectrochemical process that takes place on aburied metallic structure due to potentialdifferences which exist between points on thesame structure due to different surfaces,dissimilar electrolytes, differential aeration orthe coupling of dissimilar metals. This type ofcorrosion is often classified into categories suchas local action, long line activity, and bimetallicactivity.

Both of these corrosion mechanisms can bedetected and evaluated through the proper useof the pipe-to-soil potential measurement.

Pipe-to-soil potential measurements canindicate whether a structure is subject todynamic or fluctuating stray current corrosion.Because stray currents that come from DCpowered railways or machinery are rarely ifever constant, fluctuations in potential on aneffected structure will indicate the presence ofthis type of stray current. In order to determinethe degree of potential fluctuation and the

PIPE-TO-SOIL POTENTIAL SURVEYS AND ANALYSIS1-1

PIPE-TO-SOIL POTENTIAL MEASUREMENT

FIGURE 1-1

HIGH IMPEDANCEVOLTMETER

CURBBOX

TESTLEAD

COPPER/COPPER SULFATEREFERENCE ELECTRODE (CSE)

SOIL

PIPE

-0.85

amount of time that the structure is effected, DCvoltage recording instruments can beemployed.

Non-fluctuating stray current or static straycurrent is not as readily apparent as thedynamic type. In this case, the interferenceeffect from a foreign cathodic protection systemwill usually remain constant and unless there isexisting historical potential data, or one has theopportunity to participate in cooperativeinterference testing, this type of stray currentinterference may not be immediately detected.However, there are potential measurementtechniques that can detect this type of straycurrent interference such as lateral potentialsurveys, which will be discussed later in thischapter.

PIPE-TO-SOIL POTENTIAL MEASUREMENT

Before the various test methods and evaluationprocesses are discussed, a review of the pipe-to-soil potential measurement is required. Thismeasurement must be obtained using a highimpedance voltmeter, a calibrated referenceelectrode, an electrolyte present over the pipewhere the reference electrode can be placed,and a method by which to contact the structureunder test. A high impedance voltmeter isrequired in order to obtain the most accuratepotential reading. The reference electrodeshould be filled with distilled water (or electrodegel solution) after the copper rod has beencleaned with a nonmetallic abrasive and coppersulfate crystals have been added. Theelectrolyte for the placement of the referenceelectrode should be soil or water. Concrete andasphalt should be avoided. The connection tothe structure under test can be a valve, riserpipe, test station, etc., any point electricallycontinuous with that portion of pipe beingevaluated.

The preferred polarity convention has thereference electrode connected to the negativeterminal of the voltmeter and the structureconnected to the positive terminal. This polarityconnection will normally result in a negativevalue. It is important to note that consistency is

critical when recording these potential valueswhether it be as a negative value or a positivevalue. The methods and instrumentationemployed for this testing are critical in order toprovide meaningful data that can be properlyevaluated.

TYPES OF POTENTIAL SURVEYS

A normal survey of a pipeline system in order toobtain either static or native potential values orto ascertain the effectiveness of an existingcathodic protection system might consist ofmeasuring pipe-to-soil potentials at all availabletest locations such as those describedpreviously. However, there are occasions whenit is necessary to obtain additional potentialmeasurements between test points. This isaccomplished by placing the referenceelectrode at regular intervals over the pipelineand measuring potentials at each referencelocation. The spacing of the intervals willdepend on the type of survey being performedand the type of detail required. This type ofsurvey will indicate the anodic locations alongan unprotected structure which will be thoseareas with the most negative potentials.However, these more negative potentials mayalso correspond to a stray current “pickup”area. In the case of a pipeline under cathodicprotection, this type of survey will reveal thoselocations where protection might not beachieved based on an evaluation of thepotential values.

In order to insure the accuracy of an over-the-line potential survey, the pipeline should be firstelectronically located and staked out by thesurvey crew. This will help to ensure that thepotential measurements are taken directly overthe pipeline. Just as it is important to use theproper equipment as explained earlier, it isequally important that the reference electrodebe placed in clean soil devoid of rocks, weeds,or grass which could effect the electrodecontact resistance and thereby effect theaccuracy of the measurement. In other words,make sure to establish good cell contact withthe electrolyte.


It is also important to insure that the pipelineyou are testing is electrically continuous for thelength of pipe to be surveyed.

Detailed over-the-line potential surveys can beconducted using various methods and testequipment which will yield a variety of usefulinformation. The following is a description ofsome commonly used survey methods.

SINGLE ELECTRODE METHOD

The first method to be discussed is the singleelectrode survey, see Figure 1-2. This surveyutilizes one CSE, a high impedance voltmeter,and a reel of test wire. An additionalrequirement is a point of electrical contact tothe structure being tested.

There are two ways to perform this particularsurvey. In the first procedure, the pipeline iscontacted through a test lead or other suitableconnection point which is connected to a testreel. The test reel is connected to the positiveterminal of a voltmeter and the testingpersonnel measure potentials along thepipeline at prescribed intervals, carrying andmoving both the voltmeter and the referenceelectrode together. Once again, the referenceelectrode is connected to the negative terminalof the voltmeter. This polarity convention can bealtered as long as the convention is consistentand the test personnel are aware of the polarityof the measured pipe-to-soil potential.

In the second procedure, the test reel isconnected to the reference electrode and thevoltmeter is left at the point of connection to thepipeline. This method requires two testingpersonnel with one at the voltmeter recordingthe data and the other individual moving thereference electrode along the pipeline andcontacting the soil at set intervals. In caseswhere a long section of pipeline is to be testedor where the terrain is such that visual contactcannot be maintained, some type ofcommunications equipment is needed in orderto coordinate the timing of the readings.

It should be noted that potential readings taken

using the one electrode method may beaffected by voltage drop in the pipeline andmeasuring circuit or by stray currentinterference.

The data can be recorded manually on fielddata sheets or electronically using a recordingvoltmeter or datalogger.

Figure 1-3 shows the basic components andhookups for recording the data electronicallyusing a computerized system. In this case, thevoltage measuring and datalogging equipmentcan be carried in a compact backpackarrangement by the operator. The operatorcarries one or two reference electrodes, whichmay be affixed to the bottom of extensionrod(s), which are then “walked” forward at somespecific interval, usually 2½ to 3 feet. Potentialmeasurements are electronically recorded andstored by the instrument. The long test leadback to the connection to the pipeline at thesurvey starting point can be a one-time-usedisposable light gauge wire. During the courseof the potential survey the operator is able toelectronically note distances, terrain featuresand landmarks as well as other pertinentinformation.

The electronically collected data can then beprocessed by a personal computer in the fieldor at the office. The computers can beprogrammed to produce a potential profilecomplete with identifying notes for the pipelinesection surveyed. Some programs will allow forthe plotting of previous survey data forcomparison.

TWO ELECTRODE METHOD

Another potential survey method to bediscussed is the two-electrode survey illustratedin Figure 1-4. This survey uses two CSEs, ahigh impedance voltmeter, and a pair of testleads. These leads can be cut equal to thespan of the preselected survey distance. Thesurvey is conducted by first measuring andrecording the pipe-to-soil potential with respectto reference electrode “A” as shown in Figure1-4. Reference electrode “B” is then placed


SINGLE ELECTRODE POTENTIAL SURVEY

FIGURE 1-2

HIGH IMPEDANCE VOLTMETER

(-)(+) SINGLE REFERENCE ELECTRODEMOVED ALONG LINE

PERMANENTTEST POINT

PIPELINE

ETC. GRADE

-0.85

TYPICAL COMPUTERIZED POTENTIAL SURVEY

FIGURE 1-3

WIRE REEL

(+) (–)EXPENDABLE TEST WIRE

PERMANENTTEST POINT PIPELINE

GRADE

BACKPACK CONTAININGDATA ACQUISITION ANDLOGGING EQUIPMENT

“WALKING” REFERENCE ELECTRODES

TWO ELECTRODE POTENTIAL SURVEY

FIGURE 1-4

VOLTMETER POSITIONS

(+) (–)

PERMANENTTEST STATION

PIPELINE

ETC.

GRADE

1 2 3 4 5

“A” “B” “A” “B” “A”

NEXT PERMANENTTEST STATION

“LEAP FROGGING” REFERENCE ELECTRODES “A” & “B”

along and directly over the pipeline at somepreselected survey interval from referenceelectrode “A”. The potential difference betweenthe two electrodes is then measured andnumerically added or subtracted from theprevious reading in accordance with themeasured polarity of the forward electrode (“B”in this case). This “leap frogging” of the twoelectrodes is continued until the next permanenttest station is reached or any other locationwhere the pipeline can be electricallycontacted.

At this juncture, the cumulative potential up tothis point is compared to the actual potentialmeasured to a CSE at this second permanenttest station and adjusted as necessary. Thesurvey then continues following the abovedescribed procedure to the next contact point.

The data should be recorded in a permanentlog. See Table 1-1 for a typical recordingformat.

The two electrode method works well. However,it requires that a great deal of care be taken inrecording the data. An error made in thecalculating of potentials or the noting of polarityat any given point will cause all subsequentcalculated potentials to be erroneous. In thecase of an error, the calculated pipe-to-electrode potential will not correspond to thepotential measured at the next contact point.

Note however, that even if there are no dataerrors, a discrepancy could still exist betweenthe last calculated pipe-to-soil potential and theactual measured potential at the contact point.One possible reason is that any current flowingon the pipe will cause a voltage drop across thepipeline between the two test points whichcould account for the difference. Anotherreason for a difference could be the effect ofstray current activity on the pipeline potential.

SIDE-DRAIN MEASUREMENTS

If more detailed information is desired, anothermethod known as side-drain measurementscan often be used to troubleshoot problem

areas. Side-drain measurements are conductedutilizing two CSEs and a high impedancevoltmeter, as shown in Figure 1-5. The firstelectrode is placed directly above the pipe, incontact with the soil. The second electrode isplaced in contact with the soil at a 90º angle tothe pipe at a distance approximately equal to2½ times the pipe depth. During testing, theelectrode placed directly above the pipe shouldbe connected to the positive terminal of thevoltmeter and the other electrode to thenegative terminal. Positive side-drain readingsindicate that current is being discharged fromthe pipe at this point, making it an anodic area.Negative side-drain readings normally indicatethat current is flowing towards or onto the pipeat this point, making it a cathodic area. Theside-drain measurements should be takentypically at no more than 5-foot intervals. Testsmust be made on both sides of the pipe, untilthe extent of the problem section has beendetermined.

This technique should be used with caution.Under certain conditions, a relatively stronglocalized anodic cell could exist on the bottomof the pipe with the top of the pipe serving as acathode and negative side-drain readings couldbe measured while severe corrosion is actuallyoccurring on the bottom of the pipe at thislocation.

ANALYZING PIPE-TO-SOIL POTENTIALDATA

Upon completion of the pipe-to-soil potentialsurvey, the compiled data is normally plotted ona graph to facilitate interpretation. Potential (Vor mV to CSE) is plotted versus distance (ft).

On cathodically protected pipelines, the pipe-to-soil potential survey is used to determine ifadequate levels of protection are beingprovided to all areas of the pipeline. Adequatelevels of protection are often based on the -850mV (“instant off,” or “on” corrected for voltagedrops) versus a CSE. However, the 100 mVcriterion could be used if a static potentialprofile of the line is available.


TABLE 1-1

Typical Data Record For Two Electrode Potential SurveyConducted on a Cathodically Protected Pipeline

A B C D Comments

1 -- - -0.860(1) --

2 0.035 + -0.895 --

3 0.021 + -0.916 --

4 0.065 - -0.851 --

5 0.092 - -0.759 Unprotected Area

6 0.045 + -0.804 Unprotected Area

7 0.063 + -0.867 --

8 0.011 + -0.878 --

9 0.020 - -0.858 --

10 0.032 + -0.890 --

Where: A = position or pipeline sectionB = potential drop from electrode at last position (volts)C = polarity of forward electrodeD = pipe to copper sulfate electrode (volts)

Note: (1) Initial value measured via direct pipeline contact at Position 1

TYPICAL TEST SET UP FOR SIDE DRAIN MEASUREMENTS

FIGURE 1-5

(+)(–)

HIGH IMPEDANCEVOLTMETER

GRADE

PIPE BEING TESTED

NORMALLY 6’-10’ BUTNOT LESS THAN 2.5C

CUSO REFERENCEELECTRODE (TYP.)

4

C

-0.85

Normally, after the survey data is plotted, the-850 mV value is red-lined on the plot. All areasbelow this line are considered to beinadequately protected and in need of remedialmeasures. Figure 1-6 shows a typical pipe-to-soil potential profile of a cathodically protectedpipeline.

On pipelines which are not cathodicallyprotected, the pipe-to-soil potential profile andsurface potential surveys can be used to locatecorroding areas or “hot spots” along thepipeline. Experience has shown that when adifference in pipe-to-soil potential values existalong a pipeline, corrosion occurs at, and for agiven distance on either side of, the mostnegative or anodic points along the pipeline.

Figure 1-7 shows some typical potential andpolarity changes which may be recorded atcorroding or anodic areas on the pipe. Theexact point of “current discharge” can bedetermined by resurveying the effected areaand successively reducing the electrodespacing by one-half. When the exact point ofmaximum “current discharge” (most negativepotential) has been determined, the pointshould be staked, and all pertinent datarecorded.

Pipe-to-soil potential measurements taken overthe line can be used to indicate whether or nota pipeline is being subjected to dynamic straycurrent corrosion. This is possible however,only if the potentials are fluctuating at the timeof the survey. The two potential profile methodis sometimes used to detect the effects of staticstray currents on a section of pipe. The firstprofile is developed with the reference electrodeplaced directly above the pipe and the secondby placing the electrode some distance laterallyaway from the pipe.

Dual potential profiles, over the pipe andlaterally away from the pipe, are used tointerpret various types of corrosion activity onunprotected pipes. It should be noted, however,that such profiles should be utilized to evaluatethe effects of “long line” corrosion cells. Suchprofiles can sometimes overlook very small

localized corrosion cells where the anode andthe cathode are located very close to eachother.

PIPE-TO-SOIL POTENTIAL SURVEYS ANDANALYSIS

Figure 1-8 shows a typical potential profile on apipe on which the corrosion activity is one ofstraight forward galvanic action and is not beinginfluenced by interference currents or bimetalliccorrosion. Figure 1-9 shows a typical potentialprofile on a pipe which is exposed to damageas a result of a rectifier unit on a crossingpipeline. In comparing these two potentialprofiles, it should be noted that irrespective ofthe condition which exists, anodic areas alwaysexist at the locations where the “over the pipe”potentials are more negative than the “off thepipe” potentials. However, in a straightforwardgalvanic situation such as shown in Figure 1-8,the anodic areas occur at locations where the“over the pipe” potentials are of higher negativevalues than those measured in the cathodicareas. In an interference situation such asshown in Figure 1-9, the anodic areas are atthose locations where the potentials (both“over” and “off” the pipe) are of lower negativevalues than those at the cathodic areas.

It should be noted that although a potentialprofile similar to that shown in Figure 1-8 almostalways indicates a galvanic corrosion patternand almost always rules out a stray current orinterference situation, a profile similar to thatshown in Figure 1-9 does not necessarilyindicate a stray current situation. It is possiblethat a bimetallic corrosion condition will yield apotential profile similar to that which isencountered in an interference situation. Figure1-10 shows a potential profile on a pipeline, aportion of which is influenced by a neighboringcopper grounding system to which the pipe isconnected. It can be seen that this potentialprofile is very similar to that shown in Figure 1-9in that the anodic areas occur at locations oflow negative potential. In some situations, itmay be difficult to distinguish between aninterference situation and a bimetallic onemerely by analyzing the potential profiles. In


TYPICAL POTENTIAL PLOT OF A CATHODICALLY PROTECTED PIPELINE

FIGURE 1-6

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-1.3

-1.2

-1.1

-1.0

-0.9

-0.8

-0.7

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

PROTECTED AREAS(MORE NEGATIVE THAN -0.85

VOLT)

-0.85 VOLT

UNPROTECTED AREAS(LESS NEGATIVE THAN -0.85 VOLT)

DISTANCE ALONG PIPE

NOTE: APPLICABLE CRITERION FOR THIS LINE IS -0.85 VOLT TO Cu-CuSO4

PIPE

-TO

-SO

IL P

OTE

NTI

AL

(VO

LTS

TO C

U-C

USO

) 4

0

-1.4

SCHEMATIC SURFACE POTENTIAL SURVEY

FIGURE 1-7

CU-CUSO REFERENCEELECTRODE (TYP.)

DIRECTION OF SURVEY

GRADE

CATHODIC - ANODIC - CATHODIC

4+18MV -10MV -2MV +3MV -2MV -4MV

PIPELINE

HIGH IMPEDANCEVOLTMETER (TYP.)

+8MV +12M

CATHODIC - ANODIC - CATHODIC

POTENTIAL PROFILE SHOWINGGALVANIC CORROSION ACTIVITY

FIGURE 1-8

CURRENTDISCHARGE-110

DISTANCE ALONG PIPE (FEET)

-100

-900

-800

-700

-600

-500

-400

-300

-200

-100

CURRENTPICK-UP

CURRENTDISCHARGE

CURRENTPICK-UP

CURRENTDISCHARGE

ANODICAREA

CATHODICAREA

ANODICAREA

CATHODICAREA

ANODICAREA

25 FEET FROM PIPE

OVER PIPE

100 200 300 400 500 600 700 800 900 1000

PIPE

–TO

–SO

IL P

OTE

NTI

AL

(MIL

LIVO

LTS)

POTENTIAL PROFILE OF PIPE EFFECTEDBY STRAY CURRENT INTERFERENCE

FIGURE 1-9

-110

0


-100

-900

-800

-700

-600

-500

-400

-300

-200

-100

CURRENT CURRENTDISCHARGE

CURRENT PICK-UP

CATHODICAREA

ANODICAREA

CATHODICAREA

25 FEET FROM PIPE

OVER PIPE

100 200 300 400 500 600 700 800 900 1000

-120

PIPE

–TO

–SO

IL P

OTE

NTI

AL

(MIL

LIVO

LTS)

POTENTIAL PROFILE SHOWING BIMETALLIC EFFECT

FIGURE 1-10

-110

0


-100

-900

-800

-700

-600

-500

-400

-300

-200

-100

NORMAL GALVANICDISCHARGE TOCOPPER NORMAL

25 FEET FROM PIPE

OVER PIPE

100 200 300 400 500 600 700 800 900 1000

-120

PIPE

–TO

–SO

IL P

OTE

NTI

AL

that type of situation, a complete investigationof the conditions which exist in a given area willusually yield the information necessary toestablish whether the potentials are attributableto interference or bimetallic activity.

Even without knowledge of the surroundingconditions, there are usually sufficientdifferences between two profiles to distinguishthem. Thus, in an interference situation, theportions of the pipe immediately adjacent to theanodic area are almost always entirelycathodic. In a bimetallic situation, the profile ofthe pipe outside the area of bimetallic influenceis one which is similar to the normal galvanicprofile shown in Figure 1-8.

For the purpose of distinguishing the type ofcorrosion activity that is present, in the absenceof a cathodic protection system, the followinggeneral rules can be given.

1. Anodic areas exist at locations where thelateral (off the pipe) measurements are lessnegative than the “over the pipe”measurements. Conversely, cathodic areasexist at locations where the lateralmeasurements are more negative than the“over the pipe” measurement. This conditionalways holds, irrespective of whether thecorrosion activity is a result of an electrolyticor galvanic action.

2. When anodic areas coincide with areas ofhigh negative potential, the corrosion activityis one of “straightforward” galvanic action(this does not include bimetallic activity). Ina straightforward galvanic situation, themore negative the potential, the more anodicthe condition will be.

3. When anodic areas coincide with areas oflow negative potentials (or in severe cases,positive potentials), the corrosion activity isone of either electrolytic action or bimetalliccoupling. In an electrolytic or bimetallicsituation, the less negative (or more positive)the potential, the more anodic the area.

INTERPRETATION OF POTENTIALS UNDERNON-STRAY CURRENT CONDITIONS

After it has been determined that stray currentsare not present, potential measurements can beused in analyzing the galvanic corrosion patternwhich may exist. Surface condition of the pipe,chemical composition of the soil, and otherlocal conditions can greatly influence staticpotentials along the length of a pipeline.Therefore, the more readings taken, the betterthe evaluation. A more comprehensiveinterpretation of potential measurements can bederived from the following statements.

1. The potentials of newer pipes are morenegative than those of older pipes.

2. The potentials of coated pipes (organiccoating such a coal tar, asphalt, plastic tape,etc.) are more negative than those of barepipes.

3. The variation in potential with respect todistance is generally greater along a barepipe than along a coated pipe.

4. The normal potentials taken along a barepipe fall in the range of -500 to -600 mV. Anewly installed bare pipe will have highernegative potentials, and very old bare pipecan have much lower negative potentials.

5. The normal potentials taken along a coatedline fall in the range of -650 to -750 mV. Anewly installed coated pipe can have highernegative potentials, and older pipe or poorlycoated pipe will have less negativepotentials.

6. In highly alkaline soils, the normal potentialswill be less negative than those taken inneutral or acidic soils.

7. On bare pipe, there is usually a correlationbetween potential measurements andresistivity measurements, i.e., locations ofhigher resistivity soil correspond withlocations of lower negative potentials.


It should be understood that all of the abovestatements apply primarily to steel pipelinesthat are in non-stray current areas and are notsubject to bimetallic influences or cathodicprotection. Also, where comparisons are made,such as between coated and bare pipe orbetween old and new pipe, it is assumed thatother conditions are equal.

These statements are intended as ageneralized guide for interpretation of potentialmeasurements. They are not to be consideredas scientific principles and they do notnecessarily hold true under all circumstances.They are merely a summation of fieldexperience. They are not derived fromcontrolled experiments or rigid reasoning.Despite the apparently severe qualificationsthat have been applied to these statements, ifthey are used with care and with completeappreciation of the theory of galvanic corrosion,most corrosion problems can be evaluatedsuccessfully.

COATED CROSS-COUNTRY PIPELINEWITHOUT CATHODIC PROTECTION

The potential measurements shown in Table1-2 were recorded during a pipe-to-soil potentialsurvey along a portion of a well-coated,cross-country pipeline. Examining the data inthe table we see that about one-half of thereadings/locations fall in the range as indicatedin statement 5, measurements at five of themetering and regulator stations were lessnegative than those given in statement 5. (Thepotential of -780 mV at location 31 is slightlymore negative than the range given instatement 5, but as indicated in that statement,higher negative potentials are possible onnewly installed, well-coated pipes.)

The low negative potentials at the metering andregulator stations prompted a more detailedinvestigation.

This investigation found that the isolators thathad been installed at those stations wereshorted, and as a result, the coated line wasdirectly coupled to older uncoated distribution

piping at those locations.

Statements 1 and 2 indicate that older, barelines are of lower negative potentials thannewer, coated lines. The coupling of the coatedline to the bare line makes the potentials on thecoated line less negative at the shorted meterand regulator stations than along the balance ofthe pipeline. This situation shows theimportance of placing isolating flanges betweennew coated pipe and older bare pipe. Thisisolation is needed whether or not cathodicprotection is provided for the coated line. Ifcathodic protection is not provided, the newcoated pipe will be anodic with respect to theolder, bare pipe. As a result, if there were noisolating flange installed between the twosections, or if the flange were shorted, thenewer coated section of pipe would besubjected to accelerated galvanic corrosionattack. When using isolating flanges or devices,safety precautions must be followed to preventa spark or arc across the isolator during a faultor power surge or static discharge. Devicesexist that will maintain DC isolation while actingas a shunt under the above conditions.

GAS SERVICE LINE

The previous example shows the need for athorough investigation of a situation wherethere is an apparent contradiction between themeasurements taken and the statements listed.The next example illustrates this need in aslightly more complicated situation. In thisexample we have gas service lines in a housingdevelopment. The original service lines hadbeen installed without coating, and failuresbegan to appear after five years. The lineswhich failed were replaced with coated steelpipes, and some of the replacement pipedeveloped leaks within only two years ofservice. A potential survey conducted on thesystem found a range of potentials from -250 to-350 mV on the new coated replacement linesand potentials ranging from -500 to -600 mV onthe older bare lines.

Examination of these data tends to indicate acontradiction to statements 1, 2, and 5. Upon


TABLE 1-2

Potential Measurements

Test StationLocation No. Description of Location Pipe-to-Soil

Potential (millivolts)

21 Crestdale Regulator Station -703

22 North side of Blue River on Rt. 95 -700

24 South side of Blue River -735

26 Linden Metering Station -542

26 Valve box approximately 7.8 miles South of Blue River -730

28 Creek, 10.2 miles south of Blue River -674

30 Atlantic Regulator Station -563

31 Glendale Metering Station -780

32 Crossing of railroad near Glendale -506

33 Forest Park Regulator No. 1 -480

35 Forest Park Regulator No. 2 -537

closer inspection however, it was discoveredthat the water service lines in the developmentwere electrically continuous with the gasservice lines. Copper, being cathodic withrespect to steel, will when connected to a steelpipe, cause the potential of the steel pipe tobecome less negative than its natural potential.As a result of area relationships (the ratio ofareas of copper to steel is greater when thesteel is coated than when it is bare), the effectof copper on the potential of coated steel pipewill be far greater than that on the potential ofbare pipes.

GROUNDING SYSTEM

The next example involves a somewhat similarsituation as the previous example, but anextensive copper electrical grounding system isinvolved rather than copper pipes.

A potential profile was conducted on a newlyinstalled, underground, coated steel pipe. Theover-the-line potentials were in the range of-600 to -700 mV, which seems to be inaccordance with the previous statements. Anoff-the-line potential profile was also conductedon the line. Interestingly, the two profiles werenearly identical.

Potential profiles were also conducted alonganother newly installed, well coated pipeline ina similar environment. In this case, however,there was an extensive grounding systeminstalled along the pipeline and electricallyconnected to it. The grounding systemconsisted of copper mats installed at 50 footintervals. The potentials measured along thepipeline were considerably less negative thanexpected along a coated pipe; all of thepotentials were less negative than -500 mV,and at some locations the potentials were lessnegative than -300 mV. In addition, the off-the-line potentials were considerably less negativethan the over-the-pipe potentials.

These low negative potential levels and thelateral potential gradients are directlyattributable to the copper grounding system andindicate that severe bimetallic (galvanic)

corrosion activity exists. In fact, the condition atthis plant was so severe that leaks developedwithin the first year of operation despite the factthat the pipeline was extremely well-coated andhad been backfilled with sand. The very goodcoating no doubt contributed to the rapiddevelopment of leaks on the pipeline due to theconcentration of current discharge at holidaysin the coating. The corrosion current wasgenerated by the bimetallic coupling.

STEEL GAS AND WATER LINES

In the examples given thus far, it is possiblethat the bimetallic effect of copper could havebeen anticipated without having a completeknowledge of the potential pattern. The nextexample, however, describes a situation whichis similar to a bimetallic corrosion pattern butwhere there is no copper present. This exampleinvolves a group of school buildings. Leaksoccurred on these pipes within the first year,and these leaks were attributed to the verycorrosive soil in which resistivity was less than100 ohm-centimeters. However, potentialmeasurements taken on the gas and waterlines were all in the range of -350 to -450 mV.If the corrosion activity was one ofstraightforward galvanic action resulting fromthe corrosiveness of the soil, these potentialswould be considered as very cathodic and,therefore, not indicative of a very corrosivecondition. Therefore, it was difficult to reconcilethe very severe corrosion history with theapparent cathodic potentials.

Because the soil at this location was veryalkaline, somewhat lower potentials thannormal would be expected. However, the verylow negative potentials at this site could not beentirely attributed to the alkaline soil. Moredetailed investigation found that the lowpotentials on the gas and water lines werecaused by the radiant heat piping under each ofthe buildings. Although the radiant heat lineswere also steel, they were installed in concrete,and the normal potential of steel in concrete isapproximately -200 mV, versus the normal -500to -600 mV potential of steel in earth. Theelectrical coupling of the gas and water lines in


the soil to the radiant heat lines in the concrete(there were common structural connections inthe boiler room) made the potential on the gasand water lines less negative than normal.These gas and water lines thus behaved as theanode in a mechanism which is similar to abimetallic couple. The conditions which madethis couple particularly severe were the lowresistivity soil, the proximity of the gas andwater lines to the radiant heat lines, and thefact that the area of the gas and water lines wassmall compared to the area of the radiant heatlines. This area relationship was made stillworse by the fact that the gas and water lineswere coated.

The examples given show the manner in whichpipe-to-soil potential measurement can be usedin analyzing a corrosion problem. Theseexamples further show the need for learning asmuch about the structure under investigation aspossible and about any other structures in thearea. The remaining part of this chapter dealswith the current NACE criteria for cathodicprotection of steel and cast iron structures andsome of their applications.

CRITERIA FOR CATHODIC PROTECTION

Cathodic protection criteria are listed in NACEStandard Practice SP0169-2007 “Control ofExternal Corrosion on Underground orSubmerged Metallic Piping Systems”.

Paragraph 6.1.1 of SP0169-2007 states “Thissection lists criteria and other considerations forCP that indicate, when used either separatelyor in combination, whether adequate CP of ametallic piping system has been achieved.”

SP0169-2007 lists three (3) main criteria for thecathodic protection of steel and cast ironstructures. These criteria are as follows:

1. A negative (cathodic) potential of at least850 mV with the CP applied. This potential ismeasured with respect to a saturatedcopper/copper sulfate reference electrodecontacting the electrolyte. Voltage dropsother than those across the structure-

to-electrolyte boundary must be consideredfor valid interpretation of this voltagemeasurement.

2. A negative polarized potential of at least 850mV relative to a saturated copper/coppersulfate reference electrode.

3. A minimum of 100 mV of cathodicpolarization between the structure surfaceand a stable reference electrode contactingthe electrolyte. The formation or decay ofpolarization can be measured to satisfy thiscriterion.

SP0169-2007 also states “It is not intended thatpersons responsible for external corrosioncontrol be limited to the criteria listed below.Criteria that have been successfully applied onexisting piping systems can continue to be usedon those piping systems. Any other criteriaused must achieve corrosion controlcomparable to that attained with the criteriaherein.”

Some examples of other criteria that have beenused in the past are:

C Net Current FlowC E-log I CurveC 300 mV Shift

The criteria listed for steel and cast iron, as wellas the criteria given for other metals, requirepipe-to-soil potential measurements. Whenthese criteria are to be applied in a particularsituation and potential measurements aretaken, a number of questions that are notalways precisely answerable arise:

1. Which criterion or criteria are applicable tothe particular corrosion mechanism?

2. Where should the reference electrode beplaced to obtain valid readings?

3. How many readings are required to obtainan accurate representation of the entirepipeline or piping network?


WHICH CRITERION?

The selection of the criterion to use should begoverned not merely by what is mostconvenient for the user. It should be recognizedas a function of the particular corrosionmechanism being considered.

POTENTIAL MEASUREMENT OF -850 mV TOA Cu/CuSO4 REFERENCE ELECTRODEWITH CATHODIC PROTECTION APPLIED

Of the three criteria listed above, the -850 mVcriterion with cathodic protection applied hashistorically been the one most widely used fordetermining if an acceptable degree of cathodicprotection has been achieved on a buried orsubmerged metallic structure. In the case of asteel structure, an acceptable degree ofprotection is said to have been achieved whenat least a -850 mV potential difference existsbetween the structure and a CSE contacting thesoil directly above and as close to the pipe aspossible.

