For a better understanding of the rusting mechanism of mild steel and low alloy steels, the process of formation of α- and γ-FeOOH and Fe
3O
4, which are important atmospheric corrosion products of iron alloys, was investigated by infrared and Mössbauer spectra, and X-ray diffraction measurements and chlorine analysis of oxidation products of Fe(II) hydroxo-complexes.
The pH-value in solution before reaction was considered as the most important factor in determining the end products of oxidation. The precipitates were prepared by adding potassium hydroxide to the oxygen free aqueous solutions containing ferrous chloride, and the oxidation of the precipitates in solution was carried out at a room temperature while blowing the air at a constant rate of 1 L/min.
It was found that in the present experimental condition the process of oxidation depends on the pH-value in solution as follows: in basic solution (pH 12.5∼13.5) Fe(OH)
2→α-FeOOH, in the slightly acidic solution (pH 6.0∼6.5)β-Fe
2(OH)
3Cl→Green Rust I→γ-FeOOH, and in neutral or slightly basic solution (pH 7.0∼8.0) Fe(OH)
2→Fe
3O
4 and β-Fe
2(OH)
3 Cl→Green Rust I→Fe
3O
4.
The infrared spectra of Fe(OH)
2 and Green Rust I, an oxidation intermediate, were obtained for the first time. The strong absorption bands due to Fe(OH)
2 and Green Rust I were found at 3630 and 480 cm
−1, and 3540, 800 and 670 cm
−1, respectively. Infrared spectroscopy was thus proved to be a powerful tool for investigation of this field.
The obtained results were in good agreement with the thermodynamic considerations on the relationship between pH and the solubility of Fe(OH)
2.
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