The effect of oxide film properties on the corrosion behavior of silicon carbide/aluminum metal matrix composites

Oxide growth on pure aluminum, aluminum alloy 6061, and the aluminum-based metal matrix composite SiC/AA6061 has been studied, and the properties of the oxides related to the pit initiation behavior of the materials. The objectives of the work were to identify the effect of alloying elements and SiC reinforcement on the oxide film, and to better understand how the oxide properties control pit initiation behavior.; To this end, electrochemical and optical studies of the materials were carried out in a buffered sodium borate/boric acid solution at pH values of 8.4 and 7.2. The electrochemical studies yielded information about the effect of the alloying and reinforcement additions on the tendency to pit, on the ability to repassivate existing pits, and on the resistance to pit initiation. The optical studies allowed the characterization of oxide growth rates and optical parameters of the oxides.; The alloy and metal matrix composite showed a slightly lesser tendency to pit than pure aluminum, as measured by the pitting potential. The oxide on the composite was less resistant to pit initiation, and was found to exhibit slower repassivation rates than the other materials. The repassivation behavior and resistance to pit initiation were quite similar in the case of the alloy and the pure aluminum. Induction times for pit initiation were consistent with the predictions of Heusler's model for the breakdown of passivity.; Film growth kinetics were equal on the alloy and on the pure aluminum. Reliable kinetic data from the composite could not be obtained, due to the relative roughness of the surface. The limiting thickness at a given potential increased in the order AA6061, Al, SiC/AA6061. The oxide on the alloy was less dense than the oxide on the aluminum.; Consideration of both the electrochemical and the optical results suggests that the diffusion barrier properties of the oxides are not of primary importance in controlling the resistance to pitting for these materials. The effect of the alloying elements on film structure and the nature of the chemical bonding may be a more important consideration.