CORROSION BEHAVIOR OF A NOVEL SIC AL2O3/AL COMPOSITE EXPOSED TO CHLORIDE ENVIRONMENTS/

The electrochemical behavior of a new class of aluminum metal matrix c omposites produced by the direct metal oxidation technique, Lanxide AS -109(TM), was detailed. The composite is 69% (by volume) SiC reinforce ment, 24% oxidized metal (primarily Al2O3), and 7% retained aluminum a lloy. The corrosion behavior of the SiC/Al2O3/Al composite was found t o be dependent on NaCl concentration and pH. SiC/Al2O3/Al was suscepti ble to pitting of the Al alloy phase at open circuit during exposure t o aerated and deaerated 0.6, 0.06, and 0.006 M NaCl solutions. In cont rast, exposure to 6 x 10(-4) M NaCl did not result in pitting at open circuit, and buffered berate solution (no chloride) did not yield pitt ing at any potential examined. The pitting potential of SiC/Al2O3/Al w as estimated to decrease 80 mV per order of magnitude increase in NaCl concentration. Comparison of the composite to a model composite matri x material, Al 6061, revealed that the composite was more susceptible to localized corrosion than the matrix alloy alone which did not pit a t open circuit in deaerated 0.6 M NaCl. Polarization resistance measur ements in pH adjusted (pH 1 to 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 Al2O3, suggesting little galv anic effect of the reinforcing phases.