Many alloying elements that confer resistance to high‐temperature oxidation also prevent low‐temperature aqueous corrosion; they produce oxide films that only slowly transmit cations. Conversely sulfur, which introduces lattice defects into the films and thus favors the transmission, accelerates high‐temperature oxidation and favors the onset of aqueous corrosion. It is therefore suggested that the onset of aqueous corrosion is controlled by the relative ease with which cations can pass through the original air‐formed oxide film on the metal. A rapid supply of dissolved oxygen to all parts of the surface may arrest cations as they emerge, thus thickening the film and preventing corrosion; a far more rapid supply of oxygen is needed to prevent corrosion on iron or zinc than on stainless steel, but the supply needed is diminished if an inhibitor is present. The weakest places in the film are those where cations can most readily pass through; they become the initial corrosion anodes. The further development of the anodic zones depends on the production of anodic acidity.