Protective coatings are used ubiquitously as the primary means of corrosion defense for buried or immersed metallic structures. Cathodic protection is often used in conjunction with coatings as a secondary means of corrosion protection, however it can also have detrimental effects on the coating itself if not properly applied. Cathodic disbondment testing is used to measure a coating’s susceptibility to loss of adhesion to the substrate due to cathodic polarization.
NACE recently published a new cathodic disbondment test method, TM0115-2015, Cathodic Disbondment Test for Coated Steel Structures under Cathodic Protection. This research compares and contrasts NACE TM0115 to ASTM G8, Standard Test Methods for Cathodic Disbonding of Pipeline Coatings. The paper highlights the strengths and challenges of each test method as applied to coatings used in water infrastructure corrosion control.
In its mission to manage water resources in the western United States, the Bureau of Reclamation utilizes billions of dollars’ worth of infrastructure, much of which is steel, both in burial and immersion service. To prevent corrosion on this steel infrastructure, a combination of protective coatings and cathodic protection (CP) are implemented, offering a greater level of protection to a steel structure than either technique on its own.1
When CP is used as a secondary protection method, the protective coating must be able to withstand cathodic polarized potentials, often in the range of -0.85 ± 0.20 volts vs copper-copper sulfate reference electrode (VCSE). Polarizing the steel infrastructure surface converts available sites to cathodes by electrochemical reaction, which is primarily the reduction of oxygen or dissociation of water, Equations (1) and (2). Organic coatings may undergo cathodic disbondment (CD), or loss of adhesion, at the steel interface as a result of the reduction reaction and the corresponding reaction products. Mahdavi demonstrated that Equation (1)) is the dominant reaction for applied potentials between the open circuit potential and -1.10 volts vs saturated silver-silver chloride reference electrode (VSSC), whereas Equation (2)) dominates at potentials more negative than -1.10 VSSC and greatly accelerates CD of the coating.2 Note that Mahdavi and other authors commonly report applied potential rather than the polarized potential.