Cyclic Oxidation In Air Research Articles

Consequences of Different Mechanical Surface Preparation of Ni-Base Alloys during High Temperature Oxidation.

The influence of surface roughness on its high temperature oxidation for an Ni-base superalloy was studied using laser profilometry, atomic force microscopy, mass change measurements, glow-discharge optical emission spectrometry, scanning electron microscopy, X-ray diffraction, and positron annihilation methods. The isothermal and cyclic air oxidation tests were performed at 1000 °C and showed dependence of oxidation behavior on surface roughness. Smoother surfaces oxidation resulted in the formation of a multilayered oxide scale consisting of NiO, Cr2O3, and internally oxidized Al2O3 while a rougher surface formed protective Al2O3 scale. The factors responsible for different oxidation behaviors were determined as higher concentration of vacancies and increased residual stresses in the near-surface region of studied alloys.

Oxidation and hot corrosion studies on Fe-based superalloy A-286 pulsed current GTA weldments in gas turbine environment

Cyclic air oxidation and hot corrosion performance of pulsed current gas tungsten arc weldment (PCGTAW) of Fe-based superalloy A-286 are evaluated in the air and Na2SO4–7.5%NaVO3–5%NaCl molten salt at 700 °C. Thermogravimetric analysis is carried out to evaluate the corrosion kinetics of the weldments. Scanning electron microscope, energy dispersive spectroscopy, and x-ray diffraction methods are used to investigate the surface morphology, composition and phases of the air oxidized and hot corroded weldments. The depth of corrosion attack and distribution of alloying elements in the oxidized and hot corroded weldments are investigated at the cross sections using SEM microstructure and elemental mapping analysis. Some oxide scales spalling and sputtering are observed in PCGTA weldment in the molten salt environment during hot corrosion study which is not observed in air oxidized weldment at the end of 50 cycles. The hot corroded weldment shows a higher weight gain than oxidized weldment. Presence of molten salt increases the rate of corrosion. The cross-sectional analysis clearly explains, in both the cases higher oxidation rate is observed in the base metal zone.

Effect of hot corrosion on the bimetallic joints employed in the coal-fired boiler

Hot corrosion is the foremost problem in the thermal, gas power plants working in the high-temperature aggressive atmospheres. In the current study, the hot corrosion characteristics of AISI 904 L and Inconel 625 bimetallic joints were investigated. These dissimilar joints were produced by gas tungsten arc welding using pulsed current method (PCGTAW) employing enriched Ni-Cr-Mo fillers (ERNiCrMo-4 and ERNiCrCoMo-1). The performances of these coupons have been ascertained by exposing in the cyclic air oxidation and K2SO4 + 60% NaCl eutectic mixture of molten salt for 50 cycles at 700 °C. Thermogravimetric plots were established to find the corrosion kinetics. The scales of the oxide layer were characterized by XRD and SEM/EDS methods. The study assimilated that all the coupons exhibited better corrosion resistance when exposed in the air oxidation compared to the molten salt environment. Spallation and blistering were experienced at the ERNiCrCoMo-1 weldment and it was found to be more susceptible to degradation.

Microstructural evolution of an aluminide coating on alloy 625 during wet air exposure at 900 °C and 1000 °C

The microstructural changes of the aluminized alloy 625 during cyclic oxidation in air + 6% H2O at 900 °C and 1000 °C were analyzed using optical metallography (OM), scanning electron microscopy (SEM) with energy and wave length dispersive X-ray analysis (EDX/WDX) as well as electron backscatter diffraction (EBSD). An in-house developed thermodynamic-kinetic procedure was employed to predict the microstructural evolution of aluminized alloy 625 during high temperature exposure by considering simultaneously occurring surface oxidation and interdiffusion processes. Due to the lack of mobility data for the relevant alloying elements in the σ-phase, assumptions for the mobilities were made based on the value of the mobilities in α-Cr. Despite these assumptions, the calculated results were found to be in good agreement with experimental observations. The complete depletion of β-NiAl in the coating observed during exposure at 1000 °C was correctly predicted by the model. The model was also able to predict dissolution of the precipitate phases α-Cr and σ in the interdiffusion zone during exposures at 900 °C and 1000 °C. The model was however unable to predict the formation of the μ-phase in the alloy after 1000 h of exposure at 1000 °C. The developed modelling approach offers the potential to predict microstructural changes of aluminized nickel base alloys thus reducing cost and time consuming experimental efforts.

