Quantifying Alloy 625 Crevice Corrosion Using an Image Differencing Technique: Part III. The Transition from Diffusion to Activation Control and the Implications for the Measured Electrochemical Potentials

Abstract

An image differencing technique was used to calculate crevice area and, subsequently, surface current density, solution resistance and wall potential in an alloy 625 crevice during repassivation experiments. In these experiments, crevices were immersed in ASTM artificial ocean water and held potentiostatically in the passive region while the current was monitored with time. Once initiated, the crevice was allowed to propagate for various times, equivalent to increasing charge passed. During this period, an active front was observed to initiate and move from the tip towards the mouth. As the front moved towards the mouth, the wall potential decreased while the surface current density increased. After a fixed time, the applied potential was decreased in a stepwise fashion. At a critical potential, forward movement of the active corrosion front stopped, though crevice propagation continued in place resulting in an increase in damage depth. This potential is referred to here as ETHE. Transport calculations revealed that propagation occurs under diffusion control and ETHE is likely associated with salt film dissolution and a transition to activation/ohmic control, consistent with this being the transition potential ET (also referred to as Esat) reported for one dimensional pitting experiments (pencil electrodes). A mechanism for crevice growth and the transition to repassivation that is similar to that believed to controlled pit propagation and repassivation is proposed.