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Investigation of aqueous surface processes and corrosion mechanisms of Al-(Cu)-Fe-Cr quasicrystaline approximant
|Ewa Ura-Bińczyk 1, Krzysztof J. Kurzydlowski 1, Malgorzata Lewandowska 1, Patrik Schmutz 2, Thomas Suter 2
1. Warsaw University of Technology, Faculty of Materials Science and Engineering (InMat), Wołoska 141, Warszawa 02-507, Poland
Because of the unique properties of quasicrystals and approximants in terms of microstructure, adhesion and inherent hardness, they are promising materials for coatings of aluminium alloys when good corrosion resistance is needed. However until now, no comprehensive studies on their corrosion mechanisms have been performed. In this work, two stable approximant phases- a gamma-brass phase (γ-AlCrFe) and a mixture of two orthorhombic approximants of the decagonal phase (O-AlCuFeCr) – have been investigated.
The uniform corrosion rate, electrochemical reactions and localized attack susceptibility have been studied in aqueous environments (0.1M Na2SO4 of pH 2, pH 5.5 and pH 12, 0.1M NaCl and 1M NaCl) using Electrochemical Impedance Spectroscopy (EIS), potentiodynamic polarization and Microcapillary Electrochemistry techniques. Polarization measurements in sulphate solutions show that the quasicrystalline approximant are particularly resistant to uniform corrosion and their electrochemical behaviour is comparable to stainless steel. A passive oxide layer showing low current density and stabilizing with ageing time (EIS characterization) is forming in a wide pH range. The presence of a transpassive dissolution is also typical for chromium containing stainless steel. The used sintered samples contain a number of weak points inducing a high susceptibility to localized attack. By characterizing micrometer scale areas with the microcapillaries, the material specific localized corrosion susceptibility can be assessed and is then similar to Stainless Steel.Auger Electron Spectroscopy (AES) surface composition investigation and in particular depth profiling experiments have shown that the oxide layer is still nearly pure aluminium hydroxide. Mott-Schottky analyses indicate semi-conductive properties of the oxide layer on approximant phases in contradiction with the insulating nature of Al hydroxide. All these results point out on very special properties for these surfaces.
Presentation: Oral at E-MRS Fall Meeting 2007, Symposium D, by Ewa Ura-Bińczyk
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-14 08:47 Revised: 2009-06-07 00:44