Quantifying Alloy 625 Crevice Corrosion Using an Image Differencing Technique: Part I. Initiation and Propagation

Abstract

An image differencing technique was used to analyze optical images from an alloy 625 crevice made using an acrylic window. 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. The technique quantified the change in pixels between successive images due to corrosion damage. By calibrating the number of pixels per unit area we calculated the active area and, from this, the crevice current density. It was found that the crevice propagated in the current density range between 3 mA cm−2 at initiation and 100 mA cm−2 at steady state. It is shown that the mass loss as a function of position calculated from the current density accurately reproduces the damage profile inside the crevice. By knowing the location of the active front and the crevice current, we are also able to calculate the wall potential as a function of time and distance. It is shown that the initiation site has a wall potential that is within millivolts of the applied anodic potential and decreases as the active front moved towards the mouth. Finally, we derive an expression for the crevice shape factor which was used to show that the crevice is under diffusion control at later times in the propagation stage.