High-temperature sulfide corrosion of cobalt-chromium alloys

The kinetics and mechanism of high-temperature sulfidation of cobaltchromium alloys were investigated. It has been found that the sulfidation rate of low-chromium alloys, up to 2% Cr, is higher than that of pure cobalt. The one-layer homogeneous scale on these alloys is a solid solution of chromium sulfide in cobaltous sulfide. In the case of alloys containing more chromium, up to 43%, sulfidation rate decreases gradually with chromium content, the scale consisting of two layers. The growth mechanism of scales as well as the influence of chromium concentration on sulfidation rate is discussed.This work was carried out under contract No. 03.10 coordinated by the Institute of Physical Chemistry, Polish Academy of Sciences.     

The scaling behavior of an iron-5% aluminum alloy in sulfur vapor

The scaling behavior of an iron alloy containing 5 wt. % aluminum has been assessed (involving kinetic, metallographic, x-ray diffraction, and electron probe microanalysis studies) in sulfur vapor at 10 ton in the range 500–700°C. For comparison purposes similar experiments were also performed with pure iron. The reaction kinetics observed with the alloy were linear after an initial short period of exposure during which the kinetics were parabolic. Parabolic kinetics, however, were exhibited over the entire period of sulfidation by the pure iron, and the overall rate of sulfidation of the iron was an order of magnitude greater than that of the alloy at all temperatures. Duplex scales were formed both on the alloy and the iron after prolonged periods of exposure. However, whereas the nature and composition of the outer layer of scale formed from both materials was similar, the inner layer of scale on the alloy was significantly different from that grown on the pure iron, both morphologically and in composition containing Al₂S₃ in addition to FeS. The probable mechanism of sulfidation of the alloy is discussed in relation to current theories regarding the behavior of pure iron.     

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.     

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 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.     

High temperature sulfidation of Fe-Mn alloys and Mn in H2-H2S

The sulfidation behaviour of a range of Fe-Mn alloys and pure Mn were studied at 973 K, 1073 K, and 1173 K in H₂-H₂S atmospheres at constant pS₂ or constant pH₂S/pH₂. The sulfidation kinetics were parabolic in most cases, but two different diffusion processes affect the kinetics. In the sulfidation of the Fe-Mn alloys at 973 K and 1073 K, the diffusion of manganese atoms in the alloy is the rate-determining step and an interlocked scale/alloy interface is observed. At 1173 K there is a transition to rate-control by the diffusion of manganese in the sulfide scale connected with transition from rugged to a planar scale/alloy interface.     

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.     

Kinetics and mechanism of high-temperature sulfur corrosion of Fe-Cr-Al alloys

The influence of aluminium on the kinetics and mechanism of high-temperature sulfidation of Fe-Cr alloys containing 20 at.% chromium has been investigated. It has been found that the addition of aluminum greatly improves the scaling resistance of Fe-Cr alloys against attack by sulfur vapors at high temperatures.     

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.     

Sulfidation–Oxidation Behavior of FeCrAl and TiCrAl and the Third-Element Effect

Short-term sulfidation–oxidation exposures were conducted under high pS₂ and low pO₂ conditions for TiCrAl and FeCrAl alloys at 600 and 800 °C. Low mass gains and submicron Al-and Ti-rich oxide scales were formed on TiCrAl at 600 °C, while high mass gains and FeS-based scale formation were observed for FeCrAl. Based on the good behavior of TiCrAl, third-element effect additions of Cr are not inherently detrimental under sulfidation–oxidation conditions. Rather, differences in the mechanistic action of the third-element addition of Cr between FeCrAl and TiCrAl alloys and its relevance to low oxygen potential sulfidation–oxidation environments were the key factors in determining whether or not a protective alumina scale was established.     

Sulfidation behavior of a ferritic commercial alloy in different sulfidizing environments at high temperature

The sulfidizing behavior of Fe–22Cr–4Al–0.15Zr (wt.%) was studied in two atmospheres: S₂ vapor over the range 4.4–25.4 Pa and H₂–H₂S mixtures corresponding to aP _{S 2} range 0.2–1.297 Pa in the temperature range 973–1373 K. It was found that the constitution of the gaseous phase is of great importance on the corrosion kinetics and the morphology of the corrosion products. Furthermore, a stratification phenomenon during scale growth was observed during the initial sulfidation stage in H₂–H₂S mixtures containing a sufficiently high H₂S partial pressure. This behavior was not observed during tests in puresulfur vapor.     

Sulfidation-oxidation of advanced metallic materials in simulated low-Btu coal-gasifier environments

The corrosion behavior of structural alloys in complex multicomponent gas environments is of considerable interest for their effective utilization in coal conversion schemes. Little understanding of the degradation mechanisms of advanced high-temperature alloys in conditions typical of low-Btu coal gasifiers presently exists. Analysis of scale and subscale characteristics of several alloy types after exposure to the aggressive simulated low-Btu gasifier environments yielded a reaction model for these sulfidation-oxidation conditions. Initial competition between reactive metal oxide and base metal sulfide nuclei is followed by base metal sulfide overgrowth, chromium sulfide formation at the scale-metal interface, dissociation near voids in the subscale, and internal chromium sulfide precipitation. Additions of aluminum, titanium, and an oxide dispersion improve the sulfidation resistance by increasing the number of oxide nucleation sites and their growth kinetics on the surface in the crucial competition stage. Thermogravimetric tests carried out in three mechanistic regimes agreed with these hypotheses.     

