Kinetics and mechanism of high-temperature sulfidation of an Fe-23.4Cr-18.6Al Alloy in H2S-H2 atmospheres

Sulfidation of an Fe-23.4Cr-18.6Al (at.%) alloy was investigated in H₂S-H₂ atmospheres, Pa, at 973 K. It was found over this pressure range that sulfidation after an early transient period followed the parabolic rate law, being diffusion controlled. An investigation was carried out of the scales formed during early transient sulfidation over the sulfur pressure range Pa. Fully developed scales were multilayered consisting of an inner compact layer of equiaxed grains, an intermediate layer of equiaxed and columnar grains exhibiting a small degree of porosity, and an outer porous layer of distinct plates and needles. The grains of the inner and intermediate layers contained quarternary sulfide phases. The following phases were identified: spinels (CrFe)Al₂S₄ and (FeAl)Cr₂S₄, hexagonal (FeCr)Al₂S₄, (CrAlFe)₂S₃, and (CrAlFe)₅S₆. The plates and needles were composed of hexagonal (FeCr)Al₂S₄ and (CrAlFe)₂S₃ at and 10^–5 Pa from which pyrrhotite, FeS, grew at .     

Sulfidation properties of an Fe-26.6 at.% Cr alloy at temperatures of 973–1173 K in H2S-H2 atmospheres at sulfur Pressures 104–10−6 Pa

An investigation is reported on the sulfidation properties of an Fe-26.6 at. % Cr alloy at 973, 1073, and 1173 K in H₂S-H₂ atmospheres at sulfur pressures 10⁴ 10^–6 Pa. The sulfidation kinetics when plotted according to a parabolic relationship usually exhibited an early slow transient period before onset of parabolic kinetics. Scales contained up to three layers. A triplex (CrFe)S_x/(CrFe)₃S₄/-(FeCr)S_x scale was formed at high sulfur pressures (range I), a single-phase (FeCr)S_x or a duplex (CrFe)S_x/(FeCr)S scale at intermediate sulfur pressures (range II), and a single-phase (CrFe)S_x scale at low sulfur pressures (range III). These pressure ranges at 973 K were: range I = 10^–2Pa, 10^–2 > (range II) 10^–5 Pa, and range III .     

Sulfidation properties of an Fe-26.4Cr-8Al-6Mn alloy at 973 and 1073 K in H2S-H2 atmospheres,10^{ - 4} \leqslant {\text{P}}_{{\text{S}}_{\text{2}} } \leqslant 10^2 {\text{Pa}}

The sulfidation behavior of an Fe-26.4Cr-8Al-6Mn (at.%) alloy in H₂S-H₂ atmospheres, \(10^{ - 4} \leqslant {\text{P}}_{{\text{S}}_{\text{2}} } \leqslant 10^2 {\text{Pa}}\) .     

Solid-solid reactions,I: Experimental study of barium metatitanate synthesis

The solid-state reaction between barium carbonate and rutile powders to form barium metatitanate BaTiO₃ was studied by thermogravimetric analysis, X rays, and microscopy. Phase-stability domains were drawn in a temperature— , diagram. The dependence of the reaction kinetics on , or is discussed. In particular, the rate continuously decreases when , or increases, but it reaches a maximum as a function of .     

Sulfidation mechanism of an Fe-26.6 at.% Cr alloy at temperatures 973–1173 K in H2S-H2 atmospheres at sulfur pressures 104–10−6 Pa

An investigation has been carried out to establish the sulfidation mechanism of an Fe-26.6 at. % Cr alloy at 973, 1073, and 1173 K in H₂S-H₂ atmospheres at sulfur pressures 10⁴ .     

Hydrogen doping effects in the sulfidation of molybdenum

The kinetics of molybdenum sulfidation have been studied in H₂S/H₂ gas mixtures at 1173 K. Sulphur partial pressures were controlled by the equilibrium reaction between H₂S, H₂, and S₂. Pure molybdenum metal was sulfidized at a fixed value of 133 Pa with varying H₂S and H₂ partial pressures, and at fixed H₂S partial pressures of 5.06×10 pa⁴ and 5.06×10³ Pa with varying hydrogen and sulfur partial pressures. The gravimetric sulfidation kinetics were parabolic under all conditions. X-ray diffraction analysis of the reaction product scale revealed the presence of MoS₂ only. The sulfide scales were of uniform thickness, had a compact morphology, and were tightly adherent to the metal. The sulfidation rates at a fixed sulfur partial pressure increased with increasing hydrogen partial pressure. At fixed values, the rate was almost constant at high values, but increased substantially as was decreased. Defect models for hydrogen dissolution in MoS₂ are developed and compared with the experimental results. It is concluded that the sulfidation rate effects are due to hydride anion occupation of interlayer sites in MoS₂.     

