| Various electrochemical and non-electrochemical techniques were used to study the reaction mechanisms in pure Na(,2)SO(,4) melt and Na(,2)SO(,4) melt containing V(,2)O(,5) or CoSo(,4) at 900 C. These techniques included potentiodynamic polarization experiment, cyclic voltammetry, chronopotentiometry, chronoamperometry, A.C. impedance, acid-base titration, gas absorption rate measurement, electronic transference number measurement and TGA measurement.; In a pure Na(,2)SO(,4) melt under an atomosphere of O(,2) + 0.15%SO(,2), the primary electrochemical reactions involving gas species is the reduction of pyrosulfate ions to form SO(,3)('-). The decomposition of NA(,2)SO(,4) melt was found to form SO(,3), O(,2), and O(,2)('-) under a large positive applied potential, and to form S('2-), O('2-) and O(,2)('2-) under a large negative applied potential.; Corrosion of metals in melts containing V(,2)O(,5) is greatly accelerated, often catastrophic. The effects of V(,2)O(,5) on accelerating the corrosion rate are manifold: (1) the reduction-oxidation reactions of VO(,3)('-) + e('-) = VO(,3)('2-) and V(,2)O(,7)('4-) 2 e('-) = V(,2)O(,7)('6-) have very positive redox potentials and high exchange current densities (high reversibility); therefore they can effectively act as an electron accepter for the corrosion of metals to increase the corrosion rate. (2) the solubility of O(,2)SO(,2) and SO(,3) as well as their absorption rates in Na(,2)SO(,4) increase of V(,2)O(,5). This increases not only the rate of supply but also the amount of oxygen and sulfur oxide species required for metal oxidation and sulfidation reactions. (3) because V(,2)O(,5) is an acidic compound, the melt containing V(,2)O(,5) will become more acidic which has a significant effect on the solubility of the otherwise protective oxides.; In the CoSO(,4)-containing Na(,2)SO(,4) melt, the existence of a reduction-oxidation reaction of cobalt complex ions introduces an additional powerful cathodic reaction besides the reduction reaction for the gaseous oxidant, and therefore it accelerates the corrosion rate of a metal. However the measured corrosion rate of several metals and alloys showed that the maximum corrosion rate occurred in a Na(,2)SO(,4) melt containing 5 m/o CoSO(,4). CoO and CoS can be cathodically formed on a platinum foil from a CoSO(,4)-containing Na(,2)SO(,4) melt. The formation of CoO and CoS occurs by instantaneous three-dimensional nucleation processes, followed by a growth controlled by hemispherical diffusion. |
