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.

A Peridynamic Model for Crevice Corrosion Damage

“…This stagnation of the active site leads to deep carving into the washer near the crevice mouth. This observation is consistent with other studies on crevice corrosion in Nickel Alloy 625 washers [61][62][63].…”

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“…Such crevices were like a pit in reverse because the attack started deep in the crevice and grew toward the mouth, getting less deep with time rather than deeper. 411 The crevices were shown to grow under diffusion control initially. 410 The decreasing diffusion path associated with the growth of the crevice toward the mouth resulted in an increase in the limiting current that dominated any geometric effect on the solution resistance to cause an increase in the ohmic potential drop.…”

“…410 The decreasing diffusion path associated with the growth of the crevice toward the mouth resulted in an increase in the limiting current that dominated any geometric effect on the solution resistance to cause an increase in the ohmic potential drop. 411 The combination resulted in an unusual situation in which the crevice current density increased as the potential decreased. This continued until the crevice potential decreased to E sat , when the salt film dissolved and the crevice transitioned to activation/ohmic control.…”

“…The concepts developed in this new framework have been applied by Lillard et al to study and model crevice corrosion. − These authors first developed a novel optical method that allowed for detailed tracking of the crevice corrosion front for a crevice in Ni alloy 625 created by a transparent acrylic block crevice former . The differences in successive images could be used to determine the active area and, along with the current measured under potentiostatic conditions, the local current density.…”

“…The known geometry allowed for assessment of the ohmic potential drop and, thus, the local potential. Such crevices were like a pit in reverse because the attack started deep in the crevice and grew toward the mouth, getting less deep with time rather than deeper . The crevices were shown to grow under diffusion control initially .…”