Corrosion Behavior of Nickel‐Iron Alloys in Molten Carbonate

Abstract

Cyclic voltammetry experiments show that the corrosion behavior of binary 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 alloy is similar to the behavior of pure nickel, but during a cathodic scan, some significant differences can be observed. At an iron content of approximately 25%, characteristics of iron-rich and nickel-rich behavior 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 alloys. The first oxidation reaction of the metal is the oxidation of iron: a cubic solid solution of FeO and ct-LiFeO2 is formed. This reaction is followed by the oxidation of FeO to a-LiFeO2. At a more anodic potential a cubic solid solution of a-LiFeO2 and NiO (ca. -700 mV) and a-LiFe5O8 (ca. -500 mV in cyclic voltammograms) are formed. On the surface of the 25, 50, and 75% iron alloys a dual layer is present on the surface. The outer layer is cz-LiFe5O8, the inner layer is a cubic solid solution of NiO and a-LiFeO2. On the 10% iron alloy two oxide layers are present: both cubic solid solutions of LiFeO2 and NiO, with small amounts of a-LiFe5O5. At very anodic potentials a-Fe203 is formed. This compound is formed slowly on the ironrich alloys and its formation cannot be observed in a cyclic voltammogram. The corrosion products identified are roughly in agreement with the thermodynamic equilibrium diagram of Yokokawa et al.1 The first reduction reaction that proceeds is the reduction of trivalent nickel ions in the scale. This reaction is followed by the reduction of a-LiFe5O5. In a later stage the NiO in the cubic solid solution is reduced. At potentials of approximately -1300 mV LiFeO2 in the cubic solid solution also is reduced. At more cathodic potentials (until approximately -1500 mV) the reduction of bivalent and trivalent iron ions in the oxide proceeds until contact loss between the metal and the oxide occurs. Then, probably dissolved Ni2, Fe3, and Fe2 are reduced to the metallic state. Infroduction Molten carbonate fuel cells (MCFC5) are energy conversion devices that convert chemical energy in fossil fuels into electricity. MCFCs are operated at about 923 K. The state-of-the-art anode consists of nickel with 2-10% chromium, the cathode consists of in situ lithiated nickel oxide and the matrix of porous lithium aluminate. The cells are connected in series by a metal separator plate.

Nickel-Iron-Copper Alloy as Inert Anode for Ternary Molten Carbonate Electrolysis at 650°C

“…LiFeO 2 has a cubic structure in which Li and Fe atoms randomly occupy the octahedral sites in a cubic close packing of oxygen atoms. 20,21 NiO also has a cubic structure, Du et al found that NiO has a uniform and dense octahedron structure in Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 melt at 650 • C. 22 As the lattice parameters of NiO and LiFeO 2 are relatively similar, they tend to form solid solutions, which is in good agreement with the study of Vossen et al 17 After 100 h anodic polarization, the oxide particles are still in size of ∼1-2 μm with octahedral morphology as shown in Figures 4c and 4d. It suggests that the oxide film is physically and chemically stable during anodic polarization.…”

“…After 40 h polarization, new diffraction peaks related to NiO (PDF#04-0835) and LiFeO 2 (PDF#70-2711) appear. 17 It was reported that NiO, NiFe 2 O 4 and CuO composite film formed on Ni11Fe10Cu alloy by air oxidation, 14 and NiO, NiFe 2 O 4 and Cu three-layered scale formed on the alloy by electrochemical oxidation in binary carbonates of Na 2 CO 3 -K 2 CO 3 . 14 Interestingly, no NiFe 2 O 4 and CuO was observed in the oxide film according to the XRD patterns.…”

“…Cheng et al reported that a compact and coherent oxide layer consisting of NiFe 2 O 4 , NiO and CuO could form on Ni11Fe10Cu during air oxidation at high temperature. 14 Since CuO is soluble in molten ternary carbonates in some degree and NiFe 2 O 4 can react with Li 2 O, 15,16 it is still unknown whether a stable oxide scale is able to form in molten Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 .Vossen reported that the oxide scale of cubic solid solution of NiO and LiFeO 2 and small amount of LiFe 5 O 8 can form on the surface of Ni10Fe alloy at a fairly anodic potential in molten Li 2 CO 3 -K 2 CO 3 17 and the NiO and LiFeO 2 film can effectively protect the substrate. It is therefore interesting to know if a stable and effective oxide film can form on the Ni11Fe10Cu alloy in the ternary carbonates.…”

Pitting Corrosion of Some Stainless Steel Alloys Preoxidized at Different Conditions

“…Barrier layer may be formed by a dissolution-precipitation mechanism and/or a solid state mechanism. The results of x-ray diffraction analysis [6] MO n/2 + n/2 CO 2 + ne (7) in the case of carbonate melt and the investigated alloys (1,2,3), it can be proposed, as in previous works [8,14,15], that the oxidation and passivation of the alloys may proceed according to the following reactions:…”

Optimization of the Cathode Long-Term Stability in Molten Carbonate Fuel Cells: Experimental Study and Mathematical Modeling

“…The temperature was between 85-95°C. By varying the amount of cobalt sulphate in the bath different ternary alloys of Ni-Co-P can be obtained [19]. The Co-Ni encapsulated SS304 was rinsed with deionized water and subsequently dried at 65°C for 4 hours.…”