Nondestructive testing and imaging of corrosion thickness of steel plates using THz-TDS

Abstract

The corrosion of steel plates is one of the main reasons for the decrease of bearing capacity and structural durability. The existing nondestructive testing (NDT) approaches is difficult to accurately detect steel plate corrosion, including the location and degree of corrosion. Therefore, a non-contact nondestructive terahertz time-domain spectroscopy (THz-TDS) is utilized to evaluate the corrosion based on the properties of THz waves. The experimental results show that in the effective frequency range of 0.2–1.2 THz, the refractive index of corrosion products is around 2.8, and Fe3O4 has a significant impact on the optical parameters of the corrosion products mixture. The relative amplitude of the first signal from reflection THz-TDS exhibits a logarithmic relationship with the corrosion time of the steel plates and a linear relationship with the reciprocal of the corrosion time. By the method of extracting the delay time difference in the reflected THz signal, the THz-TDS can accurately measure the corrosion layer with a thickness larger than 40 μm, and the measurement accuracy is over 90 %. Meanwhile, the 3-dimensional (3D) visualization based on the THz method directly describes the true thickness and geographical distribution of the corrosion layer while removing the influence of coating materials. It provides a novel way to quantify corrosion degree of steel plate through NDT of corrosion under coating and extraction of corrosion thickness. The applicability and accuracy of non-contact THz-TDS for corrosion detection are therefore proved.