Stress Corrosion Cracking of Austenitic Stainless Steel in an Acidified Chloride Solution

The critical electrochemical parameters and their relation to SCC were examined in untransformed 301 stainless steel in an acidified concentrated NaCl plus Na₂SO₄ solution. Potential-time curves showed that a substantial period of time was necessary before the abrupt shift to the rest potential. During this period, there was a copious evolution of hydrogen which ceased abruptly coincident with the potential shift. A critical cracking potential was clearly identified near -375 mV, SCE. There was a true incubation period for crack initiation even after the potential had attained the corrosion potential. Current-time curves clearly indicated the formation of a pseudo-passive film where the current dropped from an initially high value to close to zero. This film, the precursor to SCC, was relatively thick and on examination after removal revealed Cr enrichment, Fe and Ni depletion, and chlorine buildup. Electron diffraction identified the film as a chromium oxyhydroxide. It is concluded that several critical electrochemical parameters must match or closely approach each other for SCC to occur, a situation that is dependent upon the degree of occlusion of a cell. These parameters are the critical cracking potential, the repassivation potential, the potential at which the current reverses, and most likely also the critical crevice potential.