AWWA JAW60846

Copper-pitting corrosion in drinking water has been the subject of relatively intense research for more than 50 years. Worldwide, several different water types are known to support pitting. In Australia, New Zealand, and some other locations around the world, however, pitting predominates in soft, unbuffered waters. The pitting corrosion of copper tubes in drinking water systems is a sporadic but persistent problem along, for example, the east coast of Australia. Historically, the lack of an adequate theory for localized soft water copper corrosion has made remedial treatments speculative and based only on longterm field trials. Even though several pitting models have been developed over time, the earlier models had their limitations. Pitting corrosion of copper in soft water is known to be dependent on water chemistry, particularly pH and buffering capacity. The randomness of the problem, however, along with the large number of chemical and physical variables involved in its initiation and propagation, required a better theoretical and practical framework for understanding more about the essential components of the pitting process. For this work, the authors used novel microelectrode electrochemical techniques, supported by field observations and thermodynamic calculations, to propose a new mechanism of copper-pitting corrosion in soft waters. The proposed model for soft water copper pitting stresses the dependency of pitting on the formation of a suitable cathodic film on the tube surface. The authors also discuss how the proposed theory may be applied to the development of a remedial water treatment strategy and used to calculate the optimum pH and alkalinity set points for a soft water. Includes 19 references, figures.