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.     

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.     

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.     

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.     

Hot corrosion behaviour of plasma sprayed YSZ/Al2O3 dispersed NiCrAlY coatings on Inconel-718 superalloy

Oxide dispersed NiCrAlY bond coatings have been developed for enhancing thermal life cycles of thermal barrier coatings (TBCs). However, the role of dispersed oxides on high temperature corrosion, in particular hot corrosion, has not been sufficiently studied. Therefore, the present study aims to improve the understanding of the effect of YSZ dispersion on the hot corrosion behaviour of NiCrAlY bond coat. For this, NiCrAlY, NiCrAlY + 25 wt.% YSZ, NiCrAlY + 50 wt.% YSZ and NiCrAlY + 75 wt.% YSZ were deposited onto Inconel-718 using the air plasma spraying (APS) process. Hot corrosion studies were conducted at 800 °C on these coatings after covering them with a 1:1 weight ratio of Na₂SO₄ and V₂O₅ salt film. Hot corrosion kinetics were determined by measuring the weight gain of the specimens at regular intervals for a duration of 51 h. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques were used to determine the nature of phases formed, examine the surface attack and to carry out microanalysis of the hot corroded coatings respectively. The results show that YSZ dispersion causes enhanced hot corrosion of the NiCrAlY coating. Leaching of yttria leads not only to the formation of the YVO₄ phase but also the destabilization of the YSZ by hot corrosion. For the sake of comparison, the hot corrosion behaviour of a NiCrAlY + 25 wt.% Al₂O₃ coating was also examined. The study shows that the alumina dispersed NiCrAlY bond coat offers better hot corrosion resistance than the YSZ dispersed NiCrAlY bond coat, although it is also inferior compared to the plain NiCrAlY bond coat.     

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.     

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     

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.     

Influence of pre-oxidation on the hot corrosion of Ti3SiC2 in the mixture of Na2SO4-NaCl melts

Polycrystalline Ti₃SiC₂ suffered from serious hot corrosion attack in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C. In order to improve the hot corrosion resistance of this material, pre-oxidation treatment was conducted at 1200 °C in air for 2 h. A duplex oxide scale with an outer layer of TiO₂ and an inner layer of a mixture of TiO₂ and SiO₂ was formed during the pre-oxidation. Because the outer oxide layer of the pre-oxidation treated specimens could inhibit hot corrosion process, they exhibited good hot corrosion resistance in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C for 50 h. However, during the hot corrosion the outer layer of TiO₂ would degrade gradually. Once the outer layer damaged, the hot corrosion rate increased sharply, the corrosion behavior was similar to Ti₃SiC₂ corroded under the same conditions. The microstructure and phase compositions of the hot corrosion samples were investigated by SEM/EDS and XRD.     

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.     

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.     

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.     

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.     

Study on hot corrosion behavior of Yb2Zr2O7 ceramic against Na2SO4 + V2O5 molten salts at temperatures of 900–1200 °C in air

Yb₂Zr₂O₇ ceramic powders synthesized by chemical‐coprecipitation and calcination method were pressureless‐sintered at 1700 °C for 10 h in air to fabricate dense bulk materials. Hot corrosion studies were performed on Yb₂Zr₂O₇ against Na₂SO₄ and Na₂SO₄ + V₂O₅ (molar ratio = 1:1) molten salts in a temperature range of 900–1200 °C for 8 h in air, respectively. Chemical reactions were investigated using X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The Yb₂Zr₂O₇ ceramic was severely corroded by Na₂SO₄ + V₂O₅ molten salt, however, no chemical reaction was found between individual Na₂SO₄ and Yb₂Zr₂O₇. Yb₂Zr₂O₇ reacted with Na₂SO₄ + V₂O₅ molten salt to form YbVO₄ and m‐ZrO₂. The thickness of hot corrosion scales formed at different temperatures was investigated to evaluate hot corrosion behavior based on fluxing mechanism. The introduction of vanadium into sulfate led to subsequent formation of NaVO₃, which was acidic enough to dissolve Yb₂Zr₂O₇ by acidic fluxing.     

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...     

The role of NaCl in the hot-corrosion behavior of Nimonic alloy 90

The influence of sodium chloride on the hot-corrosion behavior of Nimonic alloy 90 has been investigated by employing the half-immersion, crucible test. Nimonic 90 samples were hot corroded in the presence of NaCl between 700–900°C. The results showed that the weight-loss plots with both time and temperature were linear indicating the catastrophic nature of attack. An examination of the corroded samples by XRD, XRF, EPMA, SEM, and chemical analysis indicated that as the corrosion time increased, an increase in the depletion of alloying constituents like Cr, Al, Ti, and Co took place with a resultant enrichment of nickel on the alloy surface. The formation of CoCl₂ and Na₂CrO₄ was observed in all the tests. A few experiments were carried out in the presence of Na₂SO₄ and in a 1% NaCl mixture, in order to see the influence of NaCl on Na₂SO₄. The results indicated that Na₂SO₄ is innocuous when compared with NaCl. However, the severe attack was observed in the presence of the 1% NaCl mixture between 700–800°C, i.e., above the eutectic temperature and the m.p. of NaCl (800°C). The corrosion was minimum, when the salt mixture existed in the molten state. All the corroded samples were magnetic in nature. The role of NaCl on the hot-corrosion behavior of Nimonic 90 has been discussed in the light of the above crucible-test investigations.     

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.     

Aluminizing and boroaluminizing treatments of Mar-M247 and their effect on hot corrosion resistance in Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>−NaCl molten salt

The effect of surface modifications of Mar-M247 superalloy on hot corrosion resistance was examined in Na₂SO₄−NaCl molten salt. The Mar-M247 was aluminized and boroaluminized by pack cementation in Ar and underwent a cyclic hot corrosion test in Na₂SO₄−NaCl molten salt. The XRD results showed that a Ni₂Al₃ phase was formed between the aluminized layer and the substrate when the surface modification temperature was below 1273 K. However, a NiAl phase formed when the temperature was above 1273 K. The intensity of the XRD peak in the NiAl phase increased after post heat treatment. Hot corrosion resistance increased for the specimens containing NiAl rather than Ni₂Al₃ phase. The ductile NiAl phase suppressed the potential for crack initiation during thermal cycling. Post heat treatment increased the corrosion resistance of the aluminized layer for Mar-M247, which underwent surface modification at 1273 K and above. In the boroaluminized Mar-M247 specimens, corrosion resistance decreased as a result of the blocking of outward diffusion of Cr by boron and decreased cohesion between the oxide scale and the aluminized layer during thermal cycling.