Composition and formation analysis of an ultra-thin antitarnish passive film on electroless Ni-P coating

An ultra-thin passive film on electroless nickel phosphorus coating (ENPC) was obtained by treated in a passivation bath. The antitarnish and corrosion properties of the film were studied by acid exposure and potentiodynamic polarization. XPS was utilized to analyze the chemical composition and formation process of the film. The acid exposure test indicated the antitarnish property of the coating was obviously improved by passivation treatment. The potentiodynamic polarization test showed that the corrosion current for the passivated Ni-P coating was lower than that of untreated coating. By XPS measurement, the thickness of film was evaluated as a few nanometers and the film was made up of Cr, C, O and N. The N and C elements in the film suggested the nitryl aromatic compound should take part in the formation of film. The XPS spectra fitting results indicated that Cr was in the form of Cr2O3 and Cr(OH)3, while O was originated to Cr2O3, Cr(OH)3 and–NO2 in aromatic compound. And the XPS results were used to justify the formation mechanism of the passive film.