Investigation of a transparent chromate (III) passive film on electroless Ni-P coating by XPS and electrochemical methods

A transparent passive film on electroless nickel phosphorus coating (ENPC) was obtained in a chromate (Cr³⁺) bath. An ENPC sample passivated in a Cr⁶⁺-containing bath was used for comparison. The corrosion properties were tested by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). SEM, XPS, and EDX were employed to analyze the chemical composition and surface morphology of the films. SEM results indicate that the passive film is too thin to be observed by SEM. The potentiodynamic polarization tests show that the corrosion currents of the two passivated samples are only about 1/25 of the un-passivated coating. The XPS analysis illustrate that the Cr⁶⁺-treated film comprises Cr, Ni and O, while the Cr³⁺-treated film is made up of Cr, C, O, Ni, P and N. High-resolution XPS analysis show that, both in Cr⁶⁺-treated and Cr³⁺-treated films, element Cr is only in the form of trivalent compounds, no hexavalent chromium existing. By XPSpeak analyzing, trivalent chromium compounds on the two treated surfaces were fitted as Cr(OH)₃ and Cr₂O₃. However, for the Cr³⁺ and Cr⁶⁺ passivation routes, the Cr(OH)₃ and Cr₂O₃ contents in passive films are widely divergent.