Surface modification of austenitic stainless steel for corrosion resistance in high temperature supercritical-carbon dioxide environment

Surface-modification was applied to austenitic stainless steel 316LN in order to improve the corrosion resistance in high temperature supercritical-carbon dioxide (S-CO₂) environment. The surface-modification methods consisted of deposition of either a single Al layer or a NiAl bi-layer on the surface, followed by inter-diffusion heat treatments. The former resulted in formation of a surface layer composed of a mixture of NiAl and ferrite phases, and the latter resulted in formation of a continuous NiAl surface layer. Surface-modified 316LN showed reduced weight gains than the as-received 316LN alloy after exposure to S-CO₂ at 650 °C (20 MPa) for 500 h. Pre-oxidation in helium at 900 °C before S-CO₂ exposure further improved corrosion resistance by forming inner α-Al₂O₃ layer. On the other hand, the extent of inter-diffusion zone was less for the specimen with a continuous NiAl surface layer before and after S-CO₂ exposure.