Hard chromium composite electroplating on high-strength stainless steel from a Cr(III)-ionic liquid solution

A composite chromium coating has been potentiostatically electroplated on high-strength stainless steel substrates from a trivalent chromium bath. The electrolyte solvent consists of 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) to which water, hexadecyltrimethylammonium bromide (CTAB), poly(diallyldimethylammonium chloride) (PDDA), and Al2O3 particles (≈3–4 μm) were added to improve the ion transport properties, the wettability at the electrolyte–substrate interface, the metallic chromium content, and the microhardness of the coating, respectively. The X-ray photoelectron spectrum of the coating reveals 85% metallic chromium and 15% chromium oxides and chromium hydroxide. Characterization of the coating shows the existence of chromium carbide-type bonds, resulting from the incorporation of carbon atoms into the chromium crystal lattice, leading to a high degree of amorphization of the coating. Optimization of electroplating conditions yielded a uniform chromium composite coating with a Vicker's microhardness of 860 (±10) HV and a thickness of 42 μm (±4), which is comparable to a chromium coating obtained from conventional chromic acid baths. Keywords: Trivalent chromium, Electroplating, XPS, High-strength stainless steel, Ionic liquid