Room Temperature Oxidation of Iron at Low Pressures

The oxidation at 300° K of high purity iron surfaces prepared under ultra-high-vacuum conditions was studied using ellipsometry to measure film thickness. By working at low oxygen pressures (between 10–7 and 10–4 torr) it was found that the early stages of the oxidation process followed a linear growth law with rate being proportional to the pressure. This behavior is in agreement with a picture proposed by Cabrera. The sticking probability of oxygen in the pressure range studied was of the order of 5 × 10–3. Traces of water vapor lowered the limiting film thickness approached in the latter, nonlinear, oxidation stage to 18 Å from the 26 Å found at all pressures for dry oxygen.