Studies of the kinetics of nickel sulfidation in H2S-H2 mixtures in the temperature range 450–600°C

The reaction between pure nickel and H₂S-H₂ mixtures containing 1–65% H₂S has been studied over the temperature range 450–600°C. The sulfidation of nickel in the temperature range 560–600°C has been found to follow a linear rate law at low concentrations of H₂S and a parabolic rate law at higher concentrations (10% and 65% H₂S); X-ray examination of the scale formed on the metal showed it to be almost entirely -Ni₃S₂. On the basis of the kinetics and marker studies it can be concluded that the sulfide scale on nickel is formed by the outward transport of the metal and the inward transport of sulfur. In the temperature range 450–500°C the sulfidation of nickel follows a parabolic rate law. In mixtures containing 10% H₂S the scale formed contains voids, the occurrence of which is connected with formation of Ni₇S₆. It has also been shown that the rate of transport through the Ni₃S₂ layer has an essential influence on the formation of a continuous layer of Ni₇S₆. Marker studies have shown that both nickel and sulfur appear to be mobile in -Ni₃S₂.     

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 .     

The sulfidation of manganese at low sulfur pressures at 700–950°C

The kinetics of manganese sulfidation has been studied in H ₂-H₂ S gas mixtures as a function of temperature (973–1223 K) and sulfur pressure (7×10 ^–9 to 3×10 ^–4 Pa), using a thermogravimetric technique. The sulfidation of manganese at low pressures follows the parabolic rate law similar to the behavior at high sulfur pressures (10 ^–4–10⁵ Pa), although an initial nonparabolic incubation period, longer at lower sulfur pressures, was observed. The sulfidation rate constant increased with sulfur pressure and temperature according to the following empirical equation: k_P=const P(S ₂)^1/n exp(–E/RT)However, in disagreement with the results at high sulfur pressures, the exponent 1/n and the activation energy changed considerably with temperature and sulfur pressure. The results are analyzed in terms of a point-defect model of the single corrosion product—MnS—and of the possibility of a doping effect of MnS by hydrogen.     

Effect of 1 wt.% yttrium addition on high-temperature sulfidation behavior of Fe-15Cr-4Al alloy

High-temperature sulfidation studies have been carried out on Fe-15Cr-4Al with and without 1% Y in the temperature range 700–1000°C in an H ₂-H₂ S environment over the sulfur pressure range of 10 ^–9–10^–3 atm. Two-layered and three-layered sulfide scales were observed in both alloys at low and high sulfur pressures, respectively. The pegging phenomenon, similar to that occurring in high-temperature oxidation, across the innermost layer and substrate was observed in the case of the yttrium-containing alloy. Yttrium was found to be associated with aluminum and chromium sulfides. The role of yttrium was more evident at low than at high sulfur pressures and was found to reduce the parabolic rate constants by a factor of about one-half to one-seventh, respectively.     

Sulfidation behavior of austeno-ferritic steels

Two austeno-ferritic stainless steels were sulfidized at temperatures of 783, 873, and 963 K under sulfur pressures in the range 4×10^–12 to 3×10^–5 atm. In all cases a triplex scale developed, consisting of an outermost layer of (Fe, Cr)_1–xS, an intermediate layer of FeCr₂S₄, and an innermost layer of porous (Cr, Fe)_1–xS containing particles of Mo₂S₃. Parabolic kinetics were observed except at the lowest temperature where one of the steels reacted according to irregular kinetics. The Mo₂S₃ particles in the innermost layer acted as inert markers, imaging the former positions of the steels' ferrite phase in which Mo is enriched. The lamellar microstructure of the steel was thus reproduced in the innermost sulfide layer. The positions of the Mo₂S₃ particles together with the porosity of the inner layer are taken to imply inward sulfur transport through this layer and outward metal transport through all three layers.     

Sulfidation properties of an Fe-23.4Cr-18.6Al alloy at temperatures 1073 and 1173 K in H2S-H2 atmospheres of sulfur at pressures 104-10−5 Pa

An investigation is reported on the sulfidation properties of an Fe-23.4Cr-18.6Al(at.%) alloy at 1073 and 1173 K in H₂S-H₂ atmospheres, 10⁴ > P_{S 2} 10⁵Pa. The sulfidation kinetics exhibited an early transient period before onset of parabolic kinetics. Values of the parabolic sulfidation rate constants increased by as much as 10⁵ from their smallest values at low sulfur pressures, P_{S 2} 10^–4 Pa, to maximum values at sulfur pressures P_{S 2} 10² Pa. Multilayered scales were formed, the number and types of layers dependent on sulfur pressure. A fully developed scale at sulfur pressures P_{S 2} > 10^–3 Pa.     

