Case Studies on Pitting Corrosion of Copper Tubes in Central Hot Water Supply Systems

The real state of pitting corrosion was investigated on copper plumbing tubes in 15 central hot water supply systems in highstoried building located in the capital area. Results of detailed studies for pit depth were correlated to the results of corrosion potential and water chemistry surveys that were carried out in the each respective hot water systems.The results obtained are summarized as follows: (1) Pitting corrosion was found in 10 systems that were characterized both by the corrosion potential, averaged over a whole year, of not less than 115mV vs. SCE and by the HCO3-/SO42- ratio of less than 1.1, whereas no corrosion was detected in the systems that were characterized by the potential less than 115mV and/or the HCO3-/SO42- ratio of not less than 1.1. (2) Increased utilization of hot water was a companied by a potential shift towards nobler direction, when the residual chlorine became detected from the recirculating hot water. (3) It was indicated that films of amorphous copper orthosilicates, that might be formed while hot water was recirculating in the system, played an important role to cause Type II pitting corrosion.