Characterization of high-temperature oxide films on dysprosium-doped Fe-20Cr alloys by electrochemical techniques

The oxidation properties of Fe-20Cr, Fe-20Cr-0.2Dy and Fe-20Cr-1Dy alloys were studied using gravimetric and electrochemical techniques. The high-temperature oxide films of Dy-doped Fe-20Cr alloys were prepared in air at 900 °C for 24, 48 and 100 h, respectively. The electrochemical experiment was performed by a three-electrode electrochemical cell and in 0.1 mol/L Na₂SO₄ aqueous solution. Proper models were built for describing electrochemical impedance spectroscopy of the different oxide layers and the spectra were interpreted in terms of a two-layer model of the films. The results revealed that the oxide films of Dy-doped Fe-20Cr alloys became compacter than that of undoped alloys and retained their good protective ability for a relatively long time. With increasing content of Dy, the protection of the oxide films slightly decreased. Mott-Schottky curves indicated that all the oxides were n-type semi-conductors, and the N_d value of oxide film on Fe-20Cr was much larger than that of Dy-doped Fe-20Cr alloys. The results of kinetic curves and SEM were in agreement with electrochemical impedance spectroscopy and Mott-Schottky data.