Corrosion of Refractory Alloys in Molten Lithium and Lithium Chloride

Abstract

Several refractory materials have been considered over the years as containment material for lithium and lithium halides. Surface modified refractory metals are being extensively investigated for containment of reactive metals, radionuclides and their compounds. An overview of experimental observations and results of liquid lithium corrosion of selected engineering refractory materials are presented. The nature of the degradation and its mechanism has been explained. The influence of temperature, microstructure, stress, impurities and service time on the corrosion behavior for various refractory alloys have been discussed. Selection rules for materials of containment for liquid lithium and lithium compounds have been suggested. Recent experimental observations on the behavior of tantalum and niobium-based refractory metal alloys in a specific molten salt environment comprising LiCl/Li₂O/Li/Li₃N at 725^°C have been included in an effort to select suitable materials for molten salt equipment. It has been observed that oxygen contamination is particularly harmful for the refractory metal alloys where as nitrogen is deleterious to iron-based alloys.     

Determination of the Lithium Content of Molten Aluminum Using a Solid Electrolyte

A lithium probe has been used for the determination of the lithium content of molten aluminum alloys. The electrolyte is a two-phase mixture of Li₃PO₄ and Li₄Si0₄ to give an overall composition of Li_3.6Si_0.6P_0.4O₄. This electrolyte was found to be slightly attacked by pure liquid lithium, but no attack was detected in the aluminum-lithium melts. However, exposure to high humidity caused the electrolyte to degrade. A satisfactory reference electrode was found to be decomposed Li₂Ti₃0₇ consisting of Li₄Ti₅O₁₂ and TiO₂. An activityvs composition plot showed that Henry’s law was obeyed, and the activity coefficient was 0.17 at 984 K and 0.26 at 1050 K. P. C. YAO, formerly Research Student in the Department of Metallurgy and Materials Science, University of Cambridge     

非水溶剂电还原制取重稀土金属及合金的研究现状及发展

随着重稀土元素对铝、镁、铜、镍及钢铁等金属材料的功能化作用被广泛关注,使得作为战略性资源的重稀土元素在金属合金材料的应用领域拓展空间巨大.在此背景下,作为改性元素的重稀土金属及合金的市场需求必然猛增.由于重稀土单质自身的高熔点,混熔法、真空热还原法一直是生产制备重稀土金属及合金的主要方法,寻求一条低成本、高效、绿色的制备方法则至关重要.由于熔盐电化学还原法在生产低熔点轻稀土及合金工艺方面具有经济、高效等优势,近年来,通过熔盐电解方式直接制取低熔点的重稀土合金成为国内外学者研究的热点,从目前取得进展来看,具有广阔的发展前景.针对国内外重稀土金属及合金的熔盐电解法制取的研究现状进行了评述,从体系物理化学性质、电极过程机理、模拟计算等几个方面做了简要的总结和分析,以期对重稀土合金材料的制备研究提供有益的帮助.      

Corrosion resistance of vanadium alloys clad by a ferritic corrosion-resistant steel in liquid-metal heat-transfer agents

The results of studying the corrosion resistances of steels of various classes and vanadium-based alloys in liquid-metal heat-transfer agents (lithium, sodium, lead) are analyzed. These structural materials are shown to have satisfactory corrosion resistance in high-purity liquid metals. However, a nitrogen impurity in lithium and an oxygen impurity in sodium and lead cause the degradation of the mechanical properties and the fracture of the vanadium alloys. The use of these alloys clad by a ferritic corrosion-resistant steel is shown to ensure high corrosion resistance of the fuel-element can materials in fast neutron reactors with sodium and lead heat-transfer agents.     

