The improvement mechanism of rust resistance of Nb and Cu containing ferritic stainless steels was studied from the view point of chemical compositions and stabilities of passivation films. Increase of Nb content up to 0.4% enlarges the passivation limit measured by the depassivation pH in Cl-ions free solution and pitting potentials in Cl-ions containing solution. Besides, Nb addition stablilizes the passivation film measured by the time for potential decay whether Cl-ions are contained or

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... Cu addition, however, has no influence on the passivation limit. It stabilizes the passivation film in Cl-ions free solution, but deteriorates in Cl-ions containing solution. Cu and Nb addition has no influence on the thickness of passivation film. Cr content in passivation film increases with increase of Nb content in the steel, but decreases with increase of Cu content. From these results, it was considered that the improvement of corrosion resistance by Nb addition is due to the increase of Cr content in passivation films.