Deposition of ultra-black nickel phosphorus coatings

Absorbing surfaces of solar receivers are coated with black chrome, which provides the surface of steel or aluminum tubes with good absorption properties. However, the chrome coating does not protect well against corrosion because of its high porosity and high internal stresses, which cause a cracking of coatings. In this paper, we investigated two methods for formation of black coatings on chemically formed Ni-P layers: etching in nitric acid and anodic treatment in a phosphoric acid solution. It was found that in the course of processing the black coatings, their roughness and porosity increase. It was shown that nickel oxides NiO and Ni2O3 and nickel phosphates are presented in the coating composition. It was established that the heat treatment increases the wear resistance and protective ability of black coatings.