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Michael Siegwart Corrosion – Monitoring, Repair and Management

CorrosionMonitoring, Repair and

Management


Contents

1 CORROSION THEORY................................................1

1.1 PASSIVITY OF METALS ............................................21.2 UNIFORM CORROSION..............................................31.3 DEFINITION OF A CORROSION CELL ...................................41.3.1 Composition Cell........................................41.3.2 Stress Cell.............................................41.3.3 Concentration Cell......................................5

1.4 GALVANIC CORROSION.............................................51.4.1 Galvanic Corrosion of the Statue of Liberty .......................71.4.2 Stainless screw v cadmium plated steel washer...........7

1.5 TESTING FOR LOCALISED CORROSION ..................................71.5.1 Localised Attack........................................81.5.2 Pitting Corrosion.......................................91.5.3 Crevice Corrosion......................................10

1.6 STRESS CORROSION CRACKING ......................................111.6.1 Stress Corrosion Cracking (SCC) Defined................12

1.7 STAINLESS STEEL CORROSION ......................................121.7.1 Stainless Steel Weld Decay.............................13

2 REFERENCE ELECTRODES...........................................14

2.1 REFERENCE ELECTRODE: DEFINITION .................................142.2 STANDARD ELECTRODE POTENTIALS...................................142.3 NERNST EQUATION ..............................................152.3.1 Stability of Water .........................................162.3.2 Chemical Thermodynamics................................172.3.3 Pourbaix Diagrams.........................................172.3.4 Pourbaix Diagram of Iron at 25oC..............................182.3.5 E-pH Diagram of Iron at 25oC .................................192.3.6 Reference Electrode Potentials.........................19

3 CORROSION IN CONCRETE..........................................21

3.1 MAGNITUDE OF THE REBAR CORROSION PROBLEMS ............................213.2 LIME CEMENTS, PLASTERS, MORTARS AND CONCRETES .....................223.2.1 Limestone..............................................223.2.2 Lime...................................................223.2.3 Cement.................................................223.2.4 Aggregate..............................................233.2.5 Concrete and Mortar....................................233.2.6 Hydraulic Cements......................................233.2.7 Pozzolans..............................................243.2.8 Plaster................................................243.2.9 Stucco.................................................24

3.3 NATURE OF THE PROBLEM .........................................243.4 CORROSION CONTROL IN CONCRETE...................................263.5 PORTLAND CEMENT ..............................................263.5.1 TYPE I.................................................273.5.2 TYPE II................................................273.5.3 TYPE III...............................................273.5.4 TYPE IA, IIA, IIIA.....................................273.5.5 TYPE IV................................................283.5.6 TYPE V.................................................28

3.6 ACID ATTACK ON CONCRETE........................................28


4 HIGHWAY BRIDGES................................................30

4.1 HIGHWAY BRIDGES ..............................................304.2 CONVENTIONAL CONCRETE BRIDGES...................................314.3 PACK RUST ...................................................324.4 PRESTRESSED CONCRETE BRIDGES....................................324.5 STEEL BRIDGES................................................334.6 HYDROGEN EMBRITTLEMENT (PRESTRESSING STEELS!) .....................344.6.1 Sources of Hydrogen....................................344.6.2 Hydrogen Embrittlement of Stainless Steel..............34

5 ATMOSPHERIC CORROSION..........................................36

5.1 ATMOSPHERIC CORROSION MECHANISM .................................365.2 CORROSIVITY MAPS AND POLLUTION INFORMATION ........................375.2.1 Germany................................................375.2.2 Corrosion in UK........................................385.2.3 Pollution in Russia....................................39

5.3 EXPOSURE TESTING .............................................405.4 DIRECT CORROSIVITY ASSESSMENT...................................405.5 MICROENVIRONMENTS.............................................415.6 DEICING SALTS................................................415.7 NATURE OF THE PROBLEM .........................................415.8 ISO 9223...................................................43

6 CORROSION IN SOILS.............................................45

6.1 INTRODUCTION TO CORROSION IN SOILS...............................456.2 SOIL CLASSIFICATION SYSTEMS ....................................466.3 SOIL VARIABLES ...............................................476.4 CORROSION SEVERITY RATINGS .....................................486.5 NUMERICAL CORROSIVITY SCALE ....................................48

7 CORROSION BY NATURAL WATERS....................................50

7.1 WATER CONSTITUENTS............................................517.2 SATURATION AND SCALING INDICES ..................................517.3 PRIORITY POLLUTANTS ...........................................527.4 CHLORINATION OF WATER .........................................53

8 MICRO-ORGANISMS IN CORROSION...................................55

8.1 MIC BASICS..................................................558.2 BACTERIA RELATED TO MIC .......................................568.2.1 Acid Producing Fungi...................................568.2.2 Aerobic Slime Formers..................................568.2.3 Iron/Manganese Oxidising Bacteria......................578.2.4 Methane Producers......................................578.2.5 Organic Acid Producing Bacteria........................588.2.6 Sulfur/Sulfide Oxidizing Bacteria......................58

8.3 IDENTIFICATION OF MICROBIAL ACTIVITY .............................588.3.1 Direct Inspection......................................588.3.2 Growth Assays..........................................598.3.3 Activity Assays........................................59

8.4 ACCELERATED LOW WATER CORROSION (ALWC) ..........................60

9 CORROSION IN MARINE ENVIRONMENTS...............................62

9.1 IONS IN SEAWATER .............................................639.2 OXYGEN IN SEAWATER............................................639.3 PRECIPITATION OF INORGANIC COMPOUNDS FROM SEAWATER ..................649.4 ELECTROPLATING ...............................................669.5 MARINE SYSTEMS ...............................................67

10 STRAY CURRENT CORROSION......................................69

10.1 DETECTION OF STRAY CURRENTS ..................................69


10.2 STRAY CURRENTS IN TRANSIT SYSTEMS .............................6910.3 NATURE OF STRAY CURRENTS.....................................71

11 BASICS OF CORROSION MONITORING...............................73

11.1 NEED FOR CORROSION MONITORING.................................7411.2 CORROSION MONITORING TECHNIQUES ...............................7511.3 SELECTING MONITORING POINTS ..................................7611.4 DATA INTEGRATION IN CORROSION MONITORING........................7611.5 ACOUSTIC EMISSION (AE)......................................7711.6 CORROSION POTENTIAL MONITORING ................................7811.7 LINEAR POLARISATION RESISTANCE (LPR) ..........................7811.8 CORROSION INFORMATION .......................................80

12 FAILURE ANALYSIS.............................................81

13 DETERIORATION AND RISK MODELLING.............................82

13.1 STRAIGHT LINE REGRESSION.....................................8213.2 MARKOVIAN DETERIORATION MODEL.................................8213.3 RISK AND FATIGUE LIFE ASSESSMENT METHODS........................8313.3.1 Structural Reliability Models........................8413.3.2 Use of the Weibull Distribution for Fatigue Life Assessment.....................................................84

14 CATHODIC PROTECTION..........................................87

14.1.1 Corrosion Costs and Preventive Strategies Study......8714.2 SACRIFICIAL ANODES..........................................8814.2.1 Sacrificial anode material .................................8814.2.2 Anode Efficiency ........................................8914.2.3 Protective current requirements in CP design ..................9014.2.4 Designing a sacrificial anode system .........................91

14.3 IMPRESSED CURRENT CATHODIC PROTECTION ..........................9214.3.1 Non Consumable Anodes ..................................9314.3.2 Semi-Consumable Anodes .................................9414.3.3 Polarisation Behaviour...............................9514.3.4 Activation Overpotential..................................9614.3.5 Concentration Overpotential...............................9714.3.6 Conductivity Cell....................................98

14.4 CONSUMABLE ANODES...........................................99

15 ELECTROCHEMICAL CHLORIDE EXTRACTION AND RE-ALKALISATION TREATMENT.........................................................100

16 ORGANIC PROTECTIVE COATINGS.................................101

16.1 MAIN VARIABLES ............................................10116.1.1 Designing with Corrosion Protective Coatings........101

16.2 PROTECTIVE COATINGS COMPONENTS ...............................10316.2.1 Binders.............................................10316.2.2 Pigments............................................10416.2.3 Solvents............................................104

16.3 COMPARISON OF PAINT SPECIFICATIONS ............................10516.3.1 A ranking system....................................10516.3.2 The Tables..........................................106

16.4 TYPICAL PROTECTIVE COATING SPECIFICATIONS ......................10716.5 COATING FAILURE ...........................................10916.6 COATINGS FOR BURIED PIPELINES................................11016.7 APPLICATIONS OF BURIED PIPELINE COATING SYSTEMS..................11216.8 OPERATIONAL LIMITS FOR BURIED PIPELINE COATINGS..................11316.9 SUPPLEMENTARY PROTECTION SYSTEMS .............................11416.10 JOINTING COMPOUNDS AND SEALANTS...............................11416.11 WATER DISPLACING COMPOUNDS...................................114

Michael Siegwart Corrosion – Monitoring, Repair and ManagementPage 1 of 114

1 Corrosion TheoryHumans have most likely been trying to understand and control corrosion for as long as they have been using metal objects. The most important periods of prerecorded history are named for the metals that were used for tools and weapons (Iron Age, Bronze Age). With a few exceptions, metals are unstable in ordinary aqueous environments. Metals are usually extracted from ores through the application of a considerable amount of energy. Certain environments offer opportunities for these metals to combine chemically with elements to form compounds and return to their lower energy levels.

Corrosion is the primary means by which metals deteriorate. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases.

Corrosion specifically refers to any process involving the deterioration or degradation of metal components. The best known case is that of the rusting of steel. Corrosion processes are usually electrochemical in nature, having the essential features of a battery. When metal atoms are exposed to an environment containing water molecules they can give up electrons, becoming themselves positively charged ions, provided an electrical circuit can be completed. This effect can be concentrated locally to form a pit or, sometimes, a crack, or it can extend across a wide area to produce general wastage. Localised corrosion that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive agents like seawater may lead to greatly enhanced growth of the fatigue crack. Pitting corrosion also occurs much faster in areas where microstructural changes have occurred due to welding operations.

Corrosion is the disintegration of metal through an unintentional chemical or electrochemical action, starting at its surface. All metals exhibit a tendency to be oxidised, some more easily than others. A tabulation of the relative strength of this tendency is called the galvanic series. Knowledge of a metal's location in the series is an important piece of information to have in making decisions about its potential usefulness for structural and other applications. The corrosion process (anodic reaction) of the metal dissolving as ions generates some electrons, as shown here, that are consumed by a secondary process (cathodic reaction). These two processes have to balance their charges. The sites hosting these two processes can be located close to each other on the metal's surface, or far apart depending on the circumstances. This simple observation has a major impact in many aspects of corrosion prevention and control, for designing new corrosion monitoring techniques to avoiding the most insidious or localized forms of corrosion.


The electrons (e- in this figure) produced by the corrosion reaction will need to be consumed by a cathodic reaction in close proximity to the corrosion reaction itself. The electrons and the hydrogen ions react to first form atomic hydrogen, and then molecular hydrogen gas. If the acidity level is high (low pH), this molecular hydrogen will readily become a gas as it is demonstrated by exposing a strip of zincto a sulfuric acid solution.

As hydrogen forms, it could inhibit further corrosion by forming a very thin gaseous film at the surface of the metal. This "polarizing" film can be effective in reducing water to metal contact and thus in reducing corrosion. Yet it is clear that anything which breaks down this barrier film tends to increase the rate of

corrosion. Dissolved oxygen in the water will react with the hydrogen, converting it to water, and destroying the film.

High water velocities tend to sweep the film away, exposing fresh metal to the water. Similarly, solid particles in the water can brush the hydrogen film from the metal. Other corrosion accelerating forces include high concentrations of free hydrogen ions (low pH) which speed the release of the electrons, and high water temperatures, which increase virtually all chemical reaction, rates. Thus a variety of natural and environmental factors can have significant effects on the corrosion rate of metals, even when no other special conditions are involved.

1.1 Passivity of MetalsThe passivation behaviour of a metal is typically studied with a basic electrochemical testing set-up. When the potential of a metallic component is controlled and shifted in the more anodic (positive) direction, the current required to cause that shift will vary. If the current required for the shift has the general polarization behavior illustrated here, the metal is termed active-passive and can be anodically protected.

Only a few systems exhibit this behavior in an appreciable and usable way. The corrosion rate of an active-passive metal can be significantly reduced by shifting the potential of the metal so that it is at a value in the passive range. The current required to shift the potential in the anodic direction from the corrosion potential Ecorr can be several orders of magnitude greater than the current necessary to maintain the potential at a passive value. The current will peak at the passivation potential value shown as Epp.

In order to produce passivation the critical current density (icc) must be exceeded. The anodic potential must then be maintained in the passive region without allowing it to fall back in the active region or getting into the transpassive region, where the protective anodic film can be damaged and even break down completely. It follows that although a high current density may be required to cause passivation (> icc), only a small current density is


required to maintain it, and that in the passive region the corrosion rate corresponds to the passive current density (ip).

Passivity can also be readily produced in the absence of an externally applied passivating potential by using oxidants to control the redox potential of the environment. Very few metals will passivate in non-oxidising acids or environments, when the redox potential is more cathodic than the potential at which hydrogen can be produced. A good example of that behaviour is titanium, and some of its alloys, that can be readily passivated by most acids, whereas mild steel requires a strong oxidising agent, such as fuming nitric acid, for its passivation.

Alloying with a more easily passivated metal normally increases the ease of passivation and lowers the passivation potential, as in the alloying of iron and chromium in 10% sulfuric acid. Small additions of copper in carbon steels have been found to reduce ip in sulfuric acid. Each alloy system has to be evaluated for its own passivating behaviour as illustrated by the case Ni-Cr alloys where both the additions of nickel to chromium and chromium to nickel decrease the critical current density in a mixture of sulfuric acid and 0.25 M potassium sulfate.

1.2 Uniform CorrosionUniform corrosion is characterised by corrosive attack proceeding evenly over the entire surface area, or a large fraction of the total area. General thinning takes place until failure. On the basis of tonnage wasted, this is the most important form of corrosion.

The breakdown of protective coatingsystems on structures often leads to this form of corrosion. Dulling of a bright or polished surface, etching by acid cleaners, or oxidation (discoloration) of steel are examples of surface corrosion. Corrosion resistant alloys and stainless steels can become tarnished or oxidised in corrosive environments. Surface corrosion can indicate a breakdown in the protective coating system, however, and should be examined closely for more advanced attack. If surface corrosion is permitted to continue, the surface may become rough and surface corrosion can lead to more serious types of corrosion. However, uniform corrosion is relatively easily measured and predicted, making disastrous failures relatively rare. In many cases, it is objectionable only from an appearance standpoint. As corrosion occurs uniformly over the entire surface of the metal component, it can be practically controlled by cathodic protection, use of coatings or paints, or simply by specifying a corrosion allowance. In other cases uniform corrosion adds colour and appeal to a surface. Two classics in this respect are the patina created by naturally tarnishing copper roofs and the rust hues produced on weathering steels.


1.3 Definition of a Corrosion Cell Nature can stipulate corrosion in many fashions. The force behind the corrosion attraction of metals towards the formation of stable oxides or other oxidised forms of metals can be divided into three types:

1.3.1 Composition Cell

Composition cells (also known as Galvanic cells) arise when two metals with dissimilar compositions or microstructures come into contact in the presence of an electrolyte. The two most common examples follow:

Dissimilar metals: Formed by two single-phase metals in contact, such as iron and zinc, or nickel and gold. The metal that is higher on the Electrochemical Series will be the cathode. The other metal will suffer anodic reactions and will corrode.

Incidentally, dissimilar metal contact (while bathed in a suitable electrolyte) is the technology behind the construction of batteries. The voltage of a battery directly follows from the natural electrode potential of the corrosion reactions present inside the battery. Hence, controlled corrosion is a good thing!

Multi-phase alloy: Formed by a metal alloy composed of multiple phases, such as a stainless steel, a cast iron, or an aluminum alloy. The individual phases possess different electrode potentials, resulting in one phase acting as an anode and subject to corrosion.

1.3.2 Stress Cell

Stress cells can exist in a single piece of metal where a portion of the metal's microstructure possesses more stored strain energy than the rest of the metal. Metal atoms are at their lowest strain energy state when situated in a regular crystal array.

Grain boundaries: By definition, metal atoms situated along grain boundaries are not located in a regular crystal array (i.e. a grain). Their increased strain energy translates into an electrode potential that is anodicto the metal in the grains proper. Thus, corrosion can selectively occur along grain boundaries.

High localized stress: Regions within a metal subject to a high local stress will contain metal atoms at a higher strain energy state. As a result, high-stress regions will be anodic to low-stress regions and can corrode selectively.

For example, bolts under load are subject to more corrosion than similar bolts that are unloaded. A good rule of thumb is to select fasteners that are cathodic (i.e. higher on the Electrochemical Series) to the metal being fastened in order to prevent fastener corrosion.


Cold worked: Regions within a metal subjected to cold-work contain a higher concentration of dislocations, and as a result will be anodic to non-cold-worked regions. Thus, cold-worked sections of a metal will corrode faster. For example, nails that are bent will often corrode at the bend, or at their head where they were worked by the hammer.

1.3.3 Concentration Cell

Concentration cells can arise when the concentration of one of the species participating in a corrosion reaction varies within the electrolyte.

Electrolyte concentration: Consider a metal bathed in an electrolyte containing its own ions. The basic corrosion reaction where a metal atom losses an electron and enters the electrolyte as an ion can proceed both forward and backwards, and will eventually reach equilibrium.

If a region of the electrolyte (adjacent to the metal) were to exhibit a decreased concentration of metal ions, this region would become anodic to the other portions of the metal surface. As a result, this portion of the metal would corrode faster in order to increase the local ion concentration.

The net affect is that local corrosion rates are modulated in order to homogenise reduction ion concentrations within the electrolyte.

Oxidation concentration: Perhaps the most common concentration cell affecting engineered structures is that of dissolved oxygen. When oxygen has access to a moist metal surface, corrosion is promoted. However, it is promoted the most where the oxygen concentration is the least (for the reasons described in the above box).

As a result, sections of a metal that are covered by dirt or scale will often corrode faster, since the flow of oxygen to these sections is restricted. An increased corrosion rate will lead to increased residue, further restricting the oxygen flow to worsen the situation. Pitting often results from this "runaway" reaction.

1.4 Galvanic CorrosionGalvanic corrosion (also called ' dissimilar metal corrosion' or wrongly 'electrolysis') refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. It occurs when two (or more) dissimilar metals are brought into electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone. Either (or both) metal in the couple may or may not corrode by itself (themselves) in seawater. When contact with a dissimilar metal is made, however, the self corrosion rates will change:

corrosion of the anode will accelerate,


corrosion of the cathode will decelerate or even stop.

The driving force for corrosion is a potential difference between the different materials. The bimetallic driving force was discovered in the late part of the eighteenth century by Luigi Galvani in a series of experiments with the exposed muscles and nerves of a frog that contracted when connected to a bimetallic conductor. The principle was later put into a practical application by Alessandro Volta who built, in 1800, the first electrical cell, or battery: a series of metal disks of two kinds, separated by cardboard disks soaked with acid or salt solutions. This is the basis of all modern wet-cell batteries, and it was a tremendously important scientific discovery, because it was the first method found for the generation of a sustained electrical current.

The principle was also engineered into the useful protection of metallic structures by Sir Humphry Davy and Michael Faraday in the early part of the nineteenth century. The sacrificial corrosion of one metal such as zinc, magnesium or aluminium is a widespread method of cathodically protecting metallic structures.

In a bimetallic couple, the less noble material will become the anode of this corrosion cell and tend to corrode at an accelerated rate, compared with the uncoupled condition. The more noble material will act as the cathode in the corrosion cell. Galvanic corrosion can be one of the most common forms of corrosion as well as one of the most destructive.

The relative nobility of a material can be predicted by measuring its corrosion potential. The well known galvanic series lists the relative nobility of certain materials in sea water. A small anode/cathode area ratio is highly undesirable. In this case, the galvanic current is concentrated onto a small anodic area. Rapid thickness loss of the dissolving anode tends to occur under these conditions. Galvanic corrosion problems should be solved by designing to avoid these problems in the first place. Galvanic corrosion cells can be set up on the macroscopic level or on the microscopic level. On the microstructural level, different phases or other microstructural features can be subject to galvanic currents.


1.4.1 Galvanic Corrosion of the Statue of Liberty

The galvanic reaction between iron and copper was originally mitigated by insulating copper from the iron framework using an asbestos cloth soaked in shellac. However, the integrity and sealing property of this improvised insulator broke down over the many years of exposure to high levels of humidity normal in a marine environment. The insulating barrier became a sponge that kept the salted water present as a conductive electrolyte, forming a crude electrochemical cells as and Volta had discovered a century earlier. The formation of expanded material that followed was typical of confined situations found in crevice corrosion.

1.4.2 http://www.corrosion-doctors.org/Aircraft/galvseri-sea.htmStainless screw v cadmium plated steel washer

This is one of the most common forms of corrosion as well as one of the most destructive. Here’s a classic example of galvanic corrosion; a stainless screw in contact with a cadmium plated steel washer.

1.5 Testing for Localised CorrosionThese are the types of corrosion in which there is intense attack at localised sites on the surface of a component whilst the rest of the surface is corroding at a much lower rate - either because of an inherent property of the component material (such as the formation of a protective oxide film) or because of some environmental effect. Indeed the main surface may be essentially under satisfactory corrosion control. In such circumstances, if corrosion protection breaks down locally then corrosion may be initiated at these local sites.

If this event occurs under a deposit on the surface (perhaps a weld deposit or some solid debris from the environment) or at the joint of a bolted assembly etc, the attack is termed, “crevice corrosion”. If the attack initiates on the free surface of a component, it is termed “pitting”. The resistance to these two types of localised corrosion varies greatly between different materials and is extremely dependent upon environmental factors.

http://www.corrosion


The occurrence of localised corrosion is a manifest proof that the anodic surface area can be much smaller than the cathodic. The Sa/Sc ratio, or degree of localisation, can be an important driving force of all localised corrosion problems since a corrosion situation corresponds to equal anodic and cathodic absolutes currents. Corrosive microenvironments, which tend to be very different from the bulk environment, often play a role in the initiation and propagation of corrosion pits. This greatly complicates the prediction task.

In general terms, small corrosion anodic areas correspond to severe corrosion problems with low delectability. The industrial importance of localised corrosion problems has been revealed in many reports. The following pie chart summarises the findings of 363 corrosion failure cases investigated in a major chemical processing company. The importance of pitting comes second (22%) after general corrosion and before stress corrosion cracking(SCC) which is, by the way, often initiated by pitting. Crevice corrosion comes fourth at 12%.

1.5.1 Localised Attack

Stainless steels are rarely used in soil applications, as their corrosion performance in soil is generally poor. Localised corrosion attack is a particularly serious concern. The presence of halide ions and concentration cells developed on the surface of these alloys tend to induce localised corrosion damage.

The two most common mechanisms of reinforcing steel corrosion damage in concrete are:


localised breakdown of the passive film by chloride ions,

carbonation, a decrease in pore solution pH leading to a general breakdown in passivity.

Harmful chloride ions usually originate from de-icing salts applied in cold climate regions or from marine type environments/atmospheres. Carbonation damage is predominantly induced by a reaction of concrete with carbon dioxide (CO2) in the atmosphere.

Chloride induced rebar corrosion tends to be a localised corrosion process, with the original passive surface being destroyed locally under the influence of chloride ions. Apart from the internal stresses created by the formation of corrosion products leading to cracking and spalling of the concrete cover, chloride attack ultimately reduces the cross section and significantly compromises the load carrying capability of steel reinforced concrete.

1.5.2 Pitting Corrosion

Pitting corrosion is a localised form of corrosion by which cavities or "holes" are produced in the material. Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and design against. Corrosion products often cover the pits. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system. Pitting corrosion, which, for example, is almost a common denominator of all types of localised corrosion attack, may take different shapes. Pitting is initiated by:

Localised chemical or mechanical damage to the protective oxide film; water chemistry factors which can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend to render a protective oxide film less stable) and high concentrations of chloride (as in seawater),

Localised damage to, or poor application of, a protective coating,

The presence of non-uniformities in the metal structure of the component, e.g. non-metallic inclusions.

Pitting corrosion can produce pits with their mouth open (uncovered) or covered with a semi-permeable membrane of corrosion products. Pits can be either hemispherical or cup-shaped. Apart from the localised loss of thickness, corrosion pits can also be harmful by acting as stress risers. Fatigue and stress corrosion cracking may initiate at the base of corrosion pits. One pit in a large system can be enough to produce the catastrophic


failure of that system. An extreme example of such catastrophic failure happened recently in Mexico, where a single pit in a gasoline line running over a sewer line was enough to create great havoc to a city, killing 215 people in Guadalajara.

1.5.3 Crevice Corrosion

Crevice corrosion is a localised form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas) such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps. Crevice corrosion is initiated by changes in local chemistry within the crevice:

Depletion of inhibitor in the crevice,

Depletion of oxygen in the crevice,

A shift to acid conditions in the crevice,

Build-up of aggressive ion species (e.g. chloride) in the crevice.

As oxygen diffusion into the crevice is restricted, a differential aeration cell tends to be set up between crevice (microenvironment) and the external surface (bulk environment). The cathodic oxygen reduction reaction cannot be sustained in the crevice area, giving it an anodic character in the concentration cell. This anodic imbalance can lead to the creation of highly corrosive micro-environmental conditions in the crevice, conducive to further metal dissolution. This results in the formation of an acidic micro-environment, together with a high chloride ion concentration.

