Stress corrosion cracking is a form of localized corrosion that is characterized by non-homogeneous, quasi-stationary anodic and cathodic partial current densities distributions on a surface.
The anodic and cathodic regions are spatially separated and are coupled according to the Differential Aeration Hypothesis (U.R.Evans, circa 1927).
The anode occurs in that region of the system that has minimal access to the cathodic depolarizer (e.g., O2), whereas the cathode occurs in that region that has the greatest access to the depolarizer.
In order to maintain charge balance a positive current flows through the solution from the local cathode, and an equivalent electron current flows through the metal in the same direction, where they annihilate on the external surface by the reduction of the depolarizer.
In the author’s opinion, the key to understanding and predicting stress corrosion cracking lies in developing an understanding of the origin and properties of the coupling current.