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1.
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. 相似文献
2.
Loaded parts are exposed to hot corrosion to a greater extent than unloaded components. Pure nickel predeformed to various degrees by compression (up to 27%) has been oxidized in SO2 at 600°C for different periods (22 to 95 hr). It has been shown that transport properties of the scale, formed on the initial metal surface containing physical defects, depend on their surface density. A general behavior was established for the same exposure (>70 hr): the higher the preliminary strain, the greater the amount of Ni3S2 in the scale. Nickel predeformed 21% and 27%, oxidized in SO2 over 70 hr, formed scales consisting mainly of a single-phase Ni3S2 layer. An increase of the scale defectiveness accelerated attainment of heterogeneous equilibrium in a gas-scale system and intensified the formation of Ni3S2—the stable phase for the conditions used. 相似文献
3.
The reaction between preoxidized nickel and SO2 was studied at 800–1000° C. The experimental methods consisted of thermogravimetry, metallography, scanning electron microscopy, and electron microprobe analysis. Sulfur accumulates in the surface layer of the metal during the SO2 exposure. It is concluded that this is due to transport of SO2 through the oxide scale. This leads in turn to formation of a Ni-S liquid solution which eventually ruptures the scale and causes an accelerated, breakaway reaction behavior. The induction period for the accelerated reaction increases with increased preoxidation and furthermore is influenced greatly by the microstructure of the oxide scale. 相似文献
4.
J. Gilewicz-Wolter 《Oxidation of Metals》1988,29(3-4):225-238
The reaction of nickel with SO2 has been studied. The composition and morphology of the scale formed in sulfur dioxide (1.013×105 Pa) at 600°C and the transport phenomena occurring in the growing scale have been investigated. The experimental methods consisted of metallography, scanning electron microscopy, and electron microprobe analysis. The transport phenomena have been studied by the marker method and with the use of a35S radioisotope. The scale was composed of a NiO and Ni3S2 mixture and grew by the outward diffusion of nickel and inward transport of SO2 molecules through the discontinuities of the scale. It has been shown that outward transport of sulfur originating from grains of sulfide occurs. 相似文献
5.
Thermodynamics of the Na2SO4-K2SO4-CoSO4 system and their relevance to low-temperature hot corrosion
Thermodynamic calculations and experiments were performed to determine the SO3 partial pressures and temperatures at which K2SO4-CoSO4 binary mixed liquid phases form on CoO and Co3O4 in the presence of K2SO4. The calculations and experiments are in excellent agreement. Similar calculations were also made of the compositions at the liquidus surface and the associated SO3 partial pressures for the K2SO4-Na2SO4-CoSO4 ternary system. These calculations show that the presence of K2SO4 substantially reduces the SO3 partial pressures required to stabilize a liquid salt phase on the surface of oxidized cobalt alloys at 600–800°C. Consequently, at these temperatures the hot corrosion in coal-fired systems, where K levels are high, is expected to be worse than in oil-fired systems, where K levels are low. This prediction was confirmed by experiments in a pressurized fluidized bed coal combustor and in an atmospheric pressure burner rig. 相似文献
6.
A marker study of nickel oxidation in SO2 at 600°C has proved that after the reaction metallic markers (Au, Pt, W) are covered with a sulfide rim. This effect is not observed on quartz markers. The metallic markers make the intermediate NiO layer adjacent to them increase, whereas the quartz marker makes this layer disappear. 相似文献
7.
To understand the effect of K2SO4 additive in an Na2SO4 deposit on low temperature hot corrosion, the corrosion behavior of Fe-Al alloys induced by Na2SO4+K2SO4 was compared to that by Na2SO4 alone, and sulfation of Fe2O3 in the presence of either Na2SO4 or Na2SO4+K2SO4 was studied. It was found that K2SO4 additive promoted the low temperature hot corrosion, but did not change the corrosion-mechanism. Experimental results refuted the prior suggestions that the accelerated hot corrosion resulted either from the formation of K3Fe(SO4)3 or from the stimulation of sulfation of Fe3O3. The earlier formation of the eutectic melt caused the accelerated hot corrosion, or in other words, the K2SO4 additive shortened the induction stage of hot corrosion. 相似文献
8.
The oxidation kinetics of 1.25Cr-0.50Mo steel under different thermal cycling conditions in the range of 520–700°C in a 2000 ppm SO
2-argon gas mixture have been studied by thermogravimetric techniques. The influence of the number of thermal cycles experienced by the specimen has been determined on the oxidation process. Characterization of the scales was performed by scanning electron microscopy, and it was found that the nature of the scale is a function of oxidation temperature. 相似文献
9.
SO2 labeled with35S radioisotope was used to study the diffusion of sulfur in scales growing on nickel in sulfur-dioxide atmosphere at 873 K. It was observed that in addition to the inward molecular transport of SO2, outward diffusion of sulfur also occurred. This diffusion occurred mainly through the grain boundaries of the scale [DG873=3×10–7cm2/s]. The liberation of sulfur inside the scale was caused by the reaction of Ni3S2 with oxygen or with sulfur dioxide. 相似文献
10.
