Influence of Lead on the Microstructure and Corrosion Behavior of Melt-Conditioned, Twin-Roll-Cast AZ91D Magnesium Alloy

The influence of lead addition on the microstructure and the corrosion behavior of a melt-conditioned, twin-roll-cast (MCTRC) AZ91D magnesium alloy has been investigated. Electron probe microanalysis revealed that lead-containing (magnesium plumbide [Mg₂Pb]) particles were formed in the alloy and were concentrated mainly at the interdendritic boundaries. Scanning Kelvin probe force microscopy indicated increased surface potentials for the Al-Mn intermetallics, the Pb-rich particles, and the β phase, relative to the adjacent α-Mg matrix, suggesting their greater nobilities with respect to the α-Mg matrix. Corrosion testing revealed that the corrosion initiated at the α-eutectic/lead-containing particle and α-eutectic/Al-Mn particle interface, as a result of galvanic coupling between the intermetallics and α-eutectic. Corrosion subsequently propagated into the primary α-Mg matrix. The β-Mg₁₇Al₁₂ phase network at the grain boundaries acted as a corrosion barrier, restricting the propagation of corrosion from one grain to an adjacent grain, thereby contributing to an improvement in the corrosion resistance of the MCTRC AZ91D magnesium alloy.