CORROSION BEHAVIOR OF NICKEL-IRON ALLOYS IN MOLTEN-CARBONATE

Cyclic voltammetry experiments shaw that the corrosion behavior of bin ary nickel-iron alloys shows a gradual change from iron-like behavior to nickel-like behavior. The behavior of the 50 and 75% iron alloy is similar to the behavior of pure iron, the behavior of the 10% iron all oy is similar to the behavior of pure nickel, but during a cathodic sc an, some significant differences can be observed. tit an iron content of approximately 25%, characteristics of iron-rich and nickel-rich beh avior could be observed. At cathodic potentials of -1500 mV carbonate decomposition and, when scanning anodically, carbon reoxidation (-1500 mV) proceeds faster on the iron-rich alloys than on nickel-rich alloy s. The first oxidation reaction of the metal is the oxidation of iron: a cubic solid solution of FeO and alpha-LiFeO2 is formed. This reacti on is followed by the oxidation of FeO to alpha-LiFeO2. At a more anod ic potential a cubic solid solution of alpha-LiFeO2 and NiO (ca. -700 mV) and alpha-LiFe5O3 (ca. -500 mV in cyclic voltammograms) are formed . On the surface of the 25, 50, and 75% iron alloys a dual layer is pr esent on the surface. The outer layer is alpha-LiFe5O8, the inner laye r is a cubic solid solution of(8), NiO and alpha-LiFeO2. On the 10% ir on alloy two oxide layers are present: both cubic solid solutions of L iFeO2 and NiO, with small amounts of alpha-LiFe5O8. At very anodic pot entials alpha-Fe2O3 is formed. This compound is formed slowly on the i ron-rich alloys and its formation cannot be observed in a cyclic volta mmogram. The corrosion products identified are roughly in agreement wi th the thermodynamic equilibrium diagram of Yokokawa et al.(1) The fir st reduction reaction that proceeds is the reduction of trivalent nick el ions in the scale. This reaction is followed by the reduction of al pha-LiFe5O8. In a later stage the NiO in the cubic solid solution is r educed. At potentials of approximately -1300 mV LiFeO2 in the cubic so lid solution also is reduced. At more cathodic potentials (until appro ximately -1500 mV) the reduction of bivalent and trivalent iron ions i n the oxide proceeds until contact loss between the metal and the oxid e occurs. Then, probably dissolved Ni2+, Fe3+, and Fe2+ are reduced to the metallic state.