The use of rotating electrodes in the electrolysis of molten zinc chloride electrolytes

In order to decrease the resistive losses in molten salt electrowinning cells, it is necessary to reduce the interelectrode spacing. Under stationary conditions, this can only be achieved at the detriment of the current efficiency. However, by rotating 100-mm-diameter electrodes, it was found for plane disc electrodes that at a current density of 10,000 A m⁻², a separation of 4 mm, and a rotation speed of 100 rpm, a current efficiency of 75.5 pct for the electrolysis of zinc from a zinc chloride, sodium chloride, potassium chloride electrolyte was achieved. This was 31 pct above that of the equivalent stationary case. The use of conical electrodes which combined centrifugal and gravitational separation increased the current efficiency to 85.7 pct at a rotation speed of 44 rpm. Increasing the scale of the electrodes to 200 mm in diameter resulted in similar efficiencies. These results and a low temperature study using perspex discs and cones in water indicated that the removal of the anode gas may be the rate-limiting step. Formerly Research Student with the Department of Materials Science and Metallurgy, University of Cambridge.     

Activities in liquid Fe-V-O and Fe-B-O alloys

The activities in liquid Fe-V-0 and Fe-B-O alloys have been determined using the following galvanic cells Cr-Cr₂O₃(s) | ZrO₂(CaO) | Fe-V-O (l, saturated with oxide) Cr-Cr₂O₃(s) | ThO₂(Y₂O₃) | Fe-V-O (l, saturated with oxide) Cr-Cr₂O₃(s) | ZrO₂(CaO) | Fe-B-O (l, B₂O₃ saturated with Al₂O₃) The solubility of oxygen in Fe-V alloys at 1600°C decreases with increasing vanadium content to a minimum of about 180 ppm at 3 wt pct V, and then increases to over 4000 ppm at 36.3 wt pct V. Vanadium was found to decrease the activity coefficient of oxygen and the value of the interaction coefficient e_o ^V at infinite dilution of vanadium is -0.14. The activity of vanadium was calculated from the measured electromotive force, and log γ_v was found to be represented well by the quadratic formalism for N_v < 0.4: log γ_V = -0.70N ² _Fe -0.30 At 1550°C boron decreases the solubility of oxygen down to about 80 ppm at 0.67 wt pct B in Fe-B melts in equilibrium with B₂O₃ saturated with A1₂O₃ (N_{Al 2} O₃ = 0.087). The boron deoxidation product, ’K′ = (wt pct B)²(wt pct 0)³ at infinite dilution of boron is 4.4 × 10_-9 and 1.5 × 10^-8 at 1550° and 1600°C, respectively. Boron decreases the activity coefficient of oxygen in liquid iron, and the value of the interaction coefficient e_o ^B is -2.6 at infinite dilution of boron. The activity coefficient of boron at infinite dilution (γ^° _B) is 0.083 at 1550°C relative to solid boron.     

Tensile properties of directionally solidified Al-4 wt pct Cu alloys with columnar and equiaxed grains

The tensile properties of directionally solidified Al-4 wt pct Cu-0.15-0.2 wt pct Ti alloys with equiaxed grains were determined and compared with the properties of directionally solidified Al-4 wt pct Cu columnar structures. The tensile properties of the equiaxed structure were isotropic, but varied with the distance from the chill face. The mechanical properties of the equiaxed structure were generally between those of the longitudinal and transverse columnar structures. The 0.2 pct offset yield stress(σ _y, MPa) is represented as a function of the grain size,d (mm), the average concentration, C_o (wt pct), and the local concentration, C (wt pct), by σ_y = [(15.7 + 22.6 C_o) + (1.24 + 1.04 C_o)d ^-1/2] + [15.7 △C], where △C = C - C_o. The equiaxed structure exhibits inverse segregation similar to that in the columnar structure.     

