Electrochemical Examinations on the Corrosion Behavior of Boron Carbide Reinforced Aluminum-Matrix Composites

Galvanic corrosion of a 6092-T6 Al metal-matrix composite (MMC) reinforced with boron carbide (B 4 C) particulates was examined through electrochemical characterization of the MMC as well as a monolithic B 4 C. The results were interpreted based on a recently proposed electronic model for B 4 C that emphasizes a conventional non-degenerate p-type semiconductor band structure with high density of gap states. The electrochemical behavior of B 4 C in the dark that was characteristic of non-active metal electrodes was attributed to the very high density of gap states of B 4 C; while the phenomenon of the enhanced cathodic currents of B 4 C under illumination was attributed to the presence of a depletion layer at the B 4 C/electrolyte interface, characteristic of non-degenerate p-type semiconductor/electrolyte interface. The results suggested that galvanic corrosion of the MMC in the dark was limited by slow oxygen reduction kinetics at the B 4 C reinforcements and that illumination promotes galvanic corrosion of the MMC as a result of photo-enhanced cathodic activity of the B 4 C reinforcements. Scanning electron microscopy revealed that corrosion of the MMC initiated from carbon particles that are likely introduced into the matrix during the processing of the MMC. A combination of scanning vibration electrode technique and scanning ion-selective electrode technique revealed that localized anodic and cathodic sites co-exist during the corrosion of the MMC in an air-exposed 3.15 wt % NaCl solution. The localized anodic and cathodic sites have diameters of approximately 200 μm.