In-situ formation of SiC, ZrB2-SiC and ZrB2-SiC-B4C-YAG coatings for high temperature oxidation protection of C/C composites

Si3N4, (ZrO2+B4C + Si) and (ZrB2+SiC + B4C + Y2O3+Al2O3) were used as precursors for in-situ formation of SiC, ZrB2-SiC and ZrB2-SiC-B4C-YAG coatings respectively. The oxidation behavior of coated C/C composites was studied by cyclic oxidation in air at temperatures between 1500 °C and 1700 °C. The SiC coating exhibited the lowest weight loss at 1500 °C and 1600 °C. After initial mass loss for two cycles of oxidation at 1500 °C, the ZrB2-SiC coating was found to provide the best protection by the precipitated ZrO2 in SiO2 glass at 1700 °C. The ZrB2-SiC-B4C-YAG coating performed well at 1700 °C with the formation of a complex YAG glass containing ZrB2/ZrO2.

A comparative study to evaluate the corrosion performance of Zr incorporated Cr3C2-(NiCr) coating at 900 °C

In this study zirconium incorporated Cr3C2-(NiCr) coating has been sprayed on three superalloys viz. Superni 718, Superni 600 and Superco 605 using D-gun technique. A comparative study has been carried out to check the cyclic oxidation in air and hot corrosion in simulated incinerator environment (40%Na2SO4-40%K2SO4-10%NaCl-10%KCl) for the coated specimens at 900°C for 100 cycles. Oxidation kinetics has been established for all the specimens using weight change measurements. Corrosion products have been characterized using X-ray diffractometer (XRD) and scanning electron microscopy/energy-dispersive analysis (SEM/EDAX). Cr3C2-(NiCr) + 0.2%wtZr coating provides very good corrosion resistance in air oxidation for all the three coated superalloys. As all the three coated superalloys shows parabolic behaviour with parabolic rate constant as 0.07 × 10−10 (g2cm−4s−1) for Superni 718, 0.43 × 10−10 (g2cm−4s−1) for Superni 600 and 0.3 × 10−10 (g2cm−4s−1) for Superco 605 This coating is also effective in the molten salt environment but coating on Co-based superalloy Superco 605 did not perform satisfactorily. The parabolic rate constants for coated Superni 718 is 0.61 × 10−10 (g2cm−4s−1), for coated Superni 600 is 6.72 × 10−10 (g2cm−4s−1) and for coated Superco 605 is 17.5 × 10−10 (g2cm−4s−1).

Suitable sealants for cracked aluminized austenitic steels and their oxidation behaviour

Aluminizing of steels may result in the appearance of tensile cracks from the coating surface to the substrate due to the difference in thermal expansion coefficients. The cracks often need to be sealed to make the coatings impervious to chemical attack and to limit their mechanical degradation. In this work, dipping of the cracked aluminide coatings elaborated on IN-800HT in 0.5, 1 and 3.75mol·L−1 concentrations of H3PO4 in acetone and increasing the dipping time are investigated. The three concentrations resulted in the formation of P-O compounds and of Al2O3. However, the 0.5mol·L−1 allowed better wetting of the surface. Further reduction of the concentration from 0.5mol·L−1 to 0.2mol·L−1 brought about the complete sealing of both large and fine cracks. The efficiency of the sealing process with 0.2mol·L−1 and 0.5mol·L−1 was thereafter evaluated by cyclic oxidation in air at 650°C for >1000cycles. The overall microstructure and composition of the aluminide coatings remained identical after sealing and the oxidation behaviour comparable to the untreated counterparts. The oxide scale formed on the untreated samples appeared thicker compared to the sealed ones. The latter were covered by a thin layer of AlPO4 which evolved towards a thin protective alumina scale. However, for the sealed samples, some minor cracks were detected after oxidation but they appeared completely healed with Al2O3 and no propagation of them beyond the additive layer/interdiffusion zone was noticed.

