D. Mendeleev University of Chemical Technology of Russia (Moscow, Russia):

A. A. Abrashov, Cand. Eng., Associate Prof., Dept. of Innovation Materials and Corrosion Protection, e-mail: ngrig108@mail.ru
N. S. Grigoryan, Dr. Chem., Prof., Dept. of Innovation Materials and Corrosion Protection, e-mail: vetolmyan@gmail.com
Ya. V. Tolmachev, Student, Dept. of Innovation Materials and Corrosion Protection, e-mail: asnis@mail.ru
N. A. Asnis, Cand. Tech., Leading Engineer, Dept. of Innovation Materials and Corrosion Protection

A solution containing 8–12 g/l (NH₄)₆Mo₇O₂₄, 4–12 g/l Na₂B₄O₇·5H₂O, 1–2,5 g/l (NH₂OH)₂·H₂SO₄ and 3,5-4,5 g/l (NH₂)₂CO was developed. Аt pH = 5.0–7.0 and at a temperature of 85 °C it allows for 10 minutes to form protective-decorative black coatings on the surface of low-carbon steels that are not inferior in protective ability and decorativeness to coatings obtained from a standard alkaline solution of black oxidation. Solutions containing ammonium heptamolybdate (NH₄)₆Mo₇O₂₄ as the main component and sodium persulfate Na₂S₂O₈ (solution 1) or sodium tetraborate Na₂B₄O₇·5H₂O (solution 2) as oxidants were studied. It has been established that high-quality black coatings are obtained in the concentration range of 12–20 g/l (NH₄)₆Mo₇O₂₄; 12–20 g/l Na₂S₂O₈ (solution 1) and 8–12 g/l (NH₄)₆Mo₇O₂₄; 4–12 g/l Na₂B₄O₇·5H₂O (solution 2). However, the protective ability of the formed coatings was low, it was determined by the drip express method and was 12–14 s. In order to improve the protective characteristics of the coating, zinc, aluminum, magnesium, or calcium ions were introduced into the solutions. The introduction of zinc or aluminum ions into solutions in an amount of 2.5–3.5 g/l leads to an increase in protective ability by about one and a half to two times. A positive effect on the protective ability of coatings from the introduction of calcium and magnesium ions was not observed. A negative consequence of the introduction of these ions into the solutions was a partial loss of the black color of the coatings, the color depth decreased from 10 to 8 points (with the introduction of Zn²⁺ and Al³⁺ ions) and to 9 points (with the introduction of Ca²⁺ and Mg²⁺ ions). It was found that the introduction of 1.0–2.5 g/l hydroxylamine into solutions led to a substantial increase in the protective ability of coatings: the protective ability of coatings formed in solution 1 increased from 12–14 to 100 s, and in solution 2, it increased to 300 s. As for urea, it did not affect the protective ability of the coatings, but it allowed to lower the temperature of solution 2 from 98 to 85 °C when it was contained in the solution at a concentration of 3.5–4.5 g/l. The protective ability of the coatings does not change much, and the quality and color of the oxide layers do not deteriorate.
The work was carried out with the financial support of the D. Mendeleev UCTR. Project number X-2020-028.

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