Sulfidation properties of austenitic Fe-Mn and Fe-Mn-Al alloys under low sulfur vapor pressure (8 Pa) in the temperature range 873–1173 K

The sulfidation properties of austenitic Fe-Mn and Fe-Mn-Al alloys containing small amounts of carbon have been characterized with respect to the sulfidation kinetics, scale morphological development, structures, and composition of the sulfide phases. The alloys contained 21–40 wt. % Mn and 2.5–8 wt.% Al. The sulfide phase was monosulfide of manganese and iron containing the other metallic elements in solid solution. Two regimes of sulfidation categorized by slow and fast reaction rates were exhibited by all alloys when sulfidized in sulfur vapor at = 8 Pa and over the temperature range 873–1173 K. In the slow regime, a compact duplex -Mn(Fe)S/Fe(Mn)S scale evolved by a classical parabolic law associated with metal diffusion in scale. A porous microcrystalline mixed scale of the above sulfides evolved in the regime of rapid sulfidation by quasilinear kinetics associated with sulfur ingress through the porous scale.     

Sulfidation of manganese in H2S-H2 atmospheres at temperatures between 1073 and 1273 K

The sulfidation behavior of Mn in H₂S-H₂ atmospheres, atm, maintained at a total pressure of 1 atm was investigated at temperatures between 1073 and 1273 K. The reaction kinetics obeyed the parabolic rate law; the parabolic rate constant was proportional to where n=6.1±0.3. A value of 21,000±2000 cal/mole was estimated for the activation energy of the growth of the sulfide scale under constant sulfur pressure. Polycrystalline columnar -MnS scales were formed which exhibited texture with preferred (111) orientation. The parabolic rate constants at different sulfur pressures from this and previous investigations were used to evaluate the manganese self-diffusion coefficient in -MnS at .     

Sulfidation of manganese in sulfur vapor at low pressure and temperatures between 873 and 1273 K

An investigation was carried out on the sulfidation kinetics of manganese plates by thermogravimetry in pure sulfur vapor at low pressures, 4.5×10^–5< p_S2 (atm)<7.2×10^–4, at temperatures between 973 and 1173 K. The reaction kinetics were parabolic and the sulfidation rate constant at 1073 K was proportional to where n = 6.4±0.8. The activation energy for the sulfidation reaction at atm was 25,000±3000 cal·mol^–1. The -MnS developed a preferred orientation texture, and grain growth was observed even though the metal exhibited no preferred orientation.     

A method for long-term sulfidation of metal at low sulfur pressures and its application to sulfidation of an Fe-20 at.% Al alloy at 1023 K

A method of long-term Sulfidation that involves generating sulfur vapor from dissociation of Ag₂S or from transformation of NiS to Ni₃S_2+x is described. Sulfidation tests of iron samples showed the approach to be essentially valid. At 1023 K, Ag₂S dissociation of Ag₂S gives rise to atm and, in the presence of an inert carrier gas, efficient transport of sulfur to the metal was effected. Liberated silver grew in the form of discrete whiskers, hence did not interfere with continued dissociation of Ag₂S. Equilibration of NiS with Ni₃S_2+x occurs at atm at 1023 K but, with continued loss of sulfur to the metal, the nonstoichiometric Ni₃S_2±x predominates and the ambient is no longer constant. Thus for long-term studies, Sulfidation with NiS/Ni₃S_2+x mixtures is suitable only for metals that sulfidize slowly or at rates independent of the ambient . Using mainly the NiS-Ni₃S_2+x mixture as sulfur source, sulfidation tests up to 72 h at 1023 K were conducted on an Fe-20 at.% Al alloy. Sulfidation was apparently insensitive to the ambient and formation of a three-layer FeS/(Fe, Al)₃S₄/Al₂S₃ scale with internal precipitation zone proceeded at similar rates, and a relationship involving coupled diffusion is proposed. At longer exposure times, transformation of FeS to (Fe, Al)₃S₄ became extensive. Mechanical failure of the scale, followed by rapid healing beneath the original precipitation, also occured after extended reaction periods (>50 hr).     

Corrosion Behavior of Hydrogenated Iron in Oxygen-Containing Sulfuric Acid Solution

By measuring the amount of absorbed oxygen and analyzing the solution by means of photocolorimetry, the effect the cathodic hydrogenation exerts on iron corrosion in an aerated solution of 0.05 N H₂SO₄ + 1 N Na₂SO₄ at 20°C is studied. On control samples, the currents of iron dissolution (i _Fe) and oxygen reduction coincide with the limiting diffusion current of the latter . In the initial corrosion period of hydrogenated iron, the inequalities are valid, which can be explained by the interaction of active hydrogen forms with oxygen at the surface of corroding iron.     