All forms of concentration cell corrosion can be very aggressive, and all result from environmental differences at the surface of a metal. Even the most benign atmospheric environments can become extremely aggressive.

The most common form is oxygen differential cell corrosion. This occurs because moisture has a lower oxygen content when it lies in a crevice than when it lies on a surface. The lower oxygen content in the crevice forms an anode at the metal surface. The metal surface in contact with the portion of the moisture film exposed to air forms a cathode.

A special form of crevice in which the aggressive chemistry build-up occurs under a protective film that has been breached is called filiform corrosion. Another important for of crevice corrosion occurs under insulation.


1.6 Stress Corrosion CrackingStress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses. The problem itself can be quite complex. The situation with buried pipelines is a good example of such complexity.

Cold deformation and forming, welding, heat treatment, machining and grinding can introduce residual stresses. The magnitude and importance of such stresses is often underestimated. The residual stresses set up as a result of welding operations tend to approach the yield strength. The build-up of corrosion products in confined spaces can also generate significant stresses and should not be overlooked. SCC usually occurs in certain specific alloy environment-stress combinations.

Usually, most of the surface remains unattacked, but with fine cracks penetrating into the material. In the microstructure, these cracks can have an intergranular or a transgranular morphology. Macroscopically, SCC fractures have a brittle appearance. SCC is classified as a catastrophic form of corrosion, as the detection of such fine cracks can be very difficult and the damage not easily predicted. Experimental SCC data is notorious for a wide range of scatter. A disastrous failure may occur unexpectedly, with minimal

overall material loss. The micrograph on the right (X500) illustrates intergranular SCC of an Inconel heat exchanger tube with the crack following the grain boundaries. The micrograph on the left (X300) illustrates SCC in a 316 stainless steel chemical processing piping system. Chloride stress corrosion cracking in austenitic stainless steel is characterised by the multi-branched "lightning bolt" transgranular crack pattern.

The catastrophic nature of this severe form of corrosion attack has been repeatedly illustrated in many news worthy failures, including the following:

Swimming pool roof collapse in Uster, Switzerland,

EL AL Boeing 747 crash in Amsterdam.


1.6.1 Stress Corrosion Cracking (SCC) Defined

Chloride SCC One of the most important forms of stress corrosion that concerns the nuclear industry is chloride stress corrosion. Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature. It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and use of low carbon steels.

Caustic SCC Despite the extensive qualification of Inconel for specific applications, a number of corrosion problems have arisen with Inconel tubing. Improved resistance to caustic stress corrosion cracking can be given to Inconel by heat treating it at 620oC to 705oC, depending upon prior solution treating temperature. Other problems that have been observed with Inconel include wastage, tube denting, pitting, and intergranular attack.

The most effective means of preventing SCC are: design properly with the right materials; 2) reduce stresses; 3) remove critical environmental species such as hydroxides, chlorides, and oxygen; 4) and avoid stagnant areas and crevices in heat exchangers where chloride and hydroxide might become concentrated. Low alloy steels are less susceptible than high alloy steels, but they are subject to SCC in water containing chloride ions.

1.7 Stainless Steel CorrosionThere are five main types of stainless steel: ferritic, martensitic, austenitic, precipitation hardening and duplex. The ferritic and martensitic grades are so named because of their crystal structures. Both are iron-chromium-based alloys and were the type of stainless steel first developed in the early 1900’s. The ferritic and martensitic stainless steels are magnetic. The martensitic stainless steels can be hardened by a heat treatment similar to that used to harden ordinary steel, namely, heating to a high temperature, quenching, then reheating to an intermediate temperature (tempering) to achieve the desired balance of hardness and ductility.

Stainless and heat resisting steels possess unusual resistance to attack by corrosive media at atmospheric and elevated temperatures, and are produced to cover a wide range of mechanical and physical properties for particular applications. Along with iron and chromium, all stainless steels contain some carbon. It is difficult to get much less than about 0.03 % and sometimes carbon is deliberately added up to 1.00% or more. The more carbon there is, the more chromium must be used, because carbon can take from the alloy about seventeen times its own weight of chromium to form carbides. Chromium carbide is of little use for resisting corrosion. The carbon, of course, is added for the same purpose as in ordinary steels to make the alloy stronger.


Stainless steels are mainly used in wet environments. With increasing chromium and molybdenum contents, the steels become increasingly resistant to aggressive solutions. The higher nickel content reduces the risk of SCC. Austenitic steels are more or less resistant to general corrosion, crevice corrosion and pitting, depending on the quantity of alloying elements. Resistance to pitting and crevice corrosion is very important if the steel is to be used in chloride containing environments. Resistance to pitting and crevice corrosion typically increases with increasing contents of chromium, molybdenum and nitrogen.

Corrosion resistance of stainless steels is a function not only of composition, but also of heat treatment, surface condition, and fabrication procedures, all of which may change the thermodynamic activity of the surface and thus dramatically affect the corrosion resistance. It is not necessary to chemically treat stainless steels to achieve passivity. The passive film forms spontaneously in the presence of oxygen. Most frequently, when steels are treated to improve passivity (passivation treatment), surface contaminants are removed by pickling to allow the passive film to reform in air, which it does almost immediately. Most of the ferritic and martensitic stainless steels have limited corrosion resistance in marine environments, but some of the newly developed ferritic grade s (often called “superferritics”) have excellent marine corrosion resistance and are widely used in applications such as tubes for power plant condensers.

1.7.1 Stainless Steel Weld Decay

This type of intergranular corrosion can occur in the heat-affected zone of welded components and also in cast components of stainless steel due to precipitation, during cooling, of chromium carbides at the grain boundaries (and hence loss of chromium in the immediately-adjacent zone). The local loss in corrosion resistance arises because the chromium is crucial in promoting the formation of a Cr-rich passive film on the surface of stainless steels. The susceptibility to weld decay can be counteracted by carrying out a suitable post-weld heat treatment to restore a uniform composition at the grain boundaries but this is clearly often not a practicable proposition. Consequently the usual strategy in combating weld decay is by the choice of stainless steel with either of the two following features:

specification of a stainless steel containing a small amount of either titanium or niobium; which have a higher affinity than does chromium for carbon: hence carbides of these elements tend to form instead of chromium carbides, thus avoiding the Cr-depletion problem: such steels are usually termed “stabilised stainless steels”

specification of a stainless steel with low carbon content (< 0.03%); this will clearly decrease the likelihood of carbide formation in the steel. Such low-carbon grades of stainless steel are often designated by a “L” in their code; for instance the “316” grade of steel (18%Cr/10Ni/2.5Mo) is designated as “316L” when its carbon content has been limited in this way.


2 Reference Electrodes

2.1 Reference electrode: DefinitionA reference electrode is used in measuring the working electrode potential of an electrochemical cell. A reference electrode should have a stable electrochemical potential as long as no current flows through it. The most common laboratory reference electrodes are the Saturated Calomel Electrode (SCE) and the Silver/Silver Chloride (Ag/AgCl) electrodes. In field probes, a (a piece of the working electrode material) can be used. A Luggin capillary is often used to position the sensing point of a reference electrode to a desired point in a cell.

2.2 Standard Electrode PotentialsStandard potential differences are the actual cell potential differences measured in reversible cells under standard conditions. For solid or liquid compounds or elements, standard conditions are the pure compound or element; for gases they are 100 kPa pressure, and for solutes they are the ideal 1 molar (mol/liter) concentration.

Tables of standard electrode potentials can be obtained if any one electrode, operated under standard conditions, is designated as the standard electrode or standard reference electrode with which all other electrodes will be compared. This electrode is called the standard hydrogen electrode, abbreviated SHE The potential difference across a reversible cell made up of any electrode and a Standard hydrogen electrode (SHE) is called the reversible potential of that electrode, E.

If this other electrode is also being operated under standard conditions of pressure and concentration, then the reversible potential difference across the cell is the standard electrode potential E0 of that electrode. In many practical potential measurements, the standard hydrogen electrode cannot be used because hydrogen reacts with other substances in the cell or because other substances in the cell react with the catalytic platinum electrode surface upon which the H+/H2 potential is established.

It is often much more convenient to use alternative electrodes whose potentials are precisely known with respect to the SHE Two of the electrodes most commonly used for this purpose are the Ag/AgCl electrode, //AgCl(s),Cl-/Ag(c) at E0 = +0.2224 V vs. SHE, and the saturated calomel electrode (SCE) at 0.241 V vs. SHE The effect of changing the reference electrode is to change the zero of a potential scale while leaving the relative positions of all of the potentials unchanged.


Equilibrium reaction of the main reference electrodes used in corrosion

Name Equilibrium reaction

Hydrogen 2 H+ + 2 e- = H2

Silver chloride AgCl + e- = Ag + Cl-

Calomel Hg2Cl2 + 2 e- = 2 Hg + 2 Cl-

Mercurous sulfate Hg2SO4 + 2 e- = 2 Hg + SO4

2-

Mercuric HgO + 2 e- + 2 H+ = Hg + H2O

Copper sulfate Cu2+ + 2 e- = Cu (sulfate solution)

The potential corresponding to these half-reactions can be calculated from basic thermodynamic data by . first calculating the energy at a function of temperature for all chemical species involved then using Nernst equation to adjust for non standard activities or concentrations.

2.3 Nernst EquationThe Nernst equation, named after the German chemist Walter Nernst, can be derived from the equation linking free energy changes to the reaction quotient:

where, for a generalised equation of the form:

aA + bB + ... mM + nN + ..

and the reaction quotient:

where aM, aN, ..., aA, aB ... are the activities of the respective species in the generalized equation. The power terms of these activities correspond to the coefficients in the same equation. Some of the species that take part in electrode reactions are pure solid compounds and pure liquid compounds. In dilute aqueous solutions, water can be treated as a pure liquid. For pure solid compounds or pure liquid compounds, activities are constant and their values are considered to be one. The activities of gases are usually taken as their partial pressures and the activities of solutes such as ions are usually taken as their molar concentrations: i.e. ai = i [i] [i] where [i] and i are respectively the molar concentration and the activity coefficient of species i.


In the case of an electrochemical reaction, substitution of the relationships G = -nFE and G0 = -nFE0 into the expression of a reaction free energy and division of both sides by -nF gives the Nernst equation for an electrode reaction:

Combining constants at 25oC (298.15 K) gives the simpler form of the Nernst equation for an electrode reaction at this standard temperature:

In this equation, the electrode potential E is the actual potential difference across a cell in which this electrode and a standard hydrogen electrode are present. Alternatively two Nernst equations corresponding to two half-cell reactions can be combined into the Nernst equation for a cell reaction:

2.3.1 Stability of Water

The following equation describes the equilibrium between hydrogen ions and hydrogen gas in an aqueous environment:

2 H+ + 2 e- = H2

which can be rewritten as following in neutral or alkaline solutions:

2 H2O + 2 e- = H2 + 2 OH-

At higher pH than neutral, this second equation is more appropriate. However both equations signify the same reaction for which the thermodynamic behaviour can be expressed by a Nernst equation:

that transforms into the following at 25oC and hydrogen partial pressure (p) of 1 atm:

These equations delineate the stability of water in a reducing environment and are represented in a graphical form by the sloping line (a) on Pourbaix diagrams. Below line (a) the equilibrium reaction indicates that the


decomposition of H2O into hydrogen is favored while it is thermodynamically stable above that line. As potential becomes more positive or noble, water can be decomposed into its other constituent, i.e. oxygen. The equations representing respectively the acidic form and neutral or basic form of this equilibrium are written as:

O2 + 4 H+ +4 e- = 2 H2O

O2 + 2 H2O +4 e- = 4 OH-

And again these equivalent equations can be used to develop a Nernstexpression of the potential in standard conditions of temperature and oxygen pressure:

The line labelled (b) in Pourbaix diagrams represents the behaviour of E vs. pH for this last equation.

2.3.2 Chemical Thermodynamics

One can use thermodynamics, e.g. Pourbaix or E-pH diagrams, to evaluate the theoretical activity of a given metal or alloy provided the chemical make-up of the environment is known. But for practical situations, it is important to realise that the environment is a variable that can change with time and conditions. It is also important to realise that the environment that actually affects a metal corresponds to the micro-environmental conditions this metal really 'sees', i.e. the local environment at the surface of the metal.

It is indeed the reactivity of this local environment that will determine the real corrosion damage. Thus, an experiment that would investigate only the nominal environmental condition without consideration for local effects such as flow, pH cells, deposits, and galvanic effects is useless for lifetime prediction.

2.3.3 Pourbaix Diagrams

Building a Pourbaix or E-pH diagram to represent the stability of a metal or an alloy in a given environment is not an insurmountable task. However it could take a few hours of your precious time to produce what is commonly a universally accepted tool to discuss the expected behaviour of metals, many Thanks to the well known Belgium scientist that gave his name to these diagrams.


The process of building these diagrams should always follow the following steps:

1. Study background reference material on the metal/environment of choice. For the Iron-water system we found four acceptable references.

2. Decide on the species that will be considered. For the Iron-water system the data representing the species considered is abundantly available.

3. Decide on the target state of the species considered. For many metals and alloys there are different levels of hydration in the scale of stability. The Iron-water system is typically described in two states of hydration, i.e. wet and dry. The addition of extraneous soluble species such as commonly present chloride and sulfate ions can greatly complicate the thermodynamic picture.

4. Write down the equations interrelating the chemical species corresponding to the state chosen.

5. Well, now that the easy part is done, one has to go over a few sleepless nights to come to the diagram below.

2.3.4 Pourbaix Diagram of Iron at 25oC

The following diagram, produced with the KTS Thermo Excel add-on and modified for the Internet, describes the potential-pH equilibrium diagram for the system iron-water at 25oC considering only the hydrated forms of the possible s. The gray zone describes the region of stability of the base metal (Fe or iron), also called the immunity region according to Pourbaix. The orange zone indicates where one could expect to see rust, a non-protective form of corroded iron.

Thermodynamic principles can help explain a corrosion situation in terms of the stability of chemical species and reactions associated with corrosion processes. However, thermodynamic calculations cannot be use to predict corrosion rates. When two are put in contact, they can produce a voltage as in a battery or electrochemical cell causing galvanic corrosion. The material lower in what has been called the 'galvanic series' will tend to become the anode and corrode while the material higher in the series will tend to support a cathodic reaction. Iron or aluminium, for example, will have a tendency to corrode when connected to graphite or platinum. What the series cannot predict is the rate at which these metals corrode. Electrode kinetic principles have to be used to estimate these rates.


2.3.5 E-pH Diagram of Iron at 25oC

The pale blue region indicates where the most stable iron species is Fe(OH)2, a rarely encountered and highly soluble form of corroded iron, also called 'blue rust' or 'green rust'. The white regions indicate where soluble species would be predominant at concentrations of either 1, 0.01, 0.0001 or 0.000001 molar.

2.3.6 Reference Electrode Potentials

A chart for the conversion of different electrode potentials is shown on the next page.


3 Corrosion in ConcreteContrary to common belief, concrete itself is a complex composite material. It has low strength when loaded in tension and hence it is common practice to reinforce concrete with steel, for improved tensile mechanical properties. Concrete structures such as bridges, buildings, elevated highways, tunnels, parking garages, offshore oil platforms, piers and dam walls all contain reinforcing steel (rebar). The principal cause of degradation of steel reinforced structures is corrosion damage to the rebar embedded in the concrete.

Iron is unstable in nature, and because reinforcing steel used in precast concrete is made largely of iron, it, too, becomes unstable when exposed to corrosive agents such as salt, carbonation, and even air. Iron, as we commonly recognise it, is not generally found in nature because of its instability. It takes a great deal of energy to produce iron from its ore, and even then it is so unstable that it must be coated to keep it from reverting back to its ore forms (hematite, magnetite, and limonite).

The two most common causes of reinforcement corrosion are (i) localised breakdown of the passive film on the steel by chloride ions and (ii) general breakdown of passivity by neutralisation of the concrete, predominantly by reaction with atmospheric carbon dioxide. Sound concrete is an ideal environment for steel but the increased use of deicing salts and the increased concentration of carbon dioxide in modern environments principally due to industrial pollution, has resulted in corrosion of the rebar becoming the primary cause of failure of this material. The scale of this problem has reached alarming proportions in various parts of the world.

3.1 Magnitude of the Rebar Corrosion ProblemsIt was recognised by the mid 1970s that the corrosion of concrete structures was caused by the corrosion of the reinforcing steel in the concrete which, in turns, was induced by the intrusion of even a small amount of chloride from the deicing salts into the concrete. It is difficult to estimate the cost of these corrosion-related damages to conventionally reinforced and prestressed concrete bridge components in the nation. According to a 1997 report, of the 581,862 bridges in and off the U.S.A. federal-aid system, about 101,518 bridges were rated as structurally deficient. Most of these bridges were not in danger of collapse, but they were likely to be load posted so that overweight trucks will be required to take a longer alternative route.

The estimated cost to eliminate all backlog bridge deficiencies (including structurally and functionally) was approximately $78 billions, and it could increase to as much as $112 billions, depending on the number of years it takes to meet the objective. The average annual cost, through year 2011, for just maintaining the overall bridge conditions, i.e., the total number and the distribution of structurally and functionally deficient bridges, was


estimated to be $5.2 billions. While corrosion of the reinforcing steel was not the sole cause of all structural deficiencies, it was a significant contributor and has therefore becomes a matter of major concern.

The magnitude of this corrosion problem in the transportation infrastructure has increased significantly in the last three decades and is likely to keep increasing. Even though the cost of maintaining bridge decks is becoming prohibitively expensive, the benefits provided by deicing salts are too great, however, that it's use is not likely to decrease in the future. In fact, the use of road deicing salts, which are extremely corrosive due to the disruptive effects of its chloride ions on protective films on metals, has actually increased in the first half of the 1990s-after a levelling off during the 1980s.

3.2 Lime Cements, Plasters, Mortars and Concretes There is a plethora of terms used to describe the various products derived from calcined limestone. A definition of terms used in the context of this report is outlined below. The definitions here are based on features identifiable in hand specimen and are therefore intended for use by the field archaeologist. Consequently these definitions may differ somewhat from those applied by scientists employing microscopic and chemical techniques.

3.2.1 Limestone

Limestone is the natural rock type from which cements and concrete are derived. A limestone is a sedimentary rock composed of carbonates, namely the minerals calcite (calcium carbonate; CaCO3) and dolomite (calcium magnesium carbonate; CaMg[CO3]2), derived either chemically or organically. Being natural materials, limestone can have a wide range of depositional environment and components and can contain varying amounts of non-carbonate material. The type of limestone calcined to produce lime for the manufacture of cements and concrete can profoundly affect the durability and properties of the material produced.

3.2.2 Lime

"Calx", lime in Latin provides the etymological root for calcium, calcite and calcination. Strictly speaking, lime is calcium oxide. This is acquired by burning limestone, which in simplest terms, removes the carbon from the calcium carbonate (calcite). Lime forms the base for all cements and concrete. However the composition of the limestone being variable, the true composition of the lime is also variable, and the term may be generally used to described calcined limestone in general. Lime may also referred to as 'quicklime', 'unslaked lime' or 'lump lime'.

3.2.3 Cement

"Cement" is derived from caementa, which actually referred to the aggregates mixed with the slaked lime rather than the bonding agent itself. Opus caementicium refers to the masonry constructed from concrete


coursework. In the context of this work, the word cement is used exclusively in reference to the hardened binding material of any aggregate. In simplest terms, this is the slaked lime, which in the presence of air reverts to calcium carbonate. This material may also contain finely powdered admixtures, such as ash or fired ceramic. However, an alternative term for the hydrated lime binder is not put forward. In geological parlance, the term cement is used strictly to define material (often calcite, but occasionally silica) that adheres clasts in a rock, and generally the word �cement� is accepted in non-specialised use as a glue. Consequently it is defined as the adhesive binder in this report.

3.2.4 Aggregate

An aggregate is material added to a cement. In this report, the term does not include finely powdered additives to the cement such as ash. It is usually composed of rock fragments, chosen for their strengthening properties and occasionally for decorative reasons. Carefully chosen aggregates can make a concrete or mortar resemble natural rock. Organic material, including grasses, reeds and also bones can be used as aggregates, often in combination with rock material. Aggregates can be sub classed into categories of fine aggregates - that with dimensions less than 5 mm, and coarse aggregates - that with dimensions greater than 5 mm.

3.2.5 Concrete and Mortar

Both concrete and mortar are materials composed of a cement plus an aggregate. The two terms are very simply defined. A concrete is a material where the majority of the aggregate has dimensions greater than 5 mm. A mortar is a material having aggregate with dimensions less than 5 mm (Prentice, 1990). "mortar� from the Latin mortarium, originally referring to the trough in which the material was mixed, is often used wholesale to describe the bonding material of concrete masonry. This is a fair use of the term as this material binding brick-sized blocks, often laid in courses rather than haphazardly poured, is composed of lime cement with fine aggregate. Concrete should be used to describe material containing coarse aggregate generally used to fill formwork.

3.2.6 Hydraulic Cements

Hydraulic cements are waterproof and will even set underwater. Consequently most materials used in Roman and later periods for lining structures intended to carry water and for construction in marine and riparian environments will be hydraulic. Such materials are identified by the presence of finely pulverized material added to the cement binder which will cause discoloration from white to pale browns or pinks. Common additives, are volcanic ash or crushed ceramic sherds. These materials are known as pozzolans.


3.2.7 Pozzolans

A pozzolan is defined as a siliceous and/or aluminous substance that will, in the presence of water combine with lime to form cementitious compounds. Such material include clays, that are rendered active by firing (fresh geological clays would absorb too much water during the curing process, resulting in spallation and ultimately cracking of the concrete), and other substances, including waste products from blast furnaces and even rice husk ash (see Hill, et al., 1992 and references therein). However, natural pozzolans are derived primarily from volcanogenic products.

3.2.8 Plaster

The term "plaster" is one of the most universally used, describing a multitude of products, usually those which will provide a smooth coat to a wall or other surface. Plaster of Paris is powdered gypsum (CaSO4.2H2O, derived from rocks distinctly different from limestones) which, with water added, will harden and set. Lime plaster is simply a mixture of lime, water and sand, or just lime and water in the case of whitewash. To avoid confusion, it is recommended that these terms should be used in full to define compositional types.

3.2.9 Stucco

Stucco, like plaster, is a term of complex use. However it is entrenched in all literature as describing two distinct materials. Firstly it is used to refer to a fine white mortar composed of lime, crushed marble and glue-like binding additive (egg-white for example) typically used for making good surfaces for painting frescoes or generally for smoothing walls. Alternatively it is used to describe decorative work in plaster of Paris. In this report, stucco will be used to define the former case alone.

3.3 Nature of the ProblemIn order to understand the mechanisms behind corrosion of reinforcing steel in concrete, one has to examine the chemical reactions involved. In concrete, the presence of abundant amount of calcium hydroxide and relatively small amounts of alkali elements, such as sodium and potassium, gives concrete a very high alkalinity-with pH of 12 to 13. It is widely accepted that, at the early age of the concrete, this high alkalinity results in the transformation of a surface layer of the embedded steel to a tightly adhering film, that is comprised of an inner dense spinel phase in epitaxial orientation to the steel substrate and an outer layer of ferric hydroxide. As long as this film is not disturbed, it will keep the steel passive and protected from corrosion.

When a concrete structure is often exposed to deicing salts, salt splashes, salt spray, or seawater, chloride ions from these will slowly penetrate into the concrete, mostly through the pores in the hydrated cement paste. The chloride ions will eventually reach the steel and then accumulate to beyond a certain concentration level, at which the protective film is destroyed and


the steel begins to corrode, when oxygen and moisture are present in the steel-concrete interface.

In 1962, it was reported that the required minimum concentration of chloride in the concrete immediately surrounding the steel to initiate corrosion, the chloride corrosion threshold, is 0.15% soluble chloride, by weight of cement. In typical bridge deck concrete with a cement factor of 7, this is equivalent to 0.025% soluble chloride, by weight of concrete, or 0.59 kg soluble chloride per cubic meter of concrete. Subsequent research at FHWA laboratories estimated the corrosion threshold to be 0.033% total chloride, by weight of concrete.

There are indications that the chloride corrosion threshold can vary between concrete in different bridges, depending on the type of cement and mix design used, which can vary the concentrations of tricalcium aluminate (C3A) and hydroxide ion (OH-) in the concrete. In fact, it has been suggested that because of the role that hydroxide ions play in protecting steel from corrosion, it is more appropriate to express corrosion threshold in terms of the ratio of chloride content to hydroxide content, [Cl-] / [OH-], which was recently established to be between 2.5 to 6.

Once corrosion sets in on the reinforcing steel bars, it proceeds in electrochemical cells formed on the surface of the metal and the electrolyte or solution surrounding the metal. Each cell is consists of a pair of electrodes (the anode and its counterpoint, the cathode) on the surface of the metal, a return circuit, and an electrolyte. Basically, on a relatively anodic spot on the metal, the metal undergoes oxidation (ionization), which is accompanied by production of electrons, and subsequent dissolution. These electrons move through a return circuit, which is a path in the metal itself to reach a relatively cathodic spot on the metal, where these electrons are consumed through reactions involving substances found in the electrolyte. In a reinforced concrete, the anode and the cathode are located on the steel bars, which also serve as the return circuits, with the surrounding concrete acting as the electrolyte.

Corrosion can also occur even in the absence of chloride ions. For example, when the concrete comes into contact with carbonic acid resulting from carbon dioxide in the atmosphere, the ensuing carbonation of the calcium hydroxide in the hydrated cement paste leads to reduction of the alkalinity, to pH as low as 8.5, thereby permitting corrosion of the embedded steel:

The rate of carbonation in concrete is directly dependent on the water/cement ratio (w/c) of the concrete, i.e., the higher the ratio the greater is the depth of carbonation in the concrete. In concrete of reasonable quality, that is properly consolidated and has no cracking, the expected rate of carbonation is very low. For example, in concrete with w/c of 0.45 and concrete cover 25 mm, it will require more than 100 years for carbonation to reach the concrete immediately surrounding the steel. Carbonation of concrete or mortar is more of an issue in Europe than in North America.


