Cyclic Oxidation In Air Research Articles

Influence of Filler Materials on Mechanical and Hot Corrosion Properties of Gas Tungsten Arc Welded AISI 304

This paper investigated the influence of filler materials such as E308L and ENiCu-7 utilized in the GTA welded AISI 304 with respect to mechanical and corrosion properties. It was found that E308L had shown better tensile strength and corrosion resistance as compared to ENiCu-7. A comparative analysis had been made on the different regions of the weldment subjected to cyclic air oxidation at 700°C. The corrosion products were examined using XRD and SEM/EDAX analysis.

Assessment of Mechanical and Corrosion Properties of GTA Welded Monel 400 Plates Exposed to Air Oxidation at 700°C

Oxidation and hot corrosion are serious problems in marine and power plants. Hence understanding the corrosion behavior of weldment at elevated temperatures has become the vital scientific investigation nowadays. Monel 400 is one of the widely used high temperature material that can obviate the problems. The objective of the present investigation is to study the mechanical properties and corrosion behaviour of gas tungsten arc welded Monel 400 plates subjected to air oxidation. It was observed from the tensile tests that fracture occurred at the weld region. In addition, cyclic air oxidation tests were performed on the different regions of welded Monel 400 at 700°C. It was found that corrosion was found to be extremely severe on the weld zone after exposed to 50 cycles. The corrosion products were characterized using the combined techniques of optical microscopy, XRD, SEM/EDAX analysis.

Preparation and oxidation of an Al2O3-modified aluminide coating

An Al2O3-modified aluminide coating was produced by aluminizing an as-electrodeposited Ni–Al2O3 nanocomposite film using pack cementation method at 1100 C for 4 h. For comparison, aluminizing was also performed in the same condition on an as-deposited Ni film without Al2O3 nanoparticles. SEM/EDAX and TEM results indicated that the co-deposited Al2O3 nanoparticles were homogeneously dispersed in the finer-grain nanocrystalline Ni grains. The isothermal and cyclic oxidation in air at 900 °C indicated that the Al2O3-modified aluminide coatings were profoundly oxidation resistant as compared to the Al2O3-free aluminide coatings due to the faster formation of a continuous adherent α-Al2O3 scale. The effect of Al2O3 on the microstructure and the oxidation of the aluminide coating are discussed in the detail.

Oxidation behavior of plasma sprayed Al@NiCr with cyclic thermal treatment at different temperatures

Freestanding and inhomogeneous coating was prepared by atmospheric plasma spray using Al@NiCr powder. Oxidation kinetics and mechanism are discussed after cyclic oxidation in air. Slight interdiffusion occurred at 800°C but oxidation process took a dominant position at higher temperature. The oxidation of Cr turned significant above 1100°C and volatilized as CrO3. The oxidation of Ni also became severe at 1100°C. The oxidation was suppressed when internal channels were filled up with oxides. Then two stages for oxidation appeared, both of which obeyed parabolic rate law. The slow oxidation of external surface played a key role at the second stage.

Effect of annealing treatment on cyclic-oxidation of electrodeposited Ni-Al nanocomposite

An electrodeposited Ni-Al nanocomposite having a nanocrystalline Ni matrix dispersing Al nanoparticles was annealed in vacuum at 500 °C for different time (3, 5 and 8 h, respectively). The results show that the annealing treatment leads to the reaction of Ni and Al to form intermetallics and the coarsened Ni grains that are doped with a certain amount of Al atoms diffused from the nanoparticles. Cyclic oxidation in air at 1 000 °C indicates that the scale spallation resistance of the annealed Ni-Al nanocomposite increases with the increase of annealing time, due to prevention of the composite intergranular cracking during the cycling, reduction of numerous surface NiO nodules formed on the scale spalled area and prevention of internal oxidation.