Liangquan Shi 《Oxidation of Metals》1993,40(1-2):197-211
Na
2
SO
4
-induced accelerated corrosion of iron in oxygen at 750°C was observed. EDX, XRD, SEM, EPMA and some chemical examinations were carried out to understand the corrosion mechanism. The accelerated oxidation was attributed to the formation of abundant sulfide which has a highly defected lattice and allows rapid diffusion of iron ions. The sulfide resulted in turn from the formation of a liquid phase which was a eutectic melt of Na
2
SO
4
and Na
2
O. The formation of and other possible effects of the melt were discussed. The accelerated oxidation was compared with the usual low-temperature hot corrosion, showing that it has most of the characteristics of low-temperature hot corrosion except that it occurred under basic conditions developed by the removal of sulfur from the sulfate deposits instead of the usual acidic conditions established by the SO
3
in the atmosphere. 相似文献
11.
The corrosion behavior of polycrystalline Ti3SiC2 was studied in the presence of Na2SO4 deposit and water vapor at 900°C and 1000°C. The mass gain per unit area of the samples superficially coated with Na2SO4 exposed to water vapor was slightly lower than that of the samples corroded without water vapor. The microstructure and composition of the scales were investigated by SEM/EDS and XRD. Pores were observed in the corroded sample surfaces. The main corrosion phases on the sample surface were identified by XRD as TiO2, Na2Si2O5 and Na2TiO3. After Ti3SiC2 corroded in the presence of the Na2SO4 deposit and water vapor, the scale had a three-layer microstructure, which was different from the duplex corrosion scale formed on Ti3SiC2 beneath the Na2SO4 film without water vapor. Because water vapor penetrated the corrosion layer and then reacted with SiO2 to form volatile Si(OH)4, an intermediate porous and TiO2-enriched layer formed in the corrosion layer. 相似文献
12.
The short-time hot-corrosion behavior of six industrial nickel-base superalloys was investigated with static deposits of Na2SO4 or NaCl or both in still air. The oxidation kinetics and scale morphologies were measured with traditional laboratory techniques-thermobalance, metallography, electron microprobe, and x-ray analyses. Susceptibility to hot corrosion was found to be correlated to the type of scale produced during simple oxidation. Alloys forming an A12O3 scale were found to be susceptible to Na2SO4 deposits, independent of their chromium content. The quantity of Na2SO4 deposit dictated the nature of the attack and, under certain conditions, the refractory element alloy additions appeared to play an essential role. Alloys containing Cr2O3 or TiO2 in the simple oxidation scale proved to be sensitive to NaCl attack. Again, the severity of the attack within the susceptible alloy group was not related to the chromium or titanium content. Although less intensive than the Na2SO4 -induced hot corrosion, NaCl contaminations provoked extensive spalling. All of the hotcorrosion types encountered in this study were interpreted in the light of existing theories.Supported by the Délégation Générale à la Recherche Scientifique et Technique. 相似文献
13.
Austenitic Fe-18Cr-20Ni-1.5Mn alloys containing 0, 0.6, and 1.5 wt.% Si were produced both by conventional and rapid solidification processing. The cyclic oxidation resistance of these alloys was studied at 900°C in a SO
2-O
2
gas mixture to elucidate the role of alloy microstructure and Si content on oxidation properties in bioxidant atmospheres. All the large-grained, conventionally processed alloys exhibited breakaway oxidation during cyclic oxidation due to their poor rehealing characteristics. The rapidly solidified, fine-grained alloys that contained less than 1.5 wt.% Si exhibited very protective oxidation behavior. There was considerable evidence of sulfur penetration through the protective chromia scale. The rapidly solidified alloys that contained 1.5 wt.% Si underwent repeated scale spallation that led to breakaway oxidation behavior. The scale spallation was attributed to the formation of an extensive silica sublayer in the presence of sulfur in the atmosphere. 相似文献
14.
The oxidation of Fe was investigated at 500–700°C in the presence of O2 with 0–1000 ppm SO2. The exposures were carried out in a thermobalance and lasted for 24 h. The oxidized samples were investigated by grazing-angle XRD, SEM/EDX, GDOES and XPS. The rate of oxidation of pure iron is slowed down by traces of O2 in O2 below 600°C while SO2 has no effect on oxidation rate at higher temperatures. Exposure to SO2<600°C resulted in the formation of small amounts of sulfate at the gas/oxide interface. In addition, sulfur, probably sulfide, accumulated at the metal/oxide interface. The influence of SO2 on oxidation rate is attributed to surface sulfate. The sulfur distribution in the scale is rationalized in terms of the thermodynamic stability of compounds in the Fe–O–S system. Exposure to SO2 caused the formation of hematite whiskers. 相似文献
15.
