Metastable pitting behavior of aluminum single crystals

The metastable pitting behavior of aluminum single crystals (99.999% pure) with orientations of (100), (110) and (111) was followed as a function of potential at potentials below the pitting potential in a deoxygenated 0.6 M NaCl solution. The (111) crystal face exhibited the highest number of metastable events at a given potential. The (100) face showed the second highest number of events, while the (110) had the least number of events. The peak pit currents for the three surfaces were not statistically different in the potential region −0.785 to −0.745 V vs. SCE indicating that the amount of electrochemical charge passed in a breakdown event is independent of orientation. These results are discussed in relation to the nature of the oxide film and the atomic planar densities of the three orientations.