Conversion of solar to thermal energy in solar collectors is achieved through selective coatings on metal substrates. The main characteristics of the selective coatings are a high solar absorptance and low thermal emittance, metallic substrates resistant to high operating temperatures and atmospheric corrosion. In this work, selective black cobalt solar coatings electrochemically deposited onto stainless steel (AISI 304) and Inconel 600 substrates, with an intermediate layer of bright nickel as an infrared reflector, are characterized. The transformation capacity of solar-to-thermal energy depends on the thickness of the films, which depends on the galvanostatic deposition time. Hence, by controlling the charge applied in the electrochemical deposition process, the optical properties can be optimized. The black cobalt coating used in this work has the characteristic of high absorption in the solar spectrum and low emission in the infrared, even after a thermal treatment of up to 500 °C on stainless steel substrates (AISI 304).

The thermal stability of the selective coatings was studied by means of a thermal treatment in the range of 300 and 750 °C in air, observing the change in the optical properties due to the influence on the solar absorptance and the thermal emittance. The coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The Vis – IR reflectance were evaluated before and after the heat treatment using an integrating sphere (2 μm to 15 μm). The black cobalt coatings presented homogeneity, stability, good adhesion and solar absorptance of 96% and thermal emittance of 7% (calculated at 100 °C). Results suggest that the electrochemically deposited black cobalt coating (UV-Vis absorber/IR reflector/metallic substrate) can be scaled-up and applied at medium to high temperatures for the conversion of solar radiation into heat.