Some sources indicate that this criterion wasdeveloped from the fact that the most negativenative potential found for coated steel was -800mV. Therefore, the assumption was made thatif sufficient current is applied to raise thepotential of the entire structure to a value morenegative than the open circuit potential, thenthe effects of corrosion will be mitigated. Apotential of -850 mV with respect to a saturatedcopper/copper sulfate reference electrode wastherefore adopted.

A potential of -850 mV or more negative, withthe protective applied, to a CSE placed incontact with the soil directly over or adjacent tothe steel structure would be an indication thatadequate protection is being achieved at thatpoint on the structure as long as any voltagedrops (IR drops) across the electrolyte orthrough the metallic return path are taken intoaccount. The wording for this particular criteriadoes state that voltage drops other than thoseacross the structure-to-electrolyte boundarymust be considered. Consideration is thendefined as:

C “Measuring or calculating the voltagedrop(s)”

C “Reviewing the historical performance of thecathodic protection system”

C “Evaluating the physical and electricalcharacteristics of the pipe and itsenvironment”

C “Determining whether or not there is physicalevidence of corrosion”

Voltage drops can be reduced by placing thereference electrode as close to the pipe surfaceas possible. It should be noted that this means“electrically” close, not just “physically” close. Areference electrode placed physically close toa well coated pipe is not electrically close to itand voltage drops are not reduced.

The voltage drop can also be eliminated byinterrupting all sources of cathodic protectioncurrent and measuring the “off” potential. Theinstantaneous “off” potential should be free ofvoltage drop error. Comparison of the “on” and“off” potentials and noting the differencebetween them will indicate the approximatevoltage drop included in the potentialmeasurement when the measurement is madewith protective current applied. This “instant off”or polarized potential will be discussed in moredetail when the second listed criterion isanalyzed.

Voltage drops are more prevalent in the vicinityof an anode bed or in areas where dynamicstray currents are present. Unusually highresistivity soil will also cause high voltagedrops. In areas where dynamic stray currentsare present, an instantaneous reading may beimpossible to accurately record and thereforemeaningless. The use of voltage recordinginstruments is recommended so as to obtainpotential values over a 24-hour period, or atleast during the known period of highest straycurrent activity. Often times it is possible toobtain fairly stable potentials of the pipingsystem once the DC transit system stopsoperating in the early morning hours if recording


instruments are left in place overnight. Thesepotentials can provide a base-line from which toevaluate other measurements.

The -850 mV criterion with cathodic protectionapplied is the one which is almost always usedin areas with significant dynamic stray currentactivity. It is generally accepted that if thepotential of the structure to a CSE remainsmore negative than -850 mV at all times, evenif there are substantial fluctuations in potentialwith time, then the pipe can be consideredprotected at that particular test point. It ispossible that the amount of test points surveyedand the frequency at which those surveys areperformed may have to increase in heavy straycurrent areas due to ever-changing conditions.

Limitations of the -850 mV Potential to aCu/CuSO4 Reference Electrode WithCathodic Protection Applied Criterion

A limitation of this criterion for insuring totalprotection is that potentials can vary widelyfrom one area of the underground structure toanother as a result of coating damage,interference effects, etc. This suggests thepossibility of potentials being less negative than-850 mV in sections of the pipe between twoconsecutive test locations. Also, the voltagedrop component in the potential measurementhas to be considered unless the “instant off” orpolarized potential of -850 mV is used. In manycases, removal of the voltage drop componentor testing for polarized potentials may bedifficult, particularly when it is not possible toremove and/or interrupt all sources of cathodicprotection current. Piping systems protected bymultiple galvanic anode installations is anexample of a situation in which interrupting allsources of cathodic protection current may beimpractical.

More negative potentials than -850 mV can ofcourse be maintained at all test points, whichcould insure a greater degree of protectionbetween test points. However, economics is afactor to consider; higher potentials may resultin a higher power consumption. Additionally, inthis situation, care has to be exercised to insure

that the polarized potential of the pipeline doesnot reach values at which coating damagecould occur due to hydrogen evolution at thesurface of the pipeline.

Although this accepted criterion can be, and is,widely used for all types of structures becauseof its straightforward approach and simplicity,its most economical use is in the case of coatedstructures. Use of this criterion to protect an oldbare structure could require substantially higherprotective current than if one of the othercriterion were used.

Another limitation of this criterion is due to therequirement that potential readings be takenwith the reference electrode contacting theelectrolyte directly over or adjacent to thepipeline to minimize voltage drop. In cases ofriver crossings, road crossings, etc., where theelectrode cannot be properly placed, analternative criterion may have to be used.

POLARIZED POTENTIAL OF -850 mVMEASURED TO A Cu/CuSO4 REFERENCEELECTRODE

As discussed in the previous criteria section,one method of considering the voltage (IR) dropacross the electrolyte is to measure thestructure potential with all current sources “off”.The potential that is then measured would bethe polarized potential of the structure at thatparticular test location. A polarized potential isdefined in Section 2 of SP0169-2007 as, “Thepotential across the structure/electrolyteinterface that is the sum of the corrosionpotential and the cathodic polarization”. Thedifference in potential between the static ornative potential and this polarized (instant off)potential would be the amount of polarizationthat has occurred due to the operation of thecathodic protection system. In the same sectionof SP0169-2007 polarization is defined as, “Thechange from the open-circuit potential as aresult of current across the electrode/electrolyteinterface.”


Limitations of the Polarized Potential of -850mV Measured to a Cu/CuSO4 ReferenceElectrode Criterion

Polarized potentials well in excess of -850 mVshould be avoided for coated structures in orderto minimize the possibility of cathodicdisbondment of the coating. SP0169-2007 alsopoints out that, “Polarized potentials that resultin excessive generation of hydrogen should beavoided on all metals, particularly higherstrength steel, certain grades of stainless steel,titanium, aluminum alloys, and prestressedconcrete pipe.”

100 mV POLARIZATION CRITERION

The 100 mV polarization criterion, like the -850mV polarized potential, is based on thedevelopment of polarization. This causes thestructure to exhibit a more negative potentialthan in its native state.

Measurement of the polarization shift can bedetermined by either measuring its formation ordecay. Determination of the amount ofpolarization is normally made during thepolarization decay (positive shift) periodsubsequent to de-energizing the cathodicsystem, or in the case of galvanic anodes,when they are disconnected. When cathodicprotection system is de-energized, there is animmediate rapid drop of potential. Thepolarization decay does not include this drop.The potential of the structure after this initialrapid drop is recorded and used as a basis fordetermining the polarization shift. To obtain thetotal polarization shift, the final potential afterpolarization decay has taken place is measuredand subtracted from the potential readimmediately after the cathodic system has beenturned off or disconnected. If this total shift is100 mV or more, the structure is considered tobe cathodically protected.

One drawback of this method is that the timerequired for full depolarization could take fromhours to days for a coated structure to severalweeks for a bare structure. This could make thismethod very time consuming and also might

leave the structure unprotected for an extendedperiod of time. Normally however, the bulk ofthe depolarization will take place in the initialphase of the polarization decay; therefore itmay not be necessary to wait the full decayperiod except in those cases in which the totalactual polarization shift of the structure isrelatively close to 100 mV. If, during the earlyphase of polarization decay measurement, thepotential drops 100 mV or more, there is no realneed (unless the actual value is desired) to waitfor further de-polarization. If, on the other hand,the potential drop in the initial phase of thedecay period is only on the order of 50 to 60mV, it may be doubtful that the 100 mV shift willoccur. In this case, a determination should bemade as to whether a longer wait for totalde-polarization is required and justifiable.

In order to determine the formation ofpolarization on a pipeline/structure, it is firstnecessary to obtain static or native potentials(before the application of cathodic protectioncurrent) on the structure at a sufficient numberof test locations. Once the protection system isenergized and the structure has had time topolarize, these potential measurements arerepeated with the current source interrupted.The amount of polarization formation can thenbe determined by comparing the staticpotentials with the “instant off” potentials.

Applications of the 100 mV PolarizationCriterion

In many cases, the cost of the power requiredto achieve the 100 mV polarization decaycriterion may be less than that required to meeteither the -850 mV potential with cathodicprotection applied criterion or the -850 mVpolarized potential criterion. Further, thepolarized potential corresponding to 100 mV ofpolarization may be less negative than -850mV. This criterion takes into account only thepolarization film on the pipe surface and isindependent of any voltage (IR) drops in the soilor at the structure electrolyte interface. Thiscriterion should not be used in areas subject tostray currents because any outside interferencewould tend to break down the polarization at the


point of discharge. The results obtained underthese conditions could be misleading.Corrosion personnel must determine theeffectiveness of this criterion in areas of straycurrent activity.

The 100 mV polarization criterion is mostlyused on poorly coated or bare structures and insome instances could be useful in large pipenetworks as in compressor and regulatingstations where the cost of a cathodic protectionsystem to achieve either a -850 mV “on”potential or a -850 mV polarized potential maybe prohibitive. However, in an economicanalysis, the additional cost of conductingfuture periodic surveys has to be taken intoaccount; as the use of the 100 mV criterion issomewhat more complicated and costly thanthe use of the -850 mV criteria with the cathodicprotection applied.

This criterion can be used on metals other thansteel where there could be some question as towhat specific potential to use as an indication ofprotection. This criterion is often used for thoseinstallations where it is impractical to meeteither of the -850 mV criterion.

In piping networks, where new pipe is coupledto old pipe, it may be good practice to use the-850 mV polarized potential criterion for the newpipes and the 100 mV polarization criterion forthe older pipes.

Limitations of the 100 mV PolarizationCriterion

The limitations for the 100 mV polarizationcriterion are as follows:

C Interrupting all DC current sources is notalways possible.

C Stray current potential variations can makethis criterion difficult to measure.

C If dissimilar metals are coupled in thesystem, protection may not be complete onthe anodic metal.

C The only additional limitation is related to thetime required for the pipe to depolarize. Insome cases, adequate time may not beavailable to monitor the polarization decay ofthe pipe to the point where the criterioncould have been met.

CRITERIA FOR SPECIAL CONDITIONS -NET PROTECTIVE CURRENT CRITERION

This criterion is listed in Section 6 of SP0169-2007 under a special conditions section tocover those situations for bare or ineffectivelycoated pipelines where it appears that long linecorrosion activity is the primary concern.

This protection criterion is based on thepremise that if the net current at any point in astructure is flowing from the electrolyte to thestructure, there cannot be any corrosion currentdischarging from the structure to the electrolyteat that point. The principle is based on firstlocating points of active corrosion and thenmeasuring current flow to or from the structure.

Some pipeline companies use the side drainmethod for application of this criterion on thebasis that if the polarity of the voltage readingson each side of the structure indicates currentflow towards the structure, then the structure isreceiving protective current. If the electrodelocated over the pipe is positive with respect tothe other two electrodes, then current isdischarging from the pipe to the electrolyte andcorrosion is taking place.

The above statement is considered correct aslong as there are no outside sources ofinfluence such as other pipelines or othergradient sources such as stray currents. Incases where other gradient sources exist, theresults could be misleading. The results mightalso be questionable in areas with highresistivity surface soil, for deeply buriedpipelines, or where local corrosion cell exists.

Applications of the Net Protective CurrentCriterion

This criterion is normally used on uncoated


structures. The “side drain’ surface method isnormally used in cases of single isolatedpipelines to obtain an indication of whether thepipeline was receiving cathodic protection. Ifthe electrodes that are placed perpendicular tothe pipeline (remote) are positive in relation tothe electrode over the pipeline, it was assumedthat current is flowing toward the pipe.

In cases where there are sources of outsidegradients, it may be difficult to evaluate theresults of these tests properly.

Limitations of the Net Protective CurrentCriterion

The use of this criterion is to be avoided inareas of stray current activity because potentialvariations could interfere with its use. Also,extreme care must be taken in commonpipeline corridors because other gradientsources may exist that could result in erroneousor misleading measurements.

Even though the results indicate a net currentflow towards the pipe/structure, that net currentflow is indicative only of what is happening atthe specific point of test and does not representwhat may be happening at other points on thepipeline/structure. Pipeline companies that usethe side drain method for application of thiscriterion, normally require that tests beconducted at close intervals (2 to 20 feet alongthe pipeline).

OTHER CRITERIA

The two criteria listed below have been used inthe past and are presented for information.

E-LOG I CURVE CRITERION

The E-Log I technique in itself is not so much acriterion of protection as it is a means ofdetermining the correct amount of cathodicprotection current required for a structure.Being complicated in application, the E-log Icriterion is limited to specialized applicationswhere other methods may be inadequate.

Applications of the E-Log I Curve Criterion

The E-Log I Curve criterion is not generallyused by itself to evaluate existing cathodicprotection systems. Its primary use is todetermine the potential value, measured withrespect to a reference electrode, which will givea specific minimum current value required forprotection. This potential value is to be at leastas negative (cathodic) as that originallymeasured at the beginning of the Tafel segmentof the E-log I curve.

Once the current value and the potential to aremote electrode have been established, futuresurveys consist of checking the current outputof the cathodic protection system and thepotential of the structure to a remote electrode.It is important that the reference electrode belocated in the same place where it was locatedduring the E-Log I tests. Because the testmethod involved is rather elaborate, the use ofthis method is generally limited to structureswhere conventional means of determiningcurrent requirements would be difficult.Examples of such structures are pipeline rivercrossings, well casings, piping networks in aconcentrated area, and in industrial parks.

Limitations of the E-Log I Curve Criterion

The main limitation of this technique is thattesting is elaborate, thus making it relativelyslow in comparison to that required for othercriteria. Future tests are relatively simple,however. The remote reference electrode hasto be located exactly at the same locationswhere it was placed during the E-Log I tests.Because the E-Log I curve is time dependent,there is no guarantee that a repeat E-Log Icurve will yield the same minimum potential orcurrent for protection as an earlier curve.

Using this criterion in areas where straycurrents are present could result in erroneousresults as the stray currents may affect thereadings and make obtaining a goodpolarization curve impossible due to thefluctuations in the structure potential.


300 mV POTENTIAL SHIFT CRITERION

As opposed to working toward a certainminimum potential value to a referenceelectrode as discussed in some of the previoussections, the 300 mV potential shift criterion isbased on changing the potential of the structurein the negative direction by a specifiedminimum amount. The minimum potential shiftfor steel, as used in the past is 300 mV. Anystable reference electrode may be used withthis criterion because the method consists ofmeasuring a potential shift and is independentof the actual potential of the electrode.Determination of the voltage shift is made withthe protective current applied.

The development of this criterion appears tohave been largely empirical or experimental innature. Although 300 mV is the Figurecommonly used at this time, other values suchas 200 or 250 mV have also been used in thepast. There were, however, two considerationsthat supported this criterion, recognizing thatcorrosion prevention still may not be 100%complete.

The first consideration was that 300 mV may begreater than the driving potential of most of thegalvanic cells on a structure to be protected. Byshifting the structure in the negative direction by300 mV, the majority of the cells should beovercome.

The second consideration was based on testsconducted by Marcel Pourbaix. His workshowed that if a protective potential applied toa structure is made more negative by equalincrements starting from the natural or staticpotential, the first increment would yield thegreatest reduction in corrosion rate andsubsequent increments would result inprogressively smaller reductions in thecorrosion rate per increment. Experienceindicated that the 300 mV shift encompassedsufficient reduction in the rate of corrosion tomake it a somewhat effective criterion forpractical applications.

Later evaluation of this criterion by a NACE

committee concluded that the actualmeasurements or shift was not representativeof what was occurring on the surface of thestructure. This is due to the fact that when acathodic protection system is energized, animmediate shift of potential, due to a voltage(IR) drop, will be seen. In this particularapplication this voltage drop value is included inthe measured shift.

Applications of the 300 mV Potential ShiftCriterion

This criterion has been used for entirestructures and also for hot-spot protection. It ismainly used for bare steel structures whichhave undergone a slow uniform corrosion ratedue to their age. These structures/pipesnormally have a natural potential range fromabout -200 mV to -500 mV as a result ofoxidation products developing on the externalsurfaces. This criterion has also been used onsome coated pipelines where soil conditionsalter the natural or static potential of the steel to-500 mV or lower. In this case, the 300 mV shiftmay also be easier to attain than achieving apotential reading of -850 mV, and could beexpected to stop the majority of corrosion.

The 300 mV potential shift criterion is almostalways more applicable to impressed currentsystems than galvanic anode systems. Thereason is that the native state potential may besuch that a limited number of galvanic anodeswill not produce the required change ofpotential on the structure to be protected.

Limitations of the 300 mV Potential ShiftCriterion

The 300 mV potential shift in some instances isimpractical to use and in other cases, may notcorrectly indicate the level of cathodicprotection achieved. Some galvanic cells maystill be active even after the 300 mV shift isobtained and there could be areas betweenadjacent structure to electrolyte readings wherethe 300 mV shift is not being attained.Furthermore, a pipeline subject to stray currentscould have positive potentials with respect to a


CSE even after the pipe potential is shifted inthe negative direction by 300 mV. If thepotential of the structure is fluctuating morethan the shift required, this criterion cannot bevalid.

If the steel structure is coupled to a more noblemetal, a 300 mV shift might indicate that muchof the adverse effect produced by the dissimilarmetal union has been overcome, but it wouldnot necessarily indicate that complete cathodicprotection is being provided to the steelstructure/pipeline itself.

USE OF CRITERIA

It must be noted that there will be situations orconditions where a single criterion cannot beused to evaluate the effectiveness of a cathodicprotection system and it is necessary to employa combination of criteria.

There will also be situations when theapplication of the criteria listed may beinsufficient to achieve protection. Someexamples pointed out in the SP0169-2007 aresituations where the presence of sulfides,bacteria, elevated temperatures, acidenvironments, and/or dissimilar metals increasethe amount of current required for protection. Atother times values less negative than thoselisted may be sufficient. SP0169-2007 listsexamples such as a pipeline encased inconcrete or dry or aerated high resistivity soil.

REFERENCES

1. NACE Standard Practice SP0169-2007“Control of External Corrosion onUnderground or Submerged Metallic PipingSystems”, NACE International, Houston,Texas


CHAPTER 2

EVALUATION OF UNDERGROUND COATINGS USINGABOVEGROUND TECHNIQUES

INTRODUCTION

This chapter describes the indirect inspectionmethods intended for use as part of theExternal Corrosion Direct Assessment (ECDA)process for detecting coating flaws anddetermining cathodic protection levels on buriedpipelines. These methods are often morelaborious than surveys completed as part ofnormal daily operating practices as a result ofthe requirement for precise data set alignment.Standard pipeline surveys generally investigatedata trends over time or pipeline distance, whileECDA surveys look for small data variationsover short distances.This chapter describes the methods ofconducting the following surveys for abovegrade indirect inspections. Other inspectionmethods can and should be used as requiredby the unique situations along a pipeline. Thetechniques described herein are not intended toillustrate the only methods by which these toolscan be applied.

• Pipeline Locating is used to establish thelocation and centerline of the pipeline.

• Direct Current Voltage Gradient (DCVG)Surveys are used to locate and size coatingholidays.

• Alternating Current Voltage Gradient(ACVG) Surveys are used to locate andsize coating holidays.

• Close-Interval Surveys (CIS) are used todetermine cathodic protection (CP) levels,electrical shorts to other structures, static

stray current conditions, and large coatingholidays.

• Alternating Current (AC) AttenuationSurveys are used to assess coating qualityand to detect and compare coatinganomalies.

SURVEY SEQUENCE

The sequence in which the surveys areconducted is crucial to optimizing the surveytechniques and data analysis. Coating holidaysurveys (DCVG and ACVG) and AC attenuationsurveys should be completed prior to a CISsurvey. With surveys completed in this manner,pipe to electrolyte potentials can be measureddirectly above the coating holiday indicationsfound using the DCVG or ACVG method.

Some ECDA surveys require existing CPsystems to be turned off, the rectifier outputsincreased, or systems disconnected. Structurepotentials should be measured and recorded atsufficient locations to demonstrate that thepipeline has returned to the original level ofpolarization prior to completion of additionalsurveys that may be influenced by theseactions. The structure potentials should bemeasured before and after these activities havetaken place.

REFERENCED PROCEDURES

Additional information regarding the techniquescited in this chapter can be found in the latestrevisions of the following documents:

EVALUATION OF UNDERGROUND COATINGS USING ABOVEGROUND TECHNIQUES2-1

• NACE/CEA 54277, Specialized Surveys forBuried Pipelines 1988

• NACE Standard TM0497, MeasurementTechniques Related to Criteria for CathodicProtection on Underground or SubmergedMetallic Piping Systems

• NACE Standard SP0169, Control ofExternal Corrosion on Underground orSubmerged Metallic Piping Systems

• NACE Standard SP0177, Mitigation ofAlternating Current and Lightning Effects onMetallic Structures and Corrosion ControlSystems

DEFINITIONS

a) Anomaly: Any deviation from nominalconditions in the external wall of a pipe, itscoating, or the electromagnetic conditionaround the pipe.

b) Cathodic Protection (CP): A technique toreduce the corrosion of a metal surface bymaking the surface the cathode of anelectrochemical cell.

c) Close-Interval Survey (CIS): A method ofmeasuring the potential between the pipeand earth at regular intervals along thepipeline.

d) Corrosion: The deterioration of a material,usually a metal that results from a reactionwith its environment.

e) Disbonded Coating: Any loss of adhesionbetween the protective coating and a pipesurface as a result of adhesive failure,chemical attack, mechanical damage,hydrogen concentrations, surfacepreparation and application problems, etc.Disbonded coating may or may not beassociated with a coating holiday.

f) ECDA Region: A section or sections ofpipeline that have similar physicalcharacteristics and operating history and in

which the same indirect inspection tools areused.

g) Electrolyte: A chemical substancecontaining ions that migrate in an electricfield. For the purposes of this chapter,electrolyte refers to the soil or liquidadjacent to and in contact with a buried orsubmerged metallic piping system,including the moisture and other chemicalscontained therein.

h) Electromagnetic Inspection Technique: Anaboveground survey technique used tolocate coating defects on buried pipelinesby measuring changes in the magnetic fieldthat are caused by the defects.

i) External Corrosion Direct Assessment(ECDA): A four-step process that combinespre-assessment, indirect inspections, directexaminations, and post assessment toevaluate the impact of external corrosion onthe integrity of a pipeline.

j) Fault: Any anomaly in the coating, includingdisbonded areas and holidays.

k) Holiday: A discontinuity (hole) in aprotective coating that exposes thestructure surface to the environment.

l) Indication: Any deviation from the norm asmeasured by an indirect inspection tool.

m) Indirect Inspection: Equipment andpractices used to take measurements atground surface above or near a pipeline tolocate or characterize corrosion activity,coating holidays, or other anomalies.

n) IR Drop: The voltage difference betweenthe On and Off pipe to soil potential.

o) Microbiologically Influenced Corrosion(MIC): Localized corrosion resulting fromthe presence and activi t ies ofmicroorganisms, including bacteria andfungi.


p) Pipe-to-Electrolyte Potential: SeeStructure-to-Electrolyte Potential.

q) P i p e - t o - S o i l P o t e n t i a l : S e eStructure-to-Electrolyte Potential.

r) Region: See ECDA Region.

s) Structure-to-Electrolyte Potential: Thepotential difference between the surface ofa buried or submerged metallic structureand the electrolyte that is measured withreference to an electrode in contact with theelectrolyte.

SAFETY CONSIDERATIONS

Appropriate safety precautions, including thefollowing, should be observed when makingelectrical measurements.

• Be knowledgeable and qualified in electricalsafety precautions before installing,adjusting, repairing, removing, or testingimpressed current cathodic protectionequipment.

• Use properly insulated test lead clips andterminals to avoid contact with anunanticipated high voltage (HV). Attach testclips one at a time using the single-handtechnique for each connection.

• Use caution when long test leads areextended near overhead high-voltagealternating current (HVAC) power lines,which can induce hazardous voltages ontothe test leads. Refer to NACE StandardSP0177 for additional information aboutelectrical safety.

• Use caution when performing tests atelectrical isolation devices. Beforeproceeding with further tests, useappropriate voltage-detection instrumentsor voltmeters with insulated test leads todetermine whether hazardous voltages,both AC and DC, may exist.

• Avoid testing when thunderstorms are in the

area. Remote lightning strikes can createhazardous voltage surges that travel alongthe pipeline.

• Use caution when stringing test leadsacross streets, roads, and other locationssubject to vehicular and pedestrian traffic.When conditions warrant, use appropriatebarricades, flagging, and/or flag persons.

• Observe appropriate Company safetyprocedures, electrical codes, and applicablesafety regulations.

PIPELINE LOCATING

The pipeline must be located and marked toensure that subsequent measurements aremade directly above the pipeline. An inductiveor conductive pipe locating device can be used.The pipe should be located within six (6) inchesperpendicular of the pipe centerline and surveyflags or paint marks placed directly above thepipeline every 100 feet using a slack chaindistance technique or a measuring wheel. Slackchain stationing error shall be no more than 2%+/-.

The locating flags/paint marks can benumbered using a permanent marker by writingthe flag number directly on the flag or paintingthe flag number on the ground. Numbering ofthe 100 foot stations will ensure that thedifferent indirect survey techniques will beperfectly aligned by using the flag locations asthe point of alignment.

The pipeline should be stationed beginning with0+00 at the initial ECDA region and shouldprogress in increasing station numbers in thedirection of gas/product flow. The pipelinestationing should be reset to 0+00 at eachfollowing ECDA region start. As an alternative,actual pipeline as-built stationing can be used.With this method, the actual pipeline stationingis used for the initial start of the ECDA regionand stationing continued until the end of theECDA region.


PIPELINE LOCATING CREW CLEARING PATH AND MARKING PIPELINE CENTERLINE

FIGURE 2-1

PIPELINE LOCATION STATIONING FLAGS NUMBERED FOR PRECISE DATA ALIGNMENT

FIGURE 2-2

DIRECT CURRENT VOLTAGE GRADIENTSURVEYS (DCVG)

Direct current voltage gradient (DCVG) surveysare used to evaluate the coating condition onburied pipelines. Voltage gradients arise as aresult of current pickup or discharge at coatingholidays. In a DCVG survey, the DC signal iscreated by interrupting the pipeline’s CP currentor a temporary CP current, and the voltagegradient in the soil above the pipeline ismeasured. Voltage gradients are located by achange in the interrupted signal strength atgrade.

DCVG is the only method that can be used toapproximate the size of a coating holiday.DCVG signal strength is not always proportionalto holiday size, as the orientation of the holidayand other factors affect the measured signal.

DCVG surveys are capable of distinguishingbetween isolated and continuous coatingdamage. The shape of the gradient fieldsurrounding a holiday provides this information.Isolated holidays, such as rock damage,produce fairly concentric gradient patterns inthe soil. Continuous coating damage, such asdisbonded coatings or cracking, produceselongated patterns.

The DCVG system consists of a currentinterrupter, an analog or digital voltmeter,connection cables, and two probes withelectrodes filled with water. The interrupter isused to interrupt current from an existingrectifier unit, galvanic anode system, or atemporary CP system installed for the purposeof the DCVG survey.

An analog voltmeter must have a needle withthe ability to deflect in both the positive andnegative directions from the zero point, whichassists in determining the direction the currentis flowing in the soil. Digital voltmeters must besufficiently sensitive to measure 1 mV changesbetween the two reference probes and have theability to indicate the direction of current flow inthe soil. The voltmeter should have the ability toadjust the input impedance for use in high

resistance contact situations.

The current interrupter is installed in series withthe current source and set to cycle at a fast ratewith the “on” period less than the “off” period. Acommon interruption cycle is 0.3 seconds onand 0.7 seconds off. This short cycle allows fora quick deflection by the analog voltmeterneedle.

DCVG surveys can be performed withimpressed current CP systems energized.Sacrificial anodes and bonds that are notdisconnected show up as anomalies. Sacrificialanodes and bonds to other structures areusually disconnected to prevent signal loss andenhance current flow down the pipeline underinvestigation.

The IR drop is measured at the test stations inthe proximity of the DCVG survey. It isdesirable to have a minimum of 100 to 400 mVof IR drop in soil environments and more IRdrop when surveying on asphalt/concrete, in thesection of pipeline to be surveyed. If at theestimated daily survey section limits there is nota 100 to 400 mV IR drop, then the currentoutput of the CP current source should beincreased to achieve the desired result. If theoutput cannot be increased, then the section ofpipe with the 100 to 400 mV IR drop is the onlysection that can be surveyed using that CPcurrent source. Alternative CP current sourcesmay need to be temporarily installed in order toachieve the desired IR drop. It is desirable tohave as large an IR drop value as can beachieved. This will enable the surveyor todetect small holidays distant from the CPcurrent source.

A surveyor walks along the pipeline such thatthe probes can be used in a walking stickfashion. One probe is always kept near thepipeline center line while the other is heldapproximately five (5) feet away perpendicularto the pipe. The voltmeter is read when bothprobes are in contact with the soil.

If possible, the pipeline under investigationshould be electrically isolated from other


ANALOG DCVG METER

FIGURE 2-3

DCVG SURVEYOR MEASURING VOLTAGE GRADIENT ABOVE PIPELINE

FIGURE 2-4

DCVG SURVEY COMPLETED ON WET ASPHALT IN MAJOR CITY

FIGURE 2-5

parallel metallic structures by disconnectingelectrical bonds, negative drains to rectifiers,etc. In pipeline right of ways with multipleelectrically continuous pipelines or metallicconduits/structures, variations in the surveytechnique must be considered. For example, ifparallel pipelines (metallic structures) are lessthan ten (10) feet from the pipeline underinvestigation, then the perpendicular probeshould be placed at half the distance betweenthe two pipelines, however, difficulties may beencountered with current flow to the parallelmetallic structure. The perpendicular probemust be placed on the side of the investigatedpipeline without a parallel pipeline (metallicstructure). If the pipeline under investigationhas pipelines (metallic structures) on eitherside, then the probe should be placedperpendicular, but not above or in closeproximity to the parallel pipeline (metallicstructures).

If the voltmeter indicates a coating holiday,additional measurements should be made toconfirm the coating holiday is on the pipelineunder investigation and not the parallel pipeline.These tests include gradient measurements onboth sides of the pipeline and parallel with thepipeline under investigation to confirm thecoating holiday location.

When parallel metallic structures or right of wayconditions do not allow sufficient room forplacement of the probe to the side of thepipeline, an alternate method of DCVG can beconducted. Place both probes directly abovethe pipe centerline approximately five to ten feetapart from one another and measure thevoltage gradient in this manner. Leap frog therear probe to the forward position and repeatthe process continuing along the right of waydirectly above the pipe. Care must be taken topinpoint the maximum deflection or indicationwhen completing the survey in this manner. Theexact location of the indication requires movingthe probes at closer spacings once the coatingholiday is detected.

When a coating holiday is approached, anoticeable signal swing can be observed on the

voltmeter at the same rate as the interrupterswitching cycle. The amplitude of the swingincreases as the coating holiday is approachedand decreases after it has been passed.Current flow from the interrupted current sourceto the pipeline indicates a possible coatingholiday while current flow away from thepipeline indicates current flow past the pipeline.

