Abstract
Pitting of single crystals of Al and homogenized Al‐Cu alloys has been studied on low index surfaces as a function of surface orientation and alloy content in 
. The potentials at which pitting phenomena occurred were determined potentiodynamically using single‐cycle pitting tests at a scan rate of 0.5 mV/s. Galvanostatic tests were used for pit density studies. The potential at which pits initiated 
was determined, together with a pit transition potential 
that appeared as an abrupt potential discontinuity on the decreasing potential scan and was shown to be related to pit repassivation events. The observations indicated that pitting behavior was anisotropic. Pitting densities were dependent on crystallographic orientation. Consistent with this, the pitting potential showed a small dependence on surface orientation in the order 
, whereas 
was found to be independent of orientation. Walls of pits on Al were composed of {001} facets with <001> step edges. The presence of alloyed Cu raised pitting potentials, reduced the dependence of 
on surface orientation, and modified the pit morphologies. The dependence of pitting behavior on surface orientation and alloy concentration is discussed in terms of a kinetic model of pitting in which pits are considered to initiate at the base of flaws in the surface oxide film.