Iron corrosion scales: Model for scale growth, iron release, and colored water formation

This paper was presented in part by V. L. Snoeyink as the Simon W. Freese Lecture at the 2002 Canadian Society of Civil Engineers/Environmental and Water Resources Institute of ASCE Environmental Engineering Conference in Niagara Falls, Ontario, Canada, July 22, 2002. The interactions of corroded iron pipe surfaces with water are of importance because they can lead to serious water quality degradation and material deterioration. A conceptual model has been developed in this paper to describe the formation and growth of iron scales, and their reactions that lead to colored water problems. Most corrosion scales have characteristic structural features, such as a loosely held top surface layer, a shell-like layer(s), and a porous core. According to this model corrosion scales are expected to grow from inside the scale via the corrosion reaction, i.e., the conversion of iron metal to ferrous ion. The average oxidation state of iron increases with distance from the pipe wall. The scale structure and scale reactions permit the ferrous iron to be released to the bulk water, where it undergoes conversion to particulate ferric iron, which is the cause of colored water. Scale structure and composition play important roles in the reactions of iron scales that lead to iron release, and water quality control to decrease the porosity of the scale is an important means of reducing iron release. It is anticipated that the conceptual model presented here will be used as a basis for changing water quality to minimize colored water formation, and as a guide for further research.