Characterization of high-temperature oxide films on stainless steels by electrochemical-impedance spectroscopy

Oxide films formed on three stainless steels (UNS S30403; S44600; S30815) in air at 800 degrees C were characterized by electrochemical-impedance spectroscopy (EIS). The film evolution vs, oxidation time was investigated from 1 to 1000 hr. A three-electrode electrochemical cell and 0.1 M Na2SO4 solution were employed for EIS measurements. The spectra were interpreted in terms of a two-layer model of the films, where the capacitance and resistance obtained can be related to the thickness (or roughness) and defectiveness of the films. The results reveal that the oxide on S30403 grows and becomes defective, the oxide on S44600 thickens rapidly and retains its protective ability for a relatively long time, and the oxide on S30815 remains thin and resistive. The two-layer model is supported by surface characterization with SEM/EDS and in-depth profile of the oxide films obtained through glow discharge optical emission spectroscopy (GDOES).