A preliminary study of the redox behavior of tungstate and molybdate ions in Na2SO4 at 1203 K

The redox behavior of W and Mo ions on Pt electrodes in molten Na₂SO₄ at 1203 K has been studied in an O₂-SO₂-SO₃ ambient. W and Mo were added as Na₂MO₄ (M = W, Mo) to the Na₂SO₄ melt. Cyclic voltammograms showed two reduction processes for both species within the Na₂SO₄ stability range. The oxidation sequence for the tungstate ion showed one of the reduction reactions to be irreversible. For molybdate ions there are oxidation reactions corresponding to each of the reduction reactions. The characteristics of the cyclic voltammogram peaks are described.     

Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Deposit-Induced Corrosion of Mo-Containing Alloys

Disk alloys used in advanced gas turbine engines often contain significant amounts of Mo (2 wt% or greater), which is known to cause corrosion under Type I hot corrosion conditions (at temperatures around 900 °C) due to alloy-induced acidic fluxing. The corrosion resistance of several model and commercial Ni-based disk alloys with different amounts of Mo with and without Na₂SO₄ deposit was examined at 700 °C in air and in SO₂-containing atmospheres. When coated with Na₂SO₄ those alloys with 2 wt% or more Mo showed degradation products similar to those observed previously in Mo-containing alloys, which undergo alloy-induced acidic fluxing Type I hot corrosion even though the temperatures used in the present study were in the Type II hot corrosion range. Extensive degradation was observed even after exposure in air. The reason for the observed degradation is the formation of sodium molybdate. Transient molybdenum oxide reacts with the sodium sulfate deposit to form sodium molybdate which is molten at the temperature of study, i.e., 700 °C, and results in a highly acidic melt at the salt alloy interface. This provides a negative solubility gradient for the oxides of the alloying elements, which results in continuous fluxing of otherwise protective oxides.     

An electrochemical study on the influence of sodium molybdate on electrodeposition process and phase composition of ternary Zn–Ni–Mo alloy coatings

Ternary Zn–Ni–Mo alloy coatings were deposited from a citrate-sulphate bath at pH 5.7 containing different amounts of sodium molybdate. The content of molybdenum in the coatings (from 0.3 to 5.2?at.-%) can be easily controlled by increasing sodium molybdate concentration in the plating bath from 0.0025 to 0.05?mol?dm^??3, which results also in deposition of smoother deposits. An increase in molybdate concentration leads to the shift of reduction potentials towards more negative values and to the decrease in current efficiency of deposition process. XRD analyses and anodic linear sweep voltammetry (ALSV) measurements demonstrated that at least two phases are formed in the Zn–Ni–Mo alloy: a hexagonal zinc phase or solid solution of nickel in zinc and Ni₅Zn₂₁ intermetallic compound. Furthermore, the XRD analyses revealed a third phase, which could be assigned to the oxidised species of molybdenum or other alloying metals.     

Besonderheiten im Korrosionsverhalten hochchrom- und molybdänhaltiger Legierungen in heißer 92, 5%iger Schwefelsäure

Peculiarities in the corrosion behaviour of high chromium and molybdenum containing alloys in hot 92.5% sulfuric acid In laboratory tests at temperatures above 50°C unusual high corrosion rates of passivating stainless steels and nickel alloys containing more than 26% Cr were observed in 92.5% sulphuric acid. In order to investigate the cause of this phenomenon further corrosion tests and additional chemical analyses were performed. The H₂SO₄ concentration tested displays a relative maximum of the electrical conductivity, the reason being a stronger dissociation of the sulfuric acid. Electrochemical investigations revealed an enhanced activity of the cathodic reactions which lead to higher corrosion rates. The cathodic reactions are strongly dependend on alloy constitution with special emphasis on the contents of Cr, Ni and Mo. Mo containing stainless steel show potential oscillations (of the open circuit potential) between ?50 and +550 mV_H. These alloys corrode under development of SO₂ (reduction of H₂SO₄ molecules) and formation of several sulfur compounds with different oxidation numbers (6+ and 2?). Alloys with chromium contents above 26% develop additionally hydrogen gas due to a lower hydrogen overvoltage of these alloys. With increasing nickel content the overvoltage for the reduction reaction of H₂SO₄ molecules will be lowered. This fact results in an elevation of the exchange current density for the Alloy NiCr45 and therefore to the highest corrosion rate observed. Alloy B-2 shows the best resistance, i.e. very low corrosion rates. Obviously high levels of molybdenum can compensate the influence of nickel on the overvoltage of the reduction reaction or even hinder the cathodic reaction.     

