Anodic and cathodic models for interpreting polarisation behaviour of ceramic-coated substrates containing pre-existing coating breaches

Abstract

Models have been developed to help in the interpretation of polarisation diagrams for ceramic-coated substrates. The models assumed that the coatings contained pre-existing coating breaches from which anodic or cathodic currents of the substrate emanate. The anodic models for non-passivating metals indicated that the slope of the polarisation diagrams should be either equal to that of the bare substrate if coating breaches are relatively large, or equal to one third of the bare substrate value if coating breaches are relatively small and low in density. Pseudopitting potentials usually develop if the ceramic coatings support only very low anodic currents. The cathodic models indicated that the area fraction of coating breaches can only be easily obtained in deaerated solutions if the coatings do not support hydrogen evolution. In aerated solutions, it is more difficult to obtain information such as coating breach area fraction because the cathodic currents are dependent on breach size, breach density, breach distribution, and diffusion layer thickness.