FT-IR Optical Fiber Remote Detection of Aluminum Hydroxide by Evanescent Wave Absorption Spectroscopy

Remote detection of very small amounts of aluminum hydroxide has been achieved with the use of evanescent wave absorption spectroscopy. Aluminum hydroxide is a principal corrosion product of aluminum. Five absorption bands have been recorded in the spectral range from 3350 to 3650 cm⁻¹. The recorded positions, transition strengths, and widths of the absorption bands are in qualitative agreement with reference spectral data. A four-wavenumber-resolution Michelson interferometer and decoated 200-μm-core-diameter multimode chalcogenide optical fibers were combined to make the evanescent wave absorption measurements. One-to four-meter lengths of fiber were used with 15 to 25 cm of the fibers decoated. The technique is very sensitive and can detect a very small amount of aluminum hydroxide in contact with the decoated fiber core. This study is a valuable step in the development of a technique for monitoring industrial corrosion of aluminum alloys.