CONTRIBUTION OF SPECTROMETRIC METHODS TO THE STUDY OF THE CONSTITUENTS OF CHROMATING LAYERS

The amorphous constituents of a chromating layer may be defined by the simultaneous use of diffuse reflection spectrometry in the ultraviole t and visible, and of Fourier transform infrared spectrometry, The PO4 group is characterized in the infrared reflection spectrum by bands l ocated near 1070, 1030, and 900 cm(-1) for chromium phosphate, The CrO 4 group induces bands at 960, 860, and 820 cm(-1) for zinc chromate an d at about 980, 950, and 860 cm(-1) for chromium chromates, The ligand -metal charge transfer bands (LMCT) characteristic of the chromates ar e situated in the region 3.35 to 4.20 eV. The study of the thermal beh avior of chromium phosphate CrPO4 and zinc chromate ZnCrO4 shows that these compounds are stable up to 300 degrees C. Complex chromates of C r(III) may be observed in the range 150 to 300 degrees C. These analyt ical data show the contribution of these methods which, used in the re flection mode, are demonstrably the best suited to the analysis of chr omating layers on coated steel, of which the primary constituents are chromium phosphate and complex Cr(III) chromates. These results allow the interpretation of the thermal behavior of chromating layers.