AWWA JAW20484

The influence of buffer capacity, chlorine residual, and water flow velocity on the corrosion of mild steel and copper under static and dynamic conditions were investigated. An increase in buffer capacity at constant alkalinity in the pH range 6.0-9.0 decreased the corrosion rate of mild steel. However, an increase in buffer capacity at various pH levels by raising alkalinity did not result in lower corrosion rates because of the effects of higher ionic strength and conductivity. Similar effects, in addition to higher chloride content, were responsible for the relatively higher corrosion rates and pitting indexes found during field studies in some waters with higher buffer capacities. A chlorine residual accelerated the corrosion of mild steel and copper during batch studies. In the presence of oxygen, the corrosive attacks were more severe. The same effect might be observed under distribution system conditions because of high oxidation potentials of hypochlorous and hypochlorite ion. Based on laboratory and field studies, the corrosion rates of mild steel and cooper increased with flow rates, indicating enhanced corrosion damage as a result of mechanical stress and erosion effects. However, passivation may occur when velocity is increased in the presence of high dissolved oxygen concentrations. This occurs because of the greater possibility for oxygen to diffuse to the metal surface and eventually form protective oxides. Corrosion products obtained from pipe loop studies were magnetic iron oxide or magnetite, lepidocrocite and calcium carbonate. Corrosion products obtained from field studies indicated the presence of cuprite and malachite, the latter being known to cause green staining. Includes 33 references, tables, figures.