3.4 Corrosion Control in ConcreteGiven the importance of the costs associated with the corrosion of infrastructures, it is extremely important that all possible methods applicable to controlling corrosion in existing concrete bridges be developed so that these structures will not deteriorate prematurely. Equally important is developing methods to avoid this costly corrosion problem in all new concrete bridges to be constructed in the future. Accordingly, the control methods can be divided into two major areas:

Corrosion control in new concrete constructions,

Corrosion control for rehabilitation of existing concrete structures.

The use of good construction design and procedures, adequate concrete cover depth, corrosion-inhibiting admixture, and low-permeability concrete alone will not abate the problem, because concrete has a tendency to crack inordinately. Even corrosion-inhibiting admixture for concrete would likely not be of use when the concrete cracked. This situation essentially leaves the reinforcing steel itself as the last line of defense against corrosion. For this very reason, the use of a barrier system on the reinforcing steel, such as epoxy coating or other organic or even other possible metallic coatings, is even more critical in abating this costly corrosion problem.

It is likely that there may never be any organic coating that can hold up to the extreme combination of constant wetting and high temperature and high humidity that reinforcing steel is often exposed to in the marine environments. The many successful performance of embedded epoxy-coated steel bars in different projects indicates that when used in exposure conditions that do not keep the concrete constantly wet, the epoxy coating will provide a certain degree of protection to the steel bars and, thereby, delay the initiation of corrosion.

For existing chloride-contaminated concrete bridge decks, impressed-current cathodic protection-using titanium mesh anodes-provides the ultimate and permanent solution to stopping reinforcing steel corrosion in the structures, as long as associated rectifiers and electrical wiring are properly maintained. Electrochemical chloride extraction provides an alternative rehabilitation method for stopping steel corrosion in contaminated concrete, albeit less permanently. This alternative has the advantage of having no rectifier or wiring to maintain after the treatment.

3.5 Portland CementPortland cement is a closely controlled chemical combination of calcium, silicon, aluminum, iron and small amounts of other compounds, to which gypsum is added in the final grinding process to regulate the setting time of the concrete. Some of the raw materials used to manufacture cement are limestone, shells, and chalk or marl, combined with shale, clay, slate or blast furnace slag, silica sand, and iron ore. Lime and silica make up approximately 85 percent of the mass (1).


The term "Portland" in Portland cement originated in 1824 when an English mason obtained a patent for his product, which he named Portland Cement. This was because his cement blend produced concrete that resembled the color of the natural limestone quarried on the Isle of Portland in the English Channel.

Different types of Portland cement are manufactured to meet different physical and chemical requirements for specific purposes. The American Society for Testing and Materials (ASTM) Designation C 150 provides for eight types of Portland cement.

3.5.1 TYPE I

Type I is a general purpose Portland cement suitable for all uses where the special properties of other types are not required. It is used where cement or concrete is not subject to specific exposures, such as sulfate attack from soil or water, or to an objectionable temperature rise due to heat generated by hydration. Its uses include pavements and sidewalks, reinforced concrete buildings, bridges, railway structures, tanks, reservoirs, culverts, sewers, water pipes and masonry units.

3.5.2 TYPE II

Type II Portland cement is used where precaution against moderate sulfate attack is important, as in drainage structures where sulfate concentrations in groundwaters are higher than normal but not unusually severe. Type II cement will usually generate less heat at a slower rate than Type I. With this moderate heat of hydration (an optional requirement), Type II cement can be used in structures of considerable mass, such as large piers, heavy abutments, and heavy retaining walls. Its use will reduce temperature rise, an important quality when the concrete is placed in warm weather.

3.5.3 TYPE III

Type III is a high-early strength Portland cement that provides high strengths at an early period, usually a week or less. It is used when forms are to be removed as soon as possible, or when the structure must be put into service quickly. In cold weather, its use permits a reduction in the controlled curing period. Although richer mixtures of Type I cement can be used to gain high early strength, Type III, high-early-strength portland cement, may provide it more satisfactorily and more economically.

3.5.4 TYPE IA, IIA, IIIA

Specifications for three types of air-entraining Portland cement (Types IA, IIA, and IIIA) are given in ASTM C 150. They correspond in composition to ASTM Types I, II, and III, respectively, except that small quantities of air-entraining materials are interground with the clinker during manufacture to produce minute, well-distributed, and completely separated air bubbles. These cements produce concrete with improved resistance to freeze-thaw action.


3.5.5 TYPE IV

Type IV is a low heat of hydration cement for use where the rate and amount of heat generated must be minimized. It develops strength at a slower rate than Type I cement. Type IV portland cement is intended for use in massive concrete structures, such as large gravity dams, where the temperature rise resulting from heat generated during curing is a critical factor.

3.5.6 TYPE V

Type V is a sulfate-resisting cement used only in concrete exposed to severe sulfate action -- principally where soils or groundwaters have a high sulfate content. The following Table describes sulfate concentrations requiring the use of Type V Portland cement. Low Tricalcium Aluminate (C3A) content, generally 5% or less, is required when high sulfate resistance is needed.

Table:Attack on concrete by soils and waters containing various sulfate concentrations

Relative Degree of Sulfate Attack

Percentage Water-Soluble Sulfate (as SO4) in Soil

Samples

Sulfate (as SO4) in Water Samples, ppm

Cement Type

Negligible 0.00 to 0.10 0 to 150 I

Positive 0.10 to 0.20 150 to 1500 II

Severe 0.20 to 2.00 1500 to 10,000 V

Very Severe 2.00 or more 10,000 or more V plus pozzolan

3.6Acid Attack on ConcreteConcrete is susceptible to acid attack because of its alkaline nature. The components of the cement paste break down during contact with acids. Most pronounced is the dissolution of calcium hydroxide which occurs according to the following reaction:

2 HX + Ca(OH)2 -> CaX2 + 2 H2O (X is the negative ion of the acid)

The decomposition of the concrete depends on the porosity of the cement paste, on the concentration of the acid, the solubility of the acid calcium salts (CaX2) and on the fluid transport through the concrete. Insoluble calcium salts may precipitate in the voids and can slow down the attack. Acids such as nitric acid, hydrochloric acid and acetic acid are very aggressive as their calcium salts are readily soluble and removed from the attack front. Other acids such as phosphoric acid and humic acid are less harmful as their calcium salt, due to their low solubility, inhibit the attack by blocking the pathways within the concrete such as interconnected cracks, voids and porosity. Sulphuric acid is very damaging to concrete as it combines an acid attack and a sulfate attack.


Microscopic appearance

An acid attack is diagnosed primarily by two main features:

Absence of calcium hydroxide in the cement paste

Surface dissolution of cement paste exposing aggregates

Exposed aggregate at concrete surface. Ordinary polarised light.

Exposed aggregate at concrete surface. Crossed polarised light. Calcium hydroxide depletion

of cement paste. Crossed polarised light.


4 Highway Bridges4.1 Highway BridgesAccording to a 1997 report, of the 581,862 bridges in and off the U.S.A. federal-aid system, about 101,518 bridges were rated as structurally deficient. Most of these bridges were not in danger of collapse, but they were likely to be load posted so that overweight trucks will be required to take a longer alternative route. The estimated cost to eliminate all backlog bridge deficiencies (including structurally and functionally) was approximately $78 billions, and it could increase to as much as $112 billions, depending on the number of years it takes to meet the objective. The average annual cost, through year 2011, for just maintaining the overall bridge conditions, i.e., the total number and the distribution of structurally and functionally deficient bridges, was estimated to be $5.2 billions. While corrosion of the reinforcing steel was not the sole cause of all structural deficiencies, it was a significant contributor and has therefore becomes a matter of major concern.

The magnitude of this corrosion problem in the transportation infrastructure has increased significantly in the last three decades and is likely to keep increasing. Even though the cost of maintaining bridge decks is becoming prohibitively expensive, the benefits provided by deicing salts are too great, however, that it's use is not likely to decrease in the future. In fact, the use of road deicing salts, which are extremely corrosive due to the disruptive effects of its chloride ions on protective films on metals, has actually increased in the first half of the 1990s-after a levelling off during the 1980s.

According to the U.S. Department of Commerce Census Bureau, the dollar impact of corrosion on highway bridges is considerable. The annual direct cost of corrosion for highway bridges is estimated to be $6.43 billion to $10.15 billion, consisting of $3.79 billion to replace structurally deficient bridges over the next 10 years, $1.07 billion to $2.93 billion for maintenance and cost of capital for concrete bridge decks, $1.07 billion to $2.93 billion for maintenance and cost of capital for concrete substructures and superstructures (minus decks), and $0.50 billion for the maintenance painting cost for steel bridges. This gives an average annual cost of corrosion of $8.29 billion. Life-cycle analysis estimates indirect costs to the user due to traffic delays and lost productivity at more than 10 times the direct cost of corrosion. In addition, it was estimated that employing “best


maintenance practices” versus “average practices” can save 46 percent of the annual corrosion cost of a black steel rebar bridge deck, or $2,000 per bridge per year.

While there is a downward trend in the percentage of structurally deficient bridges (a decrease from 18 percent to 15 percent between 1995 to 1999), the costs to replace ageing bridges increased by 12 percent during the same period. In addition, there has been a significant increase in the required maintenance of the ageing bridges. Although the vast majority of the approximately 108,000 prestressed concrete bridges have been built since 1960, many of these bridges will require maintenance in the next 10 to 30 years. Therefore, significant maintenance, repair, rehabilitation, and replacement activities for the nation’s highway bridge infrastructure are foreseen over the next few decades before current construction practices begin to reverse the trend.

4.2 Conventional Concrete BridgesThe primary cause of reinforced-concrete bridge deterioration is chloride-induced corrosion of the black steel reinforcement, resulting in expansion forces in the concrete that produce cracking and spalling of the concrete. The chloride comes from either marine exposure or the use of deicing salts for snow and ice removal. Because the use of deicing salts is likely to continue, if not increase, little can be done to prevent bridge structures from being exposed to corrosive chloride salts. Therefore, bridge designsand concrete mixes must be resistant to chloride-induced corrosion. This can be accomplished by:

preventing chlorides from getting to the steel surface (physical barriers at the concrete surface, coating the rebar, or low chloride-permeable concrete),

making the concrete less corrosive at specific chloride levels (inhibitors or admixtures), or

making the rebar resistant to corrosion (corrosion-resistant alloys, composites, or clad materials).

Over the past 20 years, there has been a trend in new construction toward utilising higher quality concrete and more corrosion-resistant rebars. Longer bridge service life is currently achieved by using epoxy-coated rebars in the majority of new bridge construction, with the limited use of stainless steel-clad or solid rebars in more severe environments. The expected service life of a newly constructed bridge is typically 75 years and up to 120 years for stainless steel rebar construction. Admixtures to the concrete for the purpose of increased corrosion resistance have included corrosion-inhibiting admixtures and mineral admixtures such as silica fume. High-range water reducers permit the use of low water-cement ratio concretes that have lower permeability to corrosive agents and, thus, result in longer times to corrosion initiation of the rebar. Many of these methods are used in combination with each other to obtain a longer service life.


Many rehabilitation methodologies designed to extend the service life of bridges that have deteriorated due to corrosion of the reinforcing steel have been developed and put into practice within the past 25 years. These include cathodic protection, electrochemical chloride removal, overlays, and sealers. Although each of these methods have been shown to be successful, continuing developments are necessary to improve effectiveness and increase the life extension provided by these methods.

4.3 Pack RustPack rust is a form a corrosion typical of steel components that develop a crevice into an open atmospheric environment. This particular form of corrosion is often used in relation to bridge inspection to describe built-up members) of steel bridges which are already showing signs of rust packing between steel plates.

4.4 Prestressed Concrete BridgesWhereas some of the methods discussed for conventional reinforced-concrete bridges are applicable to prestressed concrete components (e.g., high-performance concrete and corrosion inhibiting admixtures), special consideration for corrosion prevention of prestressed reinforced-concrete bridges is required.

Most of these bridges are relatively new and their numbers are relatively low; therefore, the overall economic impact is not as significant as for conventional reinforced-concrete bridges. However, failure of the high-strength prestressing steel can compromise the integrity of the prestressed concrete bridge (corrosion-related deterioration compromising the structural integrity of a conventional concrete structure is highly unlikely). This makes close attention to construction details and subsequent monitoring and inspection of the prestressed concrete bridges critical.

Corrosion prevention of pretensioned structures is primarily accomplished through the use of high-performance concretes or the addition of corrosion-inhibiting admixtures. Remedial measures such as cathodic protection are possible as long as care is taken to prevent overprotection that can lead to hydrogen-induced cracking of the high-strength steel. Other measures such as electrochemical chloride removal cannot be used for prestressed concrete structures because of the relatively large amounts of hydrogen produced at the steel surface during the removal process.

Recent failures of post-tensioned structures have underscored the importance of maintaining void-free grouting of the tendons, especially near the anchorage. Maintaining the integrity of the post-tensioned tendon starts


with ensuring the integrity of the duct (typically polyethylene), followed by the application of a good-quality grout that is continuous around the strands. Placement of the grout is often more difficult when low water-cement ratio mixes and/or mineral admixtures are employed. Improved grouting practices are continuing to be developed. In addition, the use of corrosion-inhibiting admixtures can provide added protection against corrosion of the prestressing steel strands. Note that in August 2001, the American Segmental Bridge Institute conducted a 3-day training school for certifying grouting specialists. This training school will be held in the future once or twice a year.

4.5 Steel BridgesThe primary cause of corrosion of steel bridges is the exposure of the steel to atmospheric conditions. This corrosion is greatly enhanced due to marine (salt spray) exposures and industrial environments. The only corrosion prevention method for these structures is to provide a barrier coating (paint).

Changes in environmental protection regulations have brought about transformation of the approach to corrosion protection for steel bridges. Until the mid- to late-1970s, virtually all steel bridges were protected from corrosion by multiple thin coats of lead- and chromate-containing alkyd paints applied directly over mill scale on the formed steel. Maintenance painting for prevention of corrosion was rare and primarily was practised on larger bridge structures. Since the majority of the steel bridges in the interstate highway system were constructed between 1950 and 1980, most of these structures were originally painted in this manner; therefore, a large percentage of the steel bridges in the interstate system are protected from corrosion by a coating system that is now beyond its useful service life.

Moreover, the paint system commonly used for steel bridge members contains chromium and lead and can no longer be used because of the effects it has on humans and the environment. The bridge engineers have a choice of either replacing the lead-based paints with a different coating or painting over the deteriorating areas. Removal of lead-based paint incurs high costs associated with the requirements to contain all the hazardous waste and debris. Developments include:

improved and environmentally safe coating systems and

methodologies to optimise the use of these systems, such as “zone” painting (adjusting coating types and maintenance schedules based on the aggressiveness of the environment within different zones on the bridge).

Overpainting techniques to eliminate the cost of expensive paint removal also have been developed.


4.6 Hydrogen Embrittlement (Prestressing Steels!)This is a type of deterioration which can be linked to corrosion and corrosion-control processes. It involves the ingress of hydrogen into a component, an event that can seriously reduce the ductility and load-bearing capacity, cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Hydrogen embrittlement occurs in a number of forms but the common features are an applied tensile stress and hydrogen dissolved in the metal. Examples of hydrogen embrittlement are cracking of weldments or hardened steels when exposed to conditions which inject hydrogen into the component. Presently this phenomenon is not completely understood and hydrogen embrittlement detection, in particular, seems to be one of the most difficult aspects of the problem. Hydrogen embrittlement does not affect all metallic materials equally. The most vulnerable are high-strength steels, titanium alloys and aluminum alloys.

4.6.1 Sources of Hydrogen

Sources of hydrogen causing embrittlement have been encountered in the making of steel, in processing parts, in welding, in storage or containment of hydrogen gas, and related to hydrogen as a contaminant in the environment that is often a by-product of general corrosion. It is the latter that concerns the nuclear industry. Hydrogen may be produced by corrosion reactions such as rusting, cathodic protection, and electroplating. Hydrogen may also be added to reactor coolant to remove oxygen from reactor coolant systems. Hydrogen entry, the obvious pre-requisite of embrittlement, can be facilitated in a number of ways summarised below:

by some manufacturing operations such as welding, electroplating, phosphating and pickling; if a material subject to such operations is susceptible to hydrogen embrittlement then a final, baking heat treatment to expel any hydrogen is employed

as a by-product of a corrosion reaction such as in circumstances when the hydrogen production reaction acts as the cathodic reaction since some of the hydrogen produced may enter the metal in atomic form rather than be all evolved as a gas into the surrounding environment. In this situation, cracking failures can often be thought of as a type of stress corrosion cracking. If the presence of hydrogen sulfide causes entry of hydrogen into the component, the cracking phenomenon is often termed “sulphide stress cracking (SSC)”

the use of cathodic protection for corrosion protection if the process is not properly controlled.

4.6.2 Hydrogen Embrittlement of Stainless Steel

Hydrogen diffuses along the grain boundaries and combines with the carbon, which is alloyed with the iron, to form methane gas. The methane gas is not


mobile and collects in small voids along the grain boundaries where it builds up enormous pressures that initiate cracks. Hydrogen embrittlement is a primary reason that the reactor coolant is maintained at a neutral or basic pH in plants without aluminium components.

If the metal is under a high tensile stress, brittle failure can occur. At normal room temperatures, the hydrogen atoms are absorbed into the metal lattice and diffused through the grains, tending to gather at inclusions or other lattice defects. If stress induces cracking under these conditions, the path is transgranular. At high temperatures, the absorbed hydrogen tends to gather in the grain boundaries and stress-induced cracking is then intergranular. The cracking of martensitic and precipitation hardened steel alloys is believed to be a form of hydrogen stress corrosion cracking that results from the entry into the metal of a portion of the atomic hydrogen that is produced in the following corrosion reaction.

Hydrogen embrittlement is not a permanent condition. If cracking does not occur and the environmental conditions are changed so that no hydrogen is generated on the surface of the metal, the hydrogen can rediffuse from the steel, so that ductility is restored.

To address the problem of hydrogen embrittlement, emphasis is placed on controlling the amount of residual hydrogen in steel, controlling the amount of hydrogen pickup in processing, developing alloys with improved resistance to hydrogen embrittlement, developing low or no embrittlement plating or coating processes, and restricting the amount of in-situ (in position) hydrogen introduced during the service life of a part.


5 Atmospheric CorrosionComponents exposed to the elements will inevitably experience damage due to atmospheric corrosion. The severity of the corrosion and the rate at which corrosion will take place are dependant primarily upon the properties of the surface formed electrolytes, which in turn are dependant upon factors such as the humidity and pollution levels in the atmosphere.

Assessing the corrosivity of a particular atmosphere can be relatively simple. Two methods have been developed to deal with this problem:

The first method involves the exposure of metallic coupons to the environment and classifying the resultant corrosion, e.g. the coupon based method (CLIMAT) based on the wire on bolt ASTM standard.

The second method involves measuring several atmospheric parameters and classifying the atmosphere according to standardised measurements, e.g. ISO 9223.

This module contains some examples describing CLIMAT results and some weather data estimations according to the ISO methodology for locations typical of marine and rural environments.

5.1 Atmospheric Corrosion MechanismAtmospheric corrosion is an electrochemical process, requiring the presence of an electrolyte. Thin film "invisible" electrolytes tend to form on metallic surfaces under atmospheric corrosion conditions, when a certain critical humidity level is reached. For iron, this level is around 60%, in unpolluted atmospheres. The critical humidity level is not a constant - it depends on the corroding material, the hygroscopic nature of corrosion products and surface deposits and the presence of atmospheric pollutants.


In the presence of thin film electrolytes, atmospheric corrosion proceeds by balancing anodic and cathodic reactions. The anodic oxidation reaction involves the dissolution of the metal in the electrolyte, while the cathodic reaction is often assumed to be the oxygen reduction reaction. Oxygen from the atmosphere is readily supplied to the electrolyte, under thin film corrosion conditions.

5.2 Corrosivity Maps and Pollution InformationMaps have been produced for numerous geographic regions, illustrating the macroscopic variations in atmospheric corrosivity and pollution levels.

Marine atmospheres are usually highly corrosive, and the corrosivity tends to be significantly dependent on wind direction, wind speed and distance from the coast. However, an equivalently corrosive environment is created by the use of deicing salts on the roads of many cold regions of the planet. Traditional techniques for monitoring the pollutants sulfur dioxide and atmospheric chloride that influence corrosivity need to be conducted for extended periods to be site representative, and are consequently expensive. Furthermore, measurements made at limited numbers of sites do not adequately represent the variations in pollutant levels across a region or city.

However the exposure of standard metal specimens at a grid of sites and the generation of computer contoured corrosion maps has been shown to be a sensitive and cost-effective means of differentiating geographical variations in corrosivity, which is a measure of the aggressiveness of the environment. Such specimens can also be regarded as receptors for airborne pollutants and provide a means for readily characterising their geographical variations. The techniques for doing this were described in a paper "Contour Mapping the Sulfur and Chlorine Contents of Steel Corrosion Products - A New Approach for Characterising the Atmospheric Environment" by G. King, M Spicer and P Kao.

5.2.1 Germany

Despite the remarkable progress the Germans have made in reducing air pollution since the 1960s, there are still challenges to be faced. The nation’s environmental problems have clearly increased following unification with the east. The costs of unification to the economy overall have reduced their ability to address environmental issues. In addition, some areas of the east have suffered tremendous environmental degradation over the years. A number of eastern air problems, such as sulfur dioxide and particulate emissions, have already been brought under control as a result of the initial decline in economic, particularly industrial activity, but there are still significant environmental difficulties in the region.

A continuing air pollution problem nationwide for the Germans is increasing motor vehicle usage and its resulting air pollution. Despite stringent emissions controls, an increase in the number of vehicles, as well as their increased use, makes it seem unlikely that the nation will meet its air


quality standards for ozone. The eastern region of the country in particular is expected to have increasing emissions problems as motor vehicle use rises in response to increasing levels of economic activity and the opening of an extensive new highway system.

5.2.2 Corrosion in UK

The following map was very freely constructed from the Millennium map of UK corrosion of zinc.

UK Government figures released recently show urban air quality was the best since records began. In urban areas in year 2000, there were 17 days of moderate or higher air pollution on average per site, compared to 30 days in 1999 and 23 in 1998. The main causes of moderate or higher air pollution at urban sites are ozone, particulate matter and sulfur dioxide. The UK established National Air Quality Standards in 1993. The standards represent defined levels which avoid significant risks to health. As levels increase


above the standard, the likelihood of effects on health increases. Levels of ozone in the high band may cause coughing and discomfort on deep breathing during exercise in some people. In some North American cities, the problem has been attributed to premature deaths. In Toronto, for example, 1,000 people died and another 5,500 were hospitalised in 2000 because of air pollution, according to the Toronto Board of Health.

The United Kingdom has emphasised pollution control rather than pollution prevention in its environmental policy-making. The British recently have distinguished themselves among the industrialised nations in environmental policy-making through the adoption of an integrated approach to pollution prevention that collectively regulates all types of pollution from a single source. Environmental policies in the U.K. have involved the use of direct regulation, economic instruments, and international agreements. The U.K. has made an effort to adopt and implement all EC environmental directives and increasingly is a strong supporter of an even more integrated European environmental program.

As in all the other nations we have considered, despite some improvements in air quality over the previous two decades, air pollution in the United Kingdom, particularly in urban areas, continues to increases as a result of the growth in motor vehicle use (in 1970 there were some 10 million passenger cars in the country, by 1994 there were well over 20 million). The transport infrastructure has increased over this time period as well. Currently, road transport accounts for over 90 percent of passenger travel within Britain and for over 80 percent of freight delivery and the amount of traffic per year has nearly doubled. For six of the eight air pollutants identified in the air quality strategy, transport is either the major source, or a significant source, of these pollutants in urban areas.

5.2.3 Pollution in Russia

The extent of pollution and ecological collapse in Russia is due to decades of ill-considered military and industrial development undertaken in virtual secrecy and with scant concern for the environmental and health consequences. Environmental pollution clamps a stranglehold on the big cities in Russia. Pollution in Russia now threatens the health of millions of citizens and the safety of crops, water and air. In 84 of Russia's largest cities the air pollution is ten times the accepted safety levels. In some areas, especially among children, levels of respiratory problems are 50 per cent higher than the national average. Moreover, Russia is a major contributor to global ozone depletion, being the World's largest producers and consumers of ozone depleting substances (ODS). Thus, Russia's emphasis on production at all costs has cost this country its environmental integrity.

Air pollution is a severe problem in several Russian cities. In 1999, for instance, air quality in 120 cities was recorded to be at least five times above the country's own lenient standards for at least one pollutant; eight of these cities exceeded limits for three or more pollutants. Although the industrial sector remains the major contributor to Russia's air pollution, the transportation sector is playing an increasingly important role. Motor


vehicles are subject to only minimal environmental regulations, and automobile emissions in major cities, including lead, carbon monoxide and nitrogen oxides, are major sources of air pollution.

In Moscow, for instance, automobiles cause almost 90% of air pollution, and car ownership is on the rise. Additionally, most power plants in Russia are aging and lack modern pollution control equipment, resulting in large amounts of toxic emissions and waste. Several major cities are threatened by these problems, as are delicate ecosystems. Lake Baikal serves as one example of areas threatened by pollution. The lake holds 20% of the world's freshwater and is home to 1,500 species, most of which are unique to Baikal. The lake is threatened by runoff and air pollution from both a cellulose production plant on one of Baikal's major tributaries, and a coal-fired electric power plant on another.