Relationship of Ca, B, and AlP in Al–12.6Si alloy

P modification has been widely used in Al-Si piston industry, but trace of Ca element has great influence on the P modification efficiency. In this work, it is found that primary Si can be heterogeneously nucleated by AlP in near eutectic Al-12.6Si alloy, but Ca element may destroy the P modification efficiency, whereas the addition of B can recover the P modification efficiency in near eutectic Al-12.6Si alloy with high Ca containing. The microstructure transformation was related to the reaction of Ca, B, and AlP. According to the thermodynamic calculation, Ca may react with AlP and form Ca3P2 compound in Al-Si alloy, whereas, when B was added into the melt, AlP could be reformed. The reaction of Ca, B, and AlP can be shown as follows: 2AlP +3Ca→Ca3P2+2Al; Ca3P2+18B+2Al→3CaB6+2AlP. In addition, with B added into the Al-12.6Si alloy with Ca and P addition, the mechanical properties were improved compared with single Ca and/or P addition.     

Influence of prior internal oxidation on the oxidation of dilute Co-Cr alloys in oxygen

The influence of an initial preinternal oxidation treatment in Co/CoO on the subsequent oxidation behavior of a series of dilute Co-Cr alloys (containing 0–1.5 wt. % Cr) in 10⁵ and 10³ Pa oxygen at 1473–1623 Khas been investigated. Particular emphasis has been placed on determining the solubility and mobility of Cr³⁺ ions in CoO. Use has been made of subsequent annealing in argon .     

Kinetic and morphological development of oxide-sulfide scales on iron at 1073 K

The corrosion behavior of pure iron has been investigated at 1073 K in controlled gas atmospheres of SO₂-CO₂-CO-N₂. The equilibrium gas compositions were such that: (1) FeS was stable with respect to FeO, (2) FeO was stable with respect to FeS, and (3) only one of the solids was stable with respect to the gas sulfur and oxygen activities. The resultant scale morphologies are discussed along with the observed parabolic corrosion kinetics. It was shown that duplex (oxide plus sulfide) scales could be produced under all three reaction conditions. Careful adjustment of gas compositions permitted comparisons to be made among sets of experiments having (1) the same value but different and values, (2) the same value but different , and values, and (3) the same value but different and values. In this way it was confirmed that the reactant species was SO₂ over a wide range of gas compositions, and under conditions in which solid-state diffusion was rate-controlling. The exception was found at very high values, where elemental sulfur was the reactant. Catalysis of the reactant gas demonstrated that the results could be affected by the slow approach to equilibrium of the gas phase.     

Sulfidation properties of Fe-Cr alloys at 1073 K in H2S-H2 atmospheres of sulfur pressures 10−2 and 10−5 Pa

An investigation is reported on the sulfidation properties of Fe-Cr alloys over their complete compositional range in H ₂ S -H ₂ atmospheres at 1073 K. Sulfidation of alloys containing less than 60 at. % Cr obeyed parabolic kinetics and alloys of larger chromium content and chromium followed linear kinetics. Scales were composed of two distinct layers: an outer columnar grained sulfide and an inner equiaxed grained sulfide layer of increased relative thickness and porosity at larger alloy chromium contents. The scales on quenched specimens were composed of either (CrFe) ₅ S ₆ or (FeCr)S plus small amounts of (FeCr) ₃ S ₄ at =10 ^–2 Pa and of (CrFe) ₅ S ₆ at =10 ^–5 Pa. Internal sulfidation was most pronounced in the high chromium content alloys and at the low sulfur pressure. The parabolic sulfidation rate at =10 ^–2 Pa was large and of practically constant value irrespective of alloy composition. These kinetics at =10 ^–5 Pa, however, increased from a very low value by three orders of magnitude as the alloy chromium content was increased from 5–60 at.%.     

Morphological evolution during sulfidation of an iron-nickel alloy

An Fe-41 wt.% Ni alloy was reacted at 793, 888, and 938K with H₂-H₂S-N₂ mixtures corresponding to equilibrium levels of 3×10^–4–0.65Pa and varying degrees of nitrogen dilution. At 793 K and low values of and , a compact layer of almost pure Fe_1–S was the only product. At values of 5×10³–1×10⁴ Pa, large growths of nickel-rich sulfide whiskers formed on top of the compact layer. At =2.2×10⁴ and = 1.8×10^–3 Pa, the whiskers were replaced by a compact layer of (Ni, Fe)_1–S. Kinetics were irregular at 793 K, but at the higher temperatures parabolic kinetics were observed after an initial period of slow reaction, during which a compact layer of Fe_1–S was the only product. The onset of parabolic kinetics corresponded to the appearance of an additional zone of nickel-rich sulfide. This zone was made up of whiskers, except at high .     

