Effects of compound carboxylate-urea system on nano Ni-Cr/SiC composite coatings from trivalent chromium baths.

The effects of compound carboxylate-urea system on the nano Ni-Cr/SiC composite coatings from trivalent chromium baths have been investigated in ultrasonic field. These results indicated that the SiC and Cr contents and the thickness of the Ni-Cr/SiC composite coatings could be obviously improved by the compound carboxylate-urea system. The steady-state polarization curves showed that the hydrogen evolution reaction (HER) could be significantly inhibited by the compound carboxylate-urea system, which was benefit to increase the SiC and Cr contents and the thickness of the composite coatings. The cyclic voltammetry (CV) curves showed that both of the Cr(III) and Ni(II) cathodic polarization could be increased in the bath containing the compound carboxylate-urea system. Thus, a compact Ni-Cr/SiC composite coating could be obtained using this technique. The surface morphology of the Ni-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 Ni-Cr/SiC composite coating. The chemical composition of the coating was investigated by energy dispersive spectrum (EDS) analysis. The result showed the Ni-Cr/SiC composite coatings with 3.8 wt.% SiC and 24.68 wt.% Cr were obtained in this study, which had best corrosion resistance according to the results of the typical potentiodynamic polarization curves of the Ni-Cr/SiC composite coatings.