Controlof Pitting Corrosion of Copper Tubes in Potable Waters

Report No WSAA 64

August 1993

 

SUMMARY

 

1.      This research has found that a major factor in the corrosion of copperis the nature of the surface oxide which forms during exposure to water. Thefilm is a semiconductor which may have p-type or n-type conductivity. Newcopper tubes generally have thin p-type films which under most circumstancesperform well and protect the surface against corrosion. However, under certainconditions this film changes to an n-type film which promotes corrosion. Coppertubes which are performing satisfactorily have a p-type cuprous oxide film,whereas tubes which are corroding have an n-type cuprous oxide film.

 

2.      The change from p-type behaviour occurs when the corrosion potential ofexposed copper shifts significantly in the positive direction. However, thesignificant factors which determine this change remain to be determined.

 

3.      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 forthe production of highly corrosion-resistant copper tube. The peroxidetreatment may also be used in conjunction with an appropriate chemical cleaningprocedure for the passivation of new and corroded installations.

 

4.      Bicarbonate dosing of water, as practiced by the Hunter WaterCorporation, lowers the corrosion rate of copper and assists in the formationof an adherent cuprous oxide film. A thermochemical method for determiningoptimum dosing levels has been proposed. The effectiveness of bicarbonatedosing may be monitored using potentiostatic current-time measurements whichcould be automated.

 

5.      Chlorination of water causes the formation of potentially deleteriousn-type films. Additional studies of the effects of the different forms ofdisinfection are warranted.

 

6.      Photopotential and corrosion rate measurements have been found usefulin this investigation and these have potential for use in the in situ monitoring of copper corrosion.

 

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