The influence of electrode potential on the corrosion of gas turbine alloys in sulfate melts

The influence of the electrode potential on the corrosion behavior of a series of nickel- and cobalt-base gas turbine alloys has been investigated in a (mole %) 53Na₂SO₄+7CaSO₄+40MgSO₄ melt at 1073 and 1173 K and in a 90Na₂SO₄+10K₂SO₄ melt at 1173 K. Only acidic fluxing is observed in the (Na ₂,Ca, Mg)SO₄ melt at positive potentials while a protective scale rich in MgO is formed on all alloys at negative potentials. This scale prevents basic fluxing because MgO is insoluble in neutral and basic melts. The breakthrough potential for acidic fluxing is a function of the material composition. Increasing chromium content of the alloys extends the potential range of protective film formation. Acidic and basic fluxing are observed in the (Na, K)₂SO₄ melt. Acidic fluxing occurs at positive and basic fluxing at negative potentials. A protective scale is formed in an intermediate (neutral) potential range on the high-chromium alloys IN-597 and IN-738 LC. Here, too, the breakthrough potentials for acidic and basic fluxing are influenced by the composition of the alloys.     

在熔融碱金属硫酸盐中镍基高温合金的腐蚀

研究了在900℃的90Na_2SO_4-10K_2SO_4(克分子％)熔盐中,电极电位对镍基高温合金腐蚀行为的影响。在正电位条件下可发生酸性熔融(Acidic Fluxing),而在较负电位下则发生碱性熔融(Basic Fluxing)腐蚀。在中间(中性)电位区,含铬较高的合金如IN738,IN 939,IN597和IN657等可生成保护性氧化膜。合金成份将影响发生酸性和碱性熔融腐蚀的临界电位值。 含铬低的合金如IN 100和IN 713LC在任何电位下均不能生成稳定的保护性氧化膜。 鉴定了腐蚀产物层中的多种硫化物相,它们的生成与电极电位,腐蚀程度和合金的成份有关。只有在碱性熔融腐蚀条件下才能生成硫化物相NaCrS_2,它的出现可作为发生碱性熔融的标志。     

Role of platinum in the Na2SO4-Induced hot corrosion resistance of aluminum duffusion coatings

Electrochemical corrosion measurements have been carried out with Pt-containing and Pt-free Al-diffusion coatings on IN 738 LC in a 90Na₂SO₄+ 10K₂SO₄ (mol%) melt at 1173 K. Pt improves the resistance to basic fluxing while there are no significant differences between both coating types in their resistance to acidic fluxing. The corrosion resistance of the Pt-containing coating is also higher in the passive potential region where protective scales rich in Al₂O₃ are formed. The reason for the different behavior of both coating types appears to be related to the high corrosion resistance of the Pt-rich surface layer of the coating and an increased Al₂O₃ content in the scale of the Pt-containing type.     

Inhibition of Hot Corrosion by MgSO4 and BaSO4 and its potential dependence

Electrochemical corrosion measurements have been performed with IN 100 in a (mole %) 90 Na₂SO₄? 10 K₂SO₄ melt with different additions of MgSO₄ and BaSO₄ at 1173 K in order to investigate the inhibition effect of these compounds and its potential dependence. Potentiostatically controlled measurements as well as free corrosion potential measurements with the determination of the polarization resistance have been performed. The inhibition effect of MgSO₄ is strongly potential dependent. At negative potentials, where oxygen ions are produced by electrochemical side reactions, MgO is precipitated at the surface of the metal and becomes incorporated into the scale. This kind of MgO-rich scale is resistant to basic fluxing because MgO is not dissolved in basic media. At positive potentials, where acid SO₃ is produced by discharge of SO₄-ions, MgSO₄ yields no or only very little inhibition. BaSO₄ additions alone do not yield inhibition effects. However, additions of BaSO₄ to melts containing MgSO₄ improve the inhibition effect of the latter. The mechanism of this synergistic effect is not completely understood.     

Hot corrosion behaviour of a cobalt-base super-alloy K40S with and without NiCrAlYSi coating

A NiCrAlYSi coating was deposited by arc ion plating on a cobalt-base super-alloy K40S to improve its hot corrosion resistance at 1173 K in air. The K40S suffered from accelerated corrosion and formed non-protective scale with poor adherence when its surface was beneath Na₂SO₄ and Na₂SO₄ containing 25 wt.% NaCl salt deposits. After the K40S was coated with NiCrAlYSi coating, a protective α-Al₂O₃ scale was formed on the coating. Although the NiCrAlYSi coating changed into NiCoCrAlYSi during corrosion processes, it still possessed good corrosion resistance. In addition, the corrosion mechanisms were discussed on a basis of basic fluxing model.     

