Dezincificationof Brass in Potable Waters
ReportNo WSAA 84
Dezincificationof copper/zinc alloys, particularly forged duplex brass, remains a seriouseconomic problem to both the Water Industry and its customers. The presentunderstanding of the mechanism(s) of dezinfication is reviewed, together withthe influence of both the metallurgy of the brass and potable watercomposition. Previous attempts to use electrochemical methods to studycorrosion rate are also detailed.
Thedevelopment of a potentiostatic method to determine instantaneous corrosionrates of duplex brass in a variety of potable water types is described, and themethodology is shown to provide reliable estimates of dezincification rates bycorrelation with solution analysis techniques. The method has been used tocompare the dezincification propensity of a wide range of Australian drinkingwaters ranging from very soft sources to high chloride supplies that are barelypotable. The effect of a range of water chemistry parameters includingalkalinity, chloride, temperature, pH and free chlorine residual have also beenstudied.
Thiswork indicates that the traditional parameters of chloride and alkalinity haveconsiderably less effect on corrosion rate of duplex brass than previouslythought. Hard waters, with relatively high levels of chloride are in factslightly less corrosive than soft, unbuffered supplies due to the lack ofscale-forming abilities of the softer waters. However, the effects of pH and,particularly, free chlorine, are quite marked and if inadequately controlledcan lead to unacceptably high dezincification rates.
Fieldtesting of duplex brass components has shown the difficulties in correlatingmaterial composition and structure morphology with both field establishedcorrosion rates and the currently accepted accelerated test method. The lattertest showed, at best, a lack of repeatability in predicting actual fieldperformance on even a trend basis. Correlation between dezincificationpenetration and structure morphology was likewise quantitatively poor, althougha straight line correlation can be attributed to the relationship betweenvolume fraction of beta and depth of penetration for duplex brasses showingdiscrete and finely distributed beta phase. No correlation existed for brasseswith coarse, interconnected beta.
Bothelectrochemical testing and surface analysis (SIMS) on duplex, beta, alpha andarsenic-inhibited alpha brasses strongly indicates that at neutral pH themechanism of dezincification is that of preferential dissolution of zinc. Alphabrasses, however, appear to be capable of a dissolution-redeposition mechanismat lower pH as suggested by Heidersbach and Verink. Beta brasses, by contrast,only dezincify through a preferential dissolution mechanism which suggest thatarsenic inhibition will only work with alpha brasses when thedissolution-redeposition mechanism is operable.
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