Richard S. Polizzotti

An effect of chemisorbing surface reaction poisons on the transition from internal to external oxidation

We consider the effect of a chemisorbing surface reaction poison on the transition from internal to external oxidation for a model binary alloy subject to selective oxidation. We solve the diffusion equations for internal oxidation, using the rate of a surface reaction as a boundary condition. The gas-phase concentration of a strongly chemisorbing poison appears in the analysis through its retarding effect on that surface reaction. It is shown that the slow surface reaction can promote the formation of a protective oxide scale by slowing down the initial uptake of oxygen and allowing metal atoms with the highest oxygen affinity sufficient time to diffuse to the surface. As a result, a larger volume fraction of oxide forms near to the surface (with that volume fraction initially inversely proportional to the rate of the surface reaction), and the critical volume fraction required for the transition to an external scale is more easily exceeded.