Oxide Film Structure on Aluminum Alloy 6000 Series Cylindrical Ingot Surface
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OXIDE FILM STRUCTURE ON ALUMINUM ALLOY 6000 SERIES CYLINDRICAL INGOT SURFACE D. V. Pronichev,1 L. M. Gurevich,2 O. V. Slautin,3 V. A. Parfenov,4 and V. O. Kharlamov5
UDC 669.715+66.018.8
Electron microscopy and energy dispersion analysis are used to reveal structural features of an oxide film in the region of white deposit areas that are visually detected on the surface of cylindrical ingots of 6000 series aluminum alloy after homogenizing annealing. It is shown that a loose film of aluminum and magnesium oxides with an anomalously high Mg content (up to 45 at.%) and micropores are typical for these areas. Diffusion flow of magnesium towards the surface during homogenizing annealing gives rise to its chemical potential gradient occurring as a result of oxidation processes occurring during annealing. Reasons are determined for the occurrence of white deposit areas connected with formation at an ingot surface of mineral salt oxides and organic compounds from dried liquid used for cooling during ingot semi-continuous casting. Keywords: 6000 series aluminum alloy, cylindrical ingots, semi-continuous casting, oxide film, white deposit, aluminum oxide, magnesium oxide, homogenizing annealing.
The majority of aluminum alloys, in particular those of the 6000 series of the Al–Mg–Si system, exhibit good corrosion resistance in water and a moist atmosphere that is connected with formation at a surface of a dense film of Al 2O 3 that is resistant to the action of many media [1–4]. Recently there has been an increase in the demand for ingots of the 6000 series of aluminum alloys in the countries of South-East Asia and America. During marine transport there is an increased risk of surface corrosion damage for ingots whose development may provoke various versions of structural inhomogeneity at a surface, in particular visually observed individual areas lighter in color, a so called “white deposit”. It is well known [5] that the main corrosion damage mechanism for alloys of the 6000 series is pitting corrosion connected with the difference in electrochemical potential of α -solid solution based on aluminum and strengthening phases Mg 2Si and Al 5 FeSi . Research has demonstrated [6–8] that corrosion of this type has almost no effect on the loss of alloy mechanical properties during prolonged action of a corrosive medium, imitating action of a moist marine atmosphere, nonetheless pitting corrosion at a surface is acknowledged as a defect, and in an ingot subsequently subjected to hot extrusion in order to prepare aluminum shapes of complex cross section. The aim of the present work is revelation of reasons for the occurrence of structural inhomogeneity of the “white deposit” on individual areas of an aluminum alloy ingot surface and a study of the effect of different production factors on oxide film structure. FGBOU VO Volgograd State Technical University, Volgograd, Russia. 1 E-mail: [email protected]. 2 3 4 5
E-mail: [email protected]. E-mail: [email protected].
E-mail: [email protected]. E-mail: [email protected].
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