P. J. Ficalora 《Oxidation of Metals》1982,18(1-2):19-26
A differential thermal, thermogravimetric analysis (DTA-TGA) study shows that both nickel and cobalt react with their respective sulfates to form oxide (NiO or CoO) and sulfide (Ni3S2 or Co9S8 and Co4S3). In the case of the Ni+NiSO4 reaction, it is clear that the reaction occurs at T < 530°C and may be a solid-state process. A similar process is found for the cobalt+CoSO4 reaction. These reactions are briefly discussed with respect to studies of the oxidation-sulfidation of Co or Ni under conditions which produce the sulfate as a stable phase. It is proposed that the rate-determining step in these studies is the formation of the sulfate. 相似文献
16.
The oxidation behavior in air of pure vanadium, V-30Al, V-30Al-10Cr, and V-30Al-10Ti (weight percent) was investigated over the temperature range of 700–1000° C. The oxidation of pure vanadium was characterized by linear kinetics due to the formation of liquid V2O5 which dripped from the sample. The oxidation behavior of the alloys was characterized by linear and parabolic kinetics which combined to give an overall time dependence of 0.6–0.8. An empirical relationship of the form: W/A=Bt + Ct1/2 + D was found to fit the data well, with the linear contribution suspected to be from V2O5 formation for V-30Al and V-30Al-10Cr, and a semi-liquid mixture of V2O5 and Al2O3 for V-30Al-10Ti. The parabolic term is presumed related to the formation of a solid mixture of V2O5 and Al2O3 for V-30Al and V-30Al-10Cr, and TiO2 for V-30Al-10TiThe addition of aluminum was found to reduce the oxidation rate of vanadium, but not to the extent predicted by the theory of competing oxide phases proposed by Wang, Gleeson, and Douglass. This was attributed to the formation of a liquid-oxide phase in the initial stages of exposure from which the alloys could not recover. Ternary additions of chromium and titanium were found to decrease the oxidation rate further, with chromium being the most effective. The oxide scales of the alloys were found to be highly porous at 900° C and 1000° C, due to the high vapor pressure of V2O5 above 800° C. 相似文献
17.
Abstract The A2BX4 family of K2SO4-related structures have long been of interest to crystal chemists, largely due to the numerous different polymorphs and complicated sequences of phase transitions they sometimes exhibit as a function of temperature. For most such A2BX4 compounds, there are essentially only two distinct structure types or parent structures — a high-temperature, ?hexagonal‘ form isomorphous to α-K2SO4 and a lower-temperature, orthorhombic form isomorphous to β-K2SO4. In addition to these two prototype structures, however, there often exist weakly distorted, or modulated, variants. In the case of the Ba2-xCaxSiO4 system, five such modulated variants have been found via an electron diffraction study and characterized. The characteristic satellite extinction conditions associated with the weak satellite reflections have been used to determine displacement eigenvectors and atomic displacement patterns associated with each of the observed modulation wave-vectors. The widespread occurrence of modulated phases within the A2BX4 family of K2SO4-related structures suggests an almost chronic instability to displacive modulation. 相似文献
18.
The high-temperature corrosion behavior of Fe-30.1Mn-9.7Al-0.77C alloy initially coated with 2 mg/cm2 NaCl/Na2SO4 (100/0, 75/25, 50/50, 25/75 and 0/100 wt.%) deposits has been studied at 750°C in air. The result shows that weight-gain kinetics in simple oxidation reveals a steady-state parabolic rate law after 3 hr, while the kinetics with salt deposits all display multi-stage growth rates. The corrosion morphology of the alloy with 100% Na2SO4 coating is similar to that of simple oxidation. NaCl acts as the predominant corrosion species for Fe-Mn-Al-C alloy, inhibiting the formation of a protective oxide scale. For the alloy coated with over 50% NaCl in salts, NaCl induces selective oxidation of manganese and results in the formation of secondary ferrite in the alloy substrate as well as void-layers with different densities of voids layer by layer in the secondary-ferrite zone. 相似文献
19.
《腐蚀工程科学与技术》2013,48(5):395-398
AbstractThe 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. 相似文献
20.
The corrosion behavior of pure Nb and three Nb Al alloys containing 12.5, 25, and 75 at.% Al was studied over the temperature range of 800–1000°C in a H2/H2S/H2O gas mixture. Except for the Nb-12.5Al alloy consisting of a two phase structure of -Nb and Nb3Al, other alloys studied were single phase. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but fluctuated with increasing Al content. The Nb-75Al alloy exhibited the best corrosion resistance among all alloys studied, whose corrosion rates are 1.6–2.2 orders of magnitude lower than those of pure-Nb (depending on temperature). An exclusive NbO2 layer was formed on pure Nb, while heterophasic scales were observed on Nb-Al alloys whose compositions and amounts strongly depended on Al content and temperature. The scales formed on Nb-12.5Al consisted of mostly NbO2 and minor amounts of Nb2O5, NbS2, and -Al2O3, while the scales formed on Nb-25Al consisted of mostly Nb2O5 and some -Al2O3. The scales formed on Nb-75Al consisted of mostly -Al2O3 and Nb3S4 atT 900°C, and mostly -Al2O3 , Nb3S4 and some AlNbO4 at 1000°C. The formation of -Al2O3 and Nb3S4 resulted in a significant reduction of the corrosion rates. 相似文献