Reactions of chromium in SO2-containing atmospheres

The morphology and composition of scales formed on unalloyed chromium in atmospheres containing SO₂ at high temperatures have been studied. SEM images show chromium sulfides formed at the metal/scale interface. The results indicate that SO₂ may penetrate the scale as molecules, but the rate of this penetration is low. At 600°C formation of Cr₂(SO₄)₃ is also observed by SEM on the scale surface formed in SO₂+O₂/SO₃ mixtures.     

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.     

A study of the chemical composition of the passive film on a Ti-Mo alloy in HCI and H2SO4

The composition of the passive film on Ti-15Mo alloy, formed in 1 mol/L and 4mol/L HCI and 2 mol/L H₂SO₄ solutions at 70 °C (160 °F) under anodic polarization is investigated by XPS and electrochemical techniques. Anodic polarization potential is found to have an obvious influence on the content of Mo in the film. At lower anodic polarization potentials, the surface of the passive film is enriched in Mo. However, at higher anodic polarization potentials, the surface is diluted in Mo. The anions of the electrolytes influence the composition of the passive film. In HCI solution, chloride ions are incorporated with the passive film during its formation. The passive film consists of a compound containing chloride and oxide ions. While in H₂SO₄ solution, the passive film only consists of titanium-molybdenum oxide. Sulfide ions and other sulfur are not incorporated. Ti-Mo alloys have a better passivity than pure Ti in HCI and H₂SO₄ solutions. This passivity is related to the enrichment of Mo in the surface of the passive film.     

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.     

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.     

Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Deposit-Induced Corrosion of Mo-Containing Alloys

Disk alloys used in advanced gas turbine engines often contain significant amounts of Mo (2 wt% or greater), which is known to cause corrosion under Type I hot corrosion conditions (at temperatures around 900 °C) due to alloy-induced acidic fluxing. The corrosion resistance of several model and commercial Ni-based disk alloys with different amounts of Mo with and without Na₂SO₄ deposit was examined at 700 °C in air and in SO₂-containing atmospheres. When coated with Na₂SO₄ those alloys with 2 wt% or more Mo showed degradation products similar to those observed previously in Mo-containing alloys, which undergo alloy-induced acidic fluxing Type I hot corrosion even though the temperatures used in the present study were in the Type II hot corrosion range. Extensive degradation was observed even after exposure in air. The reason for the observed degradation is the formation of sodium molybdate. Transient molybdenum oxide reacts with the sodium sulfate deposit to form sodium molybdate which is molten at the temperature of study, i.e., 700 °C, and results in a highly acidic melt at the salt alloy interface. This provides a negative solubility gradient for the oxides of the alloying elements, which results in continuous fluxing of otherwise protective oxides.     

The Effects of KCl, K2SO4 and K2CO3 on the High Temperature Corrosion of a 304-Type Austenitic Stainless Steel

The oxidation of 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated at 500 and 600 °C in 5% O₂ + 40% H₂O. Prior to exposure the samples were sprayed with KCl, K₂CO₃ or K₂SO₄, the amount of salt corresponding to 1.35 ??mol K⁺/cm². For reference, salt-free samples were exposed in 5% O₂ + 40% H₂O and in 5% O₂ (N₂ was used as carrier gas). The oxidized samples were analyzed with SEM/EDX, XRD, IC and FIB. KCl and K₂CO₃ strongly accelerate the corrosion of 304L while K₂SO₄ has little influence on the corrosion rate and on the morphology of the corroded surface. KCl and K₂CO₃ react with the chromium-rich oxide on the sample surface, forming K₂CrO₄. The resulting chromium depletion of the protective oxide causes rapid oxidation and the formation of a thick duplex scale consisting of an outer hematite layer and a inner layer made up of FeCrNi spinel-type oxide. The differences in the corrosivity of the three salts are directly connected to their ability to form chromate on the surface and, hence, to the relative stability of the corresponding leaving groups (HCl, CO₂ and SO₃).     

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.     

