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.     

Metallographische Untersuchungen über den Aufbau des Zunders nach Oxydation von Eisen zwischen 700 und 900 °C in CO/CO2-Gemischen mit kleinen Gehalten an COS,H2S oder SO2

Metallographic investigations into the structure of the scale after oxidation of iron between 700 and 900°C in CO/Co₂ mixtures with minor additions of COS, H₂S or SO₂ The results of metallographic investigations are in agreement with kinetic measurements published recently (Werkstoffe u. Korrosion 21 [1970] 925) and with analytical investigations of the scale. When because of the CO/CO₂ ratio only a reaction with the sulfur compound is possible, pure sulfide layers are formed in gases containing COS and H. S. When, however, in addition to the reaction with the sulfur compound. A reaction with CO₂ is feasible, an inti-mate mixture of FeO and FeS is formed according to a linear scaling law. When the scale is high in oxide, the FeS particles are embedded in linear shape in a FeO matrix. When the scale has medium contents of oxides and sulfides a perlitic structure is formed consisting of FeO and FeS lamellae in parallel arrangement, the location changing from one grain to the other. With the transition to a parabolic law, i.e. with the transition to rate controlling diffusion of iron ions and electrons through the scale layer, only thermodynamically stable FeS is formed. Under certain conditions needles, predominantly of pure FeS, grow out from the compact scale layer. These needles have diameters between 5 and 20 lm, and may attain lengths up to 600 pn. With the transition to the parabolic law they probably grow in thickness and finally form a coherent FeS layer. In CO/CO₂ mixtures containing SO₂ essentially the same structures are formed, in this context it must be noted, that SO₂ may supply not only sulfur but also oxygen. This is why such FeO/FeS lamellae are formed in such gas mixtures where CO₂, cannot supply oxygen. Higher SO₂ and CO₂, contents in the gas the FeO/FeS lamellae “degenerate” form a coarser mixture of sulfide and oxide.     

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.     

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.     

On the sulphidation mechanism of niobium and some of Nb-alloys at high temperatures

The kinetics and mechanism of niobium sulphidation have been studied as a function of temperature (700-1000 °C) and sulphur pressure (10⁻⁴-10⁴ Pa) in pure sulphur vapour and H₂-H₂S gas mixtures, using microthermogravimetric technique. It has been found that in both different sulphidizing atmospheres the sulphidation process follows parabolic kinetics, being thus diffusion controlled. Marker experiments have shown that the slowest step of the overall reaction rate is the outward diffusion of cations. No influence of small amounts of impurities on the sulphidation rate has been observed in this study. Excellent agreement between calculated and experimentally determined parabolic rate constants has been obtained under the assumption, that the correct formula of the sulphide scale on niobium is Nb_2±yS₃ and not Nb_1+xS₂, as suggested by Gesmundo.It has been found that the rate of niobium sulphidation in H₂-H₂S gas mixtures is much higher than in pure sulphur vapour, strongly suggesting that the dissolution of hydrogen in the growing scale influences the defect structure in this sulphide.     

The hot corrosion of cobalt-base alloys in a modified Dean's rig-II. CoCrAl alloys

A range of CoCrAl alloys have been exposed at 900°C to salt-bearing atmospheres in a modified Dean's rig. The atmosphere consisted of dry air containing vapours of, respectively, Na₂SO₄, Na₂SO₄ + Na₂O, and Na₂SO₄ + NaCl. Both isothermal and 24-h cyclic exposures were used. In general, the presence of small amounts of aluminium in Cr₂O₃-forming alloys had relatively little effect. Chromium contents of 20% and above greatly enhanced the hot corrosion resistance of Al₂O₃-forming alloys. The presence of NaCl was always detrimental, leading to scale fracture and enhancing internal sulphidation.     

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.     

Effect of pre-oxidation on the hot corrosion of CoNiCrAlYRe alloy

The effect of pre-oxidation on the resistance to hot corrosion was examined by corroding the CoNiCrAlYRe alloy at 900 °C in molten Na₂SO₄. Preoxidized specimens featured strong adhesion of oxide scale with uniform multi-layered structure. The time of pre-oxidation was crucial for controlling Al content sufficient for subsequent hot corrosion. However, direct corrosion yielded a defective and non-protective oxide scale, which allowed detrimental penetration of sulfur into substrate. Sulfur migrating along phase boundary was trapped by yttrium to diminish slightly sulphidation. Thus, two advantages of proper pre-oxidation treatment were presented, as keeping repairing for Al₂O₃ scale and inhibiting sulfur penetration.     