Sulfidation kinetics and scale structures of chromium at temperatures of 973–1173 K and 104–10−6 Pa sulfur pressures

The sulfidation behavior of chromium was investigated over a temperature range of 973–1173 K in H₂S-H₂ gas mixtures of 10⁴–10^–6 Pa sulfur partial pressures using thermogravimetry, X-ray diffractometry, optical and scanning electron microscopy, and electron-probe microanalysis. Sulfidation kinetics are rapid for short periods and obey a linear rate law at low sulfur pressures, whereas at high sulfur pressures sulfidation tends to be parabolic. The surface morphologies can be divided into four types: at high sulfur pressures a petal-like crystal of Cr₂S₃(rho. and tri.) (type 1), at intermediate sulfur pressures a twinlike structure of Cr₃S₄ (type 2), at low sulfur pressures a flat surface with numerous hexagonal pits of Cr_1–xS (type 3), and a fine twinlike structure of ordered Cr_1–xS (type 4). At 973 K, the sulfur pressure ranges are type 1 at > 10^–4, type 2 at , and type 3 at . The critical sulfur pressure where type 2 was formed, 10^–5 Pa at 973 K, shifts toward higherressures at higher temperatures and becomes 10^–3 Pa at 1073 K and 10^–1 Pa at 1173K. Type 4 is observed at 1173K and 10^–6 Pa sulfur pressure. Thesulfide scale is composed of two distinct layers: an external layer, which is dense with a fine columnar structure, and an inner layer, which is porous with a layered structure of sulfides and voids. The external scale is composed offour layers at high sulfur pressures: at the scale-gas interface Cr₂S₃(rho.), next Cr₂S₃(tri.), third Cr₃S₄, and innermost Cr_1–xS. With decreasing sulfur pressures,the number of layers in the external scale was reduced. Pt markers were positioned between the external and inner scales.Emeritus Professor.     

Oxidation and Sulfidation Behavior of AlTiN-Coated Ti–46.7Al–1.9W–0.5Si Intermetallic with CrN and NbN Diffusion Barriers at 850°C

Attempts have been made to improve the high-temperature corrosion behavior of an intermetallic alloy, Ti–46.7Al–1.9W–0.5Si, in an H₂/H₂S/H₂O atmosphere at 850°C using AlTiN coating with and without CrN and NbN diffusion barriers. The oxidation and sulfidation behavior of the uncoated Ti–46.7Al–1.9W–0.5Si alloy followed protective kinetics with a parabolic rate constant of 6×10^–11 g²/cm⁴/s. A multi-layered scale developed: an outer rutile (TiO₂) layer, a continuous layer of -Al₂O₃ beneath the rutile layer, and an inner TiS layer, in which pure W was scattered. Fast outward diffusion of Ti within the substrate resulted in the formation of a zone of high concentration of aluminum (TiAl₃ and TiAl₂) between the scale and substrate.The use of an AlTiN coating greatly increased the oxidation and sulfidation resistance of Ti–46.7Al–1.9W–0.5Si. The use of NbN and CrN diffusion barriers further enhanced its corrosion resistance. The protection of the double-layer coatings persisted even after 240 hr exposure. However the mismatch of thermal expansion coefficients between the coating and substrate led to the development of cracks in some locations within the coatings. A 2.5 m thick AlTiN coating on the Ti–46.7Al–1.9W–0.5Si substrate with an embedded defect was modeled using the general finite element (FE) program ABAQUS. The modeling results showed rapid mode I failure of the coating at a temperature of 774°C. The through-fracture of the nitride film caused the nitride coating to shrink back leading to delamination around the crack in the nitride coating. The cracks formed acted as diffusion paths, for the ingress of oxygen and sulfur species and the outward diffusion of substrate elements, which resulted in the formation of nodular corrosion products with similar morphologies and microstructures to the uncoated alloy in those locations where cracks developed.     

