Electrochemical impedance studies of molten (0.9Na,0.1K)2SO4-induced hot corrosion of the Ni-based superalloy M38G at 900 °C in air

The hot corrosion behavior of the Ni-based superalloy M38G covered with a film of molten 0.9Na2SO4–0.1K2SO4 (mole fraction) has been studied by electrochemical impedance spectroscopy at 900 °C in air. For comparison, the corrosion of the alloy immersed in molten (Na,K)2SO4 was also examined. The electrochemical impedance spectra in deep molten salt during an initial stage consisted of a semicircle at high frequency and a line at low frequency indicating a diffusion-controlled reaction. With extended immersion the impedance spectra were composed of two capacitive loops, i.e., a small semicircle at high-frequency port, and a large one at low-frequency port. The change of the impedance spectra is related to the formation of a protective scale. Contrary to the corrosion in deep molten salt, the corrosion of the alloy in the presence of a film of fused salt presented the characteristics of two capacitive loops for all the duration of the experimental test. Equivalent circuits representing the corrosion of the alloy in both corrosion conditions are proposed to fit the impedance spectra and electrochemical parameters in the equivalent circuits are also calculated.