Revealing the corrosion mechanism of 321 stainless steel in alternating environments of intermediate temperature water vapor and normal temperature dilute salt solution and the effects of strain thereon

An investigation was conducted into the corrosion mechanism of 321 stainless steel (SS) without and with 10% strain in a simulated secondary circuit steam pipeline environment of nuclear-powered vessels, which involved exposure to intermediate temperature water vapor (480 °C, 95%vol) and normal temperature dilute salt solution (30 °C, 0.5 wt% NaCl). The results indicate that ferrite in 321 SS is the vulnerable site for alternating corrosion. Localized corrosion along the ferrite is caused by the combined effects of oxidation and electrochemical reactions. Residual NaCl particles from a 30 °C dilute salt solution undergo an oxidation reaction with water vapor at 480 °C, which destroys the protective oxide film, leading to the formation of a Cr-depleted zone where the Cr content in ferrite becomes lower than that in austenite. In the 30 °C dilute salt solution, the ferrite in the Cr-depleted zone exhibits a higher self-corrosion current density, and the galvanic effect between the two phases in this zone further promotes the dissolution of ferrite, thereby leading to localized corrosion. Under an applied strain of 10%, the difference in Cr content between the two phases increases. This both enhances the difference in i c and intensifies the galvanic effect between the phases, thereby accelerating localized corrosion.