Abstract
The formation of black‐oxide (magnetite) coatings on mild steel in hot concentrated solutions of sodium hydroxide and sodium nitrite was studied over the temperature range 130°–150°C. The following mechanism is proposed for the over‐all reaction: Of the total iron dissolved in step [1], only about 30% was converted to black oxide on the surface. Values of the rate constant for step [1] were obtained from plots of total iron dissolved vs. time. Plots of the initial rates (oxide‐free surface) vs. 1/T were linear, with an energy term of 41 kcal/mole. As the oxide coating built up, the rates dropped to a fraction of the initial rate and the energy term increased sharply. For thicker films the rates are apparently controlled by diffusion of the reactants through the film.
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