Potential, pH, and their gradients are important parameters in determining the electrochemical reactivity of metals in crevices. Although an IR voltage alone can be responsible for the activation of crevice corrosion that occurs immediately, changes in pH may be necessary for crevice corrosion to occur in the delayed form of crevice corrosion. In cases where acidification forms an active peak in the polarization curve, pH, corrosion products, and electrode potential, E, inside the crevice play synergistic roles in determining the onset of crevice corrosion. This synergistic relationship is explored by measuring gradients in acidity, pH(x), and electrode potential, E(x), within crevices during the induction period preceding crevice corrosion of iron in 0.2 M Na2SO4 + 0.025 M K2CrO4 of pH 9.1. Crevice corrosion was found to initiate when the IR voltage caused the E(x) to have a value within the active peak of the polarization curve of the crevice electrolyte. The active peak formed due to acidification of the crevice electrolyte. (c) 2005 The Electrochemical Society.