Copper pitting corrosion

Fig. 1.10 Copper pitting corrosion in drinking water system (a) stereomicrography of perforated membrane and (b) scanning electron micrograph of perforated pit [28].

D.A. Lyde, M.N. Nadagouda, A comprehensive investigation of copper pitting corrosion in a drinking water distribution system, Corros. Sci. 52 (2010) 1927—1938. 

Figure 6.12 Type I copper pitting corrosion. (Courtesy of Russ Green, TMi)...

Stainless steels in soil can only be attacked by pitting corrosion if the pitting potential is exceeded (see Fig. 2-16). Contact with nonalloyed steel affords considerable cathodic protection at f/jj < 0.2 V. Copper materials are also very resistant and only suffer corrosion in very acid or polluted soils. Details of the behavior of these materials can be found in Refs. 3 and 14. 

In applying electrolytic protection, galvanized tubes can be installed downstream from copper components in water boilers without danger of Cu " -induced pitting corrosion. The protection process extends the application range for galvanized tubes with respect to water parameters, temperature and material quality beyond that in the technical regulations [16, 17]. 

Metals which owe their good corrosion resistance to the presence of thin, passive or protective surface films may be susceptible to pitting attack when the surface film breaks down locally and does not reform. Thus stainless steels, mild steels, aluminium alloys, and nickel and copper-base alloys (as well as many other less common alloys) may all be susceptible to pitting attack under certain environmental conditions, and pitting corrosion provides an excellent example of the way in which crystal defects of various kinds can affect the integrity of surface films and hence corrosion behaviour. 

Mattson, E. and Fredriksson, A. M., Pitting Corrosion in Copper Tubes —Cause of Corrosion and Counter Measures , Br. Corros. J., 3, 246 (1968)... 

Obrecht, M. F., Sastor, W. E. and Keyes, J. M., Integrated Design of Field Test Panel Pilot Unit for Investigating Pitting Corrosion of Copper Water Tube by Potable Water Supplies , Proc. 4th Int. Congr. Met. Corr., 1969, 576 (1972)... 

Joseph, G., Perret, R. and Spacek, J., Pitting Corrosion of Copper Tubings , Proc. 4th Ini. Congr. Metal. Corros., 1969, 817 (1972)... 

Lucey, V, F., Developments Leading to the Present Understanding of the Mechanism of Pitting Corrosion of Copper , Br. Corr. J., 7, 36 (1972)... 

There is an increasing tendency to treat drinking waters to remove organic material. This is to minimise the formation of haloforms, produced when the water is chlorinated, which have heeilth implications . Organics are known to affect certain corrosion processes, e.g. type I copper pitting and the formation of protective corrosion product layers. However, the outcome of this development is difficult to predict as not all the organic material present is removed. 

Most simple inorganic salt solutions cause virtually no attack on aluminium-base alloys, unless they possess the qualities required for pitting corrosion, which have been considered previously, or hydrolyse in solution to give acid or alkaline reactions, as do, for example, aluminium, ferric and zinc chlorides. With salts of heavy metals —notably copper, silver, and gold —the heavy metal deposits on to the aluminium, where it subsequently causes serious bimetallic corrosion. 

Secondly, under certain conditions copper may suffer intense localised pitting corrosion, leading sometimes to perforation of the tube, in quite a short time. This form of attack is not common and depends on a combination of unusual circumstances, one of which is the possession by the tube of a fairly, but not entirely, continuous film or scale that is cathodic to the copper pipe in the supply water this can set up corrosion at the small anodes of bare copper exposed at faults or cracks in the film. Carbon films give rise to such corrosion, but since 1950, when the importance of carbon films was... 

Inadequate acid cleaning procedures also may introduce traces of copper into the boiler (typically originally present as copper-containing deposits), which can plate out onto clean metal surfaces and cause localized, anodic area pitting corrosion. 

These and other similar reactions may take place, including the corrosion of copper and nickel. The most noticeable effect is often the increased level of maintenance and pipe replacement work required resulting from the effects of pitting corrosion. 

Inorganic acids, bases, and salts with pH values <4 and >9 are not compatible with aluminum. Pitting corrosion of aluminum can occur when it comes in contact with salt water and copper halide salts. 

Customer s laboratory says there are severe pitting corrosion problems, especially under slimes. Corrosometer corrosion rates indicate 6 to 8 mpy on steel. Carbon steel coupons indicate 10 to 12 mpy. Copper is 0.4 mpy. 

The pitting corrosion of copper has been inhibited by injection of ferrous salt solution. Ferrous ion injection probably results in the formation of lepidrocrocite, FeO.OH, and provides protection. 

A common form of corrosion met with this type of metal is pitting corrosion. Pitting arises when local cells are formed. This may be due to the presence of impurities in the metal such as sulphides, silicates, or noble metals such as copper. These areas will form local cathodes, adjacent weak spots in the oxide film will break down, and the underlying metal corrodes locally. Species within the aqueous medium may also induce pitting chloride ions are particularly aggressive in this respect [21]. 

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