When a coating holiday is found, additionalgradient measurements can be beneficial toconfirm its location and that the indication is notcurrent traveling past the pipeline. Thesegradient measurements can be made on bothsides of the pipe and parallel with the pipe oneach side of the assumed coating holiday.

A straight-line attenuation effect is assumedbetween test station locations to calculate thesignal strength at intermediate coating holidaylocations. In order to calculate the coatingholiday size (%IR), the difference between theon and off potentials at each test station, valve,or other above grade appurtenance must bemeasured and recorded.

One reference electrode is placed at the baseof the test station or other electrical contactpoint, in contact with the soil while the secondelectrode porous tip contacts the test stationwire or other properly cleaned electrical contactpoint. The maximum analog needle deflectionis the test station IR drop or difference betweenthe on and off potentials when using a digitalvoltmeter.

The distance between test stations or points ofelectrical contact must be determined from thepipeline stationing and used in the calculationsfor the signal attenuation.

An example of the attenuation calculations canbe seen below:

On potential -1.45 V

Off potential -0.95 V

Signal Strength 1.45 - 0.95 V

= 0.5 V or 500 mV


The estimated signal strength can beexemplified by using the data presented inFigure 2-6.

Estimated signal strength at defect:

Precisely locating a coating holiday is achievedby marking the approximate location of theholiday at the area where the maximumamplitude is indicated. Near the approximatecoating holiday location and offset from the lineby approximately 10 ft, the probes are placedalong the voltage gradient to obtain a null (zero)on the meter. A right-angle line through thecenter of the probe locations passes over thecoating holiday epicenter, as shown in point Ain Figure 2-7. This geometrical procedurerepeated on opposite sides of the pipelinelocates the exact point above the holiday.

A survey flag, wooden stake, paint mark, orlathe is often placed at the indication epicenterand identified by a unique indication numberusing a permanent marker or paint.

After the epicenter of the coating holiday hasbeen detected, a series of continuous lateral(perpendicular) readings are measured movingtoward remote earth. Lateral readings near theholiday yield maximum voltage differenceswhere gradients are at a maximum. Readingsat remote earth are considered to be achievedwhen one has a 1 mV deflection. Thesummation of these readings is commonlyreferred to as the over-the-line-to-remote-earthvoltage. The expression “percentage IR” hasbeen adopted to give a relative indication size.

For instance, if a series of lateral millivoltreadings to remote earth are as follows, 25, 15,6, 4, 3, 1, 1, 0, the percentage IR can becalculated as follows:

The percentage IR is used to develop a coatingcondition classification system to prioritizecoating damage.

Once an indication is located, its size orseverity is estimated by measuring the potentiallost from the holiday epicenter to remote earth.This potential difference is expressed as afraction of the total potential shift on the pipeline(the difference between the “on” and “off”potential, also known as the IR drop) resultingin a value termed % IR. DCVG survey readingscan be broken into four groups based onapproximate size as follows:

Category 1: 1% to 15% IR - Indications in thiscategory are often considered of lowimportance. A properly maintained CP systemgenerally provides effective long-termprotection to these areas of exposed steel.

Category 2: 16% to 35% IR - These indicationsare generally considered of no serious threatand are likely to be adequately protected by aproperly maintained CP system. This type ofindication may be slated for additionalmonitoring. Fluctuations in the levels ofprotection could alter the status at these pointsas the coating further degrades.

Category 3: 36% to 60% IR - The amount ofexposed steel in such an indication indicates itmay be a major consumer of protective CPcurrent and that serious coating damage maybe present. As in Category 2 indications, thistype of possible coating holiday may be slatedfor monitoring as fluctuations in the levels of CPcould alter the status as the coating furtherdegrades.

= 200 mV + 1500

500 + 1500300 - 200 mV

= 200 mV + 75 mV

= 275 mV

Over the line to remote earth voltages = 25 +15 + 6 + 4 + 3 +1+1 mV

= 55 mv

Percentage IR = Over the line to remote earth voltage * 100%

Signal Strength

= 55 *100

275

= 20%


DCVG SIGNAL STRENGTH

FIGURE 2-6

DEFECT

1500 yds 500 yds

200 mV 375 mV300 mV

DCVG VOLTAGE GRADIENTS

FIGURE 2-7

Category 4: 61% to 100% IR - The amount ofexposed steel indicates that this indication is amajor consumer of protective CP current andthat massive coating damage may be present.Category 4 indications typically indicate thepotential for serious problems with the coating. These example categories are empirical innature and are based on the results of priorexploratory excavations at holiday locationsdetermined by DCVG surveys.

ALTERNATING CURRENT VOLTAGEGRADIENT SURVEYS (ACVG)

Alternating current voltage gradient (ACVG)surveys are used to evaluate the coatingcondition on buried pipelines. Voltage gradientsarise as a result of current pickup or dischargeat coating holidays. In an ACVG survey, the ACsignal is created by a low frequency transmitterconnected to the pipeline and the voltagegradient in the soil above the pipeline ismeasured. Voltage gradients are located by achange in the signal strength at grade.

ACVG signal strength is not always proportionalto holiday size, as the orientation of the holidayand other factors affect the measured signal.

ACVG surveys are capable of distinguishingbetween isolated and continuous coatingdamage. The shape of the gradient fieldsurrounding a holiday provides this information.Isolated holidays, such as rock damage,produce fairly concentric gradient patterns inthe soil. Continuous coating damage, such asdisbonded coatings or cracking, produceselongated patterns.

The ACVG system consists of a commonlyemployed, commercially available battery/ACpowered signal-generating unit that provides a937.5-Hz AC signal with a maximum output of750 mA or a 4-Hz AC signal with a maximumoutput of three amperes. A handheld receiverunit is tuned to detect the signal frequency fromthe transmitter and block other signals. Thereare two probes which connect to the receiverand are in contact with the soil.

An AC current attenuation survey may beperformed with impressed current CP systemsenergized, however by turning off the rectifierand using the positive and negative leads at therectifier station, the signal-generationcapabilities of the equipment can bemaximized. Sacrificial anodes and bonds thatare not disconnected show up as anomalies.Sacrificial anodes and bonds to other structuresare usually disconnected to prevent signal lossand enhance current flow down the pipeline.

The signal generator (transmitter) is connectedto the pipeline and appropriately grounded toearth. A constant AC signal is produced andtransmitted along the pipe. The transmitter isenergized and adjusted to an appropriateoutput. Typically, the largest attainable currentoutput is chosen to maximize the length of pipethat can be surveyed. An impressed currentanode bed or magnesium anode can be used toestablish an electrical ground.

The receiver consists of a handheld,symmetrical, multi-axis antenna array. Theelectromagnetic field radiating from the pipelineis measured by the detector.

The detector is used to measure the attenuationof the signal current that has been applied tothe pipe. An electrical current, when applied toa well coated buried pipeline, graduallydecreases as distance increases from the pointof current application. The electrical resistivityof the coating under test and the surface areain contact with the soil per unit length of pipeare the primary factors affecting the rate ofdecline and the frequency of the signal.

The two probes must be connected to thereceiver unit and plugged in. Survey intervalsshould be no greater than ten (10) feet in thoseareas deemed to have coating holidays.

The operator walks above the pipelinecenterline placing the contact probes into thesoil above the pipe and measuring the voltagegradient and direction. Probes are placedparallel to the pipe. The receiver unit will “point”in the direction of a coating holiday. The


ACVG SURVEY ABOVE PIPELINE LOCATING COATING HOLIDAYS

FIGURE 2-8

AC CURRENT ATTENUATION RECEIVER FACE

FIGURE 2-9

magnitude will increase as the holiday isapproached.

The survey continues until the receiverindicates that the coating holiday has beenpassed (signal magnitude decreases anddirection arrow reverses direction) at whichpoint the operator reverses direction andshortens the interval between readings.

When the holiday is centered between the twoprobes, the magnitude will be zero and thedirection arrows will not indicate a consistentcurrent direction.

The probe assembly can be used to the side ofthe pipe (perpendicular to the pipe) to confirmthe coating indication location. The holidaylocation is indicated by the maximum signalmagnitude with the probes placedperpendicular to the pipe.

Either store the data in the receiver unit orrecord the information in the project field book.

CLOSE-INTERVAL SURVEYS (CIS)

CIS is used to measure the potential differencebetween the pipe and the electrolyte. Data fromclose interval surveys are used to assess theperformance and operation of the CP system.CIS can also be used to detect coatingholidays.

While other indirect inspection tools may bebetter suited to detect coating holidays, CISalso aid in identifying:

1. Interference,

2. Shorted casings,

3. Areas of electrical or geologic currentshielding,

4. Contact with other metallic structures, and

5. Defective electrical isolation joints.

On and off potential surveys are used to

evaluate CP system performance inaccordance with the NACE pipeline CP criteriaas found in SP0169. On and Off surveysmeasure the potential difference between thepipe and the electrolyte as the CP currentsource(s) is switched on and off.

On and Off surveys rely on electronicallysynchronized current interrupters at each CPcurrent source, bond, and other current drainpoint that influences the pipeline potential in thesurvey area. The ratio of the On-to-Offinterruption cycle should be long enough forreadings to be made but short enough to avoidsignificant depolarization. A three second On,one second Off cycle period or similar can beused to maintain pipeline polarization over timeand allow accurate Off potentials to berecorded.

The copper sulfate reference electrodes (CSE)are placed directly over the pipeline, typically at2.5 to 5 foot intervals such that both On and Offpotentials can be measured and recorded ateach reference cell location.

The accuracy of the on and off data can beverified by recording a continuous datalog(waveprint) at test stations or points of electricalcontact such as valves or risers. This data logwill illustrate proper interrupter synchronizationand the effects of pipeline depolarization duringthe survey should it take place. The continuousdata log should measure and store thepotentials at a minimum time period as the totalcurrent interruption cycle. Waveprints can alsobe reviewed to confirm the point at which theOff potential should be recorded by the CISdatalogger. In addition, the waveprints can beanalyzed to determine the affects of:

Inductive potential spiking,

Interrupter synchronization drift,

Stray DC and AC earth currents.

CIS equipment includes, at a minimum, severalhigh input impedance data loggers/voltmeters,sufficient current interrupters for the project,


CLOSE-INTERVAL SURVEY TECHNICIAN WITH DATALOGGER, REFERENCE ELECTRODE, AND WIRE DISPENSERS

FIGURE 2-10

copper sulfate reference electrodes, smallgauge CIS wire (30 AWG), wire dispenser, andpipe/cable locating equipment (See abovesection on pipe locating).

Standard current interrupter units include 30,60, or 100 ampere AC and DC interruptioncapacity, AC or battery-powered units, withelectronic synchronization and Global PositionSatellite (GPS) timing.

Prior to the CIS, a rectifier influence survey maybe completed to determine the CP currentsources which must be interrupted for theaccurate measuring of Off potentials. Theseinclude company rectifiers, galvanic anodesystems, foreign company rectifiers, andelectrical bonds to foreign company structures.Individually, each CP current source should betested. A current interrupter is used tointerrupt the CP current source suspected ofinfluencing the CIS pipeline segment. Typically,a slow interruption cycle is used such as a tensecond On, five second Off period.Pipe-to-electrolyte potentials are measured atthe furthest test points suspected of influencefrom the suspect CP current source.

Test points are monitored moving away fromthe suspect CP current source. Once a 10 mVor less difference between On and Off pipe toelectrolyte potential is observed, the influenceof the CP current source is deemed no longersignificant for analysis of the pipe CP levels.

Several test points beyond the location wherethe influence of the CP current source isdeemed to terminate should be tested toconfirm the end of the influence from thesuspect CP current source.

To start the CIS, a current interrupter isinstalled in series in either the AC or DC circuitsof all of the current sources in the CP currentsources identified. Current interrupters maintaininterruption synchronization. All interruptersmust be synchronized together. Interruptersshould be programmed such that they areinterrupting no longer than the anticipatedsurvey duration each day. When no field

surveying is taking place, the interruptersshould be programmed to turn off in order tominimize the affects of depolarization.

A 30, 32, or 34 AWG gauge insulated wire iselectrically connected to a test station test wire,valve, or other electrically continuous pipelineappurtenance and one terminal of thevoltmeter. The other terminal of the voltmeter isattached to the reference electrode.

The pipeline is located with a pipe locator priorto collecting data to ensure that the referenceelectrode is placed directly over the pipeline(See Pipe Locating Section).

Industry standard copper sulfate referenceelectrodes (CSE) should be used for potentialmeasurements. See NACE Standard TM0497.Reference electrodes should be calibrated withan unused control reference electrode daily, theresults of which should be recorded in theproject field book. The control referenceelectrode should be a recently chargedelectrode not used to gather data in the field.To calibrate the CSE, the ceramic porouselectrode tips are placed tip to tip to measurethe voltage difference between the twoelectrodes or both tips are immersed in acontainer of potable water. A digital voltmeteron the millivolt scale is used to measure thevoltage difference. If the voltage differencebetween the two electrodes exceeds 5 mV, thefield electrode should be emptied, cleanedproperly, and recharged with unused distilledwater and copper sulfate crystals. If the voltagedifference remains greater than 5 mV, the fieldelectrode should be disposed of properly andnot used to collect data.

A datalogger can be placed at the suspectedmidpoint of each day’s CIS progress for lateranalysis of the CIS data. The datalogger shouldmeasure and record the pipe-to-electrolytepotential continuously during each day’s survey.The datalogger should be programmed torecord the pipe potential at a rate of onereading per second or greater. The dataloggershould be installed and turned on prior to thestart of CIS each day and should run until after


the completion of CIS each day. The data canbe retrieved from the datalogger each day andanalyzed for the existence of dynamic straycurrents, de-energizing of a CP source, orimproper current interrupter operation.

On and Off pipe-to-electrolyte potentials arethen measured and recorded typically at 2.5 to5 foot intervals using a high-input impedancevoltmeter/datalogger. The datalogger shouldhave the ability to adjust the time during the Offcycle at which the handheld datalogger storesthe Off potential value due to the possibility ofinductive/capacitive spiking. The dataloggershould be programmed such that the On andOff pipe to electrolyte potentials are measuredand stored from a time period beyond thespiking as determined by the waveprintsdiscussed above. Pipe-to-electrolyte potential measurementsshould be measured and recorded at each teststation and foreign pipeline crossing from eachtest wire within accessible test stations. Nearground (NG), metallic IR Drop (IR), and farground (FG) On and Off pipe-to-electrolytepotential measurements should be made ateach point of pipeline connection.

If the Off metallic IR drop exceeds 5 mV, thesurvey should be halted and an investigationmade to determine the source of the error.Possible sources for this error includeinterrupter malfunction, stray currents, andunknown foreign rectifiers or electrical bonds.

AC pipe to electrolyte potentials can bemeasured at each point of pipeline connectionand recorded in the project field book or withinthe CIS data stream. On and Off casing to electrolyte potentialsshould be measured at each casing, either bymonitoring the test wire attached directly to thecasing pipe or by temporary electricalconnection to the casing vent.

When a stationing flag is encountered, a flagcomment or code is entered into the data loggerfor later computer graphing and stationing

purposes. When a numbered flag isencountered, the flag number can also beentered into the data stream.

All permanent landmarks should be identifiedand entered into the data logger during thesurvey. These include pipeline markers, testpoints, fences, casing vents, creeks, and roadnames.

Upon completion of the survey, all CIS wireshould be retrieved. Flags can be left in placeuntil the final ECDA process/surveys arecompleted and deemed appropriate for finalremoval.

AC CURRENT ATTENUATION SURVEYS(ELECTROMAGNETIC)

AC current attenuation surveys are used toprovide an assessment of the overall quality ofthe pipe coating within a section or as acomparison of several sections. A current isapplied to the pipeline, and coating damage islocated and prioritized according to themagnitude and change of current attenuation. AC current attenuation surveys may beperformed with impressed current CP systemsenergized, however by turning off the rectifierand using the positive and negative leads at therectifier station, signal-generation capabilities ofthe equipment can be maximized. Sacrificialanodes and bonds that are not disconnectedshow up as anomalies. Sacrificial anodes andbonds to other structures are usuallydisconnected to prevent signal loss andenhance current flow down the pipeline.

Commonly employed, commercially availablebattery-powered signal-generating units includeunits that provide a 937.5-Hz AC signal with amaximum output of 750 mA or a 4-Hz AC signalwith a maximum output of three amperes. Ahandheld receiver unit tuned to detect thesignal from the transmitter and block othersignals is used to pick up the signal.

The signal generator is connected to thepipeline and appropriately grounded to earth. Aconstant AC signal is produced and transmitted


AC CURRENT ATTENUATION TRANSMITTER

FIGURE 2-11

TECHNICIAN MEASURING AC CURRENT ATTENUATION WITH RECEIVER

FIGURE 2-12

along the pipe. The transmitter is energized andadjusted to an appropriate output. Typically, thelargest attainable current output is chosen tomaximize the length of pipe that can besurveyed. An impressed current anode bed ormagnesium anode can be used to establish anelectrical ground. If operating rectifiers areinterfering with the signal, then turn the unitsoff.

Signals are measured using the receiver unit.The receiver consists of a handheld,symmetrical, multi-axis antenna array. Theelectromagnetic field radiating from the pipelineis measured by the detector. The detector isused to measure the attenuation of a signalcurrent that has been applied to the pipe. Anelectrical current, when applied to a well-coatedburied pipeline, gradually decreases asdistance increases from the point of currentapplication. The electrical resistivity of thecoating under test and the surface area incontact with the soil per unit length of pipe arethe primary factors affecting the rate of declineand the frequency of the signal.

The logarithmic rate of decline of the current(attenuation), which is effectively independentof the applied current and marginally affectedby seasonal changes in soil resistivity, providesan indication of the average condition of thepipe coating between two given points on thedate of the survey. Changes in attenuationprovide a comparative change in coatingcondition between survey sections. Suchcomparative changes can indicate “better”coating (i.e., fewer anomalies or a small singleanomaly) or “worse” coating (i.e., moreanomalies or a larger single anomaly).

Survey intervals are typically 100 feet at thepipeline locating flags. The measured currentvalue is recorded in the project field book or thetransmitter for later analysis. The receiving unitmust remain upright and perpendicular to thepipe when taking the measurements. Whenreadings are suspect, stay on peak mode andcheck both the peak and null readings andverify pipe depth. Take multiple readings in onelocation if data accuracy is questionable. The

accuracy of the readings may be affected bydistortions in the AC signal caused by otherunderground piping and conduits, traffic controlsignaling, or vibrations due to passing vehicles.

Survey data are analyzed after the survey todetermine which survey intervals exhibitreduced coating quality.

SUMMARY

The indirect above ground inspectiontechniques discussed in this chapter are usedto identify and define coating faults and in turnthose areas where corrosion activity may haveoccurred or may be occurring. Two or more ofthese inspection techniques should be usedwhen conducting this indirect inspection testingso that different types of data can be comparedand analyzed to determine whether there is anycorrelation. The effectiveness of the testingtechniques employed will depend on factorss u c h a s o p e r a t o r e x p e r i e n c e ,pipeline/coating/CP circuit conditions, depth ofpipe, and type of cover at grade.

Should significant coating damage be indicatedfrom these tests, the pipeline should beexcavated and examined for possible corrosiondamage and the appropriate remedial actionsshould be taken.

ACKNOWLEDGMENT

This chapter was written by Kevin T. Parker andwas edited by the AUCSC CurriculumCommittee.


CHAPTER 3

MATERIALS FOR CATHODIC PROTECTION

INTRODUCTION

This chapter will discuss common materialsused for underground cathodic protectioninstallations. Some of the materials haveestablished track records and some are newwith very little known about the long term lifeeffects in a particular environment. For ourpurposes, we will define "long term" as anymaterial with a successful application record inunderground use of more than 20 years.

People who specify cathodic protectionmaterials must understand the advantages andlimitations of a product and how the productrelates to any given application. Furthermore,they must be able to convey in writing, thespecifics of how the product is to bemanufactured or supplied to assure that it willconform to predetermined design life criteria.The object of this chapter is to fit the right typeof materials with the structure to be protected atthe lowest possible cost per year ofpredetermined service life.

CONSIDERATIONS FOR ALL MATERIALS

Material must be selected for each applicationwith the following interrelated items considered:

1. Design criteria

2. Life required

3. Capacity or rating of material selected

4. Economics

5. Environment - both underground and above

grade

When specifying materials, either requisitioningor purchasing, the following should be checked:

1. Specify materials completely.

2. Make sure that complete specifications areon the purchase order.

3. Check material received to ensure that itconforms to the original specifications.

Usage:

1. Follow the manufacturer's recommendationsand instructions.

2. Use compatible components.

3. Use the proper tools.

4. When a problem or question arises, ask themanufacturer or distributor for assistance.

GALVANIC ANODES

Magnesium, zinc, and aluminum can be usedas galvanic anodes for cathodic protection.Galvanic anodes are not supplied in the purestform of the metal but rather as an alloy thatcontains impurities at various concentrations.These impurities can have a profound effect onthe long term operational characteristics of theanode.

A galvanic anode must have a more negativepotential than the structure it is designed toprotect. The difference in potential between the

MATERIALS FOR CATHODIC PROTECTION3-1

anode and the structure is referred to as the"driving potential" or "driving voltage".

The amount of current produced by a galvanicanode is a function of the driving potential andthe circuit resistance in accordance with Ohm'sLaw.

Figure 3-1 shows a schematic diagram of atypical galvanic anode installation.

Table 3-1 lists metals that are used as galvanicanodes and their alloys. For each alloy the tableshows its open circuit potential, current capacityand consumption rate. Impurities and grain sizewill cause wide ranges in these theoreticalvalues.

These are representative values taken fromliterature provided by various manufacturers. Ingeneral, magnesium is preferred for use in soilsand fresh water. Zinc is generally limited to usein sea water, brackish water, sea mud, andsoils with resistivities below 1500 ohm-cm.Aluminum is generally limited to sea water,brackish water, and sea mud environments.

An important point to consider for maximumservice life is the cost per ampere hour ofcurrent capacity, once it has been establishedthat the driving potential is sufficient for thecathode metal and the resistivity of theelectrolyte in the circuit. When working in thehigher resistivity soils, long slender anodeshave a lower resistance to earth than theshorter anodes which are available. This meansthat the circuit resistance for the limited drivingvoltage available from a galvanic anode will beless, and as a result, more current output will beobtained. To say it in another way, the sameamount of anode material (such as a 20 lb.magnesium anode) in a given soil resistivityenvironment will produce more current if it hasbeen cast in a long, slender shape than it will ifcast in a short and more stocky shape. This willresult in the longer shaped anode having ashorter life expectancy - although this "shorter"life expectancy may be fully adequate forapplications in higher resistivity soils. Theanode installation design for a particular

situation will take the resistivity of theenvironment into account and will balancecurrent requirements against desired life todetermine the size, number, and weight ofanodes needed. Design of galvanic anodecathodic protection systems is covered in detailin Chapter 6 of the Advanced Course.

Prepared Backfill and Packaging

Magnesium and zinc anodes for use in soils aretypically supplied with a prepared backfillaround the anode.

The most commonly used backfill formagnesium and zinc anodes consists of:

75% Hydrated Gypsum (CaSO4 @ 2H20)20% Bentonite Clay5% Sodium Sulfate

The purpose of the prepared backfill is:

It increases the effective surface area of theanode which lowers the anode-to-earth contactresistance.

The bentonite clay absorbs and retainsmoisture.

The gypsum provides a uniform, low resistanceenvironment.

The sodium sulfate (a depolarizing agent)minimizes pitting attack and oxide film formationon the anode.

It provides a uniform environment directly incontact with the anode to assure evenconsumption.

When properly combined, the elements used tomake the backfill will provide a uniformresistivity of 50 ohm-cm when measured by theASTM G-58 Soil Box Test Method andcorrected for temperature variations. Mostreputable anode fabricators will test anddocument the resistivity values for each batchof backfill.


GRADE

TYPICAL GALVANIC ANODE INSTALLATION

FIGURE 3-1

#12 AWG TEST LEAD (TYP.)

#12 AWG ANODE LEAD WIRE

PROTECTED PIPELINE

TEST STATIONBUS BAR OR SHUNT

GALVANIC ANODE

TABLE 3-1

Capabilities and Consumption Rates of Galvanic Anodes

TypePotential*

(volts)

CurrentCapacity(A-hrs/lb)

ConsumptionRate

(lb/A-yr)

Magnesium

H-1C AZ-63D Alloy -1.4 to -1.5 250 to 470 19 to 36

High Potential Alloy -1.7 to -1.8 450 to 540 16 to 19

Zinc

ASTM B418-01

Type I (saltwater) -1.1 354 24.8

Type II (soil) -1.1 335 26.2

Aluminum

Mercury Alloys -1.10 1250 to 1290 6.8 to 7.0

Indium Alloys -1.15 1040 to 1180 7.4 to 8.4

* Copper/Copper Sulfate Reference Electrode - Open circuit practical values as shown

As an historical side note, zinc was formerlythought to perform better in a backfill of 50%gypsum and 50% bentonite. Over the years itwas determined that zinc may passivate ifsodium sulfate is not used in the backfill.

To keep the backfill uniformly around theanode, the anodes with their lead wire attachedare placed in cloth bags or cardboard boxesand the prepared backfill is then added. Clothbagged anodes are usually placed within alayered paper bag for resistance to shortperiods of inclement weather and handlingdamage. Prior to installation, the paper bag isremoved and discarded, permitting the clothbag containing the backfill to absorb moisture,allowing the anode to start putting out currentsoon after installation. During transportationand/or handling the anodes may shift in theprepared backfill. This may result in unevenconsumption of the anode, reduction of currentoutput and premature failure of the anode. Thiscondition can be avoided by carefulspecification of transportation packaging andfield handling precautions. Anodes packaged incardboard boxes or bags with centralizingdevices may tend to reduce anode shifting.

Wire and Cable Attachment

Galvanic anodes for use in soil are typicallysupplied with a lead wire that is used to connectthe anode to the structure. The most commontype of lead wire for a galvanic anode is anAWG #12 solid copper wire with TW insulation.Other wire sizes and insulations can bespecified if desired.

Proper attachment of the lead wire to the anodecore is critical for two reasons. First, theconnection must be electrically sound tominimize internal resistance. Second, it must bestrong enough to support the weight of thepackaged anode if it is lifted by the lead wire.The lead wire is usually silver soldered to thecore. Next, the connection is primed andcovered with an inert thermoplastic electricalpotting compound to prevent water migration tothe connection.

Zinc anodes do not have a recessed core, sothe cable is crimped and silver soldered to theextended rod core and coated with a piece ofheat shrinkable polyethylene tubing or severallaps of electrical tape to protect the connection.

Magnesium Alloys

Magnesium anodes are produced in a widevariety of sizes and shapes to fit designparameters. They may be cast in molds orextruded into ribbon or rod shapes, with steelspring, perforated strap or wire cores as shownin Figure 3-2. The core is important because itshould extend 85 percent or more through theanode to reduce the internal circuit resistance.Table 3-2 shows the weights and dimensions ofsome common magnesium anodes. Thechemical composition of the different alloys isshown in Table 3-3.

Four (4) magnesium alloys are available. Theirrespective potential values and consumptionrates will be an important part of thecalculations used to design a cathodicprotection system using magnesium anodes. The potential values and consumption rates ofthe 4 magnesium alloys are shown in Table 3-3.

The purchaser should request a verifiablespectrographic analysis of any anode purchaseto assure that the alloy falls within thepredetermined range of acceptable impurities.The anodes should be identified by heatnumber and date of manufacture. The chemicalanalysis should detail the heat number, the dateof manufacture, a listing of the impurities withthe percentages of each, and be signed by atechnically qualified person.

Anode current capacity and oxidation potentialmay be measured using ASTM StandardG97-97 "Standard Test Method for LaboratoryEvaluation of Magnesium Sacrificial Anode TestSpecimens for Underground Applications".

Zinc Alloys

Zinc anodes can be used in low resistivityelectrolytes where driving potential is not a


BARE AND PACKAGED MAGNESIUM ANODES

FIGURE 3-2

E

DBAGGED

MAGNESIUM ANODE

G

F

A

C

B

BOXEDMAGNESIUM

ANODEOPE

N

THIS

EN

D O

NLY

TABLE 3-2

Magnesium Anode Dimensions and Weights

Nominal Dimensions (Inches) - See Figure 3-2

Alloy A B C D E F GPackaged

Weight (lbs)

1 AZ63 3.2 Rd 2 6 6 - - 3.6

3 H.P./AZ63 3 3 6 8 6 - - 9

5 H.P./AZ63 3 3 10 12 5 - - 12

6 H.P./AZ63 3 3 10 - - 12.5 5 14

9 H.P./AZ63 3 3 13.5 17 6 - - 27

12 AZ63 4 4 12 18 7.5 - - 32

17 H.P. 3.5 3.5 25.5 30 6 - - 42

17 AZ63 3.5 3.5 28 - - 32 5.5 45

20 H.P. 2 2 60 - - 71 4.5 65

32 H.P./AZ63 5.5 5.5 21 25 8 - - 72

32 H.P./AZ63 5.5 5.5 21 - - 24 7.5 70

40 H.P. 3.5 3.5 60 64 6 - - 105

48 H.P. 5.5 5.5 32 36 8 - - 106

50 AZ63 7 7 15 24 10 - - 110

60 H.P. 4 4 60 64 5.75 - - 130

Magnesium Extruded Ribbon and Rods

Size(Inches)

Weight(lb/ft)

Core(Inches)

d x ¾ 0.24 0.125

0.750 0.36 0.125

0.840 0.45 0.125

1.050 0.68 0.125

1.315 1.06 0.125

1.561 1.50 0.125

2.024 2.50 0.125

* Data were compiled from literature provided by various vendors and may vary slightly.

TABLE 3-3

Composition of Magnesium Alloy

ElementAZ63B(H1A)

AZ63C(H1B)

AZ63D(H1C)

M1C(High Potential)

Aluminum (Al) 5.3 - 6.7% 5.3 - 6.7% 5.0 - 7.0% < 0.01%

Zinc (Zn) 2.5 - 3.5% 2.5 - 3.5% 2.0 - 4.0% -

Manganese (Mn) 0.15 - 0.7% 0.15 - 0.7% 0.15 - 0.7% 0.5 - 1.3%

Silicon (Si) < 0.10% < 0.30% < 0.30% < 0.05%

Copper (Cu) < 0.02% < 0.05% < 0.10% < 0.02%

Nickel (Ni) < 0.002% < 0.003% < 0.003% < 0.001%

Iron (Fe) < 0.003% < 0.003% < 0.003% < 0.03%

Others (each) - - - < 0.05%

Others (total) < 0.30% < 0.30% < 0.30% < 0.30%

Magnesium (Mg) Balance Balance Balance Balance

Performance Characteristics*

AZ63B(H1A)

AZ63C(H1B)

AZ63D(H1C)

M1C(HP)

Potential (Volts to Cu/CuSO4) -1.60 -1.55 > -1.40 -1.75

Theoretical Current Capacity(A-hrs/lb)

1000 1000 1000 1000

Actual Current Capacity(A-hrs/lb)

450 - 580 300 - 470 250 - 470 400 - 540

Current Efficiency (%) 45 - 58 30 - 47 25 - 47 40 - 54

Actual Consumption Rate(lb/A-yr) 18 - 15 33 - 19 35 - 19 19 - 16

* Using ASTM Standard G97-97 “Standard Test Method for Laboratory Evaluationof Magnesium Sacrificial Anode Test Specimens for Underground Applications”.

major factor. They are available in a variety ofsizes and shapes including bracelet anodes formarine pipelines, docks and piers, hull anodesfor marine vessels, and in ribbon form for use inutility ducts and for AC mitigation. Zinc is notrecommended in environments wherecarbonates or bicarbonates are found, or wherethe temperature of the electrolyte is over 120ºF. Under these situations zinc becomescathodic to steel, rather than anodic, and itsuse should be avoided.

