189.    BÉNARD, J., AND COQUELLE, O.  [Oxidation of Iron at High Temperatures Determined Metallographically.]  Rev. mét., vol. 43, 1946, pp. 113-124; Chem. Abs., vol. 41, 1947, p. 3418.

                  Sheets of Armco Fe 3 mm. thick were heated in an open electric-resistance furnace provided with a free circulation of air for 5 hr. at temperatures from 570°, at which no perceptible oxide is formed, to 1,050°, cooled, sectioned, and examined under a microscope for the nature and distribution of scale constituents.  These experiments were supplemented by isothermal heating at 700°-960° for up to 20 hr.  Oxidic film produced on heating Fe is a result of Fe diffusion outward and of O₂ diffusion inward into the metal.  These rates of diffusion, which vary with temperature, specify the character of the scale.  Structure of scale produced at or above 910°, the A₃ point of the steel, is greatly different from the scale formed under this temperature, because there is a crystallographic continuity between a-Fe and scale, that is absent in case of y-Fe.  The formation of the scale as a whole follows a parabolic function.  Individual constituents of it form according to other curves, Fe₂O₃ and Fe₃O₄ following a straight lien and FeO a parabolic function.  The influence of temperature on total scaling up to 900° can be presented by the well-known exponential equation.