The amorphous constituents of a chromating layer may be defined by the simultaneous use of diffuse reflection spectrometry in the ultraviolet and visible, and of Fourier transform infrared spectrometry. The PO<sub>4</sub> group is characterized in the infrared reflection spectrum by bands located near 1070, 1030, and 900 cm<sup>-1</sup> for chromium phosphate. The CrO<sub>4</sub> group induces bands at 960, 860, and 820 cm<sup>-1</sup> for zinc chromate and at about 980, 950, and 860 cm<sup>-1</sup> for chromium chromates. The ligand-metal charge transfer bands (LMCT) characteristic of the chromates are situated in the region 3.35 to 4.20 eV. The study of the thermal behavior of chromium phosphate CrPO<sub>4</sub> and zinc chromate ZnCrO<sub>4</sub> shows that these compounds are stable up to 300°C. Complex chromates of Cr(III) may be observed in the range 150 to 300°C. These analytical data show the contribution of these methods which, used in the reflection mode, are demonstrably the best suited to the analysis of chromating 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.
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