CORROSION OF NICKEL IN MOLTEN-CARBONATE

Cyclic voltammetry and polarization measurements on nickel under reduc ing gas atmospheres were performed to investigate the corrosion behavi or of nickel as a base metal for molten carbonate fuel-cell separator plate alloys. The anodic reactions observed and oxidation of hydrogen, oxidation of nickel to nickel oxide and to dissolved nickel ions, and oxidation of bivalent nickel to trivalent nickel incorporated in the oxide scale. The cathodic reactions are reduction of trivalent to biva lent nickel in the scale and reduction of bivalent nickel to metallic nickel. During the cathodic scan of the cyclic voltammogram a change i n the morphology of the oxide takes place. This is probably due to rec rystallization of the oxide. During the cathodic reduction of NiO the scale loses contact from the metal at approximately - 1200 mV; a small crevice is formed between the metal and the scale. The stresses in th e loose scale cause cracking. Consequently, the crevice is filled with carbonate. Then nickel oxide dissolves, forming complexes of nickel a nd carbonate ions. These complexes are reduced to nickel (and carbonat e ions) in the crevice. Due to the basic nature of the melt in the cre vice, the nickel is oxidized in the following anodic scan, at lower po tential (approximately - 1200 mV) than observed for an oxide-free surf ace (approximately - 700 mV).