I experienced the same problem on galv iron water pipe some fifty years ago in lake Jackson Texas. My problem was anodic action due to soil conditions. Dow Chemical Co, the principal employer in the area, was and is a major player in the cathodic protection of under ground lines. Copper is higher on the perodic table of elements than say lead, therefore, if there are lead covered lines in the area the copper would be sacrificial to the lead. Such is the case with magnesium vs iron which is the typical way of cathodically protecting under ground iron pipe lines in addition to wraping them with tar and paper. Normally iron pipes sacrifice to your typical copper clad ground rod. Hope this gives you some ideas for further investigation.
Raymond Ploeger P.E.
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