The role of electrode potential distribution in corrosion processes

The role of potential distribution inside cavities or recesses in a metal surface is discussed in the context of non-uniform corrosion rates where planar surfaces give way to pitted, cracked or porous surfaces or where planar surfaces contain crevices. The analysis uses the following concepts: (a) the aspect ratio (ratio of the depth to opening dimensions) of a cavity is an important factor in determining the electrode potential distribution into the cavity, and (b) since the aspect ratio is independent of the size of the cavity, large aspect ratios and hence large voltage differences (IR drops) can exist within the smallest of cavities. The consequences of a wide distribution of electrode potential on the internal cavity surface are discussed in the context of (i) crevice corrosion, (ii) hydrogen charging from inside cavities during anodic polarization of their external surface, (iii) the role of surface diffusion in dealloying corrosion and (iv) the transition from metastable to stable pitting corrosion.