The corrosion behaviour of three low index planes of an hcp magnesium (Mg) single crystal was investigated in 10 22 M NaCl/10 24 M dichromate solution. The Mg (0001) surface exhibited pitting corrosion susceptibility at the open circuit potential. However, the (101 0), and (112 0) surfaces were passive at open circuit and pitting only initiated at potentials slightly anodic to their open circuit potentials. The onset of stable dissolution in the three surfaces was marked by the formation and propagation of corrosion filaments. For the three surfaces investigated, pit shape was non-geometric, however, pit propagation was generally found to occur along preferential crystallographic directions. An analysis of the preferred crystallographic directions is presented.