Measuring the growth of oxide films on low carbon steel at 500°C by impedance spectroscopy

Abstract Oxide films, grown on low carbon steel in air at 500°C, were evaluated by concurrent room-temperature measurements of weight gain and impedance spectroscopy (IS). Weight-gain measurements displayed the expected parabolic kinetics due to growth of a relatively thick inner layer of magnetite (Fe 3 O 4 ). The Nyquist impedance spectra exhibited depressed semicircles, which were curve-fitted to obtain values of low-frequency film resistance, R , and capacitance, C . R was linear with oxidation time due to growth of a thin, porous, high-resistivity outer layer of hematite (Fe 2 O 3 ) over the underlying low-resistivity magnetite. IS shows promise for in situ non-destructive measurement of oxide scales on large low carbon steel surfaces during service by periodic measurements of oxide impedance. Further experimental measures are discussed to adapt impedance measurements to in situ determinations of oxide thickness.