Nickel (II) Oxide Solubility and Phase Stability in High Temperature Aqueous Solutions

A platinum-lined, flowing autoclave facility was used to investigate the solubility behavior of nickel(II) oxide (NiO) in deoxygenated ammonium and sodium hydroxide solutions between 21 and 315 C. Solubilities were found to vary between 0.4 and 400 nmol kg{sup -1}. The measured nickel ion solubilities were interpreted via a Ni(II) ion hydroxo-and amino-complexing model and thermodynamic functions for these equilibria were obtained from a least-squares analysis of the data. Two solid phase transformations were observed: at temperatures below 149 C, the activity of Ni(II) ions in aqueous solution was controlled by a hydrous Ni(II) oxide (theophrastite) solid phase rather than anhydrous NiO (bunsenite); above 247 C, Ni(II) activities were controlled by cubic rather than rhombohedral bunsenite

Corrosion Behavior of NiCrFe Alloy 600 in High Temperature, Hydrogenated Water

The corrosion behavior of Alloy 600 (UNS N06600) is investigated in hydrogenated water at 260 C. The corrosion kinetics are observed to be parabolic, the parabolic rate constant being determined by chemical descaling to be 0.055 mg dm{sup -2} hr{sup -1/2}. A combination of scanning and transmission electron microscopy, supplemented by energy dispersive X-ray spectroscopy and grazing incidence X-ray diffraction, are used to identify the oxide phases present (i.e., spinel) and to characterize their morphology and thickness. Two oxide layers are identified: an outer, ferrite-rich layer and an inner, chromite-rich layer. X-ray photoelectron spectroscopy with argon ion milling and target factor analysis is applied to determine spinel stoichiometry; the inner layer is (Ni{sub 0.7}Fe{sub 0.3})(Fe{sub 0.3}Cr{sub 0.7}){sub 2}O{sub 4}, while the outer layer is (Ni{sub 0.9}Fe{sub 0.1})(Fe{sub 0.85}Cr{sub 0.15}){sub 2}O{sub 4}. The distribution of trivalent iron and chromium cations in the inner and outer oxide layers is essentially the same as that found previously in stainless steel corrosion oxides, thus confirming their invariant nature as solvi in the immiscible spinel binary Fe{sub 3}O{sub 4}-FeCr{sub 2}O{sub 4} (or NiFe{sub 2}O{sub 4}-NiCr{sub 2}O{sub 4}). Although oxidation occurred non-selectively, excess quantities of nickel(II) oxide were not found. Instead, the excess nickel was accounted for as recrystallized nickel metal in the inner layer, as additional nickel ferrite in the outer layer, formed by pickup of iron ions from the aqueous phase, and by selective release to the aqueous phase

Thermodynamics of Cr2O3, FeCr2O4, ZnCr2O4 and CoCr2O4

High temperature heat capacity measurements were obtained for Cr{sub 2}O{sub 3}, FeCr{sub 2}O{sub 4}, ZnCr{sub 2}O{sub 4} and CoCr{sub 2}O{sub 4} using a differential scanning calorimeter. These data were combined with previously-available, overlapping heat capacity data at temperatures up to 400 K and fitted to 5-parameter Maier-Kelley C{sub p}(T) equations. Expressions for molar entropy were then derived by suitable integration of the Maier-Kelley equations in combination with recent S{sup o}(298) evaluations. Finally, a database of high temperature equilibrium measurements on the formation of these oxides was constructed and critically evaluated. Gibbs energies of Cr{sub 2}O{sub 3}, FeCr{sub 2}O{sub 4} and CoCr{sub 2}O{sub 4} were referenced by averaging the most reliable results at reference temperatures of 1100, 1400 and 1373 K, respectively, while Gibbs energies for ZnCr{sub 2}O{sub 4} were referenced to the results of Jacob [Thermochim. Acta 15 (1976) 79-87] at 1100 K. Thermodynamic extrapolations from the high temperature reference points to 298.15 K by application of the heat capacity correlations gave {Delta}{sub f}G{sup o}(298) = -1049.96, -1339.40, -1428.35 and -1326.75 kJ mol{sup -1} for Cr{sub 2}O{sub 3}, FeCr{sub 2}O{sub 4}, ZnCr{sub 2}O{sub 4} and CoCr{sub 2}O{sub 4}, respectively

Immiscibility in the Nickel Ferrite-Zinc Ferrite Spinel Binary

Immiscibility in the trevorite (NiFe{sub 2}O{sub 4}) - franklinite (ZnFe{sub 2}O{sub 4}) spinel binary is investigated by reacting 1:1:2 molar ratio mixtures of NiO, ZnO and Fe{sub 2}O{sub 3} in a molten salt solvent at temperatures in the range 400-1000 C. Single phase stability is demonstrated down to about 730 C (the estimated consolute solution temperature, T{sub cs}). A miscibility gap/solvus exists below Tcs. The solvus becomes increasingly asymmetric at lower temperatures and extrapolates to n - values = 0.15, 0.8 at 300 C. A thermodynamic analysis, which accounts for changes in configurational and magnetic ordering entropies during cation mixing, predicts solvus phase compositions at room temperature in reasonable agreement with those determined by extrapolation of experimental results. The delay between disappearance of magnetic ordering above T{sub C} = 590 C (for NiFe{sub 2}O{sub 4}) and disappearance of a miscibility gap at T{sub cs} is explained by the persistence of long-range ordering correlations in a quasi-paramagnetic region above T{sub C}

Investigation of flow accelerated corrosion models to predict the corrosion behavior of coated carbon steels in secondary piping systems

To investigate various methods to mitigate flow accelerated corrosion of carbon steels, we have deposited various metallic and composite coatings on the surface of carbon steels and tested their performance by exploiting flow accelerated corrosion (FAC) simulation experiments. From the results, we found that both Ni-P/TiO2 composite coating and Fe-based amorphous metallic coating exhibited outstanding FAC resistance thus they are expected to expand the life-time of secondary systems of nuclear power plants. Furthermore, to investigate their life-time in nuclear power plants, we investigated known mechanistic models and commercial models of FAC and imported the parameters of the coated carbon steels into the models

Development of a Microfluidic Setup to Study the Corrosion Product Deposition in Accelerated Flow Regions

Corrosion: Reproducing deposition in nuclear power plants A simplified micro-fluidic system can successfully emulate corrosion products deposition in nuclear reactor water circuits. A team led by Fabio Scenini at the University of Manchester in the U.K. used a stainless steel disc with a micro-orifice and a micro-fluidic cell to build a system recreating the accelerated flows of an operating power plant. They monitored the pressure drop and build-up rate of corrosion products in real time, showing that more efficient setup reproduced corrosion seen under plant conditions, and that spallation of built-up oxide was a consequence of competition between its complex hydrodynamic and electrokinetic preferential deposition and its removal at high velocities. When adding lithium to the water, corrosion oxide formation was limited. Applying this methodology may help us better understand corrosion in nuclear reactors