Behaviour of a silicon-rich coating on Ti-46Al-8Ta (at.%) in hot-corrosion environments

In this work silicon-rich coating was deposited on a Ti-46Al-8Ta (at.%) alloy in a two-step procedure comprising physical and chemical vapour deposition (magnetron sputtering and pack cementation, respectively). Protective properties of the coating were tested through cyclic oxidation in air at 800°C with samples contaminated with NaCl, Na2SO4 or a mixture of these two salts. The coated samples exhibited better hot corrosion resistance compared with the uncoated reference. This was attributed to the formation of an oxide layer composed of amorphous silica with embedded rutile and α cristobalite crystals, constituting a barrier against oxygen penetration.

Isothermal and cyclic oxidation behavior of free standing MCrAlY coatings manufactured by high-velocity atmospheric plasma spraying

In the present paper the high temperature oxidation behavior of a free standing NiCoCrAlY coating produced by high-velocity atmospheric plasma spraying (HV-APS) is investigated and compared with that produced by conventional low pressure plasma spraying (LPPS). Isothermal thermogravimetric experiments at 1000 and 1100°C in synthetic air revealed a lower oxidation rate of the HV-APS than the LPPS coating. Both coatings formed oxide scales based on alpha alumina, however, in the LPPS coating incorporation of coarse mixed Y/Al-oxide pegs into the scale occurred, increasing the oxidation rate by providing short circuit paths for oxygen diffusion probably due to higher diffusivities in the mixed oxide and/or along the interfaces between mixed oxide and alumina. In the HV-APS coatings most of the yttrium was tied-up in sub-μm Y-containing oxide particles and only minor amounts of mixed Y/Al oxide precipitates were found in the alumina surface scale. Cyclic air oxidation tests at 1100°C revealed a lower oxidation rate and better scale adherence for the HV-APS coating.The results thus show that HV-APS is a promising method for the processing of MCrAlY coatings. The specific yttrium distribution in form of fine oxide precipitates in the HV-APS material prevents the formation of deleterious Y-rich oxide pegs and promotes formation of a slowly growing, protective alumina scale with excellent adherence.

Thermal Transport Properties of New Coatings on Steels for Supercritical Steam Power Plants

In order to protect steels from oxidation and corrosion under steam- and fire-side exposures for the next generation of steam turbines, different protective coatings (slurry Al, pack Al, thermal spray CoNiCrAlY, HIPIMS CrN/NbN, sol–gel Al2O3/AlPO4) were assessed on ferritic–martensitic P92 and austenitic HR3C steels. Cyclic oxidation in air at 650 and 700 °C was subsequently conducted for the coatings to grow oxide scales. The thermal transport properties of both the as-deposited and the oxidised coatings were investigated by laser flash analysis till 900 °C to ascertain whether they exhibited any potential harmful insulation to the underlying steel substrate. The results indicated that neither the coatings nor their oxides had any impact on the thermal diffusivity in the temperature range of interest. The thermal diffusivity values were mostly dictated by those of the substrate. It is thus expected that the influence on the thermal transport properties of the coatings under steam conditions will be equivalent than in air.

Microstructure and oxidation resistance of a NiCrAlY/Al2O3-sprayed coating on Ti-19Al-10Nb-V alloy

A Ti3Al (α2) alloy (Ti-19Al-10Nb-3V, wt%) coated by air plasma spraying (APS) with Ni22Cr10AlY and Ni22Cr10AlY/Al2O3 has been tested against cyclic oxidation in air at 850°C for 10 cycles of 10h each. Effects of pre-oxidation and cyclic oxidation treatments, both in air, on the microstructure of the single α2/Ni22Cr10AlY and double coated α2/Ni22Cr10AlY/Al2O3 specimens were investigated by X-ray diffraction (XRD), SEM-EDS and Raman spectroscopy. Several distinct results appear through the cyclic oxidization of the pre-oxidized α2/Ni22Cr10AlY, namely, elemental diffusion where Ti and Al diffuses from one side and Ni and Cr from the other side, oxygen is found to be present at the support-bond coat interface and a partial reaction of the superficial NiO with α-Al2O3 to form the non-protective NiAl2O4 takes place. Conversely, pre-oxidation treatment of the double coated α2/Ni22Cr10AlY/Al2O3 specimens causes transient alumina polymorphs obtained by APS to convert to α-Al2O3 and hence to ensure the lowest cyclic oxidation rate seen by a kp value of 2.62×10−12g2cm−4s−1. Good adhesion to support of all the coated specimens is recorded following the cyclic oxidization treatment.

