The model assumes that the oxidation reaction occurs at the interface between the oxide layer and the substrate material, rather than between the oxide and the ambient gas.  Thus, it considers three phenomena that the oxidizing species undergoes, in this order:
# It diffuses from the bulk of the ambient gas to the surface.
# It diffuses through the existing oxide layer to the oxide-substrate interface.
# It reacts with the substrate.
The model assumes that each of these stages proceeds at a rate proportional to the oxidants concentration.  In the first step, this means Henrys law; in the second, Fick's law of diffusion; in the third, a first-order reaction with respect to the oxidant.  It also assumes steady state conditions, i.e. that transient effects do not appear.