Control of Pitting Corrosion of Copper Tubes in Potable Waters
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report No UWRAA 64
This research has found that a major factor in the corrosion of copper is the nature of the surface oxide which forms during exposure to water. The film is a semiconductor which may have p-type or n-type conductivity. New copper tubes generally have thin p-type films which under most circumstances perform well and protect the surface against corrosion. However, under certain conditions this film changes to an n-type film which promotes corrosion. Copper tubes which are performing satisfactorily have a p-type cuprous oxide film, whereas tubes which are corroding have an n-type cuprous oxide film.
The change from p-type behaviour occurs when the corrosion potential of exposed copper shifts significantly in the positive direction. However, the significant factors which determine this change remain to be determined.
Substantial p-type oxide films may be formed in a number of ways, by:
- controlling the corrosion potential at less than -50 mV vs SCE
- thermal oxidation using the patented UCZ (Sanco) process
- exposure to a dilute solution of hydrogen peroxide
- With further development, the latter two processes could be used fort he production of highly corrosion-resistant copper tube. The peroxide treatment may also be used in conjunction with an appropriate chemical cleaning procedure for the passivation of new and corroded installations.
Bicarbonate dosing of water, as practiced by the Hunter Water Corporation, lowers the corrosion rate of copper and assists in the formation of an adherent cuprous oxide film. A thermochemical method for determining optimum dosing levels has been proposed. The effectiveness of bicarbonate dosing may be monitored using potentiostatic current-time measurements which could be automated.
Chlorination of water causes the formation of potentially deleterious n-type films. Additional studies of the effects of the different forms of disinfection are warranted.
Photo potential and corrosion rate measurements have been found useful in this investigation and these have potential for use in the in situ monitoring of copper corrosion.