Essay: Chromium coatings

The dense oxide layer which naturally forms on the aluminum surface inhibits the direct electroplating of trivalent chromium coatings on the aluminum substrate. The trivalent chromium coatings with and without Multiwall carbon nanotubes (MWCNTs) deposits on the pure aluminum substrate using a thin zincates interlayer to improve the mechanical and corrosion properties of pure aluminum substrate. The two different zincates that applies in this research shows the considerable influence on the formation of trivalent chromium deposit on the pure aluminum substrate. The trivalent chromium improves the mechanical properties of the pure Al significantly.

The plating temperature, plating time and pH found to have significant effects on the thickness, uniformity and microstructure of trivalent chromium coatings. Results show the average thickness of homogenous chromium coating without pitting and microcracks was 35 ??m at 30?? C, pH = 2 and 40 min. Experimental results mathematically modeled with the response surface methodology (RSM) method by statistical software Design expert V8 to predict coating thickness on different coating conditions.

Compared with the trivalent chromium coatings, the Cr/MWCNTs coating has the similar amorphous microstructure. The carbon nanotubes slightly increased microhardness to HV1 N 9.2-9.8 GPa for Cr/MWCNTs from HV1 N 8.0-8.3 GPa compare to the trivalent chromium coatings. The wear results show the significant increase in wear resistance of Cr/MWCNTs coatings compare to trivalent chromium coatings, and the specific wear rate was decreased about 30-40 % under the applied normal load. A crack-bridging bonding model was proposed to explore the improvement mechanism in wear resistance by the carbon nanotubes as reinforced nanometer fiber in the trivalent chromium composite coating. The carbon nanotubes prevent the generation of cracks in the composite coating matrix.

The EIS corrosion results show that the real impedance of the trivalent chromium coating was much smaller than that of the Cr/MWCNTs composite coating, which was in accordance with OCP polarization results stating that the incorporation of the MWCNTs has greatly improved the corrosion resistance of the Cr/MWCNTs composite coating.

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