In situ impedance spectroscopic analysis of oxidation kinetics and oxide‑scale thickness evolution on Haynes‑230 at high temperature

The inductive and capacitive behavior of heat-treated Haynes-230 was investigated using electrochemical impedance spectroscopy (EIS) up to a temperature of 1000 °C and at different relaxation times during cooling to room temperature. Haynes-230 was heat treated at 1000 °C and analyzed for its in situ electrical behavior over the applied frequency range. The high thermal energy influences the formation of oxides, carbides and nitrides in the solid state, which is reflected in the inductive response associated with ionic conduction. The capacitance behavior was observed as two distinct semicircles in the Nyquist plot due to bulk and grain boundary responses. In situ oxidation behavior was recorded using impedance spectroscopy and the oxidation kinetics and evolution of oxide phases were studied using high temperature X-ray diffraction (HTXRD) and high temperature Raman spectroscopy. The surface morphology and elemental composition were examined using scanning electron microscopy (SEM), whereas Raman spectroscopy was used to identify the oxide phases and monitor their evolution during high temperature exposure. The mass gain (g/cm 2 ) was measured using thermogravimetric analysis (TGA). The EIS results were analyzed based on the inductive and capacitive responses observed in the Nyquist plots, and the oxide layer thickness was theoretically estimated from parameters such as relaxation frequency (Hz), activation energy (eV) and other relevant fitting parameters. The calculated oxide layer thickness from the EIS data was correlated with the cross-sectional oxide layer thickness obtained from SEM studies of the heat-treated Haynes-230 samples.