High-temperature sulfidation behavior of Ni-Nb alloys

The sulfidation properties of Ni-Nb alloys containing additions of niobium up to 40 wt.% have been studied at atm over the temperature range 550-700 °C. The sulfidation reactions followed the parabolic rate law; the sulfidation rates decreased with increasing amounts of niobium. An Arrhenius plot of the rate constants gave activation energies of 25.0+3.5 kcal/ mole. The scales formed on Ni-Nb alloys were multilayered, generally consisting of an outer layer of nickel sulfide ( NiS_1+x and Ni₃S₂) and an inner complex layer of NiNb₃S₆ plus NbS₂. The position of the original metal surface was notedy platinum-wire marker experiments to be the interface between the inner andouter layers. The location of the marker indicates that the outer layer, generally greater in thickness than the inner layer, grew by outward diffusion of the nickel cations, and the inner layer formed probably by the inward diffusion of sulfur. Neither preferential sulfidation nor internal sulfidation was observed. The development of the scale structures from the transient stage to steady state was also studied.     

A comprehensive investigation of the sulfidation behavior of binary Co-Mo alloys

The sulfidation behavior of Co-Mo alloys containing up to 40 wt.% Mo was studied over the temperature range 600–900°C in both 10^–2 and 10^–4 atm. sulfur vapor. All of the alloys were two-phase, with the alloys containing up to 30Mo consisting of Co₃Mo plus solid-solution Co, and the Co-40Mo alloy consisting of the two intermetallic compounds, Co₃Mo and Co₇Mo₆. The sulfide scales which formed were duplex, with an outer layer of cobalt sulfide and a complex, heterophasic inner layer whose phases were both composition- and temperature-dependent. The parabolic rate constant for the sulfidation kinetics decreased with increasing Mo content at all temperatures investigated. Three activation energies, all different from that of pure Co, were observed. Furthermore, Co-30Mo exhibited a kinetics inversion between 800 and 850°C. This inversion was largely the result of the formation of an innermost layer of Co_1.62Mo₆S₈ at the high temperatures. Specifically, the presence of this sulfide in the inner scale caused a significant decrease in the growth rate of the outer layer of cobalt sulfide. In fact, formation of a more compact, innermost layer of Co_1.62Mo₆S₈ at 900°C compared to that at 850°C resulted in a negative activation energy for the growth of the cobalt sulfide in this temperature range. The variation in the activation energies was due to both the duplex nature of the scales which formed and the phase constitution of the inner scale. A simple model has been developed to explain the changes in the activation energies. At 800°C the sulfidation rate of the Co-Mo alloys was essentially the same at the two sulfur pressures studied. The predominant phase in the inner layer of Co-10Mo and Co-20Mo was CoMoS₃, while for Co-30Mo and Co-40Mo it was MoS₂. However, in the case of the latter alloys, Co_1.62Mo₃S₄ formed in the region of the alloy/scale interface at temperatures 850°C and above. Although the MoS₂, which had formed on Co-40Mo, appeared to be a continuous layer, it was in fact found to be relatively nonprotective. Platinummarker experiments revealed the position of the original metal surface to be the interface between the inner and outer scales.     

The sulfidation behavior of Co-Mo alloys containing various ternary additions

The sulfidation behavior of Co-Mo-X alloys, where X is Al, Cr, Mn, or Ti, has been studied over the temperature range 600 or 700°C to 900°C in 10^–2 atm. sulfur vapor to determine the effectiveness of the various ternary elements at reducing the sulfidation rate relative to Co-Mo alloys. For comparative purposes, each ternary alloy contained a constant atomic proportion (i.e., 55Co, 20Mo, and 25X). All of the alloys were multiphase, and sulfidized to form complex, multilayered scales. The scales usually consisted of an outer layer of cobalt sulfide, an intermediate layer that contained primarily the ternaryelement sulfide, and an inner layer which was heterophasic. Usually, each phase within the multiphase alloy sulfidized independently of one another. In the region of the alloy/scale interface there was often a narrow band of fine porosity (transitional band) together with fine precipitates that separated the inner sulfide from the base alloy. It was found that Al and Cr improved the sulfidation resistance of the Co-Mo binary alloy, whereas Mn had the opposite effect. The Ti-containing alloy underwent a mixed sulfidation/oxidation process, so that its kinetics were inapplicable. Aluminum was found to exert the most beneficial effect. The sulfidation behavior of Co-Mo-Al alloys containing a range of Al concentrations was studied at both 700 and 900°C. It was found that for Al to be effective, a sufficient amount of the spinel, Al_0.55Mo₂S₄, had to form within the inner portion of the scale.     

