Effect of Aging Treatments on the Repassivation Potential of Duplex Stainless Steel S32205 Available to Purchase

The effect of isothermal heat treatments at 475, 700, and 870°C on the localized corrosion susceptibility of 2205 duplex stainless steel (UNS S32205) was assessed in chloride-containing environments at 60°C. Microstructural changes with heat treatment were analyzed with scanning transmission electron microscopy (STEM) for thermally treated S32205. The results of the tests indicate that exposure of S32205 to these temperatures resulted in an initial drop in the crevice repassivation potential (E_rcrev). This drop was partially recovered for specimens aged at 870°C for times longer than 5 h, but the recovery was not observed for the 700°C aged material. The drop in E_rcrev correlated with the precipitation and growth of Cr-depleted secondary austenite as well as a grain boundary chromium-depleted (GBCD) layer between a Cr-rich precipitate and the decomposing ferrite matrix. The depletion severity of the secondary austenite phase differed for S32205 aged at 700°C and 870°C, with the 700°C aged materials yielding a secondary austenite phase containing approximately 12.5% Cr and the 870°C precipitated secondary austenite containing approximately 15.8% Cr. The formation of secondary austenite results in a non-recoverable loss of localized corrosion resistance for S32205.