Kinetics of Gas/Solid Reaction: Diffusion-Controlled Case

When a metal is exposed to an oxidizing gas, say, oxygen, a tarnishing layer or oxide forms upon the metal. When a metallic alloy is exposed, an oxide of the less noble component of the alloy may form internally. Thermodynamics of oxidation is discussed, and then kinetics of such external and internal oxidation are examined when the overall kinetics is diffusion-controlled. This leads to the famous parabolic rate law with the parabolic rate law constant which can be derived in terms of the Nernst-Planck-type combination of the partial conductivities or self-diffusivities of the mobile components. The present kinetic idea may be extended to solid/solid reactions with minor modifications.