Potential-pO2- diagram for nickel, which is known to be corrosion-resistant material, in the molten sodium hydroxide system at 623K (35°C) has been studied. The solubility product of nickel (II) oxide, KSP, was determined to be 7.9×l0-15 by chemical analysis and also determined the standard electrode potential of Ni/NiO couple (E0 Ni/NiO) to De 0.73V vs. Na/Na+ (for pO2- = 0), by the use of following cell: From the experiment, following conclusions were derived. (1) In this system, nickel is more electronegative than oxygen but is more electropositive than hydrogen, so nickel does not react with dissolved water in molten NaOH. (2) Corrosion-resistant property of nickel in this system is attributed to the formation of the protective film on the metal surface, since the solubility product of nickel is very small as compared with other metal oxides. It is expected that the potential-pO2- diagram will be of considerable use in interpreting thermodynamic behavior of metals and for selecting materials to be used in this media. The same idea of constructing diagram shown here can be applied to other systems.
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