Development of multilayer oxidation resistant coatings on Cr–50Nb alloy

To protect Cr–50Nb alloys from high-temperature oxidation, the Al2O3/Si–Al multilayer coatings were produced by pack cementation process, followed by sol–gel process and hot pressing. The results indicate that the multilayer coating is dense and exhibits good adherence to the substrate, which consists of a compact Al2O3 outer layer and an inner layer composed of Si, Al, Cr, Nb. Uncoated Cr–50Nb alloy occurs catastrophic oxidation at the initial oxidation stage at 1200°C. However, the scale spalling resistance of the multilayer coating is improved significantly, and the multilayer coating exhibits good resistance to oxidation. During cyclic oxidation in air at 1200°C for 100h, the weight loss is 0.13mg/cm2 and the mass gain is 3.38mg/cm2.

Microstructure and cyclic oxidation behavior of silicide coating and Si–Al coating on Cr–50Nb alloy

To protect Cr–50Nb alloys from high-temperature oxidation, the silicide diffusion coating and Si–Al coating were produced by pack cementation. The results indicate that the two coatings have three-layer structure and the interfaces are smooth. Uncoated Cr–50Nb alloy occur catastrophic oxidation at the initial oxidation stage at 1200°C. However, the scale spalling resistance of the two coatings are improved significantly, and Si–Al coating exhibits excellent resistance to oxidation. During cyclic oxidation in air at 1200°C for 100h, the weight loss of Si–Al coating is 1.39mg/cm2 and the mass gain is 6.01mg/cm2.

The effect of grain refinement on the adhesion of an alumina scale on an aluminide coating

To understand the effect of grain refinement on the thermally grown alumina scale adhesion to the metal substrate, two δ-Ni2Al3 coatings, one coarse-grained (∼70μm) and the other ultrafine-grained (generally below ∼500nm), were prepared. The cyclic oxidation in air at 1100°C shows that the ultrafine-grained (UFG) coating is better oxidation resistant than the coarse-grained (CG) coating due to the formation of a more adherent alumina scale. The latter is intrinsically correlated with the fact that the aluminide grain refinement helps to increase the oxide/metal strength through a route to prevent the formation of large-sized voids at the interface.

Cyclic Oxidation Behavior Of Bare And Cr3C2-25 (NiCr) Coated Super Alloy At Elevated Temperature

High temperature oxidation is the main cause for the degradation of materials used at elevated temperature which is responsible for the major shut down of turbines, boilers and incinerators used in large units. An attempt has been made to use Cr3C2-25%NiCr coating deposited by D-gun technique on Superni 600 alloy. These coatings are generally used for wear applications. This coating has been studied for oxidation at 900C under cyclic conditions. Sound and adherent coatings of 150-250 μm can be obtained by D-gun process. The bare and coated alloys were subjected to cyclic oxidation in air for 100 cycles. Weight change/area has been plotted against number of cycles and to monitor the oxidation kinetics. The scale formed after oxidation both on the bare and coated alloys has been examined using FESEM/EDX, XRD, and Cross-sectional/Mapping analysis. The weight gain by coated Superni 600 was found to be about 80% less than that of bare Superni 600. D-gun sprayed Cr3C2-NiCr coating has contributed to increase in the oxidation resistance of coated superalloy. These types of coatings can be developed for high temperature applications for both corrosion and erosion resistance.

PHYSICOCHEMICAL AND MECHANICAL PROPERTIES OF CROFER 22 APU FERRITIC STEEL APPLIED IN SOFC INTERCONNECTS

The paper presents the results of investigations of the physicochemical and mechanical properties of the Crofer 22 APU steel designed for application in metallic interconnects forming the key components of solid oxide fuel cells (SOFCs). Microstructural and hardness studies of non-metallic inclusions and the matrix were carried out. Based on compression tests of raw Crofer 22 APU and the steel after 600 hrs of cyclic oxidation in air at 800°C, the composition of non-metallic inclusions and their influence on the strength properties of the steel were determined.

