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Influence of Water Velocity and NOM Composition on Corrosion of Iron Pipes

The effect of water velocity on the corrosion rate of new iron pipes was studied in the full scale. Sets with four rigs operated under different flow regimes (high, low, variable, stagnant) were installed in two locations (chemically treated surface water and groundwater) of a drinking water supply networks, and weight loss of the iron coupons was monitor over the period of one year. During the initial phase of corrosion (about 200 days), the results showed that in water which can be considered more corrosive (undersaturated in respect to calcium carbonate, high sulphate concentration) the corrosion rate of iron increased by 20-50% at low or variable flow regimes compared to at high flow rate. The chemical water quality was less important for the overall corrosion of iron coupons at high flow rate, however, it was very important for the occurrence of pitting and localized corrosion. Pitting of iron surface occurred in the more corrosive water. The influence of natural organic matter (NOM) on the corrosion of iron was studied in batch experiments by measuring the weight loss of coupons after three weeks of exposure to water containing similar concentrations but different types of NOM. The results showed that the composition of NOM affected the corrosion rate of iron pipes. Nonhumic hydrophilic NOM fractions were more corrosive than humic substances towards iron pipes at stagnant conditions. Comparing to copper pipes, iron were more affected by the type of NOM. The study allowed us to conclude that high corrosion rate and corrosion problems (new or aggressively cleaned old pipes) are more likely to occur in service pipes (low or variable flow rate) supplied with water containing high level of NOM composed of non-humic compounds. Pitting corrosion and risk of pipe failure increase having the more corrosive water.