OXIDATION OF CUPRO-NICKEL ALLOYS .2. KINETICS OF DIFFUSION IN POROUS INNER LAYERS

The oxidation of cupro-nickel alloys at temperatures > 160°C produces a duplex oxide structure. In Part I, diffusion of copper across the inner nickel oxide layer was shown to be rate controlling. This conclusion is tested in this paper by examining the oxidation rate curves of a series of nine metals and alloys extending from pure copper to pure nickel. The isochronal curves are found to pass through a maximum at 75%Cu which is explained by surface diffusion of the copper through a porous nickel oxide structure extending through the metal consumption zone. The surface diffusion coefficient derived from this model is in the range 0.8-2.0 × 10-14 m2 s-1 in accord with results of other workers. © 1979 Pergamon Press Ltd.