In Japan, generally, the lifetime of distribution pipes (ductile cast iron pipes) is considered to be 40-50 years. However, starting in 1973 in Sapporo, leakage incidents, owing to corrosion on external surface of pipes and joint bolts, started to be found in ductile cast iron pipes which had been laid for less than 10 years. Therefore we have covered the entire length of newly laid pipes with polyethylene sleeves as an anti-corrosion measure. However, leakage incidents caused by external corrosion occurred occasionally along a total of 856 km of distribution pipes laid before this measure started. Such a problem of widespread maintenance of externally corroded distribution pipes was quite rare in Japan. We investigated the cause of external corrosion and corrosion situations and derived the formula to predict depth of external corrosion. Consequently, we established an effective renewal method by building an externally corroded pipe danger map to give renewal priority to externally corroded distribution pipes depending on leakage possibility.

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