The Effect of Traces of SO2 on Iron Oxidation: A Microstructural Study

Pure iron has been exposed to pure O₂ and O₂ with 100 ppm SO₂ at 525 °C for 1 and 24 h. The samples were investigated by FIB, SEM, TEM, EDX and EBSD. The oxide scales formed on iron at 525 °C in O₂ and in O₂ + 100 ppm SO₂ are dense and adherent and consist of three layers. The outermost layer consists of hematite. Beneath it there is a duplex-magnetite scale. The two magnetite layers are separated by a straight interface. It is concluded that the inner-magnetite layer grows inward while the outer magnetite layer grows outwards. In the presence of SO₂ the inner-magnetite layer is much thinner, iron sulphate forms at the oxide surface and discrete iron sulphide grains nucleate at the metal/oxide interface. The amount of sulphide at the metal/oxide interface increases with exposure time. The oxidation of iron in oxygen at 525 °C is inhibited by 100 ppm SO₂. The inhibitive effect of SO₂ is attributed to iron sulphate that blocks active sites on the hematite surface, slowing down the formation of oxygen ions. This explains the strong inhibition of the inward growth of magnetite by SO₂. There is also a marked effect on the morphology of the outer oxide, producing hematite whisker growth and a less porous surface in the presence of SO₂.     

The sulphidation properties of titanium-, manganese- and niobium-bearing Fe---25Cr alloys in H2---H2S mixtures at 800°C

An investigation of high-temperature sulphidation properties of 4 wt% Ti-, 9 wt% Mn- and 8 wt% Nb-bearing Fe---25Cr alloys has been carried out in H₂-H₂S mixtures of sulphur partial pressure in the range of 10⁻³ < pS₂ < I Pa at 800°C. On the whole, the sulphidation kinetics of all alloys obeyed the parabolic law after the initial period of reaction. In some cases, fluctuations in the weight gain-time curves arose due to cracking of sulphide scales. Compared with the weight gain of Fe---25Cr alloy, the additions of alloying elements improved the sulphidation resistance. The effect of 8 wt% Nb was relatively major, but the effects of 4 wt% Ti and 9 wt% Mn were minimal. The addition of these elements did not change the surface morphologies or improve the structure of sulphide scales. Combining the sulphidation kinetics and the analysis of sulphide scale structure, the sulphidation mechanisms of these Fe---25Cr-base alloys have been proposed and the effects of titanium, manganese and niobium have been discussed.     

High temperature corrosion behaviour of a new Ni-30Fe-10Al-Cr-Alloy

The high temperature corrosion behaviour of a new duplex nickel-base alloy containing about 30 mass% iron, 10 miss% aluminium and 8 mass% chromium was determined in both air and hot process gases containing methane/hydrogen, sulphur dioxide and hydrogen sulphide, respectively. It was found that the corrosion resistance against carburisation, sulphidation and oxidation was excellent due to the formation of a dense, protective alumina scale. The adherence of the alumina scale was increased by an addition of 0.1 mass% hafnium. The concentration of chromium was found to have a remarkable impact on the oxidation and high temperature corrosion resistance. Alloys without chromium showed increased corrosion rates in both air and sulphur-containing gas atmospheres due to the initial formation of nickel oxides. In sulphidising SO₂- and H₂S- containing gases at least 4 mass% chromium are required to stabilise the formation of alumina and to prevent the formation of nickel/sulphur compounds.     

Application of Spectroscopic Analysis Techniques to the Determination of Slag Structures and Properties: Effect of Water Vapor on Slag Chemistry Relevant to a Novel Flash Ironmaking Technology

Flash ironmaking technology is an ecofriendly process for producing iron from iron oxide concentrates via a flash reactor that uses gaseous fuels and reductants that reduce energy consumption and minimize greenhouse gas emissions. It has the potential to achieve steelmaking in a single, continuous process. The phase equilibria and chemistry of selected slag systems were investigated during the development of a novel flash ironmaking process. Among the proposed reductants and fuels are H₂, natural gas, and coal gas. In different ironmaking processes, the molten bath (iron-slag bath) is expected to be at equilibrium with gas atmospheres of H₂/H₂O, CO/CO₂/H₂/H₂O, and CO/CO₂. The first two gas mixtures were used to represent the processes based on H₂ or natural gas/coal gas, respectively, whereas the CO/CO₂ mixture was used for a comparison. The slag composition of interest in this process was selected to resemble that of the blast furnace and is based on the CaO-MgO-SiO₂-Al₂O₃-FeO-MnO-P₂O₅ system with CaO/SiO₂ in the range 0.8–1.4. The temperature range was 1550–1650°C encompassing a wide range of expected ironmaking temperatures for the novel flash process. The oxygen partial pressure was maintained in the reducing range of 10^?10–10^?9 atm in the three gas atmospheres. It was found that H₂O dramatically affects the chemistry of the slag and strongly affects the phase equilibria in the slag as well as the equilibrium distribution of elements between slag and molten metal. The effects of water vapor on the chemistry of the slag as well as the equilibrium reactions involving the slag have been studied for the first time.     

