Non-diffusive spatial segregation of surface reactants in corrosion simulations

A metal corrosion mechanism involves several steps with their associated intermediate reactants and rates. The mechanism determines the time evolution of a metal atom through a sequence of states, beginning as soon as it is exposed to the electrolyte and ending when it dissolves, and to which we can associate the idea of a time ordering. On a perfectly smooth or flat surface, this time ordering does not generate heterogeneity in the distribution of reactants on the surface automatically. However, the random nature of individual dissolution events precludes smooth corroding surfaces, leading instead to the development of a certain degree of roughness. We show in this paper that when this is the case, a heterogeneous distribution of surface reactants does indeed emerge as the result of the interplay between the kinetics, which control the time ordering of the species, and morphology, which projects this differential aging process on a spatial dimension. This is done with the help of a 1 + 1-dimensional cellular automaton model of the dissolving metal. We analyze several simulation settings and show that a heterogeneous distribution of reactants evolves along the incisions, predominantly in the direction of their penetration within the metal. The peculiar feature of this surface segregation process is that neither diffusion nor site-dependent reactivity are involved. It is also shown that the segregation phenomenon can be well predicted from the standard macroscopic theory if its results are appropriately interpreted.