5.3 Exposure TestingThere are many factors to consider when selecting a weathering test station to conduct a test program. These can be summarized into two categories:

Location: An ideal test site should be located in a clean pollution free area, if pollution is not deemed to be a parameter, within the geo-climatic region to be used. This is important for the prevention of unnatural effects on the specimens. Within the local area chosen, there must be no isolated sources of pollution or deleterious atmospheric contamination.

Maintenance: The exposure maintenance program followed by the test site will also play a major role in determining the accuracy of testing. It is important for the specimens on the test racks to be correctly maintained. This involves ensuring the correct mounting method, and constant follow-up attention to maintain the quality.

For direct exposure the specimen is mounted on the exposure frame open backed or solid backed, and subject to all atmospheric effects. This type can be used at a number of exposure angles. The standard angles used are 45, 5, and 90 degrees, these angles being referenced from a horizontal angle of 0 degree. The angle chosen should be one that matches as closely as possible the position of the end use of the material. The racks should be cleaned on a regular basis to remove mildew and algae if these contaminate producers are present on the test site.

5.4 Direct Corrosivity AssessmentCLIMAT devices have been used for more than three decades to monitor atmospheric corrosivity. These units have been utilised successfully around the world in marine and industrial type atmospheres. The procedure was originally called a "wire-on-bolt" test, because the devices consist of a copper or steel bolt around which an aluminium wire is wrapped. It is the galvanic effect between aluminium and these bolt materials that accelerates the atmospheric corrosion of aluminium. The procedure is sufficiently sensitive to measure seasonal fluctuations in corrosion rates; the


"standard" CLIMAT test usually involves an exposure period of three months duration. Corrosion is accelerated as a result of the large cathode-anode ratio and the relatively long crevice created between the wire and the threads on the rod.

5.5 MicroenvironmentsThe corrosivity due to atmospheric conditions can be greatly affected by local conditions such as wind speed and direction, dust, debris, humidity, condensation and electrolytic species. These local conditions can change greatly within a few meters, depending on patterns in air turbulence. One extreme example of local variations due to the corrosivity of a seawater environment is the top deck of an aircraft carrier, where waves and seawater mist are abundant.

An example of local seasonal differences often exist in countries where deicing salts are used in the winter months. The following results have been obtained by exposing CLIMAT coupons in the vicinity of two important highways in Eastern Ontario. The transport and deposition of aerosols are subject to mass transport laws such as convection and turbulent diffusion. Based on these principles, the pattern of aerosol surface deposition can be modelled near obstacles. In such a modelling experiment, it was found that aerosol deposition rates onto and near an obstacle have a very localised structure due to variations in wind speed, wind speed gradients and turbulence.

5.6 Deicing SaltsEven though the cost of maintaining concrete structures is becoming prohibitively expensive mainly due to the effects of deicing salts, the benefits provided by adding these salts on icy roads are too great for their use to see any decrease in the future. The use of road deicing salts, which are extremely corrosive due to the disruptive effects of chloride ions on protective films on metals has dramatically increased in cold regions since it was introduced in the first part of the twentieth century.

The impact of deicing salts on green spaces adjacent to roads where such salts are used is quite obvious if you happen to travel on these roads in the Summer. However, there is concern that the massive use of these salts has an impact on human health. Although an alternative effective and less corrosive deicing agent, calcium magnesium acetate (CMA), is available, its price is apparently not yet reasonable enough for winter maintenance engineers to use widely. Therefore, it can be expected that the road environment would likely remain corrosive, if not more, well into the future.

5.7 Nature of the ProblemIn order to understand the mechanisms behind corrosion of reinforcing steel in concrete, one has to examine the chemical reactions involved. In


concrete, the presence of abundant amount of calcium hydroxide and relatively small amounts of alkali elements, such as sodium and potassium, gives concrete a very high alkalinity-with pH of 12 to 13. It is widely accepted that, at the early age of the concrete, this high alkalinity results in the transformation of a surface layer of the embedded steel to a tightly adhering film, that is comprised of an inner dense spinel phase in epitaxial orientation to the steel substrate and an outer layer of ferric hydroxide. As long as this film is not disturbed, it will keep the steel passive and protected from corrosion.

When a concrete structure is often exposed to deicing salts, salt splashes, salt spray, or seawater, chloride ions from these will slowly penetrate into the concrete, mostly through the pores in the hydrated cement paste. The chloride ions will eventually reach the steel and then accumulate to beyond a certain concentration level, at which the protective film is destroyed and the steel begins to corrode, when oxygen and moisture are present in the steel-concrete interface.

In 1962, it was reported that the required minimum concentration ofchloride in the concrete immediately surrounding the steel to initiate corrosion, the chloride corrosion threshold, is 0.15% soluble chloride, by weight of cement. In typical bridge deck concrete with a cement factor of 7, this is equivalent to 0.025% soluble chloride, by weight of concrete, or 0.59 kg soluble chloride per cubic meter of concrete. Subsequent research at FHWA laboratories estimated the corrosion threshold to be 0.033% total chloride, by weight of concrete.

There are indications that the chloride corrosion threshold can vary between concrete in different bridges, depending on the type of cement and mix design used, which can vary the concentrations of tricalcium aluminate (C3A) and hydroxide ion (OH-) in the concrete. In fact, it has been suggested that because of the role that hydroxide ions play in protecting steel from corrosion, it is more appropriate to express corrosion threshold in terms of the ratio of chloride content to hydroxide content, [Cl-] / [OH-], which was recently established to be between 2.5 to 6.

Once corrosion sets in on the reinforcing steel bars, it proceeds in electrochemical cells formed on the surface of the metal and the electrolyte or solution surrounding the metal. Each cell is consists of a pair of electrodes (the anode and its counterpoint, the cathode) on the surface of the metal, a return circuit, and an electrolyte. Basically, on a relatively anodic spot on the metal, the metal undergoes oxidation (ionisation), which is accompanied by production of electrons, and subsequent dissolution. These electrons move through a return circuit, which is a path in the metal itself to reach a relatively cathodic spot on the metal, where these electrons are consumed through reactions involving substances found in the electrolyte. In a reinforced concrete, the anode and the cathode are located on the steel bars, which also serve as the return circuits, with the surrounding concrete acting as the electrolyte.


Corrosion can also occur even in the absence of chloride ions. For example, when the concrete comes into contact with carbonic acid resulting from carbon dioxide in the atmosphere, the ensuing carbonation of the calcium hydroxide in the hydrated cement paste leads to reduction of the alkalinity, to pH as low as 8.5, thereby permitting corrosion of the embedded steel:

The rate of carbonation in concrete is directly dependent on the water/cement ratio (w/c) of the concrete, i.e., the higher the ratio the greater is the depth of carbonation in the concrete. In concrete of reasonable quality, that is properly consolidated and has no cracking, the expected rate of carbonation is very low. For example, in concrete with w/c of 0.45 and concrete cover 25 mm, it will require more than 100 years for carbonation to reach the concrete immediately surrounding the steel. Carbonation of concrete or mortar is more of an issue in Europe than in North America.

5.8 ISO 9223This standard classifies the corrosivity of an atmosphere based on measurements of time of wetness, and pollution categories (sulfur dioxide, airborne chlorides). The standard was not intended to be used in extreme service atmospheres such as those within chemical or metallurgical processing facilities or where there is direct contact with salt spray.

Only airborne chlorides and sulfur dioxide are considered in terms of classifying the pollution, this gives good coverage of rural, urban, industrial and marine atmospheres. Based on these measures an atmosphere is classified as being in one of five categories in terms of its corrosivity using two types of units, i.e. short term corrosion rate (CR) of steel as g m-2 year-1

(one year) or m year-1 (twenty years):

Category Short term Long term(g m-2 year-1) (m year-1)

C1 CR <= 10 CR <= 0.1

C2 10 < CR <= 200 0.1 < CR <= 0.5

C3 200 < CR <= 400 1.5 < CR <= 6

C4 400 < CR <= 650 6 < CR <= 20

C5 650 < CR 20 < CR

It is possible to deduce the corrosivity of an environment by combining TOW categories with the chloride and sulfur dioxide categories according to the following scheme when TOW is in: Category one (T1) to Category five (T5).

And convert these corrosivity ratings into short-term corrosion rates (g m-2

year-1) according to the following table.


Category Steel Copper Aluminum Zinc

C1 CR <= 10 CR <= 0.9 negligible CR <= 0.7

C2 10 < CR <= 200 0.9 < CR <= 5 CR <= 0.6 0.7 < CR <=

5

C3200 < CR <=

400 5 < CR <= 12 0.6 < CR <= 2 5 < CR <= 15

C4400 < CR <=

65012 < CR <=

25 2 < CR <= 5 15 < CR <= 30

C5 650 < CR 25 < CR 5 < CR 30 < CR

The use of the ISO guidelines and atmospheric data pertaining to these two environments have revealed the most important parameters affecting atmospheric corrosivity. In the case of the maritime Greenwood base, the time of wetness and chloride deposition rate are most important. In the rural environment, the time of wetness is likely to exert the strongest influence but chlorides transported from adjacent highways during the 'deicing' winter months can also be a factor.


6 Corrosion in Soils6.1 Introduction to Corrosion in SoilsSoil corrosion is a complex phenomenon, with a multitude of variables involved. Chemical reactions involving almost each of the existing elements are known to take place in soils, many of which are not yet fully understood. The relative importance of variables changes for different materials, making a universal guide to corrosion impossible. Variations in soil properties and characteristics across three dimensions can have a major impact on corrosion of buried structures.

Failure of Road caused by corrosion of underlying pipeline

The response of carbon steel to soil corrosion depends primarily on the nature of the soil and certain other environmental factors, such as the availability to moisture and oxygen. These factors can lead to extreme variations in the rate of the attack. For example, under the worst condition a buried vessel may perforate in less than one year, although archaeological digs in arid desert regions have uncovered iron tools that are hundreds of years old.

Some general rules can be formulated. Soils with high moisture content, high electrical conductivity, high acidity, and high dissolved salts will be


most corrosive. The effect of aeration on soils is somewhat different from the effect of aeration in water because poorly aerated conditions in water can lead to accelerated attack by sulfate-reducing anaerobic bacteria.

The effect of low levels of alloying additions on the soil corrosion of carbon steels is modest. Some data seems to show a small benefit of 1%Cu and 2.5% Ni on plain carbon steel.

The weight loss and maximum pit depth in soil corrosion can be represented by an equation of the form:

Z = aÄtm

where: Z - either the weight of loss of maximum pit depth t - time of exposure a and m - constants that depend on the specific soil corrosion situation.

6.2 Soil Classification SystemsSoil texture refers to the size distribution of mineral particles in a soil. Sand (rated from coarse to very fine), silt and clay refer to textures of decreasing particle coarseness. Soils with a high proportion of sand have very limited storage capacity for water, whereas clays are excellent in retaining water. One soil identification system has defined eleven soil types on the basis of their respective proportions of clay, silt and sand. The eleven types are:

Sand

Loamy sand

Sandy loam

Sandy clay loam

Clay loam

Loam

Silty loam

Silt

Silty clay loam

Silt clay and clay

Another identification scheme has utilised chemical composition, organic content and history of formation to define types such as gravel, humus, marsh and peat. A newer soil classification system has evolved in the USA that can be utilised to classify soils globally, at any location. In this "universal" classification system soils are considered as individual three-dimensional entities that can be grouped according to similar physical, chemical and mineralogical properties. The system uses a hierarchical


approach, with the amount of information about a soil increasing down the classification ladder.

6.3 Soil VariablesSeveral variable have been identified to have an influence on corrosion rates in soil.

Water Water, in liquid form, represents the essential electrolyte required for electrochemical corrosion reactions. A distinction is made between saturated and unsaturated water flow in soils. The latter represents movement of water from wet areas towards dry soil areas. Saturated water flow is dependent on pore size and distribution, texture, structure, and organic matter.

Degree of aeration The oxygen concentration decreases with increasing depth of soil. In neutral or alkaline soils, the oxygen concentration has an important effect on corrosion rate due to its participation in the cathodic reaction. However, in the presence of certain microbes (such as sulfate reducing bacteria) corrosion rates can be very high, even under anaerobic conditions. Excavation can obviously increase the degree of aeration in soil, compared with the undisturbed state.

pH Value Soils usually have a pH range of 5-8. In this range, pH is generally not considered to be the dominant variable affecting corrosion rates. More acidic soils obviously represent a serious corrosion risk to common construction materials such as steel, cast iron and zinc coatings. Soil acidity is produced by mineral leaching, decomposition of acidic plants (for example coniferous tree needles), industrial wastes, acid rain and certain forms of micro-biological activity. Alkaline soils tend to have high sodium, potassium, magnesium and calcium contents. The latter two elements tend to form calcareous deposits on buried structures with protective properties against corrosion. The pH level can affect the solubility of corrosion products and also the nature of microbiological activity.

Resistivity Resistivity has historically been used as a broad indicator of soil corrosivity. Since ionic current flow is associated with soil corrosion reactions, high soil resistivity will arguable slow down corrosion reactions. Soil resistivity generally decreases with increasing water content and the concentration of ionic species. Soil resistivity is by no means the only parameter affecting the risk of corrosion damage. A high soil resistivity alone will not guarantee absence of serious corrosion.

Redox Potential The redox potential essentially is a measure of the degree of aeration in a soil. An high redox potential indicates a high oxygen level. Low redox values may provide an indication that conditions are conducive to anaerobic microbiological activity. Sampling of soil will obviously lead to oxygen exposure and unstable redox potentials are thus likely to be measured in disturbed soil.


Chloride level Chloride ions are generally harmful, as they participate directly in anodic dissolution reactions of metals and their presence tends to decrease the soil resistivity. They may be found naturally in soils as a result of brackish groundwater and historical geological sea beds (some waters encountered in drilling mine shafts have chloride ion levels comparable to sea water) or from external sources such as de-icing salts applied to roadways. The chloride ion concentration in the corrosive aqueous soil electrolyte will vary, as soil conditions alternate between wet and dry.

Sulfate level Compared to the corrosive effect of chloride ion levels, sulfates are generally considered to be more benign in their corrosive action towards metallic materials. However, concrete may be attacked as a result of high sulfate levels. The presence of sulfates does pose a major risk for metallic materials in the sense that sulfates can be converted to highly corrosive sulfides by anaerobic sulfate reducing bacteria.

Microbiologically influenced corrosion (MIC) Microbiologically influenced corrosion (MIC) refers to corrosion that is influenced by the presence and activities of micro-organisms and/or their metabolites (the products produced in their metabolism). Bacteria, fungi and other micro-organisms can play a major part in soil corrosion. Spectacularly rapid corrosion failures have been observed in soil due to microbial action and it is becoming increasingly apparent that most metallic alloys are susceptible to some form of MIC.

6.4 Corrosion Severity RatingsFor design and corrosion risk assessment purposes, it is desirable to estimate the corrosivity of soils. One of the simplest classifications is based on a single parameter, soil resistivity. The generally adopted corrosion severity ratings are:

Soil resistivity (ohm cm) Corrosivity Rating>20,000 Essentially non-corrosive

10,000 to 20,000 Mildly corrosive5,000 to 10,000 Moderately corrosive3,000 to 5,000 Corrosive1,000 to 3,000 Highly corrosive

<1,000 Extremely corrosive

Sandy soils are high up on the resistivity scale and therefore considered the least corrosive. Clay soils, especially those contaminated with saline water are on the opposite end of the spectrum. The soil resistivity parameter is very widely used in practice and generally considered to be the dominant variable in the absence of microbial activity.

6.5 Numerical Corrosivity ScaleThe American Water Works Association (AWWA) developed a numerical soil corrosivity scale, applicable to cast iron alloys. The severity ranking by


assigning points for different variables. When the points total of a soil in the AWWA scale equals ten (or higher), corrosion protective measures (such as cathodic protection) have been recommended for cast iron alloys. The point system for predicting soil corrosivity according the AWWA C-105 Standard.

Soil Parameter Assigned Points

Resistivity (ohm cm)

<700 10

700 - 1000 8

1000 - 1200 5

1200 - 1500 2

1500 - 2000 1

> 2000 0

pH value

0-2 5

2-4 3

4-6.5 0

6.5-7.5 0

7.5-8.5 0

>8.5 3

Redox Potential (mV)

>100 0

50-100 3.5

0-50 4

<0 5

Sulfides

Positive 3.5

Trace 2

Negative 0

Moisture

Poor drainage continuously wet 2

Fair drainage generally moist 1

Good drainage generally dry 0


7 Corrosion by Natural WatersAbundant supplies of fresh water are essential to industrial development. Enormous quantities are required for cooling of products and equipment, for process needs, for boiler feed and for sanitary and potable water. It was estimated, in 1980, that the water requirements for industry in the USA approximated 525 billion liters per day. A substantial quantity of this water was reused. The intake of "new" water was estimated to be about 140 billion liters daily. If this water was pure and contained no contaminants, there would be little need for water conditioning or water treatment.

The fast-growing demand for clean, fresh water coupled with the need to protect and enhance the environment, has made many areas of the worldvulnerable to water shortages for various human uses.

As they interact with the electricity industry, these uses encompass agricultural irrigation, thermoelectric generation, municipal water and wastewater treatment and distribution, and industrial processes. The dependence of electricity supply and demand on water availability can impede the sustainability of economic growth, adversely affect future growth in electricity demand, cause shortages in current supplies of electricity, and have direct impact on power system planning and expansion.

Water possesses several unique properties, one being its ability to dissolve to some degree every substance occurring on the earth's crust and in the atmosphere. Because of this solvent property, water typically contains a variety of impurities. These impurities are a source of potential trouble through deposition of the impurities in water lines, boiler tubes and on products which are contacted by it.

Dissolved oxygen, the principal gas present in water, is responsible for costly replacement of piping and equipment by corrosive attack on metals with which it comes in contact. The origin of all water supply is moisture that has been evaporated from land masses and oceans and subsequently precipitated from the atmosphere. Depending on weather conditions, this may fall in the form of rain, snow, sleet or hail. As it falls, this precipitation contacts gases comprising the atmosphere and suspended particulates in the form of dust, industrial smoke and fumes, and volcanic dust and gases.

According to the American Waterworks Association (AWWA) industry database, there is approximately 1,483,000 km (876,000 mi) of municipal water piping in the United States. This number is not exact, since most water utilities do not have complete records of their piping system. The sewer system consists of approximately 16,400 publicly owned treatment facilities releasing some 155 million m3 (41 billion gallons) of wastewater per day (1995). The total annual direct cost of corrosion for the nation’s


drinking water and sewer systems was estimated at $36.0 billion. This cost was contributed to by the cost of replacing aging infrastructure, the cost of unaccounted-for water through leaks, the cost of corrosion inhibitors, the cost of internal mortar linings, and the cost of external coatings and cathodic protection.

7.1 Water ConstituentsThe concentrations of various substances in water in dissolved, colloidal or suspended form are typically low but vary considerably. A hardness value of up to 400 ppm of calcium carbonate, for example, is sometimes tolerated in public supplies, whereas 1 ppm of dissolved iron would be unacceptable. In treated water for high-pressure boilers or where radiation effects are important, as in some nuclear reactors, impurities are measured in very small units such as parts per billion (ppb). Water analysis for drinking water supplies is concerned mainly with pollution and bacteriological tests. For industrial supplies a mineral analysis is of more interest. The important constituents can be classified as follows:

Dissolved gases (oxygen, nitrogen, carbon dioxide, ammonia, sulfurous gases),

Mineral constituents, including hardness salts, sodium salts (chloride, sulphate, nitrate, , bicarbonate, etc.), salts of heavy matals, and silica,

Organic matter, including that of both animal and vegetable origin, oil, trade waste (including agricultural) constituents and synthetic detergents,

Microbiological forms, including various types of algae and slime forming bacteria.

The pH of natural waters is rarely outside the fairly narrow range of 4.5 to 8.5. High values, at which corrosion of steel may be suppressed, and low values, at which gaseous hydrogen evolution occurs, are not often found in natural waters. Copper is affected to a marked extent by pH value. In acidic waters, slight corrosion occurs and the small amount of copper in solution causes green staining of fabrics and sanitary ware. In addition redeposition of copper on aluminum or galvanised surfaces sets up corrosion cells resulting in severe pitting of the metals.

7.2 Saturation and Scaling IndicesThe saturation of water refers to the solubility product (Ksp) of a compound. For example, the ion activity product (IAP) of the ions involved when calcium carbonate is the scalant is:


The saturation level (SL) of water is defined as ratio of the ion activity product over Ksp as in the following:

In this example, water is said to be saturated with calcium carbonate when it will neither dissolve, or precipitate calcium carbonate scale. This equilibrium condition is based upon an undisturbed water, at constant temperature, which is allowed to remain undisturbed for an infinite period of time. Water is said to be undersaturated if it can still dissolve calcium carbonate. Supersaturated water will precipitate calcium carbonate from water if allowed to rest. If water is undersaturated with respect to calcium carbonate, the SL value will be less than 1.0. When water is at equilibrium, SL will be 1.0 by definition. Water which is supersaturated with calcium carbonate will have a saturation level greater than 1.0. As the saturation level increases beyond 1.0, the driving force for calcium carbonate crystal formation or crystal growth increases.

The SL definition can be simplified if the activity coefficients are incorporated into the solubility product in order to use a more practical concentration unit. The conditional solubility product (Kspc) incorporates the activity coefficients into the solubility product in which concentrations are expressed as molarities:

Six different water saturation indices have found wide acceptance. However these will not be discussed in this document.

7.3 Priority PollutantsThe concentrations of various substances in water in dissolved, colloidal or suspended form are typically low but vary considerably. Priority Pollutants refer to a list of 126 specific pollutants that includes heavy metals and specific organic chemicals. The priority pollutants are a subset of "toxic pollutants" as defined in the Clean Water Act (USA). These 126 pollutants were assigned a high priority for development of water quality criteria and effluent limitation guidelines because they are frequently found in wastewater. Many of the heavy metals, pesticides, and other chemicals listed here are on the priority pollutant list:

Heavy Metals (Total and Dissolved): "Heavy Metal" in the water treatment field refers to heavy, dense, metallic elements that occur only at trace levels in water, but are very toxic and tend to accumulate. These include Arsenic, Cadmium, Chromium, Lead in industry or in households, Mercury and Zinc.


Pesticides: Pesticides comprise a large class of compounds of concern. Typical pesticides and herbicides include DDT, Aldrin, Chlordane, Endosulfan, Endrin, Heptachlor, and Diazinon. Surprisingly, concentrations of pesticides in urban runoff may be equal or greater than the pesticides in agricultural runoff.

Polycyclic Aromatic Hydrocarbons (PAHs): Polycyclic Aromatic Hydrocarbons include a family of semi-volatile organic pollutants such as naphthalene, anthracene, pyrene, and benzo(a)pyrene. There are typically two main sources of PAHs: spilled or released petroleum products (from oil spills or discharge of oil production brines) and combustion products that are found in urban runoff.

Polychlorinated biphenyls (PCBs): Polychlorinated biphenyls are organic chemicals that formerly had widespread use in electrical transformers and hydraulic equipment. This class of chemicals is extremely persistent in the environment and has been proven to bioconcentrate in the food chain, thereby leading to environmental and human health concerns in areas such as the Great Lakes.

7.4 Chlorination of WaterAlthough three-quarters of the Earth's surface is water, only 1% is available for drinking, and this often requires treatment before it can be used safely. Water contains many kinds of microbes and organisms that can cause disease. It is estimated that 80% of all sickness and disease in developing countries is caused by unsafe water and inadequate sanitation. According to the World Health Organisation (WHO), diseases associated with dirty water cause some 25,000 deaths per day. Chlorine and its derivative water treatment products, sodium hypochlorite and chlorine dioxide, are very powerful disinfection agents and, when added to water in minute quantities, rapidly destroy bacteria and other micro-organisms.

Chlorine compounds have been used in many countries for almost a century to treat drinking water. Where they are used, they have virtually eradicated serious waterborne diseases. Different water disinfection techniques, which include disinfection using ozone, U.V. and ultrafiltration, are used around the world. These methods are often used in tandem with chlorine. A major advantage of chlorine is that it has a residual disinfection effect, and it is the only technique that can ensure disinfection right up to the tap. In fact, the residual ability to destroy and inhibit the activity of pathogenic agents is a specific characteristic of chlorine.

The action of other disinfectants, such as ozone, U.V. is only temporary. This residual property of chlorine means that it is chosen as the main technique for protecting drinking water circulating in supply networks. This is particularly important in cities, where leaks can allow germs into the system beyond the treatment station. Emergency situations involving accidental contamination of drinking water supply systems continue to demonstrate the need for chlorine as an ultimate defense against waterborne microbiological infection. In addition to its disinfecting


properties, chlorine has other benefits for water treatment - these include preventing the growth of algae and slime in pipes and storage tanks. Chlorination systems are also a comparatively economical technique and safe to use, requiring minimum maintenance. Properly used in pre-treated water (e.g. pH adjustment, flocculation, sedimentation and filtration), only very small quantities of chlorine are needed for effective purification.

Generally, it is possible to recognize the smell and taste of chlorine only in water which has not been processed properly. This is rare in modern water systems. Over decades of chlorine use, epidemiological studies carried out on populations drinking chlorinated water have failed to establish a causal relationship with any disease (including cancer), a conclusion that has been recognized by the WHO. However, it is expected that chlorine will remain a perfectly acceptable disinfectant, provided the appropriate techniques are applied. Public health is the main benefit provided by the use of chlorine to disinfect drinking water. Applied to water treatment, chlorine has four essential functions :

Elimination of undesirable matter from the water by oxidation,

Permanent protection of the hygienic and sanitary quality of the water throughout the distribution phase,

Active, immediate disinfection in cases of accidental pollution,

Continuous monitoring (of the chlorine demand) to warn of pollution.

