It is generally difficult to determine whether corrosion rates have increased or decreased based upon two general ultrasonic evaluations over a given period of time. This is simply due to the technical inability to focus the ultrasonic probe at the same pinpoint location as previously examined without some form of fixed guide. As pitting rate and variation at the interior wall surface increases, it becomes even more difficult to make a reliable comparison from a previous test location.

     While past thickness and corrosion rate information may not provide the most accurate comparison of data, it does often provide good update information if sufficient test points are presented.

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