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 ß phase, relative to the adjacent a-Mg matrix, suggesting their greater nobilities with respect to the a-Mg matrix. Corrosion testing revealed that the corrosion initiated at the a-eutectic/lead-containing particle and a-eutectic/Al-Mn particle interface, as a result of galvanic coupling between the intermetallics and a-eutectic. Corrosion subsequently propagated into the primary a-Mg matrix. The ß-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.

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