Nanoscale Morphology and Atomic Structure of Passive Films on Stainless Steel

Electrochemical Scanning Tunneling Microscopy (ECSTM) has been employed to investigate in situ the nanometer scale morphology and the atomic structure of a (100)-oriented Fe-l8Cr-l3Ni single crystal stainless steel surface passivated in 0.5 M H2SO4(aq). The results show that polarization in the pre-passive or passive domain does not destroy the typical as-prepared substrate topography but modifies the nanogranular morphology of the air-formed native oxide film initially covering the surface. In the pre-passive range (−0.06 V/SHE), dissolution of iron-rich oxide species has the main effect on the local changes, digging the surface preferentially on the substrate terraces where the native oxide film is thinner. In the passive domain, the 3D growth of oxide grains counteracts dissolution more rapidly than in the pre-passive range limiting the dissolution-induced modifications. The growth of the Cr(III)-enriched passive film is preferential along the substrate steps. Increasing the passivation potential (from 0.35 to 0.7 V/SHE) does not markedly influence the amplitude of the passivation-induced modifications of the morphology of the covering oxide film. However, it causes crystallization of the oxide. The atomic lattice measured in situ is consistent with the oxygen sublattice of the major oxide component (Cr2O3) of the Cr(III)-enriched passive film.