Anticipating effects of water quality changes on iron corrosion and red water

This study investigated the effect of disinfectant, sulfate/chloride ratios, nitrate concentration, magnesium hardness and zinc on iron corrosion. For the waters tested, free chlorine better controlled red water and microbial activity in the bulk solution. Consistent with predictions based on Larson's ratio, sulfate/chloride ratio changes did not have an effect on overall iron corrosion rates, although higher chloride was associated with increased localization of corrosion and pitting. High levels of nitrate increased the rate of chlorine decay and caused release of more iron, but had no effect on chloramine decay rates. The concentration of nitrate in the water might therefore be an important factor in considering the relative merits of chloramine versus chlorine as secondary disinfectants in real distribution systems. Increased magnesium and zinc can decrease mobilization of iron (e.g. red water) caused by silicate.