Large-time asymptotic solutions for the reaction-diffusion front between one static and one diffusing reactant are known. These states apply to single step reactions with a mean-field reaction rate proportional to ρ m α n (with m, n ≥ 1), where ρ, α are concentrations of the diffusing and static reactants respectively. Such reaction kinetics commonly arise in, for example, simple corrosion models of a porous solid, subject to a diffusing reactant. Here we address a more complex two-step corrosion reaction for oxidation of uranium in a water-vapour environment. In this case, additional complexity arises through a pair of coupled reaction fronts (one with m = 2, n = 1 and the other with m = 3, n = 1). Furthermore, we allow for material expansion owing to the corrosion process and show that the expected strong dependence of diffusion coefficients on the static reactant distribution is key to explaining experimental observation. In the large-time limit there are four main asymptotic regions, comprising two diffusion layers and two reaction fronts. Asymptotic matching of these regions allows us to construct a large-time solution that gives analytical predictions for the positions of the two propagating fronts, thickness of the diffusion layers and concentration of diffusing species outside of the fronts. This is the first mechanistic model of uranium oxidation in water vapour and (crucially) predicts a thin propagating sub-surface (hydride) layer, as recently observed in atom-probe tomography experiments.