Oxidation of metals forming more than one oxide

Oxidation of metals forming more than one oxide 

A number of metals, such as copper, cobalt and h on, form a number of oxide layers during oxidation in air. Providing that interfacial thermodynamic equilibrium exists at the boundaries between the various oxide layers, the relative thicknesses of the oxides will depend on die relative diffusion coefficients of the mobile species as well as the oxygen potential gradients across each oxide layer. The flux of ions and electrons is given by Einstein s mobility equation for each diffusing species in each layer 

In part the parabolic law may also apply to multilayer oxide systems where the cation diffusion coefficient is much higher in the lower oxide tlran in the higher oxide, which, growing as a thin layer, undergoes plastic deformation at high temperatures, thus retaining the overall oxide layer as impervious to enuy of tire gas. 

Nickel oxide, NiO, which is the only oxide formed by nickel during oxidation in air, has a very naiTow range of iroir-stoichiomen y, the maximum oxygeir/nickel ratio probably being 1.001. The oxygen dependence of the deviation from non-stoichiometry is and hence dre oxidation rate 

The addition of a small amount of cluomium at concentrations less than 1 %, increase the oxidation rate proportionately to the cluomium content. This is to be expected since the replacement of tluee nickel ions in NiO by two chromium ions in Ci 203 will introduce one cation vacancy/CiaOs molecule. 

Yttrium, on the other hand, which has a larger cation radius than Cr3+, appears to affect the grain boundary cation diffusion and not the volume diffusion of Ni2+. The effects of the addition of small amounts of yttrium to nickel is to decrease the rate of the low temperature grain-boundary dominated oxidation kinetics. 

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