The effect of K2SO4 additive in Na2SO4 deposits on low temperature hot corrosion of iron-aluminum alloys

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.     

Effect of Na2SO4 and Water Vapor on the Corrosion of Ti3SiC2

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.     

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.     

Modulated Phases in the Ba2SiO4-Ca2SiO4 System of A2BX4, K2SO4-related Structures

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 Ba_2-xCa_xSiO₄ 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.     

The identification of the sulfide NaCrS2 in nickel-base superalloys during corrosion in sulfate melts

The sulfide NaCrS₂ has been identified in the internal corrosion zone of several nickel-base superalloys under basic fluxing conditions at very negative potentials in a 90% Na₂SO₄-10% K₂SO₄ melt at 1173 K. It can also be formed in the presence of carbon-contaminated sulfate. NaCrS₂ can dissolve some Ti, Al, Ni, and Co; other elements, e.g., K, Mo, W, Nb, Ta, and Zr, could not be detected.     

Mechanism of Chromium Oxidation in SO2 at 3 × 104 Pa and 105 Pa

The kinetics, phase composition, and morphology of scales growing on chromium in SO₂ atmospheres were studied over the temperature range 1073–1273 K and SO₂pressures of 3×10⁴ Pa and 10⁵ Pa. It was found that the scales consist mainly of Cr₂O₃, with only small amounts of sulfur (probably CrS) detected next to the metallic substrate. Oxidation proceeds according to the linear rate law at 10⁵ Pa SO₂ whereas at 1173 and 1273 K at 3×10⁴ Pa SO₂ the parabolic rate law is followed. The transport phenomena were studied by means of radiotracer techniques as well as marker techniques. The oxide–sulfide scales grew mainly by outward diffusion of metal; however, inward transport of S₂ or SO₂ molecules was also observed. The mechanisms of sulfide and oxide formation are discussed on the basis of the experimental results.     

The Inhibitive Effect of Traces of SO2 on the Oxidation of Iron

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.     

Na2SO4- and NaCl-Induced hot corrosion of six nickel-base superalloys

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.     

Some aspects of the mechanism of high-temperature oxidation of nickel in SO2

A number of investigations on the mechanism of reaction of nickel with SO₂ has been summarized. The calculation results of the equilibrium gas composition in homogeneous SO₂+O₂ mixtures are described over wide ranges of temperatures (500–1100°C) and initial gas compositions. The Ni–O–S phase diagram at 540°C has been compared with data on the stability of interaction products under conditions close to equilibrium. The catalytic activity of NiO has been verified to accelerate the attainment of thermodynamic equilibrium in the SO₂–O₂–SO₃ system. The most effective catalytic activity of NiO occurred at 650–800°C. A monolayer (6 Å) of NiSO₄ was detected on the scale surface by ESCA. This surface phase is assumed to be formed either as an activated complex on the NiO catalyst or as the locally stable NiSO₄ phase. Both assumptions lead to a possible recognition of the sulfate intermediate mechanism.     

Accelerated oxidation of iron induced by Na2SO4 deposits in oxygen at 750°C—A new type low-temperature hot corrosion

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.     

The Hot Corrosion of Fe-Mn-Al–C Alloy with NaCl/Na2SO4 Coating Mixtures at 750°C

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.     

The reaction of preoxidized nickel in SO2 at high temperatures

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.     

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.     

Sulfide-forming features during oxidation of predeformed nickel in SO2

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.     

Improving the Na2SO4-induced corrosion resistance of Ti3AlC2 by pre-oxidation in air

Ti₃AlC₂ suffers severe Na₂SO₄-induced corrosion attacks at temperatures higher than 800 °C in air. A convenient and efficient pre-oxidation method is proposed to enhance the corrosion resistance of Ti₃AlC₂. The corrosion weight-changes of the pre-oxidized samples were decreased by about four orders of magnitude compared with those of the untreated specimens. The mechanism on improvement of corrosion resistance was investigated by means of thermogravimetric analysis, X-ray diffraction and scanning electron microscopy/energy-dispersive spectroscopy. A continuous and adherent α-Al₂O₃ scale was prepared by high-temperature pre-oxidation treatment in air. The preformed dense Al₂O₃ scale has good compatibility with the Ti₃AlC₂ substrate, and consequently, can act as an efficient barrier against corrosion. Long-time corrosion tests demonstrate that the Al₂O₃ scale conserves after corrosion attack and is capable of long-term stability.     

Transport of reactants during scale formation on copper in SO2+O2 atmosphere at 973 and 1073 K

The reaction between copper and a mixture of SO₂+O₂ was studied at 973 and 1073 K. The experimental methods included optical microscopy, X-ray diffraction, scanning electron microscopy (SEM), energy-dispersive X-ray analysis, and radiotracer. It is concluded that the inward transport of oxidants through the scale as well as oxygen and sulfur dioxide liberation in the reactions taking place at phase boundaries suggest that secondary processes occur inside the oxide-sulphate layer. Therefore, Cu₂O and CuOCuSO₄ appear inside this layer. In the metal-consumption zone, a Cu₂O layer forms, which contains small amounts of a sulfide phase near the metal-scale interface.     

Microstructure and desorption properties study of catalyzed NaAlH4

NaAlH₄ catalyzed by Ce(SO₄)₂ and LaCl₃ have been studied by PCT (Pressure-Content-Temperature) experiment and SEM (Scanning Electron Microscope) test method. The results show that doping with Ce(SO₄)₂ and LaCl₃ increases markedly the desorption amount of NaAlH₄. In the first desorption stage, NaAlH₄ doped with LaCl₃ display larger amount of hydrogen release than NaAlH₄ doped with Ce(SO₄)₂, while, the desorption rate of the latter is obviously faster than the former. SEM analysis shows that heating could make NaAlH₄ form a kind of porous structure. The further study indicates that different dopants have different effects on the microstructure of NaAlH₄.     

Ce(SO4)2浓度对电喷镀Ni-W-Ce合金镀层性能影响

为了研究Ce(SO4)2浓度与合金镀层表面性能的关系,采用喷射电沉积法制备了一系列Ni-W-Ce合金镀层工件。用扫描电镜(SEM)观察了镀层的表面结构,并用能谱仪(EDS)检测镀层中的元素组成。XRD分析表明,镀层存在晶格畸变。LEXT4100激光共焦显微镜观察磨损痕迹,发现磨损机理发生了变化。结果表明,添加Ce(SO4)2改善了涂层的表面微观形貌,当浓度为0.5g/L时,涂层的表面质量最佳。同时,显微硬度、耐磨性和耐腐蚀性随浓度的增加呈现先好后坏的规律。当Ce(SO4)2浓度为0.5g/L时,显微硬度达到峰值519.69HV0.1。此时,镀层耐磨性最好,其耐磨性表征参数均取得最小值。且镀层的耐蚀性也最好,腐蚀电位为-0.5537V,电弧电抗半径最小。     

Studies of the mechanism of reaction of nickel with SO2

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.