Oxidation of Flash-Anodized Al-Mg Alloy in Dry Atmospheres of N 2 , CO 2 , and O 2 and Humid Atmosphere of Ar
- PDF / 4,844,333 Bytes
- 17 Pages / 593.972 x 792 pts Page_size
- 10 Downloads / 202 Views
INTRODUCTION
IN the United States, approximately 35 pct of yearly aluminum consumption is from recycled aluminum from old scrap, such as consumer products,[1] where the use of anodized aluminum is common due to its increased corrosion[2] and wear resistance,[3] as well as its esthetic nature[4] and better adhesion for paint primers.[5] In recycling, mixtures of Al alloys with different surface compositions and properties are melted together in a large bath. The interactions between the various alloying elements and surfaces play a major role in the effectiveness and efficiency of the entire recycling process. Heat transfer[6] to the dross surface is by convection and radiation from flames and wall. The temperature on the surface exceeds the melting point of the alloy. In the current study, the oxidation of the flash-anodized Al-Mg alloy surfaces are investigated at 1072 K (800 C), taken as the atmosphere temperature in a combustion rotary furnace.[6] The rotary furnace is one of the typical furnace types used in the recovery of aluminum from secondary scrap and dross, where natural gas is burned,[7] giving mainly water and CO2 in the furnace atmosphere. Also oxy-fuel flame can be used with 50 pct excess oxygen. This work is aimed to improve the understanding of oxidation kinetics in order to reduce oxidation of the melt and to decrease the overall metal loss during remelting. Flash-anodized Al samples were oxidized in
dry N2, O2, and CO2, and Ar with 5 mol pct H2O in a Thermogravimetric Analyzer (TGA). Flash-anodized samples were chosen based on the similarity to aluminum used in recycling processes. In a well-defined system, flash-anodized samples have been shown to display consistent oxidative behavior compared to natural formed oxides.[8] The elemental composition of samples after oxidation was analyzed using X-ray photoelectron spectroscopy (XPS). Focused ion beam (FIB)-scanning electron microscopy (SEM) was used to cut out and image a cross section of the oxide layer as well as the top surface. Figure 1 is a schematic (not to scale) of a typical porous type anodized alumina, indicating the pores and hexagonal cells. The pores in the anodic aluminum oxide (AAO) extend from the oxide surface to just above the Al bulk. During heating, phase change of the oxide and vapor pressure from the molten Al will cause a build-up of stress in the oxide layer, a phenomenon well known to initiate cracks. Any liquid with good wetting properties in a sufficiently small volume will rise due to surface tension, normally referred to as capillary action. Temperatures in excess of 993 K (720 C) will cause molten Al to have a contact angle of less than 90 deg on a substrate of Al2O3, indicating wetting. Additions of certain alloying elements, such as Mg will cause the contact angle to drop, due to creation of the oxide phases on the surface.[9]
II. KYLE LAPOINTE, Internship Student, SARINA BAO, Research Scientist, and ANNE KVITHYLD, Senior Research Scientist, are with the SINTEF Materials & Chemistry, 7465 Trondheim, Norway. Contact e-mail
Data Loading...
