Lithium-aluminum-carbonate-hydroxide hydrate coatings on aluminum alloys: Composition, structure, and processing bath ch
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Lithium-aluminum-carbonate-hydroxide hydrate coatings on aluminum alloys: Composition, structure, and processing bath chemistry C. A. Drewien, M. O. Eatough, D. R. Tallant, C. R. Hills, and R. G. Buchheit Materials and Process Sciences Center, Sandia National Laboratories, Albuquerque, New Mexico 87185 (Received 30 May 1995; accepted 18 January 1996)
A new corrosion resistant coating, being designed for possible replacement of chromate conversion coatings on aluminum alloys, was investigated for composition, structure, and solubility using a variety of techniques. The stoichiometry of the material, prepared by immersion of 1100 Al alloy into a lithium carbonate-lithium hydroxide solution, was approximately Li2 Al4 CO3 (OH)12 ? 3H2 O. Processing time was shown to be dependent upon the bath pH, and consistent coating formation required supersaturation of the coating bath with aluminum. The exact crystal structure of this hydrotalcite material, hexagonal or monoclinic, was not determined. It was shown that both the bulk material and coatings with the same nominal composition and crystal structure could be formed by precipitation from an aluminum supersatured solution of lithium carbonate.
I. INTRODUCTION
The aerospace and defense industries use aluminum alloys as the major component in a variety of parts, because aluminum alloys provide lightweight, easy fabrication, moderate strength and ductility, and nonrusting properties. In saline environments, pitting of aluminum and its alloys limits their usefulness. The application of a corrosion resistant chromate conversion coating, produced by immersing the cleaned metal into a solution containing sodium dichromate, increases the life of the metal on exposure to aggressive chloride environments. However, leaching of carcinogenic chromates into the environment during the lifetime of use of chromatecoated metals, worker exposure during processing, and coating bath chemicals disposal has led to the search for alternative coating systems to replace corrosion resistant chromate conversion coatings. One possible replacement for chromate conversion coatings is based upon the novel use of lithiumaluminum-carbonate-hydroxide hydrate (hydrotalcite) as a corrosion resistant coating for aluminum alloys.1 Hydrotalcite coatings impart corrosion resistance to the underlying aluminum substrate, and processing of the coatings, like chromate conversion coatings, is performed by simple immersion of the substrate into an aqueous solution.1 These features make the lithium-aluminum-carbonate-hydroxide hydrate coating, hereafter referred to as a hydrotalcite coating, a very attractive candidate for replacement of chromate conversion coatings on aluminum alloys. In order to recommend hydrotalcite coatings, the processing, composition, structure, and properties of the material must be understood. Recent literature2–5 J. Mater. Res., Vol. 11, No. 6, Jun 1996
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addresses the structure and compositi
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