Desulfurization of iron alloys in vacuo-free energy of formation of SiS (v)

The rate of desulfurization of Fe-C-Si-S meltsin vacua (10 ώm pressure) has been investigated using 15 kg alloys melted by induction heating. For the sulfur contents from 0.008 to 0.1 pct investigated, the rate is a first-order type with respect to sulfur. Over this sulfur concentration range and in the presence of carbon up to saturation and silicon from 0 to 5 pct, the sulfur activity is sufficiently high that the fractional surface coverage by adsorbed sulfur is within 0.75 < θ_S < 1. The rate equation derived to satisfy the experimental findings for the limiting case of θ_S → 1 indicates that sulfur is evolved primarily via two activated reactions involving (SiS₂)° and (S₂)°. These complexes then produce SiS and S vapor species. At 1600°C the rates of formation of these volatile species are about one fourth of those for free vaporization of SiS and sulfur, respectively. The apparent heats of activation are ∼47 kcal for SiS and ∼37 kcal for sulfur vapor. Using an apparatus involving a Knudsen cell and the Bendix mass spectrometer, the enthalpy of reaction SiS₂(s) + Si(s) = 2SiS(v) has been measured, giving 102.2 kcal. Combining this with other thermodynamic data, the free energy of formation of SiS vapor is evaluated as Si(s) + 1/2S₂(v) = SiS(v) δF° = 15,500-19.5T from 1000° to 1686°K Si(l)+ 1/2S₂(v)= SiS(v) δF°= 3500 -12.4T above 1686°K     

Structure and mechanical properties of unidirectionally solidified Fe-Cr-C and Fe-Cr-X-C alloys

In this work four different microstructures were obtained by unidirectional solidification of Fe-Cr-C eutectic alloys. Conditions for zone coupled growth were determined in alloys containing approximately 30 wt pct chromium. Furthermore, mechanical testing indicated that the maximum strength was exhibited by Fe-30Cr-C alloys with cerium or titanium additions. These alloys had the largest volume fraction of eutectic fibers and their ultimate tensile strength was of the order of 3250 MPa. Correlations between the rate of crystal growth(u) and fiber spacing (λ) or tensile strength(Rm) were found and an expression of the typeRm = Aλ^-b2 was obtained whereb ₂ varied between 0.283 and 0.685. Finally, manganese or chromium (35 wt pct Cr) additions did not lead to appreciable improvements in composite strength for this alloy system.     

A pressure-composition-temperature study of Zr-Nb-H system

The solubility of hydrogen was determined in the (Zr + 5 wt pct Nb)-H₂, (Zr + 10 wt pct Nb)-H₂, and (Zr + 20 wt pct Nb)-H₂ systems as a function of composition, temperature (700° to 950°C) and hydrogen equilibrium pressure (0.5 to 760 mm Hg). The position of boundariesβ - (β + δ) and(β + δ)-δ were determined in each of the above three systems. Niobium significantly reduces the solubility of hydrogen in theβ andδ phases and increases the equilibrium hydrogen pressure for any fixed concentration. The equilibrium pressure-temperature relations in the two phase region (β + δ) were derived and the heat of formation ofδ-hydride from saturatedβ-Zr, ΔH _β → δ, were determined. The value of ΔH _β → δ increases up to about 5 wt pct Nb after which the effect of niobium seems to be insignificant. The maximum hydrogen pick-up of zirconium at room temperature decreases with increasing niobium content of the alloy.     

Determination of Solid Fraction from Cooling Curve

A new method to determine directly the solid fraction using the cooling curve was proposed for solidification of undercooled melts. Then, to construct three different baselines, a sudden function ξ _α (x) is introduced. In terms of the ξ _α (x) function, accordingly, the solid fractions during solidification of Ni-3.3 wt pct B, Al-7 wt pct Si, Al-14 wt pct Cu, and Fe-4.56 wt pct Ni alloys were predicted. The predictions of the primary, the regular lamellar eutectic, the anomalous eutectic, and the peritectic phases from cooling curves of the solidified samples coincide with the results of measurement or the available methods.     

The Stress-induced omega phase transformation in Ti-V alloys

The deformation behavior of bcc (β) Ti-V single crystals containing the athermal ω phase was studied using tensile tests, optical and electron microscopy and X-ray and electron diffraction. The stress-strain curves were characterized by a marked yield point and an essentially zero work hardening rate. The deformation structures consisted of wavy slip and stress-induced plates. Both X-ray and electron diffraction showed that the stress-induced plates consist of one variant of the omega phase, with the orientation relation to the parent? phase close to (0001)ω ‖(001)ω; (•1120)ω ‖(110)β. The habit plane was of the type 554. A small amount of ά (hcp) phase was present within the ω plate, with orientation relation to the β phase close to the Burgers relationshipi.e. (0001)_ά, ‖(110)β; [•1120]ά ‖[•111]_β. It is suggested that the ω phase may be an intermediate structure in the stress-induced bcc → hep transformation. R. R. AHRENS, formerly Research Assistant in the Department of Materials Science and Engineering, Cornell University     

