Corrosion Behavior of Fe<Subscript>41</Subscript>Co<Subscript>7</Subscript>Cr<Subscript>15</Subscript>Mo<Subscript>14</Subscript>C<Subscript>15</Subscript>B<Subscript>6</Subscript>Y<Subscript>2</Subscript> Bulk Metallic Glass in Sulfuric Acid Solutions

An Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ bulk metallic glass with a diameter of 5 mm was prepared with the copper-mold-casting method. The corrosion resistance of this amorphous steel in sulfuric-acid solutions was determined by electrochemical measurements. The passive film formed on the surface of the alloy after immersion in the 0.5-mol/l H₂SO₄ solution for 1 week was analyzed by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements show that the corrosion resistance of the amorphous steel in the 1 mol/l-H₂SO₄ solution is superior to a stainless steel (SUS 321), and is almost the same as Ti6Al4V, which shows that the amorphous steel has an excellent corrosion resistance in sulfuric-acid solutions. As the concentration of the sulfuric-acid solutions increases from 0.5 mol/l to 4 mol/l, the corrosion resistance of the amorphous steel decreases. The XPS result reveals that a bilayer structure of protective film formed on the surface of the amorphous steel in a H₂SO₄ solution. The compositions of the inner part of the film are MoO₂, Cr₂O₃, CoO, and FeO, and those of the outer film are MoO₃, Cr(OH)₃, Co(OH)₂, and Fe(OH)₃.     

Decomposition of Y<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> Particles in 8 Pct Cr Oxide-Dispersion-Strengthened Martensitic Steel during Tempering

Chemical composition analysis using inductively coupled plasma spectroscopy and phase identification using X-ray were performed on the extracted residues of 8 pct Cr martensitic steel strengthened by nanoscaled complex oxides, which consist of yttrium, titanium, and oxygen. Some Y₂Ti₂O₇ particles, which were stable during normalizing, decomposed into Y₂O₃ and Ti₂O₃ during tempering. This reaction reversibly occurred between normalizing and tempering. Y₂Ti₂O₇ particles formed in the steel had other constituents in solid solution as compared to the completely stoichiometric Y₂Ti₂O₇ particles synthesized artificially in air. As for the mechanism of the decomposition of Y₂Ti₂O₇ particles in the steel, segregation of oxygen to dislocations induced by normalizing caused the decomposition of Y₂Ti₂O₇ during tempering. In addition to that, the interfacial strain between Y₂O₃ particles or Ti₂O₃ particles within Cr₂₃C₆ carbides, which are formed by tempering, was lower than the strain between Y₂Ti₂O₇, which precipitated in Cr₂₃C₆ carbides or the matrix. This difference in interfacial strain could also promote the decomposition of the Y₂Ti₂O₇ particles in the steel.     

<Emphasis Type="Italic">In-Situ</Emphasis> Synthesis of Cu/Cr-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanocomposite by Mechanical Alloying and Heat Treatment

In this article, a novel method has been used to prepare a copper matrix nanocomposite containing Cu-10 wt pct Cr-10 wt pct Al₂O₃ by heat treatment of the mechanically activated Cu, Al, and Cr₂O₃ powder mixture. Structural evolutions were investigated using the X-ray diffraction (XRD) technique. The microstructure of samples was examined using scanning electron microscopy (SEM). It was found that during the milling process, Cu(Al) solid solution and Cu₉Al₄ phase were formed as the intermediate products, and therefore, Al activity was decreased. Hence, the reduction of Cr₂O₃ with Al was prevented during the ball milling stage. Further heat treatment carried out under argon atmosphere at 900 °C for 8 hours resulted in completion of Cr₂O₃ reduction by Al.     

