Comprehending the role of the S-phase on the corrosion behavior of austenitic stainless steel exposed to H₂S/CO₂-saturated liquid and vapor environments

The corrosion behavior of austenitic stainless steel after low-temperature liquid oxy-nitriding (LON) was investigated by exposing in H₂S/CO₂-saturated liquid and vapor environments up to 720 h at 60 °C. The corrosion rates before and after LON were compared by the weightlessness method, and the microstructure as well as the corrosion scales were characterized using surface analysis methods. The results indicated that the composite S-phase layer with the outer Fe₃O₄ layer and the inner nitrogen-rich sublayer could improve the corrosion performance in H₂S/CO₂-saturated environment. The base material (BM) suffered local corrosion first, which then transformed into uniform corrosion. As a comparison, The LON sample, covered with a thin corrosion product layer, indicated slight local corrosion. The excellent corrosion resistance of the S-phase should be attributed to the blocking effect of the continuous Fe₃O₄ film as well as the suppression of the atomic mobility by the nitrogen-containing supersaturated solid solution.