The growing global demand for clean energy sources encouraged scientists in industry and academia to explore renewable energy sources such as solar energy. Solar energy can be converted via solar collectors into thermal energy. This conversion might have its efficiency amplified by the use of special coatings, called selective surfaces, which absorb the maximum radiation and emit a little1. The selection of the most suitable deposition technique is crucial to ensure from the highest possible quality and good system performance. Several deposition techniques have been reported for obtaining selective solar coatings with high performance. However, among all those techniques, electrodeposition technique excels for its simplicity, low cost and possibility of large-scale production, forming films with superior hardness and mechanical resistance in addition to excellent optical and thermal proprieties. It has been reported that the maximum radiation absorption can be obtained from aluminum or copper plates coated with black paints 2.
Black chromium is one of the most commonly used coatings for solar thermal application due to its distinguished properties such as corrosion resistance3, high absorption coefficient and low emittance coefficient 4,5. In addition to these optical characteristics, it is essential that the coating does not suffer any degradations, corrosion, or erosion during the lifetime of the collector. Other researchers developed black coatings of nickel on stainless steel substrate for solar thermal cells 6. A previous study showed that when an element is “mixed” with other metals, advanced materials with amazing properties could be revealed and, from the economic point of view, the price of final products would be decreased, which might be of particular advantage for industrial applications7. Consequently, researchers showed that when Ni mixed with Co at specific percentage, the corrosion resistance and solar absorbance can be enhanced 8 9. Accordingly, this study is focusing on exploring the properties of the formed coating as a result of the deposition of a mixture of binary elements (Cr and Ni) over conventional Cr coating10. For more enhancements the incorporation of some ceramic particles to create Cr composite has a significant influence on the tribological properties11.
The ceramic Zinc oxide (ZnO) has attracted much attention as one of the most promising materials for the antireflection coating (ARC) of solar cells because of good transparency and refractive index. ZnO has a direct band gap (3.37 eV), a high excitation binding energy (60 m eV) at room temperature (RT), with such characteristics ZnO becomes a promising material in various optoelectronic applications12. Otherwise, It was reported that when ZnO doped into the coating, the corrosion resistance can significantly improve 13,14 and shift the energy converters down by light capture 15. Some researchers had reported the influence of ZnO on corrosion protection while others emphasized its effect on solar application. In this paper, we focused on both directions; influence of ZnO on binary CrNi coatings as anticorrosion and anti-emission (solar collector) of the deposited coatings. Based on the results, we determined the optimum condition in terms of microstructure, chemical composition, absorption, bending, adhesion and corrosion resistance of the deposited black films.