Effect of some ternary additions on the sulfidation of Ni-Mo alloys

Five ternary additions, Cr, Ti, Mn, V, and Al were studied at equi-atomicpercent levels (17 a/o) for their effect on the sulfidation behavior of Ni-19a/o Mo (28–30 w/o) over the range of 600–800°C in 0.01 atm S₂. Al was by far the most effective addition. A linear decrease in log k_p vs. Al content was observed up to 7.5w/o Al, beyond which no further change was observed. All alloys followed the parabolic rate law. Arrhenius plots gave activation energies of 36.9–41.2 Kcal/mol for alloys containing Ti, Cr, Mn, and V, whereas the activation energies for Al-containing alloys were 47.2 Kcal/mol, indicating that a different diffusion process was involved. Complex scales were formed on all alloys, consisting of an outer layer of Nis_1+x and complex inner layers which depended upon alloy composition. Two alloys, those with Cr and Mn, formed intermediate layers of Cr₂S₃ and MnS, respectively, but these layers had little effect on the kinetics. MoS₂ was a constituent of the inner scales except for the alloys with Al. A ternary sulfide, Al_0.55Mo₂S₄ and Al₂S₃ were observed. The presence of the mixed sulfide was always associated with the low sulfidation rates. The formation of MoS₂ on alloys results in a different, less-protective behavior than for MoS₂ formed on pure Mo. This effect is due to the intercalation of Ni into MoS₂ in octahedral positions between the weakly bonded layers of covalently bonded sheets of trigonal prisms. The size of Al⁺³ is too small to be intercalated, and thus MoS₂ is destabilized by Al.     

Effects of sulfur pressure on the sulfidation behavior of 310 stainless steel

High-temperature sulfidation behavior of 310 stainless steel was studied over the temperature range of 700–900°C above a pure sulfur pool with the sulfurvapor range of 10^–4–10^–1 atm. The corrosion kinetics followed the parabolic rate law in all cases. The corrosion rates increased with increasing temperature and sulfur pressure. The scales formed on 310 stainless steel were complex and multilayered. The outer scale consisted of iron sulfide (with dissolved Cr), (Fe, Ni)₉S₈ and chromium sulfides (Cr₂S₃ and Cr₃S₄ with dissolved Fe), while the inner layer was a heterophasic mixture of Cr₂S₃, Cr₃S₄, NiCr₂S₄, and Fe₁–_xS. Platinum markers were found to be located at the interface between the inner and outer scales, suggesting that the outer scale grew by the outward transport of cations (Fe, Ni, and Cr), and the inner scale grew by the inward transport of sulfur. The formation of Cr₂S₃, Cr₃S₄, and NiCr₂S₄ partly blocked the transport of iron through the inner scale, resulting in a reduction of the corrosion rates as compared with the results in the literature.     

