Pitting corrosion of older underground cast iron pipes

Major water supply distribution networks in many cities consist of buried cast iron pipes. Often the pipes are internally cement-lined. For these the most severe external pitting corrosion is important because this may cause leakage and eventual structural failure. Herein representative data is reported for 10 pipes, ranging in age from 34 to 129 years and from 200 to 600 mm diameter that are part of a larger data set. The pipes examined were selected by the water utilities from several critical locations. Pit depths over 1–2 m lengths of pipes were measured and examined and soil properties measured. The morphologies of pitting around the pipes were found to be very different, with often much greater pit depths and larger pit areas at the base of the pipes. There were also considerable differences along the length of individual parts of pipes. When plotted on a Gumbel plot for the maximum pit depths, the deepest pits showed a different Gumbel trend compared with the majority of pit depth maxima. Theoretically, this indicates that different pitting mechanisms are involved. This possibility is explored by reference to earlier work on the long-term pitting corrosion of steels in sea water. Comments are made also about empirical fitting with other extreme value distributions, as sometimes suggested.