Crack initiation dissolution current density

Once the crack is initiated, the metal surface inside the crack may be quite different from the normal surface of the metal. Thus, in the course of plastic deformation, the metal could have developed slip steps [see Fig. 12.77(c)] which contain crystallographic planes of high Miller index at which the specific dissolution rate (or exchange current density) may be larger than that at the normal metal surface. Anodic current densities of some 104 times those at a passive surface have been shown to appear at a metal surface that is yielding under stress (Despic and Raicheff, 1978). 

Some films are termed "passive," for stainless steels or aluminum alloys, for instance. These films can play an important role in environment-sensitive crack initiation and fracture. Under thermodynamic equilibrium conditions, the film stability may be inferred from E =/(pH) diagrams, where E is the electrical potential related to the chemical free energy G by G = -nEF, and F is Faraday s number. At equilibrium, one can define the electrode potential (related to AG) and the current density I (I e here AG is the activation energy of dissolution). 

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