Intergranular Corrosion of Austenitic Stainless Steels—An Electrochemical, Potentiokinetic Test Method

An anodic polarization test in a perchloric acid-sodium chloride electrolyte has been developed to determine the intergranular corrosion susceptibility of three austenitic stainless steels, American Iron and Steel Institute (AISI) 302, 304, and 316. A region of secondary activity is present in the polarization curves, and the magnitude and potential of the maximum of this secondary activity is related to susceptibility to intergranular corrosion. The initial work involved polarization testing of the main surface of strip and plate material in the solution-treated condition and after heat treatment at 948 K (675°C) for up to 7 days. The correlation between the penetration rate determined by ASTM C 262 and the electrochemical activity is excellent for some materials but poor for others. In the latter, it was clear that end and edge grain attack were dominant in the nitric acid test. However, if the electrochemical activity of all three orthogonal faces of the materials is measured, and a weighted sum determined to reflect the relative areas of each face exposed to nitric acid, then a consistently good correlation with penetration rate is obtained. For material heat treated to provide a range of penetration rates (0.08–14 mm/year) as measured by ASTM C 262, a relationship of mm/year = A (activity)0 5 is found. For low penetration rates (<0.5 mm/year), the power index becomes approximately 0.25. This indicates that the nitric acid test is less able to differentiate between highly resistant materials that still exhibit measurably different electro-chemical activities.