AC-Induced Pitting of Tin-Coated Copper Utility Cables Available to Purchase

Laboratory exposure tests involving chemicals commonly found in soil and an AC current flow between a tin-coated copper electrode and a surrounding electrolyte were conducted to provide data regarding the mechanism and rate of concentric neutral corrosion encountered in underground rural distribution systems. Experimental data supportive of an AC-induced pit initiation mechanism followed by autocatalytic pit growth are presented. In the absence of AC, the potential of tin-coated wire stabilizes at a more negative potential than that of bare copper wire, thus providing cathodic protection to exposed copper. In the presence of an AC signal, the potential of the tin-coated wire shifts to a more positive potential than that of the bare wire, indicating that AC-induced resistive currents on tin-coated samples passivate the tin coating, rendering it cathodic to the underlying copper. The large cathodic area leads to rapid pitting of the copper substrate at coating imperfections.