High Temperature Performance of an FeAl Laser Coated 9Cr1Mo Steel

The isothermal high temperature corrosion behavior of an FeAl coating, coated on 9Cr1Mo steel through laser surface alloying, was studied in atmospheres of pure oxygen and O₂ + 1 %SO₂. The specimens were tested at 500, 600 and 700 °C for 4–100 h. The mass change of the specimens versus time of exposure was used to study the kinetics of oxidation. The coating degradation through interdiffusion of alloying elements between the surface layer and substrate was investigated by long-term oxidation tests in air. OM, SEM, FESEM, EDS and EPMA analyses were used to study the oxidation behavior of the intermetallic coating. The results showed excellent oxidation/sulfidation resistance of the coated material due to a negligible growth rate of the oxide scale. However, the coating was degraded because of the interdiffusion of Al and Fe atoms between the coating and substrate after prolonged exposure to elevated temperatures.     

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.     

The effects of aluminum alloying additions on the sulfidation behavior of iron

As part of an overall research program to examine the possible development of sulfidation-resistant alloys, the corrosion behavior of binary iron-based alloys containing 5, 10, and 20 wt. % of aluminum has been studied in a hydrogenhydrogen sulfide atmosphere for the temperature range 800–1000°C. Details of the observed overall reaction kinetics, the scaling kinetics, and the scales' morphology and composition are given. By considering these results in relation to those reported in an earlier paper concerning the sulfidation of an iron- 5 wt. % aluminum alloy at a lower temperature range, 500–700°C, it has been possible to ascertain the usefulness of aluminum as an alloying addition whereby the sulfidation resistance of iron might be improved. It is concluded that although aluminum can be quite effective in this context at relatively low reaction temperatures at only the 5 % concentration level, at the elevated temperatures likely to be of more industrial importance its usefulness is not maintained, even at the 20% level. The possible reason(s) for the inferior high-temperature performance of these alloys are discussed.     

Kinetik und Mechanismus der Hochtemperaturschwefelung von Silber

Kinetics and mechanism of high temperature sulfidation of silver The formation of the compact sulfide scale on silver follows a parabolic law, the diffusion of silver ions in the reaction product being the rate controlling step. During this scaling reaction it is not necessary to meet all the requirements situated by Wagner's theory: in the silver/sulfur system e.g. the difference of chemical potentials at low temperatures is below the values which would be required according to Wagner's theory. The calculation of the self diffusion coefficients of the metal or the oxidant from oxidation kinetics does not yield sensible results unless — what must be shown by preliminary experiments — thermodynamical equilibria exit on either phase boundary of the scale. Contrary to existing opinion the parabolic nature of the formation of a compact one-phase scale layer does not constitute a sufficient criterion.     

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.     

The problem of sulfur in high-temperature corrosion

The role of defect and transport properties of transition metal sulfides on the kinetics and mechanism of high-temperature sulfide corrosion of metals and alloys is discussed. It has been shown, that due to the very high concentration of defects in common metal sulfides, not only pure metals but also conventional high-temperature alloys (chromia and alumina formers) undergo very rapid degradation in highly sulfidizing environments. Refractory metals, on the other hand, are highly resistant to sulfide corrosion, their sulfidation rates being comparable with the oxidation rate of chromium. Pioneering work of Douglasset al. has shown that alloying of common metals by niobium or molybdenum, and in particular combined alloying by molybdenumand aluminum, dramatically decreases the sulfidation rate. A novel Fe–30Mo–9Al alloy has been proved to be highly resistant to sulfide corrosion, its sulfidation rate being comparable with that of pure molybdenum. Even better resistance to highly-sulfidizing environments show new amorphous Al–Mo and Al–Mo–Si alloys, these materials also being simultaneously oxidation resistant. Thus, new prospects have been created for the development of a new generation of coating materials, resistant to multicomponent sulfidizing-oxidizing atmospheres, often encountered in many branches of modern technology.     

几种含铌镍基合金的热腐蚀研究

用硫酸钠沉积法研究了四种镍基合金-Ni-16Cr,Ni-16Cr-2Nb,IN738和537合金-在900～1000℃之间的热腐蚀行为。用热天平测量了热腐蚀动力学曲线,用金相、SEM、电子探针、X 射线衍射和~(3δ)S 示踪法观察和分析了腐蚀产物。研究发现,Ni-16Cr-2Nb 合金中的铌富集在表面氧化膜内,形成 NaNbO_3而降低熔盐中的 O~(2-)浓度,抑制 Cr_2O_3膜与熔盐的反应;但在复杂成份的合金中。铌的上述作用可能促进氧化膜的酸性熔融。鈪同时能促进硫在 Ni-16Cr-2Nb 变形合金晶界上的连续分布,加速内硫化过程。     

Transport properties of sulfide scales and sulfidation of metals and alloys

Oxidation of Metals - Defect and transport properties of metal sulfides are discussed, showing the differences from and similarities with oxide systems. The sulfidation kinetics and mechanism of...