The mechanism of high temperature aqueous corrosion of stainless steels

It is shown that the corrosion rate of stainless steels in high temperature, de-oxygenated water is controlled by the solid state diffusion of Fe ions along grain boundaries in the oxide layer. A duplex layered oxide is formed with the inner layer growing by the ingress of water along oxide micropores and the outer layer growing by the diffusion of metal ions. The magnitude of the corrosion rate is consistent with diffusion control by an inner layer oxide consisting of a Cr spinel rather than Cr2O3. The Fe ion flux in the oxide is carried as vacancies under alkaline conditions and as interstitials under acid conditions, respectively. The corrosion rate rises in acid and alkaline conditions because the concentration of these defects increases. The outer layer oxide can be released to solution, but as rate control resides in the inner oxide layer, the total corrosion rate is independent of fluid flow velocity and the presence of any outer layer in this model, as found in most experiments. The model is also applied to related Ni- and Co-based alloys such as Inconel 600 and Stellite, where the corrosion rates are controlled by diffusion through the simple oxides, NiO or CoO, not spinels, unlike in the steels.