The oxidation process occurring on metals has been studied b y electron diffraction bnd electron microscopy, using oxide films stripped b y procedures suggested b y Evans and co-workers. The apparatus and techniques are briefly described. The metals studied include chromium, cobalt, copper, iron, molybdenum, nickel, aluminum, columbium, and tungsten. The electron micrographs and electron diffraction patterns are presented and discussed. The oxide films are rhown to consist of small oxide crystals ranging in size from 100 to ~S W A H E physical and chemical structure of the oxide films formed T on metals and alloys is of considerable interest in our understanding of their protective properties. The authors (10, 14) have studied the structure of the oxide films formed on metals and alloys by the reflection method of electron diffraction. These studies show that chemical and physical transformations occur during the formation and heating and cooling of the oxide film. Inasmuch as the reflection method samples only the outer surface of the film, the information obtained is incomplete. The bulk structure and composition may be considerably different from that on the outer surface. In addition, the nature and size of the crystals in the oxide film can only be approximated by the reflection technique.This paper presents electron microscope and electron diffraction evidence concerning the structure of electrochemically and chemically stripped films from a series of nine metals which had been oxidized under known conditions: iron, nickel, cobalt, chromium, molybdenum, tungsten, columbium, aluminum, and copper.
ELECTRON MICROSCOPE TECHNIQUEThe use of the electron microscope for the study of the submicroscopic fine structure of matter is well known (1): Commercial instruments are available with a resolving power of less than 40 8. Thus, the shape of particles and the nature of the mosaic structure of a system of crystals may be approximated for particles of 150 8. or larger. The low penetrating power of 60-kv. electrons in matter limits its direct use to oxide films of the order of 500 8. in thickness. The preparation of such films requires the use of clectrochemical or chemical methods for stripping the oxide film from the metal.
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STRIPPING OF FILM. The presence of a film on passive ironwas proved by Evans ( 4 ) who stripped films too thin to show interference colors from metals.His first method was based on electrochemical action, using the oxidized metal as the anode in order to dissolve the metal underlying the film. This method was applied to oxidized elcctrolytic iron, copper, and aluminum to secure films which were examined with the light microscope. The second method was based on direct chemical attack of the underlying metal by a saturated solution of iodine in 10% potassium iodide.Later Evans and Stockdale (5) extended and modified the electrochemical method to remove oxide films from iron, copper, nickel, carbon steel, and stainless steel. For removal of the film I Present address, Americ...