The coating is the premier external corrosion protection method for pipelines. As such, the selection and continued integrity of any parent or rehabilitation pipeline coating is of paramount importance. It is the coating that determines cathodic protection (CP) requirements and the magnitude of pipeline maintenance expenditures to prevent failures such as metal loss and stress corrosion cracking. The fundamental contributions that the coating makes to stopping external corrosion are:
- Isolating the anode and cathode areas of the corrosion cells from each other.
- Isolating the common electrolyte – the moist soil – from both the anode and cathode areas.
These two inter-dependent activities identify water resistance as the premier property of a coating. However, the absorption of water – which is strongly influenced by temperature – can significantly influence the coating’s mechanical (secondary) properties such as soil stress resistance, adhesion and impact resistance. In addition, there is a third level of properties related to coating composition such as wetting ability, coating filler and solvent content.
Identifying a suitable coating from the supplier’s literature is complex because the properties quoted predominately relate to mechanical properties. No mention is made as to how these mechanical properties degrade as the coating absorbs water. Mechanical properties are important when building a pipeline. Good abrasion and impact resistance are desirable to reduce construction and stone damage. Unfortunately, if the coating – through handling damage – then absorbs water when buried, it can soften such that the slightest movement can cause significantly damage (Figure 1).
By understanding how parent coatings fail, we could improve the way parent and rehabilitation coatings are selected. To improve understanding, the basic electrochemical principles of several test procedures used to evaluate thin film coatings applied as a liquid or fused powder are reviewed.
Consider the major failure mechanism of thin and thick film coatings. Thin film coatings of urethane, urethane tar, liquid epoxy, polyester and FBE are typically applied 400-1,000 ? thick. The predominant failure mechanism of thin film coatings is by blister formation through water absorption. All coatings absorb a small amount of water. As an example, freshly applied fusion bonded epoxy (FBE) contains about 0.25 w/o moisture. The detrimental effects of moisture are shown during field-joint coating.
Induction heating applied to raise the temperature to approximately 250oC converts any water to steam, resulting in a big volume expansion. From basic chemistry, 18 cc of water, when converted to steam at the same temperature and pressure, occupies 22.4 litres. As the temperature is raised, the FBE softens, and at temperatures above 220oC, the strength of the FBE and its adhesion to the steel fails as the coating blisters. The only way to prevent blistering is to preheat to 125oC so the coating still has strength, then allow time for the steam generated to diffuse out of the coating. This typically takes about 15 minutes.
Applying rehabilitation coating too soon after welding at, for example, a new pipe or cut out, can also create problems. Hydrogen co-deposited during welding causes holes and blistering of thin film coatings by building up pressure in the small voids that always exist between the steel and the coating. Hydrogen can diffuse out of the steel faster than it can diffuse through thin film coatings such as urethane or FBE and will literally blow the coating off the heat-affected zone. Figure 2 shows the effect of hydrogen from a new weld joint bubbling through a liquid epoxy coating, resulting in a ring of defects at the heat-affected zone. Not all thin film coatings are good at moisture resistance, including FBE, and there are wide variations in performance, depending on the resins used and the application conditions.
Figure 2: Hydrogen from welding bubbles through a freshly applied coating leaving behind rows of small holes in the curing epoxy.
The general question is, can a coating’s water resistance be retained during the life of a good coating, and how can we predict performance? Usually on extended exposure to moist soil, a coating will absorb more and more moisture until water separates out as a single phase within the coating. In addition, the amount of bending and twisting that the pipe has received during construction, plus any mechanical damage/bruising which induces stresses within the coating, can create voids. Temperature effects are also influenced by the type and amount of filler in the coating plus the affinity of the resin itself for moisture pickup. Once void moisture separates out as a path from soil to steel, then electro-osmosis driven by the CP will force water through the coating to collect at the steel-coating interface.
As the water pressure builds up (assisted by any hydrogen and hydroxyl ions generated by the CP reaction) the coating will disbond to form a blister. The size and lifetime of a blister varies with the type of coating. Blisters in FBE can be the size of a hand and last several years. Alternatively, blisters in polyester coatings are very small and short-lived as shown by the difficulty in actually finding an un-burst blister when excavating. The factors that influence a blister’s size and lifetime must be related to the strength of the adhesion of the coating to the steel and the strength of the coating itself and how this strength is affected by water absorption and strong alkali.
Once a coating starts to develop blisters that burst, then the application of more and more CP – needed to maintain pipe-to-soil potentials of any exposed steel – just leads to an accelerated rate of coating failure. This is why it is not recommended that one continually increase the amount of applied CP to maintain pipe-to-soil potentials on FBE or other thin film-coated pipelines.
Adhesion is frequently used as the dominant property to determine what constitutes a good or bad coating, particularly when assessing a parent coating during rehabilitation. In a review of pipeline coatings by the American Gas Association, pipeline coating companies voted adhesion as the most important property.7 Water uptake resistance received only passing mention.
