Corrosion of Surface-Treated Type 304 Stainless Steel in Alkaline Subcritical Water

Surface treatment can reduce corrosion of stainless steel in hot-pressurized (subcritical/supercritical) water and, thus is being considered to control corrosion of austenitic stainless steel Type 304 (Fe-18Cr-8Ni) for application to hydrothermal liquefaction (HTL) conversion of biomass. Typical HTL conversion processes involve hot (250°C to 374°C), pressurized (4 MPa to 22 MPa) subcritical water as the conversion medium with the addition of a homogenous alkaline catalyst. The objective of this research was to determine the relative extent to which well-established surface treatments could reduce corrosion of Type 304 in simulated HTL alkaline water, with the chromia-forming Alloy 33 (Fe-33Cr-32Ni) serving as a comparative baseline. Surface treatments examined include grinding, shot peening, sandblasting, and chemical pickling. Corrosion was assessed using gravimetric measurements made after 10 d of immersion in simulated HTL alkaline water at 310°C and 10 MPa in a static autoclave test system. Analysis of the starting (preimmersion) and corroded (postimmersion) surfaces was conducted using a variety of surface characterization techniques. None of the surface treatments reduced corrosion of Type 304, relative to the mechanically-ground surface, despite achieving the desired outcomes before and during immersion. Alloy 33 is less susceptible to corrosion than Type 304 due to the formation of a more protective Cr₂O₃ sublayer at the oxide/metal interface.