Question

18B.12 Tarnishing of metal surfaces.

18B.12 Tarnishing of metal surfaces. In the oxidation of most metals (excluding the alkali and alkaline-earth metals) the volume of oxide produced is greater than that of the metal con- sumed. This oxide thus tends to form a compact film effectively insulating the oxygen and metal from each other. For the derivations that follow it may be assumed that (a) For oxidation to proceed, oxygen must diffuse through the oxide film and that this diffusion follows Fick's law. (b) The free surface of the oxide film is saturated with oxygen from the surrounding air. (c) Once the film of oxide has become reasonably thick, the oxidation becomes diffusion controlled; that is, the dissolved oxygen concentration is essentially zero at the oxide-metal surface. (d) The rate of change of dissolved oxygen content of the film is small compared to the rate of reaction. That is, quasi-steady-state conditions may be assumed. (e) The reaction involved is-O2 + M → Mo, We wish to develop an expression for rate of tarnishing in terms of oxygen diffusivity through the oxide film, the densities of the metal and its oxide, and the stoichiometry of the reaction. Let co be the solubility of O2 in the film, c, the molar density of the film, and z, the thickness of the film. Show that the film thickness is o2 -Mo,t co (18B.12-1) This result, the so-called "quadratic law," gives a satisfactory empirical correlation for a number of oxidation and other tarnishing reactions.? Most such reactions are, however, much more complex than the mechanism given above.

Answer 1

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