Passivation behaviour of nanocrystalline coating (Fe₈₀Si₂₀) obtained by in situ mechanical alloying route is studied and compared with that of the commercial pure iron and cast Fe₈₀Si₂₀ in sodium borate buffer solution at two different pH values (7·7 and 8·4). The coating reveals single passivation at a pH of 7·7 and double stage passivity at a pH of 8·4. The first passive layer is due to the dissolution mechanism and second passivity is related to stable passivation. The cast sample shows single stage passivity in the solution of pH 8·4. The difference in the passivation behaviour for the cast alloy (Fe₈₀Si₂₀) and the coating is related to the presence of highly iron-enriched localized regions, formed during the processing stage of coating.