Pitting of copper

There are several major factors contributing to the tendency for copper to undergo pitting-type corrosion these include  

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Previous considerations of pitting have been largely confined to metals and alloys that have a strong tendency to passivate, but since the pitting of copper has a number of unusual features it is appropriate to consider it in some detail. Reference to the potential-pH diagram for the Cu-H O (Section 4.2) system shows that in neutral solutions at the potentials encountered in oxygenated waters the stable form of copper is Cu O, and the corrosion resistance of copper thus depends upon whether or not the CU2O forms a protective film. 

The compositions of waters that give rise to pitting have been the subject of numerous investigations, and studies by Obrecht e/ al. of the pitting of copper tubes in the USA have shown that it is invariably a cold water phenomenon that occurs with hard well waters. These waters contained over 5p.p.m. of dissolved CO2, 10-40 p.p.m. being typical, a pH in the range... 

Lucey examined a number of examples of pitting of copper pipes and tanks from hard water districts, and found that there was no more calcium carbonate scale deposited around the pits than on other parts of the metal surface. There was, however, a large amount of CaCOs in the mound... 

Van Lear, P., Van Muylden, J., de Zoubov, N. and Pourbaix, M., Accelerated Methods for Estimating the Risk of Pitting of Copper in the Presence of Water. Application to the Study of the Influence of a Copper Contact with Another Metal or with Graphite , CEBELCOR Rappt. Tech. No. 128, May (1%5)... 

Deposit attack and pitting When water speeds are low and deposits settle on the surface (particularly at water speeds below about 1 m/s), pitting of copper and copper alloys is liable to occur by differential aeration effects. 

Pitting failures also occasionally occur in copper water cylinders and as a result of a study of this problem Lucey " has made suggestions about the mechanism of pitting of copper in supply waters. 

Analysis of Pitting of Copper with Reference to the Pourbaix Diagram... 

Fig. 7.40 Model of positions of corrosion products and reaction paths in pitting of copper in hard water (details can be found in text).

Pitting can also be counteracted by change of environment. For instance, pitting of copper and eopper alloys in hot water can be prevented by water treatment, implying pH > 8.0 and [HCO3 ]/[SO/ ] > 1 [7.29]. 

The addition of 1 to 2 mass% Fe was shown to improve the corrosion resistance impingement attack, deposit attack, pitting, of copper alloys containing 10 mass% Ni [1951Bai]. Copper precipitation has been... 

Also sulphate-reducing bacteria may cause increased pitting of copper and copper alloys by the production of hydrogen sulphide. The latter forms thick, nonadherent layers of chalcocite (Q12S) or covellite (CuSi x). When the copper sulphide film is partially removed, regions with intact CuS films act as cathodes eventually leading to pitting corrosion of the copper or copper alloy. 

A. Chamberlain and P. Angell, Influences of microorganisms on pitting of copper tube, Proceedings Microbially Influenced Corrosion and Biodeterioration (N. J. Dowling, M. W. Mittleman, and J. C. Danko, eds). University of Tennessee, Knoxville, 1990, pp. 3/65-3/69. 

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