The high-temperature corrosion behavior of Co-Nb alloys in mixed-gas atmospheres

The corrosion of Co-Nb alloys containing up to 30 wt.% Nb in H₂-H₂S-H₂O gas mixtures was studied over the temperature range of 600–800°C. The gas composition falls in the stability region of cobalt sulfide and Nb₂O₅ in the phase diagrams of the Co-O-S and Nb-O-S systems at all temperatures studied. Duplex scales, consisting of an outer layer of cobalt sulfide and a complex, heterophasic inner layer, were formed at all temperatures studied. In addition to cobalt sulfide and CoNb₃S₆, a small amount of NbO₂ was found in the inner layer. The reason for the formation of NbO₂ over that of Nb₂O₅ in the scale is that the outer sulfide scale lowers the oxygen activity within the scale into the NbO₂-stability region. Two-stage kinetics were observed for all alloys, including an initial irregular stage usually followed by a steady-state parabolic stage. The steady-state parabolic rate constants decreased with increasing amounts of Nb, except for Co-20Nb corroded at 700°C. Nearly identical kinetics were observed for Co-20Nb corroded at 600°C and 700°C. The presence of NbO₂ particles leads only to a limited decrease of the available cross-section area for the outward-diffusing metal ions. The activation energies for all alloys are similar and are in agreement with those obtained in a study of the sulfidation of the same alloys. The primary corrosion mechanism involves an outward Co transport.     

The effect of Mo on the high-temperature sulfidation of Ni

The sulfidation behavior of Ni-Mo alloys containing up to 40 wt.% Mo was studied at =0.01 atm. over the temperature range of 550–800°C. The alloys included two solid solutions (Ni-10Mo and Ni-20Mo), the single-phase intermetallic compound Ni₄Mo(Ni-29Mo), and two alloys which were two-phase, Ni-30Mo and Ni-40Mo (Ni₄Mo+Ni₃Mo). The sulfidation of all alloys followed the parabolic rate law. The rate of sulfidation decreased with increasing amounts of Mo. Activation energies for sulfidation gave values of 39.1±1.0 kcal/mol. The sulfide scales were bilayered, consisting of an outer layer nickel sulfide (NiS_1+x and Ni₃S₂) and an inner, complex layer of MoS₂ plus intermetallic particles. The rate-controlling step of the sulfidation for the alloys was inward sulfur diffusion and/or outward nickel diffusion through the inner MoS₂ layer. Neither selective sulfidation nor internal sulfidation were observed. No significant difference in the sulfidation kinetics, sulfide structure, and scale constitution could be noted between single-phase alloys and two-phase alloys. The location of the markers was the interface between the inner and outer layers, indicating that the inner layer formed by inward diffusion of sulfur, and the outer layer grew by outward nickel diffusion. The inability to form a continuous protective molybdenum sulfide layer is discussed in terms of the structure of MoS₂ and changes caused by intercalation of Ni into the layered crystal structure. The decrease in sulfidation rate with increasing Mo was attributed to increasing amounts of the intermetallic compound. The increasing volume fraction of particles decreased the available diffusion area in the inner layer and provided a blocking effect.     

Kinetics and mechanism of the sulfidation of Fe-Mo alloys

Iron-molybdenum alloys containing up to 40 wt.% molybdenum were exposed to sulfur vapor at a partial pressure of 0.01 atm at temperatures of 600–900°C. Sulfidation kinetics were measured over periods of up to 8 hr using a quartz-spring thermogravimetric method. The sulfidation kinetics of all alloys studied obeyed the parabolic rate law. The sulfidation rate of iron was found to be reduced by factors of 60 at 900°C and 120 at 600°C by the addition of 40 wt.% molybdenum. Duplex sulfide scales formed on all alloys at all temperatures, the scales consisting of an inner layer of mostly MoS₂ and an outer layer of FeS. Platinum markers were located at the interface between the outer and inner scales, showing that outward iron diffusion and inward sulfur diffusion through the inner layer occurred. The improved sulfidation resistance was attributed to the formation of the MoS₂, which acted as a partially protective barrier to the diffusion of the reacting species. Sulfidation activation energies were found to range from 24.3 to 28.5 kcal mole for the alloys compared to 20.6 kcal/mole, for pure iron. The rate-controlling step was outward iron diffusion through the outer iron sulfide layer.     

