Subsurface Corrosion Imaging System Based on LASER Generated Acoustic (LGA)

Corrosion process might occur over or beneath the surface of a material. Under-the-surface corrosion is more dangerous than other types as it is visually hidden. This research develops an imaging system to detect crevices resulting from subsurface corrosion. The system is built with a laser-generated acoustic (LGA) method, - using photoacoustic phenomena to generate acoustic waves from a laser and material interaction process, to construct a subsurface image of a metal. Though optical actuator is used, deeper penetration is achieved in the acoustic wave sensing, causing a change of intensity of the waves as they pass corrosion crevices. Measurement of the acoustic wave’s intensity is used to construct LGA image illustrating the subsurface condition of an object. There are three main components of the device, which are a laser, a microphone and a data processor. The laser beam is modulated and exposed to an object to create thermal contraction. The microphone records acoustic waves generated in the process and the data processor analyses the result. Comprehensive measurement is done over the entire surface of the object to create 2-dimensional  images. This research reports an experimental result of the imaging system on an object with subsurface corrosion crevices. LGA’s images produced showed reasonably clear evolution process of the corrosion. In the experiment, the crevices emerge at 230µm depth were detected in the first 5 hours of the corrosion process. An advance analysis showed that the LGA images produced were able to show the evolution process of the shape of the crevices during corrosion. A mathematical model, based on acoustic transmission intensity equation, is designed to further examine the LGA image resulting in a curve which fits the measurement result with MAPE 12% difference.