The removal mechanism of the oxide scale by electrolytic reaction in sodium chloride solution, was investigated electrochemically. The scale was produced on an electrolytic iron in the air at high temperature (500°C-900°C). Two kinds of electrolytes were used; 100g/
l sodium chloride solution and a mixed solution consisting of 10wt% hydrochloric acid, 25wt% sulfuric acid and 85wt% phosphoric acid. The mixed solution, which is characteristic in that the scale dissolved at room temperature but the matrix iron scarecely does, was used in comparison with the removal mechanism in sodium chloride solution. The main results are as follows.
In the case of mixed electrolyte:
1. Cathodic reduction affects the scale removal rate.
2. The rest potential-time curve is useful to known the completion of the scale removal by electrochemical pickling.
In the case of sodium chloride solution;
1. The scale is not removed by anodic dissolution only when no defect exists in the scale layer.
2. As the cathodic reduction proceedes, the amount of magnetite (Fe
3O
4) decrease but that of wüstite (FeO) does not change. After sufficient reduction (-2.0V vs SCE), the magnetite change into intermediate substance which could not be identified by X-ray diffraction. The intermediate substance is removed easily by anodic dissolution.
3. The pores and cracks produced by cathodic reduction grow during anodic dissolution. As a result, iron dissolution at the matrix ironwüstite interface occures and the scale layer is removed.
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