Model for the long-term corrosion of cast iron pipes buried in soil

The external corrosion of old cast iron water mains is a major concern for water industries in many older cities. To better manage these assets, good quality mathematical models, based on real-world observations, are required for predicting future corrosion. Herein a recently proposed model for the long-term maximum corrosion depth of cast iron buried in soil is presented. The model is based on the bimodal model, recently shown to be applicable for cast iron corrosion in soil. The model was calibrated to field data from 37 sites. It was found that the main factors influencing soil corrosion are, in order, time of wetness and inorganic nitrate concentration (for microbiological attack). Other parameters including chloride, phosphate and carbonate content were also considered, but were found to be less important. The potential influence of poor quality backfill and physical damage to the pipe on corrosion depth is also discussed.