THE INFLUENCE OF β (Mg17Al12) PHASE DISTRIBUTION ON CORROSION BEHAVIOR OF AM50 ALLOY IN NaCl SOLUTION

The effect of beta (Mg17Al12) phase distribution on the corrosion behavior of AM50 alloy in NaCl solution was studied by scanning vibrating electrode technique (SVET) in conjunction with the potentiodynamic polarization scan. The beta phase distribution was modified by casting AM50 at different cooling rates. For all cooling rates, the following phases were present: primary magnesium (a), eutectic mixture of a and beta phase and beta phase. In 0.17 wt% and 1.6 wt% NaCl solutions, the free corrosion potential (FCP) of the moderate-cooled alloy was more noble than that for the high and slow-cooled alloys. The nature of microgalvanic activity was studied in 0.17 wt% NaCl solution through SVET analysis that was conducted in-situ under both FCP and applied cathodic and anodic polarization potentials. The corrosion current densities calculated from SVET analysis at FCP were 10.28 mu A/cm(2) for fast-cooled, 4.8 mu A/cm(2) for moderate-cooled and 10.44 mu A/cm(2) for slow-cooled alloys. The corrosion current densities calculated from SVET analysis at zero current potential (ZCP) were in agreement with that determined from potentiodynamic polarization method. The FCP increased with increasing NaCl concentration. This study clearly indicated that for a given magnesium alloy composition, the corrosion resistance of the alloy can be greatly affected by the grain size and distribution of secondary phases.