INFLUENCE OF MICROSTRUCTURE ON THE CORROSION BEHAVIOUR OF MAGNESIUM ALLOYS (2011)

The influence of microstructure on the corrosion behaviour of magnesium alloys has been investigated using advanced microscopy approaches including optical microscopy, SEM, TEM and SKPFM with a focus on the effect of melt-conditioned twin roll casting (MCTRC) and friction stir welding (FSW) on the resultant microstructure of magnesium alloys.The microstructure characterization revealed that intense shearing, generated through the advanced shear technology, resulted in grain refinement and a uniform distribution of the ?-phase and reduced micro-porosity in the MCTRC Mg-Al alloys, of which were attributed to the enhanced heterogeneous nucleation, which resulted in a highly refined grain structure. The TRC Mg-Al alloys displayed a coarse grained microstructure, with a random distribution of grain sizes. Deformation features like twinning, localized shear, microporosity and centre-line segregation were some of the commonly observed defects in the TRC alloys. The general microstructure of the AZ series Mg-Al alloys was composed of ?-Mg grains, the ?-phase, rosette-shaped Al8Mn5 intermetallic particles and ?-precipitates.The MCTRC Mg-Al alloys showed improved corrosion resistance owing to the reduced grain size and the ?-phase network acting as a corrosion barrier, thereby retarding the corrosion process. The TRC Mg-Al alloys exhibited higher susceptibility to galvanic corrosion due to the coarse and random distribution of grain sizes, and segregation. The corrosion testing results showed different corrosion morphologies, including filiform-like and spherical channel-like along with overall general corrosion. However, galvanic corrosion, initiating at localized sites due to Al8Mn5 intermetallic particles and the Si/Fe impurities accounted for a major deterioration in the performance of the Mg-Al alloys. The polarization curves revealed no evidence of passivation, suggesting that the alloy surface was continuously attacked. SKPFM results indicated that the micro-constituents, namely Al8Mn5 intermetallic particles and the ?-phase exhibited higher nobility relative to the ?-Mg matrix, suggesting formation of micro-galvanic couples at localized sites leading to the initiation of galvanic corrosion.The AM60 and AZ91 Mg-Al alloys, subjected to FSW, revealed that the traverse speed had a direct influence on the weld zone microstructure, where the size of the friction stir/weld nugget zone decreased with increase in the traverse speed and the increase in the rate of deformation, led to widening of the friction stir zone, below the shoulder. The weld microstructure displayed a prominent friction stir zone, with an ultrafine grain structure of an average grain size ranging from 2-10 ?m. The localized increase in temperatures, in the TMAZ, due to the lower tool rotation rates and traverse speeds, which rise above the eutectic melting point (430�C), showed evidence of partial melting followed by re-solidification of the ?-phase and evidence of liquation below the shoulder regions in the TMAZ. The morphology of the ?-phase clearly revealed solute segregation, inconsistent with the ?-phase observed in the parent alloy microstructure.The polarization curves obtained from the weld zones in the FSW AM60 alloy showed an improved corrosion resistance compared with the parent metal zone. SKPFM results revealed that the ?-Mg matrix in the friction stir zone showed higher surface potential values compared with the parent alloy microstructure, due to the dissolution of the ?-phase, suggesting higher nobility. However, the polarization behaviour of the AZ91 alloys did not show a significant difference in the corrosion resistance in the weld zones due to the higher volume fraction of the ?-phase in the AZ91 alloys. The immersion testing results revealed higher susceptibility to corrosion in the transition zone due to the flash formation and the banded microstructure leading to failure of the weld zone.