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 .     

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.     

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.     

Comparison between the apparent chemical diffusion coefficient evaluated from relaxation experiments by means of electrical conductivity changes and the corresponding theoretical value for a pure and doped p-type oxide

The chemical diffusion coefficient for a p-type oxide either pure or doped with an aliovalent impurity as evaluated from electrical conductivity changes in simulated relaxation experiments is compared with the corresponding theoretical values obtained on the basis of Fick's first law using a convenient model to represent the defect structure of the oxide. It is found that if the relaxation process is purely diffusion controlled, the experimental value of obtained ( ) is in rather good agreement with the theoretical value calculated by considering the diffusion of lattice defects rather than with that obtained by considering the diffusion of the prevailing electronic defects , even when the latter two values differ. This is shown to be the result of relatively small departures from a proportionality (for the pure oxide) or from a linear dependence (for a doped oxide) in the relationship between the deviation from stoichiometry and the concentration of the electron holes in restricted range of oxygen activity as used in relaxation experiments.     

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 .     

Resistance of 1201 aluminum alloy to plastic deformation at various strain rates

Conclusion With an increase in the rate of plastic strain there is a change in the resistance of 1201 alloy to plastic deformation, which is revealed in a nontraditional decrease in _0.2 and increase in , and A_p. The characteristics _k and _t increase nonlinearally.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 56–61, August, 1989.     

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.     

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 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 .     

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, .     

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 .     

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).     

The diffusion of oxygen in Zirconia as a function of oxygen pressure

Diffusion of oxygen in monodinic zirconia was studied using the oxygen-18 gas-solid exchange technique. The zirconia was in the form of spheres prepared by dropping the powdered material through a high temperature plasma. Diffusion was measured as a function of equivalent oxygen pressure, using oxygen and CO-CO₂ mixtures, over the range 1 to 10^–21 atm. In pure oxygen at 700 Torr and between 600 and 1000°C, the diffusion is described by the equation,D =2.34×10^–2 exp — (45,300±1200)/RT. Experiments in CO-CO₂ mixtures at 850°C gave self-diffusion coefficients that showed a slight decrease with a decrease in D from 10^–6 atm, and finally a decrease inD from 10^–19 to 10^–21 atm. The behavior ofD with appears to be in agreement with a defect model involving anti-Frenkel imperfections, i.e., oxygen interstitials and anion vacancies, if it is assumed that the mobility for oxygen interstitials is greater than that for oxygen vacancies.     

Kinetics of equilibration in relaxation-type measurements under partial rate control by a surface reaction following a general type of rate law. I: The direct problem

The chemical diffusion coefficient for a pure binary compound is usually evaluated from relaxation-type experiments assuming that the sample surface equilibrates immediately with the surrounding atmosphere, but use of this assumption may lead to serious errors in if the reaction is partly controlled by the surface reaction. A general model based on an extension of the linear rate law is proposed as a relatively simple way of taking into account a general type of rate law for the surface reaction. It is shown that this method allows one to simulate the general kinetic behavior in case of a nonlinear type of surface rate law to a reasonable accuracy. Thus, the same approach can also be used to evaluate .     

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.     

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.     

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.     

The Corrosion of Alloy Steels under Antirust Compounds in the Presence of Microbes

The kinetic corrosion characteristics of 352-, 302, and 30 steels with rust-preventive compounds applied are investigated in an industrial neutral electrolyte, depending on whether a compound is infected with microorganisms or not. The effect of microorganisms and their metabolites on the corrosion of steels greased with the compounds are assessed.     

Development of scale morphology during wustite growth on iron at high temperature

The development of scale morphology during wustite scale growth at 1050°C and .