Influence of gas phase composition on the Hot Corrosion of steels and nickel‐based alloys beneath a (Ca‐Na‐K)‐sulfate mixture containing PbSO4 and ZnSO4

The corrosion of steels and of nickel‐based alloys was studied in exposure experiments at 600 °C beneath a molten CaSO₄‐K₂SO₄‐Na₂SO₄‐PbSO₄‐ZnSO₄ sulphate mixture in N₂‐5 vol.% O₂ with and without additions of 1000 vppm HCl, 1000 vppm SO₂ and 1000 vppm HCl in combination with 250 vppm SO₂. In the N₂‐5 vol.% O₂ atmosphere, the corrosion products are iron‐ and nickel‐rich but chromium‐free precipitates of oxides in the solidified melt. Additionally, pits filled with layered corrosion products are formed, growing into the metal substrate. These layers consists of less soluble chromium‐rich oxides, containing varying amounts of zinc, (ZnCr₂O₄) alternating with potassium‐rich sulfates, most probably K₂S₂O₇. The addition of 1000 vppm SO₂ leads to a seperation of the melt in a K₂S₂O₇ part close to the metal surface and a Ca‐rich part on top in contact with the gas atmosphere. Compared to the N₂‐5 vol. % O₂ atmosphere accelerated corrosion was observed. In the K₂S₂O₇ part of the melt dissolved iron and nickel are identified, whereas in the Ca‐rich part iron‐ and nickel‐oxide precipitates are formed. Underneath the solidified salt, thin layers of sulfides are detected. In the N₂‐5 vol.% O₂‐1000 vppm HCl containing gas, the corrosive attack is also accelerated compared to the N₂‐5 vol.% O₂‐atmosphere. Much more oxide precipitates are found in the melt on every sample and the inward growth of the zinc‐free chromium‐rich oxides is significantly enhanced. Underneath the inward growing oxide small amounts of metal‐chlorides are detected. Compared to the SO₂ containing gas, the corrosive attack is enhanced for the iron‐based materials, but retarded for the nickel‐based alloys. In the 1000 vppm HCl‐250 vppm SO₂ containing gas, the corrosive attack is similar to the atmosphere containing only 1000 vppm HCl. In addition, sulfides are formed next to chlorides at the metal/scale interface.     

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.     

Corrosion of nickel-base and iron-base alloys in a simulated fluidized-bed coal-combustion environment

The modes of initiation and propagation of corrosion attack on a series of high-temperature alloys were studied in synthetic gas mixtures at 900°C. The gas mixtures were intended to simulate the oxygen and sulfur partial pressures experienced in reducing zones in a coal-fired fluidized-bed combustor and comprised mixtures of CO, CO₂, and SO₂. The alloys studied were candidates for in-bed heat exchanger tubing for an air-heater cycle operating at 843°C and 300–500 psig and so ranged from type 300-series stainless steels to nickel-base alloys. With the exception of two FeCrAlY alloys and types 304 and 347 stainless steels, it was found that sulfidation corrosion could be initiated on all the alloys within 0.25 hr; the rate of propagation of the corrosive attack depended on the flux of SO₂ in the environment and on the nickel content of the alloys. The presence of iron in the alloys appeared to slow the initiation of sulfidation, by forming a continuous iron oxide layer. The effects of various alloying additions are discussed, and a schematic model for the initiation of sulfidation is proposed.     

The hot corrosion of cobalt-base alloys in a modified Dean's rig - III. CoCrMo alloys

A range of CoCrMo alloys have been exposed at 900°C to salt-bearing atmospheres in a modified Dean's rig. The atmosphere consisted of air containing vapours of, respectively, Na₂SO₄, NaSO₄ + NaO, and Na₂SO₄ + NaCl. Both isothermal and 24-h cyclic exposures were used. In general, the presence of molybdenum in the Cr₂O₃-forming alloys caused accelerated and sometimes catastrophic corrosion. The influence of 2.5 Mo addition to the alloys was observed to be minimal. The presence of 10% Mo in the CoO-forming alloys caused acidic fluxing in the pure Na₂SO₄, while the basic salt caused sulphidation corrosion.     

