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.     

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.     

Effects of SO2 and SO3 on the Na2SO4 induced corrosion of nickel

The effects of SO₂ and SO₃ in the environment on the hot corrosion behavior of Ni in the temperature range 750–950°C has been studied. Below the melting point of Na₂SO₄ (884°C), rapid corrosion takes place by formation of a Na₂SO₄-NiSO₄ melt, which can penetrate the porous oxide scale and give rise to sulfide information by coming in contact with the metal. The distribution of the sulfides depends on the SO₂ level in the ambient gas. Continued corrosion occurs by a sulfidation-oxidation mechanism. At temperatures above the melting point of Na₂SO₄, accelerated degradation occurs via dissolution of the surface scale, followed by reprecipitation of the oxide in a nonprotective form.Deceased     

Initial atmospheric corrosion of zinc in presence of Na2SO4 and （NH4）2S04

Initial atmospheric corrosion of zinc in the presence of Na₂SO₄ and (NH₄)₂SO₄ was investigated via quartz crystal microbalance(QCM) in laboratory at relative humidity(RH) of 80% and 25 °C. The results show that both Na₂SO₄ and (NH₄)₂SO₄ can accelerate the initial atmospheric corrosion of zinc. The combined effect of Na₂SO₄ and (NH₄)₂SO₄ on the corrosion of zinc is greater than that caused by (NH₄)₂SO₄ and less than that caused by Na₂SO₄. Fourier transform infrared spectroscopy(FTIR), X-ray diffractometry(XRD) and scanning electron microscopy(SEM) were used to characterize the corrosion products of zinc. (NH₄)₂Zn(SO₄)₂, Zn₄SO₄(OH)₆·5H₂O and ZnO present on zinc surface in the presence of (NH₄)₂SO₄ while Zn₄SO₄(OH)₆·5H₂O and ZnO are the dominant corrosion products on Na₂SO₄-treated zinc surface. Probable mechanisms are presented to explain the experimental results.     

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.     

Thermodynamic Assessments of the Al2O3-Al4C3-AlN and Al4C3-AlN-SiC Systems

A thermodynamic dataset for the Al₂O₃-Al₄C₃-AlN system was reassessed and that of the Al₄C₃-AlN-SiC system was developed in present study for the first time based on available literature data using the CALPHAD approach. In the Al₂O₃-Al₄C₃-AlN system and its subsystems the liquid was described as single phase using the partially ionic liquid model $ ({\text{Al}}^{3 + } )_{P} ({\text{Va,AlC}}_{3/4} ,{\text{AlO}}_{3/2} , {\text{AlN}},{\text{O}}^{2 - } ,{\text{N,C}})_{Q} $ , which covers compositions from the metallic liquid to the oxide, carbide and nitride liquids. The compound energy formalism was used for modeling of the solid phases in both studied systems. Ternary phases ?? and ?? of the Al₄C₃-AlN-SiC system were treated as stoichiometric compounds. A four sublattice model was proposed in order to describe the ??-solid solution forming between the isostructural Al₅C₃N and Al₄SiC₄ binary phases. Using the derived datasets the isothermal sections at 1600, 2273 and 2373?K for the Al₂O₃-Al₄C₃-AlN system and at 2133?K for the Al₄C₃-AlN-SiC system were constructed. The calculated phase diagrams of both ternary systems were compared with the available experimental data.     

The first-class reciprocal quaternary system H3PO4-K2SO4-K3PO4-H2SO4-H2O application to fertilizer fabrication problems

The first-class reciprocal quaternary system H₃PO₄-K₂SO₄-K_3PO₄-H₂SO₄-H₂O has been carefully investigated at 25 and 75°C. Representations have been given using the Jänecke coordinates. Using the established diagrams as a base, a procedure is proposed for preparing specific fertilizers containing potassium and phosphate ions by reacting phosphate rock with aqueous solutions of KHSO₄, addition of calculated amounts of water to the reaction mixture, elimination of an insoluble products, and programmed water evaporation.     

Critical thermodynamic evaluation and optimization of the MgO-Al2O3, CaO-MgO-Al2O3, and MgO-Al2O3-SiO2 Systems

A complete literature review, critical evaluation, and thermodynamic modeling of the phase diagrams and thermodynamic properties of all oxide phases in the MgO-Al₂O₃, CaO-MgO-Al₂O₃, and MgO-Al₂O₃-SiO₂ systems at 1 bar total pressure are presented. Optimized model equations for the thermodynamic properties of all phases are obtained that reproduce all available thermodynamic and phase equilibrium data within experimental error limits from 25 °C to above the liquidus temperatures at all compositions. The database of the model parameters can be used along with software for Gibbs energy minimization to calculate all thermodynamic properties and any type of phase diagram section. The modified quasichemical model was used for the liquid slag phase and sublattice models, based upon the compound energy formalism, were used for the spinel, pyroxene, and monoxide solid solutions. The use of physically reasonable models means that the models can be used to predict thermodynamic properties and phase equilibria in composition and temperature regions where data are not available.     

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.     