High-temperature sulfidation of Fe-30Mo alloys containing ternary additions of Al

Fe-30Mo alloys containing up to 9.1 wt% Al were sulfidized at 0.01 atm sulfur vapor over the temperature range of 700–900°C. The sulfidation kinetics followed the parabolic rate law for all alloys at all temperatures. For alloys containing small and intermediate amounts of Al (<4.8 wt.%), a duplex sulfide scale formed. The outer layers of the scales were found to be relatively compact FeS in all cases; whereas the inner layers were composed of the layered compound MoS ₂ (intercalated with iron), the Chevrel compound Fe _x Mo ₆ S ₈,a spinel double sulfide Al _x Mo ₂ S ₄,depending on the Al content of the alloy and the sulfidation temperature. Extremely thin scales were found on the alloys with higher Al contents. Accordingly, extremely slow sulfidation rates were observed—even slower than the sulfidation rate of pure Mo. The transition of the sulfidation kinetics from a high-rate active mode to a low-rate passive mode requires both a critical Al content in the alloy and a critical Mo content. Because of the two-phase nature of the alloys, the latter requirement implies a critical volume fraction of the intermetallic second-phase in the alloy, which has been known as the multiphase effect. Interestingly, the multiphase effect in these alloys was also a function of the Al content in the alloys.     

High-temperature-sulfidation behavior of Fe-Mo-Mn-Al alloys

The sulfidation behavior of multiphase, iron-based alloys containing up to 24 a/o molybdenum, up to 16.3 a/o manganese, and up to 24 a/o aluminum was examined in flowing H ₂ /H ₂ S gases, corresponding to a sulfur partial pressure of 4 Pa, at 800° C. An accelerated sulfidation rate was almost invariably observed on the quaternary alloys, but slow linear kinetics were found for Fe-22Mo-17Al. This behavior is due to the different products of the preferentially-attacked ferrite phase. If FeAl₂S₄ formed over the ferrite phase, the sulfur-incorporation rate into the scale was slowed down and accordingly the alloys had excellent protection, whereas formation of a MnS+FeS+MoS₂ mixture led to poor protection or breakdown of a protective scale. The nature of the ferrite reaction products was determined by the ferrite composition, which can vary widely. The molybdenum-rich R-phase and AlMo₃ reacted with sulfur slowly. When a protective preferential-sulfidation zone formed, the unreacted intermetallic phases provided a mechanical framework for FeAl₂S₄.     

Effects of yttrium and zirconium additions on the high-temperature sulfidation behavior of an Fe−10Mo−20Al−8Mn alloy

The effects of zirconium and yttrium additions on the sulfidation behavior of an Fe–10Mo–20Al–8Mn(a/o, atom percent) alloy were examined in flowing H₂/H₂S gas of 4Pa sulfur partial pressure at 900°C. Good scale protection was obtained during the initial reaction stage of the base alloy. However, after 7–8 hr, the formation of internal (Mn,Fe) Al₂S₄ platelets triggered breakdown of the protective scale. The reaction products of the zirconium-containing alloy were nonprotective. Yttrium addition resulted in an Y(Fe_1–xAl_x)₁₂ network along the alloy ferrite grain boundaries. Preferential sulfidation of this phase led to almost complete manganese depletion from the engulfed ferrite, and consequently avoided the manganese-promoted scale breakdown.After an even slower initial stage, this alloy sulfidized at a parabolic rate two orders of magnitude slower than that of pure iron. The protection during the initial and following stages was believed to be provided by an Al₂O₃-containing layer and an Al_0.55Mo₂S₄+Fe_xMo₆S_8–z layer, respectively. The formation of Al₂O₃ is thought to be due to oxygen impurities in the H₂S gas, which cannot be removed by conventional means.     