Diffusion Research in BCC Ti-Al-Ni Ternary Alloys

Interdiffusion in BCC phase of Ti-Al-Ni ternary system was investigated at 1473 K (1200 °C) by employing the diffusion-couple technique. The raw composition profiles resulting from interdiffusion treatment and retrieved from EMPA were first analytically represented by error function expansion (ERFEX), and the ternary interdiffusion and impurity diffusion coefficients were then extracted by the Whittle-Green and generalized Hall methods, respectively. The obtained main interdiffusion coefficients \( \tilde{D}_{\text{AlAl}}^{\text{Ti}} \) and two cross coefficients, i.e. \( \tilde{D}_{\text{AlNi}}^{\text{Ti}} \) and \( \tilde{D}_{\text{NiAl}}^{\text{Ti}} \), were found to increase with increasing composition of diffusing species, whereas the values of \( \tilde{D}_{\text{NiNi}}^{\text{Ti}} \) show no noticeable compositional dependence. The impurity diffusivities \( \tilde{D}_{{{\text{Al}}\left( {\text{Ti - Ni}} \right)}}^{*} \) and \( \tilde{D}_{{{\text{Ni}}\left( {\text{Ti - Al}} \right)}}^{*} \) increase with decreasing the Ni and Al compositions, respectively. The results imply that Al diffusion in β Ti-Al-Ni alloys would occur via an ordinary vacancy diffusion mechanism, whereas Ni diffusion, at least one order magnitude faster than Al, very likely benefits from interstitial diffusion as Fe and Co anomaly diffuse in BCC Titanium alloys.     

Transformations of Carbides During Tempering of D3 Tool Steel

The studies were performed on D3 tool steel hardened after austenitizing at 1050 °C during 30 min and tempering at 200-700 °C. Based on the diffraction studies performed from the extraction replicas, using electron microscopy, it was found that after 120-min tempering in the consecutive temperatures, the following types of carbides occur: $$ 200\;^\circ {\text{C}} \to \upvarepsilon + \upchi + {\text{ Fe}}_{ 3} {\text{C}},\quad 3 50\;^\circ {\text{C}} \to \upvarepsilon + \upchi + {\text{ Fe}}_{ 3} {\text{C,}} $$ $$ 500\;^\circ {\text{C}} \to \upchi + {\text{ M}}_{ 3} {\text{C }} + {\text{ M}}_{ 7} {\text{C}}_{ 3} ,\quad 600\;^\circ {\text{C}} \to \upchi + {\text{ M}}_{ 3} {\text{C }} + {\text{ M}}_{ 7} {\text{C}}_{ 3} , $$ $$ 700\;^\circ {\text{C}} \to {\text{M}}_{ 3} {\text{C }} + {\text{ M}}_{ 7} {\text{C}}_{ 3} . $$ Apart from higher mentioned carbides, there are also big primary carbides and fine secondary M₇C₃ carbides occurring, which did not dissolve during austenitizing.     

Phase Equilibria in the System Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO-CoO and Gibbs Energy of Formation of Ca<Subscript>3</Subscript>CoAl<Subscript>4</Subscript>O<Subscript>10</Subscript>

An isothermal section of the system Al₂O₃-CaO-CoO at 1500 K has been established by equilibrating 22 samples of different compositions at high temperature and phase identification by optical and scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy after quenching to room temperature. Only one quaternary oxide, Ca₃CoAl₄O₁₀, was identified inside the ternary triangle. Based on the phase relations, a solid-state electrochemical cell was designed to measure the Gibbs energy of formation of Ca₃CoAl₄O₁₀ in the temperature range from 1150 to 1500 K. Calcia-stabilized zirconia was used as the solid electrolyte and a mixture of Co + CoO as the reference electrode. The cell can be represented as: From the emf of the cell, the standard Gibbs energy change for the Ca₃CoAl₄O₁₀ formation reaction, CoO + 3/5CaAl₂O₄ + 1/5Ca₁₂Al₁₄O₃₃ → Ca₃CoAl₄O₁₀, is obtained as a function of temperature: /J mol⁻¹ (±50) = −2673 + 0.289 (T/K). The standard Gibbs energy of formation of Ca₃CoAl₄O₁₀ from its component binary oxides, Al₂O₃, CaO, and CoO is derived as a function of temperature. The standard entropy and enthalpy of formation of Ca₃CoAl₄O₁₀ at 298.15 K are evaluated. Chemical potential diagrams for the system Al₂O₃-CaO-CoO at 1500 K are presented based on the results of this study and auxiliary information from the literature.