Ultrasonic measurement of micrometric wall-thickness loss due to corrosion inside pipes

Pipelines are subject to wall-thickness loss due to corrosion along time. Ultrasonic pulse-echo techniques are widely used for thickness measurement achieving a high resolution. However, the precision of measurement is highly dependent on temperature and on the ultrasonic system. This work presents the temperature correction strategy of an ultrasonic measurement system to obtain one micron accuracy, in a pipeline corrosion long-lasting monitoring. The proposed technique is based on the fact that the coupling layer has a constant thickness during a long period of time (year) and can be used in the same way as a thermometer to compensate the changes in the ultrasonic velocity. The variation of time of flight in the coupling layer can relate to the variation of time of flight in the pipe wall to construct a correction polynomial. This function compensates the variation on propagation velocity and thermal expansion. The results are experimentally evaluated using an array of eight ultrasonic transducers (5 MHz, 10-mm diameter) operating in pulse-echo mode with a water coupling layer. Long term corrosion tests were conducted using an electrolytic bath along ten months. Good agreement was found between the theoretical corrosion rate and the results of the ultrasonic measuring system.