Tarnishing model

All of these processes can be described by a diffusion-controlled model originally derived to explain the tarnishing of metals and hence commonly called the tarnishing model. The derivation of this model is based on the assumptions that (a) the reaction site is immobile, (b) the concentration of reaction sites is independent of time and temperature in the absence of the tarnishing reaction, and (c) the reaction rate is very... 

Fig. 40.— Energy level diagram corresponding to Mott s model for tarnish reactions.

In accordance with the expressed purpose of this monograph, we shall now discuss various models to explain the chemical and physical processes of tarnishing, and we shall not be concerned with presenting the details for particular systems. Such information can be found in a large number of special monographs on this subject [1, 2, 3]. 

The slip dissolution model assumes that plastic deformation at the crack tip is responsible for the activation. But other mechanisms can have the same effect. Tensile stress at the crack tip could, for example, break a brittle tarnish film or passive oxide film, thereby exposing the base metal to the electrolyte. Selective dissolution of alloy components at the crack tip could locally weaken the metal matrix and thus permit... 

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