AC-Induced Localized Corrosion of Tin-Coated Copper Conductors in Simulated Soil Environments

Behavior of tin-coated copper electrodes in a model soil environment was studied by electrochemical impedance spectroscopy (EIS). Comparison of EIS spectra, modeled as a simple R(RC) circuit, taken under a period of time at rest potential with those taken under anodic and cathodic bias potentials shows a marked increase in electron transfer resistance under conditions of anodic bias or AC polarization. This indicates that the tin coating is passivated by the AC test signal through oxide formation during the anodic half cycle. This observation is supported by rest potential measurements. Without an AC 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 AC signal is added, the potential of the tin-coated wire shifts to a more positive potential than that of the bare wire, passivating the tin coating and making 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 early failure of the cable in the soil environment.