Empirical models for long-term localised corrosion of cast iron pipes buried in soils

Empirical models are proposed for the progression of maximum depth of localised corrosion of cast iron pipes, based on data for pipes buried in 67 different backfill clay and sandy soils for up to 129 years. Early corrosion increases with increased inhomogeneity of the backfill soil and with greater availability of free water at the soil–pipe wall interface. Longer term corrosion is correlated with free water availability and occurs at a much slower rate. In most cases, the free water is fresh, oxygenated rainwater, known to be corrosive, but may also include some groundwater. Statistical uncertainty in pit depth is estimated and factors are proposed to allow for different annual rates of precipitation.