Advances in corrosion protection by organic coatings: what we know and what we would like to know

Organic coatings are the single most widely applied method for corrosion protection of metallic materials and are of particular importance in transport and infrastructure. Nevertheless, despite over one hundred years of research and testing, the mechanisms of coatings failure are still somewhat obscure. This paper provides a brief overview of current knowledge in how protective coatings work and poses questions that we need to answer in order to develop a more predictive model for coating performance. It is shown how heterogeneity leading to electrochemical variations is important both in polymer structure (i.e. local variation in polymer conductivity influenced by the external electrolyte) and in light alloys (e.g. 2nd phase particles act as local cathodes or anodes). The conclusion is that no single protective mechanism is operative in organic coatings. Long-term performance is predicated on a number of complementary processes working holistically. By developing an understanding of the mechanisms and critical factors behind long-term performance, a predictive toolkit for “how paint fails” might usefully be created.