High-temperature corrosion of NiCrAl alloy in oxygen-chlorine mixtures

In order to decrease the attack of chlorine in O₂? Cl₂? N₂ mixtures to NiCr alloys at high temperatures, the corrosion of a preoxidized Ni 20 Cr 4 Al alloy was investigated in N₂? O₂ mixtures with 5 to 150 torr O₂ and 20 torr Cl₂ at 800°. For the generation of a protective layer of Al₂O₃ both the untreated and Al evaporated alloy sample was preoxidized in a H₂? 20 torr H₂O atmosphere at 900° C. As was found, the catastrophic corrosion by Cl₂ in a N₂? Cl₂ gas mixture with 20 torr Cl₂ could not be prevented by an Al₂O₃ layer. However, in the simultaneous presence of an oxygen content in the corroding gas mixture not lower than 10 torr, the attack of chlorine could be drastically decreased. The presence of 50 torr Cl₂ postulates an oxygen pressure which is technically unrealistic. Since the formed oxide layer spalled off during the cooling period, the Ni 20 Cr 4 Al alloy cannot be used as material for cyclic oxidation. To what extent ThO₂ particles dispersed in the alloy can prevent a spalling effect of the oxid film is at present an open question.     

Influence of the Oxygen Partial Pressure on the High Temperature Corrosion of 38Ni–34Fe–25Cr Alloy in Presence of NaCl Deposit

In biomass gasification processes, some molten salts formed during the process can promite high temperature corrosion. In this study the chromia-forming austenitic alloy Haynes^® HR-120 was oxidized with a deposit of sodium chloride for 96 h at 825 and 900 °C. Two different atmospheres were selected; one with a high oxygen partial pressure (Ar/O₂ 90/10 %vol.) and one, named syngas, with a low oxygen partial pressure (CO/H₂/CO₂ 45/45/10 %vol.). While at 900 °C the behaviour of the alloy in presence of sodium chloride was catastrophic in high oxidizing conditions, the impact of sodium chloride was insignificant in the syngas atmosphere. When exposed to the Ar/O₂ mixture, the catastrophic oxidation was attributed to the setting up of an active oxidation. At 900 °C under the syngas atmosphere, the protective behaviour of the alloy seems linked to the association of a faster evaporation of the salt and a very low oxygen partial pressure. At 825 °C a catastrophic behaviour is observed under the syngas atmosphere as the NaCl evaporation rate is much slower.     

Evaluation of High Temperature Corrosion in Simulated Waste Incinerator Environments

In this study, high temperature reactions of Fe–Cr alloys at 500 and 600 °C were investigated using an atmosphere of N₂–O₂ 8 vol% with 220 vppm HCl, 360 vppm H₂O and 200 vppm SO₂; moreover the following aggressive salts were placed in the inlet: KCl and ZnCl₂. The salts were placed in the inlet to promote corrosion and increase the chemical reaction. These salts were applied to the alloys via discontinuous exposures. The corrosion products were characterized using thermo-gravimetric analysis, scanning electron microscopy and X-ray diffraction.The species identified in the corrosion products were: Cr₂O₃, Cr₂O (Fe_0.6Cr_0.4)₂O₃, K₂CrO₄, (Cr, Fe)₂O₃, Fe–Cr, KCl, ZnCl₂, FeOOH, σ-FeCrMo and Fe₂O₃. The presence of Mo, Al and Si was not significant and there was no evidence of chemical reaction of these elements. The most active elements were the Fe and Cr in the metal base. The Cr presence was beneficial against corrosion; this element decelerated the corrosion process due to the formation of protective oxide scales over the surfaces exposed at 500 °C and even more notable at 600 °C; as it was observed in the thermo-gravimetric analysis increasing mass loss. The steel with the best performance was alloy Fe9Cr3AlSi3Mo, due to the effect of the protective oxides inclusive in presence of the aggressive salts.     

