Kinetic conditions for the simultaneous formation of oxide and sulphide in reactions of iron with gases containing sulphur and oxygen or their compounds

The scaling of pure iron has been investigated in N₂O₂SO₂ and COCO₂COS mixtures between 700 and 900°C. Simultaneous formation of FeO and FeS at the scale/gas phase boundary is observed when the diffusion in the aerodynamic boundary layer or the reaction at the scale/gas phase boundary is the rate-controlling step of the oxidation in O₂N₂ mixtures or of the sulphidation in COCOS mixtures. In those cases the addition of the second oxidant (SO₂ to O₂N₂ mixtures, and an increased CO₂ to COCO₂COS mixtures) increases the rate of the oxidation or sulphidation reactions. When, however, the diffusion of iron ions and electrons through the oxide or sulphide layer respectively, or the reaction at the metal/scale phase boundary are rate-determining, the thermodynamically stable phase (oxide or sulphide) is formed exclusively and the addition of the second oxidant has no influence on the scaling rate. These results may be understood from an evaluation of the equilibria prevailing at the scale/gas phase boundary.     

Die Verzunderung von Eisen zwischen 700 und 900 °C in CO/CO2-Gemischen mit kleinen Zusätzen von COS,SO2 und H2S

Scaling of iron between 700 and 900°C in CO/CO₂mixtures with minor additions of COS, SO₂ and H₂S Scaling of iron in CO/CO, mixtures containing less than 1.6% COS, H₂S or SO₂follows initially a linear kinetic law. The transition from the linear to the parabolic law is displaced toward shorter periods with increasing sulfur contents in the gas and with decreasing temperature. At 800 and 900°C the rate of the reaction between iron and the sul-fur compound in the gas is controlled by the mass transfer in the gas phase. In this conditions the reaction rates with COS and H₂S are practically identical, while the reaction with SO₂yields al-most double the weight increase because in this case not only sulfur, but also part of the oxygen of SO₂ react with iron. At 700°C there is a transition of the control mechanism in CO/CO₂C/S mixtures with increasing COS contents, namely from control by mass transfer in the gas phase to control by the phase boundary reaction. Some consequences concerning the heating of steel in technical furnaces are discussed.     

Morphological development of oxide-sulfide scales on iron and iron-manganese alloys

Pure iron and alloys containing 2, 15, 25, and 50 wt. % manganese have been reacted at 1073 K in controlled gas atmospheres of SO ₂-CO ₂-CO-N ₂.Equilibrium gas compositions were such that (1) FeS was stable but not FeO, or (2) both FeS and FeO were stable, or (3) FeO was stable but not FeS; in all cases, both MnS and MnO were stable. Under all reaction conditions, pure iron corroded to produce both sulfide and oxide. The resultant scale morphologies were consistent with local solid-gas equilibrium for the case in which both oxide and sulfide were stable but in the other cases indicated that equilibrium was not achieved and that direct reaction with SO ₂ (g) was responsible for corrosion. Additions of manganese did not greatly alter the scale morphologies. Under reaction conditions that were oxidizing and sulfidizing, very high levels of manganese were required to reduce the corrosion rate. On the other hand, relatively low levels had a beneficial effect both when FeO but not FeS was thermodynamically stable and similarly when FeS but not FeO was stable.     

High-temperature linear kinetics of FeS formation and reduction in COS-CO-CO2 gas mixtures

The linear kinetics of the monosulfide scale formation and reduction according to the overall reaction Fe_(s) + COS_(g) = FeS_(s) + CO_(g) in COS-CO-CO₂ gas mixtures was studied in the temperature range 750–910 C by a thermogravimetric technique. The validity of the linear rate law is limited to short times of exposure and relatively low partial pressures of COS. A proposed model for the sulfidation reaction implies that both adsorption of COS and dissociation of adsorbed COS are involved in the rate-limiting steps. For the reverse reaction it is suggested that both adsorption of CO and recombination of adsorbed CO with sulfur either in an adsorbed state or incorporated in the sulfide lattice are the rate-controlling steps. The theory of absolute reaction rates was applied to the proposed reaction model. Activation enthalpies and entropies for both the sulfidation and the reduction process were derived. From these data standard enthalpy and entropy changes for the overall reaction were evaluated and found to be in close agreement with thermochemical data from the literature.     