Scale composition and oxidation mechanism of the Ti–46Al–8Nb alloy in air at 700 and 800 °C

It is known that the oxide scale formed on TiAl alloys is generally composed of a mixture of alumina (Al 2O 3) and titania (TiO 2). The presence of niobium changes the activities of Ti and Al and influences the kinetics of oxidation and oxide layer composition. In this work, the Ti–46Al–8Nb alloy was subjected to cyclic oxidation in air at 700 °C (for 2 and 24 h) and 800 °C (for 300 h). Scale composition was analyzed by means of different techniques including X-ray photoelectron spectroscopy, X-ray diffraction and secondary ion mass spectroscopy. The scale consisted of several layers. The outer layer was built of alumina (amorphous or with very fine grains), whereas the inner layer – mainly of titania. After a longer exposure at a higher temperature (800 °C), niobium-rich precipitates and aluminum oxide grains were detected near to the alloy/scale interface and titanium nitride was found in the inner parts of the scale. Oxidation mechanism was studied by two-stage oxidation method using oxygen-18 and oxygen-16 isotopes combined with SIMS analyses. The distribution of oxygen isotopes over the alloy/scale interface indicated mixed inward/outward diffusion at the of reacting species. The experiments using Au markers showed that after longer oxidation time the inward diffusion was a predominant transport process.

Formation of WSi 2 coating on tungsten and its short-term cyclic oxidation performance in air

Microstructural aspects of WSi 2 coating on pure W and its short-term oxidation performance under cyclic heating and cooling conditions in air at 1100 and 1300 °C have been studied. Cyclic oxidation performance of this coating has been compared with its performance under isothermal oxidation. The coating was applied by using a pack siliconization method. The as-formed coating consisted of an outer WSi 2 layer and an inner W 5Si 3 layer. The WSi 2 layer had a columnar structure and had several through-thickness cracks generated due to the mismatch of coefficient of thermal expansion between the coating and the substrate. Based on the coating microstructure, the mechanism of coating growth during siliconizing has been suggested. Weight change data obtained under cyclic oxidation in air at 1100 and 1300 °C suggested that the above coating can provide protection to W substrate against oxidation for about 2 h. The oxide scale that formed on the coating during oxidation exposure consisted of SiO 2 and WO 3 at 1100 °C and only SiO 2 at 1300 °C. The protective silica layer underwent spallation during thermal cycling, leading to a diminishing of the protective capability of the coating. More importantly, localized oxidation of the W substrate through discontinuities present in the coating at sharp corners caused severe damage to the coated samples. Isothermal oxidation exposure of the coating, in comparison, resulted in a much lower degree of damage and the coating provided protection for a much longer duration (up to 10 h) at the above temperatures. In this study, apart from reporting a hitherto unreported oxide scale morphology, the microstructural degradation of the coating during oxidation has been linked to the columnar structure of the WSi 2 layer.

Microstructure and High Temperature Oxidation Performance of Silicide Coating on Nb-Based Alloy C-103

The microstructure and oxidation resistance of NbSi2 coating formed on Nb-based alloy C-103 by a pack siliconization process have been studied. The as-formed coating consists of an outer NbSi2 layer and an inner Nb5Si3 layer. A NbSi2–Nb5Si3 two-phase zone is also present between the above two layers. Weight-change data obtained under isothermal and cyclic oxidation in air at 1100 and 1300°C, suggests that the coating gives oxidation protection up to about 4 h. The oxide scale that formed on the coating during oxidation exposure consists of an outer glassy silica layer and an inner Nb2O5-silica mixed layer. Nb2O5 phase is also present in the outer silica scale in the form of elongated particles. Oxidation protection is achieved primarily by the presence of the glassy silica layer on the surface. Spallation of this layer during thermal cycling causes significant reduction in the protective life of the coating.

