CATHODIC PROTECTION

Facts

• 1 Ton of steel rusts every 90 sec• Energy required to produce 1 ton of steel

can be consumed in a household family for 3 months

• 50% steel produced worldwide is to replace rusted steel

CORROSION

• Loss of metal due to its interaction with environment.

• Corrosion is an electrochemical process.• The corrosion is a complex process which

depends on so many factors such as physical ,chemical, metallurgical, electrochemical and thermodynamics .

FACTORS WHICH INFLUENCE CORROSION RESISTANCE

CORROSION RESISTANCE

ELECTROCHEMICAL

PHYSICALCHEMICAL

METALLURGICAL

THEMODYNAMIC

CORROSION• THE LESS NOBEL METAL BECOME ANODE (POSITIVE ELECTRODE)

WHILE MORE NOBLE METAL ACT A CATHODE( NEGATIVE ELECTRODE)

• ANODIC REACTION – DISSOLUTION OF METAL M++ + 2e-• CATHODIC REACTION – DEPOSITION OF IONS N++ + 2e- N• REACTION AT ANODE WHICH CAUSES DISSOLUTION OF METAL

LEADS TO CORROSION OF METAL M

Corrosion Cell

electrical path

electrolyte (soil)

cathode anode

Conventional current

e-

e-

e-

cathode

e-

e-

e-

Iron anode

Electron migrations (e-)

Conventional current flow ( + to –v)

e- e- H2H+

e- e- H2H+

Fe(OH)2 OH- Fe+

Fe(OH)2 OH- Fe+

HOW CORROSION TAKES PLACLE

• At Anode:• 2H2O (OH)- +H + + 2e-

FE + + 2(OH) - FE(OH)2 FE(OH)2+ FE + + (OH) - FE2(OH)3

At Cathode

2e- + H + H(atom)

H + H plarisation film

• Corrosion potential – Natural potential of object with no protection or interference current present

• Polarization -A non ohmic component that will apparently exist very close to electrode/ electrolyte surface of the cathode and anode

• Polarization films are very important factor in controlling the amount of current flow . In one sense , the film of hydrogen formed on the surface of cathode may be thought of as an insulating layer which introduced ohmic resistance and reduce the flow of ohmic current

CORROSION: degradation of material, unintended attack from environment

(electrochemical mechanism)Requirements:1. Anode (oxidation)

+ve for electrolytic cell, -ve for galvanic cell

2. Cathode (reduction)-ve for electrolytic cell, +ve for galvanic cell

3. Electrolyte4. Electrical path between anode and cathode

• The amount of metal that will be removed is directly proportional to the amount of current flow

• Effective resistance in the anode cathode circuit goverens the amount of current flow in the corrosion cell which is then the function of resitivity of soil & size of anodic and cathodic area

• Polarization film formed is important in controlling the amount of current flow

• Depolarization effect tend to remove the hydrozen polarized film either by mechanical effect or by supply of dissolved oxygen

Factors affecting corrosion

1. Activation energy/polarisation2. Concentration polarisation (concentration

of ions near electrode)3. Resistance polarisation (resistance to flow

of current, e.g., painting, coating, etc).Localised corrosion is more critical than uniform corrosion

TYPES OF CORROSION CELLS ON PIPELINE

• DISSIMILAR METAL CORROSION• CORROSION DUE TO DISIMILAR SOIL• DIFFERENTIAL AERATION

CORROSION CELL • NEW & OLD PIPE

Corrosion can also occur due to

• Difference in aeration (crevice corrosion)• Contact with a foreign body• Due to stress (stress corrosion), e.g., mild

steel in alkaline environment

Control of Corrosion

• Coating• Cathodic protection• Corrosion inhibitors

Cathodic Protection

A TECHNIQUE TO REDUCE THE CORROSION OF A METAL SURFACE BY MAKING THAT SURFACE THE CATHODE OF AN ELECTROCHEMICAL CELL

… NACE

Corrosion takes place at the anode only !