Corrosion behavior of T24, T92, VM12, and AISI 304 steels exposed to KCl–NaCl–K2SO4–Na2SO4 salt mixtures

The corrosion mechanisms of T24, T92, VM12, and AISI 304 steels are studied under the influence of NaCl–KCl, NaCl–Na₂SO₄, and KCl–K₂SO₄ salt mixtures in a dry air atmosphere at 650°C for 15 days. NaCl–KCl was the most aggressive deposit and AISI 304 stainless steel exhibited the highest corrosion resistance. There was no relation between the Cr content of the ferritic steels and their corrosion resistance in NaCl–KCl. In contrast, the resistance of high-Cr steels was better when exposed to NaCl–Na₂SO₄ and KCl–K₂SO₄. The high-Cr and the low-Cr steels were more susceptible to NaCl–Na₂SO₄ and to KCl–K₂SO₄, respectively.     

A study of the Ag(s)/Ag2SO4(l) high temperature reference electrode

Ag(s)/Ag₂SO₄(l) reference electrodes for use in hot corrosion research were fabricated and their properties were studied at 927°C (1200°K). The McDanel^TM MV 30 mullite tube was used as sheath of the reference electrode and concentrations of the tested electrolytes were varied from 0.1 to 10 m/o Ag₂SO₄ (l) + Na₂SO₄ (l). 10 m/o Ag₂SO₄ was always used as electrolyte in the reference electrode. The working electrodes with less than 2 m/o Ag₂SO₄ showed marked potential drift as the Ag₂SO₄ concentration was lowered, showing poor potential stability. However, the initial values of cell voltages obeyed Nernstian behavior for all range of Ag₂SO₄ concentrations in the working electrodes, which showed that Ag(s)/Ag₂SO₄(l) electrodes behaved reversibly. Galvanostatic polarization was performed on a pair of 10 m/o Ag₂SO₄ reference electrodes to test the reversibility. The polarizability was 3.5 mV/10 μA with no hysteresis loop on the polarization curve, showing that the reversibility was satisfactory. The potential of gold electrode in molten Na₂SO₄ was measured under pure oxygen atmosphere with 10 m/o Ag₂SO₄ electrode as a reference potential. The results showed that the potential of gold electrode could be explained by the 2 Na + SO₃ + 1/2 O₂ → Na₂SO₄ cell reaction.     

Reaction mechanism of roasting Zn2SiO4 using NaOH

The reaction kinetics of roasting zinc silicate using NaOH was investigated. The orthogonal test was employed to optimize the reaction conditions and the optimized reaction conditions were as follows: molar ratio of NaOH to Zn₂SiO₄ of 16:1, reaction temperature of 550 °C, and reaction time of 2.5 h. In order to ascertain the phases transformation and reaction processes of zinc oxide and silica, the XRD phase analysis was used to analyze the phases of these specimens roasted at different temperatures. The final phases of the specimen roasted at 600 °C were Na₂ZnO₂, Na₄SiO₄, Na₂ZnSiO₄ and NaOH. The reaction kinetic equation of roasting was determined by the shrinking unreacted core model. Aiming to investigate the reaction mechanism, two control models of reaction rate were applied: chemical reaction at the particle surface and diffusion through the product layer. The results indicated that the diffusion through the product layer model described the reaction process well. The apparent activation energy of the roasting was 19.77 kJ/mol.     

Corrosion Products Formed on Copper exposed to humid SO2-containing atmospheres

X-ray diffraction analyses have been performed on samples of electrolytic copper (min. 99,9% Cu) exposed to humid atmoshperes at SO₂-supplies of 10 and 100μg SO₂ per cm² surface area per hour (10 and 100 ppm SO₂. respectively). During the SO₂ -exposures copper (II) sulphate (CuSO₄ · 5 H₂O) were the only crystalline phases formed in detectable amounts. Interruption of the SO₂- supply resulted in the formation of copper (I) oxide and antlerite (CuSO₄) · 2Cu (OH)₂. During prolonged exposure brochanite (CuSO₄ · 3Cu(OH)₂) and langite (CuSO₄· 3Cu(OH)₂) and langite (CuSO₄ · Cu(OH)₂ · 2H₂O) were also formed i. E. the Cu:S ratio of the basic copper sulphates increased with time. The formation of antlerite was preceeded by formation of an unidentified intermediate compound, probably a basic copper sulphate with a Cu:S ratio of less than three, and a simultaneous transformation of the copper (II) sulphate and copper (I, II) sulphite formed during the SO₂-exposure.     

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.     

Assessment of the Te-Tl (tellurium-thallium) system

The optimized thermodynamic data for the Te- TI binary system have been obtained by the computer operated least squares method from measured data. The Gibbs energy of the liquid phase was modeled as a two- sublattice model for ionic melt after Hillert.³¹ The intermediate compounds, Te₃Tl{2}and TeTl, were treated as stoichiometric phases, and the nonstoichiometric γ phase was expressed as a sublattice model. A strong tendency for chemical short- range order in the liquid state at the composition close to TeTh was confirmed by calculated results, but the existence of the TeTh phase was not justified. The experimental thermodynamic and phase diagram data were closely reproduced by the optimized thermodynamic data. Parameters describing the Gibbs energies of all the phases in this calculation and the calculated phase diagram and thermodynamic functions are presented and compared with experimental information.