EPR Method versus ASTM A262 Practice E for Measuring DOS in Austenitic Stainless Steels

A detailed study of AISI(1) 304 and 316 stainless steels (SSs) has been conducted to correlate the degree of sensitization (DOS) measured by the electrochemical potentiokinetic reactivation (EPR) test and by the ASTM(2) A262 Practice E test. Commercial grade AISI 304 and 316 SSs were sensitized by isothermal aging at different temperatures in the range of 823 to 1073 K. The DOS was measured using the single-loop version of the EPR test with an electrolyte of 0.5 M H2SO4 + 0.01 M NH4SCN and a potential scan rate of 6 V/h for both steels. The DOS was also determined quantitatively using a modified ASTM A262E test for AISI 316 SS aged at 923, 973, 1023, and 1073 K. In the modified A262E test, the apparent loss in the tensile strength after exposure to the Cu-CuSO4-H2SO4 solution is taken as the measure of DOS. The results indicated that, based on the EPR test, all the DOSs used in this study can be differentiated. Unlike for AISI 304 SS, the EPR curves for AISI 316 SS, sensitized by certain time-temperature combinations of heat treatment, exhibited a shoulder anodic to the reactivation peak. This has been explained with the assumption that Mo depletion width is significantly less than that of Cr in the initial stages of sensitization of AISI 316 SS. The DOS determined from ASTM A262E and EPR-DOS correlated closely for most of the heat treatments, except for 1023 K/2 h and for all the heat treatments at 1073 K. Also, the EPR charge values that correspond to failure in ASTM A262E tests varied with the temperature of sensitizing heat treatment.