Characterization of Conversion-Coated Aluminum Using Fourier Transform Infrared and Raman Spectroscopies

Abstract

Fourier transform infrared and Raman spectroscopies have been employed to define the molecular composition of chromium phosphate conversion coatings on aluminum. Attenuated total reflectance at 55° can be employed to probe the structure of conversion coatings present on aluminum at relatively high coating weights (≥23 mg Cr/m 2 ). Both reflection-absorption infrared and surface-enhanced Raman spectroscopic techniques can discern the presence of conversion coatings at coverages as low as 9 mg Cr/m 2 . On the basis of the vibrational spectra from these techniques, we have determined that hydrated chromium phosphate is the major component in these conversion coatings on aluminum. Reflection-absorption infrared and surface-enhanced Raman spectroscopies also provide a means to determine the molecular structure of the nascent oxide layer on aluminum as a function of processing conditions. Specular reflection, attenuated total reflectance at 35°, diffuse reflectance, and Raman spectroscopic methods, in general, lack the surface sensitivity necessary to probe thin (≤23 mg/m 2 ) inorganic films on aluminum.