Effects of dynamic recrystallization and strain-induced dynamic precipitation on the corrosion behavior of partially recrystallized Mg–9Al–1Zn alloys

The corrosion susceptibility of recrystallized and un-recrystallized grains in equal channel angular pressed (ECAPed) Mg–9Al–1Zn (AZ91) alloys immersed in chloride containing media was investigated through immersion testing and an electrochemical microcell technique coupled with high resolution techniques such as scanning Kelvin probe force microscopy (SKPFM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). During ECAP, dynamic recrystallization (DRX) and strain-induced dynamic precipitation (SIDP) simultaneously occurred, resulting in a bimodal grain structure of original elongated coarse grains and newly formed equiaxed fine grains with a large volume fraction of β-Mg17Al12 precipitates. Corrosion preferentially initiates and propagates in the DRXed grains, owing to the greater microchemistry difference between the β-Mg17Al12 precipitates formed at the DRXed grain boundaries and the adjacent α-Mg matrix, which induces a strong microgalvanic coupling between these phases. Additionally, the weaker basal texture of the DRXed grains also makes these grains more susceptible to electrochemical reactions than the highly textured un-DRXed grains. The influence of dynamic recrystallization and dynamic precipitation was also studied in ECAPed alloys with different levels of deformation strain through corrosion and electrochemical techniques. Increasing the strain level led to a more uniform corrosion with a shallow penetration depth, lower corrosion rate values, and higher protective ability of the oxide film. Furthermore, higher levels of strain resulted in greater hardness values of the ECAPed alloys. The superior corrosion resistance and strength of the ECAPed alloys with increasing strain level was attributed to the combination of smaller DRXed grain size, higher DRX ratio, and higher volume fraction of uniformly distributed fine β-Mg17Al12 precipitates.