Oxide Scale Formation Behavior of Stainless Steels Exposed to the Oxidative High Concentration Alkaline Solutions

ABSTRACT The corrosion behaviors of austenitic stainless steels 304 and 316 were systematically examined in a 60 wt. % NaOH solution at 240°C under an oxygen partial pressure of 3.2 MPa. After 168h of immersion corrosion, the steady corrosion rates of 304SS and 316SS were determined as 6.60 mm/y and 4.28 mm/y, respectively, with 304SS showing higher corrosion rates under these conditions. Corrosion tests conducted at different durations helped to reveal the migration and transformation patterns of key elements in stainless steel in this aggressive environment. The corrosion mechanism was identified as a multi-stage process. Firstly, during corrosion, a metallic nickel layer forms near the surface, thus preventing the formation of a stable passive protective film. Secondly, chromium (Cr) and molybdenum (Mo) dissolve and convert into their respective soluble products, CrO42− and MoO42−, thereby destroying the integrity of the material. Finally, surface corrosion products tend to crack, leading to delamination and exposure of fresh surfaces to the corrosive medium, therefore accelerating the corrosion process. The investigation results in this work demonstrated the difference in corrosion resistance between 304SS and 316SS, revealing the reasons for the lack of corrosion resistance of stainless steels in oxidative high-concentration alkaline solutions.