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THE RELATIONSHIP BETWEEN THE CORROSION OF DISTRIBUTION SYSTEM MATERIALS, AND OXIDANT AND REDOX POTENTIAL
Citation:
Stonesifer*, K., D A. Lytle*, AND M R. Schock*. THE RELATIONSHIP BETWEEN THE CORROSION OF DISTRIBUTION SYSTEM MATERIALS, AND OXIDANT AND REDOX POTENTIAL. Presented at AWWA WQTC, Seattle, WA, 11/10-14/2002.
Description:
Scale build-up, corrosion rate, and metal release associated with drinking water distribution system pipes have been suggested to relate to the oxidant type and concentration. Conversely, different distribution system metals may exert different oxidant demands. The impact of oxidizing conditions on the nature and protective qualities of metal corrosion by-product scales, and metal release rates are not well understood.
The objective of this project was to study the relationship(s) amongredox potential in terms of EH, oxidant type, oxidant concentration, metal release rate and corrosion scale properties of drinking water plumbing materials (copper, lead, iron, and red brass). In addition, the effect of oxidant type and pH on redox potential in drinking waters has been examined.
An experimental test system consisted of eight separate test loops was constructed of PVC and Teflon plumbing. Each loop consisted of a 250 mL Teflon sample cell containing two 1"x2"x1/8" metal coupons. Four metals, iron, copper, lead and brass were tested in duplicate loops. One loop in each pair contained a pH and redox electrode (platinum electrode) which was continually monitored using a computer-based system. The cells were rinsed for a short period then water allowed to sitin contact with the coupons for 23.5 hours. At the end of the stand time, samples were taken for metal leaching levels. At the end of each test period ( several months) the coupons were examined using a variety of surface and solids analysis techniques.
The reduction of chlorine and oxygen was predictable from change of redox potential as a function of stagnation time. Chlorine concentration up to approximately 0.5 mg CL2/L dramatically increased the redox potential as the boundary by which water is oxidized was approached. The rate of redox potential decrease due to chlorine reacting with the metal surface was relatively rapid when chlorine was less than approximately 0.5 mg/L. The initial reactivity of the metal surfaces with chlorine and oxygen decreased in order of iron>>copper>lead>red brass. The pH of the bulk waters remained relatively constant over extended stagnation periods with the exception of lead. The pH of water exposed to lead coupons gradually increased with stagnation time. Different redox conditions also lead to different metal release and corrosion scale characteristics.
The results are useful in understanding important but sometimes obscure interrelationships among oxidant (disinfectant) type, disinfectant residual, metal corrosion, metal release rates, and water quality changes in drinking water distribution systems.