Study of high temperature oxidation kinetics of Co–Al coating on superalloy

Nanostructured Co–Al coating on Superni-718 substrate was deposited by dc/rf magnetron sputtering. The high temperature oxidation kinetics of Co–Al coating on superalloy substrate was studied after 100 h of cyclic oxidation in air environment at 700–900°C. FE-SEM/EDS, AFM and XRD were used to characterise the microstructure, surface morphology and phase constituents of as deposited and oxidised coatings. The results indicate that the Co–Al coated superalloy substrate has higher oxidation resistance at 900°C. The presence of β-CoAl intermetallic phase in the Co–Al coating, the formation of Al2O3 and spinel phases such as CoCr2O4 and CoAl2O4 in the oxide scale are considered to be the reasons for the higher oxidation resistance shown by the Co–Al coated superalloy substrate after 100 h of cyclic oxidation in air at 900°C. Further, the oxide scale formed on the coated sample exhibits good spallation resistance during cyclic oxidation test at 900°C.

Cyclic-Oxidation of RexOy-Modified Aluminide Coating

Reactive reactive element oxide RexOy (Re=Ce, Y)-modified aluminide coatings were developed by aluminizing the as-codeposited Ni-RexOy composite film using pack cementation method at 1100°C for 4 h. By comparison, aluminizing was also performed with the same condition on an as-deposited Ni film without RexOy particles. SEM/EDAX and TEM results indicated that the co-deposited CeO2 or Y2O3 particls were homogeneously dispersed in the finer-grain nanocrystalline Ni grains. The cyclic oxidation in air at 900°C indicated that the RexOy -modified aluminide coatings were profoundly spallation resistance as compared to the RexOy -free coatings due to the formation of a continuous adherent α-Al2O3 scale.

Multi-layered silicides coating for vanadium alloys for generation IV reactors

The halide-activated pack-cementation technique was employed to fabricate a diffusion coating that is resistant both to isothermal and to cyclic oxidation in air at 650°C on the surface of the V–4Cr–4Ti vanadium alloy that is a potential core component of future nuclear systems. A thermodynamic assessment determined the deposit conditions in terms of master alloy, activator, filler and temperature. The partial pressures of the main gaseous species (SiCl4, SiCl2 and VCl2) in the pack were calculated with the master alloy Si and the mixture VSi2+Si. The VSi2+Si master alloy was used to limit vanadium loss from the surface. The obtained coating consisted of multi-layered VxSiy silicides with an outer layer of VSi2. This silicide developed a protective layer of silica at 650°C in air and was not susceptible to the pest phenomenon, unlike other refractory silicides (MoSi2, NbSi2). We suggest that VSi2 exhibits no risk of rapid degradation in the gas fast reactor (GFR) conditions.

Effect of filler materials on the performance of gas tungsten arc welded AISI 304 and Monel 400

Dissimilar metal welding involving AISI 304 and Monel 400 finds widespread use in marine and offshore environments for the fabrication of heat exchangers, evaporators, piping and vessels in petrochemical and power generation industries. However studies on the performance of such weldments are scantly found in the literature even though the individual metals have higher corrosion resistance and strength. This paper reports the work carried out on welding of AISI 304 and Monel 400 using Gas Tungsten Arc Welding (GTAW) technique to examine the weldability, mechanical and metallurgical properties. Investigations have been carried out on the hot corrosion behavior of these joints subjected to cyclic air oxidation and K2SO4+NaCl (60%) molten salt environment at 600°C. A comparative analysis was carried out on these weldments for two different filler metals such as E309L and ENiCu-7. The oxide scales formed on the various zones of the weldment have been characterized systematically using surface analytical techniques. Weld zone was found to be more susceptible to degradation than base metals used. The effect of filler materials on the hot corrosion is discussed. The studies reported in this paper would be beneficial for fabricators embracing this type of dissimilar weldments in the petrochemical and power generation industries.