The gas/oxide interface and the oxidation of metals

Attention is drawn to the importance of the structure of the surface layer of oxygen adsorbed at the oxide/oxygen interface in determining the laws of growth of thin oxide films on metals. Recent work on the chemisorption of oxygen by alumina and cuprous oxide indicates that a saturated chemisorbed layer is formed even at high temperatures and at pressures well below those used in oxidation studies. In attempting to take this into account in the current theory of metal oxidation two models of the saturated layer are discussed. The simplest assumes that the outer surface of a growing oxide film is completely covered with oxygen ions, and it is shown that, in this case, a linear law of oxidation may be obtained even though the migration of metal ions through the film is rate-determining. A more complex picture of a saturated layer is developed for p-type oxides according to which oxygen is bound to surface metal ions with an attendant valence change so that neutral pairs are formed. Some of these are converted to oxygen ions by electron transfer from the underlying metal. This model leads to laws of oxidation which are already familiar, namely a cubic law for thin films and a logarithmic law for very thin films. Particular attention is paid to the predicted pressure dependence of the oxidation rate and some experimental results for aluminium and copper are discussed.