Copper Electroplating from Vanadate- and Molybdate-Containing Electrolytes with Suspended Titanium Dioxide

Coatings with copper matrices, which are deposited from sulfate electrolytes modified by additions of either molybdate or vanadate ions and highly dispersed powder of titanium dioxide, are studied. The additives enhance the heat resistance of the coatings. The mentioned ions have a depolarizing effect on the cathodic deposition of copper and inhibit the codeposition of a dispersed phase of TiO₂. Molybdate ions in certain concentrations improve the brightness of the coatings, having been reduced to Mo compounds of lower oxidation degrees. The effect of the mentioned ions on the pH _s during the copper deposition is studied. The pH of the near-cathode layer is shown to somewhat increase in the presence of both molybdate ions and titanium dioxide.     

Structural characteristics and catalytic activities of nanocrystalline Ni-Mo-B coatings obtained by catalytic electroless reduction

The structures, chemical states of elements, and catalytic activities of Ni-Mo-B alloys with different molybdenum contents, which were obtained by catalytic electroless reduction of metal ions, were studied. The rates of the partial reactions (heterogeneous hydrolysis of dimethylamine borane, reduction of nickel ions, and evolution of molecular hydrogen) were found to make a bell-shaped curve when plotted versus the concentration of molybdate ions in solution. Original Russian Text ? V.M. Krutskikh, M.V. Ivanov, A.B. Drovosekov, E.N. Lubnin, B.F. Lyakhov, Yu.M. Polukarov, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 6, pp. 619–625.     

Chemische und elektrochemische Reaktionen von Eisensulfid und Mangansulfid in sauren und neutralen Lösungen

Chemical and electrochemical reactions of iron sulfide and manganese sulfide in acid and neutral solutions The reactions which occur upon corrosion of massive iron sulfide and manganese specimens in perchloric acid and in neutral sodium chloride solution were elucidated by measurements of current-potential curves and by coulometric and analytical investigations on the processes. In acids the sulfides are dissolved by prevailing chemical reaction under evolution of H₂S. Upon applying anodic overpotentials electrochemical reactions occur simultaneously, however, with such low velocity that the contribution to corrosion of the sulfides is insignificant. Upon applying cathodic overpotentials some hydrogen discharge is observed on iron sulfide but not on manganese sulfide. In 3% sodium chloride solution both sulfides corrode very slowly upon anodic polarization, forming elementary sulfur according to MeS = Me²⁺ + S + 2e^? (Me = Fe or Mn). At high anodic potentials additional oxidation reactions occur in which three-valent iron and tetravalent manganese ions as well as sulfite and sulfate ions are formed. Iron sulfide and manganese sulfide inclusions can he isolated from steels only by electrochemical dissolution in neutral or weakly basic electrolytes, the potential during electrolysis must not be more positive than the corrosion potential of the sulfides.     

Recovery of Mo from Ni-Mo ore leach solution with carrier co-precipitation method

In this paper, “nascent” Fe(OH)₃ as carrier precipitation and NaHCO₃ as buffer agent were used to extract molybdenum from the complex Ni-Mo ore leach solution. The effects of different variables on the molybdenum extraction, such as the dosage of FeCl₃ and NaHCO₃ and the reaction temperature and time, were studied. The results showed that over 99% of molybdenum were extracted in 2 h at 25 ± 1 °C under the conditions of mole ratio of Fe³⁺/Mo 2.2-2.5 and mass ratio of NaHCO₃/Mo 0.7-1.5. About 92% of Mo in the Fe(OH)₃ precipitation can be leached by NH₃⋅H₂O to prepare the ammonium molybdate solution with about 100 g/L Mo. After treating with NH₃⋅H₂O, the Fe(OH)₃ precipitation was dissolved with HCl to obtain the FeCl₃ solution (FeCl₃ 500 g/L) which can be reused for the next round of experiments.     

