Corrosion behaviour of nickel-chromium alloys in molten carbonate

The corrosion behaviour of nickel-chromium alloys with chromium contents ranging from 10 to 25 % in molten carbonate under reducing gas atmospheres was investigated with electrochemical methods (cyclic voltammetry). The oxide scale was investigated, both after quenching from stationary conditions at fixed potentials and shortly after interrupting a cyclic voltammogram. The composition and the properties of the corrosion products, present as oxide layers, are potential-dependent. At potentials in the range of -1500 to -900 mV the layer consists of lithium chromite (LiCrO2). For 18- and 25 %-chromium alloys a continuous layer is formed. The lithium chromite formed on the 10 %-chromium alloy at potentials in the same range does not form a continuous oxide layer. At potentials in the range of -700 to +100 mV a continuous oxide layer is formed on all alloys The corrosion product is a cubic solid solution of nickel oxide (NiO) and lithium chromite. In cyclic voltammetry experiments a hexagonal solid solution of lithium chromite and nickel oxide is formed at -700 mV. The hexagonal solid solution is an intermediate corrosion product between lithium chromite, which is stable at more cathodic potentials and a cubic solid solution of nickel oxide and lithium chromite which is stable at more anodic potentials. On the 10% chromium alloy with a non-continuous oxide scale, a cubic solid solution is formed at -500 mV. At about -300 mV the chromate formation and dissolution starts. For the 18- and 25%-chromium alloys, on which a continuous lithium chromite-scale is formed at the open circuit potential, a hexagonal solid solution of lithium chromite and nickel oxide is formed during the anodic scan at -700 mV. When the chromate formation and dissolution starts at about -300 mV, the chromium content of the scale decreases, the nickel content increases and the hexagonal solid solution becomes a cubic solid solution. The cathodic scan is very similar to the cathodic scan of pure nickel. During the cathodic scan the oxide scale mainly consists of a nickel-rich cubic solid solution of nickel oxide and lithium chromite because chromium has dissolved as chromate ions during the preceding anodic scan. The reactions are essentially the same as those on pure nickel [1, 2, 3].