High Temperature Corrosion of Ni-Based Alloys SCA425+ and IN792

This paper reports the high temperature corrosion of two Ni-base superalloys: a newly developed alloy, SCA425+, and the extensively used IN792. The composition of the two materials is quite similar, but SCA425+ contains more Cr and Al (17.1 and 10 at.% compared to 13.9 and 7.4 at.% in IN792). The results from exposures at 900 °C in SO₂ (3,000 ppm) + O₂ (69 vol%) + H₂O (31 vol%) mixed gas for 260 h using 65 h/cycle are compared with those obtained from tests in laboratory air. The microstructure of the formed oxide scales was studied using several techniques, such as XRD, SEM, FIB, EDX, STEM and XPS. It is shown that in IN792 severe internal oxidation takes place after both types of exposures. On the other hand, the newly developed SCA425+ has the tendency to form an alumina layer proving that it has more potential to be used in the aggressive environments. Surprisingly the mass gains for SCA425+ alloy exposed in SO₂-rich environment are lower than in laboratory air. The reason for this behavior is discussed.     

Microstructure and High Temperature Oxidation Property of Fe–Cr–B Based Metal/Ceramic Composite Manufactured by Powder Injection Molding Process

This study investigated the microstructure and high temperature oxidation property of Fe–Cr–B metal/ceramic composite manufactured using powder injection molding process. Observations of initial microstructure showed a unique structure where α-Fe and (Cr, Fe)₂B form a continuous three-dimensional network. High temperature oxidation tests were performed at 900, 1000 and 1100 °C, for 24 h, and the oxidation weight gain according to each temperature condition was 0.13, 0.84 and 6.4 mg/cm², respectively. The oxidation results according to time at 900 and 1000 °C conditions represented parabolic curves, and at 1100 °C condition formed a rectilinear curve. Observation and phase analysis results of the oxides identified Cr₂O₃ and SiO₂ at 900 and 1000 °C. In addition to Cr₂O₃ and SiO₂, CrBO₃ and FeCr₂O₄ formed due to phase decomposition of boride were identified at 1100 °C. Based on the findings above, this study suggested the high temperature oxidation mechanism of Fe–Cr–B metal/ceramic composite manufactured using powder injection molding, and the possibility of its application as a high temperature component material was also discussed.     

High Temperature Oxidation Behavior of Alloy 617 and Haynes 230 in Impurity-Controlled Helium Environments

The high temperature oxidation behavior of alloy 617 and Haynes 230 have been investigated for VHTR intermediate heat exchanger applications. Oxidation tests were carried out for up to 500 h at 900 °C and 1000 °C in impure helium environments containing H₂, H₂O, CO, CO₂, and CH₄. The oxidation kinetics of the alloys followed a parabolic rate law in all cases. In the impure helium environments with very low oxygen, the external oxides of alloy 617 were composed of a Cr₂O₃ layer, TiO₂ ridges on the grain boundaries, and isolated MnCr₂O₄ grains on top of the Cr₂O₃ layer. On the other hand, those of Haynes 230 consisted of a Cr₂O₃ inner layer and a protective MnCr₂O₄ outer layer, which increased the oxidation resistance. The effect of small amounts of CH₄ and H₂ on the oxidation kinetics of the alloys was insignificant. Irregular oxide morphology, such as cellular Cr₂O₃ oxides for alloy 617 and MnCr₂O₄ platelets for Haynes 230, was formed in the impure helium environment at 900 °C. For Haynes 230, along with platelets, whiskers were frequently found at the tip of the MnCr₂O₄ oxide crystals.     

