The Oxidation of Tin: III . The Mechanisms of Oxidation of Pure Tin and Their Dependence on Time and Oxygen Pressure

Abstract

The rate curves obtained for the oxidation of pure tin at oxygen pressures of 1 mm Hg and above are sigmoid‐shaped. The initial stage, in which the oxidation rate increases, is explained by nucleation and lateral growth of the oxide. Kinetic data fit an equation derived by Evans for the nucleation and lateral growth of an oxide film. Subsequently, the rate becomes logarithmic, and in this region cavities that act as diffusion barriers develop in the oxide film. The existence of cavities at the metal‐oxide interface has been revealed by electron microscopy, and mechanisms for their formation and growth are presented. An activation energy is derived that is assumed to be associated with the diffusion of tin ions through the oxide film in areas between the cavities. Thus, the mechanism operating in the region of logarithmic growth is not simple. Basically, it consists of the diffusion of tin ions through the dense regions of the film according to a parabolic or cubic relationship. It is complicated, however, by the formation and growth in the oxide film of cavities that progressively reduce the area available for the passage of tin ions through the film, as suggested by Evans. The net effect of these concurrent processes on the kinetics of oxidation is the observed logarithmic rate. For still longer times of oxidation, the thick oxide film may or may not rupture, which results in an erratic rate of oxidation.At oxygen pressures of less than 1 mm Hg, the oxidation rate increases continuously with time over the period investigated, and the dissociation of oxygen appears to be rate controlling, at least initially. The oxide film formed at these low oxygen pressures consists of fine dendritic crystals, rather than the large single‐crystal platelets formed at higher pressures. The dendritic nature of the films indicates that the amount of oxygen available for oxide formation is depleted by the reaction faster than it is replaced from the atmosphere in the reaction vessel.