External Corrosion of Ductile Iron Pipes Due To Contaminated Soils

Abstract

An experimental study is conducted to determine the interaction/exchange between metals in contaminated soils and those in metallic water pipes. Experimentation was carried out on two-100 mm diameter (ductile iron pipe) DIP sections, buried in soil and subjected to artificial rainwater (ARW) representing three years’ rainfall. The backfill soil was contaminated with controlled concentrations of heavy metals; they were Copper (Cu), Zinc (Zn), Chromium (Cr) and Lead (Pb). These were then allowed to seep through the soil to the pipe external surfaces. The results indicated that there was iron enrichment from the rainwater experiments compared to the controls whose source could only be the pipe sections; the remaining fittings were not metallic. The iron released from the pipe was then determined in the drainage. There was an increasing wash out of iron content with progressive increases in contaminant loading in each following experiment after which it decreased with time because of the contaminant flushing to have a major corrosive effect with 7.458 grams iron from the two pipe sections as a result of 0.178 g/L of Cu. Copper was shown to have the worst effect despite this was thought to be due to the respective differences between Copper and iron in the galvanic corrosion series. Most of the iron released was in particulate form indicating binding to other solids. The nature and size of these particles need further research. The Copper exerted the strongest reaction releasing 2.49 g F3/m length of one pipe, lead had the least effect. It was concluded that ion exchange at the surface was an important corrosion mechanism and the activity was a function of the metals position in the electrochemical series. A correlation between the extent of contamination and the amount of metals in the collected runoff was also obtained.