Mechanism Of Pitting Corrosion Of Copper Tubes In Soft Drinking Waters

摘要

The pitting corrosion of copper tubes in drinking waters, along with the associated problems of 'blue water' and cuprosolvency have been the subject of intermittent research for over half a century. Whilst the distinction between pitting 'types' has gradually faded with time, a single unifying theory for localized copper corrosion has yet to be developed. The present authors have used novel microelectrode electrochemical techniques supported by field observations and thermodynamic calculations to propose a view of the mechanism of pitting corrosion of copper in soft waters. The model stresses the dependency of copper pitting on the formation of a suitable cathodic film on the tube surface. In the absence of a more noble conductor, this film is typically the cubic, low pH form of cuprous oxide. The paradox of soft water copper pitting is that, whilst a low interfacial pH is a pre-requisite for pitting, so too is a relatively high bulk water pH, which favours the destabilisation of the original low porosity, protective cuprous oxide film and precipitation of a cupric salt scale. In a poorly-buffered water, cupric salt scale formation can be detrimental, as it creates a local environment shielded from the bulk solution, where formation of a low interfacial pH may occur due to copper hydrolysis. Cuprous oxide stability increases with decreasing pH, and as the interfacial pH lowers, may re-precipitate beneath the cupric salt scale. The form of cuprous oxide precipitated depends on pH, and if the interfacial pH is below 5, then the cubic, non-protective form of cuprous oxide will precipitate. In a well-buffered water the interfacial pH may be 6 or above, in which case a protective film of cuprous oxide will redevelop. Pit propagation is achieved via a galvanic couple between the large cubic cuprous oxide (low pH type) cathode and small copper anode. The high porosity of the cubic cuprous oxide film formed at low interfacial pH provides the sites for pit initiation. pH and buffering capacity are believed to be the critical water chemistry variables influencing pitting propensity in soft waters. These variables define oxide film growth and stability, conditions for cupric salt precipitation with low buffering capacity facilitating the development of local low pH conditions at the interface.