ELECTROCHEMICAL AND MICROSTRUCTURAL STUDY OF OXIDE-FILMS FORMED ELECTROCHEMICALLY AT MICROCRYSTALLINE AL-FE-V-SI ALLOYS

A recent advance in metallurgical technology has been the application of rapid solidification techniques to Al alloy production. FVS0812 is the designation given to a microcrystalline Al-based alloy consisting of 8 wt% Fe, 1 wt% V and 2 wt% Si. It is a two-phase alloy, consisting of ca. 27 vol percent of approximately spherical Fe-V-Si-rich dispers oids in ah essentially pure Al matrix. The high strength, low density properties of this advanced material, and other related alloys, have n ot yet been realized, however, due, in part, to the inability of the a lloy to form a thick, adherent, abrasion-resistant outer surface oxide film, a feature readily achieved at conventional Al alloys by normal anodizing methods. The present research has involved an electrochemica l study of oxide film growth at the 812 alloy, with the specific goals being to seek an understanding of the origin of the oxide film growth problem and ultimately to propose alternative approaches to the forma tion of a thick, stable oxide film at this material. The techniques us ed in this research have included electrochemical methodologies such a s cyclic voltammetry and electrochemical impedance spectroscopy. Cruci al information has been obtained through transmission electron microsc opy (TEM) of ultramicrotomed specimens. Experiments were carried put i nitially in neutral berate solutions to characterize the compact barri er oxide film formed in this environment and expected to be present be neath the porous oxide film formed in the normal sulfuric acid anodizi ng medium. In berate solutions, the electrochemical results implied ox ide film thicknesses which were less than seen subsequently by TEM wor k, suggesting either that the barrier film at the 812 alloy can be pen etrated by solution in very fine pores (not resolvable by conventional TEM) at its outer surface or that dispersoids trapped in the oxide fi lm cause differential oxide film thicknesses to develop across the all oy surface. In sulfuric acid solutions, dissolution of Fe and V occurs from the 812 alloy during anodization. Both impedance and TEM studies reveal the absence of a barrier film at the 812 alloy surface. Also, the thick oxide overlayer has a tortuous and more open pore structure than formed at Al and the oxide film is also substantially thinner tha n it should be. It is suggested that the absence of a barrier oxide fi lm indicates that the sulfuric acid anodizing medium is too aggressive for oxide film formation at the 812 alloy, resulting in excessive dis solution and poor oxide film qualities. (C) 1995 Wiley-Liss, Inc.