For over 100 years, chlorination of drinking water has provided a world-wide demonstration of its effectiveness in protecting public health.


8 Micro-organisms in CorrosionMicro-organisms can be categorised according to oxygen tolerance:

Strict (or obligate) anaerobes, which will not function in the presence of oxygen.

Aerobes, which require oxygen in their metabolism.

Facultative anaerobes, which can function either in the absence or presence of oxygen.

Microaerophiles, which use oxygen but prefer low levels.

Strictly anaerobic environments are quite rare in nature, while strict anaerobes are commonly found flourishing within anaerobic microenvironments in highly aerated systems. Another way of classifying organisms is according to their metabolism:

The compounds or nutrients from which they obtain their carbon for growth and reproduction.

The chemistry by which they obtain energy or perform respiration.

The elements they accumulate as a result of these processes.

A third way of classifying bacteria is by shape. These shapes are predictable when organisms are grown under well defined laboratory conditions. In natural environments, however, shape is often determined by growth conditions rather than pedigree. Examples of shapes are:

"Vibrio," for comma shaped cells.

"Bacillus," for rod shaped cells.

"Coccus," for round cells.

"Myces," for fungi like cells.

8.1 MIC BasicsMicro-organisms pervade our environment and readily "invade" industrial systems wherever conditions permit. These agents flourish in a wide range of habitats and show a surprising ability to colonise water rich surfaces wherever nutrients and physical conditions allow. Microbial growth occurs over the whole range of temperatures commonly found in water systems, pressure is rarely a deterrent and limited access to nitrogen and phosphorus is offset by a surprising ability to sequester, concentrate and retain even trace levels of these essential nutrients.

Many engineers continue to be surprised that such small organisms can lead to spectacular failures of large engineering systems. The microorganisms of


interest in Microbiologically Influenced Corrosion (MIC) are mostly bacteria, fungi, algae and protozans.

Bacteria are generally small, with lengths of typically under 10 �m. Collectively, they tend to live and grow under wide ranges of temperature, pH and oxygen concentration. Carbon molecules represent an important nutrient source for bacteria.

Fungi can be separated into yeasts and molds. Corrosion damage to aircraft fuel tanks is one of the well-known problems associated with fungi. Fungi tend to produce corrosive products as part of their metabolisms; it is these by-products that are responsible for corrosive attack. Furthermore, fungi can trap other materials leading to fouling and associated corrosion problems.

Protozans are predators of bacteria and algae and therefore potentially mitigate microbial corrosion problems.

MIC is responsible for the degradation of a wide range of materials. Bacteria can exist in several different metabolic states. Those that are actively respiring, consuming nutrients, and proliferating are said to be in a "growth" stage. Those that are simply existing, not growing because of unfavourable conditions, are said to be in a "resting" state.

Some strains, when faced with unacceptable surroundings, form spores that can survive extremes of temperature and long periods without moisture or nutrients, yet produce actively growing cells quickly when conditions again become acceptable.

The latter two states may appear, to the casual observer, to be like death, but the organisms are far from dead. Cells that actually die are usually consumed rapidly by other organisms or enzymes. When looking at an environmental sample under a microscope, therefore, it should be assumed that most or all of the cell forms observed were alive or capable of life at the time the sample was taken.

8.2 Bacteria Related to MIC

8.2.1 Acid Producing Fungi

Certain fungi are also capable of producing organic acids and have been blamed for corrosion of steel and aluminium, as in the highly publicised corrosion failures of aluminium aircraft fuel tanks. In addition, fungi may produce anaerobic sites for SRB and can produce metabolic by-products that are useful to various bacteria.

8.2.2 Aerobic Slime Formers

Aerobic slime formers are a diverse group of aerobic bacteria. They are important to corrosion mainly because they produce extracellular polymers that make up what is commonly referred to as "slime." This polymer is


actually a sophisticated network of sticky strands that bind the cells to the surface and control what permeates through the deposit.

The stickiness traps all sorts of particulates that might be floating by, which, in dirty water, can result in the impression that the deposit or mound is an inorganic collection of mud and debris. The slime formers and the sticky polymers that they produce make up the bulk of the distributed slime film or primary film that forms on all materials immersed in water.

Slime formers can be efficient "scrubbers" of oxygen, thus preventing oxygen from reaching the underlying surface. This creates an ideal site for SRB growth. Various types of enzymes are often found within the polymer mass, but outside the bacterial cells. Some of these enzymes are capable of intercepting and breaking down toxic substances (such as biocides) and converting them to nutrients for the cells.

8.2.3 Iron/Manganese Oxidising Bacteria

Bacteria that derive energy from the oxidation of Fe2+ to Fe3+ are commonly reported in deposits associated with MIC. They are almost always observed in tubercles (discrete hemispherical mounds) over pits on steel surfaces. The most common iron oxidisers are found in the environment in long protein sheaths or filaments. While the cells themselves are rather indistinctive in appearance, these long filaments are readily seen under the microscope and are not likely to be confused with other life forms.

The observation that filamentous iron bacteria are "omnipresent" in tubercles might be, therefore, more a matter of their easy detection than of their relative abundance. An intriguing type of iron oxidisers is the Gallionella bacterium, which has been blamed for numerous cases of corrosion of stainless steels.

Besides the iron/manganese oxidisers, there are organisms that simply accumulate iron or manganese. Such organisms are believed to be responsible for the manganese nodules found on the ocean floor. The accumulation of manganese in biofilms is blamed for several cases of corrosion of stainless steels and other ferrous alloys in water systems treated with chlorine or chlorine/bromine compounds.

8.2.4 Methane Producers

Only in recent years have methane producing bacteria (methanogens) been added to the list of organisms believed responsible for corrosion. Like many SRB, methanogens consume hydrogen and thus are capable of performing cathodic depolarisation. While they normally consume hydrogen and carbon dioxide to produce methane, in low nutrient situations these strict anaerobes will become fermenters and consume acetate instead.

In natural environments, methanogens and SRB frequently coexist in a symbiotic relationship: SRB producing hydrogen, CO2 and acetate by fermentation, and methanogens consuming these compounds, a necessary


step for fermentation to proceed. The case for facilitation of corrosion by methanogens still needs to be strengthened, but methanogens are as common in the environment as SRB and are just as likely to be a problem. The reason they have not been implicated before now is most likely because they do not produce distinctive, solid by-products.

8.2.5 Organic Acid Producing Bacteria

Various anaerobic bacteria such as Clostridium are capable of producing organic acids. Unlike SRB, these bacteria are not usually found in aerated macro-environments such as open, recirculating water systems. However, they are a problem in gas transmission lines and could be a problem in closed water systems that become anaerobic.

8.2.6 Sulfur/Sulfide Oxidizing Bacteria

This broad family of aerobic bacteria derives energy from the oxidation of sulfide or elemental sulfur to sulfate. Some types of aerobes, can oxidize the sulfur to sulfuric acid, with pH values as low as 1.0 reported. These Thiobacillus strains are most commonly found in mineral deposits, and are largely responsible for acid mine drainage, which has become an environmental concern. They proliferate inside sewer lines and can cause rapid deterioration of concrete mains and the reinforcing steel therein.

They are also found on surfaces of stone buildings and statues and probably account for much of the accelerated damage commonly attributed to acid rain. Where Thiobacillus bacteria are associated with corrosion, they are almost always accompanied by SRB. Thus, both types of organisms are able to draw energy from a synergistic sulfur cycle. The fact that two such different organisms, one a strict anaerobe that prefers neutral pH, and the other an aerobe that produces and thrives in an acid environment, can coexist demonstrates that individual organisms are able to form their own micro-environment within an other wise hostile larger world.

8.3 Identification of Microbial Activity

8.3.1 Direct Inspection

Direct inspection is best suited to enumeration of planktonic organisms suspended in relatively clean water. In liquid suspensions, cell densities greater than 107 cells cm-3 cause the sample to appear turbid. Quantitative enumerations using a phase contrast microscopy can be done quickly using a counting chamber which holds a known volume of fluid in a thin layer.

Visualisation of micro-organisms can be enhanced by fluorescent dyes that cause cells to light up under ultraviolet radiation. Using a stain such as acridine orange, cells separated by filtration from large aliquots of water can be visualized and counted on a 0.25 mm filter using the epifluorescent technique. Newer stains such as fluorescein diacetate, 5-cyano-2,3-ditolyltetrazolium chloride or p-iodonitrotetrazolium violet indicate active metabolism by the formation of fluorescent products.


Identification of organisms can be accomplished by use of antibodies generated as an immune response to the injection of microbial cells into an animal, typically a rabbit. These antibodies can be harvested and will bind to the target organism selectively in a field sample. A second antibody tagged with a fluorescent dye is then used to light up the rabbit antibody bound to the target cells. In effect the staining procedure can selectively light up target organisms in a mixed population or in difficult soil, coating or oily emulsion samples.

8.3.2 Growth Assays

The most common way to assess microbial populations in industrial samples is through growth tests using commercially available growth media for groups of organisms most commonly associated with industrial problems. These are packaged in a convenient form suitable for use in the field. Serial dilutions of suspended samples are grown on solid agar or liquid media.

Based on the growth observed for each dilution estimates of the most probable number (MPN) of viable cells present in a sample can be obtained. Despite the common use of growth assays, only a small fraction of wild organisms actually grow in commonly available artificial media. Estimates for SRB in marine sediments for example suggest as few as one in a thousand of the organisms present actually show up in standard growth tests.

8.3.3 Activity AssaysWhole Cell Approaches based on the conversion of a radioisotopically labelled substrate can be used to assess the potential activity of microbial populations in field samples. The radiorespirometric method allows use of field samples directly without the need to separate organisms and is very sensitive. Selection of the radioactively labelled substrate is key to interpretation of the results but the method can provide insights into factors limiting growth by comparing activity in native samples with supplemented test samples under various conditions.

Oil degrading organisms for example can be assessed through the mineralization of 14-C labelled hydrocarbon to carbon dioxide. Radioactive methods are not routinely used by field personnel but have found use in a number of applications including biocide screening programs, identification of nutrient sources and assessment of key metabolic processes in corrosion scenarios.

Enzyme Based Assays An increasingly popular approach is the use of commercial kits to assay the presence of enzymes associated with micro-organisms suspected to cause problems. For example, kits are available for the sulfate reductase enzyme common to SRB associated with corrosion problems as well as for the hydrogenase enzyme implicated in the acceleration of corrosion through rapid removal of cathodic hydrogen formed on the metal surface. Performance of several of these kits has been assessed by field personnel in round robin tests. Correlation of activity assays and population estimates is variable. In general these kits have a narrower range of application than growth based assays making it important


to select a kit with a range of response appropriate to the problem under consideration.

Metabolites An overall assessment of microbial activity can be obtained by measuring the amount of adenosine triphosphate (ATP) in field samples. This key metabolite drives many cellular reactions. Commercial instruments are available which measure the release of light by the firefly luciferin/luciferase with ATP. The method is best suited to clean aerobic aqueous samples; particulate and chemical quenching can affect results. Detection of metabolites such as organic acids in deposits or gas compositions including methane or hydrogen sulfide by routine gas chromatography can also indicate biological involvement in industrial problems.

Cell Components Biomass can be generally quantified by assays for protein, lipopolysaccharide or other common cell constituents but the information gained is of limited value. An alternate approach is to use cell components to define the composition of microbial populations with the hope that the insight gained may allow damaging situations to be recognized and managed in the future. Fatty acid analysis and nucleic acid sequencing provide the basis for the most promising methods.

Fatty Acid Profiles Analyzing fatty acid methyl esters derived from cellular lipids can fingerprint organisms rapidly. Provided pertinent profiles are known, organisms in industrial and environmental samples can be identified with confidence. In the short term, the impact of events such as changes in operating conditions or application of biocides could be monitored by such analysis. In the longer term, problem populations might be identified quickly so that an appropriate management response could be implemented in a timely fashion.

Nucleic Acid Based Methods In principle, probes could be developed to detect all possible sulfate-reducers but application of such a battery of probes becomes daunting where large numbers of field samples are to be analyzed. To overcome this obstacle, the reverse sample genome probe (RSGP) was developed. In this technique, DNA from organisms previously isolated from field problems is spotted on a master filter. DNA isolated from field samples of interest is then labelled with either a radioactive or fluorescent indicator and exposed to this filter. Where complementary strands of DNA are present, labelled DNA from the field sample sticks to the corresponding spot on the master filter.

8.4 Accelerated Low Water Corrosion (ALWC)A relatively new form of bacteriological corrosion has occurred on marine steel structures over the past 20 years or so, leading to rapid deterioration and the need for emergency repair and maintenance. These actions had often been foreseen and been budgeted for and sometimes caused significant disruption to the commercial activities on the structures in question. This form of corrosion is named accelerated low water corrosion (ALWC). It occurs at or around the lowest astronomical tide level and is induced by the growth of bacteria.


The Construction Industry Research and Information Association (CIRIA, UK) has commissioned Dr Jim Breakell and Dr Michael Siegwart of Mott MacDonald Ltd to produce a best practice guidance document for owner, operators and engineers dealing with structures affected by ALWC.

Mott MacDonald has developed and patented its own electrochemical repair technique based on the application of an inorganic coating and is, therefore, best prepared to produce such a guidance document. ALWC is not to be confused with ordinary marine corrosion (next chapter).

Figure above: ALWC at 300 above beach on Larssen Webs, Taken by M Hodgson from John Martin Construction Ltd.


9 Corrosion in marine EnvironmentsSeawater systems are used by many industries such as shipping, offshore oil and gas production, power plants and coastal industrial plants. The main use of seawater is for cooling purposes but it is also used for fire fighting, oil field water injection and for desalination plants. The corrosion problems in these systems have been well studied over many years, but despite published information on materials behaviour in seawater, failures still occur.

Most of the elements that can be found on earth are present in seawater, at least in trace amounts. However, eleven of the constituents alone account for 99.95% of the total solutes, chloride ions being by far the largest constituent. The concentration of dissolved materials in the sea varies greatly with location and time because rivers dilute seawater, rain, or melting ice, or is concentrated by evaporation. The most important properties of seawater are:

The ratios of the concentrations of the major constituents are remarkably constant world-wide.

High salt concentration, mainly sodium chloride;

High electrical conductivity,

Relatively high and constant pH,

Buffering capacity,

Solubility for gases, of which oxygen and carbon dioxide in particular are of importance in the context of corrosion,

The presence of a myriad of organic compounds,

The existence of biological life either as microfouling (e.g. bacteria, slime) or macrofouling (e.g. seaweed, mussels, barnacles and many kinds of animals or fish).

The U.S. flag fleet can be divided into several categories as follows: the Great Lakes with 737 vessels at 62 billion ton-miles, inland with 33,668 vessels at 294 billion ton-miles, ocean with 7,014 vessels at 350 billion ton-miles, recreational with 12.3 million boats, and cruise ship with 122 boats serving North American ports (5.4 million passengers). The total annual direct cost of corrosion to the U.S. shipping industry is estimated at $2.7


billion. This cost is divided into costs associated with new construction ($1.1 billion), with maintenance and repairs ($0.8 billion), and with corrosion-related downtime ($0.8 billion).

9.1 Ions in SeawaterA large part of the dissolved components of seawater is present as ion pairs, or in complexes, rather than as simple ions. While the major cations are largely uncomplexed, the anions, other than chloride, are to varying degrees present in the form of complexes. About 13% of the magnesium and 9% of the calcium in ocean waters exist as magnesium sulfate and calcium sulfate respectively.

More than 90% of the carbonate, 50% of the sulfate, and 30% of the bicarbonate exist as complexes. Many minor or trace components occur primarily as complexed ions at the pH and the redox potential of seawater. Boron, silicon, vanadium, germanium, and iron form hydroxide complexes. Gold, mercury, and silver, and probably calcium and lead, form chloride complexes. Magnesium produces complexes with fluorides to a limited extent.

Surface seawater characteristically has pH values higher than 8 owing to the combined effects of air-sea exchange and photosynthesis. The carbonate ion concentration is consequently relatively high in surface waters. In fact, surface waters are almost always supersaturated with respect to the calcium carbonate phases, calcite and aragonite.

The introduction of molecular carbon dioxide into subsurface waters during the decomposition of organic matter decreases the saturation state with respect to carbonates. While most surface waters are strongly supersaturated with respect to the carbonate species, the opposite is true of deeper waters that are often under saturated in carbonates.

9.2 Oxygen in SeawaterThe seawater oxygen content depends primarily on factors such as salinity and temperature. Relationships have been derived from which the equilibrium concentration of dissolved oxygen can be calculated if the absolute temperature T (K) and salinity S(�MO) are known:

ln [O2] (ml l-1) = A1 +A2(100/T)+A3 ln (T/100)+ A4(T/100)+S[B1 + B2(T/100)+ B3(T/100)2]

where: A1 = -173.4292 A2 = 249.6339 A3 = 143.3483 A4 = -21.8492 B1 = -0.033096 B2 = 0.014259 and B3 = -0.0017000.

The primary source of the dissolution of oxygen is the air-sea exchange with oxygen in the atmosphere, leading to near saturation. However, due mainly to biological processes, deviations may occur with seasons. The normal profile of corrosion of unprotected steel, as in the case of piling or the supporting legs for offshore oil drilling structures is based on the


measurements of the distribution of corrosion of test pilling exposed in a partially enclosed basin at Kure Beach, NC, USA.

At any location there are seasonal variations in salinity, temperature and other parameters. There are also variations with the depth of water. It should not be assumed that the variations found in these studies can be extrapolated to other oceanographic sites. For example, observations within the same depth range of depth in the Atlantic Ocean showed a much higher concentration of dissolved oxygen to the bottom, even approaching the concentration found at the surface.

9.3 Precipitation of inorganic compounds from seawater

This principle was used by engineers from Mott MacDonald to develop Latreat the patented electrochemical repair system for ALWC. The value of calcareous deposits in the effective and efficient operation of marine cathodic protection systems is generally recognized by corrosion engineers. The calcareous films are known to form on cathodic metal surfaces in seawater, thereby enhancing oxygen concentration polarization and reducing the current density needed to maintain a prescribed cathodic potential. For most cathodic surfaces in aerated waters, the principal reduction reaction is described by the following reaction:

1. O2 + 2 H2O + 4 e- --> 4 OH-

In cases where the potential is more negative than the reversible hydrogen electrode potential, the production of hydrogen as described in the following reaction becomes possible:

2. 2 H2O + 2 e- --> H2 + 2 OH-

In either case, the production of hydroxyl ions results in an increase in pHfor the electrolyte adjacent to the metal surface. In other terms, an increase in OH- is equivalent to a corresponding reduction in acidity or H+ ion concentration. This situation causes the production of a pH profile in the diffuse layer where the equilibrium reactions can be quite different from the bulk seawater conditions.

Temperature, relative electrolyte velocity and electrolyte composition will all influence this pH profile. There is both analytical and experimental evidence that such a pH increase exists as a consequence of the application of a cathodic current. In seawater, pH is controlled by the carbon dioxide system described in the following interrelated reactions:

3. CO2 + H2O --> H2CO3

4. H2CO3 --> H+ + HCO3-

5. HCO3- --> H+ + CO3

2-


If OH- is added to the system as a consequence of one of the above cathodic processes the pH adjacent to a metallic surface will increase favouring the precipitation of a calcareous deposit according to the following reactions:

6. CO2 + OH- --> HCO3-

7. OH- + HCO3- --> H2O + CO32-

8. CO32- + Ca2+ --> CaCO3(s)

The equilibria represented by Equations (3) through (8) further indicate that as OH- is introduced (Equation (1) and/or (2)) and reacts (Equations (6) and (7)), then Equations (4) and (5) are displaced to the right, resulting in proton (H+) production. This opposes any rise in pH and accounts for the buffering capacity of seawater. Irrespective of this, however, Equations (3) through (8) indicate that this buffering action is accompanied by the formation of calcareous deposits on cathodic surfaces exposed to seawater.

Magnesium compounds, Mg(OH)2 in particular, could also contribute to the protective character of calcareous deposits. However, calcium carbonate is thermodynamically stable in surface seawater, where it is supersaturated, while magnesium hydroxide is unsaturated and less stable. In fact, Mg(OH)2

would precipitate only if the pH of seawater was to exceed approximately 9.5. It is the main reason why the behaviour of CaCO3 in seawater has been so extensively studied since calcium carbonate sediments are prevalent and widespread in the oceans.

It has been demonstrated that that calcium carbonate occurs in the oceans in two crystalline forms, i.e. calcite and aragonite. Partly because calcite and magnesium carbonate have similar structures, these compounds form solid solutions, the Ca:Mg ratio of which depends on the ratio of these ions in seawater. Theoretical calculations suggest that calcite in equilibrium with seawater should contain between 2 and 7 mol% MgCO3. But although low magnesium calcite is the most stable carbonate phase in seawater, its precipitation and crystal growth are strongly inhibited by dissolved magnesium. Consequently aragonite is the phase that actually precipitates when seawater is made basic by the addition of sodium carbonate.

The degree of saturation for aragonite is described by the following solubility or equilibrium constant:

9. Ksp, aragonite = (Ca2+)Å(CO32-)

where (Ca2+) and (CO32-) are the molalities of the Ca2+ and CO3

2- ions respectively.

At 25oC Ksp, aragonite = 6.7 x 10-7

In order to understand the build-up of carbonate ions at a metallic surface under cathodic protection (CP), one can combine Equations (2), (6) and (7) to obtain an expression describing the electrochemical production of carbonate ions (Equation (10)).


10.H2O + CO2 + 2e- --> H2 + CO32-

By referring to the section of the Kinetics Chapter on concentration overpotential, one can the develop an expression for the limiting current corresponding to this reaction:

where, at neutral bulk pH, the concentration of carbonate ions in seawater is basically zero, and the expression of iL can be correctly described by:

9.4 ElectroplatingElectroplating is achieved by passing an electrical current through a solution containing dissolved metal ions and the metal object to be plated. The metal object serves as the cathode in an electrochemical cell, attracting metal ions from the solution. Ferrous and non-ferrous metal objects are plated with a variety of metals, including aluminium, brass, bronze, cadmium, copper, chromium, iron, lead, nickel, tin, and zinc, as well as precious metals, such as gold, platinum, and silver. The process is regulated by controlling a variety of parameters, including the voltage and amperage, temperature, residence times, and the purity of bath solutions.

For some metals all these demands can not be fulfilled using a water based electrolyte. Elements such as Ti and Al can only be deposited from organic electrolytes, while other metals such as Mg, Nb, Ta, and W can only be plated from molten salt electrolytes (at 700�C and above).

Metals plateable from aqueous solutions (red background). Elements with yellow background are only plateable in combination with one of the others (alloy plating). Grey text indicates that the element can be deposited both by an auto-catalytic electroless process or by electroplating. The sequence of unit operations in an electroplating operation typically involves various cleaning steps, stripping of old plating or paint, electroplating steps, and rinsing between and after each of these operations. Electroless plating uses


similar steps but involves the deposition of metal on a substrate without the use of external electrical energy.

9.5 Marine SystemsCorrosion can cause rapid failure in marine systems, and there are many examples strewn through history:

Failure of a soldered joint in a seawater system caused the loss of the USS Thresher in 1963, killing all 129 men on board, and leaving the radioactive power unit on the floor of the Atlantic.

Failure of the propulsion system on the Braer oil Tanker due to seawater corrosion leads to it foundering on the Shetland Islands in 1993, spilling 100,000 tonnes of oil.

These are just two of the largest and most damaging victims of marine corrosion, and improper corrosion design, there are many more mundane failures everyday. All these lead to downtime of subsea systems, costing hundreds of thousands of pounds in lost revenue, for the cost of a section worth maybe pennies! By considering the problems posed by corrosion from the beginning of the design process, and solving any bugs while the design is still on paper, rather than in production, or operation, the costs of making the required changes are minimised.

With the discovery of oil and gas in water depths greater than 300 m, the material selection process has become more difficult and complicated when compared to the similar process for land based operations. The costs associated with a failure in deep water are expensive and have environmental implications. Deepwater projects consist of both direct vertical access (DVA) wells and subsea projects that are completed several miles away form the host platform and tied back to it via a flowline and riser.

Historically, materials were needed to handle corrosive service consisting only of hydrogen sulfide, carbon dioxide, and chlorides. With deep-water wells and subsea systems that are being drilled and completed, chemicals are required to minimise paraffin, asphaltene, hydrates, and scale formation and provide corrosion inhibition. These chemicals however, may have adverse effects on metallic and non-metallic materials. The problem is compounded when materials have to be selected to handle produced fluids, annular fluids, and the injected chemicals.

In subsea systems the effects of hydrogen embrittlement from the cathodic protection system also have to be taken into account. In the selection of materials for wellhead equipment the following considerations have had to be taken into account:

Composition of produced fluids in contact with valve body and internal parts – all wetted parts from downhole to the flowline require identification,


Service temperature,

Operating pressure ranges,

Galvanic effects due to contact of dissimilar materials,

Crevice corrosion resistance at seal and flange faces,

Wear and galling resistance of moving parts,

Temperature and chemical resistance for non-metallic materials,

Cathodic protection (CP) on materials,

Effectiveness of coatings on materials,

Weldability for weld overlay,

Material availability and cost,

Compatibility of materials with injected fluids.


