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 .     

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 .     

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.     

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 .     

Hot-stage scanning electron microscope for high-temperaturein-situ oxidation studies

The design of a hot stage for a Cambridge S 4 scanning electron microscope (SEM) is described. Provision is made for the entry of noncondensible reactive gases to the specimen chamber through a leak tube connected to a microvalve. Preliminary oxidation studies on copper at 850°C and = 3 × 10^–4 have shown exceptional promise for the continuous observation of the scalegas interface at magnifications of up to 20,000x. Growth strains resulted in fractures at the grain boundaries of the oxide scale. Growth and random nucleation of oxides are all important features which have been observed here, and clearly show the usefulness of the modified stage of the SEM for future studies of oxidation of metals and alloys.     

Solubility of sulfur in pure Cr2O3 at 1000°C

Sulfur solubility in pure Cr₂O₃ was measured over a large range of oxygen (10^–6–10^–12 atm) and sulfur (10^–2 -10^–16 atm) partial pressures at 1273 K. Different methods of analysis were used; it was found that the neutronactivation technique produced the most reliable and reproducible results. The results obtained showed that the limiting solubility of sulfur in Cr₂O₃ varied between 16–93 ppm for the range of oxygen and sulfur pressures used in this study. The solubility was found to vary, depending on the combined effect of and . For a given , the amount dissolved increases with an increase of . An empirical equation, was found to best fit the experimental results and indicates a stronger influence of than of . A specific dissolution mechanism connot be formulated from this equation. However, a number of possibilities have been proposed, but none of these specific mechanisms seems to fit the experimental data.     

The morphology of scale growth on iron-base alloys containing chromium and silicon

A series of iron-base alloys containing 18 wt.% Cr and 0.5 to 2 wt. % Si have been oxidized in air, in a H₂/H₂O environment ( =10^–18 atm) and corroded in a H₂/H₂O/H₂S environment ( =10^–18 atm, .     

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 .     

Oxidation behavior of sus430 stainless steel in moist atmospheres at 873 k

The oxidation behavior of SUS430 stainless steel was studied in moist atmospheres at 873 K. The mass gain during oxidation was measured by using a thermobalance. An abnormal rapid oxidation occurred after an incubation period which increased with increasing oxygen partial pressure. The product on the surface was identified as -Fe₂O₃ of whisker-like morphology by X-ray diffraction and scanning electron microscopy. The abnormal oxidation was not observed under high-oxygen partial pressures. The results of the experiments conducted in various atmospheres were analyzed and are discussed.     

Sulfidation—oxidation behavior of alloy 800H in SO2-O2 and H2-H2S-CO-CO2 atmospheres

The high-temperature-corrosion behavior of alloy 800H has been studied in an oxidizing (SO₂–O₂, =0.23 atm, =1.9×10^–29 atm) and a reducing (H₂–H₂S–CO–CO₂–N₂, =1.5×10^–18 atm =4.3×10^–8 atm, a_c=0.03) sulfidizing environment, at 750°C and 850°C, respectively. When corroded in SO₂–O₂, the protective chromia scale which developed on the alloy in the early stages cracked and spalled in quite a short time period. This led to the growth of iron and nickel sulfides beneath the chromia layer, causing more chromia spallation. When correded in H₂–H₂S–CO–CO₂–N₂, the alloy exhibited breakaway corrosion in about 35hr, at which stage liquid nodules formed on the sample surface. The nodules were studied in detail and were found to consist of three layers. The growth mechanism of such nodules is proposed.     

The morphology of oxidation of alumina-forming iron-base alloys containing chromium and aluminum

A series of iron-base alloys containing different amounts of Cr and Al have been oxidized at atm and at atm. The morphologies of the A1₂O₃ scales depend on the heating rate of the specimens, the Cr content of the alloys, and the of the environment. When the specimens are slowly introduced in the hot zone and oxidized in air, all the alloys form Al₂O₃ scales with a convoluted morphology. The extent of the convolutions decreases with increase in the Cr content. When the specimens are rapidly heated and oxidized in air, the low Cr-containing alloys form internal oxides and a surface scale of Al₂O₃ which shows no convolutions. In the higher Cr containing alloys, the internal oxidation is limited. In the low .     