基于电热场耦合条件下稀土电解槽电极结构优化模拟

以3kA钕电解槽为研究模型,考虑到电热场在电解过程中的相互影响,运用COMSOL有限元模拟软件建立了的电热耦合模型,并对不同配比的电极插入深度、极间距进行仿真模拟。结果表明,在一定槽电压下,电极插入深度与极间距之间存在配比关系,配比关系影响电流和温度分布。电热场耦合后,在4.2V槽电压下,电极插入深度为220 mm、极间距75 mm时的电流效率较高,电解过程更加稳定。     

Experimental evidence for electrochemical nature of the reaction between iron oxide in calcia-silica-alumina slag and carbon in liquid iron

The electrochemical nature of the reaction between iron oxide in calcia-silica-alumina slag and carbon in liquid iron has been studied by measuring the kinetics of the slag-metal reaction. A base slag (48 pct CaO-40 pct SiO₂-12 pct Al₂O₃) containing iron oxide (≤8 wt pct FeO _t ) was reduced by an Fe-C metal bath (∼4 wt pct C) at 1400 °C. The reaction rate was calculated from measurements of the total inlet gas flow rate and the CO concentration in the outlet gas stream. The slag was “internally short circuited” by dipping an iron plate through the slag layer, and this resulted in an increase in the rate of CO evolution. An external circuit was produced by dipping a graphite rod (shielded from the slag) into the metal bath and a steel or molybdenum rod into the slag layer; the open-circuit voltage and short-circuit current were measured when iron oxide was added to the base slag layer. The reaction rate was enhanced by applying a voltage across the slag layer, and an electric arc cathode was employed in some of these “electrolysis” experiments.     

The rate of formation of intermetallic layers between aluminum and steel below 660°C

The rate of formation of intermetallic compounds between aluminum and three ferritic steels, one austenitic steel, and Inconel has been determined by an electrolytic method. The steel was held at zero potential with respect to aluminum in a NaCl-AlCl₃ melt, and the current measured. Comparison of measured thicknesses of intermetallic layers with those calculated from the integrated current gives an average deposition efficiency of 95 pct. For the Type 304 austenitic steel thickness (min), andk is given by logk= −6400/T(⁰K) +4.469. The ferritic steels show a linear rate of growth of Al₅Fe₂, with an initial higher rate such that extrapolation of the linear curve back to zero time gives an intercept of 16±7 μm. The rate constants (mm min⁻¹) may be represented by log (rate)=α/T+β, and the values of α and β are respectively −2650 and−0.788 for a plain carbon steel,−6580 and + 3.469 for a 1.3 pct Cr, 0.4 pct Mo steel, and−5950 and +2.466 for a 2.2 pct Cr, 0.9 pct Mo steel. The more highly alloyed steels are thus attacked, more slowly. Results for Inconel could not be fitted to any simple equation. With the ferritic steels growth is by aluminum diffusing inwards; with Inconel it is by nickel diffusing outward.     

Thermodynamic properties of Ni-S melts between 700° and 1100°c

The vapor pressure of sulfur over Ni-S melts of various compositions was calculated from the equilibrium weight of the melt in gas streams of known H₂S-H₂ composition. The Gibbs-Duhem equation was used to calculate the activity of nickel and other thermodynamic properties. For the reaction: 3Ni(S) + S₂(g) ⇌ Ni₃S₂(l) the suggested free energy rslationship is: ΔG° = -57,910 + 15.89T (800° to 1100°C). The Calculations were extrapolated to predict that for the reaction: Ni(s) + 1/2S₂(g) ⇌ NiS(l), ΔG° = -26.730 + 10.5T (1000° to 1100°C)     

Effect of surface roughness on low-cycle fatigue behavior of type 304 stainless steel

The effects of surface roughness on the low-cycle fatigue life of Type 304 stainless steel at 593°C in air have been investigated. It is observed that, at a strain rate of 4 × 10⁻³ s⁻¹ and a total strain range of 1 pct, the fatigue life (N _f cycles) decreases with an increase in surface roughness. Information on crack growthvs strain cycles has been generated, as a function of surface roughness, by the measurement of striation spacing on fractured surfaces of specimens tested to failure. Crack propagation follows the Ina ∞N (wherea is the crack length afterN strain cycles) relation for longer specimen fatigue lives (N_f > 2700 cycles) and departs from Ina ∞N for shorter fatigue lives. A quantitative estimate is made of the number of cycles N_o(r) to generate a crack length equal to 0.1 mm (≈ 1 grain diam). The initial surface roughness significantly affects only the initiation component of specimen life time. The effect of roughness on crack initiation is described byN ₀ (R) = 1012R^−0.21, whereR is the surface roughness (root-mean-square value) in microns.     

