The influence of interphase structure on the kinetics of oxygen reduction on graphite used in aluminum-graphite metal matrix composites

Graphite fibers are used as reinforcement in a number of metal matrix composites. In aluminum-graphite composites this causes severe galvanic corrosion when the material is exposed to a corrosive environment. The graphite fiber surface provides active sites on which the cathodic reaction of oxygen occurs in aqueous solutions. This cathodic reaction causes enhanced aluminum matrix dissolution because of galvanic coupling. This study focussed on the oxygen reduction reaction on graphite fiber surfaces and means of inhibiting it to reduce the galvanic corrosion problem. Since the reaction is sensitive to surface active sites on graphite, the first stage of the study involved testing a variety of graphitic surfaces for activity towards oxygen reduction. Graphite fibers of different elastic moduli differed in electrochemical activity towards the oxygen reduction reaction. The kinetics of the reaction were observed to be higher on the fibers of lower elastic modulus. An explanation based on the microstructure of the graphite fibers giving rise to different numbers of active surface sites was proposed. Polarization experiments and Laser Raman Spectroscopy on the fibers were performed to develop this idea. The second stage of the research was concerned with screening tests for inhibitors for the cathodic oxygen reduction reaction. Tests were performed on oriented pyrolytic graphite surfaces and a few compounds were identified as marginally effective. Further tests with ion-implanted surfaces, implanted with a few species, were tried with no significant inhibitive effects observed on the oxygen reduction reaction. However, the results suggested possible future directions for research.