10 Stray Current CorrosionStray currents, which cause corrosion, may originate from direct-current distribution lines, substations, or street railway systems, etc., and flow into a pipe system or other steel structure. Alternating currents very rarely cause corrosion. The corrosion resulting from stray currents (external sources) is similar to that from galvanic cells (which generate their own current) but different remedial measures may be indicated. In the electrolyte and at the metal-electrolyte interfaces, chemical and electrical reactions occur and are the same as those in the galvanic cell; specifically, the corroding metal is again considered to be the anode from which current leaves to flow to the cathode. Soil and water characteristics affect the corrosion rate in the same manner as with galvanic-type corrosion.

However, stray current strengths may be much higher than those produced by galvanic cells and, as a consequence, corrosion may be much more rapid. Another difference between galvanic-type currents and stray currents is that the latter are more likely to operate over long distances since the anode and cathode are more likely to be remotely separated from one another. Seeking the path of least resistance, the stray current from a foreign installation may travel along a pipeline causing severe corrosion where it leaves the line. Knowing when stray currents are present becomes highly important when remedial measures are undertaken since a simple sacrificial anode system is likely to be ineffectual in preventing corrosion under such circumstances.

10.1 Detection of Stray CurrentsDetection of stray currents, which may be causing corrosion, is somewhat involved and involves technical operations for which field staff are usually not equipped. Their presence may be suspected when large direct-current installations are in the vicinity of the structure experiencing corrosion and especially when very rapid corrosion occurs. The services of a corrosion specialist should then be requested.

10.2 Stray Currents in Transit SystemsStray current induced corrosion damage has been associated with North American DC rail transit systems for more than a century. In the United States alone, there are more than twenty transit authorities operating


electrified rail systems in major urban centers. Stray current corrosion problems continue to affect several North American cities where the transit systems are typically installed in high-density urban areas. Obviously such urban areas are associated with underground cables and piping (water and gas) systems, that can also be highly susceptible to this form of corrosion damage.

Solidly grounded systems were historically used on older transit systems, and used the "tie everything together and let the current flow" philosophy of the late 1800s. The major characteristics of a solidly grounded system are direct metallic connection of the AC rectifier negative buses to the earthing mats at the substations and the absence of insulation on the running rails. Such a design allows stray current to flow totally unrestricted between the rectifier negative bus and any available underground metallic path. Consequently, stray-current corrosion occurs frequently on the transit rails, rail fasteners, tunnels, bridges and other transit structures. The only advantage of a solidly grounded system is that the negative return voltage is at the same voltage as the earth ground, which eliminates the hazard of having electric potentials develop between station platforms and the earth ground. Electric potentials can vary from zero to 150 volts and can represent a hazard for passengers.

Ungrounded systems represent the other extreme of traction power system design. An ungrounded system has no direct metallic connection between earth and the rectifier bus at the substations. Rail fastener insulation is also important so that high, rail-to-earth resistances are maintained. In theory, stray currents from an ungrounded system should be low as long as rail shorts are not allowed to develop along the line. Practically, however, because of the thousands of fasteners in parallel on the system, an earth ground does exist. In addition, special trackwork is often difficult to isolate completely, and represents areas where grounding can occur. The one disadvantage of an ungrounded system is that sufficiently high electric potentials can develop between platforms and earth ground.

Diode-grounded systems represent a compromise between a solidly grounded and ungrounded system. They are often used to eliminate the problems of stray-current corrosion from a solidly grounded system, but also to keep electric potentials to a safe level. Diode-grounded systems contain a direct metallic connection of the rectifier bus to the earthing mats at substations, but through a diode circuit. The diode circuit allows current to flow from the earthing mat to the negative bus when a certain threshold voltage is reached. The threshold can be as low as 10 volts or as high as 50 volts depending on the conditions at the substation. In this way, electric potentials are dissipated and not allowed to build up to unsafe levels. Stray-current corrosion can still occur on diode-grounded systems, especially on the rails and rail fasteners where low rail-to-earth resistances are seen. In addition, because of the diode-ground circuit path, the return rails periodically discharge current when a threshold voltage is exceeded. It has been observed that a rail designed for 35 years life on a diode-grounded


transit system had to be replaced in seven years due to stray-current corrosion and rail cracks.

10.3 Nature of Stray CurrentsStray current problems stem from the fundamental design of electrified rail transit systems, whereby current is returned to substations via the running rails. The ground surrounding the rails can be viewed as a parallel conductor to the rails. The magnitude of stray current flow in the ground conductor will increase as its resistivity decreases. Any metallic structure buried in ground of this nature will tend to “attract” stray current, as it represents a very low resistance current path. The highest rate of metal dissolution occurs where the current leaves the structure and undesirable overprotection effects can occur at the points of current pick-up.

The stray currents tend to be very dynamic in nature, with the magnitude of stray current varying with usage of the transit system and relative position and degree of acceleration of the electrified vehicles. Fundamentally, the following factors all have an effect on the severity of stray currents: magnitude of propulsion current, substation spacing, substation grounding method, resistance of the running rails, usage and location of cross bonds and isolated joints, track-to-earth resistance and the voltage of the traction power system. At a particular location on an affected structure, the presence of stray currents can be identified when fluctuating pipe-to-soil potentials are recorded with time.

The older DC transit systems generally produce the worst stray current problems due to the following factors:

Relatively high electrical resistance of the running rails (smaller rail cross sections, bolted connections, deterioration of connections over time etc.);

Poor isolation from earth of the running rails (intentional grounded negative bus, intimate earth contact, moisture absorbing wood ties etc.);

Widely spaced substations leading to a higher voltage drop in the rails.

In modern system designs stray current problems are ameliorated with two fundamental measures: (i) Decreasing the electrical resistance of the rail return circuit and (ii) Increasing the electrical resistance between the rails and ground. The first measure makes current return through the ground less likely. Steps taken in this direction include the use of heavier rail sections, continuously welded rails, improved rail bonding and reduced spacing between substations. It is desirable to combine substations with passenger stations. At passenger stations current flow is highest due to acceleration of trains. This combination ensures that these peak currents have a very short return path. The rail to soil resistance can be increased by using insulators placed between the rails and concrete or wooden ties and by using insulated rail fasteners. Stray current concerns are particularly relevant when older


rail systems are integrated with newer designs. The higher current demand of modern, high-speed vehicles poses increased stray current risks in the older sections.


11 Basics of Corrosion MonitoringCorrosion can lead to failures in plant infrastructure and machines, which are usually costly to repair, costly in terms of lost or contaminated product, in terms of environmental damage, and ultimately it may be costly in terms of human safety. Decisions regarding the future integrity of a structure or its components depend entirely upon an accurate assessment of the conditions affecting its corrosion and rate of deterioration. With this information, an owner can make an informed decision as to the type, cost and urgency of remedial measures. Monitoring corrosion characteristics of a proposed or existing structure can lead to the proper selection of longer life materials, durable and protective coatings and corrosion control measures.

Modern corrosion monitoring technologies can emphasize the highly time-dependent nature of corrosion damage. The integration of corrosion monitoring technology in existing systems can also provide early warning of costly corrosion damage and provide information on where the damage is taking place.

In a modern business environment, successful enterprises cannot tolerate major corrosion failures, especially when these involve personal injuries, fatalities, unscheduled shutdowns and environmental contamination. For this reason considerable effort must be expended in corrosion control at all stages of a system's life, from the design table to the last day of operation. Typically, once a system, a plant or any piece of equipment is put into service, maintenance is required to keep it operating safely and efficiently. This is particularly true for ageing systems and structures that will be required to operate beyond their original design life.

Correct and effective corrosion monitoring strategies should be used as a proactive tool to assist with operating a plant more effectively, thereby prolonging its life and gaining optimum throughput. It also enables continuous monitoring of actual corrosion rates, allowing for timely preventative action if a variance is observed.

Current corrosion inspection and monitoring typically requires planned periodic shutdowns to inspect equipment. Scheduled shutdowns are costly in terms of productivity losses, restart energy and material costs. Unscheduled shutdowns are disruptive and often quite expensive. Internal corrosion failures result in costly cross contaminations of product and process streams. External corrosion leaks put process fluids into the plant environment and can create significant safety hazards.

Corrosion monitoring systems vary significantly in complexity, from simple coupon exposures or hand held data loggers to fully integrated plant process surveillance units with remote data access and data management capabilities. CLIMAT devices, for example, have been used for more than three


decades to monitor atmospheric corrosivity. These units have been utilised successfully around the world in marine and industrial type atmospheres.

Other sensors (left) or devices will produce a signal that needs to be recorded and analysed further for an estimation of the severity of a situation. By combining such signal with the knowledge of a process or environment it allows an operator to be proactive. Experience has shown that the potential cost savings resulting from the implementation of corrosion monitoring programs generally increase with the sophistication level (and cost) of the monitoring system. Corrosion monitoring is more complex than the monitoring of most other process parameters because:

There are a number of different types of corrosion,

Corrosion may be uniform over an area or concentrated in very small areas (pitting),

General corrosion rates may vary substantially, even over relatively short distances,

There is no single measurement technique that will detect all of these various conditions.

It is therefore helpful to have previous history or even a rough estimate of the types of corrosion problems to be investigated. It is also advisable to use several complementary techniques rather than rely on a single monitoring method. Real time monitoring of pipelines, vessels and other static equipment enables a near instantaneous appraisal of the corrosivity of produced and transported fluids.

On-line nature monitoring means that corrosion information is immediately available to the operator. If corrosion activity increases as a result of process non-conformities, the corrosion information can be viewed alongside process variables (including chemical injection data) such that cause-and-effect can be determined and rapid action can be taken to overcome the problem. Effectiveness of remedial action or treatment can be similarly proven.

11.1 Need for Corrosion MonitoringThe rate of corrosion dictates how long any process plant can be usefully and safely operated. The measurement of corrosion and the action to remedy high corrosion rates permits the most cost effective plant operation to be achieved while reducing the life-cycle costs associated with the operation. Corrosion monitoring techniques can help in several ways:

By providing an early warning that damaging process conditions are developing


By revealing the correlation between changes in process parameters and their effect on system corrosivity

By diagnosing a particular corrosion problem, identifying its cause and the rate controlling parameters, such as pressure, temperature, pH, flow rate, etc.

By evaluating the effectiveness of a corrosion control/prevention program

By providing management the information necessary to relate maintenance requirements to ongoing conditions of Operation.

11.2 Corrosion Monitoring TechniquesCorrosion monitoring is the practice of measuring the corrosivity of process stream conditions by the use of "probes" which are inserted into the process stream and which are continuously exposed to the process stream condition. Corrosion monitoring "probes" can be mechanical, electrical, or electrochemical devices.

These probes are an essential element of all corrosion monitoring systems. The nature of the sensors depends on the various individual techniques used for monitoring but often a corrosion sensor can be viewed as an instrumented coupon. In older systems, electronic sensor leads were usually employed for these purposes and to relay the sensor signals to a signal-processing unit. Advances in microelectronics are facilitating sensor signal conditioning and processing by microchips, which can essentially be considered to be integral to the sensor units.

Some corrosion measurement techniques can be used on-line, constantly exposed to the process stream, while others provide off-line measurement, such as that determined in a laboratory analysis. Some techniques give a direct measure of metal loss or corrosion rate, while others are used to infer that a corrosive environment may exist.

Real-time corrosion measurements refer to highly sensitive measurements, with a signal response taking place essentially instantaneously as the corrosion rate changes. Numerous real-time corrosion monitoring programs in diverse branches of industry have revealed that the severity of corrosion damage is rarely uniform with time. Complementary data from other relevant sources such as process parameter logging and inspection reports can be acquired together with the data from corrosion sensors, for use as input to the management information system. Direct techniques include:

Corrosion Coupons

Electrical Resistance

Inductive Resistance Probe

Linear Polarisation Resistance (LPR)


Electrochemical Impedance Spectroscopy

Harmonic Analysis

Electrochemical Noise

Accoustic Emission.

For practical applications on civil structures only LPR and Accoustic emission are of importance, with the former being generally significantly cheaper than Acoustic emission. More often than direct techniques indirect techniques are used in the construction industry. Indirect Techniques include:

Corrosion Potential (Halfcell),

Hydrogen Monitoring (Barnacle Electrodes),

Chemical Testing.

11.3 Selecting Monitoring PointsThe selection of monitoring points is of paramount importance as corrosion factors to be considered are often related to the geometry of systems and components. Selection of these points should be based on a thorough knowledge of process conditions, materials of construction, geometrical details of the system, external factors and historical records.

As only a finite number of points can be considered, it is usually desirable to monitor the "worst-case" conditions, at points where corrosion damage is expected to be most severe. Often, such locations can be identified by reasoning with basic corrosion principles, analysis of in-service failure records and in consultation with operational personnel. For example, the most corrosive conditions in water tanks are usually found at the water/air interface. In order to monitor corrosion under these conditions, corrosion sensors could be attached to a floating platform to maintain these conditions as the water level changes.

In practice, the choice of monitoring points is also dictated by the existence of suitable access, especially in pressurized systems. It is usually preferable to use existing access points, such as flanges, for sensor installations. If it is difficult to install a suitable sensor in a given location, additional by-pass lines with customized sensors and access fittings may represent a practical alternative. One advantage of a bypass is the opportunity of manipulating local conditions to highly corrosive regimes in a controlled manner, without affecting the actual operating plant.

11.4 Data Integration in Corrosion MonitoringReal-time corrosion measurements refer to highly sensitive measurements, with a signal response taking place essentially instantaneously as the corrosion rate changes. Numerous real-time corrosion monitoring programs


in diverse branches of industry have revealed that the severity of corrosion damage is rarely uniform with time. Complementary data from other relevant sources such as process parameter logging and inspection reports can be acquired together with the data from corrosion sensors, for use as input to the management information system.

11.5 Acoustic Emission (AE)Corrosion monitoring with AE is based on measuring acoustic sound waves that are emitted during the growth of microscopic defects, such as stress corrosion cracks. The sensors can thus essentially be viewed as microphones, which are strategically positioned on structures. The sound waves are generated from mechanical stresses generated during pressure or temperature changes. Background noise effects have to be taken into consideration and can be particularly troublesome in on-line measurements.

Acoustic Emission (AE) is a transient elastic wave generated by the rapid release of energy accumulated in stressed materials. In the early of 1960s it was recognized that growing cracks and discontinuities in pressure vessels could be detected by their AE signals. A new non-destructive testing technology was born as it was stated that small scale damage is detectable long before failure. Formally, the AE may be defined as the class of all physical phenomena where elastic waves are generated by localised sources.

Sources of AE include many different mechanisms of deformation and fracture of materials, material corrosion, surface rubbing, micro earthquakes and rockbursts, leaks from vessel, storage tanks, pipes, flanges, seals, etc. Sources in metals have been identified like crack growth, moving dislocations, slip, twinning, grain boundary sliding, fracture, decohesion of inclusions. In composites, the well-known sources are matrix cracking, breaking of fibers, delamination and interface disbonding. These mechanisms typify the classical response of materials to the applied load. The AE method offers the following advantages over the other NDT methods:

AE is a dynamic inspection method in that it provides a response to discontinuity growth under structural stress.

Static, not dangerous discontinuities will not generate AE signals.

AE can detect and evaluate the significance of discontinuities and of an entire structure during a single test.

Since only limited access is required, defects may be detected that are inaccessible to the more traditional NDT methods.

Vessels and other pressure systems can often be requalified during an in-service inspection that requires little or no downtime.


The AE method may be used to prevent catastrophic failure of system with unknown defects, and to limit the maximum pressure during system operation.

11.6 Corrosion Potential MonitoringThe measurement of the corrosion potential is a relatively simple concept, with the underlying principle widely used in industry for monitoring reinforcing steel corrosion in concrete and structures such as buried pipelines under cathodic protection. monitoring of anodic protection systems is a further application area. Changes in corrosion potential can also give an indication of active/passive behaviour in stainless steel.Furthermore, when viewed in the context of Pourbaix diagrams, the corrosion potential can give a fundamental indication of the thermodynamic corrosion risk.

The corrosion potential is measured relative to a reference electrode, which is characterised by a stable half-cell potential. The electrochemical details of important reference electrodes are explained elsewhere in this document. Corrosion potential measurements are usually classified as an intrusive indirect method. Either a reference electrode (and possibly a separate sensor of the material to be monitored) has to be introduced into the corrosive medium for these measurements, or an electrical connection has to be established to a structure in conjunction with an external reference electrode.

11.7 Linear Polarisation Resistance (LPR)Polarisation resistance is particularly useful as a method to rapidly identify corrosion upsets and initiate remedial action, thereby prolonging plant life and minimising unscheduled downtime. The technique is utilised to maximum effect, when installed as a continuous monitoring system. This technique has been used successfully for over thirty years, in almost all types of water-based, corrosive environments. Some of the more common applications are:

Cooling water systems

Secondary recovery system

Potable water treatment and distribution systems

Amine sweetening

Waste water treatment systems

Pickling and mineral extraction processes

Pulp and paper manufacturing

Hydrocarbon production with free water.


The measurement of polarisation resistance has very similar requirements to the measurement of full polarisation curves. There are essentially four different methods of making the measurement according to whether the current or the potential is controlled and whether the current (or potential) is swept smoothly from one value to another, or simply switched between two values. In addition the measurement may be made between two nominally identical electrodes (a two-electrode system), or a conventional three-electrode system (working, reference and counter) may be used.

The principle of LPR measurements is shown below, with a view of some of the fundamental pitfalls of the technique. With this widely used technique in corrosion monitoring, the polarisation resistance of a material is defined as the slope of the potential current density (DE/Di) curve at the free corrosion potential, yielding the polarisation resistance Rp. This can be itself related to the corrosion current with the help of the following approximation:

Where Rp is the polarization resistance and

Where icorr the corrosion current and B an empirical polarization resistance constant that can be related to the anodic (ba) and cathodic (bc) Tafel slopes. The Tafel slopes themselves can be evaluated experimentally using real polarization plots. The corrosion currents estimated using these techniques can be converted into penetration rates using Faraday's law or a conversion chart.

The study of uniform corrosion or studies assuming corrosion uniformity are probably the most widespread application of electrochemical measurements both in the laboratory and in the field. The widespread use of these electrochemical techniques does not mean that they are without complications. Both linear polarization and Tafel extrapolation need special precautions for their results to be valid. The main complications or obstacles in performing polarization measurements can be summarized in the following categories:

Effect of Scan Rate: The rate at which the potential is scanned may have a significant effect on the amount of current produced at all values of potential. The rate at which the potential is changed, the scan rate, is an experimental parameter over which the user has control. If not chosen properly, the scan rate can alter the scan and cause a misinterpretation of the features.

Effect of Solution Resistance: The distance between the Luggin capillary (of the salt bridge to the reference electrode) and the working


electrode is purposely minimized in most measurements to limit the effect of the solution resistance. In solutions that have extremely high resistivity, this can be an extremely significant effect.

Changing Surface Conditions: Since corrosion reactions take place at the surface of materials, when the surface is changed, due to processing conditions, active corrosion or other reasons, the potential is usually also changed. This can have a strong effect on the polarization curves.

Determination of Pitting Potential: In analyzing polarization curves the appearance of a hysteresis (or loop) between the forward and reverse scans is often thought to denote the presence of localized corrosion (pitting or crevice corrosion).

11.8 Corrosion InformationHumans are used to working with imprecise information and they naturally accept vague use of language, making continuous interpretations of the information they receive based upon context. This section introduces a generic corrosion framework linking mechanistic principles leading to corrosion damage with the observable signs of a corrosion attack.

Sixteen recognised corrosion experts accepted to complete an opinion poll listing the main sub-factors and the common forms of corrosion. The responses were then analysed.


12 Failure AnalysisThe depth of the analysis into the roots of the failure is the key to accurately unearthing all of the failure sources. Looking at machinery failures one finds that there are:

Physical Roots - The physical reasons why the parts failed.

Human Roots - The human errors of omission or commission that resulted in the physical roots.

Latent Roots (Management System Weaknesses) - The deficiencies in the management systems or the management approaches that allow the human errors to continue unchecked.

The more detailed the analysis, the better we understand all the events and mechanisms that contribute as the roots of the problem. We generally think of dividing analyses into three categories in order of complexity and depth of investigation and they are:

Component Failure Analysis (CFA), which looks at the piece of the machine that failed, for example, a bearing or a gear, and determines that it resulted from a specific cause such as fatigue or overload or corrosion and that there were these x, y, and z influences.

Root Cause Investigation (RCI) is conducted in much greater depth than the CFA and goes substantially beyond the physical root of a problem to find the human errors involved. It stops at the major human causes and doesn't involve management system deficiencies. RCI's are generally confined to a single operating unit.

Root Cause Analyses (RCA) which includes everything the RCI covers plus the minor human error causes and, more importantly, the management system problems that allow the human errors and other system weaknesses to exist. An RCA can sometimes extend to sites other than the one involved in the original problem.

Although the cost increases as the analyses become more complex the benefit is that there is a much more complete recognition of the true origins. Using a CFA to solve the causes of a component failure answers why that specific part or machine failed and can be used to prevent similar future failures. Progressing to an RCI, we find the cost is five to ten times that of a CFA but the RCI adds a detailed understanding of the human errors contributing to the breakdown and can be used to eliminate groups of similar problems in the future. However, conducting an RCA and correcting the major roots will eliminate huge classes of problems.


13 Deterioration and Risk ModellingThere a numerous numerical options available for numerical analysis of deterioration and for service life prediction and only a fraction can be presented in this leaflet as most of them are also tailored to each situation.

13.1 Straight Line RegressionMost standards, e.g. BS 8007, give deterministic corrosion rates for certain exposure conditions in mm/year/surface of exposed steels. This approach, however, is oversimplistic as corrosion does not occur at a linear rate. Furthermore, this approach does not allow for the assessment of concrete structures, where there is a significant initiation period prior to the commencement of corrosion.

13.2 Markovian Deterioration ModelBridge maintenance strategies are developed from the knowledge of the deterioration rate of each particular structure and are determined from inspection or test data. Complex models are often derived to describe the physical processes of degradation and are used in concert with test data to predict the deterioration rate. A vast quantity of data is required for these models which can only be collected from expensive physical testing techniques and, therefore, this approach is generally not feasible for use with bridge inventories containing thousands of bridges.

An alternative method makes use of inspection records collected from the assessment of the bridge stock and already exists for the majority of bridges constructed. Bridges are assessed according to a predefined set of discrete states which represent various stages in the deterioration process. Each state is assigned a numerical value and definition describing the defect condition and bridges are classified according to the defect state observed during the inspection. The condition states may be used to track the transition of bridge members form one state to another as they deteriorate during their service life. Deterioration models can be developed to calculate the probability of a given member being in a certain condition state after a particular age.

A Markovian deterioration model is mainly used to describe the transition probabilities for each condition state. Markov chains are appropriate because they divide time into discrete, equal intervals and future predictions are dependent only on the current or initial condition. This approach reduces the complexity of the modelling process and can be readily incorporated into a computational algorithms developed for maintenance strategies. This techniques has already been successfully used in various maintenance systems including bridge management systems such as POINTIS and BRIDGIT.


13.3 Risk and Fatigue Life Assessment MethodsMost of the chloride corrosion damaged structures have already been in service for a number of years, but standard methods for the assessment of hydrogen induced damage, such as CERT, are not suitable to give information on the influence of hydrogen charging on the fatigue behaviour of the structural element. The fatigue performance, however, is of utmost importance for a deteriorating structure, in particular when it is influenced by the repair method. Mechanical tests that have been discussed previously constitute a go/no-go approach because they either show or do not show any effect of hydrogen charging on steel, but a risk and fatigue life assessment should also be accomplished when the performance of electrochemical repair techniques is evaluated.

Structural reliability is defined as the probability of a structural element to survive a specified period (the design or service life) where the members’ resistance to load is higher than the applied load (or load cycles). There are two different approaches to deal with reliability issues in design. In deterministic methods a high value of load and a low value of resistance is assumed and the margin between load and resistance (the safety margin) must not fall under a specified safety factor. Probabilistic design recognises that both values have a random nature with a mean and a distribution.

Risk assessment is used as decision making tool for probabilistic designs. Theprocess of risk assessment consists in defining a system in which the process takes place and identifying the possible hazards and their probability of occurrence. Sensitivity analysis is carried out to reveal data with impact on the model. For this data a very careful examination is necessary to find the correct statistical distribution. The risk assessment is followed by the decision on the risk treatment, which can be avoidance, reduction, transfer or acceptance. The random generation of data based on one statistical distribution in a simulation will only reproduce the distribution and not give a clearer insight and therefore more than one random variable with more than one distribution should be used in simulations.

The “conservative best estimate” which is sometimes used in probabilistic risk assessments is said to be meaningless and in cases where such an estimate is likely to be employed Monte-Carlo-Simulation should be the preferred option. Monte-Carlo analysis is applied to systems that are too complex or too large to be solved deterministically.


13.3.1 Structural Reliability Models

Reliability can be defined as the probability of loss or injury to people and property due to the failure of systems. In 1940 Robert Lusser, a German mathematician, derived the product law of reliability for components in series, REs, where a series system is equal to the product of reliabilities of its components REn:

n

nns RERE

1

The reliability of structural components decreases with increasing age of the structure due to repetitive loading. Sometimes reliability is weighted with a factor to accommodate for the consequence of a risk:

failureofeConsequencfailureofobabilityRisk Pr

The reliability of a structural component will decrease with increasing age of the structure and the risk of failure will increase. The age of a structure is sometimes expressed not in years but through its load history. For structural design the load history is expressed in stress ranges from a stress spectrum and summarised using Miner’s rule as follows:

n

n n

n

Nn

1,

where n is the number of load cycles from a stress range n applied to the structure and N is the number of cycles of the stress range n to failure. Risk analysis in structural reliability models is a three-stage process. First, a preliminary hazard analysis is carried out where possible hazards, their extend and origin is identified. Then a sequence of hazards is put together in a fault tree and third, the consequences of the hazard are analysed. In Monte-Carlo Analysis a statistical function is imposed on individual branches of the fault tree.

