Front-side detection and quantitative characterization of non-visible corrosion using active infrared thermography

Active infrared thermography (IRT) has been widely investigated for corrosion detection, but most studies inspect the side opposite to where corrosion develops, enabling defect depth estimation. In coastal infrastructures or ship hulls, only the front side is accessible. This study presents a quantitative analysis of front-side thermographic inspection for non-visible corrosion, addressing defect detectability and the estimation of corrosion layer thickness, directly related to corrosion severity. Artificial corrosion with varying defect diameters and thicknesses is induced for evaluation. Two cameras in different spectral ranges, mid-wave infrared (MWIR) and long-wave infrared (LWIR), are first compared with two active techniques: pulsed thermography (PT) and long-pulse thermography (LPT). Based on detectability, thickness sensitivity and practical considerations, the LWIR camera with LPT is identified as the most suitable option, with an effective configuration of 60 cm inspection distance, 970 W heating power and 10 s excitation duration. Defect thickness is then estimated using an empirical model based on the defect diameter and the critical time, defined as the moment when the thermal responses of the defect and sound areas converge. The proposed approach achieves a thickness estimation accuracy of 0.1 mm and provides guidelines for assessing the presence and severity of corrosion under paint.