Electrochemical studies of Na2CrO4-Na2So4 melts at 1200 K

The electrochemical response of Na₂SO₄-Na₂CrO₄ solutions at a Pt working electrode was established by cyclic voltammetry and chrono-potentiometry measurements at 1200 K. Experiments were made under conditions of controlled and electrochemically measured values of melt basicity for basic, neutral, and acidic melts. Despite the thermodynamic stability of chromite ions under reducing conditions, a direct conversion of chromate to chromite upon cathodic polarization was not observed. Rather, the cathodic reduction of chromate ions proceeds by a reversible one-electron charge transfer to an electroactive intermediate species formed through an equilibrium between CrO and oxide ions in the solution. An irreversible chemical reaction reduces the chromium solute species further to precipitate a solid. As a solute in a corrosive fused salt film, a chromate to chromite transition would not be expected to stabilize the oxidation state and basicity. However, chromate ions to provide a reduction reaction more favorable than sulfate reduction to sulfide; chromate reduction leads to the precipitation of solid NaCrO₂ or Cr₂O₃ in basic or neutral solutions, respectively. A large shift in melt basicity can only occur for mildly acidic solutions with high chromate solute.     

Surface structure and catalytic activity of electrodeposited Ni-Fe-Co-Mo alloy electrode by partially leaching Mo and Fe

1 Introduction The chlorine alkaline industry and the production of hydrogen by electrolyzing water have disadvantages of high cost and energy consumption, so it is especially important to study and develop a high catalytic activity and good stability el…     

Etude de l'oxydation electrochimique du platine,du nickel,du molybdene et du tungstene dans l'eutectique fondu ZnCl2-KCl

The anodic oxydation of platinum, nickel, molybdenum and tungsten has been studied in a dehydrated ZnCl₂-KCl eutectic melt (49–51 per cent in mole) at 260 and 350°C. The reference electrode was the Ag/AgCl system contained in a closed tube of Pyrex. The anodic dissolution of these metals under potentiostatic conditions produces Ni(II), Mo(III) and W(VI). In this melt, nickel metal, molybdenum and tungsten can be electrodeposited on a platinum electrode from their salts. The analysis of the potential-time curves under galvanostatic conditions, and in open circuit, allowed to detect the superficial formation of some oxidation products on the electrode and to compare their solubilities and their chemical stabilities in the eutectic melt.     

Selectivity recovery of molybdenum(VI) from rhenium(VII) by amine-modified persimmon waste

Amine functional group was grafted to obtain modified persimmon waste gel(NH_2–CPT) with the focus of development of selective recovery of molybdenum from rhenium. The adsorption behavior of the NH_2–CPT gel for various metal ions at varying hydrochloric acid concentrations was studied. It is found that the NH_2–CPT exhibits high affinity for Mo(VI) and no affinity for Re(VII), Cu(II),Fe(III), Mn(VII), and Zn(II) under the operating conditions. The maximum adsorption capacity for Mo(VI) is 172 mgág-1, and the adsorption behavior obeys the Langmuir model. Owing to Mo(VI) as poly-anions, the adsorption mechanism of molybdenum anions could be explained as the anion exchange reactions at weak acid concentration, while neutral molecules could be explained as the complexation reactions at strong acid concentration,respectively. In addition, its excellent adsorption characteristics for Mo(VI) are confirmed by separation of Mo(VI)from Mo to Re containing industrial effluent.     

Diffusional analysis of a multiphase oxide scale formed on a Mo-Mo3Si-Mo5-SiB2 alloy

Diffusional analyses were performed to understand the oxidation at 1300 °C of a multiphase Mo-13.2Si-13.2B (at.%) alloy. During oxidation, a protective glass scale formed with an intermediate layer of (Mo+glass) between the base alloy and external glass scale. Compositional profiles across the (Mo+glass) layer and the external glass scale were determined, and interdiffusion fluxes and effective interdiffusion coefficients for the various components were determined by using “MultiDiFlux” software. The motion of the (alloy/Mo+glass) and (Mo+glass/glass) interphase boundaries after passivation was examined. Additionally, vapor-solid diffusion experiments at 1300 °C were carried out with single-phase Mo₃Si and T2 specimens in addition to a multiphase Mo-10Si-10B (at.%) alloy. These specimens were exposed to vacuum to induce silicon loss resulting in the formation of a Mo layer. An average effective interdiffusion coefficient of Si in Mo at 1300 °C was estimated from the Mo₃Si-vapor couple to be in the order of 8×10⁻¹⁷ m²/s. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Sulfidation behavior of Ni-Cr-Mo alloys at 700°C