金属锂在LiCl-KCl熔盐体系中的溶解过程

利用等温饱和法研究了温度对金属锂在LiCl-KCl熔盐体系中溶解度的影响。采用W电极,利用电流反向计时电位法和方波电流法研究了电流密度、LiCl含量对金属锂在熔盐中溶解速率的影响,并研究了极化对金属锂溶解速率的影响。结果表明,金属锂在LiCl-KCl中的溶解度随温度的升高而增大,在熔盐中的溶解反应为吸热反应,金属锂的溶解速率不受电流密度的影响,随LiCl含量的增加先增大后减小,溶解速率受熔盐体系黏度的影响较大。极化对金属锂溶解没有影响,锂的溶解没有电化学特性。     

Low Temperature Hot Corrosion of Cobalt-Base Alloys: Part II. Reaction Mechanism

This paper presents a mechanism of low-temperature hot corrosion that is based on rapid dissolution of the more noble metal or metal oxide in liquid salts. It is proposed that the rapid degradation of cobalt-base alloys results from dissolution of cobalt or cobalt oxides on the surface, which prevents the formation of a protective Cr₂O₃ or A1₂O₃ film. The reaction occurs in two stages: (a) an initial stage, during which an Na₂SO₄-CoSO₄ liquid forms on the surface, and (b) a propagation stage, during which SO₃ migrates inward and cobalt outward through the molten salt. At longer times, cobalt dissolves at the scale/salt interface and forms Co₃O₄ and/or CoSO₄(s) in different regions of the reaction product. The mechanisms of transport of various reactants and products through the liquid salt and the effects of their relative transport rates on the reaction product morphology have been considered.     

Density measurements of the lithium fluoride/lithium sulfide eutectic at high temperature

A straightforward and reliable method to determine densities of molten salts at high temperatures was de-veloped by Janz and Lorenz several years ago.^[1] This method was followed in order to determine the density of the LiF/Li₂S eutectic^[2] over the temperature range of 1176 to 1355 K in which the eutectic is liquid. The rel-ative lack of data for this eutectic is surprising given its potential usefulness in the study of advanced batteries'31 and electrowinning of metals from molten sulfides.^[41] The method is based on the fact that a solid piece of metal of known volume suspended from a pan balance into a molten salt will weigh less than if it were sus-pended in air at the same temperature. This difference in weight measured in grams will be equal to the buoyant force of the liquid at that temperature. The density of the salt bath can then readily be determined by dividing this difference by the volume of the solid piece of metal that is immersed in the bath. The procedure can be re-peated to give density values over a range of temperatures.     

Thermodynamic Analysis of Deoxidation of Titanium Through the Formation of Rare-Earth Oxyfluorides

In this work, the possibility of the direct removal of oxygen species from metallic Ti through the formation of rare-earth oxyfluorides has been investigated from a thermodynamic viewpoint. The deoxidation limit of β-Ti using rare-earth metals (M: Y, La, Ce, and Nd) as deoxidants was evaluated. It was found that Ti metal with an oxygen concentration of 200 mass ppm or less could be theoretically obtained under the M/MOF/MF₃ equilibrium at 1300 K (1027 °C), which suggested a possibility of reducing the oxygen content in Ti below 500 mass ppm utilizing a fluoride-based molten salt. Furthermore, a new deoxidation process, in which oxygen was removed in the form of MOF compounds using Mg deoxidant, was discussed as well. The obtained results revealed that the oxygen content in β-Ti could be theoretically reduced to a level below 1000 mass ppm using a MF₃-containing molten salt equilibrated with Mg. Rare-earth metals and their alloys are usually produced by the electrolysis in a fluoride-based molten salt; hence, the modern industrial electrolysis techniques can be potentially utilized for deoxidizing Ti scrap.     

Production of gradient structures in sintered TiC−(Ni,Mo) hard alloys on interaction with molten metals

Kinetic trends have been identified for mass transport of molten metals into sintered TiC?(Ni, Mo) hard alloys. Conditions have been identified for the mass transfer of the liquid by the migration and diffusion-migration mechanisms. Details are given of the formation of gradient structure in the TiC?(Ni, Mo) hard alloys on interaction with molten metals, where it is found that there are effects from the physicomechanical properties of the materials.     