Effect of H2O and CO2 on the Oxidation Behavior and Durability at High Temperature of ODS-FeCrAl

Cyclic oxidation testing was conducted on alloy MA956 and two different batches of alloy PM2000 at 1,100 and 1,200 °C in different atmospheres rich in O₂, H₂O and CO₂. Compared to 1 h cycles in dry O₂, exposure in air + 10 vol.% H₂O resulted in an increase of the oxidation rate and a decrease of the time to breakaway for all alloys at 1,200 °C, and a faster consumption of Al in the MA956 alloy. One hour cyclic testing in 49.25 % CO₂ + 50 % H₂O + 0.75 % O₂ had a smaller effect on the oxidation rate but led to increased formation of voids in alloy MA956, which had an impact on the alloy creep resistance. At 1,100 °C, exposure in 50 % CO₂ + 50 % H₂O resulted in significant oxide spallation compared with oxidation in air, but this was not the case when 0.75 % O₂ was added to the CO₂/H₂O mixture as a buffer. The control of impurity levels drastically improved the oxidation resistance of PM2000.     

KCl-Induced Corrosion of a 304-type Austenitic Stainless Steel in O2 and in O2 + H2O Environment: The Influence of Temperature

The oxidation of the 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated in the temperature range 400–600 °C in 5% O₂ and 5% O₂ + 40% H₂O. Exposure time was up to 1 week. Prior to exposure, the polished samples were coated with 0.1 mg/cm² KCl. Uncoated samples were also exposed and used as references. The oxidized samples were analyzed by gravimetry and by ESEM/EDX, XRD, IC and AES. The results show that KCl is strongly corrosive. Corrosion is initiated by the reaction of KCl with the chromia-containing oxide that normally forms a protective layer on the alloy. This reaction produces potassium chromate particles, leaving a chromium-depleted oxide on the alloy surface. At 500 and 600 °C this results in rapid oxidation, resulting in the formation of a thick scale consisting of a mixture of hematite, spinel oxide ((Fe,Cr,Ni)₃O₄) and K₂CrO₄. The thick scale is poorly protective and permeable to e.g. chloride ions. The KCl-induced corrosion of alloy 304L in dry O₂ and in an O₂ + H₂O mixture increases strongly with temperature in the range 400–600 °C. The strong temperature dependence is explained partly by the temperature dependence of the chromate-formation reaction and partly by the ability of the chromium-depleted oxide to protect the alloy at low temperature. At 400 °C, the oxide was still protective after 168 h.     

Water Vapour Effect on Ferritic 4509 Steel Oxidation Between 800 and 1000?��C

The 4509 alloy (Fe?C18Cr?CNb?CTi) was oxidised in dry and wet air in the 800?C1000 °C temperature range. Results showed that the formation of a chromia layer acts as a good diffusion barrier under isothermal conditions at 800 and 900 °C, under 7.5 vol.% water vapour and dry air. Nevertheless, a breakaway is generally observed at 1000 °C, under wet air 7.5 vol.% H₂O. It is proposed that the oxidant H⁺/OH^? species react at the internal interface with iron in the chromium-depleted alloy zone. Wüstite reacts with Cr₂O₃ to form FeCr₂O₄. Outward iron diffusion leads to Fe₃O₄ and Fe₂O₃ formation. The chromia scale was consumed by reaction with wüstite, but chromia also internally forms owing to a chromium oxidation process with the inner chromium-rich alloy area.     

Oxidation behavior of atomized Fe40Al intermetallics doped with boron and reinforced with alumina fibers

Isothermal oxidation resistance of Fe40 (at.%) Al-based atomized and deposited intermetallic alloys has been evaluated. The alloys included Fe40Al, Fe40Al + 0.1B, and Fe40Al + 0.1B + 10Al₂O₃ at 800, 900, 1000, and 1100 °C. The tests lasted approximately 100 h, although in most cases there was scale spalling. At 800 and 900 °C, the Fe40Al + 0.1B alloy had the lowest weight gain, whereas the Fe40Al alloy had the highest weight gain at 800 °C (0.10 mg/cm²) and the Fe40Al + 0.1B + 10Al₂O₃ alloy was the least oxidation resistant at 900 °C with 0.20 mg/cm². At 1000 °C, the Fe40Al + 0.1B alloy showed the highest weight gain with 0.12 mg/cm² and the Fe40Al alloy the lowest. At 1100 °C, again, as at 900 °C, the Fe40Al alloy was the least resistant, whereas the Fe40Al + 0.1B alloy performed the best, but the three alloys exhibited a paralinear bahavior on the weight-gain curves, indicating the spalling, breaking down, and rehealing of the oxides. This spalling was related to voids formed at the metal-oxide interface.     

