Defects and Heterogeneities in Corrosion Protective Organic Coatings Films and Their Effects on Film Performance

Abstract

A limiting feature of many corrosion protective coatings is how well (defect-free) and how uniformly they can be applied and cured on the metal substrate they are to protect. Failure in coatings/metal systems is very rarely global, and usually occurs at one small site of weakness in the film, and then progresses to give appearance failure or performance failure of the substrate. Careful analysis of film performance often shows that films do not fail in areas where the coating is uniformly applied at the film thickness recommended by the coating manufacturer. The distinct effects of coating heterogeneities have been observed in this laboratory (1) in studies of a multiple sample set of marine coatings by electrochemical noise methods (ENM). It is something which must be considered carefully in choosing the size of the sample set in experimental studies of coating failures (2). One good coating sample definitely does not constitute a representative sample for durability studies. Film thickness fluctuations, local pigment volume concentration fluctuations, local variations in film cross-link density, fluctuations in chemical composition (for example, from a poorly mixed two component system), and locally uncured areas are all local heterogeneities that can cause significant problems. Sampling for lifetime testing of corrosion protective coatings should include coatings with defects that are a representative of those that might be expected in actual practice. Similar arguments have been put forth for the proper analysis of automotive paint film samples that are subjected to exterior exposure testing (3).