The effect of molybdenum additions 5, 10, 15, and 20 wt. %, on the sulfidation behavior of Ni-20Cr, and the effect of chromium additions, 5, 10, 15, and 20 wt.%, on the sulfidation of Ni-20Mo were studied in pure sulfur vapor at 700°C. In general, the alloys followed a linear or near-linear rate law, the sulfidation rate of Ni-20Mo being slightly less than that of Ni-20Cr. The alloys having the lowest ternary addition, e.g., Ni-Cr-5Mo and Ni-20Mo-5Cr. exhibited the most rapid reaction rates. The highest alloying additions of 20 wt.% had no appreciable benefit on reaction rates. Scale structures were complex but generally consisted of several layers. The outer layer was always NiS_1.03, although both binaries formed Ni₃S₂ within the NiS_1.03. An inner layer of Cr₃S₄ existed in which there was considerable dissolved molybdenum. A thin, intermediate layer of Cr₂S₃ generally formed between the Cr₃S₄ and the outer nickel sulfide. An innermost layer of MoS₂ formed on all alloys containing more than 10 wt. % Mo, and a second phase of Mo₂S₃ formed within the MoS₂ on Ni-20Mo. Although the scales changed with alloy composition, no significant changes in reaction rate were observed. Notable differences in both scale structure and reaction kinetics between this study and previous studies were apparent. The differences and possible reaction mechanisms are discussed.     

Removal of vanadium from ammonium molybdate solution by ion exchange

The separation techniques of vanadium and molybdenum were summarized, and a new method of removal V(Ⅴ) from Mo(Ⅵ) by adsorption with chelate resin was presented. Nine kinds of chelate resins were used to investigate the adsorbent capability of V(Ⅴ) in ammonium molybdate solution with static method. The test results show that DDAS, CUW and CW-2 resins can easily adsorb V(Ⅴ) in ammonium molybdate solution, but hardly adsorb Mo(Ⅵ). The dynamic experimental results show more than 99.5% of V(Ⅴ) can be adsorbed, and the adsorption rate of Mo(Ⅵ) is less than 0.27% at 294-296 K for 60 min at pH 7.42-8.02. The mass ratio of V to Mo decreases to l/5 0000 in the effluent from 1/255 in the initial solution. The loaded resin can be desorbed by 5% NH3·H2O solution, and the vanadium desorption rate can reach 99.6%. The max concentration of vanadium in desorbed solution can reach 20 g/L, while the concentration of molybdenum is less than 0.8 g/L.     

Removal of tungsten from the sodium molybdate solution using nascent MnO2

Tungsten is hard to be separated effectively from molybdate solution due to their extremely similar chemical properties. In this research, a novel tungsten removal method was developed to remove tungsten from the molybdate solution using nascent MnO₂. Through comparing different preparation method of nascent MnO₂, in situ synthesized method was chosen as the optimal preparation route. And the effects of reaction temperature, Mn addition, pH value, reaction time and Mo/W mass ratio of solution on the tungsten removal were also investigated. The experimental results showed tungsten could be selectively removed quickly from molybdate solution. On the optimal reaction conditions of reaction temperature 25 °C, 50 times theoretical reagent amount of Mn addition, pH value 8.0–8.5 and reaction time 1.0 h, 95% of tungsten can be removed from the molybdate solution with different Mo/W mass ratio and the Mo loss can be controlled to only about 5%. The results showed that nascent MnO₂ was a high-efficiency tungsten removal reagent from the molybdate solution.     

Electrochemische Untersuchungen zur Hochtemperaturkorrosion von Chromstählen in Alkalisulfatschmelzen

Electrochemical investigation into the high temperature corrosion of chromium steels in alkali sulfate melts Electrochemical and corrosion-chemical investigations have been carried out with scaling resistant chromium steels, iron, chromium and platinum in a eutectic (Li, Na, K)₂-SO₄. It has turned out that sufficiently exact data concerning corrosion reactions can be obtained only from mass losses, not, however, from current density. The corrosion behaviour depends from potential. Comparable to the conditions in aqueous solutions potential ranges exist with passive and transpassive corrosion and with a rupture potential which depends from the chromium content of a steel. Protective oxide layers exist in the passive range where the mass loss becomes almost constant after a certain in cubation period. In the transpassive range corrosion follows an almost parabolic law with formation of an inner sulfide layer and a thicker external oxide layer where chromium is enriched. These layers are largely formed by oxidizing media carried to the metal surface via sulfur oxides; during this reaction inert marks in the steel remain unchanged. Sulfur oxides may be formed as secondary consecutive products by reactions between metal ions and sulfates. The solubility of metal ions in the sulfate melt is an important parameter for corrosion rates. Oxide ions (as reduction products of O₂) act as inhibitor on the anodic partial reaction, while SO₃ and ferric ions have a large lating effect, so that the anodic dissolution is autocatalyzed. Chlorides, too, act as stimulators in the transpassive range. Corrosion at the free corrosion potential is largely controlled by ferric ions which act as anodic and cathodic stimulators in acid melts. In neutral melts under oxygen an 18% chromium steel is passive.     

