Crystallographic effects on the corrosion of twin roll cast AZ31 Mg alloy sheet

The influence of crystal orientation on the corrosion behaviour of twin roll cast (TRC) AZ31 Mg alloy sheet has been investigated using correlative microscopy and immersion testing in 3.5% NaCl. Corrosion initiates as a consequence of the microgalvanic coupling between cathodic intermetallic phases, including sub-micron sized, rosette-shaped Al8Mn5 particles, β-Mg17(Al,Zn)12 phase and the Mg matrix. Subsequent development of lateral filiform-like corrosion fronts are observed running along the dendrite arms whilst being constrained by the Al- and Zn-rich interdendritic boundaries. Prolonged immersion showed variable corrosion depths across the surface. By correlating with EBSD analysis, it was confirmed that the corrosion depth is considerably higher in α-Mg grains with non-basal planes on the surface. The severity of attack suffered by α-Mg grains with non-basal planes on the surface is on average twice that for grains with basal or near-basal with planes on the surface. At the same time, corroded α-Mg grains with basal planes on the surface display nanoscale, canyon-like corrosion features developing into the alloy perpendicularly. A mechanism has been proposed to explain the sequential corrosion events.

Schematic diagrams showing (a) microstructure with Mg grains G1 (near-basal), G2 (basal) and G3 (non-basal) crystal orientations, and Al8Mn5/β/Fe particles, (b) Stage I corrosion, revealing filiform-like corrosion fronts connecting the localized corrosion initiation sites and following the dendrite arms and (c) Stage II corrosion revealing the surface degradation and the increase in corrosion depths in individual Mg grains as a function of immersion time. Filiform-like corrosion fronts observed in G1 and G3 indicate increased and preferential corrosion. G3 with non-basal crystal orientation shows increased corrosion depth followed by G1 with near-basal crystal orientation, while G2 with basal crystal orientation reveals a relatively uniform and reduced corrosion depth.203