Effects of pH and applied potential on the IGSCC susceptibility of sensitized Type 304 stainless steel in dilute Na2So4 solution at high temperature

The effects of pH and applied potential on the IGSCC of sensitized Type 304 SS were studied in dilute Na2SO4 solution at 2850C in connection with the polarization behaviors, in order to clarify the roles of crevice and dissolved oxygen in the evaluation of SCC susceptibility with double U-bend specimen in high temperature water. The results are summarized as follows. (1) The role of dissolved oxygen in the IGSCC behavior of sensitized Type 304 SS, is to make the potential of the steel noble. On the other hand, in the deaerated solution this same role was proved to be played by externally-applied potential, which is as noble as in the aerated solution. (2) The IGSCC in the inner specimen of double U-bend is caused by the environmental change inside the crevice, where the pH value is lowered to less than 3 due to the hydrolysis of metallic ions dissolved from the specimen, and by the potential shift of inner specimen to the transition region from an active to a passive state on the local cell anodic polarization curve, where IGSCC can readily be generated. (author)