High Temperature Corrosion Kinetics of Ni-Cr Alloys in HCl-containing Atmosphere

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High temperature corrosion kinetics of Ni-Cr alloys in a 1%HCl-50% O2-N2 atmosphere was investigated by thermogravimetric technique. The mass change curves of the specimens showed a slight gain in the initial period and a subsequent linear loss. Since such corrosion behavior was due to the formation of NiCl2 and its vaporization besides the oxide formation, the equation of mass change curve included the terms of mass gain and evaporation curve. However, the conventional kinetics consisting of the terms of a parabolic mass gain curve and a linear evaporation curve did not completely describe the experimental results in the 1% HCl atmosphere. Therefore, in this study, the kinetics equation was introduced on the assumption that the evaporation curve was divided into two curves; one was the curve which represented the rate of chlorination of Ni and the other was the curve which represented the chlorination of NiO. The chlorination of NiO occurred owing to the solving of NiCl2 into the NiO scale. In addition, it was presumed that some incubation periods existed until the vaporization of NiCl2 became significant. The kinetics equation introduced on these assumptions well described the experimental mass change of Ni-Cr alloys very well in the 1% HCl atmosphere.
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