Equilibria in the gas system S-O-C between 550 and 1100°C

Equilibria in the S-O-C gas system have been calculated, for a variety of starting values of CO, CO₂, and SO₂ between 550–1100° C, assuming the existence of 10 gaseous species. It is shown that the species COS, SO₃, CS, and SO may form in concentrations sufficiently high that values of sulfur and oxygen partial pressures, calculated from the initial values of CO, CO₂, and SO₂, are in error. Results are given for three sets of initial compositions and are available for 39 more.     

硫杂质对焦反应性的影响及其机理(英文)

采用以低杂质沥青焦模拟石油焦和外掺杂的方式,研究硫杂质元素对焦反应性的影响,并通过XRD、SEM和EDS等检测手段探讨其作用机理。结果表明:在无其他杂质元素干扰的情况下,硫实际上是一种对焦的空气和CO2反应性都具有明显催化性的杂质元素。其催化作用可能是通过在焦的空气和CO2反应过程中分别引发有机硫→H2S→SO2→COS和单质硫(S x)→SO2和有机硫→H2S→COS→S x→C2S→COS两组可部分循环并具有增加碳耗和增大焦比表面积作用的反应体系来实现的。     

Die Verzunderung von Eisen in O2?SO2-Inertgas-Gemischen bei 900° C

Scaling of iron in O₂? SO₂-inert gas mixtures at 900 °C . Kinetic and metallorgraphic investigations into the oxidation of iron in O₂So₂-inert gas mixtures show, that SO₂ increases the scaling rate of iron when the oxidation in the SO₂ free gas follows a linear kinetic law; in these cases the transport of oxygen from the flowing medium to the specimen surface is the rat-controlling step. Such conditions exist at 900 °C and linear flow velocities of 5.8 cm/s at oxygen contents below about 7% At constant oxygen pressure the constant of the linear kinetic law is a linear function of the SO₂.     

Behavior of oxide scales on 2.25Cr-1Mo steel during thermal cycling. I. Scales formed in oxygen and air

The acoustic-emission (AE) technique has been applied to study scale-damage processes during thermal cycling of a tube, preferentially between 600 and 300°C in air, oxygen, and air + 0.5% SO₂. The AE measurements were accompanied by optical and electron-optical investigations on tube rings exposed to the same cycling conditions. During the first period of cycling, a scale rich in hematite is formed. It suffers compressive stresses during cooling. The result is a buckled multilayered scale with separated lamellae. The scaling rate is lower than under isothermal conditions. AE signals start after 175°C cooling. After longer exposure times, the scale contains an increasing amount of magnetite and becomes more compact. The scaling rate increases and is comparable to that under isothermal conditions. AE signals are already observed after 50°C cooling and are correlated with crack formation in the magnetite caused by tensile stresses there. The addition of SO₂ to air enhances the crack-healing process due to higher Fe diffusion in FeS. The scale is more compact.     

On the reaction mechanism of nickel with SO2+O2/SO3

High-purity nickel has been reacted with 96% O₂+4% SO₂ at 700–900°C. The reaction has been studied at 700°C as a function of the total gas pressure (0.06–1 atm) and at 1 atm as a function of temperature (700–900°C). The reaction mechanism changes with the effective pressure of p(SO₃) in the gas. When NiSO₄ (NiO + SO₃ = NiSO₄) is formed on the scale surface, the scale consists of a two-phase mixture of NiO + Ni₃S₂; in addition, sulfur is enriched at the metal/scale interface. A main process in the reaction is rapid outward diffusion of nickel through the Ni₃S₂ phase in the scale; the nickel reacts with NiSO₄ to yield NiO, Ni₃S₂, and possibly NiS as an intermediate product. When NiSO₄ cannot be formed, the scale consists of NiO, and small amounts of sulfur accumulate at the metal/scale interface. It is proposed that the reaction under these conditions is primarily governed by outward grain boundary diffusion of nickel through the NiO scale, and in addition, small amounts of SO₂ migrate inward through the scale—probably along microchannels.     

The scaling of a Fe-20Cr alloy in H2-H2O-H2S mixtures

The scaling of an Fe-20Cr alloy has been studied in H₂-H₂O-H₂S mixtures between 973 and 1223 K. According to a simplified phase diagram, Cr₂O₃ and FeS should be the thermodynamically stable compounds in the gas mixtures chosen. The reaction followed a mixed rate law between linear and parabolic, indicating that the reaction rate at the scale-gas interface was comparable with the diffusion rate in the scale. At a constant H₂/H₂S ratio the scaling rate initially decreased slightly with increasing water-vapor pressure. A further increase of the water-vapor pressure resulted in localized sulfide formation, while the other parts of the surface were covered with a Cr₂O₃ film. Only Cr₂O₃ formed above a critical water-vapor content. Three zones could be distinguished when a sulfide scale is formed. The outer zone consisted of practically pure FeS; the intermediate zone was a solid solution of (Fe,Cr)S, partially decomposed to FeCr₂S₄ and metal during cooling; and the inner zone contained small Cr₂O₃ inclusions in an (Fe,Cr)S matrix.     

