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.     

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 .     

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.     

Oxidation-sulfidation behavior of Ni aluminide in oxygen-sulfur mixed-gas atmospheres

Oxidation-sulfidation studies were conducted on sheet samples of nickel aluminide, containing 23.5 at. % Al, 0.5 at. % Hf, and 0.2 at. % B in an annealed condition and after preoxidation treatments. Continuous weight-change measurements were made by a thermogravimetric technique in exposure atmospheres of air, a low- gas mixture, and low- gas mixtures with several levels of sulfur. Detailed scanning electron microscopy (SEM), X-ray, and electron microprobe analyses of the corrosion product scale layers were performed. The air-exposed specimens developed predominantly nickel oxide; the specimen exposed to a low- .     

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 .     

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 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, .     

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 .     

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.     

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.     

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—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 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 .     

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.%.     

Sodium sulfate: Deposition and dissolution of silica

The hot-corrosion process for SiO₂-protected materials involves deposition of Na₂SO₄ and dissolution of the protective SiO₂ scale. Dew points for Na ₂SO₄ deposition are calculated as a function of pressure, sodium content, and sulfur content. Expected dissolution regimes for SiO₂ are calculated as a function of Na₂SO₄ basicity, hence generated by fuels with 0.5% and 0.05% S. Controlled-condition burner-rig tests on quartz verify some of these predicted dissolution regimes. However, the basicity of Na₂SO₄ is not always a simple function of (Na₂O) show that carbon creates basic conditions in Na₂SO₄, which explains the extensive corrosion of SiO₂-protected materials containing carbon, such as SiC.     

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 .     

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 .     

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 .     

Quasiequilibrium treatment of gas-solid reactions. III. Rate of volatilization of tungsten by high-temperature oxidation

Theoretical predictions computed on the basis of the quasiequilibrium treatment of gas-solid reactions are compared with existing experimental data on the rate of volatilization (erosion) of solid tungsten by reaction with gaseous O₂ at high temperature ( 1300° T 3600° K) and low pressure (4.5 × 10^–7 11.5 Torr). The only unknown parameter in the analysis is the equilibrium probability, , defined as the fraction of the impinging O₂ molecules that attain thermochemical equilibrium at the tungsten surface rather than undergoing nonreactive scattering (e.g., reflection). An approximate expression for is estimated by a straightforward empirical procedure that is consistent with the quasiequilibrium treatment. The theoretical results based on this expression for T because appears to be an exponential function ofT; (b) In the intermediate region, the formation of volatile oxides decreases sharply with increasingT because atomic oxygen becomes the thermodynamically favored reaction product, thereby causing _W to decrease with increasingT; (c) In the highest region, _W again increases withT as a result of the formation of WO and the sublimation of W.This work was supported by the Joint Services Electronics Program [Contract DA28-043-AMC-02536(E)] and by NASA [Grant NGR-22-009-091].     

The high-temperature corrosion of alloy 800 in carburizing,oxidizing, and sulfidizing environments

This paper reports the results of corrosion experiments of a commercial iron-nickel-chromium alloy in multiple oxidant gases at 800, 900, 1000, and 1100°C. Two sets of experiments were performed at low oxygen activities; one with high sulfur activities and the other with high sulfur and with high carbon activities. The scale growth and the morphologies are interpreted in terms of the calculated thermodynamic stability of the product phases. The calculated chromium sulfide-chromium oxide equilibrium coexistence lies at systematically lower oxygen potential than the experimentally observed transition. The transition from sulfidation to oxidation occurs over a range of at constant .