The Primary Oxide Film on Iron

Abstract

The identity and structure of the invisible, air‐formed oxide film on iron has never been determined although the practical importance of a knowledge of its properties in the study of corrosion has been demonstrated. Electron diffraction experiments are now reported which show this film to be structurally identical with Fe₃O₄. In addition, it is found that the oxide has a definite orientation relationship with the underlying iron, viz., the [√210] directions in the oxide crystals are parallel to the [111] directions in the iron. Freshly abraded surfaces of electrolytic and ingot iron were investigated. Evaporated films of electrolytic iron were prepared in the diffraction camera and pictures taken before and after exposure to air show that Fe₃O₄ forms rapidly in dry air at room temperature. An air pressure of 10^—3 mm for a few minutes gave a measurable effect. Exposure to air at atmospheric pressure for periods longer than a few minutes does not greatly strengthen the oxide pattern. The factors affecting the sensitivity of this method of examining thin films on metals are discussed.