Reactions of chromium in SO2-containing atmospheres

The morphology and composition of scales formed on unalloyed chromium in atmospheres containing SO₂ at high temperatures have been studied. SEM images show chromium sulfides formed at the metal/scale interface. The results indicate that SO₂ may penetrate the scale as molecules, but the rate of this penetration is low. At 600°C formation of Cr₂(SO₄)₃ is also observed by SEM on the scale surface formed in SO₂+O₂/SO₃ mixtures.     

High Temperature Oxidation of Steels in Air and CO2–O2 Atmosphere

Three kinds of hot rolled steel slabs, viz. high strength steel, bake hardened steel and low carbon steel, were oxidized isothermally between 1100 and 1250 °C for up to 2 hr in 1 atm of air and an 85%N₂–10%CO₂–5%O₂ gas mixture. The steels were oxidized in a similar fashion in both the atmospheres. The oxidation process followed an initial linear rate law, which then gradually transformed to a nearly parabolic rate law. Thick, porous and nonadherent scales formed rapidly, due to the high oxidation temperature. The scales formed consisted of Fe₂O₃,(Fe₂O₃+Fe₃O₄), (Fe₃O₄+Fe₂O₃ +FeO) and (FeO+Fe₃O₄) from the outer surface. The presence of supersaturated oxygen beneath the scale resulted in grain boundary oxidation and the formation of internal oxide precipitates.     

Studies on the mechanism of high temperature corrosion of cobalt in SO2 atmospheres with the use of18O and35S isotopes

The composition and morphology of scales formed on cobalt in sulfur dioxide atmospheres (1.013 × 10⁵ Pa) at 850 and 900°C and transport phenomena occurring in the growing scales have been investigated. The transport phenomena have been studied by the marker method and with the use of SO₂ labeled with the oxygen isotope¹⁸O and sulfur isotope³⁵S. The scales were composed of sulfide and oxide mixtures and grew due to the outward diffusion of cobalt and inward transport of SO₂ molecules through the discontinuities in the scale. These molecules, as well as the oxidant originating from the dissociation of the outer scale layer, take part in the formation of the inner scale layer.     

Initial Stages of Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Induced Degradation of β-Ni–36Al at 700 °C: I-Intrinsic Behavior

The early-stage scaling behavior of a β-Ni–36Al alloy undergoing Na₂SO₄-deposit-induced degradation at 700 °C was systematically studied using SEM and TEM. After 20 h of exposure in an O₂–1000 ppm SO₂ ambient, the deposit-coated alloy formed a dense but thin Al₂O₃ scale on most areas of the surface; however, large nodules formed locally. Nodule formation occurred where the scale had lost its protective character, with rapid internal oxidation ensuing. The presence of sulfur both in the environment and in the salt played a key role in nodule formation. Removal of SO₂/SO₃ from the gas mixture, or of the Na₂SO₄ deposit from the surface, prevented nodule formation, while removing the sulfur source after nodule formation prevented further nodule growth. The degradation could be linked to the dissolution of reaction products in the Na₂SO₄ deposit and the formation of a low-temperature eutectic liquid. Further, when an Na₂SO₄–48% MgSO₄ deposit was used, the nodule density increased.     

The mechanisms of breakaway oxidation of three mild steels in high-pressure CO2 at 500°C

The mechanisms of breakaway oxidation of a rimming steel, a low-alloy steel (1 Cr-0.5 Mo), and a free-cutting steel (En1A) have been studied in high-pressure CO₂ at 500°C. Average compressive stresses up to 170–280 ton in^–2. in the scale have been derived from foil elongation and creep data. Carbon contents of up to 6 and 15% of the weight gain have been found in protective and breakaway scale and are larger than previously reported.¹ Most of the carbon is deposited near the scale-metal interface, showing that CO₂ diffuses through the scale. Oxidation in CO₂-tritiated water mixtures gives a maximum tritium content in the metal at 250–500 h, which declines thereafter. Treatment with some sulfur compounds before oxidation, or the presence of sulfur in the metal, reduces the rates of protective and breakaway oxidation in wet CO₂ and carbon transfer to the metal, but not to the scale. It is proposed that breakaway is initiated by an effect of hydrogen such as the accumulation of hydrogenous gases at the scale-metal interface under pressures sufficient to rupture the inner scale. Carbon deposition may assist initiation, and is probably the main factor in propagating breakaway oxidation.     

