Pronounced corrosion damage occurs in supercritical water oxidation reactors, and few materials are immune to attack. Ln this paper, we describe a phenomenological model for the corrosion process, and we discuss the effect of electrolyte dissociation and water density, as influenced by temperature and pressure, upon the kinetics of corrosion of metals and alloys in supercritical water. The corrosion process at near-critical temperatures is believed to involve acid attack, with the concentration of H+ being a function of the dissociation constant of HCl, which is a major product of the oxidation of chlorinated organic waste, and of the density of the solution. We show that the competing effects of temperature on the heterogeneous rate constant and on the concentrations of H+ and O-2 leads to a pressure (and hence density)-dependent maximum in the corrosion rate in the vicinity of the critical temperature. This result is in general agreement with experimental data on corrosion in aqueous solutions at near-critical temperatures.