Oxidation of Minor Elements from an Iron–Nickel–Chromium–Cobalt–Phosphorus Alloy in 17.3% CO2–H2 Gas Mixtures at 700–1000 °C

Fe–Ni–Cr–Co–P alloys were exposed to 17.3% CO₂–H₂ gas mixtures to investigate the oxidation of minor elements in metallic alloys in the early solar system. Reaction temperatures varied between 700 and 1000 °C. Gas-phase equilibrium was attained at 800, 900, and 1000 °C, yielding H₂–H₂O–CO–CO₂ gas mixtures. Experiments at 700 and 750 °C did not achieve gas-phase equilibrium and were performed in H₂–CO₂ gas mixtures. Reaction timescales varied from 1 to 742 h. The experimental samples were characterized using optical microscopy, electron microprobe analysis, wavelength-dispersive-spectroscopy X-ray elemental mapping, and X-ray diffraction. In all experiments Cr experiences internal oxidation to produce inclusions of chromite (FeCr₂O₄) and eskolaite (Cr₂O₃) and surface layers of Cr-bearing magnetite [(Fe,Cr)₃O₄]. At 900 and 1000 °C, P is lost from the alloy via diffusion and sublimation from the metal surface. Analysis of P zoning profiles in the remnant metal cores allows for the determination of the P diffusion coefficient in the bulk metal, which is constant, and the internally oxidized layer, which is shown to vary linearly with distance from the metal surface. At 800 and 900 °C, P oxidizes to form a surface layer of graftonite [Fe₃(PO₄)₂] while at 700 and 750 °C P forms inclusions of the phosphide-mineral schreibersite [(Fe,Ni)₃P].     

Oxidation Resistance of a Cr0.50Al0.50N Coating Prepared by Magnetron Sputtering on Alloy K38G

A Cr_0.50Al_0.50N coating has been prepared by a reactive-magnetron-sputtering method on alloy K38G. The coating possesses mainly the B1 type with a small amount of B4-type crystal structure phase. Isothermal oxidation tests were performed at 900–1,100 °C for 20 h by thermogravimetric analysis (TGA) in air. The results reveal that the coated samples have much lower mass gain than that of the bare alloy. The parabolic rate constants of the coated samples decrease by 2 orders of magnitude compared with the bare alloy at 1,000 and 1,100 °C. During the oxidation of the coated samples below 1,000 °C, the main oxide is Cr₂O₃, but above 1,000 °C, the scale changes to α-Al₂O₃. The observed oxidation behaviors demonstrate that the Cr_0.50Al_0.50N coating can provide good protection against corrosion over a wide temperature range.     

Cyclic oxidation behavior of Ni3Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025% B between 900 and 1100°C

A Ni₃Al-based alloy, the composition of which was Ni-16.0% Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025%B, was cyclically oxidized in the temperature range of 900 to 1100°C in air for up to 500 hr. The alloy displayed good cyclic oxidation resistance up to 1000°C, with little scale spallation. It, however, lost cyclic oxidation resistance during oxidation at 1100°C after about 200 hr, displaying large weight losses due to serious scale spallation. NiO, α-Al₂O₃, NiAl₂O₄ and ZrO₂ were formed. The oxide scales consisted primarily of an outer Ni-rich layer which was prone to spallation, and (Al, Cr, Zr, Mo, Ni)-containing internal oxides which were adherent due mainly to the formation of (Al₂O₃, ZrO₂)-containing oxides that keyed the oxide scale to the matrix alloy.     