Sulfidation of iron at high temperatures and diffusion kinetics in ferrous sulfide

The kinetics and mechanism of iron sulfidation have been studied as a function of temperature (950–1200 K) and sulfur pressure (10^–3-0.065 atm). It has been stated that a compact Fe_1–yS scale on iron grows according to the parabolic rate law as a result of outward lattice diffusion of metal ions through cation vacancies. The activation energy of sulfidation increases with sulfur pressure and the 1/n exponent increases with temperature. This nontypical dependence of iron sulfidation kinetics on temperature and pressure results from the analogous effect of both these parameters on defect concentration in ferrous sulfide. The chemical diffusion coefficients,D_FeS, and diffusion coefficients of defects, D_d, in ferrous sulfide have been calculated on the basis of parabolic rate contacts of iron sulfidation and deviations from stoichiometry in ferrous sulfide. It has been shown thatD_FeS is practically independent of cation vacancy concentration whereas the diffusion coefficient of defects depends strongly on that parameter. A comparison of self-diffusion coefficients of iron in Fe_1–yS calculated from the kinetics of iron sulfidation to those obtained from radioisotopic studies indicates that within the range studied of temperatures and sulfur vapor pressures the outward diffusion of iron across the scale occurs preferentially along the c axis of columnar ferrous sulfide crystals.     

Oxidation of an Fe—19 wt. % Ni alloy in CO2 at 700–1000°C

The oxidation of an Fe—19.34 wt. % Ni alloy in dry CO₂ has been studied at 700—1000°C using thermogravimetry, metallography, and EPMA. Weight gains for oxygen consumption followed a linear-parabolic-linear sequence at all temperatures. During the initial linear stage the scale consisted mainly of magnetite and the activation energy of 133±25 kJ · mole^–1 is considered to be due to dissociation of CO₂ into CO and adsorbed oxygen on the outer magnetite surface. During the parabolic oxidation stage a continuous Ni-rich layer containing 70% Ni forms a barrier to the diffusion which has an activation energy of 192±79 kJ · mole^–1. The breakdown of the barrier layer causes a return to linear kinetics with an activation energy of 138±42 kJ · mole^–1 for dissociation of CO₂ on the outer surface. During the final linear stage there is pronounced general and intergranular subscale formation. Detailed information is presented of the Ni redistribution and concentrations during oxidation and its correlation with the kinetics and morphology.     

Protection of Fe-Cr-Al alloys in sulfidizing environments by means of an α-Al2O3 scale

The sulfur corrosion behavior of ferritic Fe-22.2Cr-5.5Al and Fe-10.2Cr-5.1Al (wt.%) alloys was studied in sulfur vapor and in a 10%H ₂ S -H ₂ (vol. %) atmosphere at 900°C after preoxidation of the alloys at 1000°C in oxygen to form an -Al ₂ O ₃ scale. The immunity time before onset of sulfidation attack to form a layered scale containing chromium and iron sulfides was dependent upon the -Al ₂ O ₃ scale thickness and the nature of the sulfidizing atmosphere. In pure sulfur at low vapor pressure, =8.1×10 ^–5 atm, the sulfide scale initially developed by the diffusion of metal cations through the -Al ₂ O ₃ barrier. On the other hand, the sulfide was nucleated and grew beneath the Al ₂ O ₃ barrier when the alloys were exposed in the H ₂ S -H ₂ atmosphere at =1.5×10 ^–5 atm. It was possible to demonstate by calculations from a gas-oxide-metal model for sulfur adsorption and diffusion in the solid phases that the types of initial sulfidation attack in these atmospheres were determined by sulfur threshold concentrations at the alloy-oxide and oxide-gas interfaces.     

The sulfidation of pure Co, Y, and of a Co-15 wt.% Y alloy in H2-H2S mixtures at 600–800°C

