Critical Discussion on Applicability of Extreme Value Statistics to Localized Corrosion-Use of Individual Depth Data for Evaluation of the Applicability

摘要

The applicability of extreme value statistics tolocalized corrosion was studied based on a actual case. Thelocalized corrosion under tubercles on inner surface of the steelchannel in which river water flowed was investigated. Their depthsobeyed the distribution of exponential type, while the maximumdepths did the double exponential distribution. When the similardata appeared in the previous papers, extreme value statistics wasoften assumed to be applicable. However, according to the theoryof extreme value statistics, the maximum depths are necessary tobe sampled from a single population.Then, it was evaluated by calculating the confidence intervalsfor the population mean whether the data belong to the samepopulation or not. Moreover, the theoretical distribution ofmaximum depths was calculated and was compared with themeasured one. The results of these analyses showed that extremevalue statistics was inapplicable to the present case. Thus, thepresent case study demonstrates that extreme value statistics is notalways applicable to the maximum depths of localized corrosion,even if the depths obey the distribution of exponential type and themaximum depths do the double exponential distribution.