PITTING BEHAVIOR OF A NOVEL SiC/Al_2O_3/Al COMPOSITE EXPOSED TO CHLORIDE ENVIRONMENTS

摘要

The electrochemical behavior of a new class of aluminum metal matrix composites produced by the direct metal oxidation technique, Lanxide AS-109~(TM), was detailed. The composite is 69% (by volume) SiC reinforcement, 24% oxidized metal (primarily A1_2O_3), and 7% retained aluminum alloy. The corrosion behavior of the SiC/Al)2O_3/Al composite was found to be dependent on NaCl concentration and pH. SiC/Al_2O_3/Al was susceptible to pitting of the Al alloy phase at open circuit during exposure to aerated and deaerated 0.6, 0.06, and 0.006 M NaCl solutions. In contrast, exposure to 6X10~(-4) M NaCl did not result in pitting at open circuit, and buffered borate solution (no chloride) did not yield pitting at any potential examined. The pitting potential of SiC/Al_2O_3/Al was estimated to decrease 80 mV per order of magnitude increase in NaCl concentration. Comparison of the composite to a model composite matrix material, Al 6061, revealed that the composite was more susceptible to localized corrosion than the matrix alloy alone which did not pit at open circuit in deaerated 0.6 M NaCl. Polarization resistance measurements in pH adjusted (1 - 13) 0.6 M NaCl solutions revealed that the corrosion rate is minimized in neutral solution. Pits were neither confined to regions adjacent to SiC nor A1_2O_3, suggesting little galvanic effect of the reinforcing phases.