Investigation of transfer correction for in-service inspection of coated steel welds using ultrasonic method

Abstract

Welding is commonly used for fabricating tanks or piping systems, and the weld joints are inspected immediately without painting. After all welds have passed the acceptance criteria, the welds are painted to prevent corrosion. Oil and gas pipelines, fuel tanks and associated steel structures are prone to stress and corrosion during operation. Additionally, unforeseen events such as accidents, earthquakes, or plate movements may result in damage, leading to leaks and cracks that can have significant environmental implications. Annual or periodic inspections are essential, as they require the removal of paint for evaluation because sound waves are attenuated by the paint layer. However, paint stripping is a costly and time-consuming process. This study aims to investigate the appropriate transfer correction for compensating the ultrasonic energy attenuation caused by coating thickness at different frequencies and probe angles for in-service inspection of coated steel weld joints using the ultrasonic method. Carbon steel A36 was used as the calibration specimen according to ASME 2021 criteria. Alkyd resin enamel paint was sprayed onto the specimens to simulate coating layers ranging from 0-900 microns in thickness. Weld joints were also used to simulate a lack of fusion discontinuity, with a design coating thickness of 0-400 microns. This study utilized frequencies of 2.25, 5 and 10 MHz, and the chosen angles included the normal angle (0 degrees) and angles of 45, 60 and 70 degrees. Increasing coating layer thickness resulted in greater energy attenuation and reflected energy when the coating layer was more than half of the sound wavelength. Attenuation was not observed in the normal probe at 2.25 MHz. However, at frequencies of 5 and 10 MHz, attenuation was observed starting from the designed coating thickness of 500 and 100 microns, respectively. Attenuation was present at all frequencies and angles for angle probes, increasing with angle and frequency. Equations and transfer corrections were provided to compensate for energy attenuation due to coating layers, thereby improving the accuracy of signal testing for weld joints in industrial applications. It may be possible to develop more reliable testing methods to ensure the safety and integrity of fuel transportation systems without the need for costly and time-consuming paint removal.