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The purpose of this study was to determine the mechanism by which austenitic iron-and nickel-based alloys oxidize in pure supercritical water. Austenitic stainless steel alloys Type 304 (UNS S30400) and Type 316L (UNS S31603) and nickel-based Alloy 625 (UNS N06625) and Alloy 690 (UNS N06690) were exposed in deaerated supercritical water over the temperature range from 400°C to 550°C. Exposure experiments resulted in the formation of a two-layer oxide on Type 304 and Type 316L in which the outer layer is porous magnetite (Fe 3 O 4 ) and the inner layer is a denser iron-chromium spinel. In Alloy 690 the outer layer is iron-rich and the inner layer is composed of chromia (Cr 2 O 3 ), nickel oxide (NiO), and Ni(Cr,Fe) 2 O 4 . The oxide on Alloy 625 was too thin to measure. Inner and outer oxides have the same grain orientation as the matrix on the stainless alloys, and on Alloy 690, the inner oxide has a common orientation with the matrix but the outer layer consists of randomly oriented oxides. The inner/outer oxide interface is coincident with the original metal surface. The activation energies for oxide growth on each of the alloys, determined from weightgain measurements, are ~210 kJ/mol for the stainless alloys and ~140 kJ/mol for the nickel-based alloys. In both alloy systems, the outer layer grows by outward iron cation diffusion (iron in stainless steel and nickel in nickel-based alloys) and the inner layer grows by inward diffusion of oxygen to the oxide-metal interface. The likely rate-limiting step of the corrosion mechanism is inward oxygen diffusion by a short-circuit process.

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