Prediction of stress corrosion cracking susceptibility of stainless steels based on repassivation kinetics

Repassivation kinetics of rapidly scratched scars on the surface of type 304 (UNS S30400) stainless steel (SS) in a chloride solution was examined using on ampero-chronometric method. Its relationship to stress corrosion cracking (SCC) susceptibility measured by slow strain rate tests (SSRT) was explored. The repassivation kinetics was analyzed in terms of the current density flowing from the scratch (i[t]) as a function of the charge density that flowed from the scratch (q[t]). The log i(t) has a linear relationship with 1/q(t) in which the slope, determined from the linear relationship was very effective as a measure of repassivation kinetics. The alloy/ environment system with a lower value of the slope showed a faster repassivation rate with formation of a thinner arid more protective passive film during repassivation. Willi an increase in applied potential, the slope increased gradually and reached asymptotically a limiting value beyond which an inflection point appeared in the log i(t) vs 1/q(t) plots. The change in the slope with applied potential was correlated with the SCC susceptibility. Based on this correlation. a new method was proposed for the prediction of SCC susceptibility in terms of repassivation kinetics. The validity of this method was confirmed by applying the relationship between changes in the slope and SCC susceptibility to effects of solution temperature and Cl- concentration on repassivation kinetics arid SCC susceptibility of type 304 SS.