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.     

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.     

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.     

Acidic and basic fluxing of Ni-base superalloys in a 90Na2SO4-10K2SO4 melt at 1173 K

The influence of the electrode potential on the corrosion behavior of a series of Ni-base superalloys has been investigated in a (mole %) 90Na₂SO₄-10K₂SO₄ melt at 1173 K. Acidic fluxing occurs at positive potentials and basic fluxing at negative potentials. A protective scale is formed in an intermediate (neutral) potential range on high chromium-containing alloys such as IN-738LC, IN-939, IN-597, and IN-657. The breakthrough potentials for acidic and basic fluxing depend on the composition of the alloy. Alloys with low chromium contents such as IN-100 and IN-713LC do not form stable protective scales at any potential. Numerous sulfide phases have been identified in the scale and subscale, depending on potential, severity of attack, and material composition. NaCrS₂ only forms under basic fluxing conditions. Its presence can therefore be considered as an indication that basic fluxing conditions have existed.     

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.     

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.     

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.     

Ni/Cu定向结构涂层在不同浓度H2SO4中的耐蚀性能

为了研究Cu元素对Ni基合金定向结构涂层耐腐蚀性能的影响,向Ni60合金粉末中添加了5%Cu(质量分数,下同),制备了定向结构Ni60/Cu复合涂层。采用电化学试验和浸泡试验,评估了涂层在不同浓度H₂SO₄溶液中的电化学腐蚀特性和浸泡腐蚀性能,探讨了涂层在不同浓度H₂SO₄溶液中的腐蚀行为。结果表明,涂层在不同浓度H₂SO₄溶液中的腐蚀均表现为活化-钝化-过钝化的过程,电化学阻抗谱在整个时间常数内具有典型的容抗特征,H₂SO₄溶液浓度从5%增至80%时,电荷转移电阻先减小后增大,涂层的耐腐蚀性呈现先降低后升高的趋势。随着H₂SO₄溶液浓度的增加,涂层表面的腐蚀程度先加剧后逐渐减缓,且在H₂SO₄溶液浓度为40%时,腐蚀电位移至最负,腐蚀电流密度增至最大。但在H₂SO₄溶液浓度达到80...     

Corrosion products of tin in humid air containing sulfur dioxide and nitrogen dioxide at room temperature

To obtain a fundamental understanding of the corrosion behavior of tin in corrosive gas environments, in situ infrared reflection absorption spectroscopy measurements were carried out on tin in humid air containing SO₂ and NO₂ at room temperature. A series of time-resolved in situ IR spectra in air of 90% relative humidity (RH) containing 10-22 ppm SO₂ suggested that the oxide films on tin specimens had a protective effect and that no significant corrosion occurred. The corrosion products in air of 80-90% RH containing 10-22 ppm NO₂ were SnO₂, SnO, nitrate and hyponitrite. The synergistic effect of SO₂ and NO₂ on corrosion of tin was not observed in humid air (RH of 90%) containing 0.84 ppm SO₂ and 1.8 ppm NO₂.     

Transport of sulfur in the scale growing on nickel in sulfur dioxide atmosphere at 873 K

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 [D_G873=3×10^–7cm²/s]. The liberation of sulfur inside the scale was caused by the reaction of Ni₃S₂ with oxygen or with sulfur dioxide.     

Evaluation of cyclic hot corrosion behaviour of detonation gun sprayed Cr3C2-25%NiCr coatings on nickel- and iron-based superalloys

In the present investigation, Cr₃C₂-NiCr cermet coatings were deposited on two Ni-based superalloys, namely superni 75, superni 718 and one Fe-based superalloy superfer 800H by detonation-gun thermal spray process. The cyclic hot-corrosion studies were conducted on uncoated as well as D-gun coated superalloys in the presence of mixture of 75 wt.% Na₂SO₄ + 25 wt.% K₂SO₄ film at 900 °C for 100 cycles. Thermogravimetric technique was used to establish the kinetics of hot corrosion of uncoated and coated superalloys. X-ray diffraction, FE-SEM/EDAX and X-ray mapping techniques were used to analyze the corrosion products for rendering an insight into the corrosion mechanisms. It was observed that Cr₃C₂-NiCr-coated superalloys showed better hot-corrosion resistance than the uncoated superalloys in the presence of 75 wt.% Na₂SO₄ + 25 wt.% K₂SO₄ film as a result of the formation of continuous and protective oxides of chromium, nickel and their spinel, as evident from the XRD analysis.     

