Abstract
The maximum depth of pitting corrosion on bottom plates of oil storage tank was evaluated by extreme value statistics. The maximum pit depth of each sample fitted in with a doubly exponential distribution. But, the distribution of extreme values is affected by the original distribution which is exponential and therefore the estimated value Xmax becomes small, when sample size is too small. In case of this tank, sample size 0.5m square or larger is desired.
Samples shall be removed from the region where pitting corrosion took place heavily, as much as possible.
The parameters of cumulative distribution function were calculated in accordance with some methods. It was suggested that the MVLUE method was available for N≤20 and the maximum likelihood method for N>20.
The life of storage tank was predicted by the growth speed of pitting corrosion. Time dependence of the estimated maximum pit depth was linear within the period of 12.8 years use.
