The rate of oxidation of pure iron increases with decreasing oxygen pressure in the temperature range of 220°–450 °C and the pressure range 100–0.01 Torr. forms initially on the surface of the iron, and subsequently nucleates in the surface of the . As the layer becomes continuous, diffusion through the layer is reduced, resulting in a substantial reduction of the oxidation rate. At low oxygen pressures (0.01 Torr), the rate of nucleation of is low, resulting in relatively thick films of and high oxidation rates, whereas at high oxygen pressures (100 Torr), the rate of nucleation of is high, resulting in thinner films, and lower over‐all oxidation rates.