Potential and Chloride Effects in the Stress Corrosion Cracking of Type 304 Stainless Steel in 290 C Water

The stress corrosion cracking (SCC) of stainless steel in high temperature water was studied by anodically polarizing stressed (partially flattened), hollow thinwall Type 304 stainless steel electrodes to the point of SCC in 0.001N to 5N NaCl at 290 C. The potential of SCC increased progressively as the chloride concentration was lowered, cracking occurring at about 600 mV (with respect to Pt) with 5N NaCl, but not until about 1100 mV with 0.001N NaCl. On the other hand, SCC was always preceded by pitting where the current rose sharply, and the current required to produce cracking was progressively less as the NaCl concentration decreased. There was no rise in hydrogen flux through the electrode wall prior to cracking, nor any other indication of a hydrogen effect in SCC. The pitting preceding SCC was revealed by SEM to actually involve a type of filiform corrosion where chloride was concentrated under the surface oxide in filament-like occluded cells. A Cr-rich corrosion phase is formed beneath these cells, and it is within this phase that SCC apparently initiates. With 5N NaCl, a thin, amorphous electrobrightening surface film formed in place of the crystalline spinel film which developed with lower concentrations. SCC appeared to initiate through filiform corrosion in this film also, however.