Characterization of Hot Corrosion of Fe-20Cr Coated with A Fused (Na,K)_2SO_4 Film By Electrochemical Impedance Spectroscopy

摘要

Electrochemical studies of hot corrosion were conducted mostly in deep molten salts, which are not identical to a fused salt film in service conditions. In order to establish electrochemical measurements under a fused salt film, a two-electrode probe was proposed for electrochemical impedance studies of hot corrosion of Fe-20Cr alloy induced by a fused 0.9Na_2SO_4-0.1K_2SO_4 (mole fraction) film at 1173K in air. The corrosion of the alloy was characterized by an initial incubation stage with a relatively low corrosion rate and a later stage with rapid material degradation, accompanied with quite different impedance features. In the initial stage, the alloy formed a protective Cr-rich oxide scale, and the corresponding impedance spectra were composed of two capacitive loops. With extended exposure, the electrochemical impedance spectra showed the typical characteristics of a diffusion-controlled reaction, which was associated with the degradation of Cr-rich scale and the formation of a no-protective mixed scale of oxides and sulfides owing to fluxing of oxide scale as well as internal sulfidation of Cr. Two equivalent circuits were proposed to fit the impedance spectra in two stages. Based on the transport of molecular O_2 in the melt, Warburg impedance was introduced to calculate the product (C_(O_2) D_(O_2)~(1/2) ) of solubility and diffusion coefficient for oxygen in the melt, and the main reduction reaction was also discussed.