Table 3-4 shows the weights and dimensions ofsome common zinc anodes.

The chemical composition of the different alloysis shown in Table 3-5.

When used strictly as an anode, zinc is wellsuited for low resistivity environments such assea water, salt marshes, and brackish water.Zinc normally becomes impractical forprotecting large bare areas when the resistivityof the electrolyte exceeds 1500 ohm-cm.

Zinc anodes are also used as grounding cellsfor electrical protection of isolators. Zincgrounding cells are usually made from 2 or 4zinc anodes separated by a 1" isolating spacerand packaged in a cloth bag with backfill. Eachanode has a lead wire (typically AWG #6) whichis connected across an isolation device. Faultcurrent and lightning will pass between theanodes without causing damage to the pipelineor the isolation device. Figure 3-3 shows twozinc anodes used in a grounding cell installedacross an isolating flange.

Zinc can also be used as a permanentlyinstalled reference electrode.

Aluminum Alloys

Aluminum has been generally limited toapplications of sea water and chloride richenvironments such as offshore petroleumplatforms, marine pipelines and onshore oilproduction separation equipment using asaltwater tank. There are two common alloyfamilies. One uses mercury, the other uses

indium, to reduce the passivating effect of theoxide film that forms on aluminum. Thecomposition of each alloy is shown in Table 3-6.

The mercury alloys are effective in high chlorideenvironments where chloride levels areconsistently greater than 10,000 ppm. Seawater has an average chloride level of 19,000ppm.

Indium alloys are effective in waters containing1000 ppm or more of chlorides.

The mercury alloy is used in free flowing seawater and its principle advantage is its highcurrent capacity. The current capacity of eachalloy is shown in Table 3-6. Its primarydisadvantages are that it cannot be used inbrackish water, in silt/mud zones, or at elevatedtemperatures. Under high humidity atmosphericstorage conditions some grades have beenknown to degrade and become totally unusableprior to installation. There is some controversyalso concerning the long term environmentaleffects of this alloy in sea water since itcontains mercury.

The indium alloy is used in free flowing seawater, brackish water, silt/mud zones, and atelevated temperatures. Its basic limitation is alower current capacity than the mercury alloy.

Aluminum anodes have a variety of standardcores such as pipe, strap, bar, end type, sidetype, etc. The type of core must be specified.Cores should never be galvanized prior topouring the anode.

On major projects it is advisable to monitor thequality control process and inspect the finishedmaterials prior to shipment.

Just as with the magnesium, it is important thata certified analysis accompany each anodeheat.

The purchaser should request a verifiablespectrographic analysis of any anode purchaseto assure that the alloy falls within thepredetermined range of acceptable impurities.


TABLE 3-4

Zinc Dimensions and Weights

Weight (lbs) Height Width Length Core (dia.”)

Bare Zinc Anodes

5 1.4 1.4 9 0.250

12 1.4 1.4 24 0.250

18 1.4 1.4 36 0.250

30 1.4 1.4 60 0.250

30-A 2.0 2.0 30 0.250

45 2.0 2.0 45 0.250

60 2.0 2.0 60 0.250

Zinc Ribbons

2.4 1.0 1.250 -- 0.185

1.2 0.625 0.875 -- 0.135

0.6 0.500 0.563 -- 0.130

0.25 0.344 0.469 -- 0.115

TABLE 3-5

Zinc Alloy Compositions

ElementASTM B418-01

Type I(sea water)

ASTM B418-01Type II(soil)

Aluminum 0.1 - 0.5% < 0.005%

Cadmium 0.025 - 0.07% < 0.003%

Iron < 0.005% < 0.0014%

Lead < 0.006% < 0.003%

Copper < 0.005% < 0.002%

Others — 0.1%

Zinc Balance Balance

TABLE 3-6

Aluminum Alloy Composition and Performance

ElementMercury Family

AI/Hg/ZnIndium Family

Al/In/Zn

Zinc (Zn) 0.35 - 0.60% 2.8 - 6.5%

Silicon (Si) 0.14 - 0.21% 0.08 - 0.2%

Mercury (Hg) 0.035 - 0.060% ---

Indium (In) --- 0.01 - 0.02%

Copper (Cu) 0.004% max 0.006% max

Iron (Fe) 0.10% max 0.12% max

Aluminum (Al) Balance Balance

Consumption Rate (lb/A-y) 6.8 - 7.0 7.4 - 8.4

Current Capacity (A-hrs/lb) 1250 - 1290 1040 - 1180

Potential to Ag/AgCl -1.05 -1.10

to Cu/CuSO4 -1.10 -1.15

4

TWO UNIT ZINC GROUNDING CELLS

FIGURE 3-3

5

6

1

2

3

The anodes should be identified by heatnumber and date of manufacture. The chemicalanalysis should detail the heat number, the dateof manufacture, a listing of the impurities withthe percentages of each, and be signed by atechnical ly qualif ied person. Somemanufacturers identify each anode with a stampor tag; others may label each pallet of bareanodes with an identity number painted on theside.

IMPRESSED CURRENT CATHODICPROTECTION SYSTEMS

Unlike galvanic systems, impressed currentsystems utilize an external power source suchas a rectifier, electrically connected to anodeswith a low consumption rate. The anodes areconnected to the positive terminal of the rectifierand the circuit is completed by attaching thenegative terminal to the structure to beprotected. Figure 3-4 shows the components inan impressed current system.

An impressed current system is used to protectlarge bare and coated structures and structuresin high resistivity electrolytes. Care must betaken when employing this type of system sothat cathodic coating damage and the potentialfor the development of stray currents, adverselyaffecting other foreign structures, areminimized.

Prior to the 1970's there were only three typesof anodes primarily used for impressed currentanode beds: high silicon cast iron, graphite andscrap steel. As technology progressed, so didthe materials used for impressed currentanodes. When installation and operating costsare assessed, very few anodes can be useduniversally for any type of application and stillachieve a desirable design life.

In most soils, anodes evolve oxygen and theanode oxidizes as the current is discharged. Inchloride containing soils or water, anodesevolve chlorine gas which can form hydrochloricacid, and the anodes can breakdownchemically. Some anodes perform well in thepresence of oxygen and others in the presence

of acids. It goes without saying that if corrosionpersonnel do not effectively detail the type ofanode construction they expect, they willprobably get less than they expected. Thefollowing are minimum details to address in ananode specification.

1. Anode Type, Size and Shape - composition,dimensions and weight, within 2%.

2. Lead Wire - size, type, insulation/thickness,length.

3. Type of Lead Wire Connection - end, center,<0.02 ohms resistance.

4. Lead Wire Termination - epoxy cap, fullencapsulation, shrink cap.

5. Packaging - bare, canister size/gauge,backfill type, weight.

6. Palletizing - size, padding, number perpallet.

7. Options - lifting rings, centering devices.

The sections of the specification covering thevarious types of impressed current anodesshould include additional information onconstruction methods.

TYPES OF IMPRESSED CURRENT ANODES

The three (3) most common anode materials foruse in soil are high silicon cast iron, graphiteand mixed-metal oxide.

Other impressed current anode materialsinclude platinum, aluminum, lead silver,magnetite, and polymer conductive.

High Silicon Cast Iron

The typical alloy for cast iron anodes is ASTMA518 Grade 3. Table 3-7 shows themetallurgical composition of this alloy.


TYPICAL IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM

FIGURE 3-4

CATHODIC PROTECTION

RECTIFIER

BURIED ANODEBED

CATHODIC PROTECTION

CURRENT FLOW

NEGATIVERETURN CABLE

IMPRESSED CURRENT ANODE (TYP)

AC POWER SUPPLY TO RECTIFIER

( - ) ( + )

POSITIVEHEADER CABLE

TABLE 3-7

Cast Iron CompositionASTM A518 Grade 3

ELEMENT COMPOSITION

Silicon 14.2 - 14.75%

Carbon 0.70 - 1.10%

Manganese 1.50% max

Molybdenum 0.20%

Chromium 3.25 - 5.00%

Copper 0.50% max

Iron Balance

TABLE 3-8

Typical Cast Iron Rod Anode Dimensions

NOMINALWEIGHT lbs (kgs)

NOMINALDIAMETER

in (mm)

NOMINALLENGTHin (mm)

NOMINALAREAft² (m²)

1.0 (.5) 1.1 (28) 9 (230) .22 (.02)

5.0 (2.3) 2.0 (51) 9 (230) .39 (.04)

9.0 (4.1) 2.5 (64) 9 (230) .50 (.05)

26 (12) 1.5 (38) 60 (1520) 2.0 (.19)

43 (20) 2.0 (51) 60 (1520) 2.6 (.24)

44 (20) 2.0 (51) 60 (1520) 2.6 (.24)

60 (27) 2.0 (51) 60 (1520) 2.7 (.25)

110 (50) 4.0 (102) 60 (1520) 4.0 (.37)

220 (100) 4.5 (114) 60 (1520) 5.5 (.51)

The principal reason for the good performanceof cast iron anodes is the formation of a siliconoxide (SiO2) film on the anode surface, reducingthe rate of oxidation, and retarding theconsumption rate. Chromium is added toproduce a chlorine resistant alloy and improveits life in chloride containing soils and water.

Cast iron anodes have good electricalproperties and the resistivity of the alloy is 72micro ohm-cm at 20º C. In soils, the anodes areusually backfilled with metallurgical or calcinedpetroleum coke breeze to increase the effectiveanode surface area and provide uniformconsumption. By increasing the diameter orlength of a cylindrical anode, the anode toelectrolyte resistance is lowered. Increasing thelength has a greater effect than increasing thediameter.

The high tensile strength of the anode is anasset in some circumstances, but it is brittleand subject to fracture from severe mechanicaland thermal shock.

Cast iron anodes are manufactured in a widevariety of dimensions, shapes and weights.Table 3-8 shows the weights and dimensions ofsome common high silicon cast iron anodes,

Solid cast iron anodes have the lead wiresattached at the end as shown in Figure 3-5. Theconnection is encapsulated with epoxy or aheat shrinkable cap to protect it from electricaldischarge.

Current typically discharges from conventionalend connected anodes at a greater rate (3:1)from the area of the cable connection than fromthe body of the anode. This causes anexaggerated attack, called "end effect". Aconnection in the center of a tubular anode,suitably sealed at each end, has beendemonstrated to be effective against this rapidloss of the anode at the cable connection. Adrawing of a center connected tubular anode isshown in Figure 3-6.

Testing of chromium bearing tubular anodes inartificial sea water has shown the consumption

rate to be about 0.7 pounds per ampere year ata discharge rate of 3.5 amps per square foot ofanode surface area. No data has been providedfor soil or mud conditions but consumption inthese environments could double or triple theconsumption rate.

Tubular anodes range in size from 2.187" to4.750" in diameter, and from 46 pounds to 175pounds in weight, with surface area rangingfrom 50 percent - 170 percent greater thanconventional rod type anodes. The anode cableconnection strength should be tested to beequal to the breaking strength of the cablewithout any change in the 0.004 ohms or lessconnection resistance.

Thus, for the following cable sizes, connectionpull test values should be:

Connection Strength Cable Break Strength

No. 8 - 799 pounds 525 pounds



Graphite Anodes

Graphite rods have been used as an impressedcurrent material for many years. The basicconfigurations consist of round or square rods,manufactured from a slurry of powderedpetroleum, coke and coal tar resin. The coal taris used as a bonding agent to hold the graphiteparticles together and then baked at hightemperatures to fuse the mixture. This processincreases the resistance to oxidation andsubsequent breakdown.

There are many types of graphite compositionsand the type used for cathodic protection anodebeds is one of the most porous. Graphitecathodic protection anodes should have aspecific gravity of 1-6 grams/cc minimum. Theporosity allows moisture penetration to migrateto the connection, causing failure at the cableconnection. The porosity is reduced byimpregnating the rods with filler of linseed oil,


TEFLON (FEP) SLEEVE WITH POLYETHYLENE WASHER (OPTIONAL)

1” EPOXY CAP (OPTIONAL)

EPOXY SEALANT

POTTING COMPOUN

D

TAMPEDTELLURIUM

LEAD CONNECTIO

N

SHRINK CAP OR FULL EPOXY

ENCAPSULATION (OPTIONAL)

LEAD WIRE(LENGTH AND INSULATION AS SPECIFIED)

ANODE

WELL TAMPED COKE BREEZE

PLYWOOD END PLUG SECURED WITH 4 SCREWS

84”OR96”

8”

28 GA. GALVANIZED SPIRAL CANISTER

TYPICAL CAST IRON ANODE LEAD WIRECONNECTION AND PACKAGING

FIGURE 3-5

L

D

D CENTER CONNECTION

CENTER CONNECTION

NOMINALWEIGHTlbs (kgs)

NOMINALDIAMETER

in (mm)

NOMINALLENGTHin (mm)

NOMINALAREAft² (m²)

31 (14) 2.6 (66) 41 (1067) 2.4 (.22)

50 (23) 2.6 (66) 60 (1520) 3.5 (.33)

46-50 (21-23) 2.2 (56) 84 (2130) 4.2 (.39)

63-70 (29-32) 2.6 (66) 84 (2130) 4.9 (.46)

85-95 (39-43) 3.8 (97) 84 (2130) 7.0 (.65)

110-122 (50-55) 4.8 (122) 84 (2130) 8.8 (.82)

175-177 (79-80) 4.8 (122) 84 (2130) 8.8 (.82)

230 (104) 4.8 (122) 84 (2130) 8.8 (.82)

260 (118) 6.7 (170) 76 (1981) 11.4 (1.06)

270 (122) 6.7 (170) 84 (2130) 12.3 (1.14)

TYPICAL TUBULAR ANODE DIMENSIONS

FIGURE 3-6

micro-crystalline wax, or phenolic based resin.

There is controversy concerning the best typeof filler and even whether a filler really reducesmoisture penetration over long periods of time.Whatever filler is used, it should be applied bya method that assures complete penetration ofthe cross-section of the graphite anode. Insoils, graphite anodes must be backfilled withmetallurgical or calcined petroleum coke breezeto increase the effective anode surface areaand provide a uniform low consumption rate.

The anode to lead wire connection is just ascritical as with a cast iron anode. Figure 3-7shows a typical graphite anode to lead wireconnection. Both end and center connectionsare available. Additional specification detailsshould include:

1. Type of connector - lead, brass, molten,compression, centered.

3. Connect ion sealant-thermoplast ic,thermosetting (epoxy).

4. Cable sealant - TFE tubing, shrink cap,encapsulation.

5. Impregnation - wax, linseed oil, resin.

6. Sizes - 3" x 30" - 2 sq. ft., 3" x 60" - 4 sq. ft.,4" x 40" - 3.5 sq. ft., 4" x 80" - 7 sq. ft.

Graphite should not be operated at currentdensities exceeding one ampere per squarefoot in soil with coke breeze backfill. Foroptimum life in soils, most engineers designgraphite anodes for a maximum density of 0.20amperes per square foot, or one ampere per 3"x 60" rod. If current densities are within theseranges, the consumption rate will beapproximately 2 pounds per ampere-year(lb/A-yr). Exceeding these limits causes thematerial to become mushy and less conductive,due to chemical breakdown of the crystalboundary.

As with cast iron, graphite is brittle and may beeasily damaged during transportation, either

bare or packaged. Special handling andpadding is necessary to prevent cracking andbreaking.

Mixed Metal Oxide Anodes

Mixed metal oxide anodes were developed inEurope during the early 1960's for use in theindustrial production of chlorine and causticsoda. The first known use of the technology forcathodic protection use occurred in Italy toprotect a sea water jetty in 1971. These anodesexhibit favorable design life characteristicswhile providing current at very high densitylevels. The oxide film is not susceptible to rapiddeterioration due to anode acid generation,rippled direct current, or half wave rectification,as is common with other precious metalanodes.

The composition of the anode consists of atitanium rod, wire, tube or expanded mesh(refer to Figure 3-8) with the oxide film bakedon the base metal. Sometimes these anodesare referred to as dimensionally stable orceramic anodes.

For oxygen evolution environments such assoils, the anode oxide consists of rutheniumcrystals and titanium halide salts in an aqueoussolution that is applied like a paint on the basemetal and baked at 400 to 800º C forming arutile metal oxide. After baking, the rutiledevelops a matte black appearance and ishighly resistant to abrasion.

For chlorine evolving environments such as seawater, the oxide consists of an aqueoussolution of iridium and platinum powder that isalso baked at high temperatures to achieve adesirable film.

Some manufacturers produce variations of theoxide films specifically for chloride or non-chloride electrolytes and they are notinterchangeable.

Normally, titanium will experience physicalbreakdown around 10 volts, but the oxide film isso highly conductive (0.00001 ohm-cm


TYPICAL GRAPHITE ANODELEAD WIRE CONNECTION

FIGURE 3-7

Potting Compound

Zone

Tamped TelluriumLead Connection

EpoxySealant

Zone

Connection LockRings

BALLAST RING

MIXED METAL ANODE

FIGURE 3-8

COPPERELECTRICAL CONTACT

TITANIUMSTRING ANODE

CABLE

resistivity) that the current is discharged fromthe oxide rather than the base metal even witha rectifier voltage of 90 volts in soils. This is incontrast to the insulating titanium dioxide filmthat naturally forms on the surface of baretitanium. When the mixed metal oxide film hasbeen consumed, the insulating titanium dioxidefilm will cover the anode and not allow currentto discharge unless the applied voltage isgreater than 10 volts in sea water or 50 to 70volts in fresh water.

The maximum recommended current densitiesfor various electrolytes are:

Soil, Mud, Fresh Water: 9.0 amps/ft2 (20 years)Sea water: 55.7 amps/ft2 (15 years)

Anodes in soil or mud must be backfilled withfine, low resistance, calcined petroleum cokebreeze for maximum life and performance.Even when the anode is pre-packaged withpetroleum coke, conservative engineeringjudgment would dictate that the anode packagebe surrounded with metallurgical coke, prior tofinishing the backfilling with native soil.Consumption rates at these densities rangefrom 0.5 mg/Ay in sea water to 5 mg/Ay in cokebreeze, fresh water and sea mud.

As with any anode, the connection must beconstructed so as to be moisture proof, watertight and have no more than 0.001 ohmresistance.

Advantages of mixed metal oxide anodes:

C Lightweight and Unbreakable

C Dimensionally Stable

C Negligible Consumption Rate

C High Current Density Output

C Inert to Acid Generation

C Cost Effective

Mixed metal oxide anodes are available in avariety of shapes including wire, rods, tubesand mesh.

Platinum Anodes

Platinum can be used as an anode coating formany types of cathodic protection installations.Structures in a vast array of environments suchas underground, offshore, concrete, powerplants, and the internal surfaces of piping, tanksand machinery have utilized platinum forcathodic protection systems. In sea water,platinum has a low consumption rate, 0.00018pounds per ampere year, so only a smallamount is needed for a twenty-year anode life.In soil and fresh water, the consumption mayrange up to 0.005 pounds per ampere yeardepending on the resistivity of the electrolyteand the amount of coke breeze surrounding theanode in soil applications. Pure platinum, byitself, would be too expensive. Therefore, it isnormally coated over noble base metals suchas titanium and niobium. When anodes are inthe form of wire and rods, there may be acopper core to increase the conductivity forlengths in excess of 25 feet since titanium andniobium are relatively poor electrical conductorscompared to copper. Wire anodes for use indeep anode beds, and packaged anodes forsurface anode beds are manufactured. Figure3-9 shows different types and sizes of platinumanodes.

The passive film on titanium starts to breakdown at 10 volts, anode to cathode potential,therefore, these anodes are limited to lowresistance environments such as sea water.Niobium has a break down voltage of 120 volts,anode to cathode potential, and is used inhigher resistivity electrolytes.

Current densities range from 50 amps persquare foot in soils to 500 amps per square footin sea water, with a platinum thickness of 300micro inches, depending on the anode surfacearea and the method of coating application.Platinum has been coated on base metalsusing many techniques including sputteredelectrode position, cladding, and metallurgicallybonded.

In metallurgically bonded anodes, the metalsare compressed together in an oxygen free


20% NiobiumDiameterInches

Nb ThicknessInches

Resistancemicrohm/ft

Pt Thicknessµ-in (2X)*

.750 .038 22 300 (600)

.500 .025 50 200 (400)

.375 .019 89 150 (300)

.250 .013 201 100 (200)

.188 .009 356 75 (150)

.125 .006 806 50 (100)

40% Niobium

DiameterInches

Nb ThicknessInches


Pt Thicknessµ-in (2X)*

.375 .038 113 150 (300)

.250 .025 256 100 (200)

.188 .019 453 75 (150)

.125 .013 1025 50 (100)

.093 .010 1822 38 (75)

.063 .007 4102 25 (50)

.031 .0035 16,408 12.5 (25)

100% Titanium

DiameterInches

Ti ThicknessInches


Pt Thicknessµ-in

.750 Solid 468 300

.500 Solid 1054 200

.375 Solid 1874 150

.250 Solid 4215 100

.188 Solid 7454 75

.125 Solid 16,862 50

* Double Platinum Thickness

PLATINUM ANODES

FIGURE 3-9

vacuum. This provides an oxide free, lowresistance bond between the metals.

Cladding involves wrapping a thin sheet ofplatinum around a rod and spot welding theplatinum to the base metal at the overlap area.

Electrodeposition techniques plate a film ofplatinum on the base metal.

Thermal decomposition and welded techniquesexhibit the same problems as cladding, and asof the late 1980's are rarely used.

Each process has its advantages anddisadvantages and the corrosion worker shouldevaluate them for each application.

The anode to cable connection is critical andimproper connections will result in very earlypremature failure. Users should assure that theanodes are manufactured in compliance withtheir specifications by skilled personnel underthe guidance of established quality controlmethods.

The major disadvantage to platinum is poorresistance to anode acid evolution in staticelectrolytes, rippled direct current, and halfwave rectifiers. Use of a 3-phase transformerrectifier in sea water systems has been knownto double the life of platinum anodes byreducing the ripple in the DC output.

Very rapid deterioration will occur if the anodeis driven at current discharges exceeding themanufacturer's limitations. In undergroundapplications, platinum anodes have had onlylimited.

Aluminum Anodes

Aluminum anodes have been used in the pastas an impressed current anode for protectingthe interior of water tanks, particularly where icedestroys the anodes annually. The consumptionrate of 9 pounds per ampere year limits the costeffectiveness of aluminum anodes compared toother anode systems. There are severalproprietary systems for water tank cathodic

protection using other anode materials that willwithstand icing.

Lead Silver Anodes

Lead alloy anodes are only used in free flowingsea water applications and may employ variousmetals such as antimony, lead, tin, and 1 or 2%silver. Commonly supplied in rod or strip form of1.5" diameter by 10" long, they have been usedextensively in Europe with a 2% silver alloy,which doubles its life.

Upon initial startup, the consumption rate isabout three pounds per ampere year andeventually a black, passive film of lead peroxideforms to extend the life of the anode, resultingin consumption of about 0.2 pounds per ampereyear. Normal current density ranges from 3 to25 amps per square foot. In silting or lowchloride conditions, this oxide film does notform and the anode is consumed rapidly.

Cable connections are made by drilling a holeand silver soldering the lead wire at the base ofthe hole. The connection cavity is then filledwith epoxy to prevent moisture penetration.

Installation is accomplished by hanging theanodes from a structure, dock or pier in aperforated fiberglass pipe or by a supportdevice to maintain its position. This support isimportant to prevent ice damage and keep theanodes from coming in contact with mud or silt.

Magnetite Anodes

Magnetite (Fe3O4), a European inspired anode,is itself an iron based corrosion product.Therefore, there is little or no corrosion involvedduring current discharge.

The anodes are manufactured and shippeddirect from the factory with the lead wireattached and distributors splice extra wire tomeet user requirements. The material is brittleand a factory installed lead wire attachment tothe internal wall of the anode is critical. Figure3-10 shows the anode construction.


ANODELEAD WIRE

TYPICAL MAGNETITE ANODE

FIGURE 3-10

MAGNETITE

COPPERLAYER

MAGNETITEANODE

PLASTICCOMPOUND

PLASTICCAP

POROUS BODY

Magnetite anodes installed in free flowingsea-water (25 ohm-cm) is a normal application.

Polymer Conductive Anodes

In 1982, an anode material was test marketedto provide a small amount of current inrestricted spaces such as internal pipesurfaces, heat exchangers, utility ducts, andareas shielded from conventional anode bedcurrent.

The material resembles electrical cable butactually consists of a stranded copperconductor with an extruded, conductivepolyethylene (see Figure 3-11). This concept isused in underground concentric power cablesas a conductive shield around the ground wires.The polymer contains carbon granules whichdischarge the current. The anode should bebackfilled in carbonaceous coke breeze formaximum life.

An optional plastic mesh is available, toseparate the anode from the cathode inrestricted spaces, preventing electrical shortingbetween the anode and cathode. Currently thematerial is available in four different diametersand the current output ranges from 3 to 9milliamperes per linear foot.

Scrap Steel Anodes

For companies who have access to scrap pipeor old rails, this type of anode may beeconomical if properly installed. If scrap pipe isused, multiple lead wire connections are madeat close intervals and carefully coated at eachlocation to prevent premature connectionfailure. The major cost of the system is in labor,welders and heavy construction equipmentnecessary to install the components.

An abandoned pipeline can also be used as animpressed current anode.

A major disadvantage in the use of scrap steelanodes is the high consumption rate of 20lb/A-yr.

Backfill for Impressed Current Anodes

Special carbonaceous backfill is used tosurround most impressed current anodesinstalled in soil in order to reduce the anodebed resistance and extend the anode life. Theonly exception applies to anodes used inphotovoltaic power supply systems, wheremagnesium anode backfill is used due to thehigh back voltage encountered withcarbonaceous backfill.

Coke breeze is conductive and transfers currentfrom the anode to the interface of native soil.Some types pass the current more efficiently bymore electronic conductance and others areless efficient and current passes electrolytically. The first type is metallurgical coke breeze,derived as a waste by-product of coking(heating) coal, associated with steel production.The composition and particle size of coke maygreatly enhance the life of an anode and shouldbe considered at the design stage. Most oftenthe particle size is about 0.375" (d") and smallerfor surface anode bed backfill. The smaller theparticle size, the greater the compaction andconductivity is. Metallurgical coke should notexceed 50 ohm-cm when measured usingASTM G-58 Soil Box Test Method, temperaturecorrected. The carbon, iron, copper, sulfurcontent and weight per cubic foot are importantvalues to analyze prior to installation. Refer toTable 3-9. Metallurgical coke breeze isavailable bagged or bulk and is often used inprepackaged impressed current anodes.

Calcined petroleum coke consists of finelyscreened particles of coke that are typicallyderived as a waste by-product of crude oilrefining. It consists predominately of roundcarbon granules and is much more conductivethan metallurgical grades of coke. Theresistance should be less than 2 ohm-cm usingASTM G-58 Soil Box Test Method, temperaturecorrected. Since resistance values are low, highcurrent density is less prone to gas generationand the incidence of gas blocking in deepanode beds using a 92 percent carbon contentcoke is rare. Some grades incorporate asurfactant, similar to detergent, to reduce water


POLYMER ANODE

FIGURE 3-11

POROUS ABRASION-RESISTANT NON-CONDUCTIVE BRAID

STRANDED COPPERCONDUCTOR

HEAT SHRINKABLE END CAP

POLYMER GRAPHITE ANODE MATERIAL

TABLE 3-9

Coke Breeze Composition

TypeBulk Density

(lb/cu ft)Porosity

(%)Carbon(lb/cu ft)

Metallurgical 45 48.0 32.51

Petroleum, calcined

Delayed 48 59.5 47.76

Fluid 54 56.7 49.93

70 44.0 64.73

74 40.8 68.53

TABLE 3-10

Wire And Cable Insulation Designations

Designation Insulation Thick (in) Cable Size Specification

HMWPE High Molecular Weight Polyethylene .110.125

No 8 - No 2No 1 - 4/0

D-1248, Type 1Class C, Cat. 5

TW Polyvinyl Chloride (PVC) .030.034.060

No 14 - No 10No 8

No 6-No 2

U.L. Standard 83(60º C wet/dry)

THW Polyvinyl Chloride (PVC) .045.060

No 14 - No 10No 8 - No 2

U.L. Standard 83(75º C wet/dry)

THHN PVC/Nylon Jacket(.0004” Nylon)

.015

.020No 14 - No 12

No 10U.L. Standard 83

(90º C dry)

THWN PVC/Nylon Jacket (.004” Nylon)

.015

.020No 14 - No 12

No 10U.L. Standard 83

(75º C wet)

PVF/HMWPE Polyvinylidene (.020”)HMWPE jacket (.065”)

.085 No 8 - No 2 Kynar™ASTM D-257

ECTFE/HMWPE Ethylene Chlorotriflora-ethylene (.020”)HMWPE jacket (.065”)

.085 No 8 - No 2 Halar™

tension and promote pumping and compactionin deep anode beds.

WIRE AND CABLE

Conductors typically used for undergroundservice are made of solid or stranded copper,with a variety of insulation materials designedfor the type of electrical and chemical exposureto be encountered (see Table 3-10).

Conductors are rated on their ampacity undercertain temperature and service conditions (seeTable 3-11). Insulation values also varydepending on wet or dry conditions. Thefollowing data are general in nature, coveringthe most common types, and specification datashould be obtained directly from manufacturerpertaining to their product.

CABLE TO STRUCTURE CONNECTIONS

The most common method of connectingcables to structures is the exothermic weldingprocess.

Exothermic welding uses a graphite mold tocontain a mixture of copper alloy andmagnesium starter powder. After igniting thepowder with a flint gun, the powder becomesmolten and drops on the cable and structure.The slag on the connection is removed bylightly striking the weld with a chipping hammerafter cooling. Figure 3-12 shows a typical set-upfor an exothermic weld.

Newer systems utilize a ceramic mold with anelectronic ignition.

In order to prevent damage to the pipe, weldmetal charges are limited to 15 grams for steelpipe and 32 grams for cast iron and ductile ironpipe.

Connecting large cables may require separatingthe cable strands and using multiple exothermicwelds.

Another method of connecting cables tostructures is pin brazing.

The technique of pin brazing is based mainlyupon electric-arc silver soldering using a pinbrazing unit, a hollow brazing pin containingsilver solder and flux.

A thin layer of silver is transferred from thebrazing pin to between the pipe and the cable.It creates a metallurgical bond between thematerials.

The pin brazing process uses a lowertemperature than the exothermic weldingprocess.

Structures that contain certain types offlammable substances may require the use ofgrounding clamps to make the cableattachment.

Special attention should be given to coating anyattachment or cable splice to prevent bi-metalliccorrosion attack. The coating should be solventfree to prevent deterioration of the insulation.

