A Study of Hot Corrosion of Fe-Cr Alloys Induced by a Thin Film of Fused (Na,K)2SO4 by Electrochemical Impedance Spectroscopy

To understand the hot corrosion mechanism of materials it is of significance to establish electrochemical measurements under a thin film of fused salts. In this paper, the hot corrosion behavior of Fe-Cr alloys containing 5, 10 and 25wt% Cr, respectively, in the presence of a fused film of 0.9Na SO -0.1K SO (mole fraction) at 1173 K in air has been investigated by electrochemical impedance spectroscopy using a two-electrode system. The Nyquist plots for the corrosion of both Fe-5Cr and Fe-10Cr are composed of a very small semicircle at high frequencies and a line at low frequencies indicating that the corrosion is controlled by the diffusion of oxidants in the fused salt film. The alloys were severely corroded, forming a thick porous corrosion layer containing large amounts of non-protective precipitated Fe oxide particles. Conversely, the Nyquist plots for the corrosion of Fe-25Cr are composed of double capacitive loops, with significantly larger impedance values, which are associated with the formation of a protective Cr scale. Based on the experimental diffusion impedance results and the theoretic solubility and diffusion coefficient of oxygen in molten salts, respectively, a theoretical value of the diffusion flux of molecular O has been calculated.