Abstract
A film formed on oxygen-free copper plates immersed in 1000 ppm NaCl and 1000 ppm Na2S solutions under room temperature has been assessed qualitatively and quantitatively using cathodic reduction at a constant current density of −1 mA/cm2 in deaerated 0.1 M KCl solution. Reagent-grade purity powder specimens of cuprous oxide (Cu2O), cupric oxide (CuO), and cuprous sulfide (Cu2S) were also reduced cathodically to determine the respective reduction potentials of Cu2O, CuO, and Cu2S. The reduction potential of the film formed in 1000 ppm NaCl solution was approximately −0.8 V vs. Ag/AgCl, which is close to the reduction potential of Cu2O powder. In contrast, that of the film formed in 1000 ppm Na2S solution was approximately −1.15 V vs. Ag/AgCl, which coincided with that of Cu2S powder. Therefore, the films formed in NaCl and Na2S solutions were composed mainly of cuprous oxide and cuprous sulfide, respectively. The thickness of the film formed in NaCl solution increases logarithmically with immersion time, although the thickness of the film formed in Na2S solution increases linearly with immersion time. The film thickness, as calculated from the cathodic reduction curves, shows good agreement with data obtained using optical microscopy and EPMA analysis for the cross-sectional surface of the films.
