ASTM B825-19,Standard Test Method for Coulometric Reduction of Surface Films on Metallic Test Samples.
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ASTM B825-19 covers procedures and equipment for determining the relative buildup of corrosion and tarnish films on metal surfaces by the constant-current coulometric technique, also known as the cathodic reduction method.
Summary of Test Method
In constant-current coulometry, a fixed reduction-current density is applied to the sample in an electrolytically conductive solution, and the resulting variations in potential-measured against a standard reference electrode in the same solution – are followed as a function of time. Typically, with well-behaved surface films, the voltage-time plot should show a number of horizontal portions, or steps, each corresponding to a specific reduction potential or voltage (Fig. 1).The final potential step, which is always present with all substances,corresponds to the reduction of hydrogen ions in the solution, and represents a limit beyond which no higher potential reduction process can occur.
Many of these environmental test methods require monitoring of the conditions within the chamber during the test in order to confirm that the intended environmentally related reactions are actually taking place. The most common type of monitor consists of copper, silver, or other thin metalliccoupons of a few square centimeters that are placed within the test chamber and that react with the corrosive environment in much the same way as the significant surfaces of the parts under test.
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