Effect of Grain Boundaries on the Diffusional Growth of Oxides on Metals

SEVERAL mechanisms1–3 for the growth of thin films of oxides on metals have yielded equations of the form y3 = kt, where y is the film thickness, k a rate constant, and t, time. However, none of these mechanisms can be satisfactorily extended to the case of thick films formed on such metals as titanium, zirconium and tantalum4. It is proposed that this so-called ‘cubic’ oxidation can result from a combination of diffusion along grain boundaries and normal bulk diffusion in relatively thick oxide films.