The corrosion behaviour of copper in neutral tap water .1. Corrosion mechanisms

The corrosion behaviour of copper in neutral aerated simulated tap water is investigated using electrochemical methods and XPS. Semi-infinite diffusion behaviour is found in electrochemical impedance spectroscopy (EIS) from intermediate to low frequency. Polarization resistance (R(p)) obtained at very low frequency (0.0005 Hz) increases with immersion time, which coincides with the growth of oxide film on copper. The growth of oxide film with immersion time has little effect on the cathodic process, but considerably reduces the anodic dissolution current. The observed results suggest that the diffusion of copper ions in oxide film controls the overall corrosion rate. Under rotating conditions polarization resistance decreases and anodic current increases, which was caused by the decrease of the oxide film thickness. The phenomena of the mixed diffusion of copper ion in oxide film and in solution has been observed at a low pH of 5. Passivation is found when pH = 10. XPS spectra show that the oxide film formed is composed mainly of cuprous oxide. An equivalent circuit representing the corroding interface is proposed with a discussion of theoretical approaches to the calculation of diffusion impedance and polarization resistance.