CORROSION BEHAVIOR OF CHROMIUM IN MOLTEN-CARBONATE

The corrosion behavior of chromium in molten carbonate was investigate d with electrochemical techniques in combination with quenching after polarization at fixed potentials. Between -1700 and -1500 mV carbon de position takes place on the surface. The stationary corrosion product formed on chromium after polarization at -1700 mV is probably chromium carbide. Between -1600 and -300 mV a LiCrO2-layer is present on the s urface of the chromium. The layer is continuous between -1600 and -500 mV; at -300 mV the scale is nonadherent and porous. At potentials abo ve approximately -500 mV chromate formation and dissolution take place . At cathodic potentials point defects (oxygen vacancies and bivalent chromium ions) are assumed to be present in the scale, causing a high ionic conductivity. The corrosion rate is expected to be determined by a combination of applied electrode potential and electrical transport properties of the oxide layer. When the potential increases, the oxid ation rate of the chromium increases due to the larger driving force f or oxidation. In the potential region where oxygen vacancies are fille d and bivalent chromium ions are oxidized (-1100 to -1000 mV), the con ductivity of the scale decreases and the oxidation rate is determined by the transport properties of the scale: the passive properties of th e LiCrO2-scale have improved. At potentials above -500 mV chromate dis solution takes place. In the anodic scan of a cyclic voltammogram two peaks can be observed, corresponding with the oxidation of point defec ts (-950 mV), and the formation of instable intermediate chromium oxid e (-700 mV). These reactions are accompanied by the formation of lithi um chromite. While scanning cathodically, first chromate ions are redu ced (-600 mV). This is probably followed by small changes in the oxide scale. At very cathodic potentials (-1300 mV) trivalent chromium ions are reduced to bivalent chromium ions and point defects (oxygen vacan cies and bivalent chromium ions), which are incorporated in the LiCrO2 -lattice, and water is reduced. These reactions may be accompanied by the reduction of the instable chromium oxide formed during the precedi ng anodic scan. Near -1700 mV carbonate decomposes, Lithium chromite i s reduced and possibly carbide formation also takes place.