On the Corrosion Mechanisms of Al-Cu-Fe Quasicrystals
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On the Corrosion Mechanisms of Al-Cu-Fe Quasicrystals A. Rüdiger, U. Köster * Department of Chemical Engineering, University of Dortmund, D-44221 Dortmund, Germany * Corresponding author: tel.: ..49-231-7552678, fax.: ..49-231-7555978, e-mail: [email protected] Abstract The mechanisms of dissolution and passivation of bulk polycrystalline icosahedral Al63Cu25Fe12 during electrolytic corrosion in 0.1 N NaOH and 1 N H2SO4 were studied in detail. In 1 N H2SO4 selective dissolution of Al and Fe occurs at the open circuit potential, which leads to a porous layer of recrystallized fcc Cu; after anodic polarization dissolution of the alloy is followed by redeposition of Cu and formation of Cu2O. In 0.1 N NaOH selective dissolution of Al was observed and a nanocrystalline layer consisting of Cu2O and FeOOH forms at the open circuit potential as well as during anodic polarization up to -150 mVSHE. At higher potentials Cu-ions dissolve into the electrolyte, which allows building up an Al-hydroxide layer. Presumably due to the loss of Al during polarization phase transformations of the quasicrystals (e.g. continuous transformation or precipitation) were observed by transmission electron microscopy. In order to clarify the influence of the quasicrystalline structure on the corrosion, open circuit potentials and current densities of the quasicrystalline and three ternary crystalline phases with different Al-content, but approximately constant ratio Cu:Fe of 2:1 were compared. Lower current densities, but a less noble open circuit potential for the quasicrystals than expected from the trend over the Al-content indicate a small influence of the quasicrystalline structure. Keywords: Quasicrystal, corrosion, immersion, polarization, continuous phase transformation, Al-Cu-Fe
Introduction For any application of quasicrystalline coatings because of their good wear resistance and exciting low coefficient of friction a high corrosion resistance is of utmost importance. Therefore, it is necessary to
ensure that the quasicrystals can be used in an environment or at least, that they do not corrode faster than comparable crystalline alloys. Corrosion was studied mainly in very strong media in order to achieve measurable results during reasonable times. One of the objectives was to clarify the corrosion mechanisms of icosahedral Al63Cu25Fe12 under various conditions. It is well known that the composition of an alloy has the strongest influence on the corrosion resistance even in the case of quasicrystalline material [1,2]. As observed at a polarization of different crystalline Cu-Al-phases in 1 N H2SO4 by Langer et al. [3] or in salt spray tests of quasicrystalline and crystalline Al-Cu-Fe alloys by Rüdiger et al. [4] it appears that the structure also may effect the corrosion behavior under certain conditions. In this context it will be of interest to quantify the influence of structure. Experimental details Bulk icosahedral quasicrystals (polycrystalline single-phased) Al63Cu25Fe12 and crystalline AlCu-Fe phases were produced by
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