Non-destructive Testing of Paint Coatings on Steel Plates by Ultrasonic Reflectometry

An ultrasonic reflectometry method has been used successfully to detect elasticity changes following the curing of 100 \(\upmu \)m thick epoxy films (DGEBA-PAA) coated onto 1.5 mm thick steel plates. The method employs a goniometric apparatus to measure the reflection coefficient amplitude around 5 MHz followed by a standard FFT analysis of the reflected signal. A specific ultrasonic wave mode was identified which was dependent on, and correlated with, the presence of the coating layer. The goniometer angle associated with this mode was different from that associated with Lamb modes of the plate, enabling the new mode to be detected reliably. The sensitivity of the new mode to variations in the paint mass density, longitudinal and transverse velocities and thickness has been quantified numerically by using Brekhovskikh’s model with due account taken of the finite width of the ultrasonic fields of the transducers. The method was tested for the detection of the evolution of the coating elasticity during curing at 80 \(^{\circ }\)C for 400 h. Compensation was applied to correct for the effect of the natural swelling of the paint layer on the angular position of the tracked mode, and this was validated experimentally. The evolution of the angular position was found to offer a reliable means to detect elasticity changes during the cure of the coating. The mass density variation in the coating during cure only weakly affected the angular position. This method will provide a promising tool for the non destructive evaluation of paint coatings, particularly in service for the detection of ageing effects in the longer term.