No abstract

The present paper presents the results of iron aluminide coatings formed on pure iron by low pressure chemical vapour deposition. The coatings were characterised using X-ray diffraction, optical microscopy, TEM and electron microprobe analyser. Three distinct layers were seen in the optical micrographs in the coatings where deposition was carried out above the α– γ transformation temperature. The smaller thickness of the top layer compared to that of the second layer has been attributed to the smaller diffusivity of Fe compared to Al in FeAl. The formation of the third layer has been rationalised based on the fact that at deposition temperatures >1185 K, solid solution with Al above 1·5 at-% transforms to α with a definite boundary, while solid solution with less that 1·5 at-%Al will remain as γ close to the substrate. Those deposited below this temperature will remain in α at all compositions, and no boundary is formed.

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.