Studies on the effect of the composition of wrought alloys (D16, AMg6 and V95) on the formation kinetics, structure, and electrochemical behavior of black plasma-electrolytic coatings

• A.G. Rakoch, Tran Van Tuan and E.P. Monakhova

Department of Metallurgy of Steel, New Production Technologies and Protection of Metals, College of Environmentally Sound Technologies and Engineering, National University of Science and Technology “MISiS”, Leninsky pr. 4, 119049 Moscow, Russian Federation

Abstract: In this study, the kinetics of anodic voltage growth and coating thickness on wrought alloys D16, V95 and AMg6 upon plasma-electrolytic oxidation in an aqueous solution containing 3 g/l NaOH, 5 g/l Na₂O·2.9 SiO₂·18 H₂O and 1.5 g/l CoOOH·H₂O were compared. Moreover, the structure of black plasma-electrolytic (PE) coatings formed on their surface and the type of dependence of electrochemical characteristics on their thickness and time of exposure in 0.5% aqueous NaCl solution were studied. The resulting nearly identical kinetic and electrochemical plots and the structure of black PE coatings on various wrought alloys allowed us to assume that the model perceptions of their formation, the type of dependence of the protective ability on their thickness and on the duration of exposure in aqueous solutions containing chloride ions do not depend on the nature of these alloys. At the same time, the phase composition and microhardness of the black PE coatings formed on these wrought alloys differ significantly. The values of electrochemical characteristics (impedance modulus, anodic and cathodic currents) also differ significantly, since they depend on the nature and concentration of alloying elements in these alloys. We suggest the most likely causes of this influence: 1) considerable removal of the elements from coatings having a much lower affinity to oxygen than aluminum and magnesium. This leads to an increase in their porosity, and, consequently, to a decrease in the protective properties of the coating inner layers, rapid solidification of molten local areas in them and, as a consequence, a decrease in the probability of α-Al₂O₃ formation; 2) formation of substitutional solid solutions: as oxides of alloying elements (Mg, Cu, Zn) dissolve in Al₂O₃, they “stabilize” its low-temperature modification (γ-Al₂O₃) since they increase the concentration of electron vacancies in the coatings and, as a result, the heat transfer to the metal base increases.

Keywords: plasma-electrolytic treatment, black coatings, aluminum alloys, electrochemical behavior, alloying elements

Int. J. Corros. Scale Inhib., 2022, 11, no. 4, 1763-1786
doi: 10.17675/2305-6894-2022-11-4-23

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