Corrosion Behaviour of Fe–Cr–(Mn,Si) Ferritic Alloys in Wet and Dry CO<Subscript>2</Subscript>–SO<Subscript>2</Subscript> Atmospheres at 650 °C

Model alloys of Fe–9Cr, Fe–20Cr and their ternaries containing 2Mn or 0.5Si (wt%) were exposed to Ar–20%CO₂ and Ar–20%CO₂–0.5%SO₂ at 650 °C, and the results were compared with those in wet CO₂, with and without SO₂. In S-free, dry CO₂, all 9Cr alloys went into breakaway, and the 20Cr alloys formed regions of protective Cr₂O₃, but also fast-growing Fe-rich nodules. Addition of SO₂ to dry CO₂ led to little change for 9Cr alloys and less protective scaling of 20Cr alloys, but decreased the extent of internal carburisation. Addition of H₂O to CO₂ caused breakaway for all alloys. Simultaneous addition of both H₂O and SO₂ to CO₂, however, achieved the best results: passivation of 20Cr alloys, and partial protection of 9Cr alloys. The detection by XPS of sulphide species within the chromia scale allows a discussion of competitive adsorption of S-, C- and H-bearing species on oxide grain boundaries.     

High-temperature corrosion in mixed gas environments

Scaling reactions between pure metals and multiple oxidant gases are reviewed briefly. It is recognized that elemental oxidant activities are usually so low that the actual reactant species are heteronuclear molecules such as SO₂, CO₂, etc. The formation of duplex, sulfide-oxide scales on iron and manganese, even when sulfide is unstable with respect to oxide, is attributed to direct reaction with SO₂. The persistence of the metastable sulfide is due to its preservation by the rapidly growing scale. The reaction of pure chromium with a number of mixed gases is also discussed. The continued formation of carbides and nitrides beneath an external Cr₂O₃ scale layer indicates that the latter material is permeable to gas species. Interaction among different gas species is observed, and is attributed to selective adsorption on internal surfaces within the chromium oxide. New work on the reaction of alloys with mixed gases is reported. Several austenitic heat-resistant alloys were exposed at 1000°C to gases containing one, two or all of the oxidants carbon, sulfur and oxygen. Gases containing two or more oxidants produced multiple zones of internal precipitation. The precipitates were chromium-rich oxides, sulfides and carbides arranged in order of thermodynamic stability: oxides beneath the external scale, carbides deepest within the alloys and sulfides in an intermediate zone overlapping the oxide zone. Each precipitate zone widened according to parabolic kinetics. This finding confirms the as yet untested prediction made by J. L. Meijering in 1971. However, the rate at which a particular zone grows changes according to presence of other oxidants. Interactions between the oxidants can be large and reaction rates are currently not predictable.     

Long term corrosion of X70 steel and iron in humid supercritical CO2 with SO2 and O2 impurities

Abstract

The corrosion of X70 steel and iron in supercritical CO₂/SO₂/O₂/H₂O environment were investigated after a 454 h exposure. Optical microscopy was applied to observe the morphology of etch pits and synthesise the three-dimensional morphology. X-ray diffraction and X-ray photoelectron spectroscopy were employed to detect the composition of product scales. Experimental results verified that the localised corrosion occurred on the X70 steel sample under corrosion product deposits. Ferrous sulphate, sulphur and iron sulphide were detected as the corrosion products.     

Degradation Behaviour of HR-120 Alloy Exposed to CH4–CO–H2–H2O Mixed-Gas Environments at 950 °C

The HR-120 alloy is a candidate structural material for heat-exchanger reactors of steam methane reforming. Consequently, the behaviour at high-temperature of this alloy in oxidizing/carburizing atmosphere is of considerable interest for industrial applications. In this study, the behaviour of HR-120 alloy was evaluated in Ar, CH₄, CO, H₂, H₂O oxidant/carburizing gas mixtures at 1,223 K, with or without pre-oxidation. In the former case, the as-grown scale layer consisted of inner Cr₂O₃ layer and an outer MnCr₂O₄ spinel layer. This scale structure, which is completely transformed into carbide layer in Ar–CH₄ atmosphere, exhibits an excellent stability in CH₄, CO, H₂, H₂O gas mixture. However, the oxidation of particles rich in Nb promoted cracking and spalling of protective scale layer, resulting in exposure of substrate metal.