The effect of aluminum content on the corrosion behavior of Fe-Al alloys in reducing environments at 700 °C

The high-temperature corrosion behavior of monolithic Fe-Al alloys, with 0 to 20 wt pct Al, was investigated at 700 °C in a reducing atmosphere (p(S₂) = 10⁻⁴ atm, p(O₂) = 10⁻²⁵ atm) for up to 100 hours. Postexposure characterization of the corrosion reaction products consisted of surface and cross-sectional microscopy, in combination with energy dispersive spectroscopy, electron probe microanalysis, and quantitative image analysis. From the kinetic data, three stages of corrosion behavior (i.e., inhibition, breakdown, and steady state) were found with the observance and/or duration of each stage directly related to the aluminum content of the alloy. The first stage, labeled the inhibition stage, was characterized by low weight gains and the absence of rapid degradation of the alloy. Typically observed for compositions with 10 to 20 wt pct Al, protection was afforded due to the development of a thin, continuous alumina scale. For alloys with 7.5 wt pct A1, the ability to maintain the initially formed alumina scale was not observed, resulting in the breakdown stage. Localized corrosion product nodules, containing iron sulfide (Fe_1-x S) and the spinel-type tau phase (FeAl₂S₄), developed through the alumina scale due to sulfur short-circuit diffusion. These growths were accompanied by relatively high corrosion rates. Further decreasing the aluminum content to 5 wt pct and below lead to the formation of a continuous sulfide scale whose growth was controlled by iron and sulfur diffusion through the previously formed product. The alloy wastage rates in the steady-state stage were relatively high when compared to the previous two regions.     

Diffusion of cobalt,chromium, and titanium in Ni3Al

Diffusion studies of cobalt, chromium, and titanium in Ni₃Al (γ′) at temperatures between 1298 and 1573 K have been performed using diffusion couples of (Ni-24.2 at. pct Al/Ni-24.4 at. pct Al-2.91 at. pct Co), (Ni-24.2 at. pct Al/Ni-23.1 at. pct Al-2.84 at. pct Cr), and (Ni-24.2 at. pct Al/Ni-20.9 at. pct Al-3.17 at. pct Ti). The diffusion profiles were measured by an electron probe microanalyzer, and the diffusion coefficients of cobalt, chromium, and tita-nium in γ′ containing 24.2 at. pct Al were determined from those diffusion profiles by Hall’s method. The temperature dependencies of their diffusion coefficients (m[su2]/s) are as follows: ~D_(Co) = (4.2 ± 1.2) × 1O^-3exp {-325 ± 4 (kJ/mol)/RT} ~D_(Cr) = (1.1 ± 0.3) × 10^-1 exp {-366 ± 3 (kJ/mol)/RT} and D_(Ti) = (5.6 ± 3.1) × 10¹ exp {-468 ± 6 (kJ/mol)/RT} The values of activation energy increase in this order: cobalt, chromium, and titanium. These activation energies are closely related to the substitution behavior of cobalt, chromium, and titanium atoms in the Ll₂ lattice sites of γ′; the cobalt atoms occupying the face-centered sites in the Ll₂ structure diffuse with the normal activation energy, whereas the titanium atoms oc-cupying the cubic corner sites diffuse with a larger activation energy that includes the energy due to local disordering caused by the atomic jumps. The chromium atoms which can occupy both sites diffuse with an activation energy similar to that of cobalt atoms.     