固态氧化物熔盐电解的研究进展

本文介绍了熔盐电解固态氧化物技术在稀有难熔金属制备方面的应用,结合Ti的制备分析了FFC法的发展、工艺特点以及所面临的问题,重点阐述了导电性对FFC法生产的影响。以太阳能级硅(SOG-Si)的制备和多孔膜有效应用为例介绍了熔盐电解固态氧化物技术应用的最新研究进展。综合熔盐电解固态氧化物技术的研究现状,提出室温离子液体中电脱氧制备金属及合金,是实现冶金过程绿色生产的新方法,电解固态氧化物技术的应用有着很好的工业化前景,但要实现大规模工业生产仍然面临着诸多问题。     

THE PREPARATION AND PROPERTIES OF COPPER,NICKEL, AND IRON CONTAINING A DISPERSED OXIDE PHASE

Abstract

Three methods are described for the preparation of a stable oxide in copper, nickel, or iron. The first, which has been applied to all three metals, requires a reaction between the matrix metal oxide and the refractory oxide at high temperatures such that subsequent reduction will yield a dispersion of oxide in the matrix metal powder. In the second method, copper hydroxide or carbonate is precipitated in the presence of an aluminium salt. Ignition of the precipitate to the oxide followed by hydrogen reduction gives a copper powder containing a dispersion of alumina. In the third process, a salt that can be decomposed to a stable oxide is dissolved in molten nickel nitrate, which on rapid cooling eitherretains the additive in solid solution or as an intimate dispersion. Nickel powder containing the dispersed oxide is obtained on subsequent decomposition.

The results on sintered and worked materials show a substantial improvement in mechanical properties compared with the pure metals. Improvements are also achieved in the sintered nickel alloys, which can be sintered to high densities. The effect of the dispersion on the structure and recrystallization behaviour of these materials is discussed.     

Thermodynamic activity of magnesium in several highly-solvating liquid alloys

An emf technique is reported using porous alumina tubes to contain reference metals and alloys in a molten salt electrolyte. Data are reported for the activity of magnesium and its temperature derivative in several liquid metal solvents. These include lead (650 °C), tin (800 °C), bismuth (850 °C), antimony (850 °C), and mixtures of tin and antimony (800 °). In all cases, the solvents show strong negative deviations from ideal thermodynamic behavior, due to the effects of strong solvation caused by the formation of intermetallic compounds. The extent of this effect is Pb < Sn < Bi < Sb. The data reported are useful in evaluating solvents for the possible carbothermic reduction of magnesium oxide.     

Examination of Galvanic Action between Fe-Based Bulk Metallic Glass and Crystalline Alloys

Fe-based bulk metallic glasses (amorphous metals) have been developed, and several compositions are shown to have excellent corrosion resistance in chloride solutions. Further, thermal-spray amorphous metals are being developed for use as a barrier coating layer, to protect substrate materials from corrosion. Galvanic action between dissimilar metals and the coating/substrate for the amorphous-alloy coatings is of practical interest for a number of applications. The mixed-potential theory provides a useful approach for examining the corrosion behavior of the component materials in the galvanic couple and is applied in this study. Galvanic action was studied for an Fe-based structurally amorphous metal (SAM) 1651 and several crystalline alloys that included 1018 C-steel, stainless steel (SS) 316L, and alloy 22. Anodic and cathodic polarization curves of each of the metals were measured by potentiodynamic polarization. Based on the mixed-potential theory, the behavior of the component materials in a galvanic cell was predicted. The predictions are compared to the measured behavior of galvanic couples with the crystalline alloys. This article is based on a presentation given in the symposium entitled “Iron-Based Amorphous Metals: An Important Family of High-Performance Corrosion-Resistant Materials,” which occurred during the MSandT meeting, September 16–20, 2007, in Detroit, Michigan, under the auspices of The American Ceramics Society (ACerS), The Association for Iron and Steel Technology (AIST), ASM International, and TMS.     

