Porosity tests generally make use of the relative corrosivities of the coating and the substrate to determine the integrity of the former. This paper is concerned with coatings, such as gold, that are cathodic to the substrate. Coatings of this type are usually applied to protect the substrate from the environment and provide a surface having other beneficial properties, such as wear resistance, electrical conductivity, and an attractive appearance. Porosity is detrimental to the intended functioning of such coatings in that it provides openings through which atmospheric corrodants can attack the substrate and degrade the integrity of the plating.

Different methods of evaluating such tests are presented, including laboratory investigations of the critical parameters and interlaboratory (“round robin”) comparison testing. These methods are described with reference to the nitric acid vapor and sulfur dioxide porosity tests, respectively. It will also be shown how results and insights from these programs are incorporated into ASTM test specifications.