Check if the pipelines are ...

Check if the pipelines are properly grounded. At pH 6 water is not buffered, pH excursion, high chloride if this is a possibility and dissolved oxygen could be causing pitting. Calcite contactor could be tried out. The process would be slow. Precipitation of ferric hydroxide is unavoidable at pH 7-7.5 corrosion. Mn+4 precipitates > pH 9.5, may not be an issue. Your target pH of water should be 7.7-7.8; this will give about 70-80 ppm bicarbonate hardness. LSI turns positive. If calcite contactor does not work you can try injection of controlled quantity of lime water and CO2 injection keeping final pH