Effect of Aggressive Anions on the Stress Corrosion Cracking of Stainless Steel in Hot-Pressurized Alkaline Water

U-bend stress-corrosion cracking (SCC) testing of low Cr ferritic (Type 409) and austenitic (Type 304L) stainless steel was conducted in a hot-pressured alkaline water to study the effect of aggressive anions (Cl⁻ and HS⁻) on the relative susceptibility. SCC was only observed in Type 304L when immersed in the solution that contained both aggressive anions. Critical factors were identified based on a cross-section examination of the U bends after exposure using complementary electron microscopy techniques. These factors include (i) preferential oxidation of deformation bands (arising from cold working), (ii) Ni-S compound formation at the oxide/metal interface and (iii) S and Cl incorporation into the inward-growing Cr-rich oxide. These critical factors were considered within an overall slip dissolution-type mechanism to account for the SCC observed.