Morphological Development of Subscale Formation in Fe–Cr–(Ni) Alloys with Chloride and Sulfates Coating

Three types of stainless steel (430, 304, and 310) with a coating of NaCl, NaCl/AlCl₃, or NaCl/Al₂(SO₄)₃ are exposed at 750 and 850°C. Results show that NaCl has a major effect on corrosion and sulfur plays an important role in intergranular corrosion. After high-temperature exposure with a 100% NaCl coating, the morphologies of alloys 304 and 310 show typical uniform subscale attack the depths of attack increasing with temperature, while alloy 430 showed a planar attack. Alloy 310 has the highest chromium content and has the least metal loss. After high-temperature exposure with a NaCl/AlCl₃ coating, the corrosion morphologies and depths of attack are similar to those associated with an NaCl coating, but only voids are larger in the subscale. When coated with NaCl/Al₂(SO₄)₃, the alloys are attacked simultaneously by sulfur and chlorine at 750°C, resulting in a typical sulfur-attack intergranular corrosion. However, as the temperature increases to 850°C, the corrosion morphology changes to a uniform subscale attack.     

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.     

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.     

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.     

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.     

Corrosion behavior of select MAX phases in NaOH, HCl and H2SO4

In this paper we report on the electrochemical corrosion of select MAX phases, namely Ti₂AlC, (Ti,Nb)₂AlC, V₂AlC, V₂GeC, Cr₂AlC, Ti₂AlN, Ti₄AlN₃, Ti₃SiC₂ and Ti₃GeC₂ in 1 M NaOH, 1 M HCl and 1 M H₂SO₄ solutions. Polarization characteristics recorded in 1 M NaOH show that V₂AlC, V₂GeC and Cr₂AlC undergo active dissolution at potentials more positive than the corrosion potential, while Ti₂AlC, (Ti,Nb)₂AlC, Ti₃SiC₂ and Ti₃GeC₂ passivate. In the 1 M HCl solutions, Ti₂AlC, V₂AlC and V₂GeC actively dissolve; Ti₃SiC₂ and Ti₃GeC₂ passivate. Depending on potential, (Ti,Nb)₂AlC and Cr₂AlC showed trans-passive behavior. In 1 M H₂SO₄ solutions, Ti₂AlC, (Ti,Nb)₂AlC, Ti₃SiC₂ and Ti₃GeC₂ passivate, V₂AlC and V₂GeC show active dissolution, while Cr₂AlC exhibits trans-passive behavior. Ti₂AlN and Ti₄AlN₃ were passive in all solutions except in 1 M HCl, where Ti₂AlN showed trans-passive behavior. Given that the corrosion behavior of (Ti,Nb)₂AlC is unlike either Ti or Nb, the behavior of the former cannot be predicted from that of the latter.     

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.     

Preparation and characterization of spinel Li4Ti5O12 anode material from industrial titanyl sulfate solution

Spinel Li₄Ti₅O₁₂ anode material is successfully synthesized by a solid-state method using lithium carbonate and titanium precursors which are prepared by the low cost industrial titanyl sulfate solution. The characters of H₂TiO₃ and TiO₂ precursors are determined by TG/DTA and SEM methods. TG-DAT and EDS methods show that H₂TiO₃ can absorb sulphate ions which can be present as impurities. XRD method shows that the impure phases of Li₂SO₄ and rutile TiO₂ appear in Li₄Ti₅O₁₂ synthesized by H₂TiO₃. The formation of Li₂SO₄ is identified in thermodynamics during the process of calcination. Owing to the formation of Li₂SO₄ impurity, the capacity of the Li₄Ti₅O₁₂ synthesized by H₂TiO₃ is low. One effective way that can tackle this problem is to remove the sulphur by calcining H₂TiO₃, after calcinations, the production will have a thermal treatment with Li₂CO₃. The obtained Li₄Ti₅O₁₂ shows better electrochemical performance. The specific capacities can be increased by 20 mAh g⁻¹ at 0.1, 0.5 and 1C rates.