CABLE SPLICES

Cables can be spliced together usingexothermic welding, high compression crimps,or split-bolts as shown in Figure 3-13.

SPLICE ENCAPSULATION

There are many methods available for moistureproofing cable splices:

1. Hand wrapping

2. Epoxy mold encapsulation

3. Shrink tubing and sleeves

4. Elastomer/urethane impregnated wrap

Hand wrapping has historically been the mostreliable method if done properly, although it isalso the most expensive since more labor timeis required. The exposed splice area is cleanedwith a clean rag, dampened with solvent toremove release oil and human oil on thestrands. The area is wrapped with 3 laps of


TABLE 3-11

Conductor Ampacities And ResistancesNACE Corrosion Engineer’s Reference Book

SizeAWG

Ampacity*(Copper)

Resistance(Ohms/1000 ft @ 25º C)

No. 16 6 4.18

No. 14 15 2.62

No. 12 20 1.65

No. 10 30 1.04

No. 8 50 0.652

No. 6 65 0.411

No. 4 85 0.258

No.2 115 0.162

No. 1 130 0.129

No. 1/0 150 0.102

No. 2/0 175 0.0811

No. 3/0 200 0.0642

No. 4/0 230 0.0509

250 MCM 255 0.0423

300 MCM 285 0.0353

350 MCM 310 0.0302

400 MCM 335 0.0264

500 MCM 380 0.0212

* Ampacity based on THW and HMWPE insulation

TYPICAL EXOTHERMIC WELD PROCESS

FIGURE 3-12

MOLDHANDLE

MOLD

STARTING POWDER

CADWELD

WELD

SEALING DISK

SLEEVE (IF REQUIRED)

NEGATIVE RETURN CABLEORTEST LEAD

PIPE WALL /STRUCTURE

HOLD MOLDFIRMLY AGAINST

CLOSE COVER AND IGNITE STARTING POWDER WITH

INSULATE SPLICE WITH TWOHALF LAPPED LAYERS OF

RUBBER ELECTRICAL TAPEAND TWO HALF LAPPED LAYERSOF PLASTIC ELECTRICAL TAPE

CABLE SPLICE CONNECTIONS

FIGURE 3-13

SELF-ADHERING FLEXIBLE RUBBER

INSULATING MATERIAL

HEAT SHRINKABLEWRAP SLEEVE

EXOTHERMIC WELD CONNECTION

INSULATED STRANDEDCOPPER CABLE

EXOTHERMICALLY WELDED SPLICE

COMPRESSION CONNECTION

COMPRESSION CONNECTOR

2 HALF LAPPED LAYERS OF PLASTIC TAPE

INSULATED STRANDEDCOPPER CABLE

SPLIT BOLTCONNECTOR

SPLIT BOLT SPLICE CONNECTION

electrical rubber insulating tape extending 2inches on adjacent insulation, paying specialattention to the crease area if wrapping a "Y"connection, as in an anode header cable splice.The entire area is then wrapped with 3 laps ofPVC electrical tape and coated with anelectrical sealant to complete the splice.

Epoxy molds (Figure 3-14) have been usedextensively over the years with some mixedresults. Epoxy does not bond to the release oilsused on wire, permitting moisture to traveleventually into the connection. Since there is nochemical bond to polyethylene, the mold mustextend far enough over the adjacent insulationto prevent hygroscopic moisture migration.

Epoxy molds should not be disturbed orbackfilled for 30 minutes at 70º F so that themixture begins to cross link and bonds to thesurfaces. Any movement will distort the materialand reduce the adhesion. During cold weather,this time will be extended to approximately 3hours at 32º F.

Heat shrinkage tubing of irradiated polyethylenewith an effective electrical sealant is used toinsulate in-line cable splices. This method hasproven to be very effective and inexpensive.Care must be taken when using a propanetorch so as not to melt or distort the cableinsulation while shrinking the sleeve.

Elastomer sealant kits are a recentdevelopment that involves wrapping a pliablestrip of sealant around the splice area tomoisture-proof the connection. An outer wrap ofelastic fabric impregnated with a quick setting,moisture cured urethane is wrapped around thesealant and sprayed with water to harden thefabric. This forms a hard shell to eliminate cold flow of the sealant and prevent rocks or soilstress from damaging the encapsulation.

POWER SUPPLIES

Many sources of direct current (DC) power areavailable, as discussed in detail in Chapter 7 ofthe Intermediate Course, for use withimpressed current systems as follows:

1. Transformer-Rectifier

2. Solar Photovoltaic Cells

3. Thermoelectric Generators

4. Turbine Generator Units

5. Engine Generator Units

6. Wind Powered Generators

Transformer-rectifiers, commonly referred to assimply "rectifiers", are the most frequently usedpower source for impressed current anodesystems. The unit consists of a step downtransformer to reduce alternating current to anacceptable level, and a rectifying element toconvert the alternating current (AC) to directcurrent (DC). Output terminals and a host ofoptions and accessories complete the finalassembly of the unit. Figure 3-15 shows atypical rectifier circuit.

Specifying the various options is easilyaccomplished using the following options menu.Selecting them properly requires knowledge.

Options1. Enclosure Type Utility

Swing OutCabinetsSlide Out RacksCustomSmall Arms ProofPole MountWall MountPad MountPhotovoltaic (solar panel)

2. Cooling Type Air CooledOil CooledExplosion ProofSubmersibleFan CooledForced Oil Cooling

3. Control Type Standard ControlCurrent RegulatedVoltage Regulated


EPOXY ENCAPSULATED SPLICE

FIGURE 3-14

EPOXY MOLD

INSULATINGRESIN

1/4” MIN.1/2” MAX.

FILLING ORIFICE

COMPRESSIONSPLICE

CONNECTOR

TYPICAL RECTIFIER CIRCUIT

FIGURE 3-15

TO AC LINE

LIGHTNING ARRESTERS

CIRCUIT BREAKERS

INPUTCHANGEOVER

BARS

TRANSFORMER

SECONDARYTAP CHANGE

CIRCLE

RECTIFIERSTACK

E X Y Z C

AC

LIGHTNINGARRESTER

SHUNT

FUSE

DC OUTPUT

A V

F D

Y4 Z4 Z1 Z3 Z2

X3 X1X4 X2

X

Y3 Y1Y4 Y2

Y

Z3 Z1Z4 Z2

Z

X4 X1 X3 X2

460 230 460 230 460 230

C C C

B B B

A A A

Automatic Potential ControlSolid State Control (no taps)IR Drop Free

4. Rectifying Selenium BridgeElement Selenium Center Tap

Silicon BridgeSilicon Center Tap

5. Circuit Type Center Tap, Single PhaseBridge, Single PhaseThree Phase WyeThree Phase BridgeMultiple Output Circuit

6. AC Input 115 volts230 volts, single or

3 phase460 volts, single or 3 phase115/230 volts230/460 volts, 3 phase115/460 volts

7. DC Volts Specify maximum DC output in volts

8. DC Amperes Specify maximum DCoutput in amperes

9. Options AC and DC LightingArresters

AC Arresters onlyDC Arresters onlyCommunications FilterEfficiency Filter (choke)Meter SwitchesPilot LightDC Failure LightDC FusePainted CabinetHot Dipped Galvanized

CabinetAnodized Aluminum CabinetStainless Steel CabinetCabinet Gauge -

16,14,12,11

Cabinet LegsCabinet Side DoorSlide Out RacksHigh Ambient OperationSpecial TapsCoolant - Oil cooled unitsNon-Standard Access KnockoutsAC Frequency -

other than 60 hzElapsed Time MeterConvenience Outlet -

115 volts

JUNCTION BOXES

Junction boxes are an enclosure for terminatingmultiple cables to a common electrical bus bar.

An anode junction box can be used to connectmultiple anode cables to a common electricalbus bar. The rectifier positive cable is fedthrough a conduit to the main anode bus and aperpendicular bus terminates the lead wires atindividual terminals. Leads may be attached tofixed value current limiting resistors and thecurrent output can be measured by connectinga DC voltmeter across each respective shunt.A typical anode junction box is shown in Figure3-16.

Where several anode circuits are supplied fromone power supply, it may be desirable to controlthe current to each anode using variableresistors connected in series with the outputcable. Many of these control options can nowbe built into the rectifier circuitry.

A junction box can also be used to connectmultiple structures together.

A junction box can be installed at a rectifierwhere multiple structure cables are connectedto a bus bar which is in turn connected to therectifier negative terminal.


TYPICAL JUNCTION BOX WITH SHUNTS

FIGURE 3-16

TO POSITIVETERMINAL OF

RECTIFIER POSITIVEBUS

RESISTOR (TYP.)SIZE BASED ON

CURRENT REQUIRED

ANODE LEADWIRES (TYP.)

VOLTMETER (USED TO DETERMINEANODE CURRENT OUTPUT)

WIRESHUNT(TYP.)

V

DISTRIBUTIONBOX

(+)(-)

PERMANENTLY INSTALLED REFERENCEELECTRODES

There are 3 types of permanently installedreference electrodes that are used formonitoring the potential of a structure or as asensor in conjunction with potentially controlledrectifiers. They are:

High Purity Zinc

Copper/Copper Sulfate Electrode

Silver/Silver Chloride Electrode

If the electrode is to be buried, it must besurrounded with special backfill to increase theion trap area. Zinc is particularly subject to theeffects of polarization and should besurrounded with a 75%/20%/5% mixture ofprepared backfill to prevent this passivationeffect.

Copper bearing electrodes with a saturatedsulfate solution will become contaminated in thepresence of chlorides and are limited to freshwater and chloride free environments formaximum life.

Silver bearing electrodes with a saturatedchloride solution are used in sea water andbrackish waters where chlorides are found.Other types utilize a perforated plastic tubecontaining a chloridized silver rod and dependon the natural chlorides in sea water to providemeaningful readings. This second type will giveerroneous readings in brackish waters due tothe constant changes in chloride levels.

TEST STATIONS

As simple as a test station is, it seems to causea great deal of confusion, particularly to thosepeople who are unfamiliar with the concept ofcathodic protection. The variables are manyand here is a partial listing of the variations.

Choices1. Box Type Post Mount

Flush Mount

Flange MountRoundSquareExplosion Proof -

Specify Type, ClassCondulet

2. Construction PolycarbonateFlake Filled PolyesterCast IronCast aluminumABS PolymerThermosettingComposite

3. Terminals Specify NumberPlated BrassCopperBrassBanana PlugsStainless SteelLock Washers

4. Hub Type Slip Fit, sizeThreaded, sizeDouble HubSingle Hub

5. Hub Size Specify size and quantity

6. Post Type Treated wood, sizePVC Conduit, sizeSteel Conduit, sizeWith or without anchors

7. Post Length Specify Length

8. Flush Type Cast Aluminum TopCast Iron TopPlastic Top, magnetic

9. Wiring NonePre-wired, specify size,

length, insulationShunts - specify quantity,

size, type

10. Covers Name cast on top“O" Ring Seal, round onlyWeather Proof Seal


Waterproof SealSlip FitExposed TerminalsLocking Type

11. Terminal Polyester Laminate - 0.125"Blocks Polycarbonate

12. Options Shorting Bars - specify quantity, materialLightning Arresters -

specify typeFixed Current Limiting

Resistors -specify amperage

Variable Slide WireResistor - specifyamperage

Rotary Rheostat - specify amperage

Meters - specify type, rangeLocking DevicesShunts - specify amperage

Figure 3-17 shows some typical test stations.

FLANGE ISOLATORS AND DIELECTRICUNIONS

To isolate cathodically protected structuresrequires the mechanical insertion ofnonconductive materials between the metalcomponents. The isolation material should notdeform or deteriorate due to the operationalconditions of the external or internalenvironment or the structure itself. Once again,there are a wide variety of materials to choosefrom depending on the particular application.

Flange gaskets (Figure 3-18) are selected onthe basis of pipe size, ANSI pressure rating, fullface or raised face, and material composition tosuit the pipeline product. The components of aflange isolation kit consist of a gasket, sleeves,isolating washers, and steel washers.

Components

1. SizeS Specify

2. Gasket TypeS Full Face “E” (with bolt holes)S Ring “F” (without bolt holes)S “D” for RTJ flanges

3. MaterialsS Plain Phenolic - 225º F, max.S Neoprene Faced Phenolic - 175º F, max.S Glass/Phenolic - 350º F, max.S Aramid Fiber - 450º F, max.S Epoxy/Glass - 350º F

4. Sealing ElementS Nitrile Sealing Element - 250º F, max.S Viton - 350º F, max.S Teflon - 450º F, max.

5. Pressure RatingS ANSI Class - 150 lbs to 2500 lbs

6. Isolating SleevesS Polyethylene - 180º F, max.S Phenolic - 225º F, max.S Mylar - 300º F, max.S Acetal - 180º F, max.S Nomex - 450º F

7. Isolating WashersS Glass/Phenolic - 300º FS High Temp. Phenolic - 350º FS Silicon/Glass - 350º FS Epoxy/Glass - 280º F to 350º F

8. Steel WashersS Cadmium Plated, c" thick

Check with the manufacturer for more specificand absolute data. Flange isolation kits are bestsuited for above ground electrical isolation onnew installations.

Dielectric isolating unions (Figure 3-19) performthe same basic function as flange gaskets byinserting a high resistance plastic between theunion faces. They are normally available as “O”Ring Type or Ground Joint Type in pressureratings of 150 to 3,000 psi working pressure.Common installations include hot waterheaters, service station pumps, natural gas


PERMANENTREFERENCEELECTRODE

LOW RESISTIVITY

BACKFILLCLOTH SACK

LEAD WIRE

BUSHING

TEST WIRE

PROTECTED PIPE

CONDUIT, GALVANIZEDSTEEL OR LEXAN

GRADE

WOODENPOLE

GALVANIZEDSTEEL STRAP (TYP.)

POLE MOUNTEDTEST STATION

TERMINALBOARD

FLUSH MOUNTEDTEST STATION

GRADE

TEST WIRE

PROTECTED PIPE

LEAD WIRE

PERMANENT REFERENCEELECTRODE

LOW RESISTIVITYBACKFILL

CLOTHSACK

TYPICAL CATHODIC PROTECTION SYSTEM TEST STATION INSTALLATIONS

FIGURE 3-17

TYPICAL ISOLATING FLANGE ASSEMBLY

FIGURE 3-18

ISOLATING GASKET

CATHODICALLYPROTECTEDSIDE

ISOLATING WASHER(DO NOT USE ON THIS SIDEIF FLANGE IS BURIED)

STEEL WASHER

ISOLATING SLEEVEISOLATINGWASHER

STEEL WASHER

STANDARD BOLT OR STUD

COAT FLANGESTO BE BURIED

UNPROTECTEDSIDE

TYPICAL ISOLATING UNION DETAIL

FIGURE 3-19

THREADING

ISOLATING MATERIAL(MOLDED NYLON)

NUT

FLANGEDMETALBODIES

distribution service lines, and hydraulic lines.

MONOLITHIC WELD IN ISOLATORS

Monolithic weld in isolators are factoryassembled, one piece, isolating devicesdesigned to eliminate underground isolatingflanges. They eliminate bolts and washerstherefore providing a uniform surface to easilyapply an exterior dielectric coating.

They are available in pipe sizes up to 120inches diameter in all ANSI pressure ratings.Factory pretested, they assure high dielectricqualities and are available with an internalcoating to reduce bridging of the insulating gapwhen the product carried in a conductive liquid.

CASING ISOLATORS AND END SEALS

Since the mid-1980's, the effects of electricallyshorted casings has received enhancedscrutiny from regulatory agencies. The causeand effect relationship of electrically shortedcasings should be well known to the reader bynow, and the purpose of this section is toacquaint the reader with the types of materialsthat are currently available. Careful selectionand installation of the components duringconstruction, either new or rehabilitative, willreduce the possibility of a shorted casing.

Casing isolators (Figure 3-20) generally consistof molded polyethylene segments, joinedtogether to form rings at frequent intervals onthe pipe, to support the weight of the pipe withproduct and keep the carrier (product line)centered in the casing. On occasion thesesegments become damaged during installationor during operation due to soil shifting and pipemovement, resulting in a shorted condition.

Other types are made of steel, shaped like asaddle, and are lined with a thick rubber or PVCsheeting to isolate the carrier pipe from thesaddles. These too may become damaged andineffective as mentioned previously if notproperly specified. To order, specify a highcompressive strength runner suitable for thepipe and product weight and the length of the

casing or crossing. The isolator should have aquality coating and be properly designed for theparticular project.

Another form of casing isolation is anonconductive wax filler, used in conjunctionwith isolating rings, to fill the annular spacebetween the carrier and the casing. If doneproperly, the filler will prevent ground waterpenetration and condensation from filling theannular space with electrolyte, thus preventingion flow. It will also prevent atmosphericcorrosion from occurring within the casing.

If the casing has been in service, it should beflushed with water to remove debris. The endseals are usually replaced with new seals thatcan accommodate the weight and installationpressures encountered during the fillingprocedure. For new and rehabilitated casings,the vents are temporarily sealed off and thecasing is pressurized to determine if the fillerwill be contained.

Complications that may arise are, stitch weldedcasing, perforated casing, out of round casing,improper seals and constricted vent openings.

Casing end seals (Figure 3-20) havetraditionally consisted of rubber boots held inplace with hose clamps, shrink sleeves withsupport skirting, and a proprietary rubber linksystem. Only careful selection and supervisionduring installation can prevent the end sealsfrom being the weakest link in the system.

CONCLUSIONS

It is important for corrosion control personnel tounderstand the characteristics of the variousmaterials and equipment associated withcathodic protection systems.

Before designing a system, the corrosionworker should acquire catalogs from variouscathodic protection equipment suppliers todetermine what materials and equipment areavailable and to determine their characteristics.


CASING

ISOLATINGSPACER (TYP.)

PAVEDROADWAY

END SEAL

GRADE

METALLIC VENTPIPE (TYP.)

PIPE

TYPICAL ISOLATED CASING DETAIL

FIGURE 3-20

CHAPTER 4

DYNAMIC STRAY CURRENT ANALYSIS

INTRODUCTION

This chapter will review the causes andcommon means of detecting and mitigatingdynamic stray current effects. Steady state, orstatic, stray current has been covered inChapter 5 in the Intermediate Course text.

This chapter presents detailed information ondynamic stray currents emanating from directcurrent (DC) sources. While alternating current(AC) may create a potential safety hazard, itseffect on corrosion of ferrous structures is notfully understood. A brief discussion on stray ACappears at the end of this chapter.

STRAY CURRENTS

Stray currents are defined as electrical currentsflowing through electrical paths other than theintended paths. Stray, or interference currentscan be classified as being either static ordynamic.

Static interference currents (Chapter 5,Intermediate Course text) are defined as those,which maintain constant amplitude andgeographical path. Examples of typical sourcesare railroad signal batteries, HVDC groundelectrodes and cathodic protection systemrectifiers.

Dynamic interference currents, on the otherhand, are defined as those which arecontinually varying in amplitude, direction orelectrolytic path. These currents can bemanmade or caused by natural phenomena.Typical examples of man-made sources are DCwelding equipment, electrical railway systems,

chlorine and smelter plants. Telluric, or naturalsources of dynamic stray currents, are causedby disturbances in the earth’s magnetic fielddue to sun spot activity. Telluric effects havenot been shown to contribute to corrosion, butthey do, however, create measurementdifficulties and can interfere with one’s ability toassess cathodic protection systemperformance.

THE EARTH AS A CONDUCTOR

In order to understand the control of straycurrent, one must first understand what thesestray currents are. In underground corrosion,we are dealing with the earth as a hugeelectrolytic medium in which various metallicstructures are buried and often interconnected.The earth is an electrolyte because of the watercontained in the soils. This water causesionization of various soluble salts and makesthe earth an electrolytic conductor.Conductance is measured in electrical unitsknown as Siemens, or mho centimeters. Morecommonly, the reciprocal of conductivity,resistivity, is measured and expressed as ohmcentimeters. The more conductive the earth, thelower the resistivity. Seawater, for example, hasa resistivity of 25 to 30 ohm centimeters.Natural soils, depending on the amount ofionizable material present and the moisturecontent, range in resistivity from near that ofsea water up to the hundreds of thousands ofohm centimeters. When one considers straycurrents, it should be borne in mind that thelower the resistivity of the soil, the more severethe effects of stray currents may be.

From the values of resistivity cited, it is quite

DYNAMIC STRAY CURRENT ANALYSIS4-1

evident that a metallic structure in the earthmay be a far better conductor of electricity thanthe earth itself, even seawater. Resistivities forcopper and iron are less than 10 x 10-6 ohmcentimeters, while that for lead is approximately22 x 10-6 ohm centimeters. It is readilyunderstandable, therefore, that should currentflowing in the earth produce a potentialdifference (voltage gradient) crossed by such ametallic conductor, the conductor will readilyacquire a part of the current that is flowing. SeeFigure 4-1. Thus, pipelines and cables canbecome major conductors of stray currents inthe earth environment.

POTENTIAL GRADIENTS IN THE EARTH

How do the stray currents that accumulate onmetallic structures underground get into theearth? As in any electrical circuit, it isnecessary to have potential differencesbetween two points in order to have a currentflow. One also must have an electrical circuit,that is, a source of the current and a return forthat current. The magnitudes of current will bedependent upon the electrical potential andelectrical resistance. The amount of currentwhich will flow on any one structure will beinversely proportional to the resistance of thevarious parallel electrical paths that join the twopoints of different electrical potential in thecircuit. This is true of any electrical circuit whereparallel resistances are involved. A metallicelectrode in earth that is positive with respect tothe earth around it will discharge current intothe earth and onto any other metallic structuresthat may be in the vicinity. Any element that isnegative to earth will have the opposite effect ofcollecting current from the earth or from metallicstructures in its vicinity.

Many companies feel that any positive pipe-to-soil voltage swing of less than 10 millivolts on astructure is tolerable and does not requiremitigation, since interference is negligible. TheBritish “Joint Committee for Coordination ofCathodic Protection of Buried Structures”, hassuggested that the allowable limit of positivechange be 20 millivolts. It is possible thatpositive voltage changes within these ranges

may be indications of strong adverseelectrolytic currents, but in the majority of caseswhere voltage swings are less than 10 millivoltsnegligible interference exists.

In practice, if the positive swing does not causea potential less negative than -0.850 volts (IRdrop free), then negligible corrosion can beexpected.

DETECTION OF DYNAMIC STRAYCURRENTS

The easiest way to keep abreast of possibleinterference problems in an area is throughcontact with Corrosion or ElectrolysisCoordinating Committees. These committeesact as clearinghouses for information onunderground plant locations and knowninterference sources.

Dynamic stray currents are present if thestructure-to-soil potential is continuallyfluctuating while the reference electrode is keptin a stationary position in contact with the soil.See Figure 4-2. These potential changes are adirect result of current changes at the source ofthe interference due to loading variations. Thiswould also be detected by constantly changingline current measurements (Figure 4-3).

In efforts to locate the source of man-madedynamic stray currents, the following stepsshould be taken:

1. Determine if any DC electrical railwaysystems, mines or industrial plants(aluminum, chlorine, etc) exist in the areawhere the interference was detected.

2. Inquire of other pipeline operators, cableoperators or corrosion committees in thearea if any dynamic earth current sourcesin the area, such as rapid transit systemsor electrical DC substations exist.

3. If there are mines or DC railways in thearea, contact them to determine thelocation of their operations and currentsources.


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-2.00

-1.80

-1.60

-1.40

-1.20

-1.00

-0.80

-0.60

12:00 PM 2:24 PM 4:48 PM 7:12 PM 9:36 PM 12:00 AM 2:24 AM 4:48 AM 7:12 AM 9:36 AM 12:00 PM

Vg

(V

olt

s to

Cu

/Cu

SO

4)

Time

Pipe-to-Soil Potential vs Time

Vg-Min Vg-Ave Vg-Max -0.85 V

TYPICAL PIPE-TO-SOIL POTENTIAL (Vg) PROFILEINDICATING DYNAMIC STRAY CURRENT

FIGURE 4-2

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V

E

gsc

sc

4. Trace the current flow along the structureback to its source. This can be done byobserving the current flow at intervalsalong the interfered structure by utilizingmillivolt drop test station lead wires, todetermine the direction of the current flow.This technique is illustrated in Figure 4-3.

Let us assume that a situation exists where asingle generating unit is causing interferenceproblems. A voltmeter connection, to be usedfor pipe-to-soil potential (Vgsc) measurements, ismade between the pipeline and a referenceelectrode, somewhere within the earth currentpattern of the generator and its load. Observingthe fluctuating potential readings at this pointalone would not enable one to determine if thereadings are being taken at a point where thepipeline is picking up or discharging current. Ifmeasurements are being taken at a point ofcurrent pickup, a negative potential swing wouldoccur. A positive swing would indicate adecrease of current flow and a condition of thepipe returning to its steady state condition.Readings observed at a discharge point,swinging in the positive direction would indicatean increase in current leaving the line and anegative swing would indicate a decrease incurrent discharge.

The determination of whether test locations areeither pick-up or discharge points is done byobtaining correlations of the pipe-to-soilpotential to the open circuit voltage between theinterfered pipeline and the stray current source.This is known as a Beta Plot. Figure 4-4illustrates a typical Beta plot measurementconnections. The locations for the pipe-to-soilmeasurements may be remote from the opencircuit measurements. Communication must beestablished between the locations, as bothmeasurements must be made simultaneously.Sufficient data points must be obtained toindicate trends in the data. Figure 4-5 is anexample of a Beta Plot of one locationindicating a current pick up area and Figure 4-6shows a current discharge area at anotherlocation.

As the connection shown in Figure 4-4

indicates, a positive value for Esc occurs whencurrent flows from the stray current source tothe pipeline. Conversely, a negative value of Esc

indicates a flow from the pipeline to the straycurrent source. The slope of a straight linedrawn through the data points is the value ofBeta (β):

As indicated, this type of testing requires thatvoltage measurements be taken at two differentlocations. Many readings should be taken at thetwo locations simultaneously. The meters usedat the two locations must be identical or elsecomparison of the two sets of readings will bedifficult.

In most cases, a dual channel recorder such asan X-Y plotter is used or a multi-channel datalogger is used.

Interpreting Beta Curves

Beta curves, which are sloped from the bottomright hand side of the graph to the top left side,would indicate an area of current pick up. Whatthis means is that as the current output of thesource increases, the current pick up by theinterfered structure also increases. An exampleof such a curve is shown in Figure 4-5.

An area of current discharge is indicated by acurve which slopes from the bottom left to thetop right of the graph, indicating that the rate ofcurrent discharge increases as the currentoutput of the source increases. An example ofsuch a curve is shown in Figure 4-6.

If the plotted points describe a vertical line, thiswould indicate a neutral curve. This means thatthere is no influence on Vg by the outputfluctuations of the current source. Such a curveis rare in practice and if one is plotted, thepossibility of equipment malfunctions or errorsduring testing should be investigated.

If the plotted points do not describe a straightline, this may indicate that the interfered


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FIGURE 4-5

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TYPICAL BETA CURVE - DISCHARGE AREA

FIGURE 4-6

0.0

0.5

1.0

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2.0

2.5

3.0

3.5

4.0

-1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20

ES

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OL

TS

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Vgsc2 (VOLTS)

∆Vgscβ =

∆Esc

structure may be picking up stray currents froman additional source or that the suspectedsource is not causing the interference. Plotsdescribing two or more distinct lines wouldindicate the presence of more than oneinterfering current sources. The slopes of theselines will indicate whether currents aredischarging or being picked up according totheir slopes and the polarity of the meterconnections.

Determining the Point of MaximumExposure

The point of maximum exposure can bedetermined based on the slopes of the plotteddischarge beta curves. The beta curve whichhas the most horizontal (largest) slope indicatesthe location of maximum exposure. Figure 4-7shows this graphically.

In practice, the calculated slopes can be plottedversus distance to indicate clearly the locationof the largest slope (indicative of the mosthorizontal line).

METHODS OF MITIGATING THE EFFECTSOF DYNAMIC STRAY CURRENT

There are several general methods used toreduce or mitigate the effects of dynamic straycurrent. These methods include: control at thesource, installation of mitigation bonds andreverse current switches, use of sacrificialanodes, and use of impressed current cathodicprotection to counter the stray current. Each ofthese methods is discussed in the followingsections.

Controlling Stray Currents at the Source

The most effective way to minimize dynamicstray current is to minimize the electrical currententering the earth in the first place. In the caseof transit and any other systems involving railreturns, the best procedure is to ensure that therails are installed with insulated fasteners onwell-ballasted roadbeds or concrete inverts.The DC negative bus of the traction powersubstation should be ungrounded, and the

substations should be spaced at a maximum ofabout one-mile apart. Modern rail transitsystems use welded rail; this reduces thevoltage drop in the rail, which in turn reducesthe stray current induced in the earth.

Similarly, when dealing with equipment, ifisolated electrical positive and negative circuitscan be used, stray current problems will beavoided because the currents never reach theearth. When welding is done, care should betaken to assure that the welding electrode andthe ground are relatively close together and theelectrical path between them is of negligibleresistance. Thus, no voltage drop of anysignificance will exist between them.

Mitigation (Drainage) Bonds and ReverseCurrent Switches

Mitigation bonds and reverse current switchesare used to mitigate the effects of stray currentcorrosion on a structure.

The purpose of a mitigation, or drainage bondis to eliminate current flow from a metallicstructure into the electrolyte by providing ametallic return path for such current. Thedrainage bond will allow the stray current toflow through the structure and back to thesource, through the bond. Figure 4-8 shows thetypical current flow when a drainage bond isinstalled. As a result, corrosion will not occur ifthe current is not allowed to flow from the metalsurface into the electrolyte.

It is not good practice to install drainage bondson modern rail transit systems that have beendesigned to minimize stray current at thesource. The high resistance to ground of thesesystems minimizes stray current. Installation ofa bond lowers this resistance and in turnincreases stray current flow.

When designing a bond, the goal is to find theproper size resistance to be inserted betweenthe affected structure and the source so thatstray current discharge into the electrolyte iseliminated at all points. The proper bond willcause the affected structure to return the pickup


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current to the interfering structure through thebond.

Before the mitigation bond can be sized, thepoint of maximum exposure must be located.This is done as described earlier under“Determining the Point of Maximum Exposure”.

As previously indicated, dynamic stray currenttesting requires potential measurements takensimultaneously at two different locations. Thesereadings can be taken by two technicians whoare in communication with each other by radioso that one can tell the other when to take areading. Many readings should be taken at thetwo locations simultaneously. Meters used atthe two locations must have identicalcharacteristics or comparison of the two sets ofreadings will be difficult.

To facilitate the taking of these potentialreadings, strip chart recorders can be set up atthe two locations. Recordings obtained over a24-hour period will normally provide all the datarequired. The corresponding readings taken atthe two locations can then be plotted on lineargraph paper. This plot of points is known as a“beta curve”. Several locations should be testedlongitudinally along the pipeline within theexposure area. The location of the recorder atthe source remains constant. A separate set ofsimultaneous readings should be made at eachtest location. This type of survey is known as abeta profile.

State-of-the-art computerized multi-channel testequipment is available to facilitate obtaining abeta profile. These computerized recorders canbe set to record the potentials at as many as 28different locations simultaneously for anydesired length of time. The recorder will storeall readings in a permanent memory until it isdown-loaded into a computer terminal.Computer programs are available which willthen plot beta curves. Similarly X-Y recorders,which simultaneously record pipe-to-soilpotential and pipe to source voltage arefrequently used.

