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

Abstract

Galvanic corrosion of a 6092-T6 Al metal-matrix composite (MMC) reinforced with boron carbide (B4C) particulates was examined through electrochemical characterization of the MMC as well as a monolithic B4C. The results were interpreted based on a recently proposed electronic model for B4C that emphasizes a conventional non-degenerate p-type semiconductor band structure with high density of gap states. The electrochemical behavior of B4C in the dark that was characteristic of non-active metal electrodes was attributed to the very high density of gap states of B4C; while the phenomenon of the enhanced cathodic currents of B4C under illumination was attributed to the presence of a depletion layer at the B4C/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 B4C reinforcements and that illumination promotes galvanic corrosion of the MMC as a result of photo-enhanced cathodic activity of the B4C 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.