Influence of HCl in the Atmosphere on Hot Corrosion of Ni-Cr-Mo Alloy under a Coating of Molten NaCl-KCl-Na2SO4

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Hot corrosion behavior of Ni-10mass%Cr alloys having several Mo content under a molten NaCl-KCl-Na2SO4 mixture in 75vol%O2-N2 atmosphere containing 0.1vol%HCl was examined by thermogravimetry, chemical analyses of scale formed on the alloys and electrochemical technique. Especially, the effect of Mo addition and the influence of HCl in atmosphere on the hot corrosion behavior were considered.In the atmosphere with HCl, corrosion resistance of the Ni-Cr-Mo alloy increased with an increase in the amount of Mo in the alloy. In this case, it was found that a protective Cr2O3 layer was formed as an inner scale on the alloy. The analyses of the scale formed in the initial stage showed that for Ni-Cr-Mo alloy exposed at the gas mixture with HCl, in the initial stage less than 6ks, MoO2 was formed at the scale/alloy interface, followed by a growth of Cr2O3 layer. The electrochemical measurement showed that the anodic current with increasing polarization potential measured in the atmosphere containing HCl rose less than that in the atmosphere without HCl. Moreover, the inhibition of the rise in the anodic current was also observed when H2O vapor was introduced into the atmosphere. Consequently, the mechanism, that the protective Cr2O3 layer was formed by the anodic oxidation reaction of the alloy with H2O vapor produced by the reaction of HCl and O2- in the molten salt, was proposed.
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