Abstract
Crevice corrosion of Inconel 625 and other metals at high temperatures is currently being investigated. In order to model the effects of temperature on the initiation of crevice corrosion, values for a large number of parameters including diffusion coefficients, ion mobilities, equilibrium constants, passive currents and critical crevice pH values must be determined.
Potentiodynamic studies have been used to determine the critical pH for crevice corrosion, using both the Oldfield and Sutton, and Crolet criteria. It has been found that the critical pH as defined by Oldfield and Sutton seems to be a function of both temperature and chloride ion concentration, but the same relationship does not appear to exist between these variables and the critical pH when the Crolet criterion is used.
Modelling studies of the incubation period of Inconel 625 at 25°C indicate that the duration of the incubation period is strongly dependent on the value of the critical pH assigned to the model. During simulation studies the chloride concentration in the crevice increases to levels greater than 4 M. At this concentration of chloride ions and at a temperature of 25°C potentiodynamic studies indicate that the critical pH would vary between 0.7 and 1.0 depending on the criteria used. This wide variation in critical crevice pH values yields crevice corrosion incubation times of 120 hours and 80 hours respectively for geometrically identical crevices.
