External and Internal Corrosion of Large-Diameter Cast Iron Mains

Abstract

Deterioration in cast iron mains manifests itself in the form of corrosion. External corrosion is typically found to occur in pipes buried in corrosive soils while internal corrosion is dependent on water chemistry and flow characteristics. In the literature, corrosion of cast iron pipes (external and internal) is typically characterized by corrosion pit depth even though corroded area and corroded pit volume as well as pit location may enhance this characterization. Knowledge of these corrosion pit properties permits the assessment of its structural integrity. Typically, internal and external corrosion pits and corroded areas are observed to occur in many irregular shapes and sizes, which make their characterization a challenge. This paper describes extreme value statistical models that can be used to estimate external and internal corrosion pit depths in cast iron pipes using indirect properties or parameters. The goal of these corrosion models is to be able to predict corrosion pit depth based on available data with an acceptable degree of confidence. The external corrosion model is calibrated using external corrosion data collected by Thames Water Utilities Ltd (TWUL) and subsequently validated with data obtained from the inspection of four cast iron pipe lengths. The internal corrosion model was calibrated using internal corrosion data but sufficient appropriate data was not available for its validation.