Extreme value statistics for pitting corrosion of old underground cast iron pipes

Many major city water supply distribution networks consist of buried cast iron pipes. In many cases the pipes are internally cement-lined and the predominant corrosion is by external pitting. This may cause leakage and eventual structural failure. It is conventional to use the Gumbel extreme value distribution to represent the statistics of maximum pits depth and to use it to estimate the probability of pipe wall perforation. Herein data obtained for maximum pit depths for large-sized (1-2 m long) samples of 10 pipes exhumed from different, apparently randomly selected, locations after 34-129 years of service are examined for consistency with the Gumbel probability distribution. This was the case for the deepest pits, but the data for less deep pits show a consistent pattern of departure from the Gumbel distribution. Some extreme pit depth data, inconsistent with the rest are interpreted as possibly caused by material imperfections.