I was involved in a similar situation where a brass coupler on a two inch soft copper water main had suffered a leak, due to the copper pipe becoming eroded away at the joint. A mechanic who used to work in the metal plating field seemed to believe this erosion was due to the copper water pipe having a voltage on it and being electricaly sacraficial, as in plating. I questioned the original soundness of the connection and associated the etching away within the fitting to the effects of a slow but steady leak, similar to what is evident on faucet valve seats when a leak will cut through brass and over a relatively short period of time, cut a groove right through and deeply into the valve seat. You may want to check to see if the unballanced service current is on the copper pipe rather than on the neutral at the service, causing the copper to carry voltage and be sacraficial. This was the suggested cause in my situation, but never varified. Tim Micsak
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