Effect of Externally Added Molybdate on Repassivation and Stress Corrosion Cracking of Type 304 Stainless Steel in Hydrochloric Acid

The effect of externally added molybdate (MoO42–) on the repassivation and SCC behavior of type 304 (UNS S30400) stainless steel (SS) was investigated in 1 M hydrochloric acid (HCl) at open-circuit potential (OCP). With the addition of MoO42–, the time to failure initially decreased but then gradually increased. From studies using potential-vs-time data, the scratching electrode technique, and electron spectroscopy for chemical analysis (ESCA), the initial decrease in the time to failure at lower MoO42– concentrations was attributed to an increase in the cathodic reaction rate and a consequent increase in anodic dissolution. The increase in the time to failure at higher MoO42– concentrations was attributed to the reduction in anodic dissolution with a strong tendency for MoO42– deposition on the bare metal surface.