Low-temperature deposition process for black phosphate-selenide coatings

Abstract

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 Na2SeO3; 1–4 g/l CuSO4⋅5H2O; 0.5–5 g/l NaH2PO4 and 0.25–2.5 g/l Na2HPO4 ([NaH2PO4]/[Na2HPO4] 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. The work was carried out with the financial support of the Mendeleev University of Chemical Technology. Project number X-2020-027.