On the Application of Lithium Additives in the Electrolytic Production of Primary Aluminum

The behavior of carbon–graphite subjected to treatment in the lithium carbonate Li₂CO₃ melt without cryolite and alkali-metal fluorides is studied to reliably estimate the influence of lithium on the surface layers of a carbon-containing cathode lining. The chemical composition and the structure of the carbon–graphite material after its interaction with lithium in the Li₂CO₃ melt have been studied. The high-temperature interaction of the system components in the melt is found to be accompanied by fracture of the operating surface of the carbon–graphite material, while the carbon–graphite surface does not failed upon interacting with lithium vapors. Based on the obtained data, a model for the formation of lithium ions during the reduction of lithium and its interaction with a carbon–graphite sample during the electrolysis of lithium carbonate is proposed.     

Effects of Different Calcium Compounds on the Corrosion Resistance of Andalusite-Based Low-Cement Castables in Contact with Molten Al-Alloy

Andalusite containing low-cement castables (LCCs) have been used in aluminum casthouses for several decades. CaF₂ is commonly added to the refractory to improve its corrosion resistance mainly because of its role in the formation of anorthite (CaAl₂Si₂O₈); the latter has been reported to decrease the penetration of molten aluminum alloys into refractories. This article investigates the effect of the addition of different calcium containing compounds (CaO, CaCO₃, CaSO₄, CaF₂, Clinker white cement, calcia feldspar, wollastonite, and Ca₃(PO₄)₂) on reactions with the refractory constituents to form anorthite as well as the effect of the additives on both the subsequent physical properties and the corrosion resistance of andalusite LCC refractories. Corrosion tests using the Alcoa cup test at temperatures (1123 K [850 °C] for 150 hours and 1433 K [1160 °C] for 72 hours) were conducted to determine the extent of penetration, whereas immersion tests in boiling water were conducted to determine the extent of open porosity in the material. Scanning electron microscopy coupled with energy dispersive spectrometer, optical microscopy, and X-ray diffraction techniques were employed to characterize the phase formations in the materials after the tests. The study demonstrated that both calcia feldspar and clinker white cement had the potential to be used as new additives for decreasing the penetration of molten Al-alloy into the refractory materials. Anorthite formation (in the refractory matrix), along with the absence of glassy phases, were responsible for the improvement in the corrosion resistance of the castables containing calcia feldspar. However, in the sample containing cement, the presence of calcium silicate phases were observed to resist reactions with molten aluminum. The observed results were validated using thermodynamic calculations, which indicated that tricalcium silicates (3CaO.SiO₂) and dicalcium silicate (2CaO.SiO₂) phases were more resistant than wollastonite (CaSiO₃) for applications involving contact with molten aluminum.     

热压法制备的(Mo1-xMbx)5Si3合金组织及抗腐蚀性能

以Mo粉、Nb粉、Si粉为原料,采用热压法制备(Mo<,1-x>Mb<,x>)<,5>Si<,3>(x=0,0.2,0.4)难熔金属硅化物合金,利用电化学测试及盐酸浸泡腐蚀实验,研究该合金在浓度为1 mol/L的HCl溶液中的腐蚀行为以及Nb含量对合金组织和耐蚀性的影响.结果表明,随Nb含量增加,(Mo<,1-x>Mb...     

