Abstract
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 (250-374 °C), pressurized (4-22 MPa) subcritical water as the conversion medium with the addition of 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 days of immersion in simulated HTL alkaline water at 310 °C and 10 MPa in a static autoclave test system. Analysis of the starting (pre-immersion) and corroded (post-immersion) surfaces were 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 prior to and during immersion. Alloy 33 is less susceptible to corrosion than Type 304 due to the formation of a more protective Cr2O3 sub-layer at the oxide/metal interface.
