Passive films solid film theory

In view of the fact that there are two opposing views on the mechanism of passivity it is not surprising that a similar situation prevails concerning the mechanism of breakdown of passivity. The solid film theory of passivity and breakdown of passivity is dealt with in some detail in Section 1.5, so that it is appropriate here to discuss briefly the views based on the adsorption theory. 

Figure 40. Plot of the fluctuation-diffusion current J vs. iwr.91 id is the slope of the fluctuation-diffusion current given by Eq. (115). Solid and dotted lines correspond to the theoretical and experimental results, respectively. (NiCljJ = 0.1 mol nT3. [NsCl] = 7 mol m 3. V = 0.1 V, T= 300 K. (Reprinted from M. Asanuma and R. Aogaki, Nonequilibrium fluctuation theory on pitting dissolution, n. Determination of surface coverage of nickel passive film, J. Chem. Phys. 106, 9938, 1997, Fig. 8. Copyright 1997, American Institute of Physics.)...

Based on conventional electrochemical observations, two models have been proposed in the literature to account for the structure and properties of these passive films. The first is based on the formation of an inorganic surface layer in which the transport is described by solid electrolyte theory, whereas the second assumes that the film has both compact and porous layers with polymer-like characteristics. 

The Cr-depletion theory has been accepted as the mechanism by which a sensitized stainless steel becomes susceptible to intergranular corrosion [60]. Recall that a minimum of 10 to 13wt.% Gr in solid solution is required to form protective passive films on Fe-Ni-Gr alloys... 

Faraday published a theory in which he suggested that the metal surface of certain passive metals was oxidized and the oxide layer was very thin (Phil. Mag., 9 (1836), p. 5). If a metal is converted to an oxide and the oxide which is formed is stable, the metal is considered passive as this oxide forms a barrier between the metal and the environment. For example, iron is not attacked in concentrated HNO3, as a very thin film of an oxide is formed on its surface and causes loss of reactivity. A film of solid hydroxide or oxide may be precipitated from an aqueous solution if there are metal ions in the solution... 

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