Anodic metal dissolution concentrates hydrated metal salt at the anode interface and makes the interfacial diffusion layer more or less anion-selective or cation-selective for ion transport. The anion-selective diffusion layer, formed with monovalent chloride or hydroxide in acid chloride solution, contributes chloride ions concentrated at the interface to form a chloride film, leading eventually to either the chloride-film-induced passivation if the chloride is insoluble (eg, Ag/AgCl) or the transition from the active state to the polishing state dissolution if soluble (eg, Fe/FeCl2). The cation-selective diffusion layer, formed with multivalent phosphate or sulfate, contributes hydrogen ions depleted at the interface to form an oxide film leading to the oxide-film-induced passivation (eg, Ni/NiO). Furthermore, the transformation from a salt film to an oxide film on metal anodes takes place only if the salt film is cation-selective, and does not if it is anion-selective.