Studies On Electrodeposition Of Chromium Coatings Based On Cr(Ⅲ) Bath Using L-ethyl-3-methylimidazolium Ionic Liquid

Chromium coatings have been widely applied in aerospace, automotive, shipbuilding because of their wear-resisting, heat resistance, corrosion resistance and reflective ability. At present, the problem of the chromium prepared from water-based trivalent chromium solution has a disadvantage of thin-coating with poor-quality. However, compared with the ordinary aqueous solution system, ionic liquid as a kind of green environmental protection electrolyte has a wide electrochemical window, high electrical conductivity and low volatility, etc. Therefore, the electrodepositing of chromium materials has been a worldwide hot spot based on Cr(III)-ionic liquid system. However, at present, the electrodepositing of chromium coatings based on Cr(III)-1-ethyl-3-methylimidazolium hexafluorophosphate([EMIM]PF6) and Cr(III)-1-ethyl-3-methylimidazolium bisulfate([EMIM]HSO4) are scarcely reported. Therefore, the technology and electrochemical mechanism of chromium plating using [EMIM]PF6 and [EMIM]HSO4 ionic liquid as electrolytes are investigated by cyclic voltammetry, linear sweep voltammetry, chronopotentiometry, electrochemical impedance spectroscopy and chronoampererometry measures. The main contents and conclusions as follows:1-ethyl-3-methylimidazolium hexafluorophosphate([EMIM]PF6) and 1-ethyl-3-methylimidazolium bisulfate([EMIM]HSO4) ionic liquid are synthesized using two step method, respectively. Their structures are characterized and confirmed by IR. Electrochemical windows of [EMIM]PF6 and [EMIM]HSO4 are measured by cyclic voltammetry. Their values are 3.0 V and 3.6 V, respectively.Secondly, the electrochemical mechanism of Cr(III) reduction and chromium nucleation/growth process on a glassy carbon(GC) electrode in [EMIM]PF6 and [EMIM]HSO4 are studied by cyclic voltammetry, linear sweep voltammeter, chronopotentiometry, electrochemical impedance spectroscopy and chronoamperometry. These results from the cyclic voltammetry show that the reduction of Cr(III) occurs by a two-step process, Cr(III) to Cr(II) and Cr(II) to Cr(0), respectively, the two steps are an irreversible process with a diffusion coefficient of Cr(III) in solution of 4.58 × 10-7 cm2 s-1 and 4.70 × 10-7 cm2 s-1 at 323 K, the apparent activation energy(Ea) for the diffusion process is about 41.8 k J mol-1 and 29 k J mol-1. Moreover, the two-step reduction reaction has been identified by linear sweep voltammetry, electrochemical impedance spectroscopy and chronopotentiometry in the two ionic liquids. Moreover, the electrochemical impedance spectroscopy aslo indicated chromium electrodeposition in [EMIM]PF6 and [EMIM]HSO4 are the diffusion control process at low frequencies and charge transfer step at high frequencies. Chronoampererometry experiments reveal that the growth/nucleation mechanism of chromium in [EMIM]PF6 is a three-dimensional instantaneous nucleation behavior, the growth/nucleation mechanism of chromium in [EMIM]HSO4 is dependent on deposition voltage of more progressive in character at lower overpotentials and a three-dimensional instantaneous nucleation behavior at higher overpotentials.At last, the composition and morphology of Cr deposits are investigated using scanning electron microscope(SEM), X-ray diffractogram(XRD) and energy-dispersive spectroscopy(EDS). The result reveals that the coatings obtained from [EMIM]PF6 are amorphous chromium and the coatings obtained from [EMIM]HSO4 are crystalline chromium. These effects of electrodeposition current density, electrolyte temperature and electrodeposition time on these coatings thickness and electrodeposition rate in [EMIM]PF6 and [EMIM]HSO4 ionic liquid have been studied. The optimum condition for preparing chromium coatings from [EMIM]PF6 ionic liquid as follow: electrodeposition current density is 45 m A·cm-2, electrolyte temperature is 45℃, electrodeposition time is 50 min. The optimum condition for preparing chromium coatings from [EMIM]HSO4 ionic liquid as follow: electrodeposition current density is 50 m A·cm-2, electrolyte temperature is 50℃, electrodeposition time is 50 min. The polarization result indicate the corrosion resistance of chromium coatings obtained from [EMIM]HSO4 is better than obtained from [EMIM]PF6.