The corrosion of pure Co and Y and of a Co-15 wt.% Y alloy in H₂-H₂S mixtures providing a sulfur pressure of 10^–8 atm. at 600–800°C and also of 10^–7 atm. at 800°C was was studied to examine the effect of yttrium on the sulfidation resistance of pure cobalt. The alloy was nearly single phase, containing mostly the intermetallic compound Co₁₇Y₂ plus a small amount of cobalt solid-solution. For all conditions except for 800°C under 10^–8 atm. S₂, the alloy formed multilayered scales consisting of an outer region of pure cobalt sulfide, an intermediate region of a mixture of cobalt sulfide with yttrium oxysulfide and finally an innermost layer of a mixture of yttrium oxysu fide with cobalt metal. At 800°C under 10^–8 atm. S₂, below the dissociation pressure of cobalt sulfide, the alloy formed only a single layer composed of a mixture of metallic cobalt with yttrium oxysulfide. Pure yttrium produced only the oxysulfide Y₂O₂S, as a result of the large stability of this compound and of the presence of some impurities in the gas mixtures used. The corrosion kinetics were generally rather complex, but except at 800°C under 10^–8 atm. S₂, the addition of yttrium reduced the sulfidation rate of cobalt, even though the formation of a continuous protective external layer of a pure yttrium compound was never achieved. Finally, when the gas-phase sulfur pressure was above the dissociation of cobalt sulfide the corrosion rate of yttrium was significantly lower than that of Co-15 Y. The internal sulfidation of Y in Co-15 Y was not associated with depletion of Y in the alloy. This difusionless kind of internal attack is typical of binary A-B alloys presenting a very small solubility of the most-reactive component B in the base metal A, which restricts severely the flux of B from the alloy toward the alloy-scale interface.     

Reactivity of Ni20Cr30Al metallic felts with S2

The interaction of Ni20Cr30Al metallic felts with sulfur was investigated between 680 and 920°C at S₂ pressures over the range 2×10^–2 to 18×10^–2 torr. Two principal steps were observed in the sulfidation process, each one leading to a parabolic law. First, solid-state diffusion occurs in each fiber of the material; Ni₃S₂ is formed and reacts with Ni, giving a liquid solution. Second, when a continuous layer of liquid is formed around each fiber, the maximal S₂ pressure at the solid sulfide-liquid sulfide interface is the Ni-Ni₃S₂ vapor pressure. Hence, the sulfidation rate is reduced and no pressure law is observed.     

The corrosion of Fe-Mo alloys in H2/H2O/H2S atmospheres

The corrosion of Fe-Mo alloys containing up to 40 wt.% Mo was studied over the temperature range 600–980C in a H₂/H₂O/H₂S mixture having a sulfur pressure of 10^–5 atm. and an oxygen pressure of 10^–20 atm. at 850C. All alloys were two-phase, consisting of an Fe-rich solid solution and an intermetallic compound, Fe₃Mo₂. The scales formed on Fe-Mo alloys were bilayered, consisting of an outer layer of iron sulfide (FeS) and of a complex inner layer whose composition and microstructure were a function of the reaction temperature and of the Mo content of the alloys. No oxides formed under any conditions. The corrosion kinetics followed the parabolic rate law at all temperatures. The addition of Mo caused only a slight decrease of the corrosion rate. Platinum markers were always located at the interface between the inner and outer scales, indicating that outer scale growth was primarily due to outward diffusion of iron, while the inner scale growth had a contribution from inward diffusion of sulfur.     

Oxidation-sulfidation behavior of iron-chromium-nickel alloys

Oxidation-sulfidation studies of Fe-Cr-8Ni alloys with 4, 12, and 22 wt. % Cr were conducted at 750 and 875°C in multicomponent gas mixtures that contained CO, CO₂, CH₄, H₂, and H₂S. The reaction processes resulted in parabolic kinetics. A chromium concentration in the range 0–12 wt. % in the alloy had a negligible effect on the parabolic rate constant; however, the rate constant for the alloy with 22 wt. % Cr was significantly lower. For a given sulfur partial pressure, the oxygen partial pressures required for the formation of a continuous oxide layer in an Fe-22Cr-8Ni alloy were 10² to 10³ times those calculated for Cr-Cr₂O₃ equilibrium at temperatures of 875 and 750° C, respectively.Work supported by the U.S. Energy Research and Development Administration.On visiting appointment from the Academy of Mining and Metallurgy, Cracow, Poland through a fellowship of the International Atomic Energy Agency.     

Sulfidation properties of Fe-Al alloys at 1173 K in H2S-H2 atmospheres

The sulfidation kinetics and morphological development of reaction products are reported for Fe-9 and 18 at.% Al alloys exposed at 1173 K to H₂S-H₂ atmospheres at sulfur pressures in the range 10^–1–10³ Pa. The Fe-9 Al alloy sulfidized parabolically at Pa giving rise to a duplex scale composed of an outer Al-doped FeS layer and an inner FeS + FeAl₂S₄ lamellar layer and to an internal sulfidation zone containing Al₂S₃ precipitates. The Fe-18 Al alloy which was sulfidized at .     