Cyclic oxidation of an ultrafine-grained and CeO 2-dispersed δ-Ni 2Al 3 coating

A novel ultrafine-grained and CeO 2-dispersed δ-Ni 2Al 3 coating was fabricated through aluminizing a CeO 2-nanoparticle-dispersed nanocrystalline Ni matrix film using an NH 4Cl-activated pack cementation method at 600 °C. Two CeO 2-free δ-Ni 2Al 3 coatings, one coarse-grained and the other ultrafine-grained, were also prepared. Compared with the later, the ultrafine-grained and CeO 2-dispersed coating profoundly increased scale spalling resistance during cyclic oxidation in air at 1000 °C. The dispersed CeO 2, together with the ultrafine-grained coating structure, helped prevent the formation of cavities at the scale/coating interface, which was proposed as a main cause for improvement of the cyclic oxidation resistance.

Cyclic Oxidation Behavior of Co-rolled Al-Ferritic Stainless Steel

The lifetime of thin-walled catalyst-carrying foils is directly linked to the overall amount of aluminum present in the base alloy. Ferritic stainless steel containing more than 7 wt. % of Al represents an interesting material for the production of substrates in catalytic converters because of its good corrosion resistance at high temperature. Despite that, the considerable brittleness of the alloy makes production by means of traditional metallurgical processes difficult. Enriching the steel surface with aluminum avoids these problems and due to the high superficial aluminum concentration, enhances the Al2O3 scale formation in the early stages of the oxidation process. Among several techniques, the co-rolling process appears to be a promising technology for enriching ferritic stainless steel surfaces with aluminum without affecting the structural and mechanical properties of the substrate. This process involves cold rolling the annealed AISI 430 ferritic stainless steel together with aluminum foil to form an Al-steel-Al sandwich. After a preoxidation treatment the resultant assemblies were subjected to cyclic oxidation in air. Their oxidation behavior was then characterized by the weight gain versus the number of cycle curves. Oxides were examined by SEM and the phases were identified by X-ray diffraction (XRD). The results highlight that in the lower range of temperatures (T

Cyclic oxidation of alumina-forming Ni–Al nanocomposites with and without CeO 2 addition

CeO 2 nanoparticles were co-electrodeposited into an alumina-forming Ni–Al nanocomposite that was composed of nanocrystalline Ni matrix and dispersed Al nanoparticles. Cyclic oxidation in air at 1000 °C for 126 h showed that the alumina-forming nanocomposite scaled faster than its counterpart which contained a CeO 2 addition, due to the formation of numerous surface NiO nodules and internal oxidation of aluminum as a result of the composite intergranular cracking during the cycling. The intergranular cracking was profoundly prevented when the CeO 2 was present.

Cyclic Oxidation of Heat Resisting Steels

Standard cast, heat resisting steels containing 25–29 w/o (weight percent) chromium and 30–36 w/o nickel together with cast alloys containing 45 or 60 w/o nickel plus low levels of aluminium were subjected to cyclic oxidation in air at 1000 and 1150°C. The standard materials suffered rapid weight loss which was somewhat mitigated by the presence of cerium. The 45 w/o nickel alloys were much more resistant and the 60 w/o nickel alloys showed superior resistance to cyclic oxidation. This improvement was due to alumina formation at or near the alloy surface. In the absence of aluminium, alloys underwent subsurface chromium carbide oxidation at a rate independent of alloy chromium content. This effect is shown to be a consequence of rapid oxygen diffusion along internal phase boundaries.

Application of image analysis and image simulation for quantitative characterization of scale spallation during cyclic oxidation of a Pt-aluminide coating

An image analysis and simulation of the spalled alumina areas at the surface of a Pt modified β-NiAl aluminide coating RT22 during cyclic oxidation in air is presented. The size distribution of the freshly spalled areas and of the unspalled zones is studied for the same coating deposited on three different nickel base superalloys which were exposed to various numbers of 300 h cycles at 1050 °C (up to 10,500 h) in a previous study [Vialas N, Monceau D. Oxid Metals, submitted for publication]. It is shown for this coating that the distribution of the freshly spalled area depends on the nature of the coated substrate and on the number of cycles. It is also shown that spallation of these three systems can be simulated by a two-parameter model which describes the spalling during the cooling step as successive microspalling events through nucleation and growth processes. This approach is consistent with previous in-situ observations and allows the introduction of a time-dependent damaging process in the models of cyclic oxidation kinetics.

