The critical pitting potential, defined as the least positive potential at which pits can be grown, may be that potential at which the metal salt of the aggressive ion in solution is in equilibrium with the metal oxide. Such an equilibrium may be achieved when the activity of the anion in the pit has been increased by the potential difference between the inside and outside of the pit. The theory successfully predicts the critical pitting potentials for aluminum, magnesium, iron, and nickel, but fails for zirconium, titanium, and tantalum.