The effect of water quality on pitting corrosion of copper tube in hot soft water

Laboratory reproduction of pitting on copper was attempted in a hot, circulating water loop for the purpose of clarifying the mechanism and establishing countermeasures to the pitting. Type 2 pitting was observed to occur on the internal surface of copper tubes after long-time exposure to Tokyo water, when it was heated to 60°C and residual chlorine of 2～3 mg/l was added. After prolonged loop testing, the electrode potential of copper exceeded the critical value, +150 mV(SCE). The pits were deep and acute, and were covered with greenish blue products, which were identified as basic copper sulfate. Pitting did not occur in water free of residual chlorine even under air-saturated conditions, in which copper retained a low potential value.