Adhesion assessment by a test such as the cross hatch is easy. On the other hand, assessment of the water uptake of a coating and its effect on long-term performance is difficult. Test procedures range from simple weight gain to measurement of the AC or DC resistance or capacitance. Many of the test procedures are covered by ASTM specifications.
The best water uptake test method is probably the change in DC resistance (or conductivity) because it allows repeat tests on the same sample as the specimen ages so predictions as to useful life can be made from extrapolation of experimental observations. As moisture is absorbed by a coating, there usually is some delay before the coating begins to pass a measurable ionic current.2 This delay is thought to represent the time taken for the moisture to reach the surface of the underlying steel pipe. This is an important parameter. Once this point has been reached for a pipe coating under CP, the electro-osmosis becomes dominant in forcing more water into the coating, ultimately to form blisters.
No test at the moment looks at water absorption under the influence of CP (except indirectly by cathodic disbondment) and how the failure of the coating by water absorption is accelerated by varying amounts of CP. An additional parameter to the test is temperature. The higher the temperature, the more exacting the test in separating good from bad coatings. The CP is purely added to a test to enable the limiting initial exposure time to be better defined because penetration of water through the coating to the steel surface will not be uniform across the whole specimen surface. A test that comes close to the above is cathodic disbondment testing, but unfortunately, no measurements of the change in electrical parameters of the coating itself are made.
Cathodic Disbondment Tests
When it comes to cathodic disbondment testing, what are we actually measuring? Cathodic disbondment of a parent or rehabilitation coating is a widely applied selection test procedure. In the procedure, an artificial holiday is created in the coating to expose a standard amount of bare steel that is made cathodic in a small electrolytic cell. After a number of days of exposure at set levels of CP and temperature, the radius of coating disbonding from the edge of the original holiday is measured. The higher the test temperature, the more exacting the test conditions.
There are standard specifications describing the test conditions, but no matter what test procedure is used, all suffer from major differences when compared to real practice. All the tests utilize salt solutions simulating sea water. This is the wrong electrolyte for land-based pipes. In practice, at a coating defect the cathodic protection electrochemical reaction generates hydroxyl ions. Hence, the electrolyte at a coating fault is a sodium hydroxide solution containing various amounts of carbonate and bicarbonate generated by the absorption of carbon dioxide from the surrounding soil. Such an electrolyte in the presence of CP is strongly reducing and generates a protective magnetite film on the steel surface.1 Alkaline solutions also have a much greater effect on the organic materials used as a protective coating.
Some test procedures have the anodic and cathodic reactions taking place in the same bulk of electrolyte. This is incorrect. The anolyte and catholyte must be separated in a two-compartment cell (Figure 3). 8 Prolonged electrolysis in a single cell produces hypo-chlorous acid at the anode which will affect the cathode by neutralizing any alkali formed. This effectively reduces any attack that an otherwise alkaline environment would have on disbondment. Coating systems demonstrate very different performance in acid compared to alkaline conditions. Unfortunately, there is no recognized land-based electrolyte composition in which to conduct tests.
1. Holes For Filling / Emptying / Over Potential And Temperature Measurement.
2. 4 x Platinised Titanium Anodes In Anode Compartment.
3. Blank Sealed Anolyte And Catholyte Chamber Tops.
4. 4” X 3” PVC Reducing Socket
5. 3” PVC Pipe.
6. Test Specimen.
7. Catholyte Chamber.
8. Anolyte Chamber.
9. 8 off 2mm Ø Catholyte / Anolyte Bridging Holes with porous ceramic plugs
Figure 3: Diagram of a cathodic disbondment cell that has separate anode and cathode compartments to prevent acidity generated at an anode from influencing test results.
The application of CP to any exposed steel surface generates a film that reduces the exposed steel surface area. The morphology and composition of the film varies with the applied potential and electrolyte composition.6 The formation of this film is shown during the first few hours of the test where the potential at the holiday becomes more cathodic as the current decreases. The best film is formed at potentials of -950 to -1150 mV (Cu/CuSO4) and is composed of magnetite but – when freshly formed – also contains ferrous hydroxide which makes the magnetite film poorly conducting.
After 5-10 days, a poorly conducting magnetite/ferrous hydroxide film is produced. The electrochemical reaction can then take place either through holes in the magnetite film, on the surface of the poorly conducting magnetite film itself or at the miniscule ring of steel surrounding the holiday where disbondment can occur. The electrochemical reaction should take place preferentially at any exposed film-free steel surface, but the minuteness of the liquid-filled discontinuities in the magnetite film and their electrical resistance limits the initial passage of current to the steel, forcing the reaction either to the steel ring or to the surface of the magnetite film itself.
Generally, a considerable proportion of the current probably flows to the magnetite film surface as stearic effects from the hydrogen gas given off by the CP reaction and the minuteness of the bare steel ring limit access. In a test there is usually no way of controlling what proportion of current flows to what part of the holiday surface, and this is thought to be one parameter that gives rise to the widely varying results given by cathodic disbondment tests.