The points when plotted should describe a

straight-line plot referred to as the beta curve.The slope of the plotted line will indicatewhether the interfered structure in the area ofthe test is picking up current, dischargingcurrent or neither.

Once the location of maximum exposure isdetermined and its negative slope or beta curveplotted, the size of the resistance bond can bedetermined. The required size of the resistancebond is such that its installation will cause thebeta curve at the point of maximum exposure toassume a neutral or pick-up slope. Figure 4-9shows a beta curve at a point of maximumexposure as well as the required mitigationcurve.

Sizing the Mitigation (Drain) Bond

Two methods are available to size the bond,trial and error and mathematical. Both areexplained below.

Trial and Error Method

In relatively simple cases, such as where asingle source of stray current is involved, a trialand error solution may be possible. The size ofthe resistance bond can be determined byinstalling temporary cables and variableresistors and determining when stray currentcorrosion has been mitigated. Since draincurrents of up to 200 to 500 amperes can beinvolved, test cables must be sized accordingly.A mitigation curve such as that shown in Figure4-9 must be obtained from the test bond.

When using the trial and error method on straycurrent problems involving transit systems, careshould be exercised to ensure that thetemporary bond does not affect signaling.

Mathematical Method

Where a more complex interference problemexists that precludes the use of the trial anderror method, the following mathematicalmethod can be used to size the resistancebond. As noted earlier, dynamic stray currentsmay involve large values of current flow, so the


TYPICAL BETA CURVE - DISCHARGE AREA(MITIGATION CURVE)

FIGURE 4-9

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

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Vgsc (VOLTS)

∆Vgscβ =

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MITIGATION CURVE

x E I x V

Isc 1

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1

anticipated currents must be known to sizecable and associated electrical equipmentproperly. A mathematical solution can providethe designer with these anticipated values.

This method is based upon the electricalrelationships between the source and thepipeline. The measured potential-to-earth of apipeline is a combination of its natural potential-to-earth plus the sum of all potential changescaused by the DC sources influencing it.

The test set ups for the mathematical methodare shown in Figures 4-10 and 4-11. Figure4-10 explains the test procedure for determiningthe internal resistance (Rint) between thepipeline and the stray current source. Figure 4-11 shows the procedure to determine thechange in pipe to soil potential per ampere ofdrain current.

The beta curve at the point of maximumexposure, such as the one shown in Figure4-12, is plotted. The equations used in reachingthe solution are as follows:

Equation 1

Vg = ∆Vgo + ∆Vgcp + ∆Vgsc + ∆Vgb

Where:

Vg = Pipe-to-soil potential with all currentsources operating

∆Vgo = Natural pipe-to-soil potential of thepipeline

∆Vgcp = Change in pipe-to-soil potentialcaused by cathodic protection ofpipeline

∆Vgsc = Change in pipe-to-soil potentialcaused by stray current sources

∆Vgb = Change in pipe-to-soil potentialcaused by mitigation bond

In the case of a dynamic stray current:

Equation 2

∆Vgsc = β x ∆Esc (See Figure 4-4)

Where:

β = ∆Vgsc /∆Esc

If no cathodic protection is present, Equation 1simplifies to Equation 3:

Equation 3

Vg = Vgo + β + ∆Esc + ∆Vgb

For the dynamic stray currents to be mitigated:

Equation 4

β x ∆Esc = ∆Vgb (Vg - Vgo must = 0)

From Figure 4-12, the value for Vgo = 0.570volts and β (the slope of the plotted points) =0.017.

∆Vgb is the change in potential-to-soil of theinterfered structure caused by a bond current.The value of ∆Vgb can be calculated for anygiven bond current from Equation 5:

Equation 5

(See Figure 4-11)

V I x V

Igb 1

g

1

Combining Equation 4 with Equation 5 yieldsEquation 6:

Equation 6

The value of the bond current, Ib, is calculatedfrom Equation 7:


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BETA CURVE PLOTTED AT POINT OF MAXIMUM EXPOSURE

FIGURE 4-12

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4.0

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scint

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R 0.00169

0.017 0.070 = 0.0294b

E I x (R R )

I E

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0.070 0.0294

12.0

0.0994

I 120.7 amperes

sc b int b

bsc

nt b

b

b

Equation 7

I x EV

Ib sc

g

The resistance of the bond can be calculatedfrom Ohm’s Law:

Equation 8

Esc = Ib x (Rint + Rb)

Or, rearranging:

Equation 9

R E

I Rb

sc

bint

Substituting:

Equation 10

Or:

Equation 11

Tables 4-1 and 4-2 present sample data for asingle bond stray current problem. From Table4-1, the average resistance between structuresis found to be 0.070 ohms. Table 4-2 shows thevalue of ∆Vg/I1 to be 0.00169 V/A. Using the βvalue from Figure 4-12 of 0.017 and substitutingthe values into Equation 11, the bondresistance, Rb, is calculated:

The bond resistance value, 0.0294 ohm for thisexample, can be a simple cable or acombination of cable and variable or fixedresistors. The bond must also be sized topermit the maximum current flow and stillremain in the ampacity range for the bond. In adynamic stray current situation, the maximumcurrent can be calculated once the maximumopen circuit voltage between the structures isknown. This value is usually obtained by usinga recording voltage instrument over severaloperating cycles of the stray current source tomeasure the peak value. For most stray currentsources the typical cycles is 24 hours.

For example, if the maximum value of opencircuit voltage, Esc, is 12.0 volts, substitution ofthis value into Equation 8 and rearranging theequation gives the value of the maximum straycurrent through the bond 120.7 amperes asshown below:

Table 4-1 also shows some typical measuredvalues for E1 and I1, using the test setup shownin Figure 4-10, as well as the calculated internalresistance.

Table 4-2 also shows some typical measuredvalues for Vg and l1, using the test setup shownin Figure 4-11, and the correspondingcalculated ∆Vg/I1.

In many instances of stray current work wheretraction systems are involved, there may belocations where drainage bonds are required inareas where electrical reversals of potentialcould occur. Therefore, it is often necessary to


TABLE 4-1

Rint Data - See Figure 4-10

E1 (Volts) I1 (Amperes) Rint (ohms)

On +1.30 36.0

Off -1.15 0

Delta +2.45 36.0 0.068

On +0.80 39.0

Off -1.80 0

Delta +2.60 39.0 0.067

On +0.50 41.0

Off -2.30 0

Delta +2.80 41.0 0 .068

On +2.10 29.0

Off 0.00 0

Delta +2.10 29.0 0.072

On +2.00 29.5

Off -0.15 0

Delta +2.15 29.5 0.073

On +1.30 35.0

Off -1.25 0

Delta +2.55 35.0 0.073

On +0.35 43.0

Off -2.65 0

Delta +3.00 43.0 0.070

Average Rint = 0.070

TABLE 4-2

)Vg/I1 Data - See Figure 4-11

Vg (Volts) I1 (Amperes) )Vg/I1 (V/A)

On 0.645 34.0

Off 0.590 0

Delta 0.055 34.0 0.00162

On 0.635 23.0

Off 0.600 0

Delta 0.035 23.0 0.00150

On 0.770 82.0

Off 0.620 0

Delta 0.150 82.0 0.00183

On 0.770 86.0

Off 0.625 0

Delta 0.145 86.0 0.00168

Average )Vg/I1 = 0.00169

install an electrolysis switch or silicon diode intothe circuit to prevent current flow from therailroad back onto the pipeline through thebond. The resistance to the forward flow ofcurrent created by these devices must beincluded in the sizing of the bond.

Use of Sacrificial Anodes

Sacrificial or galvanic anodes may be used tomitigate stray current effects in situations wheresmall current flows or small voltage gradientsexist. A misconception surrounding the use ofgalvanic anodes for stray current mitigationstates that the anode draws the stray currentsand prevents currents from entering into theelectrolyte from the interfered structure. Thisover-simplification does not consider the entireelectrical network involved. In effect, a potentialgradient field produced by the galvanicanode(s) counteracts the interference current.The effect is a net current flow to the interferedwith structure. A version of the electrical circuitis shown in Figure 4-13.

The potential gradients of the stray currentsource and of the anodes counteract eachother. Galvanic anodes produce limitedvoltages, however, and as such are limited tothe stray current voltages, which they canovercome.

Another consideration when using a galvanicanode system to overcome stray currents is theexpected life of the anodes. As the anodesdissipate, their resistance to ground increases.The increased resistance reduces the currentflow from the anode and decreases theresulting voltage gradients. Sacrificial anodesshould be sized to provide a sufficientanticipated life span. As with any other straycurrent mitigation procedure, the anodes shouldbe placed on an active monitoring schedule.

Galvanic anode drains are commonly used inlieu of bonds where small drain currents areinvolved. In areas of large current drains, theuse of galvanic anode drains would beimpractical due to the high consumption rate ofthe anode material; frequent anode

replacement would be required. Galvanicanodes would also be impracticable wherevoltage gradients are encountered that aregreater than those galvanic anodes canproduce.

Use of Impressed Current Systems

When the magnitude of stray currents arebeyond the ability of galvanic anodes tocounteract, impressed current systems cansometimes be utilized. Impressed currentsystems have much higher voltage capacitiesthan galvanic anodes and a greater life perpound of anode material. Built-in control andmonitoring circuits in the impressed currentrectifier can be used to adjust the protectivecurrent output based upon the voltage-to-earthfluctuations of the interfered pipeline.

The design of galvanic or impressed currentmitigation systems is done by the trial and errormethod. Simulated systems are placed in thefield and the results measured. Based upon theresults, a full-scale system can be designed.

STRAY CURRENT FROM ALTERNATINGCURRENT (AC) SOURCES

Stray current from AC sources can causecorrosion as well as be a safety hazard. Bothsteady state and dynamic stray AC current maybe found.

Several laboratory and field studies haveindicated that above a certain minimum ACcurrent density, normal levels of cathodicprotection will not control AC corrosion toacceptable levels and that AC mitigation isoften required to prevent serious corrosion.1

Methods of mitigating AC corrosion are beyondthe scope of this text.

Steady state stray AC is most commonlyassociated with pipelines laid in close proximityand paralleling high voltage AC electricaltransmission lines. AC induction on suchpipelines can create dangerous voltages.Mitigation of the voltage on the line as well assafety grounding of test stations and above


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ground appurtenances to buried equipotentialmats are recommended if the AC pipe-to-soilpotential exceeds 15 volts.

Dynamic AC stray currents may be generatedby AC welding operations, bad grounds inbuildings or AC electrified railroads.

For information on AC mitigation, the reader isreferred to NACE Standard PracticeSP0177-2007 “Mitigation of Alternating Currentand Lightning Effects on Metallic Structures andCorrosion Control Systems”.2

CONCLUSIONS

Stray current interference on a pipeline cancause severe local corrosion that could lead toa hazardous situation, product leakage andcostly repairs. Therefore, every effort should bemade to avoid interference problems and, ifthey exist, to mitigate their effects.

Stray currents can be classified as being eitherstatic or dynamic. Static interference currentsare defined as those, which maintain a constantamplitude, direction and electrolytic path.Dynamic interference currents are defined asthose, which continually vary in amplitude,direction and electrolytic path. Staticinterference is always caused by man-madesources; dynamic interference, however, canalso be caused by telluric activity.

Stray current control at the source is the bestmethod of stray current mitigation.

The use of bonds or cathodic protectionsystems to mitigate stray current corrosion canbe very effective when properly designed andinstalled. It is very important that all field testingand remedial actions be conducted or taken inthe presence of, or with the permission of,representatives of all companies/ownersinvolved.

Sacrificial anode or impressed current cathodicprotection can also be used to mitigate straycurrent.

Stray AC can cause corrosion and be a safetyhazard.

REFERENCES

1. R. A. Gummow, “AC Corrosion – AChallenge to Pipeline Integrity,” MaterialsPerformance, 38, 2 (Feb. 1999)

2. NACE Standard Practice SP0177-2007“Mitigation of Alternating Current andLightning Effects on Metallic Structures andCorrosion Control Systems”, NACEInternational, Houston, Texas


CHAPTER 5

DESIGN OF IMPRESSED CURRENT CATHODIC PROTECTION

INTRODUCTION

The objective of this chapter is to providecorrosion control personnel with informationnecessary to design an impressed currentcathodic protection system for undergroundstructures.

This chapter will discuss the variousparameters which must be considered in thedesign of an impressed current cathodicprotection system. These parameters includethe following:

1. Protective current requirements

2. Anode bed resistance

3. Anode bed layout

4. Anode life expectancy

5. Rectifier sizing

REVIEW OF IMPRESSED CURRENTSYSTEM FUNDAMENTALS

Definition

Cathodic protection can be defined as thereduction or elimination of corrosion by makinga metallic structure cathodic with respect to theelectrolyte in which it is installed. This isaccomplished by applying current to thestructure by means of an impressed directcurrent or attachment of a galvanic anode to thestructure.

Cathodic protection makes a metallic structurethe cathode of a purposely designedelectrochemical cell thereby protecting it from

corrosion. The protective current is eitherdeveloped from within the cell or introduced tothe cell from an external source which is largeenough to overcome the effect of corrosioncurrents being discharged from the anodicareas of the structure. When the surface of thestructure is fully polarized, corrosion of thestructure is controlled. In a sense, the corrosionprocess is not eliminated but merely transferredto an expendable, or sacrificial, material(anode).

Theory of Operation

The impressed current cathodic protectionsystem utilizes an external power source toprovide a potential difference between theanode(s) and the protected structure. Theanodes are connected to the positive terminaland the structure to the negative terminal of thepower source. Current flows from the anodesthrough the electrolyte and onto the surface ofthe structure. It then flows along the structureand back to the power supply through anelectrical conductor. Since the structure ispicking up current from the electrolyte, thestructure is protected. The protective currentoutput of the power supply is adjusted to deliversufficient current to overcome the corrosioncurrents trying to leave the anodic points on thestructure. As the anodes discharge current theyare consumed. Therefore it is desirable to useanode materials that are consumed at lowerrates than magnesium, zinc, and aluminumwhich are used in sacrificial or galvanicsystems. Special alloys and combinationmaterials are used in impressed currentsystems to obtain a long, useful life expectancyfrom the system.

DESIGN OF IMPRESSED CURRENT CATHODIC PROTECTION5-1

Application

The impressed current cathodic protectionsystem, in most cases, utilizes a variable powersource. Impressed current systems are capableof protecting large structures or structureswhich require greater magnitudes of currentthan can be provided economically by agalvanic anode system. These systems can bedesigned to protect bare (uncoated) structures,poorly coated, or well coated structures. As theamount of surface area to be protectedincreases, the total current requirementincreases and impressed current systemsbecome the system of choice.

Advantages and Disadvantages

Impressed current systems can be designed fora variety of applications with a reasonabledegree of flexibility. This is a distinct advantageof this type of system, along with the ability toincrease or decrease the current output, eitherautomatically or manually, by changing theoutput voltage. The disadvantages of thesystem are the increased maintenancerequirements, tendency for higher operatingcosts, and possibility of contributing to straycurrent interference on neighboring structures.

INFORMATION USEFUL FOR DESIGN OF ANIMPRESSED CURRENT CATHODICPROTECTION SYSTEM

Before embarking on the design of a system,certain information should be gathered,analyzed, and factored into the design:

1. What is the pipe material? Is it steel(including grade of steel), cast iron,wrought iron or other material? What is theknown electrical resistance?

2. Is the line bare or coated? If coated, whatis the coating material and what coatingspecifications were used?

3. If it is an existing line, is there a leakrecord? If so, information on the locationand date of occurrence of each leak will

positively indicate the more seriousproblem areas.

4. What is the pipe diameter, wall thicknessand weight per foot? Is there any data onany changes in these items along the routeof the line?

5. What are the location and constructiondetails of all corrosion test points whichhave been installed along the line? If notest points have been installed forcorrosion test purposes, determinelocations where contact can be made withthe pipeline for test purposes (other thanby driving contact bars down to the pipe).

6. Is the line of all-welded construction or aremechanical couplers used?

7. What are the locations of branch taps?

8. What are the locations of purposelyisolating flanges or couplers, if any, used tosectionalize the line or to isolate itelectrically from other portions of thesystem or from piping of other ownership?

9. Obtain route maps and detail maps givingas much data as is available.

10. What are the locations of undergroundstructures of other ownership that cross thepipeline to be surveyed? If any of thesestructures are cathodically protected,determine the location of cathodicprotection current sources (particularlyrectifiers) that may be in close proximity tothe structure to be protected.

11. Location of possible sources of man-madestray current (such as DC electric transitsystems or mining operations) that couldaffect the line under study.

12. Do any sections of the pipeline closelyparallel (within 200 feet or so) high-voltageelectric transmission lines? If so, what isthe length of such exposure, how close isthe pipeline to the towers, at what voltage


does the electric line operate, and whatmethod is used for grounding the towers?(This information is significant becauserectifier installations and isolating joints inwell coated pipelines, closely parallel tohigh-voltage electric lines, may bedamaged by induced AC voltage surgesunder electric system fault conditions ifpreventive measures are not taken.)

13. Is the line now operated at elevatedtemperatures, or will it be so operated inthe foreseeable future? (High temperaturecould cause deterioration of coatings usedand increase the current requirement toachieve corrosion control.)

14. Is AC power available to provide power tothe rectifier?

DESIGN OF AN IMPRESSED CURRENTANODE BED

General

There are several steps to be taken indesigning an impressed current anode bed. Theinitial steps include:

C Selecting an anode bed site.

C Selecting an anode bed type.

Once the anode bed site and type have beenselected, the designer can proceed to evaluatethe different parameters required to completethe design of the selected impressed currentanode bed.

Selecting an Anode Bed Site

The following factors should be taken intoaccount when considering an anode bed site:

a. Soil Resistivity - Soil resistivity is one of themost important factors to consider whenselecting an anode bed site. Soil resistivity isone major component in evaluating theanode bed resistance. The anode bedresistance is a critical component in the

determination of protective current outputand rectifier size. Usually, the lower theresistivity, the fewer number of anodesrequired to deliver a given current. In manycases, the area of lowest resistivity isconsidered a prime location for an anodebed site.

b. Soil Moisture - Another factor to consider inthe selection of an anode bed site is the soilmoisture at the proposed anode depth. Inmost cases, soil resistivity decreases as themoisture increases until the soil becomessaturated. When possible, anodes should beinstalled below the water table (assumingthe water resistivity is not too high), or inareas of high moisture content such asswamps, ditches, creek beds, etc.

c. Interference With Foreign Structures -Structures such as pipelines, undergroundmetallic sheathed electrical cables, or wellcasings, may be subjected to stray currentinterference problems. Therefore, anodebeds should be located away from foreignstructures whenever possible. Local utilitiesshould be contacted to determine ifunderground structures exist in the area ofthe proposed anode bed location.

d. Power Supply Availability - The selectionof an anode bed site may be influenced bythe availability of an economical source ofpower to be used to provide the requiredcathodic protection DC current.

e. Accessibility for Maintenance andTesting - The anode bed site selected mustbe accessible to construction vehicles foranode bed installation, testing and repairs.

f. Vandalism - Anode bed locations should beselected so as to minimize the potential forvandalism.

g. Purpose of Anode Bed and SiteAvailability - Two additional factors thathave to be considered in the process ofselecting an anode bed site are the intendedpurpose of the anode bed and the possibility


of acquiring the required right-of-way for theinstallation. Is the intent of the anode bed toprotect a long section of the pipeline(assuming the pipeline is well coated), or isit the intent to afford localized protection to ashort section of a pipeline (poorly coated) ora structure? In the first case, the approachmay be the installation of one remote typeanode bed near the midpoint of the pipelinesection, or perhaps the installation of a deepanode bed near the midpoint area. In thesecond case, the approach could be theinstallation of a distributed anode bed.

Once a determination has been made as tothe intent and purpose of the installation andthe most suitable type of installation, theland availability should be investigated.

Selecting Anode Bed Type Based on SiteSelection

Prior to selecting the anode bed type, it is wiseto consider the major types of anode beds usedand application conditions associated witheach.

Distributed Anode Bed

A distributed anode bed is used to protect alimited area of the pipeline. The anode bed isgenerally installed close to the structure and,hence, is sometimes called a “closely coupledanode bed”. Distributed anode beds are used toreduce potential interference effects onneighboring structures, to protect sections ofbare or ineffectively coated pipelines, andpipelines in congested areas where electricalshielding might occur with other anode bedtypes.

A distributed anode bed can be made ofmultiple individual anodes connected to aheader cable. A distributed anode bed can alsobe one long continuous anode. This is referredto as a “linear anode”.

Remote Anode Bed

A remote anode bed, sometimes called a

“conventional anode bed” or a “point-type”anode bed, consists of anodes installed in theearth at a distance away from the pipeline. Thistype of anode bed is normally used to distributeprotective current over a broad area of thepipeline to be protected.

A remote anode bed used to protect a pipelineis usually installed perpendicular to the pipeline,and the first anode may be more than 100 feetaway. The anodes are typically installed on 10to 30 foot spacings.

Remote anode beds can also be installedparallel to or at an angle to the pipeline,depending on right-of-way availability. The maindifference between a perpendicular and aparallel installation is that the gradient effect onthe pipeline from a perpendicular anode bedwould be less than from a parallel anode bed(assuming the line of parallel anodes is at adistance from the pipeline equal to that of thefirst anode in a perpendicular anode bed).Figure 5-1 is an example of a remote anodebed design with the anodes parallel to the linesto emphasize the anodic gradients around theanode bed and the cathodic gradients aroundthe pipeline.

The term “remote”, as associated with this typeof anode bed design, means that the pipeline isoutside of the anodic gradient of the anode bedcaused by the discharge of current from theanodes to the surrounding soil (referred to as“remote earth”). This would allow a betterspread of the protective current to be achieved,thus protecting a larger section of a pipelinefrom one installation. In real life however,remote anode beds are installed at distancesfrom a pipeline where the remaining gradientaround the pipeline is normally small (not zero).

Deep Anode Bed

A deep anode bed utilizes anodes installedelectrically remote from the structure in avertical hole drilled to a minimum depth of 50feet and, in some cases, as deep as 1,000 feet.This approach achieves similar results as aremote surface anode bed.


REMOTE ANODE BED OPERATION

FIGURE 5-1

ANODE BED

CATHODIC PROTECTIONGRADIENT FIELD SURROUNDING

PROTECTED STRUCTURE

ANODIC GRADIENTFIELD SURROUNDING

ANODE BED

NEGATIVERETURN CABLE

DIRECTIONOF CURRENTFLOW (TYP.)

PROTECTEDPIPELINE

RECTIFIER

(+)

(–)

(+)(+)(–)

(–)

POSITIVE HEADER CABLE

Deep anode beds are usually effective whenhigh surface soil resistivities exist over deeperlow-resistivity areas where the anodes areinstalled.

Hybrid Anode Bed

In some cases, on complex structures, differentanode bed types are used in combination toafford complete protection to the pipeline. Anexample may be the case where a remoteanode bed has been used to protect severalmiles of a well coated pipeline, but shieldingfrom foreign structures occurs in a localizedarea. In this case, a distributed anode bedmight be considered to supplement theeffectiveness of the remote anode bed. Thiscombination of anode beds is sometimesreferred to as a hybrid approach.

The designer may find himself involved in caseswhere site availability is scarce and where thetype of anode bed to be selected is dependenton the sites available. A similar case woulddevelop if the size of the site available isrestricted. These conditions may develop inurban areas or in instances in which the landowner may not wish to sell or lease any land forthe installation.

Land limitations may dictate that the installationhas to be restricted to the existing right-of-way,in which case a distributed type anode bed maybe selected for a localized condition or a deepanode installation may be selected forprotection of extended areas.

In cases where the site availability may dictatethe type of anode bed, it would be advisable toanalyze further the soil conditions to be able todetermine which is the most economical type ofdesign that would do a proper job.

As previously mentioned, soil with a highresistivity layer near the surface and a low orlower resistivity layer underneath would be wellsuited for a deep anode type of installation.This type of soil condition can be encounteredalong a pipeline at areas where a relativelyshallow (sometimes slightly more than pipeline

depth) surface soil overlies a high resistivitylayer of rock which, in turn, overlies a layer ofsoil of relatively low resistivity. The currentdischarge from a deep anode bed in this type ofsoil arrangement could extend laterally forconsiderable distances and work its way upthrough the higher resistivity rock layer to thesurface soil where the structure to be protectedis installed.

In contrast with the above example, the casemay be where the surface resistivity is low andthe underlying soil is of high resistivity. In thiscase, a surface remote or distribution type ofanode bed can be effectively utilized and adeep anode bed should not be considered.

In the cases where there are no restrictions tothe type of anode bed to be utilized from any ofthe parameters listed in the preceding sections,economics would then be the deciding factor.

Tests Required for Anode Bed Design

The design of an impressed current anode bedis normally based on test data acquired in thefield. The basic tests that should be conductedare soil resistivity and current requirement tests.

Soil resistivity tests are conducted along theroute of the pipeline and at the locationsselected for the anode bed installation.

Referring to the soil resistivity data shown inTable 5-1, let us assume that we are going toinstall a vertical anode bed to a depth of ten(10) feet. The weighted average soil resistivityto that depth (pin spacing) is shown as 19,533ohm-cm. For design calculation purposes, anaverage resistivity of 20,000 ohm-cm should beused.

It should be noted again that the resistivityvalue used for design purposes should beobtained in the specific area where the anodebed will be installed, as there can beconsiderable variations in soil resistivity withinshort distances. An actual case example of thisproblem was an anode bed designed on thebasis of soil resistivity data taken about 50 feet


TABLE 5-1

Typical Soil Resistivity Data

GalvanometerDial Reading Multiplier Setting Resistance

(Ohms) Spacing (ft) Factor Resistivity(Ohm-cm)

2.4 10 24 2.5 191.5 11,490

1.6 10 16 5 191.5 15,320

10.2 1 10.2 10 191.5 19,533

6.2 1 6.2 15 191.5 17,801

2.6 1 2.6 20 191.5 9,958

8.2 0.1 0.82 25 191.5 3,926

I I x V

VT A

REQ

g

I 10 x 400

260 15.385 AmperesT

away from the foot of a hill. Because of right-of-way considerations, the anode bed was moved30 feet towards the foot of the hill without newsoil resistivity measurements. When the systemwas energized, the resistance of the anode bedwas found to be about ten (10) times higherthan it would have been at the locationoriginally selected.

The second set of tests that should beconducted are current requirement tests todetermine the amount of current required toachieve protection of a pipeline/structure or asection thereof. In the event that the structureor pipeline is proposed and not yet installed, thecurrent requirement must be estimated byassuming a current density.

A temporary anode bed must first be set up,preferably at the location of the proposed anodebed installation. The temporary anode bed mayconsist of several ground rods driven into theground and connected to a portable rectifier orother DC source. Soil resistivity test pins mayalso be used. Temporary power sources, suchas batteries and DC welding machines, mayalso be used.

A creek or small lake, in close proximity to thestructure requiring protection, can be used toset-up a temporary anode bed. Householdgrade aluminum foil unrolled into the water andconnected to a DC power source using clipleads can provide an excellent temporaryanode bed for use when conducting currentrequirement tests.

Current requirement tests are conducted byenergizing the temporary anode bed and taking“ON” and “OFF” potential readings along thepipeline/structure using a high input resistancevoltmeter and a portable copper-copper sulfate(Cu-CuSO4) reference electrode. While takingpotential readings, the current output of thetemporary anode bed is increased untilprotection is achieved in the entirepipeline/structure, based on establishedcathodic protection criteria, as discussed inChapter 1 of this course. Figure 5-2 shows atypical setup for current requirement tests.

At times, it may be difficult to obtain an outputfrom the temporary anode bed that is largeenough to achieve full protection of the entirepipeline/structure. Should this be the case, theactual current requirements can be calculatedby extrapolating the data obtained to thedesired protective levels.

As an example, let us consider a test setup inwhich the output of the temporary anode bed is10 amperes, and the potential shifts obtainedare:

Location Vg-ON Vg-OFF ∆V

East End -0.75 V -0.45 V -0.30 V

Midpoint -0.95 V -0.55 V -0.40 V

West End -0.71 V -0.45 V -0.26 V

From the above data it can be seen that thelargest shift required to meet the -0.85 voltprotective criterion is 400 millivolts, [0.45 (Eastor West End) - (-0.85)] and the lowest shiftobtained with 10 amperes is 260 millivolts. Thetotal current required can be calculated asfollows:

Where:

IA = Applied Test Current

∆VREQ = Potential Shift Required to Meet-0.85 Protective Criterion

∆Vg = Lowest Voltage Shift ObtainedDuring Test

IT = Total Current Required

A total of approximately 16 amperes would berequired as a minimum to meet the -0.85 voltcriterion.

It is important to note that the amount of currentrequired to cathodically protect a pipeline or


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structure is greatly affected by the condition ofthe coating on the structure. A bare structuremay require many times the amount of currentto protect the same pipeline/structure, if it wereproperly coated. Table 5-2 shows typicalcurrent requirements for a 5 mile section of18-inch diameter steel pipe with variousdegrees of coating effectiveness.

DESIGN CALCULATIONS

The following examples show the various stepsnecessary to design a cathodic protectionsystem for underground pipelines.

Existing Structures

For existing structures, the design can bebased upon data obtained in the field throughthe previously mentioned current requirementtesting to determine the actual currentnecessary to achieve cathodic protection.

Current Requirement

A temporary anode bed is set up using 5ground rods and a portable rectifier. The groundrods are driven approximately 1½ feet into theground at 10-foot spacing. The measuredoutput of the rectifier is 34 volts and 0.40amperes. While interrupting the rectifier output,the following pipe-to-soil potential readingswere taken at various locations with respect toa portable copper-copper sulfate (CuCuSO4)reference electrode, placed directly over thepipeline in the area under the influence of thetemporary anode bed, and the change inpotential (∆V) was calculated.

PIPE-TO-SOIL POTENTIAL (Volts)

Location On Off ∆V

1 -0.592 -0.561 -0.031

2 -0.570 -0.523 -0.047

3 -0.603 -0.545 -0.058

4 -0.598 -0.527 -0.071

5 -0.693 -0.575 -0.118

6 -0.833 -0.635 -0.198

7 -0.865 -0.650 -0.215

8 -0.814 -0.611 -0.203

9 -0.731 -0.605 -0.126

10 -0.655 -0.590 -0.065

11 -0.630 -0.575 -0.055

12 -0.640 -0.580 -0.060

Step No. 1

Using the lowest change in potential shift (∆V)measured during the tests, calculate the voltageshift required to satisfy the -0.85 volt cathodicprotection criterion.

Lowest Potential Shift = -0.031 VoltsObtained at Location No.1 (based on data intable)

Off Potential = -0.561

Required Voltage Shift =-0.850 - (-0.561) = -0.289 Volts

Step No. 2

Using the previously calculated values,calculate the required current using thefollowing relation:

Substituting the Values:

Step No. 3

Verify that, with an output of 3.73 amperes, apotential of -0.85 volts can be achieved at thepoint of lowest “OFF” potential obtained duringthe test.


