An Evaluation of the SCC Susceptibility of Chromized Type 409 Stainless Steel with Relevance to Materials Selection for Hydrothermal Liquefaction of Biomass

Biomass hydrothermal liquefaction (HTL) is operated in a harsh reaction medium, which contains hot pressurized water, inorganic acidic or basic catalyst, and inorganic/organic corrosive components released during the conversion. Candidate alloys for this application shall have acceptable resistance to both general corrosion and stress corrosion cracking (SCC) to withstand the HTL process conditions (250-374°C and 4-22 MPa). Ferritic Fe-Cr steels are more prone to corrosion but less susceptible to SCC compared to austenitic Fe-Cr-Ni steels. Our recent study has shown that chromizing can significantly reduce corrosion of Type 409 stainless steel (Fe-11Cr) in a simulated aqueous HTL solution. The objective of this study is thus to determine the SCC susceptibility of chromized Type 409 stainless steel, relative to the bare case. The slow strain rate testing (SSRT) technique was used for this purpose. For simplicity of experimentation, SSRT was conducted using simulated HTL water containing 800 ppm KCl, 1 M K2CO3 and 10 wt.% acetic acid at ambient temperature and pressure. Complementary potentiodynamic polarization measurements and surface analysis by X-ray photoelectron spectroscopy (XPS) were also made to help interpret the SSRT results. The SSRT results show no significant difference in SCC susceptibility, regardless of the starting surface. Thus, chromizing, while significantly reducing the corrosion of Type 409 stainless steel, does not adversely affect SCC susceptibility, at least under the conditions tested.