Electrochemical aspects of black chromium electrodeposition from 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid

The electrochemical behaviour of trivalent chromium (Cr3+) in 1-butyl-3-methylimidazolium tetrafluoroborate [BMIm][BF4] ionic liquid solutions was studied by cyclic voltammetry and chronoamperometry. The reduction of Cr3+ occurs in two steps, Cr3+ to Cr2+ and Cr2+ to Cr0, respectively. The first step is quasi-reversible with a diffusion coefficient of Cr3+ in solution of 3.13 × 10−8 cm2 s−1 at 303 K and 25.8 × 10−8 cm2 s−1 at 358 K, estimated from cyclic voltammetry data.Black chromium films were electrodeposited on copper, stainless steel and carbon steel substrates at a constant potential of −1.5 V vs Pt quasi-reference electrode. The films consist of aggregates of nanosized particles. The coatings in the as-deposited condition present an amorphous structure but after annealing for 4 h, a nanocrystalline Cr2O3 phase is formed, with an average crystallite size of 17 nm.

► Black chromium electrodeposition was enabled on copper, nickel and steel substrates using [BMIm][BF4] ionic liquid as a solvent and a Cr(III) salt precursor, without additives.
► The reduction of Cr(III) in [BMIm][BF4] ionic liquid occurs in two steps with an intermediate Cr(II) species.
► The diffusion coefficient of Cr(III) species was determined from cyclic voltammetry and chronoamperometry data with excellent agreement.
► The variation of the diffusion coefficient and viscosity with temperature within the range of 303-358 K has been analyzed.