Electrodeposition of nanocrystalline chromium coatings from 1-butyl-3-methylimidazolium-hydrogen sulfate ionic liquid

• [BMIM]HSO4 shows a good electrochemical stability for chromium electrodeposition.
• Cr(III) reduction occurs by a two-step process, and the process is an irreversible.
• Nanocrystalline Cr coatings can be electrodeposited directly from [BMIM]HSO4.
• Chromium coatings obtained at higher potential present better corrosion resistance.

The electrochemical behavior of trivalent chromium reduction from 1-butyl-3-methylimidazolium-hydrogen sulfate ([BMIM]HSO4) ionic liquid is investigated. The result of cyclic voltammetry reveals that the Cr(III) reduction is irreversible and occurs in two steps, Cr(III) to Cr(II) and Cr(II) to Cr(0), respectively. In the electrochemical impedance spectroscopy measurements, the preferable equivalent circuit is made to fit the experimental data. These effects of electrodeposition potential and electrolyte temperature on the coating thickness and electrodeposition rate are studied by potentiostatic method. These results show that the temperature and electrodeposition potential have great effect on the coating thickness and electrodeposition rate. The composition and morphology of electrodeposited Cr are investigated using scanning electron microscope (SEM), X-ray diffractogram (XRD) and energy-dispersive spectroscopy (EDS). SEM, XRD and EDS observations reveal that the coatings are composed of nanocrystalline chromium with a ball-like structure. Moreover, the corrosion resistance of the as-deposited chromium layer is evaluated using polarization curves. The result shows that the corrosion resistance of the chromium coatings obtained at higher potential is better.