Nondestructive testing of corrosion thickness in coated steel structures with THz-TDS

The corrosion of steel plate structures usually occurs under the coating materials, and it is difficult to be accurately detected by the existing non-destructive testing (NDT) technologies. A novel non-contact terahertz time-domain spectroscopy (THz-TDS) is therefore proposed to measure the corrosion thickness under different coatings, which provides a method to quantify the corrosion degree of steel structures. In this study, the optical parameters of the corrosion products and the common coating materials (epoxy resin, rubber, and cement paste) are investigated by transmission THz-TDS, and the method of extracting the delay time difference in the reflection THz-TDS is proposed to measure the thickness of corrosion layer. The experiments show that the detection of the steel plate corrosion can be achieved from the reflected THz signal based on the peak number change and a significant attenuation of the signal peaks. Moreover, the reflected THz signal can measure the corrosion thickness with an accuracy of more than 90% and the minimum identification thickness is 40 μm. Besides, the thickness of the coating layer is simultaneously measured irrespective of the coating material types. It proves the applicability and accuracy of THz-TDS for NDT corrosion thickness of coated steel structures.