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A model of passivity breakdown including the role of the inter-granular boundaries of the barrier oxide layer on the redistribution of the potential at the metal/oxide/electrolyte interfaces in the passive state is presented. Different mechanisms of breakdown at the oxide grain boundaries are considered, depending on which interface governs the potential drop: (i) local thinning and dissolution of the oxide layer, (ii) metal voiding or (iii) particle growth at the metal/oxide interface followed by rupture of the barrier layer. The role of chloride ions is discussed in each case. The key experimental observations made at the nanometre scale and validating the model are discussed.