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This paper reports comparative investigations of the effects of Dy2O3 and MgO additives on the hot corrosion of nickel-based alloy K3 beneath Na2SO4-V2O5 film at 1073K in air by using electrochemical impedance spectroscopy. It was shown that the Dy2O3 additive could react with V2O5 to form a refractory compound DyVO4, and reduced dramatically the corrosion of the alloy due to the formation of a porous, friable deposit and of a protective Cr2O3-rich layer. Correspondingly, the electrochemical impedance spectra were composed of two capacitive loops. In contrast, the electrochemical impedance spectra for the corrosion in both Na2SO4-V2O5 and Na2SO4-MgO environments exhibited the typical features of a diffusion-controlled reaction. The MgO additive could decrease the corrosiveness of Na2SO4-V2O5 by the formation of magnesium vanadates and promoting the formation of a protective Cr-enriched scale, but with less effectiveness than the Dy2O3 additive.