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The effect of pH adjustment on the internal corrosion rate of cast iron and copper water distribution pipes Millette, Louise


Two experimental sessions were conducted to examine to effects of pH adjustment on internal corrosion of cast iron and copper water distribution pipes. The Greater Vancouver Regional District uses surface lakes as its potable water source. As confirmed by chemical water characteristics monitoring, the supplied tap water has several of the attributes of an aggressive water: low pH (4.9 to 5.7), low alkalinity (10 to 15 mg/L as CaCO₃), low hardness (6 to 7 mg/L as CaCO₃), and dissolved oxygen consistently near saturation. Because of this aggressive nature, the tap water has a tendency to dissolve water distribution pipes, and in particular domestic copper pipes (municipal cast iron mains are cement lined for corrosion protection). This accelerated corrosion of copper pipes not only increases the maintenance costs for home owners, it also encourages high levels of copper in their tap water. This last finding was confirmed by a preliminary tap water metal concentration survey wherein, after a month of sampling of six dwellings, it was found that the recommended maximum copper level of 1.0 mg/L was exceeded in 67% of the cold water morning first flush samples. The investigated corrosion control measure consisted of pH adjustment to target values of 6, 7 and 8, through the addition of hydrated lime (Ca(OH)₂). Cast iron and copper pipe samples were exposed to pH adjusted water for durations ranging from one to twelve weeks. For the most part, the adjusted pH experimental flow-through units were gravity-fed; however, to examine the effects of the much higher normal distribution system pressure, another set of control units were maintained at system pressure. Although it was found that the absolute winter corrosion rates were higher than the summer rates, analysis of the relative coupon weight loss variations, with reference to the gravity control unit, lead to two major findings. The corrosion rates of cast iron were ten times those of copper and the increased pH acted to enhance these cast iron corrosion rates by approximately 15%. However, pH adjustment successfully decreased copper corrosion by 68%. The effects of the increased pressure on corrosion were different for both metals. The corrosion rates of cast iron coupons in the control pressurized cells were twice the rates of the coupons in the gravity control units. The effect of the increased pressure on copper was not as marked, but the rates were found to be slightly lower than expected from the pressurized water lower pH.

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