The high-temperature corrosion of Fe-Nb alloys in a H2/H2O/H2S gas mixture

The corrosion of Fe-Nb alloys containing up to 40 wt.% Nb has been studied over the temperature range 600–980°C in a mixed gas of constant composition having sulfur and oxygen pressures ranging from 10^–8 to 10^–4 atm. and from 10^–27 to 10^–18 atm., respectively. All alloys were two-phase, consisting of an Fe-rich solid solution and an intermetallic compound, Fe₂Nb. The scales formed on the Fe-Nb alloys were duplex, consisting of an outer layer of iron sulfide (FeS) and an inner complex layer of Fe_xNbS₂(FeNb₂S₄ or FeNb₃S₆), FeS and unreacted Fe₂Nb. No oxides were detected at any temperature. The addition of Nb reduced the corrosion rate. The corrosion kinetics of Fe-Nb alloys followed the parabolic rate law, regardless of alloy composition and temperature. Platinum markers, attached to the original alloy surfaces, were always located at the interface between the inner and outer scales.     

The high-temperature corrosion behavior of Nb-Al alloys in a H2/H2S/H2O gas mixture

The corrosion behavior of pure Nb and three Nb Al alloys containing 12.5, 25, and 75 at.% Al was studied over the temperature range of 800–1000°C in a H₂/H₂S/H₂O gas mixture. Except for the Nb-12.5Al alloy consisting of a two phase structure of -Nb and Nb₃Al, other alloys studied were single phase. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but fluctuated with increasing Al content. The Nb-75Al alloy exhibited the best corrosion resistance among all alloys studied, whose corrosion rates are 1.6–2.2 orders of magnitude lower than those of pure-Nb (depending on temperature). An exclusive NbO₂ layer was formed on pure Nb, while heterophasic scales were observed on Nb-Al alloys whose compositions and amounts strongly depended on Al content and temperature. The scales formed on Nb-12.5Al consisted of mostly NbO₂ and minor amounts of Nb₂O₅, NbS2, and -Al₂O₃, while the scales formed on Nb-25Al consisted of mostly Nb₂O₅ and some -Al₂O₃. The scales formed on Nb-75Al consisted of mostly -Al₂O₃ and Nb₃S₄ atT 900°C, and mostly -Al₂O₃ , Nb₃S₄ and some AlNbO₄ at 1000°C. The formation of -Al₂O₃ and Nb₃S₄ resulted in a significant reduction of the corrosion rates.     

等离子喷涂Al-Mo涂层的高温硫化和氧化行为研究

烧嘴是水煤浆气化系统的重要部件。运行过程中的高温硫化经常导致烧嘴提前失效,进而影响设备的安全稳定运行。本文采用等离子喷涂方法制备了Mo为粘结层的Al-Mo涂层,测量其在973 K, 1073 K and 1173 K的硫化和氧化行为,并与Mo涂层和Inconel合金进行比较。结果表明Al-Mo涂层的高温硫化抗力和氧化抗力均优于Mo涂层,高温硫化抗力优于Inconel合金。该涂层的提出为喷嘴失效问题的解决提供了便捷、有效的技术方案。     

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.     

The defect structure of 2s Nb1+xS2 and the high-temperature sulfidation of niobium

An approximate model for the defect structure of the niobium sulfide 2s Nb_1+xS₂ is developed on the basis of experimental data concerning its deviation from stoichiometry as a function of sulfur pressure. The model involves the presence of doubly-charged metal interstitials and metal vacancies as well as of free electrons and electron holes. The possible effects of hydrogen dissolution on the concentration of the intrinsic defects in this compound are also evaluated for various hydrogen species with an effective charge different from zero, but no final conclusion concerning the existence of an actual doping effect of this compound is reached. The rate constant for the sulfidation of niobium at 950°C is calculated on the basis of this defect structure as a function of the sulfur pressure and is compared with the experimental results concerning the sulfidation of niobium in H₂-H₂S mixtures. It is concluded that both metal vacancies and metal interstitials contribute significantly to the growth of the sulfide 2s Nb_1+xS₂     

A new thermobalance for studying the kinetics of high-temperature sulfidation of metals

A new thermobalance for studying the high-temperature sulfidation of metals and alloys is described. Results of preliminary investigations of the sulfidation kinetics of iron confirm that this apparatus is entirely suitable for such gravimetric measurements. The weight gains of the sample are recorded exact to 10^–5. The application of the membrane manometer makes possible direct measurements of the sulfur vapor pressure with an accuracy of ±0.2 (tr). Owing to a special construction of vacuum valves, easy access to the reaction chamber is secured, thus eliminating any necessity of cutting the reaction tube.     

