Possible Distribution of Potential inside Corroding Crevices

摘要

One of the principal tasks in theoretically describing crevice corrosion is to develop the ability to predict which geometries will be susceptible to this form of attack and to determine the critical crevice geometry that separates the region where crevice corrosion initiates from the one where crevice corrosion does not occur (1). It has been suggested that different mechanisms must be invoked to describe the initiation of crevice corrosion in different systems (geometries). In accordance with the classical point of view, the initiation of crevice corrosion is connected with the development of a differential aeration cell and with the acidification of the crevice solution and/or migration of aggressive anions (for example, Cl") into the cavity (2, 3), as embodied in the differential aeration hypothesis (DAH). However, as it was noted in Ref. 3 "this classical theory is not able to explain cases of immediate corrosion or cases of crevice corrosion in systems that show no significant acidification or aggressive ion buildup in the crevice solution. In these cases, crevice corrosion can be caused by IR (ohmic) potential drop in the cavity, which places the local metal potential existing in the crevice wall in the active pit region of the polarization curve (1, 3-6). Generally speaking, in the absence of significant concentration drops in the crevice, the IR drop can be calculated by solving a Poisson-type differential equation relative to the potential in the solution by using a numerical method.