Anodic current flow at a passive electrode lowers the surface pH and increases the corrosion and film dissolution rates. Analytical expressions are obtained to permit the calculation of the rates of formation of new film and soluble species. It is shown that when equilibrium is achieved between the film and the solution, as with magnesium, polarization of only a few tens of millivolts can result in current densities of the order of 1 A/cm2. For metals forming films which dissolve more sluggishly it is often necessary to exceed the oxygen evolution potential before large currents can be achieved. Thick anodic films can be formed when the film blocks oxygen evolution and when the film dissolution rate constant is small. Buffers suppress the pH change at the surface and may prevent rapid dissolution and corrosion.