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The oxidation characteristics of Zr-xNb (x=0, 0.2, 1.5, 3.0, 10 wt.%) alloys were investigated in the LOCA temperature ranges of 800 to 1200℃ under the steam condition, using a modified thermo-gravimetric analyzer. In all temperature ranges, the oxidation rate constants of the alloys were controlled by parabolic rate law. The rate constants increased gradually as the Nb contents increased at temperatures below 1000℃. This is the reason why the oxidation reaction of Zr-xNb alloys was enhanced by the formation of Zr (3phase at temperatures of 800 to 1000℃. But the rate constants decreased slowly with increasing the Nb contents at tempeartres above 1000℃. The prior 13-Zr region of Zr-xNb alloys were consisted with an a-Zr(0) phase having grain-boundary shape and a needle-like Widmansta¨tten phase incorporated into the a-Zr(0) phases above 1000℃. And, above 1000℃, the oxides of the ZrxNb alloys were duplex structures, i.e. outer oxide: columnar structure, inner oxide: network structure. The inner network-structured oxides would be caused by the formation of niobium oxides in the oxide and would retard the oxygen diffusion through the oxide. This could result in reducing the rate contants of Zr-xNb alloys in the temperature range of 1000 to 1200℃. (Received April 9, 2004)

keyword : Zirconium alloy, Zr-Nb system, High-temperature oxidation, Steam oxidation, Oxidation kinetics, Oxidation rate constant, Loss-of-coolant accident, LOCA

The objective of this study is to understand the effect of Si, on the corrosion property of the Ca-modified weathering steel and to determine the proper range of such alloying elements to maximize the effect of Ca on the corrosion resistance of the steel. The pH of a thin water film covering the steel was measured to evaluate the effect Ca addition on pH value. The electrochemical impedance spectroscopy (EIS) was employed to evaluate the corrosion resistance of the Ca-modified weathering steel. The characteristics of rust formed on the weathering steels having different alloy chemistry were examined. Analysis on the inclusion morphology and composition showed that the Ca-containing inclusions were mainly composed of Ca-Al-Mn-O-S,and Si was not founded in them except the specimen containing 0.6%Si. In distilled water, the Ca-modified specimens with or without Si showed higher pH values as Ca content was increased. The effect of Ca addition was not greatly affected by the addition of Si when the steel contained a proper Ca content of 50-70 ppm. In the impedance test, the specimens containing higher amounts of Ca showed higher polarization resistance, meaning better corrosion resistance. The analysis on the rust layers formed on the Ca-modified steel surface showed that the dense inner layer was mainly composed of a-FeOOH and y-FeOOH, whereas the loose outer layer was composed of ν-FeOOH and β-FeOOH. Fe₃O_(4) was distributed randomly throughout the rust layer. The specimens containing a high amount of Ca showed a dense and compact morphology, and the specimens containing Si also showed a dense and compact one. The result of Mossbauer analysis showed that more α-FeOOH was formed as Ca content was increased. (Received May 24, 2004)

keyword : Weathering steel, Ca addition, Corrosion resistance, α-FeOOH

The synthesis process of iron carbide was proceeded in two stages of reduction of hematite fines by H₂ gas, and carburization of the reduced metal by CO-H₂ gas mixture at 600℃. The surface of reduced iron was pre-treated with an addition of 0.05 vol% NH₃-Ar before carburization. The synthesized iron carbides were comprehensively explored by C/S analyzer (Low C/S determinator), Mossbauer spectroscopy, X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy at various reaction times of 5, 10, 15, 20, 25, 30, and 35 min. By addition of a small amount of NH3 gas, the super iron carbides contained 10 wt% carbon were made, and such addition stabilized iron carbides. It was found that addition of NH3 gas played a major role in the formation of iron carbide without decomposition (Fe₃C→ 3Fe+C) of iron carbides and precipitation of free carbon. From these results, we succeed in synthesis of super iron carbide, Fe_(5)C₂, with a stable single phase without Fe and Fe₃C phases. (Received August 2, 2004)

keyword : Super iron carbide(Fe5C2), Free carbon, NH3 gas, Synthesis

The solvent extraction to separate and recover cobalt and nickel from sulfuric acid solution was studied using Cyanex 272 in kerosene. The solution processed was equivalent to that expected from leaching of a matte, intermediate product of manganese nodules. The extraction of both metals increased with increasing equilibrium pH and extractant concentration. However, cobalt was preferentially extracted over nickel. The extraction efficiencies of cobalt and nickel were 80% and 1.9% respectively with 0.2M Cyanex and O/A of 1 at equilibrium pH 5.0. The separation factor of Co/Ni increased with raising the extraction temperature (205 at 20℃, and at 441 at 50℃). The saponification of Cynanex 272 increased the extraction of cobalt due to the increase of equilibrium pH. Nickel (0.40 g/l) loaded in an organic phase could be scrubbed with a solution containing 10 g/l Co at equilibrium pH of 6.0. The stage numbers and cobalt recoveries in extraction and stripping processes required for the counter current operations are predicted using the equilibrium isotherm plots. (Received May 19, 2004)

keyword : Solvent extraction, Cyanex 272, Cobalt, Nickel, Scrubbing, Stripping, Manganese nodules, Leaching solution