Corrosion behaviour of nickel in molten Li2CO3-Na2CO3-K2CO3- ternary mixture

ABSTRACT

The corrosion behavior of nickel was studied in molten Li2CO3-Na2CO3-K2CO3 mixture at temperatures of 475, 500, 525 and 550°C. The following techniques of measurements used were corrosion potential under open circuit, galvanic current, galvanostatic polarization, and impedance. The amount of nickel dissolved in the melt was determined by atomic absorption spectroscopy. These measurements showed that the oxidation and passivation of nickel may be due to the formation of NiO, which can be formed by dissolution-reprecipitation mechanism. It was proposed that the dissociation of the carbonate ions and diffusion of nickel oxide near the surface are probably the slowest steps in the oxidation and passivation of nickel, while the other steps proceed fast. At temperatures of 525 and 530°C, under open circuit conditions some NiO, oxide may dissolve in the melt, this as indicated from the obtained results. Under high polarization conditions the oxidation of divalent nickel to trivalent nickel, may occur in the solid state. The reaction occurring at passivation breakdown region is represented by the anodic decomposition of carbonate ions and evolution of CO2 and O2 gases. The high resistance of oxide scales, formed on nickel, and diffusion processes occurring there greatly affect the results of impedance measurements