Influence of current density and duration of PET of AA2024 alloy on the rate and growth mechanisms of a coating’s wear-resistant anticorrosive inner layer

• A.G. Rakoch¹, Z.V. Khabibullina¹, O.V. Volkova¹, A.V. Borko¹, Tran Van Tuan¹, I.V. Suminov² and S.V. Zhukov²

¹ Department of Metal Protection and Surface Treatment, National University of Science and Technology “MISiS”, Leninsky pr. 4, 119049 Moscow, Russian Federation
² Moscow State University of Technology “STANKIN”, Vadkovskiy per. 1, 127055 Moscow, Russian Federation

Abstract: In this study the growth mechanisms of two-layered hard anticorrosive PE coatings on AA2024 alloy are given. During PET the following phenomena take place: i) The formation of a coating’s outer layer mainly occurs as a result of the melted aluminum oxide transportation onto a coating’s surface, accompanied with the influence of a large amount of the gas-vapor phase on it, which happens right after the microdischarges extinction in coating’s transverse pores. ii) The growth of coating’s inner layers occurs via two mechanisms. The first one is based on the formation of melted aluminum oxide without its transportation onto a coating’s surface. The ignition of anodic plasma microdischarges under the coating’s outer layer results in the sealing of coating’s inner layer’s pores and the adjacent ones that are located in the outer layer. This process takes place during the ignition of microdischarges under the anodic “semiperiod” until the voltage reaches its maximum value. The second mechanism is based on the convective and diffusive transfer of aluminum and oxygen ions through the melted parts of the coating’s inner layers which are adjacent to the powerful microdischarges. These microdischarges appear in the coating’s transverse pores generally when the voltage reaches its peak values. The second mechanism of the PE coatings’ growth is prevalent when the pores amount in the inner layer decreases significantly. iii) The rapid growth of a coatings’ average thickness from 50 to 80 μm with or without a slight average dimensional one-side change of the samples might be caused by the consumption of aluminum for both the growth of PE coating and the sealing of the voids and transverse pores which are particularly located in the inner layers. iv) The significant raise of a current density which is applied for processing PE treatment of AA2024 alloy, results only in the increase of the coatings’ growth rate. There is no impact on the growth of coatings’ thickness which occurs via different mechanisms. It was demonstrated that to synthesize a wear-resistant anticorrosive inner layer, the PE processing under the alternating current densities of 15–45 A/dm² should be conducted till the coating’s thickness reaches more than 80 μm.

Keywords: aluminum alloy, plasma electrolytic treatment, coating’s growth mechanism

Int. J. Corros. Scale Inhib., 2021, 10, no. 4, 1621-1637
doi: 10.17675/2305-6894-2021-10-4-15

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