Crystallographic pitting in magnesium single crystals

The low index planes, (0001), (10 (1) over bar0), and (11 (2) over bar0) in hcp Mg, were investigated. The corrosion behavior for each orientation was investigated in a solution of .01 M NaCl + 10(-4) M Na2Cr2O7. The Mg basal plane orientation exhibited corrosion initiation and propagation at open circuit. However, the type I (10 (1) over bar0) and type II (11 (2) over bar0) prismatic planes were resistant to corrosion at open circuit but exhibited stable dissolution upon polarization at potentials immediately positive to the open circuit potential. Corrosion initiated and propagated via the formation a critical pitting solution in an occluded site geometry facilitated by a pit cover. Corrosion observed in the (0001), (10 (1) over bar0), and (11 (2) over bar0) planes propagated as filaments. Additionally, the propagation of the corrosion filaments in all planes investigated exhibited a crystallographic directional dependence.