Korrosionsreaktiorren von elementarem Schwefel mit unlegiertem Stahl in wäßrigen Medien

Corrosion reactions between elemental sulphur and plain carbon steel in aqueous media Plain carbon steels are rather severely attacked by elemental sulphur at room temperature in the presence of aqueous media. The corrosion occurs preferentially at the places where the two solid substances iron and sulphur are in contact with each other and results in shallow pit formation. At the same time the pH is also decreased slightly and small amounts of H₂S and sulphate ions are formed. Neutral salts stimulate the corrosion process whereas phosphates inhibit it and the alkaline media such as Na₂CO₃ and ethylamine with pH > 12 prevent it completely. The latter can be made use of for corrosion protection. At high salt concentrations (c > 1 mol/l) the corrosion rate, however, decreases with increasing salt concentration. The corrosion rate may increase with increasing flow velocity of the medium, but the corrosion takes place uniformly. The results of electrochemical investigations show that the reduction of sulphur occurs at the corrosion product FeS and is the rate controlling step. No sulphur reduction is observed on platinum electrodes when no FeS is present. It is assumed that the starting reaction to initiate corrosion in the system Fe/S/H₂O is a slight disproportionation of S to H₂SO₄ resulting in the formation of FeS.     

Effect of nanocrystallization on the corrosion resistance of K38G superalloy in CO + CO2 atmospheres

The corrosion resistance of the cast superalloy K38G and a sputtered nanocrystalline coating of the same material was investigated in pure CO in the temperature range, 850–1000°C and in CO-20 vol.% CO₂ at 900°C. The cast K38G alloy formed Cr₂O₃ and TiO₂ scales, and a zone of internal Al₂O₃ precipitation. Weight-gain kinetics followed the parabolic rate law under all conditions investigated. The sputtered K38G nanocrystalline coating, however, formed a single-phase Al₂O₃ scale and no internal-oxidation zone. The parabolic rate constants for nanocrystalline coating oxidation were about one order of magnitude smaller than those of the cast alloy. The changes in reaction morphology and rate are attributed to the more rapid grain-boundary diffusion of aluminum in the nanocrystalline material.     

CO+CO2 mixtures and diffusional transport in MnO

Studies of MnO at high temperatures (1000–1200?C) suggest that diffusional transport can be different when the oxide is exposed to carbon-free environments and to CO/CO₂ mixtures, respectively. In the phase field of MnO near the MnO/Mn₃O₄ boundary it is concluded that defect clusters (consisting of four manganese vacancies + one interstitial manganese ion) are the important defects. Under these conditions it is proposed that carbon can dissolve in the oxide in association with the defect clusters. A defect structure model is proposed to account for the differences in properties. In keeping with this interpretation it is shown that parabolic rate constant for growth of MnO scales in CO/CO₂ mixtures is not only dependent upon the oxygen activity, but also up on the carbon activity in the gas. The electrical conductivity is also affected by changes in the carbon activity.     

Amounts and Distribution of Phases in Sulfide Plus Oxide Scales on Iron

Pure iron was exposed at 800°C to flowing, catalyzed-gas mixtures of N₂/CO₂/CO/SO₂ adjusted to control the partial pressures of SO₂, S₂ and O₂. The equilibrium gas compositions were such that iron oxide was thermodynamically stable with respect to sulfide. The reaction product scale was invariably a mixture of oxide plus sulfide, and grew according to parabolic kinetics at high P_SO2 values and by linear kinetics in dilute gases. In both cases the reactant gas species was SO₂, not molecular oxygen or sulfur. The relative amounts of sulfide and oxide corresponded to stoichiometric reaction of SO₂ at high P_SO2 values, but not in dilute gases. At low P_SO2 values, the relationship between scale-sulfide volume fraction and P_SO2 corresponded to two independent scale-SO₂ reactions leading to oxide and sulfide growth. The two-phase mixture was lamellar, with platelets oriented approximately parallel to the mass-transfer direction. An inverse relationship between lamellar spacing and linear scaling rate is interpreted as evidence of a cooperative (cellular) growth mechanism.