Corrosion Behavior of Si Diffusion Coating on an Austenitic Fe-Base Alloy in High Temperature Supercritical-Carbon Dioxide and Steam Environment

In order to enhance corrosion resistance of stainless steel (SS) 316LN at high temperature environments, surface modification was carried out by Si deposition and subsequent heat treatment at 900 °C for 1 h. This resulted in the formation of Fe5Ni3Si2 phase on the surface region. The surface-modified alloy was exposed to high temperature S-CO2 (650 °C, 20 MPa) and steam (650 °C, 0.1 MPa) for 500 h and evaluated for its corrosion behavior in comparison to the as-received alloy. In S-CO2 environment, the as-received SS 316LN showed severe oxide spallation and thick Fe-rich oxide formation, while the surface-modified alloy formed a continuous and adherent Si- and Cr-rich oxide layer. In steam, as-received SS 316LN formed very thick duplex Fe- and Cr-rich oxide layers. On the other hand, surface-modified SS 316LN formed notably thinner oxides, which could be attributed to the formation of Si-rich oxide under outer Fe-rich oxides on the surface-modified alloy. Thus, in view of the weight changes, oxide thickness, and morphologies of the two conditions, it was found that Si diffusion coating was effective in improving the corrosion resistance of SS 316LN in both S-CO2 and steam environments.