High-temperature sulfidation of Mo-50 Wt.% Re

Mo-50Re was sulfidized over the range of 1000–1100°C in sulfur vapor at pressures of 10^–4 and 10^–2 atm. The reaction kinetics followed the parabolic rate law with an activation energy of 55.4 kcal/mole for and 48.2 kcal/mole for atm. The pressure dependence varied between +1/4 to +1/6 for the slope of a plot of log K_p vs log .Analysis of the diffusional processes occurring in both the scale and the alloy substrate gave an expression for the ratio of the thickness of the scale and of the -phase as a function of the corresponding rate constants for the growth of each layer. Finally, the conditions required for the formation of the -phase layer between the outer scale and the alloy substrate were obtained in terms of the ratio between the diffusion coefficients of the two metals in the intermetallic compound.     

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.%.     

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.     

The oxidation and decarburizing of Fe-C alloys in air and the influence of relative humidity

Isothermal oxidation treatments were carried out on an Fe-C alloy (0.4 % C): (a) in almost dry air around A₁, and also with an Fe-C alloy (0.5% C) and IRSID pure iron; (b) in dry air ( nm Hg); (c) in almost dry air(1–2% water vapor) at 700° C; and (d) in moist air (31% water vapor). Theresults are as follows: The rate of oxidation at a temperature below A₁depends chiefly on alloy structure, i.e., on thermal history of the sample.The water vapor content of the air strongly influences the scale adherenceas well as the rate of oxidation of the Fe-C alloys below A₁, but has virtuallyno effect on the rate of oxidation of pure iron. Under the same conditions, avery light decarburization of metal occurs in air, whereas no decarburizationoccurred in air with 13% water vapor.     

The kinetics and mechanism of catastrophic oxidation of metals

A set of independent methods has been used to study the catastrophic oxidation of copper in the system Cu–Me_xO_y (where Me is Bi, W, Mo, or V). Two stages of the catastrophic oxidation have been revealed: a rapid stage (K10^–4 kg² m^–4 sec^–1) and a super rapid stage when the metal is oxidized within1–5 sec. The weight ratios of metal to oxidizer and the partial oxygen pressure for the superrapid copper oxidation have been established. The mechanism of the catastrophic oxidation of metals is considered.     

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.     

Electron re-emission Mössbauer study of the oxidation of metallic iron in oxygen

The results of an electron re-emission Mössbauer (ERM) spectroscopy study of the oxidation of metallic Fe at temperatures from 150 to 500°C in pure oxygen ( Torr) are presented. The oxidized samples were prepared from a metallic Fe sample enriched to 70% in the Mössbauer isotope Fe⁵⁷ and the oxidation kinetics were determined volumetrically. Scanning electron microscopy was employed to characterize the morphology of the oxide films. The hematite, magnetite, and total-oxide thicknesses were determined from the ERM spectra using a theory recently developed in this laboratory, and the results were found to agree well with the measured oxygen content of the oxide films. For Fe and Fe-base alloys it is shown that the ERM technique is most useful in the oxide-thickness range of approximately 20–3000 Å within that range, oxide-phase identifications and thickness determinations are perhaps more readily determined by ERM spectroscopy than by any competing technique. In particular, the ERM measurement is nondestructive, and no supplementary measurements (such as surface-roughness determinations) or special sample preparations (such as oxide-film stripping) are required     

Oxidation of copper and electronic transport in copper oxides

Oxidation of copper and electronic transport in thermally-grown large-grain polycrystals of nonstoichiometric copper oxides were studied at elevated temperatures. Thermogravimetric copper oxidation was studied in air and oxygen at temperatures between 350 and 1000°C. From the temperature dependence of the oxidation rates, three different processes can be identified for the oxidation of copper: bulk diffusion, grain-boundary diffusion, and surface control with whisker growth; these occur at high, intermediate, and low temperatures, respectively. Electrical-conductivity measurements as a function of temperature (350–1134°C) and oxygen partial pressure (10^–8–1.0 atm) indicate intrinsic electronic conduction in CuO over the entire range of conditions. Electronic behavior of nonstoichiometric Cu₂O indicates that the charge defects are doubly-ionized oxygen interstitials and holes. The calculated enthalpy of formation of oxygen ( ) and hole-conduction energy (E_H) at constant composition for nonstoichiometric Cu₂O are 2.0±0.2 eV and 0.82±0.02 eV, respectively.This work was supported by the U.S. Department of Energy, under Contract W-31-109-Eng-38.     

Effect of strain rate on the resistance of steel 08Kh15N5D2TU-Sh to plastic deformation

Conclusions The effect of increased on the resistance of steel 08Kh15N5D2TU-Sh to plastic deformation is manifested in increased strength and reduced deformation and energy characteristics. The coefficients of variation of all mechanical characteristics increase significantly. A certain martensite-block orientation (in the fracture zone) in the direction of load application is observed in specimens tested at and , and is not observed at .Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 46–48, September, 1986.