Design of chemical composition and optimum working conditions for trivalent black chromium electroplating bath used for solar thermal collectors

Black chromium is one of the selective surfaces which are commonly used in solar thermal systems. The aim of this research was to design an electroplating bath, using chromium trivalent ions instead of its hexavalent ions because of its toxicity, for black chromium coating to produce a good metallurgical and light properties at low cost and reproducibility. The effect of the additives was investigated and a coating with an absorption coefficient of 0.96 was prepared. The bath contained inexpensive constituents such as chromium sulfate, cobalt chloride, sodium fluoride, sodium hypophosphite and sodium dihydrogen phosphate and did not need any pH control. Thermal resistance, corrosion resistance and adhesion of the coatings as well as throwing power of the bath were investigated. The effect of bright Ni as an undercoat before black chromium electroplating was also investigated. SEM and XRD techniques were employed to characterize the surface microstructure and chemical composition. Spectroscopy was also used to measure absorptance of the coatings.