Electroplating of Thick Hard Chromium Coating from a Trivalent Chromium Bath Containing a Ternary Complexing Agent: A Methodological and Mechanistic Study

Abstract

Trivalent chromium electroplating is considered a greener alternative to hexavalent chromium electroplating. However, the electrochemical reduction of Cr(III) in aqueous solution is extremely unstable and the thickness of the coating can hardly grow beyond 10 μm, which limits its industrial applications. We herein report a trivalent chromium bath containing a ternary complexing agent, which yields bright and uniform chromium coatings with thickness exceeding 30 μm, as well as high hardness and corrosion resistance. The electroplating behaviors and first principal calculations reveal that the Cr(II) intermediate plays a vital role in sustained electroplating in Cr(III) baths. The composition of complexing agents has a profound effect on the geometry and electron accepting ability of the Cr(II) complex ions. The failure to grow thick Cr coating is attributed to the accumulation of inactive hydroxo-bridged complexes. In the bath with ternary complexing agent, the Cr(II) complex ions are dsp3 or dsp2 hybridized with high electron affinity, which can be readily reduced to metallic Cr. The bath also shows a strong competitive ligand-binding effect that renders Cr(II) to preferentially bind with an organic ligand rather than a hydroxy ligand. Furthermore, the planar geometry of dsp2 hybridization does not favor the formation of hydroxo-bridged complexes because of the steric hindrance effect. The above merits of the ternary complexing agent lead to sustained electroplating, and thick Cr coatings are obtained.