Electron Diffraction and Electron Microscope Study of Oxide Films Formed on Metals and Alloys at Moderate Temperatures Stripped Oxide Films of Metals

Abstract

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. SURVEY OF THE LITERATURE 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...

Role of Crystal Orientation in the Oxidation of Iron

“…The fact that an oxide film is made up of many small crystallites on each grain of metal has been studied in some detail (29). Crystals of oxide which were oriented to the metal structure were more stable thermodynamically than the unoriented crystallites of the oxide.…”

The Diffusion of Hydrogen Through Materials of Construction

“…Considerable work has been done on oxide coatings of metals over a wide range of temperatures, particularly by Gulbransen and other co-workers. (31)(32)(33)(34)(35)(36)39,40) Stratified Coatings…”

The Isolation of Surface Films from Copper

“…An over-all equation may be written for the corrosion of copper in bromine-free alcoholic ammonium acetate containing dissolved air by combination of reactions [1], [2], and [4] and follows Cu + YzO~ + 2CH~COONH~ = Cu(NH3)~(CH~COO)~ + I4_~O [5] A similar equation may be written, as follows, for the corrosion of copper in a dry alcoholic ammonium acetate-bromine solution freed from dissolved air Cu + Br~ + 2CH~COONH, = Cu(NH~)~(CH,COO)~ § 2HBr [6] Table II shows that detached cuprous oxide films are dissolved by ammonium acetate. Equation [6] shows that hydrobromic acid is a reaction product during film stripping.…”

“…Satisfactory methods for isolating surface films currently are confined to relatively few metals (1). The iodine-anhydrous methanol method (2) has been used extensively for the isolation of oxide films from iron.…”