Evanescent wave absorption measurements of corroded materials using ATR and optical fibers

Abstract

The purpose of this research effort is to develop an in-situ corrosion sensing capability. The technique will permit detection of corrosion on and within aircraft structures. This includes component junctions that are susceptible to corrosion but which are not accessible for visual inspection. The prototype experimental configuration we are developing includes long wave infrared transmitting optical fiber probes interfaced with a Fourier Transform Infrared (FTIR) interferometer for evanescent wave absorption spectroscopic measurements. The mature and fielded technique will allow periodic remote sensing for detection of corrosion and for general onboard aircraft structural health monitoring. An experimental setup using an Attenuated Total Reflection (ATR) crystal integrated with an FTIR spectrometer has been assembled. Naturally occurring corrosion including Aluminum Hydroxide [Al(OH) 3 ] is one of the main corrosion products of aluminum the principle structural metal of aircraft. Absorption spectra of our model corrosion product, pure Al(OH) 3 , have been collected with this ATR/FTIR experimental setup. The Al(OH) 3 spectra serve as reference spectral signatures. The spectra of corrosion samples from a simulated corrosion process have been collected and compared with the reference Al(OH) 3 spectra. Also absorption spectra of naturally occurring corrosion collected from a fielded corroded aircraft part have been obtained and compared with the spectra from the simulated corrosion.