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Free chlorine residual and corrosion

Hypochlorous acid is a strong oxidizing agent used for the disinfection of drinking water and is the predominant form of free chlorine below pH 7.5. Free chlorine species (i.e., hypochlorous acid and hypochlorite ion)

can also act as primary oxidants towards lead and thus increase lead corrosion (Boffardi, 1988, 1990; Schock et al., 1996; Lin et al., 1997). However, a pipe loop study on the effect of chlorine on corrosion demonstrated that a free chlorine residual (0.2 mg/L) did not increase lead concentrations (Cantor et al., 2003). A survey of 94 U.S. water companies and districts also revealed no relationship between lead levels and free chlorine residual concentrations (in the range of 0-0.5 mg/L) (Lee et al., 1989).

Significant lead dioxide deposits in scales were first reported by Schock et al. (1996) in pipes from several different water systems. Suggestions were made as to the chemical conditions that would favour these tetravalent lead deposits and the changes in treatment conditions (particularly disinfection changes) that could make the tetravalent lead scales vulnerable to destabilization. Schock et al. (2001) found deposits in lead pipes of the Cincinnati, Ohio, distribution system that contained lead dioxide as the primary protective solid phase. Subsequent to these findings, different attributes of the theoretical solubility chemistry of lead dioxide were expanded upon, particularly the association with high free chlorine residuals and low oxidant demand.

Following the discovery of elevated lead concentrations after sections of Washington, DC, converted to chloramination, Renner (2004) described the link of the disinfectant change to the previous U.S. EPA research on tetravalent lead scale formation (Schock et al., 2001). Schock and Giani (2004) reported the results of tap monitoring history and scale analysis from the Water and Sewer Authority system in Washington, DC, confirming lead dioxide as the primary starting material; this validated the hypothesis that the lowering of ORP by changing from high dosages of free chlorine to chloramination caused high rates of lead dissolution. The laboratory experiments of Edwards and Dudi (2004) and Lytle and Schock (2005) confirmed that lead dioxide deposits could be readily formed and subsequently destabilized in weeks to months under realistic conditions of distribution system pH, ORP and alkalinity. A more recent laboratory study by Switzer et al. (2006) demonstrated that water with free chlorine oxidized lead to insoluble lead dioxide deposits and that lead was almost completely dissolved in a chloramine solution. These study findings further support the hypothesis that a change from free chlorine to chloramine can cause lead dissolution.

When hypochlorous acid is added to a water supply, it becomes a dominant oxidant on the copper surface (Atlas et al., 1982; Reiber, 1987, 1989; Hong and Macauley, 1998). Free chlorine residual was shown to increase the copper corrosion rate at lower pH (Atlas et al., 1982; Reiber, 1989). Conversely, free chlorine residual was shown to decrease the copper corrosion rate at pH 9.3 (Edwards and Ferguson, 1993; Edwards et al., 1999). However, Schock et al. (1995) concluded that free chlorine species would affect the equilibrium solubility of copper by stabilizing copper(II) solid phases, which results in a substantially higher level of copper release. The authors did not observe any direct effects of free chlorine on copper(II) solubility other than the change in valence state and, hence, the indirect change in potential of cuprosolvency.

Several authors reported an increase in the iron corrosion rate with the presence of free chlorine (Pisigan and Singley, 1987; Cantor et al., 2003). However, a more serious health concern is the fact that iron corrosion by-products readily consume free chlorine residuals (Frateur et al., 1999). Furthermore, when iron corrosion is microbiologically influenced, a higher level of free chlorine residual may actually decrease corrosion problems (LeChevallier et al., 1993). No information was found in the literature correlating iron levels with free chlorine residuals.

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