Effects of Compound Carboxylate-Urea System on Nano Cr/SiC Composite Coatings from Trivalent Chromium Baths in Ultrasonic Field

The effects of compound carboxylate-urea system (sodium citrate, sodium oxalate, sodium acetate and urea) on the nano Cr/SiC composite coatings from trivalent chromium baths have been studied in ultrasonic field. These results indicated that the SiC content and thickness of the Cr/SiC composite coatings could be significantly improved by the compound carboxylate-urea system. The steady-state polarization curves showed that the hydrogen evolution reaction (HER) could be obviously inhibited by the compound carboxylate-urea system, which was benefit to increase the SiC content and the thickness of the composite coatings. The cyclic voltammetry (CV) curves showed that the Cr(III) cathodic polarization could be increased in the Cr(III) bath containing the compound carboxylate-urea system. Thus, a compact Cr/SiC composite coating could be obtained using this technique. The surface morphology of the Cr/SiC composite coatings checked with the scanning electron micrographs (SEM) showed that the surface smoothness could be also improved and the microcracks and pinholes could be decreased due to the presence of the compound carboxylate-urea system. The phase composition of the as-posited coating was measured by the X-ray diffraction. XRD data showed that the as-posited coating was Cr/SiC composite coating. The chemical composition of the coating was investigated by energy dispersive spectrum (EDS) analysis. The result showed the Cr/SiC composite coatings with 5.5 wt.% SiC content were obtained in this study, which had best corrosion resistance according to the results of the typical potentiodynamic polarization curves of the Cr/SiC composite coatings.