Fault trees used to assemble the different distributions in a Monte Carlo Analysis are heuristic and based on human judgement. The predictive qualities of Monte-Carlo analysis increases with increasing number of tests, not only in terms of Monte-Carlo cycles, but also for the amount of different statistical distributions imposed on the system. The model will only repeat the distribution and not give new knowledge if only one statistical distribution is used throughout the system.

13.3.2 Use of the Weibull Distribution for Fatigue Life Assessment

The two-parameter Weibull distribution model is used for fatigue damage assessment of structures, but the assessment obtained from this distribution is not very accurate. Other authorities, on the other hand, make extensive use of the two-parameter Weibull distribution for their probabilistic life assessment. The cumulative density function of this distribution is given as:


S

s

ct

natetF

1)(

where tc is the live, s the scale parameter and s the shape parameter. The shape parameter indicates whether the failure rate is constant (s=1), increasing (s>1) or decreasing (s<1). Decreasing failure rate thereby indicates “infant mortality” and is typical for the “burn-in” period of electrical components. The scale parameter gives the point in time where 63.2 % of the elements will have failed. The reliability or the survival probability is given as:

s

s

ct

natetF

)(

The two-parameter Weibull distribution assumes that total failure (collapse) can occur from the beginning of the life. This assumption might be correct for electrical components such as light bulbs, but it cannot be transferred to civil engineering structures because this would mean that a bridge for example could collapse from the very first moment of its completion. Such events are very rare and are always related to gross design or construction errors, and must therefore be excluded.

A shift parameter, s, is introduced into the two-parameter distribution and gives the three-parameter Weibull distribution. The shift parameter marks a certain point in time before which failure does not occur. When data points correspond better to a three-parameter distribution than a two-parameter distribution, the plot of data on a Weibull graph appears to have a convex shape rather than a straight line. By shifting the points towards higher age a straight line is obtained and the distance between original data points and shifted distribution (age) is the shift parameter. The cumulative density function of the three-parameter Weibull function is given as:

s

s

sct

natetF

1)(

and the reliability or the survival probability is given as:

s

s

sct

natetF

)(

It is sometimes recommended plotting the survival probability in order to compare the performance of systems given by different distributions. The plot of the survival graph in a three-parameter Weibull distribution starts after the shift parameter, s, is exceeded.


The Weibull distribution is used to model data that results from fatigue tests, where specimens are tested to complete failure. The goodness of fit of data to a statistical distribution is obtained with a test. The most common test is the Xi2 – goodness-of-fit test, but it requires a minimum amount 15 of data points. Sometimes, however, tests are very time consuming or expensive and less data points are obtained. The goodness-of-fit for less than 15 data points is obtained using the Kolmogorov-Smirnov or the Anderson-Darling goodness-of-fit tests.


14 Cathodic ProtectionThe science of cathodic protection (CP) was born in 1824, when Sir Humphrey Davy made a presentation to the Royal Society of London: "The rapid decay of the copper sheeting on His Majesty's ships of war, and the uncertainty of the time of its duration, have long attracted the attention of those persons most concerned in the naval interest of the count. ... I entered into an experimental investigation upon copper. In pursuing this investigation, I have ascertained many facts ... to illustrate some obscure parts of electrochemical science... seem to offer important application." Davy succeeded in protecting copper against corrosion from seawater by the use of iron anodes. From that beginning, CP has grown to have many uses in marine and underground structures, water storage tanks, gas pipelines, oil platform supports, and many other facilities exposed to a corrosive environment. Recently, it is proving to be an effective method forprotecting reinforcing steel from chloride-induced corrosion.

The basic principle of CP is simple. A metal dissolution is reduced through the application of a cathodic current. Cathodic protection is often applied to coated structures, with the coating providing the primary form of corrosion protection. The CP current requirements tend to be excessive for uncoated systems. The first application of CP dates back to 1824, long before its theoretical foundation was established. Cathodic protection has probably become the most widely used method for preventing the corrosion deterioration of metallic structures in contact with any forms of electrolytically conducting environments, i.e. environments containing enough ions to conduct electricity such as soils, seawater and basically all natural waters. Cathodic protection basically reduces the corrosion rate of a metallic structure by reducing its corrosion potential, bringing the metal closer to an immune state. The two main methods of achieving this goal are by either:

Using sacrificial anodes with a corrosion potential lower than the metal to be protected,

Using an impressed current provided by an external current source.

14.1.1 Corrosion Costs and Preventive Strategies Study

The cost of cathodic protection of metallic structures subject to corrosion can be divided into the cost of materials and the cost of installation and operation. Industry data have provided estimates for the 1998 sales of various hardware components totalling $146 million. The largest share of the cathodic protection market is taken up by sacrificial anodes at $60 million, of which magnesium has the greatest market share. Major markets for sacrificial anodes are the water heater market and the underground storage tank market. The costs of installation of the various cathodic protection (CP) components for underground structures vary significantly depending on the location and the specific details of the construction. For


1998, the average total cost for installing CP systems was estimated at $0.98 billion (range: $0.73 billion to $1.22 billion). The total cost for replacing sacrificial anodes in water heaters and the cost for corrosion-related replacement of water heaters was estimated at $1.24 billion per year; therefore, the total estimated cost for cathodic and anodic protection is $2.22 billion per year.

14.2 Sacrificial AnodesThe earliest experiments on cathodic protection were performed with zinc anodes that were electrically connected to copper plates immersed in seawater. As can be seen on the galvanic series, such an arrangement would produce a cathode (copper) and an anode (zinc). In the large galvanic cell so formed, the zinc cylinder corroded away in a manner to protect the copper substrate. This method of cathodic protection can be used with other combination of metals providing the necessary current to the metal to be protected, as Sir Humphry Davy and Michael Faraday illustrated almost two centuries ago.

When two metals are electrically connected to each other in a electrolyte e.g. seawater, electrons will flow from the more active metal to the other, due to the difference in the electrical potential, the so called "driving force". When the most active metal (anode) supplies current, it will gradually dissolve into ions in the electrolyte, and at the same time produce electrons, which the least active (cathode) will receive through the metallic connection with the anode. The result is that the cathode will be negatively polarised and hence be protected against corrosion. To calculate the rates at which these processes occur, one has to understand the electrochemical kinetics associated with the complex sets of reactions that can all happen simultaneously on these metals.

14.2.1 Sacrificial anode material

The galvanic series shows that magnesium heads the list as the most anodic metal and is widely separated from iron in the galvanic series. Magnesium coupled to iron provides sufficient galvanic potential to provide positive protection. An important feature of a sacrificial anode system is that it is inherently a safer system than impressed cathodic protection systems because the normal potentials generated are insufficient to damage coatings present on the surface to be protected. Because of the low potentials generated, sacrificial systems can be used only in low-resistance soils, i.e., with a resistivity less than 3000 cm.

Zinc Anodes The equilibrium potential of zinc is -763 mV vs. SHE (-1000 mv vs SCE) and therefore it is anodic to iron (equilibrium potential -440 mV vs. SHE). However, the polarity of the zinc-iron couple could reverse, with the iron becoming anodic to the zinc when the temperature is raised above 60oC in an aerated electrolyte. This limits the use of zinc anodes to ambient temperature applications. Zinc anodes are generally specified to strict chemical compositions because of harmful impurities, which impair the


performance of the anode. The most detrimental impurity is iron. If present in excess of 0.0014%, iron will precipitate out as discrete intermetallic particles which will set up local galvanic cells on the zinc anode.

This has the effect of reducing anode efficiency. Moreover, the corrosion products of this self corrosion, namely the electrically non-conductive zinc hydroxide, tend to induce passivity making the anode redundant. This problem is effectively overcome by the addition of aluminium and silicon which combine with the iron to form less harmful intermetallics. Cadmium is also added to promote uniform corrosion and the formation of non-adherent corrosion products. As a result anode efficiencies greater than 90% can be achieved. Zinc anodes, because of their driving voltage, are suitable for the protection of coated pipelines in high conductivity electrolytes, such as in seawater, and at ambient temperatures. Zinc anodes are also used in applications where spark hazards need to be avoided as in storage tanks containing flammable hazards.

Aluminium Anodes Aluminium has an equilibrium potential of -1.90 V vs. SCEand therefore thermodynamically it is a very reactive metal. However, in most natural environments aluminium has been found to be stable due to the formation of a thin protective oxide.

This film has a high electrical resistance and thus renders pure aluminium useless as a sacrificial anode. Even when galvanically coupled to a more active metal in a chloride containing environment aluminium corrodes by pitting rather than uniform dissolution. But in the light of aluminium other favourable properties, including a large electrochemical equivalent, relatively low cost and low density, attempts have been made to modify this oxide film. To disrupt the physical integrity of the protective film several aluminium alloys have been developed. Alloying elements which were found to induce activation included Hg, In, Sn, Ga, Bi, Zn, Cd, Mg and Ba. Commercially successful aluminium anodes mainly contain zinc (0.5-4%) with either indium (0.01-0.05%) or mercury (0.030-0.050%). The current capacity of such alloys is in the region of 2500 A hr/kg and anode efficiencies can be greater than 90%.

The role played by the zinc in the enhancement of aluminium activation is thought to be due to a modification of the passive film on aluminium. For mercury, it is believed that Hg2+ ions dissolved near the anode interface are plated scale (as Hg) at the weak points. The amalgamated areas formed may either allow direct corrosion through the amalgam or enhance the adsorption of Cl- ions and cause film breakdown. Similarly, indium ions are also thought to plate at film defects and thus prevent repassivation by enhancing chloride adsorption. Further since mercury and indium are poor cathodes for hydrogen evolution, this results in higher anode efficiencies.

14.2.2 Anode Efficiency

A prospective sacrificial anode must possess a large number of electrons per unit mass and should deliver these electric charges efficiently. Thus the electrical output of an anode is given by current capacity which is expressed


in Ampere-hours kg-1 or kg A-1 y-1. The value of the current capacity is determined by the electrochemical equivalent, the density and the efficiency of the anodic material. The electrochemical equivalent, which is dependent on the atomic weight and valence, is a characteristic of the anode material. However efficiency is determined by a number of factors including nature of the environment, operating current density and metallurgical microstructure. It is apparent that if the cathode reaction rate on the anode is low then the efficiency will be high, so that there is minimum self corrosion. Similarly large operating currents will yield high anode efficiency. It should be added that the type of corrosion attack experienced by the anode also significantly affects the magnitude of the anode efficiency. For instance, severe pitting and intergranular attack may result in a chunk of the anode to become detached without complete consumption of the electric charge in that piece.

14.2.3 Protective current requirements in CP design

Assumptions of protective current requirements and bare metal areas. To obtain a starting point, certain general assumptions have been found helpful:

For bare metal in the ground, a current of 11 to 22 mA/m2 of bare metal surface has been found adequate, except under extreme or unusual conditions. This value must then be modified to suit the particular conditions.

For coated pipe, the current required is difficult to estimate without field tests. The primary reason is the unknown condition of the protective coat which can vary from nearly 0% to 98% coverage. For a fairly new protective coat properly applied, assume 2 percent bare and 22 mA/m2 for use in tentative calculations. Field test may show that this figure should be modified.

Bare pipelines can usually be protected by 11 to 22 mA/m2. This is seldom justifiable economically for extensive or long lines, however, and the necessary protection is usually afforded by the application of cathodic protection to localized areas called "hot spots."

Bare steel tanks are treated the same as bare pipelines. Inside steel surfaces in contact with fresh water at zero or low velocities require from 22 to 65 mA/m2, depending on the nature of the water. The low value is used for water which is scale forming. That is, the water will form a calcareous coating on the surface of the metal.

Protecting steel surfaces in contact with water in motion presents another problem. Water in motion produces a scouring effect which prevents the formation of the above-mentioned coating and even the formation of a hydrogen film. Therefore, surfaces exposed to water in motion require a higher current density. The amount required is hard to predict. In this case, an experimental determination of the current requirement should be made.


14.2.4 Designing a sacrificial anode system

Several factors enter the determination as to how many sacrificial anodes may be required for a given structure and corrosion problem and the manner of distributing them with respect to the location where corrosion is occurring. The anode requirements for a small installation will normally involve the steps taken in the following examples. For cathodic protection of larger structures involving use of six or more anodes or an impressed current (rectifier) system, additional steps must be taken to assure proper functioning of the system, i.e., proper distribution of the anodes, prevention of damage to other buried metal work, design of an economic system, and proper operation and maintenance.

Example Determine the galvanic anode requirements for a cathodic protection system of 45.7 m of 0.1 m diameter coated pipe buried in the ground for a service life of ten years. Assume that the magnesium anodes considered for this application weight 3.6 kg and can provide 1000 Ah per 0.9 kg of metal, i.e. approximately 50% of the faradic capacity.

Required data

1. Knowledge of the condition of pipe protective coating

2. Soil resistivity in ohm-centimetres (do not use sacrificial anodes in soil whose resistivity exceeds about 3,000 ohm-centimetres.

3. Assume a current demand.

4. Protective current required is equal to area of bare metal to be protected times the required current.

5. Number of anodes required must be computed.

Data and assumptions for the problem

1. Pipe surface 5% bare.

2. Soil resistivity determined as 1,000 ohm-centimetres.

3. Assume 11 mA/m2 current density demand for bare steel.

Solution:

The protective current required is the total area of bare steel in square meters times the required current per square meter.

Surface exposed = length of pipe (m) x pipe circumference (m) x fraction exposed

Current demand = Surface exposed x current density For the example of 45.7 m of pipe in 1,000-ohm-centimeter soil:

Surface exposed = 45.7 x 0.3597 x 5/100 = 0.822 m2

Current demand = 0.822 x 11 = 9.04 mA or 0.00904 A.


Number of anodes equal to total current required times the installation life divided by the ampere-hour rate of the magnesium anodes.

Protection capacity required (Ah) = Current demand x life of the system to be protected

Number of anodes = Capacity required / capacity of each anode

The ampere-hour rating varies with different conditions but 0.90 kg of magnesium can be rated at about 1000 ampere-hours (Ah). Thus, a 3.6-kg magnesium anode would be expected to deliver about 4,000 ampere-hours.

Protection capacity = 0.00904 x 24 (hours/day) x 365 (days/year) x 10 years

Protection capacity = 792 Ah

In this case, a single 3.6-kg magnesium anode delivering 4000 Ah could provide five times the required life with an indicated useful life of 50 years.

14.3 Impressed Current Cathodic ProtectionCathodic protection can be also applied if the metal to be protected is coupled to the negative pole of a direct current (DC) source (schematic), while the positive pole is coupled to an auxiliary anode. Since the driving voltage is provided by the DC source there is no need for the anode to be more active than the structure to be protected. There are basically three types of anode materials:

Inert or non consumable anodes,

Semi-consumable anodes,

Consumable anodes.

Schematic of a CP system (right).

All items to be protected shall be electrically connected and should have a welded or brazed connection to an anode. For bolted or clamped assemblies without an all welded brazed electrical grounding, the electric resistance should be less than 0.10 ohm. Coating on contact surfaces shall be removed prior to assembly. In the impressed current CP, the large electrochemical is formed between an anode and the structure to be protected by a power supply that is controlled by reading a reference electrode close to the structure.


14.3.1 Non Consumable Anodes

This type of anode supports other anodic reactions on their surfaces. In environments where water and chloride ions are present, chlorine evolution and oxidation of water are possible.

Platinised substrates: Platinum is the ideal permanent impressed current anode material. It is one of the most noble metals and in practically all environments forms a thin invisible film, which is electrically very conductive. In addition, the exchange current densities of most anodic reactions on the Pt surface are greater than on other anode materials. Due to its high cost, platinum is applied as a thin coating (1-5 m) on metallic substrates such as titanium, niobium and tantalum.

Platinised titanium is often used in marine environments. To avoid the dissolution of titanium at unplatinised locations on the surface, the operating voltage of the anode is limited by the anodic breakdown potential of titanium which is in the range of 9 to 9.5 V in the presence of chlorides. Hence the maximum recommended operating voltage of platinised titanium anodes is 8 V. The corresponding maximum current density output is approximately 1 kA m-2. For cathodic protection systems where operating voltages are relatively high, niobium and tantalum based anodes are generally selected. This is because these two substrates have anodic breakdown potentials greater than 100 V in chloride containing electrolytes. The wastage rate of platinised anodes is approximately 8 mg A-1 y-1.

The rate of platinum consumption has been found to accelerate in the presence of AC current ripple. Most wastage was observed to occur with AC frequencies of less than 50 Hz. The repeated oxidation/reduction processes result in the formation of a brownish layer of platinum oxide. To avoid the occurrence of this phenomenon, a single or a three phase full-wave rectification is recommended. The consumption rate of platinised anodes is also adversely affected by the presence of organic impurities such as sugar and diesel fuel.

Magnetite: Magnetite is a cheap and naturally occurring material. It is a non-stoichiometric oxide and has an electrical conductivity of 1.25 -1 m-1. Due to its brittleness, the anode is cast as a hollow cylinder and closed at one end. The inner surface is then copper plated and the cylinder is filled with polystyrene. Epoxy resin is used to fill any remaining space. The anode cable is soldered to the copper plate. Magnetite anodes have been successfully used in the cathodic protection of buried structures and those immersed in seawater. The maximum operating current density is 0.115 kA m-2 and the anode consumption rate is approximately from 1 to 4 g A-1y-1.

Lida: This is a recently developed anode. It is claimed that it has superior mechanical, consumption and electrochemical properties compared with conventional anodes. The anode is composed of an inert metal oxide , ruthenium oxide coated titanium. The operating current density is 0.8 kA m-

2 and the consumption rate is in the range of 0.8 mg A-1 y-1.


14.3.2 Semi-Consumable Anodes

Semi-consumable anodes such as graphite and high silicon iron have been in service since the first industrial electrochemical systems were built.

Graphite: Graphite anodes are widely used. Carbon has been used as an anode in chlorine production since the end of the nineteenth century. Graphite, which is less porous and more electrically conductive, is now preferred for use in impressed anode materials. However, graphite can still be highly porous, with the porosity being exacerbated by gas evolution. For this reason, graphite is often impregnated with resins to reduce solution ingress and improve mechanical strength. Graphite anodes are inert when chlorine evolution is occurring, chlorine being produced efficiently at low polarisation. But if oxygen formation predominates, as in low chlorine media, graphite is oxidised to carbon dioxide. Graphite deterioration also increases with decreasing pH and increasing sulfate ions concentration.

To eliminate the possibility of galvanic corrosion caused by detached pieces, graphite is not recommended for use in closed systems. In addition, graphite suffers high consumption rates in water at temperatures above 50oC. Consumption rates measured for graphite depend on the environment and thus range from 0.045 in seawater to 0.45 kg A-1 y-1 in freshwater. Similarly the corresponding operating current densities vary from 2.5 to 10 A m-2. The maximum operating voltage for graphite anodes is only limited by excessive consumption rate and brittleness of the material. The main disadvantages of graphite compared to other impressed anodes are low operating current densities and inferior mechanical strength. Graphite is generally used in conjunction with carbonaceous back-fills in soil based impressed anode systems.

High silicon iron (HSI) alloys: These anodes are widely used. HSI anodes contain about 14.5% silicon and certain alloys have 4.5% chromium. Chromium has now replaced molybdenum as an alloying element in this type of anode. The high silicon content ensures that the alloy forms a protective film containing silicon dioxide, SiO2. A prerequisite for the formation of this film is that the anode must initially corrode during the first few hours of operation. The mechanism of this passivating film is not well understood. The high electrical conductivity of the film is believed to be due to the presence of iron oxides. Silicon dioxide is highly resistant to acids but it is readily dissolved in alkaline conditions. High silicon iron anodes are extremely hard and cannot be machined easily. They are generally cast and then stress relieved by annealing. Although brittle these anodes have superior abrasion and erosion characteristics compared to graphite.

High silicon iron anodes are widely used usually in conjunction with carbonaceous backfills in soils. They have also found limited use in marine and freshwater environments. The maximum operating current density is determined by the type of alloy and the environment. For instance, in groundbeds with backfills the current density is limited to between 10 and 20 A m-2 because of problems caused by gas entrapment. In marine environments, a high iron chromium anode can be operated up to 50 A m-2.


As for graphite, the maximum operating voltage is limited by excessive consumption and brittleness of the material. The consumption rate of these anodes is influenced by the operating current density and the nature of the environment. Generally, a lower current density reduces the consumption rate. Wastage rates range from 0.10 to 0.50 kg A-1 y-1. Sulfate ions in particular have been noted to enhance the dissolution rate of these materials.

Lead alloys: The function of lead as an impressed current anode depends on the formation of a protective and electrically conductivity film (101-102 Sm-1) of lead dioxide, PbO2. This film is non-stoichiometric oxide and exists in two forms:

alpha - PbO2 (orthorhombic)

beta - PbO2 (tetragonal).

Lead dioxide is surprisingly stable in the presence of chloride ions. The insoluble lead chloride, PbCl2, is believed to be responsible for healing the defects in the film. This ensures that Pb/PbO2 behaves as an inert electrode and hence allowing at high polarization the evolution of chlorine and oxygen. To form an adherent and stable film of PbO2, lead is generally alloyed with Ag and Sb. A typical alloy composition is Pb 6 Sb 1 Ag.

Owing to the low overvoltage of chlorine evolution on the surface of these anodes, lead alloys are mostly used in seawater applications. Maximum operating voltage and current density of these anodes are 24 V and 1 kA m-2

respectively. The consumption rate is in the range 1-10 g A-1 y-1. It should be added that lead alloy anodes are sometimes used with platinum pins. It has been found that a platinum microelectrode inserted into the surface of the lead enhanced the formation of PbO2. It is also worthwhile noting that the performance of lead alloy anodes (with and without Pt pins) is adversely affected at operation depths greater than 30 m in seawater.

14.3.3 Polarisation Behaviour

Metallic surfaces can be polarised by the application of an external voltage or by the spontaneous production of a voltage away from equilibrium. This deviation from equilibrium potential is called polarisation. The magnitude of polarisation is usually described as an overvoltage () which is a measure of polarisation with respect to the equilibrium potential (Eeq) of an electrode.

This polarization is said to be either anodic, when the anodic processes on the electrode are accelerated by changing the specimen potential in thepositive (noble) direction or cathodic when the cathodic processes are accelerated by moving the potential in the negative (active) direction. There are three distinct types of polarization in any electrochemical cell, the total polarization across an electrochemical cell being the summation of the individual elements:

E(applied) - Eeq = total = act +conc +iR


Where act is the activation overpotential, a complex function describing the charge transfer kinetics of the electrochemical processes. act is predominant at small polarization currents or voltages and conc is the concentration overpotential, a function describing the mass transport limitations associated with electrochemical processes. act is predominant at large polarization currents or voltages and iR is often called the ohmic drop. iR follows Ohm's law and describes the polarization that occurs when a current passes through an electrolyte or through any other interface such as surface film, connectors ... The ohmic drop is the simplest of the three polarization terms and can be evaluated either directly with a conductivity cell or using conductance data.

14.3.4 Activation Overpotential

Both the anodic and cathodic sides of a reaction can be studied individually by using some well established electrochemical methods where the response of a system to an applied polarisation, current or voltage, is studied. A general representation of the polarisation of an electrode supporting one redox system is given in the Butler-Volmer equation:

where i is the anodic or cathodic current; = charge transfer barrier or symmetry coefficient for the anodic or cathodic reaction. values are typically close to 0.5; act = Eapplied - Eeq, i.e. positive for anodic polarisation and negative for cathodic polarisation; n = number of participating electrons; R = gas constant; T = absolute temperature and F = Faraday = 96485 C mol-1.

The exchange current density reflects the electrocatalytic performance of a surface towards a specific reaction and can vary over many orders of magnitude. The exchange current density for the production of hydrogen on a metallic surface, for example, varies between 10-2 A cm-2, for a good electrocatalytic surface such as platinum, and 10-13 A cm-2 for electrode surfaces containing lead or mercury.

Added, even in small quantities, to battery electrode materials, mercury will stifle the dangerous production of confined gaseous hydrogen. Mercury and lead were also, for the same hydrogen inhibiting property, commonly used in many commercial processes as electrode material before their high toxicity was acknowledged a few years ago.

When act is anodic, i.e. positive, the first term in the Butler-Volmer Equation becomes negligible and ia can be more simply expressed the following equation:


or its logarithmic form:

in which ba is the Tafel slope or coefficient. ba has a positive value that can be obtained from the slope of a plot of against log i, with the intercept yielding a value for i0, i.e.

Similarly, when is cathodic, i.e. negative, the second term in the Butler-Volmer Equation becomes negligible and ic can be more simply expressed by the following equation:

or its logarithmic form:

in which bc is the Tafel slope or coefficient. bc has a negative value that can also be obtained from the slope of a plot of against log i, with the intercept yielding a value for i0, i.e.

14.3.5 Concentration Overpotential

When the cathodic reagent at the corroding surface is in short supply, the mass transport of this reagent could become rate controlling. A frequent case of this type of control occurs when the cathodic processes depend on the reduction of dissolved oxygen. Because the rate of the cathodic reaction is proportional to the surface concentration of the reagent, the reaction rate will be limited by a drop in the surface concentration. For a sufficiently fast charge transfer (small activation overvoltage), the surface concentration will fall to zero and the corrosion process will be totally controlled by mass transport. For purely diffusion controlled mass transport, the flux of a species O to a surface from the bulk is described with Fick'sfirst law:


where JO = flux of species O (mol s-1 cm-2), DO = diffusion coefficient of species O (cm2 s-1) and CO/x = concentration gradient of species O across the interface (mol cm-4). The diffusion coefficient of an ionic species at infinite dilution can be estimated with the help of Nernst-Einstein relating DO with the conductivity of the species (O):

where zO = valency of species O, R = gas constant, i.e. 8.314 J mol-1 K-1 and T = absolute temperature (Kelvin) and F = Faraday's constant, i.e. 96485 C mol(e-)-1.

