Cathodic polarization-induced structural evolution of oxide film on the 654SMO super austenitic stainless steel

• Cathodic polarization can remove Cr-enriched oxide film rather than induce defective. • The gradual thinning and ultimate removal process of oxide film is observed directly. • Chloride ions enhance the reductive dissolution of oxide film . • Cr 2 O 3 component is reduced by forming soluble ions rather than low-valence Cr oxide. Cathodic polarization is often used as a pretreatment procedure before electrochemical measurement of a certain passive metal material, aiming to remove the natural oxide film and obtain a fresh metal surface. However, cathodic polarization is proposed to only render the oxide film more defective rather than complete removal. This controversy has not come to an end due to lacking the direct experimental evidence. Here, we primarily employed aberration-corrected transmission electron microscopy to systematically investigate the structural and chemical evolution of transpassive oxide films induced by cathodic polarization. It is found that cathodic polarization can thin and almost remove the Cr-enriched oxide film, and the Cr/Fe ratio within the outer oxide layer remains invariant during thinning, contrary to theoretical predictions suggesting minimal reducible dissolution of chromium oxides. Our results confirm that cathodic polarization induces simultaneous reductive dissolution of both Cr oxide and Fe oxide, leading to thinning and eventual removal of the oxide film. This work provides direct experimental evidence for the structural evolution of oxide films under cathodic potential at the nanometer and atomic scale, and enriches the theoretical system of the structural evolution of passive films under external field effects.