The high rate anodic dissolution of 100Cr6 steel in NaNO3 electrolytes of various concentrations and at different temperatures was investigated. Galvanostatic flow channel experiments were used to examine the current efficiency of the steel substrate. Below 6 A cm(-2) (zone A), oxygen evolution dominates, while at higher current densities iron dissolution prevails (zone C). Potentiodynamic polarization studies indicated a complete substrate surface passivation up to +1.8 V (vs NHE), and periodic fluctuations of the current density at higher anode potentials (> +1.8 V) due to severe oxygen evolution. Rotating cylinder measurements served for polarization studies at lower current densities in the region of dominating oxygen evolution. Scanning electron micrographs revealed a correlation between the current efficiency and the coverage of the substrate surface with an electronically conductive film at current densities of 2, 9 and 20 A cm(-2). The microstructure of the black, solid surface film developing during the high rate anodic dissolution of the steel was found to be heterogeneous and very porous. The main film components, as determined by X-ray diffraction and Auger electron spectroscopical measurements, were amorphous iron oxides, FexOy, and inert carbides, M3C, originating from the steel matrix. An activation-repassivation process is proposed, which is responsible for the development of the complex multilayer (multiphase) structure observed at the steel substrate surface.