Hongbo Ding

Electrochemical Behavior of Boron Carbide and Galvanic Corrosion of Boron Carbide Reinforced 6092 Aluminum Composites

Electrochemical behavior of boron carbide (B4C), which is a degenerated semiconductor, in 0.5 M Na2SO4 solution was studied using potentiodynamic polarization, cyclic voltammetry, and impedance spectroscopy. Polarization diagrams revealed three anodic current peaks and a gradual increase of anodic currents with increasing applied potential. Cyclic voltammograms revealed that the three anodic current peaks appeared only during the first anodic potential scan. During the subsequent potential scans, the electrode behaved like an inert electrode within a wide potential window. Impedance spectra taken at various applied potentials revealed a potential-independent electrode/electrolyte interface double layer capacitance value approximately of 10 μf/cm, typical value of the Helmholtz capacitance of concentrated electrolyte solution, indicating that the chargecarrier concentration of boron carbide is higher than that of the 0.5 M Na2SO4 solution. The galvanic corrosion of particulate B4C reinforced 6092-T6 Al metal matrix composite (MMC) induced by the B4C reinforcement was examined with a zero resistance ammeter (ZRA) technique and a platinization method. The ZRA measured a galvanic current between the B4C and aluminum couple. By soaking the MMC specimen in a 0.5 M Na2SO4 solution containing small amount of H2PtCl6, platinum microparticles of approximately 200-300 nm in diameter were precipitated primarily on the B4C reinforcement particles. Both results indicated that B4C particles were cathodic sites and induced galvanic effects on the corrosion of the 6092-T6 Al MMC reinforced with B4C. Figure 1. SEM backscattered electron image of

Effect of Embedded Titanium-Containing Particles on the Corrosion of Particulate Alumina Reinforced Aluminum-Matrix Composite

The microstructure and corrosion initiation sites of particulate alumina reinforced 6092 T6 Al metal-matrix composites (MMCs) (1, 2) were examined with scanning electron microscopy (SEM) and energy dispersive X-ray Analyzer (EDXA). The MMC contained a variety of Ticontaining micro-particles (Figure 1) that were very likely introduced into the Al matrix as impurities of the alumina reinforcements. The reduction product of CuSO4 precipitated onto the Ti-containing particles, suggesting that the Ti-containing particles conducted cathodic reactions. When the MMC was immersed in salt water in open-circuit conditions, micro-crevices formed around both the Ti-containing particles as well as Fe-Si-Al intermetallic particles. With the semi-quantitative EDXA technique, the Ti-containing particles that induced microcrevices can be roughly divided into three categories: 1) titanium suboxides with compositions close to that of Ti6O, Ti3O, Ti2O and TiO; 2) Ti-Zr-Al oxides; and 3) TiO2. Although little is known about the electrical properties of the titanium suboxides and the Ti-Zr-Al oxides, the results suggested that these compounds were either electrically conducting or semi-conducting. As for the TiO2 particles (3), it was hypothesized that 1) the TiO2 particles were reduced and consequently n-doped by the processing of the MMC (e.g., hot-pressing and T6 heat treatments); and 2) the flatband potential of the TiO2/electrolyte system might be close to the open-circuit potentials of the MMC in air-exposed salt water, making cathodic reactions at the TiO2 surfaces feasible. Microcrevices also formed around some particles with an oval structure (Figure 2). The core region of the oval structure appeared to be a Ti-Si-Al phase while the shell region of the oval structure appeared to be an Fe-Si-Al phase.