Monitoring the Cumulative Process of Corrosion Defects at the Elbow of a Welded Pipe Using Magnetostrictive-Based Torsional Guided Waves

Abstract

Ultrasonic guided waves (UGWs) of T(0,1) mode were generated by magnetostrictive sensors (MsS) to perform 36-day monitoring of a defective welded pipe elbow. A hydrochloric acid pool was employed to accelerate the corroded defect accumulation process, and a total of 11 defect levels were recorded. The envelope of UGWs was found to be insufficient to indicate the defect level lower than 6 due to the instability of monitoring signals. To achieve the high-quality display of the corrosion accumulation process, the energy of concerned wave packets was introduced to act as a new indicator. To suppress the influences of sensor deterioration, the energy of wave packet corresponding to the elbow region (Edefect) was divided by the energy of wave packet representing the feature reflectors (such as obstacle, free end and welds) in the straight pipe section. The compensated energy of wave packet demonstrated the stepped growth at each level of defect under the operation frequency of 64 kHz. The smoothed curve of Edefect/Eweld was the self-compensation to MsS deterioration and could achieve the highly sensitive display of the corrosion defect accumulation process since the value of Edefect increased at the cost of subsequent weld waves’ energy.