Corrosion Ferritic Stainless Steel in a Simulated Hydrothermal Liquefaction Bioconversion Aqueous Solution: Effect of Surface Cr Content

The objective of this study was to determine the effect of surface Cr content on the corrosion of ferritic stainless steels in hot-pressurized alkaline water (310°C at 10 MPa) simulating a hydrothermal liquefaction bioconversion medium. Two methods to increase the Cr surface content were investigated: (i) selecting commercial grades of ferritic stainless steel with an increasing Cr content and (ii) applying a Cr coating (chromizing) to a low-Cr (Type 409) ferritic stainless steel. The observed parabolic-like corrosion kinetics were analyzed and discussed in terms of the structure and composition of the double-layered oxide films that formed. The (surface) Cr content is a critical factor affecting corrosion. Corrosion was reduced by 66% (after 20 d exposure) when increasing the Cr content from 9 wt% (P91) to 21 wt% (SS443) in the commercial grades of ferritic stainless steel. Moreover, corrosion was reduced by 84% (after 20 d exposure) by chromizing the surface of a low-Cr (Type 409) ferritic stainless steel. Improved corrosion protection was attributed to increased Cr incorporation into the inner (barrier) Fe(Fe_1-nCr_n)₂O₄ layer, with the formation of a Cr₂O₃ layer (resulting from chromizing) being most beneficial.