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This paper describes improvement of corrosion resistance of CoNi films by surface passivation. The CoNi films were obliquely evaporated on polyethylene terephthalate (PET) films, and passivated by a newly developed mild oxidation method. The passivated films were subjected to environmental exposure test at a relative humidity of 90% and a temperature of 60 °C. No changes of recording performance were observed even after a one‐month exposure. The chemical structure of the passive layer formed on the CoNi films was studied by x‐ray photoelectron spectroscopy and reflected electron diffraction. It was concluded that their strong corrosion resistance was attributed to the very thin, amorphous cobalt(III) oxyhydroxide formed on the surface, containing a small amount of Co2+ ions.