Investigation of Copper Pitting Propensity Using Bench and Field Scale Testing
A range of techniques designed to monitor copper pitting propensity were applied at two water utilities with known pitting disposition. In addition to traditional approaches including ECorr rise, chlorine decay kinetics, and surface analyses, a novel method was attempted to develop "pitting currents" between copper samples exposed to differential flow. This method allowed pitting current, potential, and resistance between "pits" and sections of copper pipe to be measured. As part of this evaluation, effects of different corrosion inhibitors and secondary disinfectants were investigated in an attempt to identify water quality modifications that might alleviate copper pitting at each utility.
At a Florida utility where customers were experiencing a severe pitting problem, experiments investigated the effectiveness of corrosion inhibitors including orthophosphate, an orthophosphate-polyphosphate blend, and zinc polyphosphate. Results suggested that zinc polyphosphate provided the greatest likelihood of mitigating copper pitting corrosion, whereas orthophosphate and the orthophosphate-polyphosphate blend actually increased electrochemical indications of pitting. According to theory, orthophosphates can increase pitting intensity if applied in insufficient quantities, whereas cathodic inhibitors, such as zinc, can only reduce corrosion rates. Surface analyses determined that zinc polyphosphate produced the least amount of scale, whereas the control produced the greatest amount. Further, surface analyses also suggested that zinc and phosphorus may behave synergistically during precipitation reactions which decrease copper release. Subsequent testing at Virginia Tech laboratories confirmed that some of the benefits from the zinc polyphosphate product were directly attributed to zinc. Consistent with previous research, pitting propensity of the water in the absence of inhibitor decreased at pH 7.5 relative to pH 8.5 (Marshall, 2004). In addition, higher concentrations of chloramines increased the pitting propensity of the water, although the ratio of chlorine to ammonia had little effect.
Similar experiments were also conducted at a utility in Iowa. Copper pitting has always existed in this area at some low level; however an outbreak of copper pinhole leaks recently occurred that is temporally correlated with high chlorine and chloramine concentrations. Experiments investigated combinations of disinfectant type (free chlorine or chloramines) and corrosion inhibitor (orthophosphate or zinc polyphosphate) in an attempt to decrease pitting propensity. Results indicated that the addition of zinc polyphosphate decreased pitting propensity in free chlorine systems as well as systems dosed with chloramines. In contrast, the addition of orthophosphate seemed to be ineffective in either system. Final surface analyses confirmed that inhibitors performed most effectively in the free chlorine system, whereas no clear benefits were realized in chloramine systems.