A study of the mechanism of internal sulfidation-internal oxidation during hot corrosion of Ni-base alloys

The hot corrosion of wrought Ni-16Cr-2Nb was studied at temperatures of 910–1020°C using an autoradiography technique. The autoradiographic pictures of the deposits of Na₂SO₄ enriched with³⁵Sshow that sulfur diffuses along the grain boundaries of the alloy preferentially, where it forms metallic sulfides. The sulfides are then oxidized; sulfur atoms are released, forming new sulfides at the grain boundaries or dissolving in grains and migrating inward by volume diffusion. These results provide new evidence for the sulfidationoxidation mechanism of hot corrosion.     

Sulfur effects on the internal carburization of Fe-Ni-Cr alloys

Several commercial and laboratory-cast model austenitic alloys have been exposed in both sulfur-free carburizing environments and also in carburizing atmospheres to which additions of H₂S have been made. These studies were concentrated over the temperature range 1223–1323 K at a fixed carbon activity (a_c=0.8) with sulfur activities ranging from 2.2×10^–12 bar to 1.4×10^–9 bar. Under conditions of sulfur adsorption, e.g., 5.5 × 10^–11 bar at 1273 K, the blocking of adsorption sites for methane resulted in a transition from the parabolic kinetics observed during sulfur-free carburization to surface controlled linear kinetics. Higher levels of H₂S promoted the formation of a surface layer of chromium sulfide which reduced internal carburization but became a problem itself. The role of minor alloying elements has been established and the use of thermodynamic phase stability diagrams in defining the optimum conditions for sulfur inhibition of carburization evaluated.     

The corrosion behavior of Fe-Al alloys in H2/H2S/H2O atmospheres at 700–900°C

The corrosion behavior of five Fe-Al binary alloys containing up to 40 at. % Al was studied over the temperature range of 700–900°C in a H₂/H₂S/H₂O mixture with varying sulfur partial pressures of 10^–7–10^–5 atm. and oxygen partial pressures of 10^–24–10^–2° atm. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants decreased with increasing Al content. The scales formed on Fe-5 and –10 at.% Al were duplex, consisting of an outer layer of iron sulfide (FeS or Fe_1–xS) and an inner complex scale of FeAl₂S₄ and FeS. Alloys having intermediate Al contents (Fe-18 and –28 at.% Al) formed scales that consisted of mostly iron sulfide and Al₂O₃ as well as minor a amount of FeAl₂S₄. The amount of Al₂O₃ increased with increasing Al content. The Fe 40 at.% Al formed only Al₂O₃ at 700°C, while most Al₂O₃ and some FeS were detected at T800°C. The formation of Al₂O₃ was responsible for the reduction of the corrosion rates.     

Phase relations in an Fe-Cr-S system at temperatures of 1073 and 1173 K in the sulfur pressure range from 100 to 10−5 Pa

Phase relations and stability fields in the Fe-Cr-S system were investigated at 1073 and 1173 K in the sulfur pressure range 10⁰–10^–5 Pa. The sulfides, produced by the sulfidation-annealing process of Fe-Cr alloys followed by rapid quenching, were characterized using X-ray diffraction powder analysis at room temperature. The Cr₃S₄ in the Cr-S system extends beyond the FeCr₂S₄ stoichiometry in the Fe-Cr-S ternary system at intermediate sulfur pressures. The spinel-monoclinic transition of the FeCr₂S₄ was observed at sulfur pressures of 10^–3–10^–3.5Pa at 1073 K and at 10⁰–10^–0.5 Pa at 1173 K. The free energy change for formation of the spinel, FeCr₂S₄, from a monoclinic (Cr, Fe)₃S_4–y, hexagonal (Fe, Cr)_1–xS, and sulfur vapor is given by the relation G = –1523 + 1.09 T (kJ/mol). The phase-transition mechanism of FeCr₂S₄ is discussed on the basis of an enhancement of the cation coordination numbers from 4-6-6 for the spinel to 6-6-6 for the monoclinic, when the sulfur partial pressure decreases.Emeritus Professor.