Technology And Mechanism Of The Electrodeposition Of Chromium Coatings From Imidazolium Ionic Liquids

Chromium coating has been widely used for electronic material,corrosion prevention, decorative material because of their high hardness,corrosion resistance and excellent decorative characteristics. At present,the technology of electroplating chromium plating from aqueoussolutions has two major problems: firstly, plating solutions need to addsome organic additives, the chromium electrodeposition from Cr3+electrolytes is the participation in the process of organic additives fromsolution. This participation results in the appearance of chromium carbidecompounds in deposits indicating that they are generally amorphous;secondly, the chromium electrodeposition from Cr3+aqueous solutions isalways accompanied by intensive hydrogen evolution reaction resultingin profound effect on the quality of chromium deposits. In contrast toaqueous solutions, ionic liquids can provide an ideal kind of solvents forthe electrodeposition of chromium since they usually provide a wideelectrochemical window, low volatility and high electrical conductivity.Thus, using ionic liquid as for electrolysis is not only can improve thequality of the coating without the occurring of hydrogen evolution, butalso will prevent chromium carbide from forming because of theirexcellent electrochemical stability, and the crystal chromiumelectrodeposition can be obtain without annealing at high temperature.Thus, the technique using ionic liquid as Cr3+electrolytes for the crystalchromium electrodeposition is simpler. Therefore, the technology andelectrochemical mechanism of chromium plating in1-butyl-3-methylimidazolium bromide ([BMIM]Br) and1-butyl-3-methylimidazolium hydrosulfate ([BMIM]HSO4) wereinvestigated, respectively.Firstly, two kinds of ionic liquid,[BMIM]Br and [BMIM]HSO4,were synthesized with one step and two steps, respectively. Theirstructures were characterized and confirmed by IR. Electrochemicalwindows of [BMIM]Br and [BMIM]HSO4were measured by cyclicvoltammetry. Their values were2.7V and3.0V, respectively.Secondly, the optimum operating conditions for electrodeposition of chromium from [BMIM]Br and [BMIM]HSO4were obtained by meansof orthogonal test method. The best parameters for the preparation ofchromium plating from [BMIM]Br ionic liquid: deposition voltage is-1.80V, time80min, temperature80℃; The best parameters for thepreparation of chromium plating from [BMIM]HSO4ionic liquid:deposition voltage is-1.90V, time100min, temperature80℃. Under theoptimum conditions, the chromium coating with the thickness of6.2μmin [BMIM]Br and the thickness of9.4μm thickness in [BMIM]HSO4were obtained respectively. The electrodeposits were characterized bySEM, EDS and XRD. SEM analysis showed that the chromiumelectrodeposits obtained on Cu electrodes were smooth, dense andcrack-free in [BMIM]Br and [BMIM]HSO4. EDS analysis showed thatthe coatings were composed of Cr without other elements, and XRDpattern of the electrodeposited layers revealed the characteristic peak ofcrystal Cr in the two ionic liquids.At last, the electrodeposition and electrocrystallization mechanismsof chromium reduction in [BMIM]Br and [BMIM]HSO4wereinvestigated by cyclic voltammetry, linear sweep voltammeter,chronopotentiometry, electrochemical impedance spectroscopy andchronoamperometry. The result from the cyclic voltammetry showed thatthe reduction of trivalent chromium (Cr3+) occurs by a two-step process,Cr3+to Cr2+and Cr2+to Cr0, respectively, and the two steps were anirreversible process with a diffusion coefficient of2.3×10-6cm2/s in[BMIM]Br and1.49×10-7cm2/s in [BMIM]HSO4. Moreover, the two-stepreduction reaction had been identified by linear sweep voltammetry andchronopotentiometry in the two ionic liquids. Electrochemical impedancespectroscopy indicated chromium electrodeposition in [BMIM]Br and[BMIM]HSO4were the diffusion control process at low frequencies andcharge transfer step at high frequencies. Chronoamperometry resultsprovided direct evidence that chromium electrodeposition took placethrough an instantaneous nucleation and three-dimensional growthmechanism with diffusion controlled in the two ionic liquids.