Characterization of oxide scales formed on heating equipment in supercritical water gasification process for producing hydrogen

The external- and internal walls of stainless steel heating equipment installed in supercritical water gasification plants for converting fossil fuels or renewable biomass to produce hydrogen-rich gases, respectively, are exposed to high-temperature air and reducing subcritical aqueous systems, all confronting severe corrosion issues. In these two harsh environments, the oxidation characteristics of typical stainless steel 304 were investigated by series of analytical methods. The oxide scale formed on stainless steel 304 in air at 550–650 °C exhibits a three-layer structure: an outmost layer consisting of Fe-rich corundum-type oxides, an inner layer comprising Cr-rich oxides and unoxidized metals, and an intermediate layer. The reducing subcritical aqueous experiments indicate that the relatively severe corrosion occurs at 350 °C rather than at 250 °C and 425 °C, which can be attributed to the reduced H⁺ concentration at 425 °C comparing to that at 350 °C and the increased Arrhenius rate constant of corrosion reaction with further increasing temperature from 250 °C to 350 °C. A proper pH range (6.5–10.5) for improving the corrosion resistance of Fe/Ni-based alloys in reducing subcritical aqueous environments was obtained, of which the upper limit decreases with increasing oxidicability of solutions.