Hydrotalcite formation on aluminum sheet and powder
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M. S. Weather Department of Geological Sciences, Cornell University, Ithaca, New York 14853-1504 (Received 10 July 1992; accepted 10 August 1992)
We have observed the formation of the hydrotalcite-like phase of lithium dialuminate, LiAl 2 (OH) 6 OH • 2H 2 O, and also the carbonate analog, by the oxidation of aluminum sheet and also aluminum powder in aqueous lithium hydroxide or lithium carbonate solutions. A secondary phase, bayerite, was also observed following the oxidation process, except when the aluminum was treated with lithium oxalate solutions where it is the principal phase. Results have been obtained for the time required to form a passivating film to hydrogen formation as a function of temperature and oxidizing solution. Grazing incidence x-ray diffraction of aluminum sheet samples, combined with polycrystalline x-ray diffraction on similarly treated aluminum powder, were used to evaluate the formation of the films. Both transmission and reflectance infrared absorption spectra on powder and sheet samples were used to support the x-ray observations. Scanning and transmission electron microscopies show morphological differences between preparations, film thicknesses of 10-20 fim, and also film defects. Additional SIMS analysis determined the relative lithium and aluminum concentrations in the films, suggesting that a higher concentration of lithium occurs when lithium carbonate is present in the reacting solutions.
I. INTRODUCTION Chemically deposited films on aluminum sheet are used for various applications such as food and beverage cans. It has been reported that a superb rust inhibitor for steel is the complex mineral, pyroaurite, having the chemical composition Fe(ii)6Fe(m)2(OH)16CO3 • 4H2O.1 This species belongs to a group of minerals that are referred to as anionic clays since the exchangeable ions are anions such as hydroxyl, carbonate, or halides. Crystallographically, they have a crystal structure similar to hydrotalcite {Mg 6 Al 2 (OH) 16 CO 3 • 4H2O} and hence are often referred to as hydrotalcite-like phases. These structures are comprised of positively charged metal hydroxide sheets between which are located charge compensating anions and water molecules.2 5 Most of these compounds are composed of divalent trioctahedral sheets (as in brucite) in which there is substitution by trivalent cations such as aluminum giving the positively charged layers. In contrast, there is a lithium dialuminate, LiAl 2 (OH)6 • 2H 2 O or a comparable carbonate analog, in which
lithium ions occupying sites in the dioctahedral sheets of bayerite or gibbsite give positively charged sheets that are compensated by interlayer hydroxyl or carbonate ions resulting in a hydrotalcite-type structure.6 Hydrotalcite powders have been synthesized by the reaction of magnesium or aluminum hydroxides with substituting cations and compensating anions in basic media. The purpose of this work is to present results on the formation of hydrotalcite-like lithium dialuminate films on aluminum sheet and powder by the rapid oxidation an
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