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    Electron Microscopy and Electron Diffraction Studies of Oxide Films Formed on Iron in Water and Oxygen Atmospheres

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    Electron microscope and electron diffraction methods are useful techniques for the identification of water-formed deposits and corrosion products. More important, these methods can determine the crystal structure and habit of the crystals making up the deposits or corrosion products, from which in turn much can be learned about the chemical and physical conditions of formation of the corrosion product and the metal structures responsible. A study of the localized corrosion products formed in the vapor state reaction of water vapor and oxygen with pure iron is presented. Three types of localized growths occur on iron when the reaction occurs between 400 and 500 C. These are: (1) long oxide whiskers 200 or 300 Å in diameter; (2) thin blade-shaped platelets of about 100 Å in thickness; and (3) rounded platelets of oxide about 100 Å in thickness. Oxide whiskers form in many oxidizing environments and in a wide variety of stress and metallurgical conditions of the metal. They are formed alone on pure iron when it reacts with dry oxygen. Thin blade-shaped platelets form in water vapor atmospheres, while rounded oxide platelets form during the initial stage of reaction of impure iron with oxygen. These localized growths were identified by electron diffraction as hexagonal α-Fe2O3. This oxide was formed in dry oxygen and in water vapor. The crystal habit of the corrosion product was found to depend upon the oxidizing atmosphere, the pretreatment of the iron, and the impurities present in the metal. The localized growth habits of the oxide can lead to an understanding of the genesis of water-formed deposits.

    Author Information:

    Gulbransen, Earl A.
    Westinghouse Electric Corp., Pittsburgh, Pa.

    Copan, Thomas P.
    Westinghouse Electric Corp., Pittsburgh, Pa.

    Committee/Subcommittee: D19.03

    DOI: 10.1520/STP46310S