The a.c.-induced corrosion of copper neutrals

Abstract

Electrochemical behaviour of tin-coated copper electrodes in a model soil environment was studied by means of electrochemical impedance spectroscopy (EIS), d.c. polarization, and long-term exposure testing. Comparison of EIS spectra, modelled as a simple R(RC) circuit, taken over a period of time at rest potential, with those taken under anodic and cathodic bias potentials, shows an increase in the electron transfer resistance under conditions of anodic bias or a.c. polarization. This indicates that the tin coating is passivated by the a.c. signal through oxide formation during the anodic half cycle. This observation is supported by rest potential measurements. Without an a.c. signal the potential of tin-coated wire stabilizes at a more negative potential than that of bare copper wire and provides cathodic protection to copper exposed through coating flaws. However, when an a.c. signal is added, the potential of the tincoated wire shifts to a more positive potential than that of the bare wire, passivating the tin coating and rendering it cathodic to the underlying copper. The large area of the tin cathode relative to the small area of exposed copper substrate leads to rapid pit initiation in the copper, resulting in premature failure of the cable in the soil environment.