The influence of trace impurities on the passivity of iron revealed by photoelectrochemical microscopy

Photoelectrochemical microscopy shows substantial surface heterogeneity, dependent upon the solution composition and upon the sample origin and pretreatment, of the passive film on high-purity iron in 0.1 M KOH. An effect of both the microstructure and the purity of the metal is demonstrated. The non-uniformity is characterised by regions of varying photo-activity, which in the extreme are evident as photo-inactive zones. The general contrast is interpreted as the consequence of the non-uniform spreading over the surface of a porous oxide layer on top of the barrier layer. An additional, more localised contrast could be induced by potential cycling in the passive range. This additional contrast was one obtained on a slightly impure iron and not on more pure material. The effect on the impure material could be eliminated by annealing of the specimen. By comparison with the effects of increasing the concentration of KOH or adding EDTA, this particular contrast was presumed to be due to small particles of oxide developing on the surface of the passive layer. It is interpreted as an incipient local breakdown, nucleated at the metal/oxide interface around impurity particles in the grain boundaries of the metal and triggered by changes in the electrostatic stresses in the oxide.