ESCA study of oxidation and hot corrosion of nickel-base superalloys

A study of the high-temperature oxidation and Na₂SO₄-induced hot corrosion of some nickel-base superalloys has been accomplished by using ESCA to determine the surface composition of the oxidized or corroded samples. Oxidation was carried out at 900 or 1000°C in slowly flowing O₂ for samples of B-1900, NASA-TRW VIA, 713C, and IN-738. Oxidation times ranged from 0.5 to 100 hr. Hot corrosion of B-1900 was induced by applying a coating of Na₂SO₂ to preoxidized samples, then heating to 900° C in slowly flowing O₂. Corrosion times ranged from 5 min to 29 hr. For oxidized samples, the predominant type of scale formed by each superalloy was readily determined, and a marked surface enrichment of Ti was found in each case. For corroded samples, the transfer of significant amounts of material from the oxide layer to the surface of the salt layer was observed to occur long before the onset of rapidly accelerating weight gain. Some marked changes in surface composition were observed to coincide with the beginning of accelerating corrosion, the most striking of which were a tenfold decrease in the sulfur to sodium ratio and an increase in the Cr(VI) to Cr(III) ratio. Supported by NASA Grant No. NSG-3009     

Stability of External α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Scales on Alloy 602 CA at 1100–1200 °C

An external ultrathin α-Al₂O₃ scale grown on the Ni-base alloy 602 CA during air oxidation at 800 °C was characterized by means of high-resolution TEM/EDX and electron diffraction. Alloy samples pre-oxidized at 800 °C were subsequently exposed at 1100, 1150 and 1200 °C for up to 100 h. Whereas the external alumina remained stable at 1100 °C, with the increasing exposure temperature, the pre-grown alumina scale tended to break down resulting in an external chromia scale accompanied by internal alumina precipitation. The transition from external to internal Al oxidation was investigated using SEM/EDX/EBSD. The critical Al depletion at the scale-alloy interface during the post-exposure at 1100–1200 °C was modeled using the CALPHAD-based thermodynamic-kinetic approach.     

Effects of 7·5 vol.-% water vapour on oxidation of nickel based alloy between 900 and 1100°C

Abstract

A Ni based SY 625 alloy was oxidised at 900, 1000 and 1100°C under dry and wet conditions. Water vapour has little effect on the oxidation rate and scale composition. At 900 and 1000°C, the outer scale is composed of Cr₂O₃, and a continuous NbNi₄–Ni₃Mo subscale is found at the oxide/alloy interface. At 1100°C, the scale is composed of an outer chromia scale and an internal CrNbO₄ subscale. Nevertheless, the oxide scale morphology differs between dry and wet conditions. Under dry conditions, the oxide scale appears to be compact, and chromia pegs are observed at the internal interface. The oxide scales formed under wet conditions show that porosities spread inside the scale, and the chromia grain size is smaller. At 1100°C, some scale spallation is observed under dry and wet conditions probably due to the molybdenum oxidation, leading to MoO₃ evaporation and void accumulation at the internal interface.     

Kinetic studies of coke formation and removal on HP40 in cycled atmospheres at high temperatures

An austenitic FeNiCr alloy, HP40Nb, has been preoxidized and subsequently exposed to an alternating carburizing/oxidizing/carburizing atmosphere. During the oxidation at 1000°C a thick Cr₂O₃ layer was formed which partly spalled off during cooling to room temperature, in this way chromium depleted areas resulted at the surface. The carburizing and reducing condition was established by a C₂H₆/C₂H₄/H₂ mixture at a temperature of 850°C while the oxidation for decoking was conducted in air at 800°C. The exposure times were relatively short, respectively 90/30/180 minutes. During the first exposure of the preoxidized alloy to the carburizing atmosphere, coke formation took place, and underneath the coke layer the alloy was carburized, however, only locally. After the decoking in air at 800°C, during the second exposure to the carburizing atmosphere much more catalytic coke formation was observed compared to the first exposure. The coke formation was initiated by the reduction of (Fe,Ni,Cr)-spinels formed in the oxidizing atmosphere. The reduction of the oxides gives rise to the formation of (Fe,Ni)-particles which show strong catalytic activity towards coke formation.     

