Synchrotron Radiation Based Grazing Angle Infrared Spectroscopy of Chromate Conversion Coatings Formed on Aluminum Alloys

For the first time, synchrotron radiation based Fourier transform infrared spectroscopy using a grazing angle objective has been performed on a chromate conversion coating (CCC) formed on an aluminum alloy. Synchrotron radiation (SR) based grazing angle infrared spectroscopy (GAIRS) has been compared with the most commonly performed near-normal reflection-absorption infrared spectroscopy (RAIRS). Our results indicate that the use of grazing angle incidence increases the sensitivity to the surface species. The SR-GAIRS technique was observed to have nearly an order of magnitude higher sensitivity to chromate compared to the SR-RAIRS technique. Significant amounts of the Cr-O bonds associated with the hexavalent chromium seem to be oriented in a direction normal to the surface. Thickness of the CCC was calculated using theoretical expressions. Calculated thickness agrees with reported values, thereby validating the use of SR-GAIRS as a tool for rapid, nondestructive thickness measurements. Hence, synchrotron radiation based grazing angle infrared spectroscopy can prove to be a highly effective tool to study the local chemistry on surfaces under ambient conditions.