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Extreme value analysis, the basic principle and application of which had been described in the book written by E. J. Gumbel, has been successfully applied to estimate the maximum pit depth on the oil tank base plate. This method is quite useful to predict corrosion failure life for such a large apparatus that detailed inspection of corrosion damage for the whole area could not be easily done due to its large surface area. Maximum pit depth distribution was determined from 10 small inspection sites chosen at random, the mean area of which was 1.85 m2. From the distribution the maximum pit depth for the whole area of 1.04 x 103 m2 of oil tank base plate was estimated to be 5.56 mm at 50 % probability after 10 years exposure. Penetration of the maximum pit into the base plate was concluded to occur at the inspection time with high confidence because the original plate thickness of 6 mm failed within lc control limit of the extreme value plot.

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