TABLE 5-2

Typical Current Requirements Based On Coating System Effective Resistance

Effective Coating Resistance (1)

(ohm-ft²) Current Requirement

(Ampere)

Bare Pipe (2) 187.50

10,000 3.73

25,000 1.49

50,000 0.75

100,000 0.37

500,000 0.075

1,000,000 0.0373

5,000,000 0.007

“Perfect Coating” 0.000015

(1) Effective coating resistance, as defined in the above table, of 10,000 to 25,000 ohm-ft² indicates poorapplication or handling during installation. Resistance of 100,000 to 5,000,000 ohm-ft² indicates goodto excellent application. Installation in 1,000 ohm-cm soil.

(2) Bare pipe in this table is assumed to require a minimum of 1.5 milliamperes per sq. ft. of pipe surface.In practice, most design engineers use 2 milliamperes per sq. ft. for pipe-in-soil, unless theenvironment is acidic, contains high concentrations of chlorides, bacteria, or the pipe is operating atelevated temperatures. In these cases, as much as 3.5 to 5.0 milliamperes per sq. ft. may berequired.

V

V

I

I or

g

REQ

A

T1

Lowest Voltage Shift

Required Voltage Shift

Applied Current Used

Current Required (I )T1

0.047

0.327

0.40

I I 2.783 amperes

TT

Lowest “OFF” Potential = -0.523 Volts atLocation No.2

Potential Shift (∆V) at That Location DuringTest = -0.047 Volts

Potential Shift Required = -0.850 - (-0.523) =0.327 Volts

Total Current Required can be calculated asfollows:

Substituting the Values:

Step No. 4

The current required for achieving -0.850 voltsthroughout the tested section of the pipe is 3.73amperes.

There are several points that the designershould bear in mind when interpreting theresults of current requirement tests:

1. The results indicate current requirement onlyin the area where the temporary anode bedwas installed, and where the pipe-to-soildata indicated an influence from the anodebed.

2. If two or more anode beds are going to beinstalled for protection of an electricallycontinuous pipeline/structure, the effects ofthe anode beds are additive. Therefore, if ata point of the pipeline, 1 anode bed with 1ampere output achieves half of the requiredshift, and a second anode bed with 1ampere output at a different location alsoachieves half of the required shift at thesame location, the total current required is 2

amperes and not 2 amperes per anode bed.

3. The tests do not take into account futurechanges which might necessitate morecurrent, such as additional deterioration ofthe coating.

In cases where the pipeline/structure has notbeen installed, or when current requirementtests are not feasible, current requirements canbe estimated by selecting a current density andapplying that current density to the pipelinesurface area.

There are many different tables that have beenpublished outl ining current densityrequirements. One such table is shown asTable 5-3, and was taken from the NACECORROSION ENGINEER’S REFERENCEBOOK.

As an example, if a proposed pipeline were tobe coated and buried in relatively neutral (notalkaline nor acidic) but corrosive soil (due tonon-homogeneous conditions), one mightselect 2 ma/ft2 as the appropriate currentdensity to establish the total currentrequirement. If the pipeline was anticipated tohave a 95% coating efficiency, then 2 mA wouldbe applied to the 5% of the pipeline that wasbare.

EXAMPLE:

A 24-inch diameter pipeline two miles in lengthis being installed with a 95% efficient coating.Determine the total current required to achievecathodic protection.

Step No. 1

Calculate the area to be protected using thefollowing formula:

AS = B A d A L A 0.05

Where:

AS = Total Surface Area (ft2)


TABLE 5-3

Approximate Current Requirements for Cathodic Protection of Steel

Environment mA/ft²

Sea Water - Cook Inlet 35 - 40

- North Sea 8 - 15

- Persian Gulf 7 - 10

- US - West Coast 7 - 8

- Gulf of Mexico 5 - 6

- Indonesia 5 - 6

Bare Steel in Soil 1 - 3

Poorly Coated Steel in Soil or Water 0.1

Well Coated Steel in Soil or Water 0.003

Very Well Coated Steel in Soil or Water 0.003 or less

d = Pipeline Diameter (ft)

L = Length of Pipeline (ft)

.05 = 5% of Pipe Requiring Protection(Bare Area)

B = 3.1415

AS = B A 2 A 5280 A 2 A 0.05

AS = 3317.5 ft2

Step No. 2

Calculate the current required based on a 2ma/ft2 current density as follows:

IT = Id x AS

Where:

IT = Total Current Required (Amperes)

Id = Current Density (mA/ft2)

AS = Total Surface Area (ft2)

IT = (.002)(3317.5)

IT = 6.64 Amperes

Regardless of the method that is used todetermine the current required to achieveprotection, it is wise to consider increasing thatvalue in the design stage for futurerequirements over the life of the system. Thisincrease may be necessitated by coatingdegradation or additions to the structure. Mostdesigners add 20% to cover theserequirements.

Determining the Number of Anodes

After obtaining all the field data required for thedesign of the anode bed, the next step is theselection of the type of anode to be used andthe calculation of the number of anodesrequired. The factors to be considered whencalculating the number of anodes are:

1. Anode Material

2. Rate of Consumption of Anode

3. Current Required for Protection

4. Shape and Size of Anode

5. Maximum Anode Material Density

6. Life Expectancy

Of the above items, Nos. 1, 2, 4, and 5 areinterdependent. The consumption rate andmaximum allowable density depend on theanode material as do the shape and size of theanodes.

The first step in this phase of the design should,in most instances, be the selection of the anodematerial. Items to be considered for thisselection are environmental conditions,consumption rate, and sizes available.

Although not a general problem with mostunderground structures, there could beinstances where the anode material could besubjected to acidic or alkaline soil environmentswhich could be detrimental to the anodematerial. Under these conditions, it would bewise to check with the anode manufacturers toinsure that the actual environment will notadversely affect the anode material. Anotherenvironmental factor that could affect thematerial selection is whether the anodes will beinstalled in soil or water, or a combination ofboth.

As mentioned in Chapter 3 of this course, thereare many different anode materials that couldbe used for impressed current systems. Evenscrap steel has been used for anode material.Scrap steel, however, has a high consumptionrate (approximately 20 lbs/amp-year). The mostcommonly used anode materials forconventional anode bed installations aregraphite and high-silicon chromium-bearingcast iron anodes. Normal operating densitiesrange between 0.2 and 1.0 amp/ft2 for graphitewith an average rate of consumption of 2.0lb/amp-year, and between 0.5 and 2.5 amp/ft2

for the high-silicon chromium-bearing cast ironanode with an average consumption rate of0.75 lb/amp-year. These consumption rates aremarkedly reduced if the anode is installed in


Desired Life Weight x Utilization Factor

Req'd Current Output x Consumption Rate

or

Weight Consumption Rate x Desired Life x Req'd Current

Utilization Factor

0.75 x 20 x 15

0.60 375 lbs.

Number of Anodes 375

60 6.25 7 anodes

Number of Anodes Required Current

Max. Output per Anode

15

2.5 6 anodes

coke breeze backfill. Other anode materials thathave been used for soil installations in recentyears are the platinized, mixed metal oxide andmagnetite anodes.

The next step in the selection of the anode is todetermine the sizes and shapes available. Forsoil installations, the 5-foot long anodes arenormally used, and they are available indiameters ranging from 1½" to 4" (graphite andhigh-silicon iron only). The selection of theanode size will depend on the desired anodeoutput and the required life expectancy.

Another variable to consider is the use ofprepackaged canister anodes. These anodesconsist of graphite, high-silicon chromium-bearing cast iron or other suitable anodesencased in a steel stove pipe filled with a selectbackfill. As discussed in Chapter 3, “AnodeMaterials”, there are advantages anddisadvantages to the use of canister anodes.

Now that we have selected the anode materialand the type and size of the anode, we canproceed with calculating the number of anodesrequired.

EXAMPLE:

The current required to protect a pipeline,based on current requirement tests, is 15 ampsincluding spare capacity for future demand.High-silicon chromium-bearing cast iron anodes(2" x 60") are to be used in the anode beddesign. Required anode bed life expectancy is20 years.

Step No. 1

Calculate the number of anodes required basedon a consumption rate of 0.75 pounds per amp-year and a utilization factor (amount of anodematerial that can be lost to consumption beforeanode replacement is required) of 60%, and ananode weight of 60 lbs., using the following lifeexpectancy formula:

Step No. 2

Calculate the number of anodes required basedon a nominal current discharge per anode of2.5 amps per anode (recommended output bymanufacturer). Since the surface area of theseanodes is 2.8 ft2, the anodes would beoperating at a current discharge density ofapproximately 0.9 amp/ft2. This value fallswithin the range of 0.5 to 2.5 amp/ft2 mentionedearlier in this chapter as the output range forthis type of anode.

Based on the above calculations in Step 1, thenumber of anodes required is 7.

An alternative approach to the method outlinedabove for the determination of the number ofanodes required for a conventional anode bedis the “cut and try” method. With this method,an actual anode installation is made and testsare conducted to ascertain the degree ofprotection achieved. The number of anodes isincreased until the desired level of protection isreached. Calculations should then be made todetermine if the life expectancy would be metwith this method. The size of the rectifier can bedetermined from the actual field tests.


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R 0.00521

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dS

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Calculating the Anode Resistance-to-Earth

The next step in the design process is thecalculation of the resistance-to-earth of theanodes. There are exceptions to this approach,such as in those cases in which the utility or thecompany has established a preferred range ofanode bed resistance. If this is the case, thenthe preferred anode bed resistance becomesthe primary factor in calculating the number ofanodes required for the installation.

There are two basic approaches to calculatingthe resistance of an anode bed:

1. By using existing formulas, and

2. By using existing charts.

The anode bed resistance-to-earth can becalculated using one of the following formulas,as applicable, for the anode bed design:

Single Vertical Anode (H. B. Dwight formula)

Where:

RV = Resistance-to-earth of single verticalanode in ohms

ρ = Effective soil resistivity in ohm-cm

L = Anode length in feet

d = Anode diameter in feet

Multiple Vertical Anodes in Parallel (Erling D.Sunde Formula)

Where:

RV = Resistance-to-earth, in ohms, of thevertical anodes connected in parallel


L = Anode length in feet

N = Number of vertical anodes in parallel


S = Spacing between anodes in feet

Single Horizontal Anode (H. B. Dwight formula)

Where:

RH = Resistance-to-earth, in ohms, of thehorizontal anode


L = Horizontal anode length in feet

N = Number of vertical anodes in parallel


S = Twice anode depth in feet

NOTE: When using prepackaged anodes oranode backfills (such as coke breeze), thedimensions of the canister or the backfillcolumn must be substituted in the aboveformulas for the anode dimensions.

Using the anode bed/anode resistance-to-earthformulas listed above can be somewhatcomplicated and time consuming. Therefore,graphs have been developed such as the onesshown in Figures 5-3 and 5-4 to expedite theprocess of determining anode bed/anoderesistance-to-earth.

Figure 5-5 shows a typical vertical anodedesign chart for an impressed current anodebed. The chart was developed on 2" x 60"anodes in an 8" x 7' column of 50 ohm-cm cokebreeze backfill. The chart is based on anodesinstalled vertically in a straight line.

EXAMPLE:

The following example shows how both theformulas and the graphs can be used todetermine the anode bed resistance-to-earth.

A conventional anode bed consisting of twenty-


5.0

6.0

7.0

8.0

9.0

10.0

ST

AN

CE

-TO

-EA

RT

H I

N O

HM

S R with ρ = 10000 ohm-cm

R with ρ = 8000 ohm-cm



RESISTANCE-TO-EARTH OF 3" X 60" ANODES IN A VERTICAL INSTALLATIONIN 8" X 7' COLUMN OF COKE BREEZE.

SPACING = 10 FEET.

ρ = 10000 ohm-cm

ρ = 8000 ohm-cm

ANODE BED RESISTANCE-TO-EARTHOF 8" x 7' ANODES AT 10 FT SPACING

AT VARIOUS SOIL RESISTIVITY VALUES

FIGURE 5-3

0.0

1.0

2.0

3.0

4.0

0 5 10 15 20 25 30

AN

OD

E B

ED

RE

SIS

NUMBER OF ANODES

ρ = 6000 ohm-cm

ρ = 4000 ohm-cm

0.5

0.6

0.7

0.8

0.9

1.0

ST

AN

CE

-TO

-EA

RT

H I

N O

HM

S R at 10' Spacing

R at 15' Spacing

R at 20' Spacing

R at 25' Spacing


ρ = 1000 OHM-CM.

10' Spacing

15' Spacing

20' Spacing

ANODE BED RESISTANCE-TO-EARTHOF 8" x 7' ANODES IN 1000 OHM-CM SOIL

AT VARIOUS SPACINGS

FIGURE 5-4

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25 30

AN

OD

E B

ED

RE

SIS

NUMBER OF ANODES

25' Spacing

0.5

0.6

0.7

0.8

0.9

1.0

TA

NC

E-T

O-E

AR

TH

IN

OH

MS R at 10' Spacing

R at 15' Spacing

R at 20' Spacing

R at 25' Spacing

10' Spacing

15' Spacing

20' Spacing


ρ = 1000 OHM-CM.

USING A GRAPH TO DETERMINE ANODE BED RESISTANCE-TO-EARTH

FIGURE 5-5

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25 30

AN

OD

E B

ED

RE

SIS

T

NUMBER OF ANODES

25' Spacing

25 Anodes at 15' SpacingR = 0.18 ohms

R 0.00521

N L ln

8 L

d1

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Sln 0.656 NV

R 0.00521 10000

25 7 ln

8 7

0.6671

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15ln 0.656 25V

R 10000

10000.18 1.8 ohmsV

five (25) 2" x 60" anodes is to be installed in an8" x 7' column of 50 ohm-cm coke breeze.Anodes are connected in parallel at 15'spacings. Soil resistivity is 10,000 ohm-cm.

Method No. 1 -- Using Sunde’s formula tocalculate the anode bed resistance-to-earth:

Substituting the values in the above formula:

After making all the calculations, the result is:

RV = 1.8 ohms

Method No. 2 -- Using the graph in Figure 5-5to determine the anode bed resistance-to-earth:

Step No. 1:

Locate the number of anodes on the horizontalaxis of the graph (25) and draw a vertical lineup to the curve representing anodes at 15'spacing.

Step No. 2:

Draw a horizontal line from that point on thecurve to the vertical axis of the graph and readthe resistance which, in this case, is 0.18 ohmin 1,000 ohm-cm soil.

Step No. 3:

The resistance is directly proportional to theresistivity of the soil. Therefore, the resistancein 10,000 ohm-cm soil would be:

Calculating Total Circuit Resistance

The total circuit resistance includes thefollowing:

C Resistance-to-earth of the pipeline orstructure

C Resistance of the CP system cables

C Resistance-to-earth of the anodes (anodebed resistance)

The resistance-to-earth of the pipeline/structuremay be significant or negligible, depending onlength of pipe and soil resistivity. Using thevalues on Table 5-2 and assuming, forexample, that the current requirements listedare necessary to shift the pipe-to-soil potentialapproximately 0.40 volt, then, applying Ohm’sLaw, the resistance-to-earth values for thoseconditions range from 0.0021 ohm (0.40/187.5 -negligible) to 57.14 ohms (0.40/0.007 - verysignificant). The mid-range current requirement(0.37 ampere) represents a resistance-to-earthvalue of 1.081 ohms (0.40/0.37) which might bea significant percentage of the total circuitresistance.

The last factor to calculate for the circuitresistance is the resistance of the positive andnegative cables. The first step would be toselect the proper size of the cable. The cablesize should be appropriate to carry the outputcurrent and to minimize the IR drop in the cabledue to the current flow. Once the size of thecable is determined, the total length of cable tobe used is multiplied by the per-foot resistanceof the cable selected.

The next step is to calculate the total circuitresistance using the following formula:

RT = RABed + RC + RS

Where:

RT = Total circuit resistance

RABed = Anode bed resistance (usingdimensions of carbonaceous backfillcolumn for calculations)


R 0.00521

N L ln

8 L

d1

2 L

Sln 0.656 N

R 3.08 ohms

ABed

ABed

RC = Cable resistance

RS = Pipeline/structure-to-earth resistance

EXAMPLE:

Calculate the total circuit resistance of theconventional anode bed shown in Figure 5-6,using ten (10) 2" x 60" graphite anodes in 8" x7' columns of 50 ohm-cm coke breeze backfill.Soil resistivity is 8,000 ohm-cm. Currentrequired for protection is 15 amperes. The pipeto be protected is 3-mile long, 16-diametercarbon steel pipe. Assume a coating resistance of 500,000 ohm-ft2.

Step No. 1

Calculate RABed:

Step No. 2

Calculate RC, based on the effective systemcable length and the resistance of the cable perlinear foot. The effective cable length of thesystem (referring to Figure 5-6) can bedetermined using the following formula:

Lcable = Lneg + Lpos1+ ½ Lpos2

Where:

Lcable = Effective cable length (ft)

Lneg = Negative return cable length (ft)

Lpos1 = Length of positive cable to first anode(ft)

Lpos2 = Length of positive cable between firstand last anodes (ft)

Lcable = 300 +16 + 135/2 =383.5 ft

The cable resistance (RC) can be calculatedusing the following formula:

RC = Rcable x Lcable

Where:

Rcable = Resistance per linear foot of cablebased on a standard cable resistancetable such as the one shown in Table5-4. In this example, No.4 AWG cableis being used with a resistance of0.2540 ohm per 1,000 ft.

RC = 383.5 x 0.2540/1000

RC = 0.0974 ohm

Step No. 3

Calculate RS for the 3 miles of coated 16-inchdiameter carbon steel pipe, assuming a coatingresistance of 500,000 ohm-ft2.

RS = R/AWhere:

R = Coating resistance (ohm-ft²) = 500,000ohm-ft²

A = Surface area of pipe (ft²) = 66,350 ft²

RS = 500,000/66,350 = 7.53 ohms

Step No. 4

Calculate RT:

RT = RGbed + RC + RS

RT = 3.08 + 0.0974 + 7.53

RT = 10.71 ohms

Sizing the Rectifier or DC Power Supply

The final step in designing a conventionalanode bed system is sizing the DC output of thepower supply to be used. The following factorsmust have been determined in order to size theDC output of the power supply.

1. Total Circuit Resistance. As indicated in


TYPICAL REMOTE ANODE BED DESIGN

FIGURE 5-6

2” X 60” GRAPHITEANODE (TYP.) INSTALLED

IN 8” X 7’ COLUMNSOF COKE BREEZE

(50 OHM-CM)

15 FT.(TYP.)

TO ACFEED

CATHODICPROTECTION

RECTIFIER

LPOS 116 FT.

R(+)

LPOS 2135 FT.

300 FT.

NO. 4 AWGNEGATIVERETURN CABLE

(–)

ISOLATINGFLANGE (TYP.)

3 MILES

16 INCH DIAMETERCARBON STEEL PIPE

TABLE 5-4

Concentric Stranded Single Conductor Copper Cable Parameters

SizeAWG

Overall Diameter NotIncluding Insulation

(Inches)

Maximum DCResistance @

20º C (Ohms/1000 ft)

Maximum AllowableDC Current Capacity

(Amperes)

14 0.0726 2.5800 15

12 0.0915 1.6200 20

10 0.1160 1.0200 30

8 0.1460 0.6400 45

6 0.1840 0.4030 65

4 0.2320 0.2540 85

3 0.2600 0.2010 100

2 0.2920 0.1590 115

1 0.3320 0.1260 130

1/0 0.3730 0.1000 150

2/0 0.4190 0.0795 175

3/0 0.4700 0.0631 200

4/0 0.5280 0.0500 230

250 MCM 0.5750 0.0423 255

the previous section, the total circuitresistance consists of the anode bed-to-soilresistance, the cable resistance and thepipeline (structure)-to-earth resistance.

2. Total Current Required. The total currentrequired to protect the pipeline/structurebased on current requirement tests. Asmentioned earlier, the test results arenormally increased by 20% or more toprovide for future requirement increases.

3. Back Voltage. The back voltage is thevoltage that exists in opposition to theapplied voltage between the anodes and theprotected structure. Anode bed anodes withcarbonaceous backfill will usually have aback voltage of about 2 volts. In areas ofunusual soil composition, the back voltagemay be slightly higher, but, for designpurposes, 2 volts should be used unlessexperience in a specific area dictatesotherwise.

Using these three factors, the minimum DCoutput of the power supply can bedetermined/calculated. The required currentoutput of the power supply is equal to the totalcurrent required plus a percentage for laterdeterioration. The DC output voltage can becalculated using the following formula:

V0 = IR x RT + Vb

Where:

V0 = Required Power Supply Voltage

IR = Total Current Required

RT = Total Circuit Resistance

Vb = Back Voltage

EXAMPLE:

Assume the pipeline in the previous examplehas a current requirement of 0.003 mA/ft² andcalculate the required power supply voltage

Step No. 1

Calculate total current required, plus 20% for

deterioration.

IR = Surface Area of Pipe x Current Density

IR = 66,350 ft2 x 0.003 mA/ft2

IR = 0.199 Amp

With 20% spare capacity IR = 0.239 Amp.

Step No. 2

Calculate required power supply voltage usingRT from previous example.

V0 = IR x RT + Vb

V0 = (0.239x10.71) + 2

V0 = 4.56 Volts

The required output rating as calculated is notwhat would be considered a standard off-the-shelf rating. The actual rating of the rectifier unitpurchased should be based, wheneverpossible, on standard available ratings. In thiscase, the closest rating might be 4 amps and 8volts.

DESIGN OF DISTRIBUTED ANODE BED

The distributed anode bed is installed in closeproximity to the pipeline/structure to beprotected. Distributed anode beds are usedprimarily when it is desired to protect a limitedarea of a pipeline, or to protect sections of abare or coated pipeline in areas whereelectrical shielding precludes effectiveprotection with remote anode bed installations.Figure 5-7 shows a typical distributed anodecathodic protection system.

Many companies designed this type of anodebed using a combination of the current pickupby the structure and the gradient established inthe soil by the anodes. The current pickupmakes the potential of the pipeline/structureswing in the negative direction, while the anodegradient makes the soil surrounding thepipeline/structure swing in the positive direction.The result is an additive effect.

Figure 5-8 shows how one anode swings the


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potential of the earth in the vicinity of astructure. The earth potential swing conceptinvolves having the protected structure interceptthe anodic gradient field surrounding thedistributed anode bed, as shown in the figure.The part of the structure that is within thegradient field of the anode bed is in soil whichis electrically positive with respect to remoteearth. This means that, within the anodicgradient field, the structure is negative withrespect to adjacent earth, even though it maynot be made more negative with respect toremote earth. Within the anodic gradient field,that part of the structure closest to the anodebed is normally most negative with respect toadjacent earth. Figure 5-9 shows how thegradient from the anode bed influences thepotential of the pipeline-to-earth.

In designing a distributed anode bed, thedesigner considered that each individual anodecreated a gradient around itself and that theeffect on the pipeline/structure is additive.Figure 5-10 shows the additive effect of twoadjacent anodes.

A similar gradient effect is also created by adistributed anode bed that uses a linear anode.

The design of a distributed anode bed is similarto that of the remote anode bed as far asdetermining current requirements, calculatinganode bed resistance-to-earth and sizing theDC output of the power supply.

In designing a distributed anode bed, the sizingof the header cable is more critical than for aremote anode bed. This is due to the length ofthe header cable and the associated voltagedrops across the it.

DESIGN OF DEEP ANODE BED

When it is desirable to install an anode bedsystem which has many of the operatingcharacteristics of a conventional anode bed, butfor which there is no space available at thedesired site location, the only way to install theanodes at a distance from the pipeline/structureis to install them deep below the

pipeline/structure.

Deep anode beds are also used when the soilstrata are so arranged that the upper layers ofsoil are of high resistivity with underlying layersof lower resistivity.

A deep anode bed is usually more expensive tobuild than a conventional anode bed ofcomparable capacity. In addition, the anodes ofa deep anode bed are well below grade leveland are inaccessible for maintenance shouldfailure occur in the anode material or in theconnecting cables. Although there arereplaceable deep anode bed systems on themarket, it may be more economical to abandona malfunctioning system in place and install anew anode bed close to the original one, ratherthan to attempt system repairs.

The design procedure for a deep anode bed is,for all practical purposes, the same as that fora conventional anode bed. The deep anode bedoperates on the same concept as theconventional anode bed, making the protectedpipeline/structure more negative with respect tothe surrounding soil, as illustrated in Figure5-11.

Graphite and high silicon-content cast ironanodes in coke breeze backfill are often usedfor deep anode beds (see Figure 5-12). Inrecent years, platinized anodes andmixed-metal oxide anodes in fluidized cokehave become available for this use.

Other designs include the use of a steel casingas the anode material itself, or in cases wherethe drilled hole stays open and the water tableis high enough, plain anodes without specialbackfill are installed inside the hole. In allcases, the upper portion of the installation(normally 50 to 100 feet) does not dischargecurrent in order to avoid the gradients in thearea of the pipeline/structure. This can beaccomplished by installing a plastic casing inthat section of the hole or by covering thatsection of the steel casing with a high dielectrictape/coating.


POTENTIAL GRADIENT FIELD AROUND A VERTICAL ANODE

FIGURE 5-9

SUM OF EARTH POTENTIALCHANGE (GRADIENT) ANDSTRUCTURE POTENTIAL TOEARTH PRIOR TO ENERGIZINGANODE

ANODE AT DESIGN DISTANCEFROM PROTECTED STRUCTURE

-0.85 VOLT MINIMUMPROTECTIVE POTENTIAL

DISTANCE IN EITHER DIRECTION ALONG STRUCTURE

STR

UC

TUR

E PO

TEN

TIA

L TO

CLO

SEC

OPP

ER S

ULF

ATE

ELEC

TRO

DE

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-1.0

-0.50

PROTECTED

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R 0.00521

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R 0.00521 10000

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0.6671

R 6.73 ohms

V

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However, it should be noted that, whendesigning a deep anode bed, the anode bedresistance-to-earth should be calculated usingthe total length of the backfill column install asillustrated by the following example:

EXAMPLE:

A deep anode bed consists of four 2" x 60"high-silicon chromium-bearing cast iron anodeswith a column 40' long of coke breeze backfill inan 8" diameter hole. The effective soil resistivityat anode depth is 10,000 ohm-cm.

The deep anode bed resistance-to-earth can becalculated using H. B. Dwight’s formula for asingle vertical anode as follows:

Where:

RV = Resistance-to-earth of single anode

ρ = Effective soil resistivity = 10,000ohm-cm

L = Anode length = 40 ft

d = Anode diameter = 0.667 ft

Many deep anode beds use conventionalanodes with coke breeze backfill. One mainproblem that can develop with this type ofinstallation is gas blocking. Chlorine gasevolves from the anode as a result of thecurrent discharge. Gas blocking can render adeep anode bed useless. In order to eliminate,or at least reduce this problem, it is standardprocedure to install a vent pipe which allows forthe dissipation of the gas. Plastic materials notsubject to deterioration by chlorine gas shouldbe used for this vent pipe. Figure 5-12 showsthe details of a typical deep anode bed.

Other problems associated with deep anodebeds are:

1. Cable Insulation - Some types of cableinsulation, such as the HMWPE, areattacked by chlorine gas and ozone.Special insulation, such as Kynar® orHalar®, should be used.

2. The anode-lead connection is a verycritical point of the system, and stepsshould be taken to preclude later problems.

3. As the steel casing consumes, theresistance of the anode bed may increasebecause of the layer of iron oxide thatdevelops.

CONCLUSIONS

Many parameters must be considered whendesigning an impressed current cathodicprotection system. As previously discussed,these parameters include the following:

1. Current density required for protection,

2. Life expectancy of installation,

3. Anode bed layout,

4. Anode bed resistance, and

5. Rectifier output.

The corrosion control designer must be awareof these parameters to determine the mosteffective and economical system design.

There are basically three (3) different types ofanode bed designs:

1. Remote or conventional,

2. Distributed, and

3. Deep anode.

Each type of anode bed has its ownapplications, and it is up to corrosion controlpersonnel to ascertain which type would bemore suited for each installation beingconsidered.


DESIGN OF GALVANIC ANODE CATHODIC PROTECTION6-1

CHAPTER 6

DESIGN OF GALVANIC ANODE CATHODIC PROTECTION

INTRODUCTION

Galvanic anodes remain an important anduseful means for cathodic protection ofpipelines and other buried structures. Theapplication of cathodic protection utilizinggalvanic anodes is nothing more than theintentional creation of a galvanicelectrochemical cell in which two dissimilarmetals are electrically connected while buried ina common electrolyte. In the “dissimilar metal”cell, the metal higher in the electromotive series(or more “active”) becomes anodic to the lessactive metal and is consumed during theelectrochemical reaction. The less active metalreceives cathodic protection at its surface dueto the current flowing through the electrolytefrom the anodic metal. The design of a galvaniccathodic protection system involvesconsideration of all factors affecting the properselection of a suitable anode material and itsphysical dimension, placement, and method ofinstallation to achieve a sufficient level ofprotection on the structure.

GALVANIC ANODE APPLICATION

General Uses

Galvanic anode installations are normally usedfor cathodic protection applications whererelatively small amounts of protective currentsare required and the resistivity of the electrolyteis low enough to permit their use. If currentrequirements are such that a large number ofgalvanic anodes would be required, it may bemore economical to use an impressed currentsystem, if possible.

Specific Uses

The following are some specific uses forgalvanic anode cathodic protection systems:

1. For protecting structures where a source ofDC or AC power is not available.

2. For clearing interferences caused byimpressed current cathodic protectionsystems or other DC sources, where theinterferences are not of great magnitudes.

3. For protecting sections of well-coatedpipelines.

4. For providing hot-spot protection on barepipelines where complete cathodicprotection is impractical.

5. For supplementing impressed currentsystems where low spots or unprotectedareas remain on the structure due toshielding effects or other reasons.

6. For temporary protection during constructionof a pipeline until the impressed currentsystem is installed.

7. For AC mitigation.

G A L V A N I C A N O D E C A T H O D I CPROTECTION DESIGN PARAMETERS

Galvanic Anode Selection

The first step in designing a galvanic anodesystem is the selection of the anode material.Corrosion control personnel must consider the


following to determine the most economicalanode material to use:

1. Required amount of protective current

2. Total weight of each type of anode

3. Anode life calculations

4. Desired life of installation

5. Efficiency of anode types

6. Theoretical consumption rate

7. Driving potential

8. Soil or water resistivity

9. Cost of anode material

10. Shipping costs

11. Degree of skill required for installing thegalvanic anode system

Table 6-1 shows some of these parameters forseveral anode materials. The order of priority ofthe above factors will probably change for eachinstallation.

Anode Current Efficiency

Anode current efficiency is a measure of thepercentage of the total anode current outputwhich is available in a cathodic protectioncircuit. The remaining current is dissipated inthe self-corrosion of the anode material.

From Table 6-1 it should be noted that zinc hashigh current efficiency. Magnesium on the otherhand, is shown to have an efficiency ofapproximately 50 percent. Although thisefficiency figure is used quite commonly, it isnot a constant and can be even less at lowcurrent outputs.

Current Requirements

The determination of the amount of currentrequired to protect a structure is one of themost important design parameters. Currentrequirements for a structure can be determinedby conducting current requirement tests on thestructure. This method was discussed in detail

in the previous chapter for the design of theimpressed current system. These approachesare identical for the galvanic anode system.

