Development of long-term localised corrosion of cast iron pipes in backfill soils based on time of wetness

Parameters characterising the properties of soils used as backfill in immediate contact with cast iron pipes exhumed from a variety of soils after up to 129 years exposure are used together with average annual precipitation to estimate the average time of wetness. This is shown to correlate closely with pipe wall corrosion penetration. The possibility of microbiologically influenced corrosion is dismissed owing to lack of critical nutrients. The presence of chlorides in some soils most likely arises from pipe wall corrosion penetration and subsequent leakage of chlorinated drinking water. The predictions from the derived model are compared with an independent set of 16 pipe-wall corrosion penetration observations for pipes with up to 98 years exposures and found to agree well. The potential influences of soil temperature, physical damage and poor quality backfill are discussed.