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.
Oxide coatings are widely used as protective and decorative coatings on ferrous metals under non-harsh operating conditions. For protective and decorative oxidation, a chemical oxidation method is widely used using concentrated alkaline solutions (600-800 g/l NaOH) operating at very high temperatures (130-150 °C). The obvious disadvantages of the process are the high energy consumption, difficult working conditions, the aggressiveness of the solutions used and low protective ability of the coatings due to their high porosity. In addition, it is known that to harden the surfaces of machine parts in order to increase wear resistance, they are thermally laser treated. The surface hardening is the more effective, the higher the absorption coefficient of the treated surface. For laser heat treatment, high-temperature (95-98 °C) processes of applying black phosphate coatings with a high absorption coefficient (0.8-0.9) are used. The disadvantages of these processes are also the instability of solutions and the irreproducibility of the results. This work is about to the study of the process of formation of phosphate-selenide coatings on steel in order to replace high-temperature solutions of black oxidation and phosphating. A low-temperature process has been developed for the deposition of black selenide-containing phosphate coatings on steel, corresponding to a point of 10 on a ten-point color scale, in a solution containing: 4-8 g/l Na 2 SeO 3 ; 1-4 g/l CuSO 4 ⋅5H 2 O; 0.5-5 g/l NaH 2 PO 4 and 0.25-2.5 g/l Na 2 HPO 4 ([NaH 2 PO 4 ]/[Na 2 HPO 4 ] molar ratio is 2); 5 min at pH = 2-3, t = 18-25 °C. Oiling the coatings in I-20A industrial oil for 2 min leads to an increase in the protective ability of the coatings according to Akimov from 1 to 25 min. Corrosion tests of coatings in a salt spray chamber in accordance with ASTM B117 have shown that oiled phosphate-selenide coatings have the greatest protective ability: the first spots of red corrosion appear after 20 hours of testing, while oxidized, oiled samples begin to corrode after 18 hours. After 100 hours of testing, the area of the oxidized sample affected by corrosion is 90 %, and the oiled phosphated sample is 50.
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