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A study of corrosion of a rotating iron disk electrode has been made in slightly acidified 0.1N Na2SO4 and 0.1N NaCI solutions at 15C. It was found that in the sulfate solutions of pH>3.9 and chloride solutions of pH>3.4 pits appear less than ten minutes after the immersion of the electrode in the solutions and then U-shaped brown walls of rust form as aggregates of precipitates around pits. In the pH 4-6 sulfate solutions well-defined and vortex-shaped rust-free parts of surface are established in the steady-state corrosion, while in the chloride solutions (pH 3.9) yellow and black vortex-shaped rust belts cover almost the entire surface of the electrode. The rate of corrosion of iron under a given rotational speed of the disk decreases with time of immersion and reaches steady-state values in both solutions. The rate of corrosion of the iron electrode in the chloride solutions was higher than that in the sulfate solutions under the same hydrodynamical conditions and pH. Measurements of the fraction of uncorroded part on the iron surface showed that the fraction decreased with time and that it become higher at a lower rotational speed. Microscopic observations showed that for sulfate solutions the brown rust film, non-porous, entirely covered inner parts of the U-shaped wall while for chloride solutions the yellow rust particles were fine and of dispersive nature. The parts of substrate under the yellow rust were found to be left uncorroded. The rate of corrosion of iron can be well interpreted under the assumption that the diffusion of oxygen dissolved in the solutions toward the surface of iron is rate-determining and that the electrochemical reduction of oxygen takes place at the uncorroded parts of surface under yellow rust in the chloride solutions. The relation between the rest potentials of the electrode and the corrosion current densities at the sites of iron dissolution is shown by Tafel lines with the Tafel coefficient of 0.03-0.04 in the sulfate solutions. The Tafel coefficient in the chloride solutions during the period of pit growth and steady-state corrosion is in the range 0.03-0.06.

Effect of Anions on the Reduction Rate of Oxygen on an Iron Electrode with Precipitated Films