High-temperature sulfidation of Fe-Nb alloys

The sulfidation behavior of Fe-Nb alloys containing up to 30 w/o Nb was studied over the range of 600–900°C in 0.01 aim. S₂ vapor. All alloys were two-phase, consisting of an Fe-rich solid solution and Fe₂Nb, and followed the parabolic rate law at all temperatures. Scales consisted of two layers-an outer layer of FeS and an inner, complex layer which contained some FeS, FeNb₂S₄ (possibly some FeNb₃S₆), NbS₂, and intermetallic particles which were either completely or only partially sulfidized. Platinum markers were located always at the interface between the two layers, which corresponded to the original metal surface. Activation energies were 18±3 kcal/mol in close agreement with the 19.8 reported for pure iron. The sulfidation rate decreased markedly with increasing Nb content of the alloys. The decrease is attributed to increasing amounts of Fe₂Nb with increasing Nb, the net effect being that the diffusion path for outward iron diffusion through the inner layer is reduced as the Nb content increases. An analysis of the structure of NbS₂ reveals that it is easily intercalated with Fe between loosely bonded layers of S-Nb-S. The S-Nb-S layers are covalently bonded which results in very low diffusivities of either S or Nb in pure NbS₂. Although intercalated Fe tends to change the Van der Waal's type bonding between layers to more ionic or covalent, Fe diffuses readily between the layers in NbS₂. Intercalation of Fe also increases the concentration of sulfur defects in NbS₂, which in turn increases the diffusivity of sulfur. Nb was observed to be immobile. Thus, it is thought that either outward iron diffusion or inward sulfur diffusion in the inner layer is the rate-controlling step, in spite of the close agreement of activation energies with that of the sulfidation of pure iron.     

Influence of Zr or Hf additions to Fe-23Cr-5Al on the protectiveness of preformed Al2O3 scales against sulfidation

An Fe-23Cr-5Al alloy and those containing 0.17 w/o Zr or 0.12 w/o Hf were oxidized to form -Al₂O₃ scales in a flow of pure O₂ at 1300 K for specified periods up to 400 ks, and subsequently sulfidized at 1200 K in an H₂ –10% H₂S atmosphere without intermittent cooling. The protectiveness of the preformed scale was evaluated by the protection time after which a remarkable mass gain takes place owing to the rapid growth of sulfides. In general, the protection time increases as the scale thickens. Both additives increase the protection time to some degree by forming more structurally perfect scales. However, ZrO₂ particles on or near the outer surface of the scale on the Zr-containing alloy provide sites for sulfide formation. The scales formed on the grain boundaries of the Hf-containing alloy are ridged. The tops of the ridges are associated with cracks, which provide preferential sites for sulfide growth.     

The effect of preoxidation on the sulfidation of Ni-20Cr (2–5)Al Alloys

Ni-20Cr alloys with 2, 3.5, and 5 wt.% Al have been preoxidized up to 100 hr at 1000°C in dry H₂, in H₂/23% H₂O and in air and subsequently exposed to an H₂/5% H₂S atmosphere at 750° C. During the preoxidation treatment different types of oxide scales were formed which affect the sulfidation protection in different ways. Optimum results were obtained for alloys with 3.5 and 5 wt.% Al after 20 hr exposure to dry H₂ at 1000°C. A thin Al₂O₃ scale is formed which decreases the sulfur attack by more than one order of magnitude. Preoxidation conditions for Ni-20Cr-2Al alloys in H₂ and for Ni-20Cr-2Al and Ni-20Cr-3.5Al in H₂/H₂O were observed to be less effective. No improvement was found for preoxidation in air or for Ni-20Cr-5Al alloys preoxidized in H₂/H₂O.