Phosphate inhibition of soluble copper corrosion by-product release

doi:10.1016/S0010-938X(01)00112-3

Corrosion Science

Volume 44, Issue 5, May 2002, Pages 1057-1071

https://doi.org/10.1016/S0010-938X(01)00112-3 Get rights and content

Abstract

Copper corrosion by-product release to potable water is a complex function of pipe age, water quality, stagnation time, and type of phosphate inhibitor. Moderate (≈1 mg/l as P) doses of polyphosphate or orthophosphate generally decreased copper release in pipe rig tests of >3 years duration. The exception was pH 7.2 and alkalinity 300 mg/l as CaCO₃, in which phosphate dosing significantly increased copper release over long time periods by hindering the formation of relatively insoluble malachite scale. Dosing of polyphosphates, which is invariably a blend of orthophosphate and polyphosphate due to reversion, was not as beneficial as orthophosphate. These relative effects are consistent with expectations based on simplistic solubility models, in which copper complexation by polyphosphate tends to increase soluble copper release, whereas higher doses of orthophosphate tend to decrease copper solubility.

Introduction

Leaching of copper from potable water plumbing has become a focus of worldwide regulation due to concerns over human dietary inputs and environmental impacts of copper. The concentration of copper from corrosion in water supplies, variously termed the “copper uptake” or “copper corrosion by-product release”, is measured through a variety of protocols with sampling locations and stagnation times delineated by appropriate authorities. In the United States, the Environmental Protection Agency specifies an “Action Limit” based on the concentration of copper in drinking water after greater than 6 h stagnation. If more than 10% of samples collected from selected homes exceed the Action Limit of 1.3 mg/l copper, utilities must take action to reduce corrosion by-product release through adjustment of pH, alkalinity, or by dosing corrosion inhibitors such as silicates or phosphates.

Phosphates can be dosed as either phosphoric acid, combinations of orthophosphoric acid and zinc orthophosphate, polyphosphates, or blends of orthophosphoric acid and polyphosphate. Very little is currently known regarding phosphate inhibition of copper corrosion by-product release, which is disconcerting given that 53% of US utilities reported using these inhibitors for copper corrosion control in a 1992 survey [1]. Usage has likely increased since that time as utilities complied with the EPA action limit. About 33% of utilities dosing phosphate inhibitors used either orthophosphate or zinc orthophosphate, while the remaining 67% dosed polyphosphate alone or a blend of polyphosphate and orthophosphate.

The selection of inhibitors is interesting because it might be expected that orthophosphates and polyphosphates would have contrary effects on soluble copper release. That is, dosing of orthophosphate is believed to act through formation of a Cu₃(PO₄)₂ or similar scale on the copper pipe surface [2], [3]. This scale is often of lower solubility than fresh Cu(OH)₂ solids which are now believed to control the maximum concentration of copper in relatively new copper plumbing during stagnation (Fig. 1) [3], [4], [5].

Practical research has demonstrated the usefulness of orthophosphate dosing in meeting the copper Action Limit at utilities, although critical information is often missing and it is currently uncertain as to how the inhibitors act. In general, benefits of orthophosphates are thought to be limited to pH<8.0 [2], [3], [6], [7]. Another topic of recent interest is whether orthophosphate dosing might produce short term benefits relative to systems without inhibitor, allowing the Action Limit to be met, but at the expense of long term detriments. In partial support of this hypothesis, researchers have demonstrated adverse impacts of orthophosphates on copper release, although this was for relatively small diameter pipe and at very low dissolved oxygen [8].

In contrast, while even less is known about the effects of polyphosphates on copper corrosion by-product release, research has clearly established the theoretical potential of polyphosphate to increase lead leaching to potable water [9], [10], [11]. Though similar reactions would be expected for copper, no one has yet proven that polyphosphates are detrimental relative to orthophosphate, or even determined their relative impact relative to waters without inhibitors. Research is complicated by inevitable reversion of polyphosphates to orthophosphates [12]. Although the rate of reversion varies markedly and is not always significant, water sampled at consumers' homes will invariably contain significant concentrations of orthophosphate even if dosed with polyphosphate at the treatment plant [13], [14].

This research is aimed at clearly establishing the role of phosphate inhibitor type, as well as pipe age, on copper corrosion by-product release.

Section snippets

Experimental

Pipe rig tests were conducted using a modified pipe rig apparatus to illustrate changes in copper corrosion by-product release in response to water quality changes and aging [15]. The rigs were 12^′′ long, 3/4^′′ diameter Type L tubes with a volume of ≈100 ml. Number two rubber stoppers were used to plug the ends of each tube. All pipes were initially rinsed three times with 0.1 N sodium hydroxide (NaOH) and then rinsed five times with reagent grade (Milli-Q) water. Boulder, CO tap water (pH 7.8,

Results and discussion

Two approaches were used in this work to gain insight to effects of time on by-product release phenomena. First, by-product release was tracked from a single pipe for a time period as long as 4 years. Second, since a real tap water was used for the study, with inherent typical seasonal and other variations in water quality, another experiment was conducted in parallel in which pipes were exposed for a few months or a few weeks. By measuring by-product release from all these pipes on the same

Conclusions

•
Moderate (≈1 mg/l as P) doses of polyphosphate or orthophosphate generally decreased copper release in pipe rig tests of >3 years duration. The orthophosphate acted to reduce the solubility of copper solids in equilibrium with water, presumably by formation of a cupric phosphate scale.
•
At pH 7.2 and alkalinity 300 mg/l as CaCO₃, phosphate dosing significantly increased copper release relative to pipes not dosed with inhibitor after a time period of years, but reduced copper release in the short

Acknowledgements

This work was supported by the American Water Works Association Research Foundation (AWWARF). The opinions, findings, conclusions or recommendations are those of the authors and do not necessarily reflect the views of AWWARF.

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Phosphate inhibition of soluble copper corrosion by-product release

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Corros. Sci.

Wat. Res.

J. AWWA

J. AWWA

J. AWWA

J. AWWA