MgO as an alkaline earth oxide with high chemical stability can inhibit significantly got corrosion, especially vanadate-induced corrosion of alloys. It is shown that rare earth element Dy and its oxide can greatly improve the corrosion resistance of alloy. This paper presents comparative investigations of the effect of Dy2O3 and MgO on the air oxidation of type 304 stainless steel and on the Na2SO4-V2O5 induced hot corrosion of nickel based alloy K3.
Sol-Gel method was employed to prepare MgO, Dy2O3 and MgO-15%Dy2O3 oxide films on 304 stainless steel, and the effect of the oxide films on the oxidation of the alloy at 900℃ in the air was examined by thermogravimetry (TGA), X-ray diffraction (XRD) and scanning electron microscopy equipped with an energy dispersive X-ray analyzer (SEM/EDX). Experimental results indicated that MgO and Dy2O3 films decreased greatly the oxidation rate of the alloy by promoting the formation of a Cr2O3-rich scale and improving the adhesion of the oxide scale, with a better effectiveness for the Dy2O3 film.
Electrochemical impedance spectroscopy is a technique which has proved effective in understanding reaction mechanisms and kinetics, and by which more information for corrosion processes may bi obtained. In this paper, two-electrode system was established to study the effect of Dy2O3 and MgO additives on the hot corrosion behavior of nickel-based alloy K3 in molten 0.4Na2SO4-0.6V2O5 (in mole fracton) at 800℃. Experimental results indicated that K3 suffered from fast corrosion in Na2SO4-0.6V2O5 environment, forming a porous scale. Correspondingly, electrochemical impedance spectra for the corrosion of K3 exhibited the typical features of a diffusion-ontroled reaction.
The addition of 10 and 20wt% Dy2O3 or MgO to Na2SO4-0.6V2O5 could decrease greatly the corrosion rate of K3, but with different inhibiting effect. MgO could react with V2O5 to form Mg3V2O8 with a high melting point. However, the formation of liquid Na6Mg(SO4)4 was also observed. The liquid phases can act as binders between the ash particles, and give rise to compaction of the salt deposits. The corrosion of the alloy was still controlled by the diffusion of oxidation through the salt layer. In contrast to MgO, Dy2O3 additive could just react with V2O5 to form refractory DyVO4, leading to the formation of a porous, friable deposit, which can be removed easily. Moreover, the Dy2O3 additive could promote the formation of a protective Cr and Al enriched oxide layer. As a result, electrochemical impedance spectra for the corrosion in the presence of Dy2O3 were composed of double capacitive loops. Conclusively, Dy2O3 exhibits better inhibiting effectiveness than MgO.
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