Corrosion Monitoring and Thickness Measurement

INTRODUCTION During the last three years my interest in the use of ultrasonics for the detection of corrosion and the measurement of remaining wall thickness has been re-awakened. The method has been extensively used to verify and quantify Magnetic Flux Leakage (MFL) results. Discrepancies between ultrasonic and MFL results have usually been put down to "Another MFL false call." However, having watched many ultrasonic tests being carried out, I have come to believe that the boot is often on the other foot and that we place too much faith in the ultrasonic method for corrosion monitoring. The techniques used by many practitioners give rise to serious shortcomings in both probability of detection, and accuracy of remaining wall assessment. These shortcomings are not confined to corrosion monitoring of flat plate, but apply equally to pipe and vessel inspection. This paper looks at the root causes of poor performance with ultrasonics and suggests some methods to improve the situation. REFLECTIVITY As with any proposed ultrasonic procedure, we first need to consider the nature of the target reflecting surface since this will affect signal amplitude and thus the probability of detection. Reflectivity and other factors to be discussed later can also have a significant influence on the accuracy of remaining wall thickness measurement. The amplitude of an echo from a target reflector depends on the following factors  :-