Study of Molten Salt Corrosion of High Velocity Oxy-Fuel Sprayed Cermet and Nickel-Based Coatings at 900 °C

Absract Samples of Cr₃C₂-NiCr cermet and NiCrBSi coatings formed by the high velocity oxy-fuel (HVOF) process on Superni 718 superalloy have been corroded in the Na₂SO₄-V₂O₅ molten salt environment at 900 °C under cyclic conditions. The hot corrosion behavior of the coatings has been investigated by means of thermogravimetric analysis, X-ray diffraction, scanning electron microscopy/energy dispersive spectroscopy, and electron probe microanalyzer techniques. Efforts have been made to formulate the mode of corrosion attack. Both the coatings protected the substrate superalloy Superni 718 completely. While protecting the superalloy, the Cr₃C₂-NiCr cermet coating partially oxidized along the splat boundaries up to the coating substrate interface, whereas only the upper part of the coating, to about 100 μm from the surface, oxidized in the case of the NiCrBSi coating. The hot corrosion resistance of both the coatings has been attributed to the formation of protective oxides of chromium/silicon at the surface and at the splat boundaries of the coatings.     

Evaluating the Diffusion Coefficient of Sulfur in Low-Silica CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag

The chemical diffusion coefficient of sulfur in the ternary slag of composition 51.5 pct CaO-9.6 pct SiO₂-38.9 pct Al₂O₃ slag was measured at 1680 K, 1700 K, and 1723 K (1403 °C, 1427 °C, and 1450 °C) using the experimental method proposed earlier by the authors. The P_\textS2 P_{{{\text{S}}_{2} }} and P_\textO2 P_{{{\text{O}}_{2} }} pressures were calculated from the Gibbs energy of the equilibrium reaction between CaO in the slag and solid CaS. The density of the slag was obtained from earlier experiments. Initially, the order of magnitude for the diffusion coefficient was taken from the works of Saito and Kawai but later was modified so that the concentration curve for sulfur obtained from the program was in good fit with the experimental results. The diffusion coefficient of sulfur in 51.5 pct CaO-9.6 pct SiO₂-38.9 pct Al₂O₃ slag was estimated to be in the range 3.98 to 4.14 × 10⁻⁶ cm²/s for the temperature range 1680 K to 1723 K (1403 °C to 1450 °C), which is in good agreement with the results available in literature     

Thermodynamics on the formation of spinel (MgO·Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) inclusion in liquid iron containing chromium

The stability diagram of MgO, spinel solid solution (MgO·(Al _X Cr_1−X )₂O₃), and sesquioxide solid solution ((Al _Y Cr_1−Y )₂O₃) as a function of Mg, Al, and O contents at a constant chromium content (18 mass pct) in liquid iron is drawn at 1873 K. The interaction parameters between Mg and other solutes (Al, Cr, Ni, Ti, Si, and C) are determined by the experimental method, which assures equilibrium between Mg vapor and liquid iron, were applied to calculate the diagram. Titanium deoxidation is not recommended for the prevention of spinel formation, because Ti accelerates Mg dissolution from refractory or slag due to its high affinity for Mg (e _Mg ^Ti = − 0.64). The standard Gibbs free energies of formation for the three inclusions (periclase, spinel, and sesquioxide solid solutions) and the tielines between two solid solutions were calculated with the aid of the regular solution model and the thermochemical F*A*C*T database computing system, respectively. The phase stability regions and oxygen content in steel for the current Fe-Mg-Al-Cr (18 mass pct)-O system are compared with those of the previous non-Cr system. Detailed information on the spinel composition according to Mg and Al contents is also available from the present stability diagram.     

Distribution Ratios of Phosphorus Between CaO-FeO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Na<Subscript>2</Subscript>O/TiO<Subscript>2</Subscript> Slags and Carbon-Saturated Iron

In order to effectively enhance the efficiency of dephosphorization, the distribution ratios of phosphorus between CaO-FeO-SiO₂-Al₂O₃/Na₂O/TiO₂ slags and carbon-saturated iron (\( L_{\text{P}}^{\text{Fe-C}} \)) were examined through laboratory experiments in this study, along with the effects of different influencing factors such as the temperature and concentrations of the various slag components. Thermodynamic simulations showed that, with the addition of Na₂O and Al₂O₃, the liquid areas of the CaO-FeO-SiO₂ slag are enlarged significantly, with Al₂O₃ and Na₂O acting as fluxes when added to the slag in the appropriate concentrations. The experimental data suggested that \( L_{\text{P}}^{\text{Fe-C}} \) increases with an increase in the binary basicity of the slag, with the basicity having a greater effect than the temperature and FeO content; \( L_{\text{P}}^{\text{Fe-C}} \) increases with an increase in the Na₂O content and decrease in the Al₂O₃ content. In contrast to the case for the dephosphorization of molten steel, for the hot-metal dephosphorization process investigated in this study, the FeO content of the slag had a smaller effect on \( L_{\text{P}}^{\text{Fe-C}} \) than did the other factors such as the temperature and slag basicity. Based on the experimental data, by using regression analysis, \( \log L_{\text{P}}^{\text{Fe-C}} \) could be expressed as a function of the temperature and the slag component concentrations as follows:

$$ \begin{aligned} \log L_{\text{P}}^{\text{Fe-C}} & = 0.059({\text{pct}}\;{\text{CaO}}) + 1.583\log ({\text{TFe}}) - 0.052\left( {{\text{pct}}\;{\text{SiO}}_{2} } \right) - 0.014\left( {{\text{pct}}\;{\text{Al}}_{2} {\text{O}}_{3} } \right) \\ \, & \quad + 0.142\left( {{\text{pct}}\;{\text{Na}}_{2} {\text{O}}} \right) - 0.003\left( {{\text{pct}}\;{\text{TiO}}_{2} } \right) + 0.049\left( {{\text{pct}}\;{\text{P}}_{2} {\text{O}}_{5} } \right) + \frac{13{,}527}{T} - 9.87. \\ \end{aligned} $$

     

Development of Wear-Resistant Cu-10Cr-3Ag Electrical Contacts with Nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Dispersion by Mechanical Alloying and High Pressure Sintering

Cu-10Cr-3Ag (wt pct) alloy with nanocrystalline Al₂O₃ dispersion was prepared by mechanical alloying and consolidated by high pressure sintering at different temperatures. Characterization by X-ray diffraction and scanning electron microscopy or transmission electron microscopy shows the formation of nanocrystalline matrix grains of about 40 nm after 25 hours of milling with nanometric (<20 nm) Al₂O₃ particles dispersed in it. After consolidation by high pressure sintering (8 GPa at 400 °C to 800 °C), the dispersoids retain their ultrafine size and uniform distribution, while the alloyed matrix undergoes significant grain growth. The hardness and wear resistance of the pellets increase significantly with the addition of nano-Al₂O₃ particles. The electrical conductivity of the pellets without and with nano-Al₂O₃ dispersion is about 30 pct IACS (international annealing copper standard) and 25 pct IACS, respectively. Thus, mechanical alloying followed by high pressure sintering seems a potential route for developing nano-Al₂O₃ dispersed Cu-Cr-Ag alloy for heavy duty electrical contact.     

The solubility of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> in calcium ferrite slags at 1573 K

Continuous converting of copper matte based on calcium ferrite slag has many attractions for the copper smelting industry. However, a serious drawback is that this slag leads to shorter furnace campaigns because it is aggressive toward the magnesia-chromia refractories that form the furnace lining. As part of an investigation into the causes of this aggressiveness with a view to devising strategies to mitigate it, the solubility of Cr₂O₃ in calcium ferrite slag has been determined. The standard drop-quench experimental technique was employed at a temperature of 1573 K and a relatively high oxygen partial pressure of 3.7×10⁻⁴ atm, conditions similar to those used in continuous converting. It was found that approximately 2 wt pct of Cr₂O₃ can dissolve in calcium ferrite slag under these conditions. The Cr₂O₃ solubility was found to be unaffected by the Cu₂O content of the slag, but may decrease as CaO content decreases. The implications of these findings on the mechanism of attack of magnesia-chromia refractories by calcium ferrite slag are discussed.     

