The semi-conductor property and corrosion resistance of passive film on electroplated Ni and Cu–Ni alloys

Mott–Schottky analysis, electrochemical impedance spectroscopy in conjunction with the Point Defect Model (PDM), were performed to characterize the semi-conductor property of passive films formed on Cu–Ni alloys in 1 mol/L NaOH solution. The acceptor density, flat potential and vacancy diffusion coefficient in the passive films formed on Cu–Ni alloys were determined. Results indicated that the passive films displayed p-type semi-conductive characteristics, where metal vacancies (over oxygen vacancies and interstitials) preponderated. With the increase of Cu, the flat potential moved towards positive, while the acceptor density and impedance of passive films decreased due to its steady oxidation-from Cu(I) to Cu(II), which increased the susceptibility to corrosion. In addition, the acceptor density and flat potential increased when the formation potential moved towards negative. The calculated value obtained for D0 was around 10−14 cm−2 s−1.