Kinetics of the chloride-induced oxidation ofiron at 300°C

Abstract

Iron was oxidized at 300°C in the presence of solutions containing chloride and/or iron(II) ions, but in the absence of oxygen. The oxidation reaction, 3Fe + 4H 2 O → Fe 3 O 4 + 4H 2 Δ G 300 ° C = − 10 · 3 kcal / mole Fe 3 O 4 , was conveniently studied by the hydrogen-effusion method 7 which determines the rate at which hydrogen is evolved. The electrolytes in solution affect the reaction rate and except for iron(II) do not enter into the final reaction products. The generally accepted concept that chloride ions accelerate the oxidation or iron was not experimentally verified. It was found that, in addition to chloride ions, the system must contain iron(II) ions. An empirical equation has been derived that describes the extent of iron oxidation, W, with time, t, as a function of the iron(II) chloride (molar) concentration, viz. W = a log ⁡ ( b t + 1 ) + k ′ [ Fe 2 + ] 1 / 2 [ Cl − ] 1 / 2 t where a and b are adjustable parameters, k' the kinetic rate constant for the steady state condition of constant oxidation rate. The first term, the log term, has been described 11,12 as an oxidation mechanism wherein an oxide film, containing pores that are continuously being filled and regenerated, influences the reaction rate. Magnetite or Fe 3O 4 is deposited as a tightly adherent crystalline film at the metal/solution interface and apparently also quenches the reaction rate by serving as a physical barrier. The differential with respect to time of the second term of equation (2) has been reported 23 as describing a rate mechanism that is proceeding under the diffusive control of ions migrating across the Nernst boundary layers of anodic and cathodic regions of an oxide-free metal/solution interface.