Observation of the crystallization behavior of a slag containing 46 wt pct CaO, 46 wt pct SiO<Subscript>2</Subscript>, 6 wt pct Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, and 2 wt pct Na<Subscript>2</Subscript>O using the double hot thermocouple technique

In this work, isothermal crystallization of a synthetic slag containing 46 wt pct CaO, 46 wt pct SiO₂, 6 wt pct Al₂O₃, and 2 wt pct Na₂O has been investigated by means of double hot thermocouple technique (DHTT). The effect of Na₂O content on crystallization time was confirmed. Two different types of calcium silicate crystals were observed. Calcium di-silicate forms at temperatures above 1150 °C and calcium tri-silicate precipitate at temperatures below 1050 °C. A mixture of the two types of calcium silicate has been observed between the two temperatures. The tendency of crystals to become richer in calcium at low temperatures that has also been observed in previous published works has been confirmed. No effect of the cooling rate on crystallization start time was confirmed in the range of cooling rates applied in this investigation.     

High-Temperature Oxidation of Alloy 617 in Helium Containing Part-Per-Million Levels of CO and CO<Subscript>2</Subscript> as Impurities

The objective of this study was to determine the mechanisms of carburization and decarburization of alloy 617 in impure helium. To avoid the coupling of multiple gas/metal reactions that occurs in impure helium, oxidation studies were conducted in binary He + CO + CO₂ gas mixtures with CO/CO₂ ratios of 9 and 1272 in the temperature range 1123 K to 1273 K (850 °C to 1000 °C). The mechanisms were corroborated through measurements of oxidation kinetics, gas-phase analysis, and surface/bulk microstructure examination. A critical temperature corresponding to the equilibrium of the reaction 27Cr + 6CO ↔ 2Cr₂O₃ + Cr₂₃C₆ was identified to lie between 1173 K and 1223 K (900 °C and 950 °C) at CO/CO₂ ratio 9, above which decarburization of the alloy occurred via a kinetic competition between two simultaneous surface reactions: chromia formation and chromia reduction. The reduction rate exceeded the formation rate, preventing the growth of a stable chromia film until carbon in the sample was depleted. Surface and bulk carburization of the samples occurred for a CO/CO₂ ratio of 1272 at all temperatures. The surface carbide, Cr₇C₃, was metastable and nucleated due to preferential adsorption of carbon on the chromia surface. The Cr₇C₃ precipitates grew at the gas/scale interface via outward diffusion of Cr cations through the chromia scale until the activity of Cr at the reaction site fell below a critical value. The decrease in activity of chromium triggered a reaction between chromia and carbide: Cr₂O₃ + Cr₇C₃ → 9Cr+3CO, which resulted in a porous surface scale. The results show that the industrial application of the alloy 617 at T > 1173 K (900 °C) in impure helium will be limited by oxidation.     

Temperature dependence of the refractive index of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Na<Subscript>2</Subscript>O-SiO<Subscript>2</Subscript> melts: Role of electronic polarizability of oxygon controlled by network structure

Refractive indexes for the Al₂O₃-Na₂O-SiO₂ system have been measured using an ellipsometer for a wavelength of 632.8 nm over a wide temperature range (1100 to 1800 K). Two kinds of sample were used: xAl₂O₃-(40-x)Na₂O-60SiO₂ and yAl₂O₃-yNa₂O-(100-2y)SiO₂, where x ranged between 6 and 20 mol pct and y between 12.5 and 25 mol pct. In the former samples, the temperature coefficient of refractive indexes changed from negative to positive on increasing the concentration of Al₂O₃. In the latter samples, the refractive indexes increased monotonically with decreasing concentration of SiO₂, and the temperature coefficient was always positive. It has been found that the temperature dependence of refractive indexes in these melts is determined by the coefficient of thermal expansion, which would be relevant to the degree of polymerization of the melts. In addition, the electronic polarizability of oxygen derived from the refractive indexes increased with increasing temperature in each melt. This suggests that the basicity of the alumino-silicate melts increases as temperature increases. The positive temperature coefficient of the electronic polarizability of oxygen can be attributed to an increase in the distance between cation and oxygen ion due to thermal expansion. The dependence of the electronic polarizability of oxygen on the concentration of Al₂O₃ has also been discussed in terms of the electronic polarizabilities of three types of oxygen contained in the melts. This article is based on a presentation given in the Mills Symposium entitled “Metals, Slags, Glasses: High Temperature Properties & Phenomena,” which took place at The Institute of Materials in London, England, on August 22–23, 2002.