Ab initio modelling of localized corrosion: Study of the role of surface steps in the interaction of chlorides with passivated nickel surfaces

Periodic DFT calculations have been performed on a (5 3 3)-oriented hydroxylated NiO surface to model the influence of step edges on the adsorption and sub-surface insertion of Cl at the surface of the passive film on nickel. With increasing Cl coverage, substructures of Ni(OH)2, Ni(OH)Cl or Ni(Cl)2 composition are formed and detached from the surface, suggesting a major role of the step edges in the Cl adsorption-induced thinning mechanism of the oxide film. The calculated energies show the promoting effect of the adsorption of Cl on the energy of detachment of the substructures. At surface saturation, sub-surface insertion becomes energetically more favourable than adsorption. The results suggest a possible bifurcation from the Cl adsorption-induced oxide thinning mechanism to the penetration-induced mechanism of passivity breakdown at saturation in adsorbed Cl of the step edges.