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

Abstract

For the first time, synchrotron radiation based Fourier transform infrared spectroscopy using a grazing angle objective has been performed on a chromate conversion coating ͑CCC͒ formed on an aluminum alloy. Synchrotron radiation ͑SR͒ based grazing angle infrared spectroscopy ͑GAIRS͒ has been compared with the most commonly performed near-normal reflection-absorption infrared spectroscopy ͑RAIRS͒. 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. Dichromate based solutions have been used as corrosion inhibitors on aluminum since the beginning of the last century. 1-4 Since then, the mechanism of protection offered by chromate-based solutions has been under investigation. Toxicity and mutagenic effects of hexavalent chromium have stimulated the development of alternative coatings, 5-7 the success of which depends on understanding precisely the underlying mechanism of protection offered by hexavalent chromium. A variety of surface analytical tools have been utilized to study the interaction of chromates with aluminum, including Auger electron spectroscopy ͑AES͒, 8,9 X-ray photoelectron spectroscopy ͑XPS͒, 8,10-14 Fourier transform infrared ͑FTIR͒ spectroscopy, [15][16][17][18][19] Raman microscopy, 18,20,21 UV-visible spectroscopy, 22 ellipsometry, 23 atomic force microscopy, 24,25 transmission electron microscopy, 25-27 Rutherford back scattering, 8 secondary ion mass spectroscopy ͑SIMS͒, 28 X-ray absorption spectroscopy ͑XAS͒, 17,29 and X-ray absorption near edge spectroscopy ͑XANES͒. 17,[30][31][32][33] Infrared spectroscopy has gained prominence due to its ability to determine the chemical nature, and in some cases, the geometry of adsorbates on surfaces. Infrared microspectroscopy is a nondestructive technique that allows chemical analysis of materials at a lateral resolution of few microns. This feature combined with stability of chromate conversion coatings ͑CCCs͒ under these experimental conditions have resulted in numerous studies. [15][16][17][18][19]23 The ability of FTIR spectroscopy to be operated in various modes makes it a very attractive analytical technique. However, only reflection-absorption and transmission modes are routinely employed to study thin films and coatings. The grazing angle objective ͑GAO͒ provides infrared radiation at grazing incidence ͑65°-85°͒, which enhances the...

Performance Comparison of Steam-Based and Chromate Conversion Coatings on Aluminum Alloy 6060

“…Acidification of pH inside the cavity due to hydrolysis may lead to the partial dissolution or dealloying of the intermetallic particles due to selective leaching of aluminium or other active elements25,26 . Under present conditions, high temperature of steam produces rapid oxidation of the leached elements, ultimately forming mounds around or over the intermetallic particles.The FT-IR analysis of the formed oxides revealed that the layers generated by chromate and chromatephosphate surface treatments were mixed Cr VI Cr III oxide/hydroxide and chromate-phosphate, which has been already reported in the literature35,36 extensively. On the other hand, the coating formed by…”

“…The assignment of the peaks in each FT-IR spectra was performed using the existing literature[35][36][37][38][39] .The comparison of the potentiodynamic polarization curves of the conversion coatings showed a distinct behaviour related to the microstructural changes of AA6060 caused by chromate, chromatephosphate and steam surface treatments. It is reported in the literature 40 that presence of Fe containing intermetallic particles increase the cathodic activity of the surface and they act as preferential sites for pit initiation in the aluminium alloy matrix.…”

Surface Pretreatments of Aluminum Alloy AA2024-T3 and Formation of Chromate Conversion Coatings

“…XPS probes only the top ϳ10 nm ͑100 Å͒ of the coating ͑surface͒ while the coating is approximately 0.5 m ͑5000 Å͒ thick. 23 Therefore, while XANES probes the entire coating, XPS analyzes only the outer surface, as the photoelectrons measured by XPS are easily attenuated by scattering in the coating. This indicates that the surface of all the CCCs is highly enriched in hexavalent chromium.…”

Morris Cohen Graduate Student Award Address- Surface Chemistry and Corrosion Behavior of Aluminum-Copper Systems: Air Formed Films to Complex Conversion Coatings

“…This study has pioneered the use of synchrotron radiation based grazing angle infrared spectroscopy (SR-GAIRS) to enhance the detection of chromate in a CCC by an order of magnitude as compared to other IR techniques (5). This technique can be used as a non-destructive tool for thickness measurements.…”