• Anode: the electrode from which current leaves to the electrolyteAnode gets corroded

• Cathode: the electrode where current is collected from the electrolyteNo corrosion takes place at the cathode

IMPRESSED CURRENT GROUND BEDS

Insulated header cable

Cable trench

Insulating joint of header cable & pigtail of anode

High silicon cast iron anode

Carbonaceous backfill material, well temped

Angured hole for anode & backfill

Impressed Current CP

• Requires external power supply• TRU output can be controlled (automatic /

manual)• Can be used in high resistivity soil, can

protect uncoated structures• Wide choice on anode material; anodes

have long life

Anode

Protected Pipe Pipe

CP unit

Anode Cable

Cathode Cable

TYPICAL CP UNIT

+

-

Ground bed

Anodic Area

Half cell

• Copper / copper sulphate (used for land PSP)

• Silver / silver chloride (used in marine environment)

• Hydrogen (used in laboratory)• Calomel (used in laboratory)

On Potential

on potential

PSP

corrosion potential (550 mV)

days 0 2 4 6

CURRENT MEASUREMENTS

-V+-V+ ++ ++

BAA A

Buried Pipeline- Use of two electrode method

Foreign Pipelines close to Cathodic Protection Ground Bed and crossing

Protected line

Rectifier

Remote GB

Influence of ground bed surrounding foreign line

Current flow from foreign structure to protected line in crossing area

Foreign line

Foreign Pipelines close to Cathodic Protection Ground Bed but not crossing

 

Protected line

Rectifier

Remote GB

Foreign line

Current discharge from foreign line

+

-

Pipe-to-soil potential measurement

PSP

pipe earth

copperelectrode

CuSO4solution

porous plug

FOREIGN LINES CROSSING BARE CATHODICALLY PROTECTED LINE

Foreign line

Influence of GB

P/LineForeign line tends to become positive to soil. Current picked up by foreign line in electrically remote sections and discharge to the protected line in the crossing area

Most intense damage to foreign line

PEARSON TECHNIQUE

NB: 1Receiver indicates maximum as operator A passes directly over defect.2Receiver indicates full reading when both operators are equidistant from defect.3Receiver indicates maximum as operator & passes directly over defect.

Electrically connected

Coating defect

soil

transmitterOperator A Operator B

Receiver

EarthOperators use cleat shoes

CLOSE INTERVAL PIPE TO SOIL POTENTIAL SURVEYS (CIPS

Pipeline under investigation

Trailing wire

Long Half Cell

Mounting Board

Back pack

Protection criteriaNACE RP-0169-96

• A negative 850 mV PSP w.r.t. Cu/CuSO4 reference electrode with CP current applied

• A negative 850 mV polarised PSP w.r.t. Cu/CuSO4 reference electrode

• A minimum 100 mV cathodic polarisation• A net protective current• 300 mV shift (not per NACE)

IR Drop in On Potential

power supply

PSP half cell

anode pipe IR drop

Off Potential

IR drop PSP 100 mV min corrosion potential

time 0

On-Off PSP Survey

• Current interrupters are used• on:off time = 4:1• Off potential should be measured at least

after 0.5 sec.• Off period should be less than 3 sec to

avoid depolarisation.

Current Interrupters and Synchronisation

• GPS- Costly

• SCADA- Not available everywhere

• Quartz clock- Can loose synchronisation by 0.5 sec/day

Sacrificial System CP

• No external power requirement• Suitable in low resistivity soil / marine

environment only• Choice on anode material is limited, e.g.,

magnesium, zinc, aluminium, etc.;anodes get corroded fast;

• Can protect well coated structures only, protected area is small

CP Design

• Impressed current system• Sacrificial anode system• Hybrid system

Designing CP Installation-impressed current

• Establish soil resistivity• Estimate total current requirement• Select suitable anode material• Calculate anode bed size, shape and

configuration / calculate total mass of anode for design life

• Select location of CP / test stations• Consider facilities required for control

monitoring

Span of protection for CP Stations

CP stn 1 CP stn 2 CP stn 3

l l l l

pipeline

Required Protective Current

• 2I0 = 2πDJL

I0 = current to one side of CP station(A)

D = diameter of pipeline( m)J = protective current density(A/m2)L = spread of protection on one side of CP station

SPREAD PROTECTION• 2L= 8 U L/ 3.14 D J R’

