Black decorative anticorrosion coatings obtained on AA2024 alloy by plasma–electrolytic treatment and inhibition

• A.G. Rakoch¹, Yu.I. Kuznetsov², Tran Van Tuan¹, Z.V. Khabibullina¹, A.A. Gladkova¹, A.A. Chirkunov² and A.M. Semiletov²

¹ Department of Metal Protection and Surface Treatment, National University of Science and Technology “MISiS”, Leninsky pr. 4, 119049 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: This research work is concerned with developing a model of black anticorrosion coatings growth during plasma electrolytic treatment of a widely used industrial alloy AA2024 and investigation of corrosion inhibitor use efficiency depending on a coating thickness. The inhibitor used in this study is an equimolar composition of sodium oleoyl sarcosinate and sodium flufenamate. Assuring that addition of 1.5 g/l Co(OH)₃ into an alkaline aqueous solution (pH~12.6), containing 5 g/l of technical water glass, leads to the formation of visually black coatings with higher anticorrosion properties, the following assumptions were made: 1) the visual black color of a coating is mainly provided with the existence of CoO and Co₃O₄ oxides; 2) the increase in the anticorrosive properties of the coatings is caused by the filling of many open pores in the coatings with cobalt orthosilicate which is more fusible than aluminum oxide. Major filling of the pores with the double oxide takes place on the surface of a forming inner layer prior to its intensive growth under the outer layers of the coatings. As a result, a coating with a thickness of 17 μm has higher anticorrosion properties than the ones with thicknesses of 38 and 70 μm. Results of energy dispersive X-ray spectroscopy and review of the data given in the published scientific works confirm the possibility of the abovementioned hypothesis. The efficiency of the corrosion inhibitor is also higher for a coating with a thinner inner layer, when it has already started to be formed under porous outer layers of the coating. The black coating with an average thickness of 17 μm, loaded with the mixed inhibitor, shows the maximum decrease (by approximately 467.4 times) in the corrosion current density, i.e. the greatest protection of AA2024 alloy against corrosion.

Keywords: aluminum alloy, plasma electrolytic oxidation, corrosion protection, corrosion inhibitor

Int. J. Corros. Scale Inhib., 2021, 10, no. 2, 562-579 PDF (1 142 K)
doi: 10.17675/2305-6894-2021-10-2-5

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