On the nature of the Fe-bearing particles influencing hard anodizing behavior of AA 7075 extrusion products

The deleterious effects of Fe-bearing constituent particles on the fracture toughness of wrought A1 alloys have been known. Recent studies have shown that the presence of Fe-bearing, constituent particles is also determental to the nature and growth of the hard anodic oxide coating formed on such materials. The present study, using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA), was made to examine the influence of the nature of the Fe-bearing particles on the hard anodizing behavior of AA 7075 extrusion products containing varying amounts of Si, Mn, and Fe impurities. It was found that, in the alloy containing 0.25 wt pct Si, 0.27 wt pct Mn, and 0.25 wt pct Fe, the Fe-bearing constituent particles are based on the Al₁₂(FeMn)₃Si phase (bcc with α=1.260 nm). These particles survive the hard anodizing treatment, add resistance to the electrical path, causing a rapid rise in the bath voltage with time, and cause a nonuniform growth of the anodic oxide film. In the materials containing 0.05 wt pct Si, 0.04 wt pct Mn, and 0.18 wt pct Fe, on the other hand, the formation of the Al₁₂(FeMn)₃Si-based phase is suppressed, and two different Fe-bearing phases, based on Al−Fe−Cu−Mn-based (simple cubic with a=1.265 nm) and Al₇Cu₂Fe, respectively form. Neither the Al−Fe−Cu−Mn-based phase nor the Al₇Cu₂Fe-based phase survive the hard anodizing treatment, and this results in a steady rise in the bath voltage with time and a relatively uniform growth of the anodic oxide film. Consideration of the size of the Fe-bearing, particles reveals that the smaller the particle, the more uniform the growth of the anodic oxide film.     

Stability of the chromia layer on steels in flowing nonequilibrium H2-H2O-CH4-CO mixtures at 950 °C

The stability of the protective oxide layer formed on Fe-12.5 pct Cr, Fe-18 pct Cr-10 pct Ni, and Fe-25 pct Cr-20 pct Ni was investigated in a flowing nonequilibrium H2-H2O-CH4-CO mixture at 950 °C. The samples were preoxidized in H2-H2O, and subsequently the carbon activity was raised in stepwise manner by adding methane. CO formation occurred on the sample surface by the catalytic reaction CH4 + H2O → CO + 3H2. The conversion of oxide to carbide commenced at the end of the sample wherePco has its highest andPH2O its lowest value. Oxidation, conversion, and catalytic reaction were monitored by gravimetric measurements and CO-determination during the experiments. Partial equilibria can be calculated for (ac)1 =K ₁ P _{ch 4}/(Ph2)2, (ac)2 =K2(PcoPh2)ph{2o}and(Po2)1\2 = K3Ph2o/Ph2. The conversion of oxide to carbide started when the steady state ofac and(Po2)1/2 on the surface approached the value for the equilibrium of chromium oxide and chromium carbide. The results show that this steady state value is much nearer to the point(ac)2-(Po2)1/2 in the thermochemical stability diagram than to the point (ac)1 — (Po ₂)^1/2- The oxide layers were stable even at very high values of (ac)₁ which demonstrates that there is only very little interaction of CH₄ with the oxide surface. P. C. Prasannan formerly Research Scientist at the Max-Planck-Institut für Eisenforschung This paper is based on a presentation made at the “Stephen R. Shatynski Memorial Symposium on Surfaces and Interfaces” held at the 114th annual AIME meeting in New York, February 24-28, 1985, under the auspices of the ASM-MSD Thermodynamic Activity Committee.     

Structural superplasticity in a fine-grained eutectic intermetallic NiAl−Cr alloy

A rapidly solidified and thermomechanically processed fine-grained eutectic NiAl−Cr alloy of the composition Ni₃₃Al₃₃Cr₃₄ (at, pct) exhibits structural superplasticity in the temperature regime from 900°C to 1000°C at strain rates ranging from 10⁻⁵ to 10⁻³ s⁻¹. The material consists of a B2-ordered intermetallic NiAl(Cr) solid solution matrix containing a fine dispersion of bcc chromium. A high strain-rate-sensitivity exponent of m=0.55 was achieved in strain-rate-change tests at strain rates of about 10⁻⁴ s⁻¹. Maximum uniform elongations up to 350 pct engineering strain were recorded in superplastic strain to failure tests. Activation energy analysis of superplastic flow was performed in order to establish the diffusion-controlled dislocation accommodation process of grain boundary sliding. An activation energy of Q _c=288±15 kJ/mole was determined. This value is comparable with the activation energy of 290 kJ/mole for lattice diffusion of nickel and for ⁶³Ni tracer selfdiffusion in B2-ordered NiAl. The principal deformation mechanism of superplastic flow in this material is grain-boundary sliding accommodated by dislocation climb controlled by lattice diffusion, which is typical for class II solid-solution alloys. Failure in superplastically strained tensile samples of the fine-grained eutectic alloy occurred by cavitation formations along NiAl‖‖Cr interfaces.