A crack in a metal undergoing stress corrosion cracking (SCC) can be envisioned as consisting of a film free crack tip and oxide covered crack walls. Galvanic interaction between crack tip and crack wall will cause the crack tip to progress by anodic dissolution. This is the corrosion or electrochemical part of SCC. The smaller the ratio of the area of the crack tip (anode) to the area of the crack walls (cathode), the greater the galvanic interaction. In order for a crack to propagate at a high rate, it is therefore essential, as pointed out recently by Engell and Speidel,1 that the crack walls passivate fast, the passivating oxide following the advancing crack, thereby keeping the anode to cathode area ratio at a very small value.
In general, it is not possible to measure quantitatively the galvanic interaction between crack tip and crack wall. It was shown recently,...
