A black chrome plating process using trivalent chromium and water-tolerant, ethaline-based ionic liquid baths

In this study, chromium plating was conducted using a deep eutectic solvent ionic liquid mixture of choline chloride, ethylene glycol, and hydrated trivalent chromium salt, containing varying amounts of added water (i.e., 3, 6, 9, and 12 mol). Physicochemical analysis showed that the addition of water improved the conductivity and reduced the viscosity of the ionic liquid bath. Cyclic and linear scan voltammetry revealed that chromium deposition involves a two-step reduction for the Cr +3 ion. The plating system’s cathode efficiency initially increased with water content, reaching a maximum of about 45% in the bath containing 6 mol of added water. Beyond this water addition, cathode efficiency subsequently declined. A black chromium deposit with a nodular microstructure was observed. The best deposit was also derived at the bath with an additional 6 mol of water. The chromium coatings possessed an average microhardness of ca. 580 HV, which approximates the properties of industrial hard chrome. Graphic abstract