Testing Sensitization and Predicting Susceptibility to Intergranular Corrosion and Intergranular Stress Corrosion Cracking in Austenitic Stainless Steels

In this paper, degree of sensitization (DOS) of austenitic stainless steels (SS) is defined in terms of the characteristic parameters of the chromium depletion zones: coverage (proportion of the grain boundary length covered by chromium depletion zones), width, and depth (the minimum level of chromium in the depletion zones). A sample matrix was developed that provides heat-treated samples of Type 304 SS (UNS S30400) having the same coverage developed at four different temperatures of heat treatments. The coverage was measured by quantitative metallography. The absolute value of depth was measured by the quantitative potentiostatic electrochemical test. Other electrochemical (potentiodynamic and potentiostatic) techniques were used to assess the depth of chromium depletion zones. New parameters have been developed to express the results of the potentiodynamic, potentiostatic, and electrochemical potentiokinetic reactivation (EPR) tests that reflect the contributions attributable to either coverage and depth or only the depth of chromium depletion zones. It has been shown that these new parameters are sensitive in the range of 7.5% to 13.5% Cr in the depletion zones. For the EPR test, the new parameters make it more sensitive to discriminate highly sensitized samples, which was also not possible by the parameter P_a. The results of these test techniques were correlated with the results of the intergranular corrosion (IGC) (ASTM A262, Practice C) and intergranular stress corrosion cracking (IGSCC) (ASTM G123) tests. The cutoff values of various parameters of these tests have been established, enabling screening of heats of Type 304/304L SS (UNS S30403) that resist IGC in nitric acid (HNO₃) environments as well as IGSCC in hot chloride environments. These techniques are fast, can be used as nondestructive tests, and have been quantified.