Untersuchungen zum Verhalten von Hochtemperaturwerkstoffen gegenüber Aufstickung

Investigations of nitridation behaviour of high temperature materials Furnace rolls and inner tube walls of industrial bright annealing furnaces are often subject to attack by nitrogen atmospheres. In order to select appropriate materials for application in nitriding atmospheres, three commercial stainless steels (AISI 314, alloy DS, alloy 800H) and four nickel base alloys (alloy 45-TM, alloy 600H, alloy 601H and alloy 602CA) with different concentrations of nickel, chromium, silicon and aluminium were exposed to both N₂/H₂ gas atmospheres at temperatures of 1000°C, 1100°C and 1200°C. The impact strength, the mass change due to nitrogen pick-up and the depth of internal nitridation were determined after exposure. At 1000°C nitrogen pick-up and loss of impact strength, was low for all alloys investigated. At 1100°C and 1200°C, however, all alloys suffered internal nitridation. Both internal nitridation and loss of ductility were more severe in the iron base alloys than in the nickel-base alloys. The corrosion attack by nitridation decreased with increasing nickel content. The highest resistance to nitridation was found in the nickel base alloys 600 H and 602 CA.     

Corrosion Testing of Ni Alloy HVOF Coatings in High Temperature Environments for Biomass Applications

This paper reports the corrosion behavior of Ni alloy coatings deposited by high velocity oxyfuel spraying, and representative boiler substrate alloys in simulated high temperature biomass combustion conditions. Four commercially available oxidation resistant Ni alloy coating materials were selected: NiCrBSiFe, alloy 718, alloy 625, and alloy C-276. These were sprayed onto P91 substrates using a JP5000 spray system. The corrosion performance of the coatings varied when tested at ~525, 625, and 725 °C in K₂SO₄-KCl mixture and gaseous HCl-H₂O-O₂ containing environments. Alloy 625, NiCrBSiFe, and alloy 718 coatings performed better than alloy C-276 coating at 725 °C, which had very little corrosion resistance resulting in degradation similar to uncoated P91. Alloy 625 coatings provided good protection from corrosion at 725 °C, with the performance being comparable to wrought alloy 625, with significantly less attack of the substrate than uncoated P91. Alloy 625 performs best of these coating materials, with an overall ranking at 725 °C as follows: alloy 625 > NiCrBSiFe > alloy 718 ? alloy C-276. Although alloy C-276 coatings performed poorly in the corrosion test environment at 725 °C, at lower temperatures (i.e., below the eutectic temperature of the salt mixture) it outperformed the other coating types studied.     

Salzschmelzkorrosion gerichtet erstarrter eutektischer CO-Cr-C-Basis Superlegierungen bei hohen Temperaturen

Corrosion of directionally solidified eutectic Co-Cr-C-Superalloys by molton salts at high temperatures The corrosion behaviour of various directionally solidified 73C-class eutectic alloys (Co-Cr₇C₃) and the conventionally cast nickel base alloy IN 738* were investigated using a eutectic sulphate melt (sodium, calcium, and magnesium with 2% sodium chloride). As these materials are designed for high temperature applications, tests were carried out at 900°, 1000°, and 1100°. The additions to 73C were nickel, aluminium, and manganese. Corrosion surface attack for 73C and IN 738 was found to be similar. The grain boundary formation of sulphides and oxides in IN 738 is shown up as a disadvantage when compared with 73C as 73C has no grain boundaries perpendicular to the surface. This could possibly be compensated by directionally solidifying IN 738. A 10% nickel addition to 73C was found to increase the corrosion resistance, a 2% aluminium addition showed a minor improvement, and a 4.7 or 10% manganese addition to 73C to influence the corrosion resistance considerably.