A Fringe Projection Based Approach for Corrosion Monitoring in Metals

Corrosion detection and monitoring is a relevant task in many different fields including automotive, naval, civil etc. In the case of artifacts, corrosion is a relevant topic because this process can introduce huge damages on really ancient objects so that preventive procedures and long term monitoring are required. In this specific field the monitoring task presents, in addition, some extra requirements. In fact, monitoring the surface condition without contact, without sampling the object and, possibly, without diminishing the fruition of the handwork is preferable. Optical techniques, in general, are successfully used in many fields where non-contact, high accuracy measurements are required. In particular, an approach based on fringe projection will be explored in this paper. This method mainly consists in projecting a given intensity pattern on the object and in observing the fringe modulation introduced by the object itself; modulation contains information about the contour of the object itself. In this study corrosion tests on bronze alloy were performed both under salt spray and acid rain condition. A sinusoidal fringe pattern was projected on the analyzed samples before starting the measurement and at each advancement step of the corrosion test. Fringe patterns were analyzed using two approaches. Fast Fourier Transform of the recorded patterns was studied to detect if formation of corrosion patina can be related to some variation in the frequency spectrum. Moreover, continuous wavelet analysis was performed to detect where corrosive patina forms.