Electrolyte Resistivity

The selection of the galvanic anode material isprimarily dependent upon the resistivity of thesurrounding electrolyte (soil). As discussed inChapter 3, the low driving potentials of thevarious galvanic anode material alloys limit theireconomical use to the lower resistivity soils.Zinc anodes are seldom used in soils withresistivity values greater than 1,500 ohm-cm.Magnesium anodes are generally used in soilswith resistivity values between 1,000 and10,000 ohm-cm. Ideally, the galvanic anodebed should be installed in the area of lowestresistivity. Soil resistivity surveys should beconducted as described in Chapter 5 of theBasic Course.

Total Circuit Resistance

Total circuit resistance depends upon thefollowing:

C Anode bed resistance - Rabed

C Resistance of interconnecting cables - Rckt

C Resistance to earth of the cathode(structure) - Rcathode

The electrical circuit equivalent of the currentoutput of a galvanic anode is depicted in Figure6-1.

It should be noted that as corrosion personneldevelop experience, they will be able todetermine the conditions under which some ofthe above factors can be considered negligible.

Anode Bed Resistance - (Rabed)

The resistance of the galvanic anode bed ismore critical than that of the impressed currentsystem due to the fact that the driving voltage ofthe anodes Is limited. The current output islimited by the potential difference between thestructure and the anode material.

TABLE 6-1

Typical Operating Characteristics of Galvanic Anodes

GalvanicAnode Material

Theoretical OutputCharacteristics

(amp-hr/lb)

Actual Output*Characteristics

(amp-hr/lb)

ConsumptionRates

(lb/amp-yr)

CurrentEfficiency

NegativePotential (Volts)

to CuCuSO4

Zinc(MiI-A-18001 U) 370 370 23.7 90% 1.10

Magnesium(H-1 Alloy) 1000 250 - 580 15 - 35 25 - 58% 1.40 - 1.60

Magnesium (High Potential) 1000 450 - 540 16 - 19 45 - 54% 1.70 - 1.80

* Based on shown current efficiencies.

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R 0.00521 1500

5.5 6 ln

8 5.50.417

12 5.5

15ln 0.656 6

R 1.10 ohms

abed

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Anode bed resistance of a galvanic anodesystem can be calculated using the sameformulas used to determine the resistance ofthe impressed current anode bed.

Example:

Calculate the resistance of a galvanic anodebed consisting of six prepackaged 60 lb zincanodes (5" x 66" long) spaced 15' apart,center-to-center, average soil resistivity in thearea is 1,500 ohm-cm.

Where:

Rabed = resistance of the total number ofvertical anodes in parallel (ohms)

D = electrolyte resistivity (ohm-cm)

L = length of anode (feet)

d = anode diameter in feet

N = number of anodes in parallel

S = spacing between anodes in feet

Resistance of Interconnecting Cables (Rckt)

The resistance of the interconnecting cables(Rckt) is normally negligible in the design ofgalvanic anode cathodic protection systems.The IR drops in the cables are usuallyinsignificant because of the low current outputof this type of installation. When the resistanceof the interconnecting cables is to be includedin the overall system resistance, Rckt can becalculated for all practical purposes, using thefollowing formula:

Rckt = Resistance per ft of cable (Table 6-2) xLength of cable

Cathode-to-Earth Resistance (Rcathode)

The cathode-to-earth resistance is resistancebetween the structure and electrolyte.

On a coated pipeline this resistance can becalculated by first determining the effectivecoating resistance (Rc) (as discussed inChapter 2) and then using the followingformula:

Rcathode = Rc/n

Where:

Rcathode = resistance of pipeline to earth (ohms)

Rc = effective coating resistance - ohmsper average square foot of surfacearea (ohm-ft² )

n = the number of square feet of pipelinesurface area

Example:

Assuming a 10-mile section of 36" diameterpipe with a coating resistance of 50,000 ohms(Rc) for one average square foot. The pipe-to-earth resistance (Rcathode) can be calculated asfollows:

Step No. 1Calculate surface area of pipeline (n):

n = p A pipe diameter (ft) A length of pipe (ft)

n = 3.14 x (36/12) x (10 x 5280)

n = 497,376 sq. ft.

Step No. 2Calculate pipe-to-earth resistance:

Rcathode = Rc/n = 50,000/497,376

Rcathode = Rc/n = 0.10 ohm

Anode Life

Having arrived at an anode configuration thatwill produce the required current output is not

TABLE 6-2

Resistance of Concentric Stranded Copper Single Conductors

Size AWG Max. DC Resistance@ 20º C (ohms/1000 ft)

14 2.5800

12 1.6200

10 1.0100

8 0.6400

6 0.4030

4 0.2540

3 0.2010

2 0.1590

1 0.1260

1/0 0.1000

2/0 0.0795

3/0 0.0631

4/0 0.0500

250 MCM 0.0423


Years Life Th A E UF

Hr I=

× × ××

Years Life 0.114 * A E UF

I=

× × ×

Years Life 0.0424 * A E UF

I=

× × ×

Years Life 0.114 10 0.50 0.85

0.10 4.8 years=

× × ×=

Years Life 0.0424 10 0.90 0.85

0.10 3.2 years=

× × ×=

sufficient in itself. An examination of theestimated life of the anodes must beundertaken in order to determine whether thedesign will provide protection for a reasonableperiod of time. The following expression may beused to calculate the estimated life of theanode:

Where:

Th = Theoretical amp-hours/lb

A = Anode weight (lbs)

E = Current Efficiency expressed as adecimal

UF = Utilization Factor - typically 85 percentexpressed as a decimal (0.85) -- thepoint at which the anode should bereplaced even though the anode metalmay not be entirely consumed.

Hr = Hours per year

I = Current (amps)

The utilization factor accounts for a reduction inoutput as the surface area of the anodedecreases with time, limiting the anode output.This factor is usually assumed to be 0.85.

1. For magnesium:

2. For zinc:

* These constants are derived by dividing thetheoretical ampere-hours per pound by thenumber of hours in a year.

As an illustration, compare the lives of ten

pound anodes of magnesium and zinc, eachdischarging 0.1 amp. Assume 90% currentefficiency for zinc and 50% for magnesium.Each anode is assumed to require replacementwhen it is 85% consumed.

Using the above formulas, the life expectancieswork out as follows:

1. For magnesium:

2. For zinc:

The calculated life figures reflect the effect ofdifferent rates of consumption of the two metalsas well as the effect of current efficiency.

G A L V A N I C A N O D E D E S I G NCALCULATIONS

A well-coated pipe is 36 inches in diameter and2,500 feet long. The average soil resistivity inthe area is 1,000 ohm-cm. The desired systemlife expectancy is 20 years.

Step No. 1

Choose the anode material. Based on the factthat magnesium has a higher amp-hr/lbcharacteristic and lower consumption rate thanzinc (see Table 6-1) magnesium is selected.

Step No. 2

Calculate the surface area (n) to be protected:


n = 3.14 x (36/12) x (2500)

n = 23,550 ft2


Years Life 0.114 W E UF

Iprotect=

× × ×

20 0.114 W 0.50 0.85

0.283=

× × ×

R 0.00521

N L ln

8 Ld

12 LS

ln 0.656 Nabed =⋅

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⋅⋅ ⋅⎛

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R 0.00521 1000

7 2 ln

8 20.625

12 215

ln 0.656 7abed =⋅

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⋅⋅ ⋅⎛

⎝⎜⎞⎠⎟

Step No. 3

Calculate current requirements. The anode bedshould be designed to achieve a satisfactoryamount of polarization. After polarizationgalvanic anodes tend to self-regulate and thecurrent output at the anode bed will declinewhile protection is maintained. As a rule ofthumb, the amount of current required topolarize the pipe is four times the amount ofcurrent required to maintain protection.

a. Current required to maintain protection(lprotect) on the pipe can be calculated asfollows, assuming a current requirement of0.003 mA/ft2, typical for a well coated,electrically isolated pipe. This is based on1.5 mA/ft2 of exposed metal and on 0.2%coating damage.

lprotect = 0.003 mA/ft2 x surface area

lprotect = 0.003 mA/ft2 x 23,550 ft2

lprotect = 71 mA = 0.071 Amperes

b. The current required to polarize (Ipolarize) thepipe, by rule of thumb is based on a currentrequirement of 0.012 mA/ft2 (four timescurrent required for protection). This alsoapproximated the current required shouldcoating deteriorate to a total of 0.8%damage.

lprotect = 0.012 mA/ft2 x surface area

lprotect = 0.012 mA/ft2 x 23,550 ft2

lprotect = 283 mA = 0.283 Amperes

Step No. 4

Determine the minimum amount of anodematerial (W) required to provide an anode bedlife of 20 years, based on the current requiredto polarize the pipe.

W = 117 lbs of magnesium

Based upon the above calculated anodematerial weight requirement, it appears that 7 -17 lb anodes (4" x 4" x 17"), with backfill outerdimensions of 7.5" x 24", will provide sufficientanode material for the desired life.

Step No. 5

Calculate the anode bed resistance (Rabed) foran anode bank assuming center-to-centeranode spacing of 15 feet.

Rabed = 0.99 ohm

Step No. 6

Calculate the cathode to earth resistance(Rcathode) assuming an effective coatingresistance (Rc) of 15,000 ohm-ft2.

Rcathode = Rc/surface area

Rcathode = 15,000 ohm-ft2/23,550 ft2

Rcathode = 0.64 ohm

Step No. 7

Calculate the connecting cable resistance (Rckt)assuming the use of approximately 105 feet ofNo. 6 AWG wire.

Rckt = Resistance of wire x length

Rckt = 0.403 ohm/1000 feet x 105 ft

Rckt = 0.042 ohm

Step No. 8

Calculate anode bed current output (Iabed).Assume the anode potential to be -1.55 voltsand you want to polarize the pipe to -1.0 volt.


I Anode Potential Cathode Potential

R R Rabed

abed cathode ckt=

−+ +

I 1.55 1.0

0.99 0.64 0.042 0.329 Ampereabed =

−+ +

=

i 120000 f y

m =⋅ ⋅

ρi 150000 f y

m =⋅ ⋅

ρ

i 50000 f y

z =⋅ ⋅

ρ i 40000 f y

z =⋅ ⋅

ρ

The anode bed current output (Iabed) calculatedabove exceeds the previously calculatedcurrent values required to polarize and protectthe structure, therefore the design will beeffective in providing cathodic protection. Oncethe pipe polarizes (usually within a few months),the anode current will drop to the protectivelevel of about 0.071 amperes, thus furtherextending anode life. Also, using the polarizingcurrent in these calculations allows for coatingdeterioration over time.

Figure 6-2 shows a detail of a typical systemlayout for this design.

Simplified Calculations

From the previous example, we can see that itcan be rather time consuming to calculate thevarious resistive factors required to design agalvanic anode bed, and often certainassumptions must be made that result in anapproximate current output calculation. Theoutput of magnesium and zinc anodes hasbeen fairly well documented under varyingconditions, and many graphs, tables, and chartshave been developed. These references can beused by corrosion control personnel to providea simplified and reasonably accurate means ofdetermining anode current outputs undernormal operating conditions. One suchreference, widely used, was developed by D. A.Tefankjian(1). The following equations can beused to determine the current output formagnesium or zinc anodes, utilizing correctionfactors provided in Tables 6-3 and 6-4.

Bare or Poor Coating Good coating

Where:

im = current output for magnesium anode inmilliamperes

iz = current output for zinc anode inmilliamperes

D = soil resistivity in ohm-cm

f = anode shape factor from Table 6-3

y = driving voltage factor from Table 6-4

The equations assume a minimum soilresistivity of 500 ohm-cm and a distancebetween anode and structure of 10 feet. Theequations and factors were developed basedon bare or poorly coated structures. Because ofthe increased circuit resistance occurring withwell coated structures, the calculated currentvalue should be reduced by 20 percent. Thiscan be done by multiplying i as shown above,by 0.80.

Example

Calculate the number of anodes required toprotect 10,000 feet of well coated, electricallyisolated 16" pipe at a pipe-to-soil potential of0.85 volts in a soil of 2,500 ohm-cm. Assume acurrent requirement of 1.5 mA/exposed squarefoot of metal and coating damage of 1%. Use32 lb pre-packaged standard alloy magnesiumanodes and design for a 20 year anode life.

Step 1

Calculate the surface area (n) to be protected.


n = p A (16/12) A 10000 = 41,888 ft2

Surface area to be protected = 41,888 x 1%

= 419 ft2

Step 2

Calculate the current required:

419 ft2 x 1.5 mA/ft2 = 629 mA

TYPICAL GALVANIC ANODESYSTEM INSTALLATION

FIGURE 6-2

THREADED TERMINAL WITH TWOASA BINDING NUTS (TYP.)

TEST BOX

BUS BAR ORSHUNT

TERMINALBOARD

ANODE LEAD WIRE

PIPE LEAD WIRES

GRADE

TEST BOX - SEE DETAIL

CREOSOTED POSTCONDUIT STRAP (TYP.)

RIGID CONDUIT

MAGNESIUMANODE PIPELINE

2” MIN 18”

C4” BA

TABLE 6-3

Anode Shape Factors (f)

Anode Weight (lbs) Anode Factor (f)

Standard Anodes

3 (Packaged) 0.53

5 (Packaged) 0.60

9 (Packaged) 0.71

17 (Packaged) 1.00

32 (Packaged) 1.06

50 (Packaged - anode dimension 8" dia x 16") 1.09

50 (Packaged - anode dimension 5" x 5" x 31") 1.29

Long Anodes

9 2.75" x 2.75" x 26" backfill 6" x 31" 1.01

10 1.50" x 4.50" x 72" backfill 4" x 78" 1.71

18 2.00" x 2.00" x 72" backfill 5" x 78" 1.81

20 2.50" x 2.50" x 60" backfill 5" x 66" 1.60

40 3.75" x 3.75" x 60" backfill 6.5" x 66" 1.72

42 3.00" x 3.00" x 72" backfill 6" x 78" 1.90

Extra-Long Anodes

15 1.6" dia x 10' backfilled to 6" dia 2.61



TABLE 6-4

Driving Voltage Correction Factors (Y)

P/S Standard Magnesium High-Potential Magnesium Zinc

-0.70 1.21 2.14 1.60

-0.80 1.07 1.36 1.20

-0.85 1.00 1.29 1.00

-0.90 0.93 1.21 0.80

-1.00 0.79 1.07 0.40

-1.10 0.64 0.93 --

-1.20 0.50 0.79 --

TABLE 6-5

Multiple Anode Adjustment Factors

No of Anodesin parallel

Adjustment Factors(anode spacing in feet)

5' 10' 15' 20'

2 1.839 1.920 1.946 1.965

3 2.455 2.705 2.795 2.848

4 3.036 3.455 3.625 3.714

5 3.589 4.188 4.429 4.563

6 4.125 4.902 5.223 5.411

7 4.652 5.598 6.000 6.232

8 5.152 6.277 6.768 7.035

9 5.670 6.964 7.536 7.876

10 6.161 7.643 8.304 8.679


i 120000 f y

i 120000 1.06 1.00

2500m m=

⋅ ⋅⇒ =

⋅ ⋅ρ

Number current requirement

anode output

Number 629 mA

50.9 mA / Anode

Number 12.4 or 13 anodes

=

=

=

Life 0.114 W E UF

Current (amps)

Life 0.114 32 0.50 0.85

0.0509

=× × ×

=× × ×

Step 3

Calculate the anode current output: (im)

im = 50.9 mA (0.0509 amps)

Step 4

Calculate the number of anodes required:

Anodes could be spaced evenly along the lineat intervals of 10,000/13 or about every 769feet.

Step 5

Calculate anode life:

Life = 30.5 years

The twenty year design life is satisfied.

Other Factors

The calculated result assumes an evendistribution of current over the calculated lengthof pipe, which is not realistic. The greatest is inclose proximity to the anode and decreaseswith distance. Therefore, assuming uniform

coating and soil, a greater amount of current isavailable to the pipe closest to the anode. Theattenuation of current is more pronounced ascoating quality decreases, so an individualgalvanic anode protecting a “hot spot” on a barepipe will protect a much smaller area of pipethan that indicated above. For a series ofindividually connected anodes, the potentialincrease on the pipe that results betweenadjacent anodes, is additive.

Anodes are commonly connected in parallel inorder to achieve higher current output at agiven location. The approximate current outputof a group of anodes can be calculated bymultiplying the calculated current output of asingle anode by the appropriate adjusting factorindicated in Table 6-5.

Example

The current output (It) of six 32 lb pre-packagedstandard alloy magnesium anodes connected inparallel can be calculated as follows, based ona current output of 50.88 mA for an individualanode. Assume an anode spacing of 10 feet.

It = 50.88 mA x Adjusting Factor (Table 6-5)

It = 50.88 mA x 4.902

It = 249.41 mA

SPECIFICATION AND MAINTENANCE OFGALVANIC ANODE INSTALLATIONS

Specifications must be written to insure that thematerials and methods, upon which the designhas been predicated, are utilized. There shouldbe no deviation from these specificationsunless approved by the person in charge ofdesign. Inspection should be performed duringsystem installation to guarantee that thespecification of material and procedures areadhered to.

The maintenance of galvanic anode systemsconsists of routine surveys and, whennecessary, repairs/replacement. A survey todetermine the operating levels of the systemshould be conducted annually and the data


recorded in a permanent log.

Current measurements will enable the life of theanode to be predicted, and their timelyreplacement scheduled accordingly. Additionalanodes should be installed where low potentialswarrant their installation. Damaged facilitiesshould be repaired to ensure that adequatelevels of protection are maintained.

CONCLUSIONS

As was the case with the design of theimpressed current cathodic protection system,the design of the galvanic anode systemwarrants the consideration of various factors.These factors include the following:

1. Soil resistivity

2. Current Requirements

3. Anode Material Selection

4. Life Expectancy

The resistivity of the soil and the resistance toearth of the structure being protected have agreat impact on design and systemperformance when using galvanic anodes. Thisis due to the fact that the driving potential of thesystem is limited to the natural potentialdifference between the anode material and thestructure being protected.

Reference

1. Tefankjian, D.A., 1974. “Application ofCathodic Protection”. Proceedings of the 19th

Annual Appalachian Underground CorrosionShort Course, pp 122-138.

CHAPTER 7

MIC INSPECTION AND TESTING

INTRODUCTION

Microbiologically induced corrosion (MIC) hasbeen actively studied in recent years becauseof its potentially deleterious effect on importantunderground structures, such as pipelines. Themost well known bacteria associated withpipeline corrosion are the sulfate reducingbacteria (SRB). These bacterial are active indeaerated environments often associated withwet, poorly drained soils (that promote oxygenexclusion), require a nutrient that is near thepipe and often the carbonaceous material of thecoating, and have a sulfurous (rotten eggs)odor associated with the sulfate reductionprocess. If conditions change, they maybecome dormant and another type of bacteriamay become active. Because bacterial live incommunities, there is also a synergistic effectamong them whereby the reaction products ofone may act as a nutrient for another. Thus, itis important to test for more than one type ofbacteria. However, it is important to recognizealso that the presence of bacteria does notnecessarily mean that they are active or thatthey pose a threat to the integrity of thestructure.

Research studies have shown that MIC can becharacterized by the chemical, biological, andmetallurgical features associated with thesuspected site. The type and form of pittingcorrosion is often distinctively associated withMIC. Chemical and biological data aresupportive, and are useful for estimatingwhether a potential for MIC exists.

The following sections give a procedure forinspecting the dig site, assessing the coating

and steel at coating damage and suspectedMIC areas; sampling the soil, groundwater, orpipe-surface products for MIC testing; carryingout the MIC testing using commerciallyavailable field test kits; and visually inspectingthe corroded areas for MIC characteristics.

SITE INSPECTION

Before excavation of the pipeline, thetopography of the surrounding area of the digand the soil type should be noted. Thisinformation is useful in assessing the likelyconditions that can support various types ofbacteria. For example, low, wet areas with claysoils collect and hold water, and tend toexclude oxygen to promote deaeratedconditions. Soils with more sand or rock allowwater and/or air to migrate more readily to thepipe and promote more aerated conditions.Higher elevations with rocky soils tend to bebetter drained and drier, and tend to promotemore aerated conditions.

Exposure of the pipe should be slow andcareful to avoid damage to the pipe and areasof sampling. Remove soil from the pipe surfacecarefully to expose the coating surface. Do notremove any products adhering to the coatingsurface. Note and record the following on anappropriate form:

• Coating type (e.g., coal tar, asphalt,bitumen, tape), type of damage (e.g.,disbonding, holidays, blistering, seamtenting, cracking, and wrinkling), extent ofdamage (% of exposed area), and location(circumferential and longitudinal position onpipe in relation to weld seams and coating

MIC INSPECTION AND TESTING7-1

seam, if present)

• Corrosion and/or cathodic protection surfaceproducts present

N Location (see above)

N Type (e.g., deposit, nodule, or films)

N Color (e.g., brown, black, white, or gray)

N Smell (e.g., none, earth, rotten eggs)

• Soil type (e.g., sand, gravel, silt, clay, rock)

• Soil moisture (e.g., wet, dry).

Do not remove any coating or surface productsat this time. Only the person performing thefield test should remove coating or productsfrom the pipe surface at his discretion untiltesting is complete.

COATING INSPECTION

Coating inspection for MIC testing purposesshould precede any other type of coatingevaluation planned.

Identify likely area of MIC or other forms ofcorrosion from the visual inspection above.Carefully remove the damaged coating using aclean knife to expose the steel beneath. If aliquid is present, take a sample using a syringeor cotton swab following procedures to bedescribed below for testing purposes. If thearea is not to be tested for MIC, the steelsurface condition and liquid pH should beevaluated.

Visually inspect the steel surface for corrosion.Where possible, use a gauge to measure thedepth of corrosion. Also, determine the lengthof the corroded area in relation to thecircumferential and longitudinal position.

The pH of the liquid may be tested usinghydrion paper or its equivalent. Carefully slicethe coating to a length to allow the test paper tobe slipped behind the coating. Press the

coating against the pH paper for a few secondsand remove, then remove the pH paper. Noteand record the color of the paper in relation tothe chart provided with the paper. Determinethe pH of groundwater away from the pipe inthe ditch if possible for reference. Compare thetwo pH values to determine if the pH near thepipe is elevated. An elevated pH indicates thepresence of cathodic protection currentreaching the pipe. A pH above about 9 wouldbe considered elevated for most soils. It is notuncommon to determine a pH of 12 to 14 forwell protected steel.

MIC AND DEPOSIT SAMPLING

Once the pipe is exposed, the soil and anysuspected deposits must be sampled andtested immediately. The coating around thesuspected area of corrosion should be carefullyremoved using a knife or similar instrument.Sample contamination must be kept to aminimum. Therefore, avoid touching the soil,corrosion product, or film with hands or toolsother than those to be used in sample collectionand provided with the test kits.

MIC SAMPLING AND TESTING

A minimum of three samples should becollected and tested. Two samples should betaken from one or more of the followinglocations:

• Undisturbed soil immediately next to theexposed pipe steel surface or at an area ofcoating damage

• A deposit associated with visual evidence ofpipe corrosion

• A scale or biofilm on the steel surface or thebackside of the coating

• Liquid trapped behind the coating.

The third sample should be taken from fresh,undisturbed soil at pipe depth, at least 1 mtransverse to the pipe. This location, such asthe ditch wall, will act as a reference from which


to determine if the bacteria count near the pipeis elevated.

Additional samples may be taken and testedfrom other locations where bacterial activity issuspected.

Collect the soil, film, or liquid sample followingthe detailed procedures outlined in the test kit.Inoculate the vials at the dig site. Mark the sideof each vial with the appropriate dilutionnumber, as well as the tray next to it, using apermanent marker. Each tray should be markedto identify the following:

• Date and time sample collected• Location where the sample was taken• Location of the dig site.

This same information must be put on theoriginal box in which the sample tray is to bestored. The box of samples should be stored atambient temperature with the box top closed; itis not necessary to incubate the samples atelevated temperature. All unused test kitsshould be stored in a refrigerator to prolongshelf life.

After one (1), two (2), and five (5) days, viewthe sample bottles and record the result. Usethe rating number (1 through 5) correspondingto the highest bottle number to give a positiveresult. Also, record the number of the type ofresponse, such as 2-Cloudy, shown on thePositive Reaction Sheet supplied with the testkit. At the end of 15 days, record the finalresults. Typically, the results are unchangedafter about two to five days of incubation.

Each test kit tests for the presence and count(bacterium/ml) of four (or five) types of viablebacteria.

A test kit that tests for four types of bacteriatests for:

• Sulfate reducing bacterial (BlO-SRB)• Acid producing bacteria (RIO-APB)• Aerobic bacteria (BIO-AERO)• Anaerobic facultative bacteria (BlO-THIO)

A test kit that tests for five types of bacteriatests for:

• Sulfate reducing bacterial (BlO-SRB)• Acid producing bacteria (BlO-APB)• Aerobic bacteria (BlO-AERO)• Iron-related (depositing) bacteria (BlO-IRB)• Low-nutrient bacteria (BlO-LNB).

The presence and viability of these four (or five)types of bacteria are tested in each of the four(or five) colored strings of bottles in each tray.The bottles in each string contain a fast-actingnutrient specific to the bacterium tested. Sometest kits give a quantitative indication of theviability of each bacterium from 10 to 105

bacterium/ml by the serial dilution of the sampleand subsequent growth in each string; othertest kits give a quantitative indication from 10 to104 bacterium/ml.

SUPPORTING ANALYSES

Corrosion and other types of deposits on thepipe may be analyzed in the field using the testkit to assist in interpretation of MIC and othercorrosion, or cathodic protection products. Thiskit can be used to qualitatively analyze for thepresence of carbonate (CO3

+2), sulfide (S-2),ferrous iron (Fe+2), ferric iron (Fe+3), calcium(Ca+2), and hydrogen (H+1, pH) ions.

Collect a sample of soil, deposit, film, or liquidfrom the area of interest. Use only a clean knifeor spatula provided with the test kit. The films ordeposits may be from the steel surface, coatingsurface, interior of a corrosion pit, or thebackside of the coating. In all cases, note thecolor and type of sample. Carefully transfer thesample to the test kit vial for testing. Follow thedetailed procedure given in the test kitinstruction sheets. For comparison purposes,obtain a reference sample taken at least 1 mfrom the previous collection site.

Record the results of the analysis on theResults Sheet provided with the kit or in acompany designated format. Determine thelikely type of deposit according to thecharacteristics shown below. A corrosion film


other than a carbonate, an oxide for example,may only show the presence of ferric or ferrousion, but with a pH that is similar to that of thereference sample or lower. In addition, cathodicprotection films may not necessarily show thepresence of calcium or carbonate, but will havean elevated pH compared with the referencesample. For most types of soils, the pH isgreater than about nine (9). An elevated pH isnot usually associated with the presence ofSRB.

PositiveResult

SRBCorrosion

FilmCorrosion

OxideCPFilm

CO3-2 Yes Yes

S-2 Yes

Fe+2 Yes Yes Yes

Fe+3 Yes Yes

Ca+2 Yes

pH Elevated

METALLURGICAL INSPECTION

The form of the corrosion pits associated withMIC is reasonably distinctive. These featurescan be observed in the field with the unaidedeye or a low power microscope.

After any films or products sampled above havebeen obtained from a corroded area, theremaining product should be removed using aclean spatula or knife, being careful not toscratch the metal. Clean any remaining materialwith a clean, dry, stiff brush, such asnylon-bristle brush. Do not use a metal brush ifpossible, because the metal bristles will mar thepit features. If not all of the product can beremoved with this method, use a brass bristlebrush in the longitudinal direction only. Dry thearea with an air blast or an alcohol swab. Ashiny metallic surface of the pit suggests thepossibility of active corrosion. However,judgment must be used to differentiate thiscondition from one created by scraping thesteel surface with a metallic object, such as the

knife or spatula use to clean the surface orobtain the sample product.

Examine the corroded area newly cleaned firstvisually with the unaided eye. Then use a lowpower magnifying lens at 5X to 50X power toexamine the detail of the corrosion pits. MICoften has the following features:

• Large craters up to 2 or 3 inches (5 to 8 cm)in diameter or more

• Cup-type hemispherical pits on the pipesurface or in the craters

• Sometimes the craters or pits aresurrounded by uncorroded metal

• Striations or contour lines in the pits orcraters running parallel to longitudinal pipeaxis (rolling direction)

• Sometimes tunnels can be seen at the endsof the craters also running parallel to thelongitudinal axis of the pipe.

Some examples of these features are shown inother materials. The cupping action of the pitsis a result of the local action of the bacteria.The striations follow the rolling direction of thesteel made into the pipe and are believed to bea result of preferential attack of the steelmicrostructural components.

ACKNOWLEDGMENT

This chapter was written by K. C. Garrity andwas edited by the AUCSC CurriculumCommittee.


MIC OF 304 SS WELD Under Tubercle Illustrated

From Little, Wagner, and Mansfeld

TUBERCLE BUILD-UP WITH IRON-RELATED BACTERIA

Fe-Related Bacteria

Aerated

Chloride

Deaerated

Low pH

IRB Create a Differential Oxygen Corrosion Cell with Low pH Environment

METALLURGICAL Pope and GRI

MICROBIOLOGICAL Aerobic and Anaerobic

• Aerobic Require O2 and Nutrient

• Consume O2 and Produce Organic Acids Corrosive to Steel

• Protection Requirements Less Well Defined

• Occur with Anaerobic Bacteria

• Form Complex Communities Dominated by Acid Producing Bacteria

• MIC a Result of Microbe Community, Not SRB Alone

MICROBIOLOGICAL COMMUNITY Example APB and SRB

• Organic Nutrients Feed APB

• APB Products Nutrients for SRB

• SRB Fed by Organic Acids and Sulfate

• Produce more APB and SRB

• Chlorides with Acids (H+) Lower pH and Corrode Steel

APB

Organic Acids (Lactic, formic, acetic, etc)

SRB Sulfate

APB APB

SRB

H2S CO2 Acids

Organic Nutrients

MICROBIOLOGICAL SRB Theory (Anaerobic)

• 8H2O8OH-+8H+

• 4Fe4Fe+2+8e- (A)

• 8H++8e-8H (C)

• SO4-2+8H +Bacteria

S-2+4H2O

• Fe+2+S-2FeS (A)

• 3Fe+2+6OH- 3Fe(OH)2

(A)

• FeS Depolarizes

• Need Nutrient – Soil biomass

– Coating adhesive

• Need No or Low O2

– Wet soil

– Low areas

– Crevices

• More Protection (Current) Needed to Overcome Effects

MIC TESTING

• Field Test Kits for Viable Bacteria – SRB, APB, anaerobic, aerobic, iron related

– Bacteria count—10 to 105 (104) colonies/ml

• Test Soil or Surface Product

• Create Slurry

• Inoculate Culture Media Vials

• Compare After 2 - 5 Days, 15 Days

TEST KIT INTERPRETATION

• Presence of Bacteria Not Conclusive of MIC

• SRB Often Do Not Dominate • Not Uncommon to Find All Tested Bacteria

Present • Interpret Indications with Caution

– 1 to 3 bottles – may have problem – 1 to 5 bottles – possible problem

CULTURE TEST Results

Aerobic Bacteria Test

APB Bacteria Test

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