I0 = current to one side of CP station(A)D = diameter of pipeline( m)J = protective current density(A/m2)L = spread of protection on one side of CP station

R’= Resistance of the pipe (ohm/m) ^UL= 0.30 Volts(max . Potential diff between

drain point and end protection)

Types of Anode Bed

• Horizontal anode bedLower resistanceR = (ρ/2πL)ln(L2/tD)

• Vertical anode bedR = (ρ/2πL)ln(2l/d×(4t+3l)½/(4t+l) ½)

Sacrificial Anode Materials

• Zinc Alloy (C-Sentry)Sea water, low resistivity soil

• Aluminium Alloy (Galvalum I, Galvalam II, Galvalum III, Alanode)Sea water

• Magnesium Alloy (Galvomag)soil

Anode Life

L = Ecm/IL = anode lifeE = efficiencyc = capacity(Ah/Kg)m = anode weight (kg)I = anode output current(A)

Anode Life

L = Ecm/I (HOURS)L = anode lifeE = 50% say for Mg anode, 90% for ZnC = 2200Ah/Kg for Mg & 820 Ah/ Kg Zn

m = anode weight (kg)I = anode output current(A)

Impressed Current Anode Materials

• Platinum and platinised metals• High silicon iron• Lead-platinum• Lead-silver• Graphite• Iron• Cast iron

GALVANIC ANODE CURRENT OUTPUT

The output current from a sacrificial anode can be calculated by Ohm’s law as follows:

• Ia = ^V / R• Ia = anode out current (A)• V= driving voltage (V)• R = Current resistance • V is the voltage difference between the potential of the

cathodically protected surface and the potential of the anode. The circuit resistance comprises resistance of anode to earth, resistance of protected structure to earth and resistance of cables and structure itself.

Anode Bed

anode lead

anodebackfillcanister

Backfill

• Effective size of anode increases• Anode loss partly shared by bedding

material• Porous surrounding for gases to escapeCarbonacious backfill – coal coke breezeCalcined petroleum coke breeze

Ground Bed

– A system of anode beds• Swallow / deep ground bed• Horizontal / vertical type ground bed• Distributed / closely spaced ground bed• Close / remote ground bed

CP Field Surveys

• Pearson survey• Current attenuation Test (CAT) survey• Direct Current Voltage Gradient survey• Close Interval Potential Logging (CPL)

survey

Sources of Power Supply

• AC power grid• Solar power• Wind power• Thermo electric generator• CCVT (closed circuit vacuum turbine)

Controls

• DC output controlManual output voltage controlConstant voltage controlConstant current controlConstant PSP control

• Multi reference control

Annunciations

• Reference fail• Overprotection• Under protection

Stray Current Interface

Stray current: current through path other than the intended circuitSources:– DC traction system– Geomagnetic / telluric current– HVDC interference– EHV AC interference

Anodic Interference

power supply

anode design current p/l

stray currentmetallic object

Cathodic Interference (more critical)

power supply

anode design current p/l

stray current metallic

object

Geomagnetic Current

• Also called telluric current• Caused by the effect of solar winds• Varies from time to time

Safe Potential

Etouch = Ib×{Rb+Rfeet}/duration of shock

Rb = 1000 Ω human body (adult)

Ib = Safe current 15mA let go

117mA ventricular fibrillation

CP of Offshore Structures

• Resistivity is extremely low (16-40 Ω-cm) • CP current requirement is very high

(thousands of amperes)• Depolarisation due to temperature, oxygen,

sea currents• Steel corrodes at the rate of 0.13 mm/year

under laboratory conditions

Galvanic Anode material

• AluminiumLow driving potentialExtensively used for marine applications

• MagnesiumLow efficiencyCan affect rapid polarisation of offshore structures in combination with aluminium

Galvanic Anode Material

• ZincLow driving potentialHigh efficiencySuitable for low resistivity soils, fresh water and marine applications (widely used)Protection of ship hulls

Choice of Half Cell

• Silver / Silver Chloride half cell is most common0.8V w.r.t. Ag/AgCl electrode in aerated seawater, 0.9V under anaerobic conditions

• Zinc / Zinc Chloride half cell can also be used

• Cu/CuSO4 half cell will polarise due to the presence of chloride ions

Thank You