High-temperature oxidation of materials

The high-temperature oxidation of a wide variety of materials has been investigated, viz: group I-VIII metals and their alloys; refractory compounds involving carbides, nitrides, borides, and silicides of the group IV-VI transition metals, and materials based on them; and composite materials for various applications. The basic laws and mechanisms of interaction of these materials with gaseous media have been determined. Heat resistance vs composition relationships for various types of binary systems have been defined, based on studies carried out over a broad range of temperatures (500–1600°C), concentrations of second component (0–100%), and an oxidation time of 10 hours. The redistribution of alloying elements during oxidation has been studied, and its effect on oxygen solubility, heat resistance, phase composition and morphology of the scale, and structure of the scale—alloy interface clarified. The definitive role of metal — ion diffusion in the oxidation behavior of titanium and zirconium, and the catastrophic oxidation of zirconium and its alloys, has been demonstrated. Alloying group IV metals with elements of higher valency produces a reduction in oxygen solubility due to the increase in electron concentration, and a reduction in the extent of scaling. The relationship of the heat resistance vs composition diagrams to the phase diagrams of the binary systems has been elucidated.Institute of Materials Science, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7/8(380), pp. 121–128, July–August, 1995.     

难熔高熵合金:制备方法与性能综述

从加工方法、微观结构以及各类性能三方面介绍了难熔高熵合金(Refractory high-entropy alloys,RHEAs),最后对难熔高熵合金的发展和未来进行了展望。以MoNbTaVW为代表的难熔高熵合金在高温下表现出优于传统镍基高温合金的压缩屈服强度,且屈服强度随温度的变化更加缓慢,高温力学性能优异;以MoNbTaVW、MoNbTaTiZr、HfNbTiZr等为代表的难熔高熵合金,与商用高温合金、难熔金属、难熔合金以及工具钢相比,展现出更优的耐磨性能。以W₃₈Ta₃₆Cr₁₅V₁₁合金为代表的难熔高熵合金在辐照后,除了析出小颗粒第二相外,不存在位错环缺陷结构,抗辐照性能优异。提出了难熔高熵合金未来发展的两大方向:建立高通量的实验和计算方法继续探索更多的难熔高熵合金组成和结构模型;探索多场耦合环境下难熔高熵合金的服役行为。      

Reoxidation of Ni‐ and Ni‐Fe‐Alloys by Al2O3‐SiO2 Refractory Materials

Reactions at the refractory/melt interface during ingot casting of Ni‐ and Ni‐Fe‐alloys were studied. The casts were performed using different alumino‐silicate bricks as refractory materials. Samples taken from the casting channel before and after casting were investigated using light and scanning electron microscopy with XPS. Thermodynamic calculations were performed with FactSage and the results were compared with the results from industrial tests. After the melt has infiltrated the surface layer of the bricks, refractory corrosion starts with an attack of Mn and Mg of the melt on SiO₂ and Fe₂O₃ of the refractory bonding matrix. Despite the presence of elements with higher oxygen affinity in the melt, low‐melting alumino‐silicate phases are predominantly built by the reaction with Mn and Mg. In a second step this liquid phase either traps non‐metallic inclusions from the melt or, at higher contents of Zr, Ti, Mg, Y etc. in the melt, causes massive reoxidation and inclusion formation. The refractory materials investigated show an increasing trend for reoxidation with an increasing amount of SiO₂ in glassy phases of the refractory bonding matrix. By the use of a refractory material with higher mullite content in the bonding matrix or by use of alumina bricks a strong reoxidation of the melt and intense inclusion formation can be avoided. These observations are also valid for other alloys with higher contents of elements with high affinity to oxygen.     

Use of Thermodynamic Modeling for Selection of Electrolyte for Electrorefining of Magnesium from Aluminum Alloy Melts

With United States Department of Energy Advanced Research Project Agency funding, experimental proof-of-concept was demonstrated for RE-12^TM electrorefining process of extraction of desired amount of Mg from recycled scrap secondary Al molten alloys. The key enabling technology for this process was the selection of the suitable electrolyte composition and operating temperature. The selection was made using the FactSage thermodynamic modeling software and the light metal, molten salt, and oxide thermodynamic databases. Modeling allowed prediction of the chemical equilibria, impurity contents in both anode and cathode products, and in the electrolyte. FactSage also provided data on the physical properties of the electrolyte and the molten metal phases including electrical conductivity and density of the molten phases. Further modeling permitted selection of electrode and cell construction materials chemically compatible with the combination of molten metals and the electrolyte.