The region near the metallic surface where the concentration gradient occurs is also called Nernst diffusion layer (). Since the concentration gradient is greatest when the surface concentration of species O is completely depleted at the surface, i.e. CO = 0, it follows that the cathodic current is limited in that condition, as expressed by the following equation.

For intermediate cases, conc can be evaluated using an expression derived from the Nernst equation.

where 2.303�R�T/F = 0.059 V when T = 298.16 K.

14.3.6 Conductivity Cell

In theory, a conductivity measuring cell is formed by two 1-cm square surfaces spaced 1-cm apart. Cells of different physical configuration are characterised by their cell constant, K. This cell constant (K) is a function of the electrode areas, the distance between the electrodes and the electrical field pattern between the electrodes. The theoretical cell just described has a cell constant of K = 1.0. Often, for considerations having to do with sample volume or space, a cell's physical configuration is designed differently. Cells with constants of 1.0 cm-1 or greater normally have small, widely spaced electrodes. Cells with constants of K = 0. 1 or less normally have large closely spaced electrodes. Since K (cell constant) is a "factor"which reflects a particular cell's physical configuration, it must be


multiplied by the observed conductance to obtain the actual conductivity reading. For example, for an observed conductance reading of 200 �S using a cell with K = 0. 1, the conductivity value is 200 x 0. 1 = 20 �S/cm. In a simplified approach, the cell constant is defined as the ratio of the distance between the electrodes, d, to the electrode area, A. This however neglects the existence of a fringe-field effect, which affects the electrode area by the amount AR. Therefore K = d/(A + AR). Because it is normally impossible to measure the fringe-field effect and the amount of AR to calculate the cell constant, K, the actual K of a specific cell is determined by a comparison measurement of a standard solution of known electrolytic conductivity. The most commonly used standard solution for calibration is 0.01 M KCl. This solution has a conductivity of 1412 �S/cm at 25oC. In summary, the calibration of a conductivity probe is to compensate for the fact that:

K is not specifically known,

K changes as the electrode ages,

Calibration simply adjusts the measured reading to the true value at a specified temperature.

14.4 Consumable AnodesExamples of this type of anode include scrap iron or steel and cast iron. The anode is deliberately dissolved to provide the electrons required to polarizethe structure. Consumable anodes can be used in buried or under immersed conditions. They have consumption rates of approximately 9 kg A-1 y-1. and maximum current densities are in the order of 5 A m-2. Due mainly to their high consumption rate the use of such anodes is rather rare unless a redundant source of iron or steel is readily available such as an old ship beached below low tide, a disused pipeline, well string etc. However, since such structures are frequently massive, they represent a very low resistance to earth and therefore can make the cathodic protection engineer's life much simpler.


15 Electrochemical Chloride Extraction and Re-Alkalisation Treatment

The Special Service Division of Mott MacDonald is experienced in the design and specification of various electrochemical repair techniques and able to assist with any issues regarding electrochemical repair.

Electrochemical Chloride Extraction (ECE) was first investigated in the early 1970s in the United States by the Federal Highway Administration and later used in the U.S. under the Strategic Highway research program (SHRP) and in Europe by Norcure. Re-alkalisation is the sister technique of electrochemical chloride extraction, but it is applied for much shorterduration.

The duration of ECE varies between 4 to 8 weeks. The applied current density is usually in the order of 1 A/m� of the concrete surface and the total charge varies between 650 and 2000 Amphrs/m� of concrete surface. Different electrolytes such as water or lime solution are used. The duration of re-alkalisation is in the order of some days with sodium or potassium carbonate electrolyte being used and the total charge varies between 70 and 200 Amphrs/m�.

Between one third and two third of externally penetrated chlorides can be removed during ECE and it reduces in particular the chloride content of the concrete of the immediate surrounding of the steel and corrosion is generally stopped. The current density has a significant influence of the success on the treatment because it is the driving force in migration. The applied current density also determines side effects and 1 A/m� of concrete surface have been reported for laboratory studies on ECE.

Current densities, based on the area of the concrete surface, are calculated on the assumption that in the treated structure the ratio of the reinforcement area to concrete area is equal to one. However, in most cases the reinforcement to concrete ratio is much less. This leads to unnecessary high current densities and undesired side effects. For some cases, 1 A/m� of concrete surface can be equal to 5 A/m� of steel surface. Therefore, current densities for field applications have been calculated as current per square metre of steel surface and are in the range of 1 to 2 A/m2.

Potential side effects are reduced bond strength, altered concrete pore size, HE and HISCC failure of high strength steel and alkali-silica reaction. Findings, however, are inconclusive with regard to the effect on bond strength and HE.


16 Organic Protective CoatingsMany paints, coatings and high performance organic coatings have been developed as a need to protect equipment from environmental damage. Of prime importance in the development of protective coatings was the petroleum industry that produced most of the basic ingredients from which most synthetic resins were developed.

The cracking of petroleum produced a multitude of unsaturated workable compounds that are important in the building of large resin polymers such as vinyls and acrylics. The solvents necessary for the solution of the resins were also derived from petroleum or natural gas. The building blocks for epoxies and modern polyurethane coatings are other derivatives produced by refining petroleum products. Some important concepts for designing corrosion resistant coatings are:

Coating protection

Component design

Component function

Coating formulation

According to the U.S. Department of Commerce Census Bureau, the total amount of organic coating material sold in the United States in 1997 was 5.56 billion L (1.47 billion gal), at a value of $16.56 billion. The total sales can be broken down into architectural coatings, product original equipment manufacturers (OEM) coatings, special-purpose coatings, and miscellaneous paint products. A portion of each of these was classified as corrosion coatings for a total estimate of $6.7 billion. It is important to note that raw material cost is only a portion of a total coating application project, ranging from 4 to 20 percent of the total cost of application. When applying these percentages to the raw materials cost, the total annual cost of coating application ranges from $33.5 billion to $167.5 billion.

16.1 Main variables

16.1.1 Designing with Corrosion Protective Coatings

Many coatings contain as many as 15 to 20 ingredients with their own range of functionality. Some of the main variables used to design corrosion protective coatings are:

Impermeability: The ideal impermeable coating should be completely unaffected by the specific environment it is designed to block, be it most commonly humidity, water or any other corrosive agent such as gases, ions or electrons. The ideal impermeable coating should have a high dielectric constant and also have perfect adhesion


to the underlying surface in order to avoid any entrapment of corrosive agents.

Inhibition: In contrast with coatings developed on the basis of impermeability, inhibitive coatings function by reacting with a certain environment to provide a protective film or barrier on the metallic surface. The concept of adding an inhibitor to a primer has been applied to coatings of steel vessels since these vessels have been first constructed. Such coatings were originally oil based and heavily loaded with red lead.

Cathodically protective pigments: As with inhibition, cathodic protection in coatings is mostly provided by additives in the primer. The main function of these additives is to shift the potential of the environment to a less corrosive cathodic potential. Inorganic zinc based primers are good examples of this concept.

The structure should be designed to facilitate the application of maintenance coatings in the future. Some details of structures can make it virtually impossible to apply a continuous coating:

Structural Steel Shapes: The outside of an angle always presents a problem, being difficult to coat because coatings tend to pull away from a point or sharp edge. The interior of a square angle is difficult to coat as dirt accumulates here and it is often a difficult area to reach by spray or brush.

Sharp Edges: Sharp edges should be eliminated wherever possible. Remember coating materials tend to run away from an edge. If the coating is applied by brush and the applicator brushes away from the edge, the coating is invariably brushed off, leaving a thin area. Brushing should be towards an edge. When spraying, double coating of edges should take place where possible.

Welded Joints: Welds must be given special attention when coatings are specified. One of the major difficulties along the welds occurs because of weld splatter. Weld splatter should be carefully removed by blasting or chipping. Where resistance to corrosion is required, all rough welding should be ground smooth. All welds should, if possible, be double coated.

Brackets: Brackets and other temporary fabricating aids are frequently welded on the surface of structures during construction. They are sometimes left in place after the job is completed. If the brackets are cut from the surface a rough spot usually remains, thus starting a corrosion problem. If left in situ and even though thoroughly cleaned by blasting, these fixtures are extremely difficult to coat properly. All brackets and extra metal should be removed and previous contact areas ground smooth.


Discontinuous, Tack or Skip Welds: In a corrosive atmosphere these welds are vulnerable since they cannot be properly coated. Skip welding consists of welding a 5 cm bead and then skipping from 5 cm to 30 cm before welding another 5 cm bead. Skip welding is used mainly for reinforcing purposes when a continuous weld is not considered necessary. Structures, which will be exposed to a corrosive environment, should have continuous welds.

Lap Welds: Lap welding consists of continuous welding on the outside surfaces only, leaving the steel plates lapped on the inside thus forming crevices which are difficult to coat properly. If a coating is to give best results, all joints should be completely sealed.

Steel Angles: Steel angles placed back to back are often used to form trusses. These angles are usually separated by washers or other members of the truss. The resultant gap is difficult to protect in a corrosive atmosphere. Trusses should be designed with a minimum of crevices between steel members or alternatively adequately coated before joining.

Weld Flux: Weld flux is a hygroscopic material. Left on a weld it absorbs moisture and creates a spot where early coating failure can be anticipated. Specifications should ensure complete removal of all weld flux, by wire brushing and washing with copious quantities of fresh water.

Pipeline Design: Pipe supports, flanges, threaded joints and pipe hangers are all potential points of corrosion. Crevices are formed in threaded couplings, which allow the penetration of moisture. Pipe hangers and supports cause local areas of severe corrosion since the ring of the hanger or support never fits accurately enough to prevent a crevice.

16.2 Protective Coatings Components

16.2.1 BindersIn order to perform in a practical environment, a coating must convert, after its application, into a dense, solid, and adherent membrane with all the properties discussed previously. The binder is the material which makes this possible. It provides uniformity and coherence to the coating system. Not all binders are corrosion resistant so that only a few serve in the formulation of protective coatings. The binder ability to form a dense, tight film is directly related to its molecular size and complexity. Binders that have the highest molecular weight will form films by the evaporation of the vehicle while binders with smaller molecular weight will generally be reacted in situ. Binders cam be classified according to their essential chemical reactions. Oxygen reactive binders include alkyds epoxy esters Urethane - or Silicone alkyds. Lacquers may consist of polyvinyl chloride polymers, chlorinated rubbers, acrylics or bituminous materials. Heat Conversion binders may consist of hot melts, organisols and plastisols or


powder coatings. Co-reactive binders can be made of Epoxies or Polyurethanes. There are also condensation binders coalescent binders and inorganic binders, which may comprise post-cured silicates self-curing water silicates or self-curing solvent based silicates.

16.2.2 Pigments

Pigments are essentially dry powders that are insoluble in the paint medium and that consequently need to be mixed in it by a dispersion technique. They range from naturally occurring minerals to man made organic compounds. Pigments contribute several properties essential to the effective use of protective coatings. Several different pigments may be used within the same coating, all of them contributing to the coating general characteristics to perform important functions such as providing:

Colour,

Protection to resin binder,

Corrosion inhibition,

Corrosion resistance,

Film reinforcement,

Non-skid properties,

Sag control,

Increased coverage,

Hide and gloss control, and

Adhesion.

Zinc phosphates are now probably the most important pigments in anti-corrosive paints. The selection of the correct binder for use with these pigments is very important and can dramatically affect their performance. Red lead is likely to accelerate the corrosion of non-ferrous metals, but calcium plumbate is unique in providing adhesion to newly galvanized surfaces in the absence of any pre-treatment, and is claimed to behave similarly on other metals.

16.2.3 Solvents

Most coatings are made with multiple solvents and rarely with a single solvent. The choice of solvents influences viscosity, flow properties, drying speed, spraying or brushing characteristics, and gloss. There is no universal solvent for protective coatings, the best solvent in one system being often impractical for another. Asphalts, for example, can be readily dissolved by hydrocarbons but are insoluble in alcohols. One of the most serious problems associated with coatings is the wrong choice of solvent since it can severely affect the curing and adhesion characteristics of the final coating.


One convenient way to describe solvents is to regroup them into the following categories:

Aliphatic hydrocarbons: aliphatic hydrocarbons or paraffins such as naphta or mineral spirits are typically used with asphalt, oil and vinyl based coatings.

Aromatic hydrocarbons: aromatic hydrocarbons, such as toluene, xylene or some of the higher boiling homologs, are typically used with chlorinated rubbers, coal tars and certain alkyds.

Ketones: ketones such as acetone, methyl ethyl ketone, methyl iso-butyl or amyl ketone and many others, are very effectively used with vinyls, some epoxies and other resin formulations.

Esters: esters such as ethyl, n-propyl, n-butyl or amyl acetates are used commonly as latent solvents (a type of solvent that just swells the binder at room temperature) with epoxy and polyurethane formulations.

Alcohols: alcohols such as methyl, propyl, iso-propyl or butyl alcohols and cyclo-hexanol are good solvents for highly polar binders such as phenolics.

Ethers and alcohol ethers: ethers such as ethyl ether are excellent solvents for some of the natural resins, oils, and fats.

Water: the recent regulations to reduce the emission of volatile organic compounds (VOCs) produced by organic solvents are forcing the coating industry to reconsider the applicability of water as a solvent.

16.3 Comparison of paint specifications

16.3.1 A ranking system

It should be possible to take each component of a paint specification and assess it in comparison to other specifications by providing a ranking system. Such a system will not give an answer to a specific question or the suitability of a particular specification for a particular application. However, as an educational tool to demonstrate the important parameters in a paint specification it could be useful. The following is a brief outline of such a ranking system; it is based on only a few resin and pigment types but shows how the ranking system could work. As an educational tool the tables below could be developed into something more interactive such as a spreadsheet, database or web based application and this could also include guidance on the use of various specifications. In such a form it would also be easy to add more resins, pigment etc.


The tables follow a logical progression dealing with each coat in a specification (primer, barrier(s) and finishes). For each coat an assessment is made on the following key parameters:

Resin Type

Pigment Type

Thickness (in the tables thickness is given in microns as the Minimum Dry Film Thickness)

Each variable within each coat is assigned a value and the sum of these values for individual coats and the whole specification gives an overall ranking of the coat or specification. The tables have been compiled such that the higher the final total value the "better" that specification is for a given service condition. But note it is only a ranking system. It is possible that one could compare a primer only specification with a high build Glass Flake Epoxy (GFE) and conclude that the GFE is better. This may well be the case but GFE would be inappropriate for use in an internal environment whereas a primer only scheme would be appropriate. Similarly, a primer only scheme would be totally inappropriate for long-term immersion service in seawater.

In the current form the ranking system is only really appropriate for conventional multi-coat specifications given in things like the SSPC manual and ISO 12944 Part 5. The current form it does not really deal with modern specifications based on high film thickness specifications such elastomeric urethanes nor does it really do justice to things like polysiloxane coatings. However, the system could be developed to include these things.

As it stands, the framework is intended to assess and rank specifications that are commonly used to protect structural steel against corrosion in applications such as building, bridges etc. Other Tables could be developed using a similar ranking procedure to cover other types of coatings or substrates.

The ranking system is also based on conventional and high solids solvent borne coatings and does not include water borne coatings. The ranking system for materials such as water borne epoxies may be different. Similarly, the typical specifications do not take account of water borne coatings and it is possible that to achieve the same performance with a water borne system additional coats would be required to achieve the same total film thickness.

16.3.2 The Tables

The tables are really a matrix, for example under primers one could have an Epoxy Zinc Phosphate Primer with a thickness of >100 microns. These tables should not be used to say that the Epoxy based specification is 2 times better than the Alkyd based specification! As a teaching tool for students this system would allow uses to get a feel for the important parameters in paint specifications by changing individual variables and seeing the effect


that this has on overall rankings. This would help bring understanding to what is important in coating specifications in the real world of engineering. No doubt someone would come up with a wonderfully bizarre and durable specification that could not be produced! Final comment, all the tables assume that steel has been blast cleaned to Sa 2� of ISO 8501-1.

16.4 Typical Protective Coating Specifications The framework to assess and rank specifications presented in the previous section is based on commonly used specifications to protect structural steel against corrosion in applications such as building, bridges etc. The following are some typical corrosion protection specifications all based on blast cleaning to Sa 2� of ISO8501-1.

a) Conventional Epoxy specification

Coat Material Thickness (m)

Primer Epoxy Zinc Phosphate 75

Barrier Epoxy MIO 125

Finish Polyurethane 50


Standard UK Highways agency specification for use on motorway bridges. Expected life to first major maintenance in excess of 20 years, used on the Blythe Valley Bridge.

b) Zinc rich epoxy specification


Primer Epoxy Zinc Rich 40




Bridge specification. Used on Oresund crossing and Channel Tunnel Rail link structures

c) Zinc rich for buildings





Standard specification for external steel on buildings.


d) Zinc rich only specification



Cavity and perimeter steel in buildings

e) Epoxy Phosphate specification


Primer Epoxy Zinc Phosphate 75



In swimming pool environments. External steel easily maintainable.

f) Alkyd specification


Primer Alkyd Zinc Phosphate 40

Barrier Alkyd MIO 75

Finish Alkyd undercoat 75

Finish Alkyd Gloss 50

g) Glass Flake Epoxy (GFE) specification for bridges


Primer Epoxy Zinc phosphate 25

Barrier GFE 400


h) Glass Flake Epoxy (GFE) specification for offshore platforms


Primer Epoxy Zinc phosphate 25

Barrier GFE 500

Barrier GFE 500



16.5 Coating Failure Although paints offer a significant enhancement to the life of metallic-coated steel, they do eventually “fail” in some fashion. This can take the form of chalking or fading to a colour that is no longer acceptable to the user. It can also take the form of blistering or flaking. Both blistering and flaking can occur by separation along the paint primer bond line, the primer pre-treatment bond line or the pre-treatment metallic coating bond line. The specific nature of blistering or flaking depends on many factors associated with the specific combination of paint, primer, pre-treatment, metallic coating, and the environmental conditions. Loss of paint adhesion can take several forms. The most common ways are:

1. Lateral undercutting corrosion at a scratch in the paint, or at a sheared edge (where the paint/primer/metallic coating/steel are all exposed to potential corrosion). The net effect of this lateral undercutting corrosion is the loss of adhesion between the paint and the metal substrate. The corrosion can occur by (a) chemical reaction along the paint/ metallic coating interface which can cause the chemical adhesion bond to be degraded, or (b) bulk corrosion of the metallic coating leaving the paint totally “unconnected” to the steel sheet.

2. Blistering beneath the paint caused by corrosion reactions beneath the paint film. Remember, paints are not impervious; water can penetrate through the paint to the substrate surface during times of wetness. If the initial bond strength is not real good, if the pollutants in the environment are particularly insidious for the type of paint system used, etc., blisters can develop beneath the paint even though there are no discontinuities in the paint. As the blisters grow larger and begin to combine, the net effect can be gross flaking of the paint in large areas.

To minimize the tendency for loss of paint adhesion through undercutting corrosion or blistering, one needs to take into account very specific recommendations from the steel supplier and paint manufacturers. The “best” coated-product design requires that the user pay attention to the type and thickness of the metallic coating, the type of pre-treatment, the type and thickness of the primer, and the type and thickness of the topcoat. The recommendations from the suppliers will take into account issues such as:

Types and concentrations of corrosive contaminants such as acid rain, coastal salts and manufacturing plant effluents in the area.

Wetness of the environment, Amount of UV light exposure, Customer expectations with respect to performance and aesthetics

(paint fading, chalking of the paint and rust stains at shared edges)

Desired product life.


16.6 Coatings for Buried PipelinesIn addition to corrosion protection, many pipelines require thermal insulation to prevent hydrocarbons to produce waxes or hydrates. These heavier components can clog lines and require immediate attention. There is thus a continuous need of improvements in coating as oil and gas operations extends to unprecedented depths and temperatures. Deep water wells put stringent requirements on insulation products to withstand high compressive loads and tolerate exposure to particularly aggressive environments.

Over the past fifty tears, pipelines have been coated with a variety of protective coatings with a wide of performance. The advantages and disadvantages of the main coating types used for pipeline protection are summarized here.

Asphalt / Coal tar: 1940 to 1970

Advantages Disadvantages

Easy to apply Subject to oxidation and cracking

Minimal surface preparation required Soil stress has been an issue

Long track record in certain environments without failure

Limitations at low application temperatures

Permeable to cathodic protectionin event of failure

Environmental and exposure concerns

Associated with corrosion and stress crack corrosion failures

Tape wrap (two layer): 1960 to now


Simple application Poor shear stress resistance

Many documented failures related to corrosion and stress crack corrosion

Shielding of cathodic protection

Adhesives subject to biodegradation


Two-layer extruded polyethylene: 1960 to now


Excellent track record Limited temperature range

Good handling Poor shear stress resistance

Limited pipe sizes (<24 in. [610 mm] outside diameter)

Fusion-bonded epoxy: 1975 to now


Excellent adhesion and corrosion resistance Low impact resistance

Does not shield cathodic protection High moisture absorption and permeation

Three-layer polyotefin: 1986 to now


Excellent combination of properties

Best suited for electrical resistance welded pipes

High thickness to eliminate weld tenting

Composite coating: 1990 to now


Excellent combination of properties

Suitable only for large diameter pipes and is not designed for small diameter pipes (<406 mm OD)

Conforms well to external raised weld profiles

It should be noted, however, that election of thermal insulation for a particular application is always a balance between the required thermal and mechanical performance.


16.7 Applications of buried pipeline coating systems Main Application

Polyethylene Sub-sea flowlines, non-insulated

Polypropylene Sub-sea flowlines, non-insulated

Polychloroprene Risers, clamps, guides, and special flowlines

EPDM High-temperature flowlines and risers

Polypropylene (thick) Oil: land and sub-sea

Polychloroprene (thick) Oil: land and sub-sea

EPDM (thick) Oil: land and sub-sea

Solid polyurethane Oil: land and sub-sea

Foamed polypropylene Multi-phase land and sub-sea

Syntactic polypropylene Multi-phase land and sub-sea

Syntactic polyurethane Conventional Multi-phase land and sub-sea

Glass beads Deep water

Glass syntactic epoxies Deep water

Glass syntactic epoxies Deep water

Pipe in pipe / jacket Gas: land and sub-sea

EPDM = ethylene propylene diene monomer


16.8 Operational limits for buried pipeline coatingsDepth Limit

Temperature Range U

(m) (oC) (W/m2/K)

Anti-corrosion

Polyethylene >1,000 m 60-80

Polypropylene >1,000 m 110-135

Polychloroprene >1,000 m 90-100

EPDM >1,000 m 135-150

Insulation (High U)

Polypropylene (thick) >1,000 m 110-135 U < 10

Polychloroprene (thick) >1,000 m 90-100 U < 10

EPDM (thick) >1,000 m 135-150 U < 10

Solid polyurethane >1,000 m 100-115 U < 8

Insulation (Medium U.)

Foamed polypropylene <600 m 50-120 4 <U<10

Syntactic polypropylene >1,000 m 110 5 <U<10

Syntactic polyurethane Conventional <300 m 100-115 2.5

<U<10

Glass beads >1,000 m ~100 2 <U<4

Insulation (Low U)

Glass syntactic epoxies >1,000 m ~100 2 <U<4

Pipe in pipe / jacket various 100-150 U <1

EPDM = ethylene propylene diene monomer, U is the thermal transmittance (higher Us mean less insulating).

However, the development of coatings for deep sea applications is really dynamic and, as the range of these materials is broadened, the selection of an appropriate technical solution becomes less constrained.


16.9 Supplementary Protection SystemsSupplementary protection is provided to surfaces which already have some form of permanent or semi-permanent form of protection such as cladding or conversion coating. The supplementary protection may be in the form of a material that can be easily applied and removed, and which will be replaced periodically during the life of the system. Jointing compounds and sealants are examples of this type.

16.10 Jointing compounds and sealantsJointing compounds are used for protection at joints where they act by excluding dirt and moisture, and by providing a reservoir of soluble passivators which act as inhibitors. Sealants are applied to joints to prevent the escape of fluids, such as fuel, but they also exclude moisture. Jointing compounds are required to remain flexible so as to allow easy disassembly of parts. Various synthetic resins are used for this purpose. The compounds harden sufficiently at edges to take paint, but they remain tacky within the joint so that flexure does not cause cracking. Sealants of the type now being specified are also elastomeric, and the most popular are polysulphide sealants containing corrosion inhibitors. The inhibitive sealants are very effective when used in faying surfaces and butt joints, for wet installation of fasteners and over fastener patterns. They are also effective in insulating dissimilar metals.

16.11 Water displacing compoundsWater displacing or displacement compounds may be useful in providing supplementary protection for paint systems that have deteriorated or become damaged in service. They are applied as fluids by wiping, brushing, spraying or dipping, and they are usually immiscible with water and displace water from surfaces and crevices. A number of fluids used are based on lanolin and contain various solvents and inhibitors. The evaporation of the solvents leaves either thin soft films, semi-hard films or hard resin films providing varying degrees of protection. Some of these fluids may be used to provide short-term protection. They should then exhibit excellent water displacing characteristics and leave a thin oily film providing short-term corrosion protection. One widespread method of application of such compounds is called 'fogging.' The fog created by the applicator can be distributed evenly in the most intricate components as illustrated in the following pictures:

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