Crystalline phosphate coatings with enhanced protective properties

• A.A. Abrashov, N.S. Grigoryan, T.A. Vagramyan, A.V. Sundukova, D.V. Mazurova and T.A. Chudnova

Mendeleev University of Chemical Technology of Russia, 9 Miusskaya Sq., 125047 Moscow, Russian Federation

Abstract: A technology in which crystalline phosphate coatings are deposited at room temperature on 08ps grade steel was developed. A solution widely used in Russian industry has been selected as a prototype. This solution based on the KF-7 concentrate is used to apply independent crystalline anticorrosion coatings before oiling and formation of adhesion phosphate layers under paint-and-varnish coatings. The solution contains (g/L): ZnO 9.94; H₃PO₄ 18.98; HNO₃ 7.93; Ni(NO₃)₂·6H₂O 0.30; FeCl₃·6H₂O 0.28; A_free,eq 4.0–4.3; A_total,eq 34–40; pH 2. Coatings formed in this solution at a temperature of 70°C in 12 minutes are characterized by a phosphate layer mass of 5–10 g/m² and a protective ability (PA) of 35–45 s. It was found that in the base solution in which nickel cations are replaced with lanthanum ones with a concentration of 0.06–0.15 g/L, coatings are deposited at a temperature of 70°С with a mass of 6–6.5 g/m² and a PA of 45–50 s. If the solution temperature is lowered to 35°С, the quality of coatings deteriorates and their mass and protective capability are reduced. It was determined that if lanthanum cations are added to the phosphating solution together with an additional nitrogen-containing promoter, namely hydroxylamine, the solution temperature can be lowered to 20°С without deteriorating the coating properties. A low-temperature (t=20°С) crystalline anticorrosion phosphating solution was developed. A solution that contains La³⁺ ions makes it possible to deposit phosphate coatings with a mass of 4.5–4.9 g/m², which can be used in combination with an oil or another inhibiting impregnating agent for the protection of the metal base against corrosion. It was been shown that coatings can be deposited from the developed solution without a preliminary activating stage while preserving the coating properties.

Keywords: corrosion protection, phosphate coatings, anticorrosion coatings, low-temperature phosphatizing, rare-earth metals, lanthanum

Int. J. Corros. Scale Inhib., 2023, 12, no. 1, 160-169
doi: 10.17675/2305-6894-2023-12-1-9

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