Single crystals of 99.99%, 99.999%, and 99.9999% Al have been grown from the melt under conditions controlled to produce either homogeneous solute distributions, or microsegregation (on the scale ∼100μ) of the dominant residual impurities Fe, Cu, and Si. Exposure of samples to resulted in a fine‐scale pitting of segregated specimens while homogeneous specimens remained free of such pits. Various mechanical and annealing treatments did not disturb this contrast and, in general, the pitting could be quantitatively related to a previous segregation study in these metals. Polarization measurements showed that the main effect of segregation is to produce a decrease in the cathodic overvoltage which shifts the corrosion potential into a range where pitting occurs. Since the two less pure grades exhibited segregation in the same size scale, and were relatively indistinguishable, it seems that the distribution, rather than the total amount of residual impurity, is more important in producing a tendency for pitting.