Iron release from corroded iron pipes in drinking water distribution systems: effect of dissolved oxygen.

Iron release from corroded iron pipes is the principal cause of "colored water" problems in drinking water distribution systems. The corrosion scales present in corroded iron pipes restrict the flow of water, and can also deteriorate the water quality. This research was focused on understanding the effect of dissolved oxygen (DO), a key water quality parameter, on iron release from the old corroded iron pipes. Corrosion scales from 70-year-old galvanized iron pipe were characterized as porous deposits of Fe(III) phases (goethite (alpha-FeOOH), magnetite (Fe(3)O(4)), and maghemite (alpha-Fe(2)O(3))) with a shell-like, dense layer near the top of the scales. High concentrations of readily soluble Fe(II) content was present inside the scales. Iron release from these corroded pipes was investigated for both flow and stagnant water conditions. Our studies confirmed that iron was released to bulk water primarily in the ferrous form. When DO was present in water, higher amounts of iron release was observed during stagnation in comparison to flowing water conditions. Additionally, it was found that increasing the DO concentration in water during stagnation reduced the amount of iron release. Our studies substantiate that increasing the concentration of oxidants in water and maintaining flowing conditions can reduce the amount of iron release from corroded iron pipes. Based on our studies, it is proposed that iron is released from corroded iron pipes by dissolution of corrosion scales, and that the microstructure and composition of corrosion scales are important parameters that can influence the amount of iron released from such systems.

Physico-chemical characteristics of corrosion scales in old iron pipes.

Corrosion scales play an important role in modifying water quality in drinking water distribution systems. The corrosion scales from old iron/steel pipes were analyzed for their structure and composition. Scales were studied both before and after drying. and goethite, magnetite and lepidocrocite were identified as the primary constituents of the dried samples. High concentrations of readily soluble ferrous phases were detected in wet-scale samples. The corrosion scales had a shell-like, enveloping layer, covering porous deposits of iron oxide phases. Our studies were able to identify important differences between corrosion scales found in two different water distribution systems. Further studies are needed to establish the role of corrosion scales in the mechanism of iron release from corroded pipes.