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This paper describes a standard procedure for using constant-current electrolytic reduction (“coulometric reduction”) to determine the relative buildup of corrosion and tarnish films on control coupons from environmental tests, and discusses the types of results and correlations that may be expected. Examples of the applications of this proposed ASTM standard method will be presented for two types of environmental exposure: the mixed flowing gas test and the humid sulfur vapor (“flowers-of-sulfur”) test.

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.