Electrodeposition of black chromium from environmentally electrolyte based on trivalent chromium salt

Thin films of black chromium have been prepared by electrodeposition technique on steel substrates. Deposition of 1 µm thickness is conducted in a solution of chromium trivalent salt and oxidizing agent fluorosilicic acid (H2SiF6) operated at a current density 350 mA cm− 2 at 25 °C and 1 min. The influence of H2SiF6 concentrations on the trivalent chromium electrodeposition was studied by the potentiodynamic technique. The phase structure and surface morphology of the deposited films were examined by using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The chemical composition of these thin films was determined by energy dispersive X-ray analysis (EDS) and X-ray photoelectron spectroscopy (XPS) analysis. The spectral reflectance in the visible light, for films coating was characterized. From the SEM analysis, it was found that the black chromium has nano lamellar morphology that leads to a strong dispersion level. The XRD results showed that chromium, chromium oxide and cobalt oxide are the main bulk chemical compounds in the films. However, from XPS analysis of these surfaces, it was possible to determine that the most external layers of the films are made of different kinds of chromium and cobalt compounds. The black chromium–cobalt alloy thin film has better optical properties to transform solar energy into thermal energy, and these properties remain practically constant even when heat treated to a high temperature, 400 °C for 24 h.