Effect of applied potential on changes in solution chemistry inside crevices on type 304L stainless steel and Alloy 825

The changes in the pH, chloride concentration, and potential inside a rectangular crevice of metal against polymethyl methacrylate (PMMA) are monitored using microelectrodes as a function of time and externally applied potential. It is found that the environment inside the crevice is altered within the experimental time frame only when the external potential is maintained above a certain value. When the external potential exceeds a certain value, there is an incubation period followed by a rapid increase in the current density which is succeeded by a decrease in pH and the potential inside the crevice. The current density also decreases rapidly upon reversal of the external potential, while a significant reversal of pH occurs over a much longer period of time. The kinetics of these changes in the environment is a function of crevice tightness. A decrease in crevice gap results in a greater decrease in pH. The changes in chloride concentration are much more modest, perhaps due to the formation of chloride complexes which can not be detected by the Ag/AgCl microelectrode. Presence of chromium depletion on the surface of alloy 825 in the crevice results in a more rapid decrease in pH even when the surface is rougher. These observations are explained in terms of crevice corrosion nucleation in small zones of narrow gap between the peaks of surface asperities which then propagate laterally to adjoining zones of wider gaps between the peaks and valleys of the asperities