Quantitative Evaluation of Remaining Thickness of Anticorrosion Coatings Based on Active Lock-in Infrared Measurement

Abstract

Application of heavy-duty anticorrosion coating is a typical method to prevent corrosion in steel bridges. It is required to repaint before coating deterioration becomes prominent because the anticorrosive performance of the coating decreases over a long term. Thus, it is important to develop a remote and nondestructive inspection method for the deterioration of coating of entire bridges at an early stage. This study proposed a new inspection method for the deterioration of the top coat of the heavy-duty anticorrosion coating by short-wave (SW) infrared spectral characteristics on the absorption and reflection. The remaining thickness of the coatings can be quantitatively evaluated by measuring the infrared reflected energy from the top coat because the absorption of the coating depends on the thickness. We measured the infrared reflected energy from the specimen consisting of various thicknesses of the top coat. Moreover, the infrared reflected energy for the in-service bridge cannot be ignored the effect of the reflections from the sea surface and the shadows of bridge members. Thus, we developed the measurement method independent of the lighting condition that combines the active infrared measurement with a halogen lamp and the lock-in processing that can extract only the fluctuating component of infrared intensity, thereby reducing the influence of disturbances. It was found that the remaining thickness of the top coat for the actual bridge in-service can be quantitatively evaluated based on active lock-in infrared measurement even in the artificially reproduced disturbance condition.