Proposed mechanisms for disbondment involve interface steel dissolution (to form a magnetite film) releasing the coating, or the interface layer between the steel and coating or the layer just inside the coating dissolves/saponifies, releasing it from the steel surface. Exactly what happens is uncertain, but we do know that the formation of a protective film can block off the steel surface from being a region where the electrochemical reaction takes place preferentially. This fact also provides a mechanism for the advancement of any disbonding and provides a mechanism for significantly reducing disbondment by precoating the steel surface with a magnetite film before applying an organic coating. As there would be no bare steel, there would be no means of propagating the disbondment reaction. The chromate film applied before FBE coating tries to simulate this and does improve disbondment test results.
More worrisome is the test interpretation. What do disbondment test results actually mean in practice? In the test, the general levels of CP applied to the holiday are typical to those that can be applied in practice to a real pipeline. Therefore, if the test produces 2 mm of disbondment in 28 days, what is going to be produced on a real pipeline in 10 years? The only satisfactory result from the test should be that there will be no disbondment, but this is impossible to achieve in a test where there is no way of controlling the electrochemical reactions and where they take place on the steel surface.
What we can say about the test is that the difference in coating performance between one coating that has 20 mm of disbondment and another that may have 2 mm can be determined. Such discrimination is enhanced by testing at elevated temperatures. What is not possible at present is to argue over the acceptance or rejection of a coating where there is only 1 or 2 mm difference in the disbondment between test results and specification requirements. It is worth commenting that there is no justification for the limits to disbondment set out in specifications. Limits are just ideas of what it is thought maximum disbondment should be.
Are cathodic disbondment tests worth carrying out? It is the authors’ opinion that the test in its current variety of forms is unacceptable. This is because results for identical tests can vary widely and are operator-dependent. In addition, the tests measure the wrong property. We should be measuring – with time – the water uptake by the coating under the influence of CP and the amount of disbondment produced by any blister formation. Such a test comports more with reality and would be even more discriminating as the test temperature is increased.
Rehabilitation Coating Selection
Let us consider problems in selecting a rehabilitation coating. In this article, the absorption of water by the protective coating has been considered in detail together with cathodic disbondment. There are many other tests that have to be considered to arrive at what is considered to be the best choice of coating for the proposed application.4,5 In the decision-making process, there are three particular problems that confront the selector:
- What tests are to be used? There are many tests and many variations of any one particular test.
- Where should the testing be conducted? There are very few laboratories with the proper facilities to evaluate pipeline coatings. Even those that exist lack experience and up-to-date information as to how coatings fail. Because there are so few coating evaluations carried out, there is no one particular center of excellence. There is not enough available work to justify a full-time laboratory staff for such evaluations. Consequently, pipeline coating evaluations tend to be attached to the facilities where the main operation is pipe coating or decorative paint evaluations.
- What coatings are to be tested? There is a wide variety of coatings but most of them are unsuitable. Generally, there are fewer than 10 suitable coatings on which any extensive laboratory and field evaluations have been done.
The best advice to the planner of any proposed coating rehabilitation project is to be very careful in the selection of the rehabilitation coating. Do your own tests. Don’t believe any unverified assertions. Do actual field trials because any coating that cannot be put onto the pipeline under field conditions is impractical and should not be selected.
Dr. John Leeds is founder and managing director of Pipeline Integrity Management Ltd and DC Voltage Gradient Technology and Supply Ltd. He obtained his Ph.D. from London University and has more than 40 years of experience in corrosion, particularly in the oil and gas industry specializing in cathodic protection, coatings, survey techniques, data analysis and pipeline integrity and rehabilitation. He can be contacted at email@example.com.
Dr. Sarah Leeds is technical director of Pipeline Integrity Management Ltd and DC Voltage Gradient Technology and Supply Ltd. She obtained her Ph.D. from Manchester University studying the influence of surface films on cathodic protection and also has a MSc from London University. She has 14 years of industrial experience. She is the British Standards Institute GEL/603 Co-opted Expert Committee Member on Cathodic Protection and can be reached at firstname.lastname@example.org.
1. Leeds, Dr. J.M., Cathodically Generated Film Protects Pipe Surface, Pipe Line Industry Sept, Oct 1992.
2. Kairaitis, A.J., Private Communication, Moisture Absorption of Paint Films.
3. Cope, G.R., Holiday Testing for Fusion Bonded Epoxy, Transactions of the Australasian Corrosion Institute, Perth, 1988.
4. Klechka, I.W., Testing Coatings for Below Ground Pipeline Applications, NACE Corrosion Conference 1993, paper no 440.
5. Roche, M.G. and L’Archer, E.E., Comparative Tests for the Selection of Pipeline Coatings, NACE Corrosion Conference 1984 paper no 357.
6. Leeds, Dr. S.S., Influence of Surface Films on Cathodic Protection, PhD Thesis, University of Manchester UK, 2007.
7. Anon., Line pipe Coating Analysis, American Gas Association. November 1978.
8. Bates, C., Private Communication.