Passivation of the surface of AZ31B magnesium alloy in solutions based on salts of rare-earth metals

• A.A. Abrashov¹, N.S. Grigoryan¹, M.A. Simonova¹, T.A. Vagramyan¹ and I.A. Arkhipushkin²

¹ Federal State Budgetary Educational Institution of Higher Education “Russian Chemical-Technological University named after D.I. Mendeleev”, Miusskaya sq., 9, 125047 Moscow, Russian Federation
² A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky pr. 31, 119071 Moscow, Russian Federation

Abstract: A solution has been developed for applying protective-adhesive lanthanum-, manganese-containing coatings under paintwork on the surface of magnesium alloy AZ31B instead of high-toxic chromate coatings, as well as independent anticorrosive coatings under mild operating conditions, for example, for inter-operation storage of products. A solution containing 1–6 g/l La(NO₃)₃∙6H₂O and 0.5–2 g/l KMnO₄, allows at рН 2.0–3.0 and at a temperature 18–30°С to form conversion coatings consisting of La₂O₃, MnO₂, MgO, Mg(OH)₂ on the surface of AZ31B magnesium alloy in 2 minutes. Titanium-, zirconium- and lanthanum-, manganese-containing coatings can be used as adhesion layers under paintwork instead of high-toxic chromate coatings, and also as independent anticorrosive coatings under mild operating conditions, for example, for inter-operation storage of products. Using ellipsometry, it was found that the thickness of the cerium-containing coatings is 175±5 nm. Corrosion tests in a salt-spray chamber according to ISO 4536 (SD test) ASTM B117 have shown that the developed lanthanum-, manganese-containing coatings are superior in protective ability to the currently widely used chromate coatings and are not inferior to titanium-, zirconium-containing coatings. The developed coatings also have good adhesion properties. Ti-, Zr- and La-, Mn-containing coatings in terms of protective characteristics and adhesion strength with paintwork are not only not inferior to the currently used chromate coatings but even surpass them.

Keywords: corrosion protection, conversion coatings, cerium-containing coatings, lanthanum-containing coatings, titanium-, zirconium-containing coatings, surface treatment, chromate-free passivation, rare-earth metals

Int. J. Corros. Scale Inhib., 2021, 10, no. 3, 961-975
doi: 10.17675/2305-6894-2021-10-3-9

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