Reactions at the corroding nickel electrode in molten sodium carbonate under CO/CO2 atmosphere

Anodic reactions at the CO, CO₂|Ni electrode in molten sodium carbonate at 1000°C are examined in detail. Experimental polarization curves are fitted via a computer graphics procedure to a model employing five anodic reactions with activation, concentration and resistance polarization, and passivation. The resultant empirical parameters are interpreted in terms of absolute rate expressions describing possible anodic reactions. Three of the anodic reactions are found to be in accord with the findings at inert electrodes by previous investigators, i.e. oxidation of CO by two mechanisms and oxidation of carbonate, while the cathodic reaction is consistent with reduction of CO₂. The remaining two anodic reactions were associated with the oxidation of nickel to Ni²⁺ with formation of a NiO film above a passivation potential, and, at higher anodic overpotentials, oxidation of NiO to a higher oxide, e.g. Ni₂O₃. There is an indication of the formation of a third oxide at still higher anodic overpotentials.     

Reduction roasting–magnetic separation of vanadium tailings in presence of sodium sulfate and its mechanisms

Reduction roasting with sodium sulfate followed by magnetic separation was investigated to utilize vanadium tailings with total iron grade of 54.90 wt% and TiO_2 content of 17.40 wt%. The results show that after reduction roasting–magnetic separation with sodium sulfate dosage of 2 wt% at roasting temperature of 1150 °C for roasting time of 120 min, metallic iron concentrate with total iron grade of 90.20 wt%, iron recovery rate of 97.56 % and TiO_2 content of 4.85 wt% is obtained and high-titanium slag with TiO_2 content of 57.31 wt% and TiO_2 recovery rate of 80.27 % is also obtained. The results show that sodium sulfate has a catalytic effect on the reduction of tailings in the novel process by thermodynamics, scanning electron microscopy(SEM) and X-ray diffraction(XRD) and reacts with silica and alumina in the tailings to form sodium silicate and sodium aluminosilicate. Migration of elements and chemical reactions destroy the crystal structures of minerals and promote the reduction of vanadium tailings, resulting in that iron grains grow to large size so that metallic iron concentrate with high total iron grade and low TiO_2 content is obtained.     

Dissolution kinetics of molybdite in KOH media at different temperatures

The dissolution kinetics of synthetic molybdite (MoO₃) in a potassium hydroxide (KOH) medium was studied by varying the system temperature, KOH concentration, and particle size. Additionally, the effects of the stirring rate and different reagents such as barium hydroxide (Ba(OH)₂), calcium hydroxide (Ca(OH)₂), and sodium hydroxide (NaOH) were also evaluated. The experiments were performed in a reactor with controlled temperature and agitation. The results indicated that the dissolution reaction mechanism of molybdite generates potassium molybdate (K₂MoO₄) without intermediate compounds. Temperature (6–80 °C), KOH concentration (0.0005–0.025 mol/L), and particle size (5–40 μm) positively affected the dissolution of molybdite. The maximum Mo recovery was 67.5% in 0.25 h for 80 °C and 0.01 mol/L KOH. At the lowest temperature (6 °C), which is near the freezing point of water (0 °C), a substantial amount of Mo was recovered (17.8% in 45 min). The kinetics equation describing the molybdite dissolution in a KOH environment indicated that diffusion occurs through the porous layer. The activation energy was calculated to be 47.81 kJ/mol. A reaction order of 1.0 with respect to KOH concentration was obtained and was found to be inversely proportional to the squared particle size. The kinetics equation was obtained. The dissolution of molybdite resulting from the oxidation of a molybdenite concentrate (MoS₂) led to a low molybdenum recovery, which was primarily caused by the consumption of KOH by impurities such as CaCO₃ and Cr(MO₄)₃.