Surface Chemistry of Oxide Scale on IN-738LC Superalloy: Effect of Long-Term Exposure in Air at 1173 K

The oxidation kinetics of IN-738LC at 1173 K in dry air up to 1500 hr followed parabolic law. Surface morphology and the oxide phases present in the scale were characterized by SEM, XRD, EDS, FIB, and XPS. FIB investigation exhibited a compact and adherent oxide layer. XRD analysis revealed the presence of NiO, NiAl₂O₄, NiCr₂O₄ spinel, and Al₂O₃ on the top scale surface formed at 1173 K in dry air. Extensive XPS analyses revealed the presence of Cr₂O₃, CrO₂, and CrO₃ on the top scale surface formed on IN-738LC after 10 hr of exposure. The presence of TiO₂, Al₂O₃, Cr₂O₃, NiO, and NiAl₂O₄ and NiCr₂O₄ spinels along with the oxides of Ta at the top surface of the scale was observed after 100 hr of oxidation. The TiO₂ content was high on the surface and the entire scale cross section was composed mostly of Cr₂O₃, NiO, TiO₂, and Al₂O₃ after 100 hr of exposure to dry air at 1173 K. The concentration of Al₂O₃ on the surface of the oxide scale was found to increase after 100 hr of exposure and remained constant thereafter. After 300 and 1500 hr of exposure, the surface oxide was mainly Al₂O₃ along with oxides of Ni, Ti, and Cr. The oxide scale cross section consisted mostly of Al₂O₃ along with other oxides such as Cr₂O₃, NiO, and TiO₂. The oxide-scale composition was found to vary significantly with the duration of exposure to dry air at 1173 K.     

Hochtemperaturkorrosion hochlegierter Stähle unter simulierten Müllverbrennungsbedingungen

High temperature corrosion of high alloy steels in simulated waste incineration environments The corrosion of high alloy steels was investigated in discontinuous exposures in N₂-O₂-HCl-SO₂ at 600°C with and without deposits, taken from a waste incinerator. Additions of 500–3000 vppm HCl to N₂-O₂ leads always to accelerated corrosion due to the formation of Cl₂. The Cl₂ diffuses through cracks and pores of the oxide scale and mainly FeCl₂ is formed at the metal/scale interface; by evaporation and outward diffusion through the oxide scale it is oxidized to Fe₂O₃ (“active oxidation”). After addition of SO₂ the corrosion process is retarded by sulfide formation on the metal-chloride. At 600°C the main corrosion process under deposits is “hot corrosion” caused by a sulfate melt. In N₂-O₂ oxide scales are dissolved by basic fluxing, the corrosion products are PbCrO₄ and zinc-rich spinel. In N₂-O₂-SO₂ the oxide scales are dissolved by acidic fluxing as sulfates; under these conditions Cr₂O₃-layers are sufficiently resistent. In N₂-O₂-HCl, (K, Na) Cl is formed in the deposits and therefore catastrophic corrosion occurs by Cl₂. After addition of SO₂ to N₂-O₂-HCl the corrosion rate is decelerated and acidic behaviour of the melts is induced.     

Corrosion of nickel with small alloy additions of Si,Fe, and Mn in SO2+O2/SO3 at 700°C

The corrosion of nickel with alloy additions of Si, Fe, and/or Mn up to 4 wt% has been studied in SO ₂+O₂/SO₃ at 700°C. All alloy additions greatly improve the corrosion resistance of nickel in oxygen-rich atmospheres (O₂ with about 4% SO₂); the best improvements are achieved with Si, Fe+Si, and Fe+Mn+Si additions. High-purity nickel corrodes rapidly under these conditions; the scale then consists of NiO+Ni₃S₂, and the sulfide forms a three-dimensional network along the grain boundaries of the NiO grains and serves as the diffusion path for rapid outward migration of nickel. From studies of the microstructure and distribution of the alloying elements in the protective scales, it is proposed that the alloying additions exert their beneficial effects by accumulating/segregating at the grain boundaries of NiO (e.g., as silicates) and thereby influence the wetting characteristics and disrupt the sulfide network.     

两种渗铝涂层在高温熔融碱金属硫酸盐中的腐蚀

研究了在1173K温度的90Na_2SO_4-10K_2SO_4熔融盐中,单渗铝涂层和铂改性的渗铝涂层LDC-2的腐蚀行为和它们的电位关系。 两种涂层都有一钝化电位区,在此电位区内,涂层表面可能生成保护性氧化膜,但单渗铝涂层的腐蚀速度高于LDC-2涂层。后者的钝化电位区较前者宽300mV。 这两种涂层的酸性熔融腐蚀临界电位值十分接近。但碱性熔融临界电位差达约400mV,LDC-2涂层耐碱性熔融性能远优于单渗铝涂层。 对于铂改进渗铝涂层抗碱性熔融腐性蚀能的机构,也进行了探讨。