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Extreme Value Statistics (EVS)
EVS in the Analyzer Studio
The service life of industrial systems being subject to localized corrosion is determined by an extreme parameter value, such as maximum corrosion depth or minimum time to failure.
Approximately 50 % of the cost of localized corrosion could be avoided by better application of existing corrosion control technology, if only we knew in advance where and when corrosion damage would occur.
For example, systems may be serviced during scheduled down time thereby avoiding costly, unscheduled down time.
The Figure illustrates how the existing OLI software allows clients to extrapolate the data, obtained in short-term experiments (within one month) to the long periods of time (up to one year) and theoretically to the infinity.
On the basis of Deterministic Extreme Value Statistics, a method has been developed for extrapolating experimentally measured corrosion damage into the future.
The simplest data set to which the OLI techniques could be applied consist of n samples (xi,ti,si), i = 1,…,n, where xi is the depth of the deepest pit over a sample with the area, si, of metal exposed to a corrosion environment for time ti. The separate areas si could be either distinct coupons from designed experiments or else random samples at various times from regions of a metal surface thought to be representative of the whole system of area S.
The key feature of this method is that it is deterministic, in that the output is constrained by the natural laws, rather than being empirical as is the classical extreme value statistical method. Further, the method can be used for relatively small total numbers of experimental points (n).
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The OLI Code Yields:
• Probability of failure, Pf, for given observation time, t, and thickness of the wall, d, for the whole system with area S.
• The mean value of the deepest pit for given t and S.
• Time to first perforation (service life) for given Pf, d, and S.
• Number of perforations for given d, t, and S.
• Area of perforation for given d, t and S.
The current version of the code can be used only under constant external conditions. In future versions, changes in the external conditions (corrosion potential, electrolyte composition, temperature, etc.) may be taken into account.
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