Cast Iron and Steel

Dissolved oxygen can either enhance or inhibit the rate of corrosion of steel, depending on the mineral content of the water and the dissolved oxygen concentration (Singley et al., 1985; Benjamin, Sontheimer, and Leroy, 1996). Oxygen is also very important in causing concentration cell corrosion. Chlorine can accelerate the rate of attack of iron either by direct increase of the redox potential of the electrolyte that favors the conversion of iron to ferrous and then ferric ions, or through a sequence of chemical reactions that produce hydrogen ions, hypochlorous acid, hypochlorite ions, and chloride. The effect of chlorine, like that of oxygen, diminishes after the corroding surface is passivated with corrosion by-products. A secondary effect of chlorine is its significant mitigating impact on microbiological corrosion phenomena. The same is true for other disinfecting agents. Changes from oxic conditions (chlorine residual or oxygen present) to anoxic conditions fosters the production of ferrous iron, which is much more soluble than ferric iron. Redevelopment of oxidizing conditions, or contact of the water with air, will then result in red water. Overall, red water formation is a very complex phenomenon still under investigation (Trussell, 1985; Singley et al., 1985; Benjamin, Sontheimer, and Leroy, 1996; Larson, 1975; Stumm, 1960; L grand and LeRoy, 1955; Rossum, 1987).

The effect of pH is generally through its role in secondary reactions, such as the oxidation of ferrous iron, and on the formation of scales and corrosion products. This effect influences the...