Pitting of single crystals of A1 and homogenized A1-Cu alloys has been studied on low index surfaces as a function of surface orientation and alloy content in 0.5M NaC1. 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 (Epit) was determined, together with a pit transition potential (Ept~,) 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 (Epit)~0el~ > (E,it){~H} > (Epit){111}, whereas Eptp was found to be independent of orientation. Walls of pits on A1 were composed of {001} facets with <001> step edges:The presence of alloyed Cu raised pitting potentials, reduced the dependence of Epit 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.

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The columnar to equiaxed transition has been examined experimentally in lead-tin alloys. The effect of thermal conditions during solidification, melt superheat, and alloy composition on the transition have been considered. In directionally solidified alloys, the position at which the transition occured was dependent on the temperature distribution in the system, occuring when a specific low temperature gradient was reached at the advancing interface. Melt superheat did not influence the transition position. The alloy content affected the transition position, the transition occuring earlier at higher alloy contents. Adding particles to the melt did not change the position of the transition.