Characterisation of oxide scales formed on nickel superalloy using impedance spectroscopy

Impedance spectroscopy has been used to measure the electrical properties o f oxide scales formed from oxidation of TN738LC superalloy at high temperat ure. Electrical resistance and capacitance of the oxide scales were obtaine d from the simulation of the measured impedance diagrams based on the equiv alent circuit model, which represents the features of the oxide scales. For oxidation of IN738LC superalloy, the electrical resistance of oxide scales increased with increasing oxidation time for the specimens exposed to air at 900 degrees C. However, for the specimens oxidised at 1,200 degrees C, t he oxide scales showed very low electrical resistance, which indicated that cracking and spallation in oxide scales occurred continuously. By using sc anning electron microscopy and X-ray diffraction techniques, the compositio n and microstructure of the oxide scales were examined. It was found that e lectrical properties were determined, not only by the microstructure of oxi de scales, but also by the composition of the oxide scales. By determining the relationship between electrical properties, microstructure and composit ion of oxide scales, impedance spectroscopy could be used as a non-destruct ive technique for monitoring the oxidation of metallic alloys at high tempe rature.