Oxidation of stainless steels and nickel-base alloys in supercritical water containing H2O2 environments

Supercritical water oxidation (SCWO) is a promising technology being able to completely decompose and oxidize most organic materials at very short reaction times ranging from seconds to minutes. However, the SCWO also leads to severe corrosion of most reactor materials such as stainless steels and nickel-base alloys, which has become one of the major problems hindering the development and industry application of SCWO technology. The chemical compositions and structures of the corrosion scales are important for understanding of corrosion process and possible environmentally assisted cracking mechanisms. In the present work, corrosion behaviors of stainless steels and nickel-base alloys were investigated in H₂O₂-containing SCW environments. The effects of temperature, exposure time and H₂O₂ concentration were examined. The mass gain and the morphologies, microstructures, compositions and structures of the oxide scales were investigated using weight measurement, X-ray photoelectron spectroscopy, grazing incidence X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. Duplex layers of the oxide scales were observed for both the stainless steel and nickel-base alloy after exposure in SCWO environments, consisting of an outer layer of large-grain oxides and an inner layer of fine-grain oxides. The duplex-layer structure was identified to be Ni(OH)₂/NiO/NiCr₂O₄/Cr₂O₃/alloy matrix from outer to inner layer for the nickel-base alloy, and to be FeOOH/(Fe,Cr)₂O₃/ (Fe,Cr)₃O₄/Cr₂O₃/Ni-rich layer/steel matrix for the stainless steel. A nickel enrichment was observed at the oxide/matrix interface for the stainless steel, especially at higher temperatures. The growth mechanisms of the oxide scales on the stainless steel and nickel-base alloy are discussed.