Effect of cold working on oxidation behavior and nature of oxide film formed on carbon steel
Carbon steel is extensively used as a structural material in the secondary circuit of nuclear reactors. During fabrication of components such as elbows, T‐joint, expander/reducer, etc., the material is subjected to cold working. Aim of this study is to systematically investigate the effect of cold working on the oxidation behavior of carbon steel in alkaline environment at high temperature and pressure. Specimens from cold worked (up to 30%) carbon steel samples were oxidized in an autoclave at 270 °C for durations of up to 240 h in water of pH25 °C 10–10.2 obtained by adding lithium hydroxide. Oxidation rate, as calculated by weight gain, increased with extent of cold working. Potentiodynamic polarization and electrochemical impedance measurements were done in a borate buffer solution of pH 9.2 at room temperature, to characterize the oxide film formed on cold worked specimens. Impedance measurement followed by Mott–Schottky analysis showed that the defect density in the oxide increased with extent of cold working. Detailed examination established that the type and size of crystallites formed on the outermost oxide layer was the same irrespective of the extent of cold working.
Oxidation behavior and defect density in the oxide film of carbon steel was determined using weight measurement method and Mott–Schottky analysis respectively. Oxidation rate was increased with extent of cold working. Defect density in the oxide film determined by Mott−Schottky analysis was found to increase from 8.5 × 1021 to 2.2 × 1022 cm−3 with increase in extent of cold working up to 30%.
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