Oxidation characteristics of a platinized MCrAlY bond coat for TBC systems during cyclic oxidation at 1000 °C

The scale growth mechanisms of a platinized NiCoCrAlY bond coat (BC) beneath a ceramic thermal barrier coating (TBC) were studied during cyclic oxidation in air at 1000 °C. The thermally grown oxide scales (TGO) after exposure times ranging from 100 to 9100 h were characterized by scanning electron microscopy and laser fluorescence spectroscopy. The TGO appeared to consist of alumina and no oxides of the other elements present in the NiCoCrAlY coating were detected. Up to exposure times of 1000 h substantial amounts of metastable alumina were found in the outer part of the TGO. This is probably the reason for the formation of a porous, mixed alumina/zirconia layer at the TBC/TGO interface. This layer was prone to micro-crack formation, which did, however, not result in macroscopic TBC failure. The latter occurred after crack formation at the TGO-BC interface.

Electrochemical impedance spectroscopy of thermal barrier coatings as a function of isothermal and cyclic thermal exposure

Electrochemical impedance spectroscopy (EIS) is being developed as a non-destructive evaluation technique of thermal barrier coatings (TBCs) for life-remaining assessment and quality control. In this investigation, EIS was employed to non-destructively examine TBCs, as a function of thermal exposure during isothermal and cyclic oxidation in air at 1121 °C. TBCs examined in this study include electron beam physical vapor deposited (EB-PVD) and air-plasma sprayed ZrO 2–7wt.%Y 2O 3 (YSZ) on NiCoCrAlY or (Ni,Pt)Al bondcoat. Electrochemical impedance response of TBC specimens at room temperature were acquired and analyzed with an AC equivalent circuit based on the multi-layered micro-constituents of TBCs. Microstructural characteristics of TBCs were also examined by optical and electron microscopy. Initial increase in the resistance of YSZ with thermal exposure was observed and related to the high temperature sintering of YSZ. The parabolic growth of TGO during high temperature oxidation was inversely proportional to the capacitance of TGO. An explanation based on electrolyte penetration into sub-critical damage is proposed for the gradual decrease in the resistances of YSZ and TGO with prolonged thermal exposure. Observation of exposed metallic bondcoat surface on the fracture surface, which provides conduction readily through electrolyte contact, was related to the abrupt and large increase in the capacitance of YSZ and TGO.

Significance of minor alloying additions and impurities on alumina scale growth and adherence in FeCrAl alloys

AbstractUltra-high purity Fe–Cr–Al–Y model alloys with controlled additions of impurities such as phosphorus and carbon, and potentially more beneficial elements such as titanium and zirconium have been prepared by induction melting in water-cooled, silver crucibles. 1 mm thick samples were then prepared by hot and cold rolling and annealing prior to cyclic oxidation in air at temperatures in the range 1100–1300°C. Other impurities were kept to a minimum of <10 ppm. scanning electron microscopy, Auger surface analysis and Scanning Transmission Electron Microscopy were used to characterise the samples both before and after oxidation.Weight gain studies during oxidation showed that the high phosphorus containing alloy went into breakaway very quickly, after only 200 hours at 1300°C, while the Ti and Zr rich samples lasted for 1900 hours and 3300 hours respectively. In some cases, chromium or titanium rich precipitates were found along the alloy grain boundaries, often associated with carbon, while in other ...

Significance of minor alloying additions and impurities on alumina scale growth and adherence in FeCrAl alloys

Ultra-high purity Fe–Cr–Al–Y model alloys with controlled additions of impurities such as phosphorus and carbon, and potentially more beneficial elements such as titanium and zirconium have been prepared by induction melting in water-cooled, silver crucibles. 1 mm thick samples were then prepared by hot and cold rolling and annealing prior to cyclic oxidation in air at temperatures in the range 1100–1300°C. Other impurities were kept to a minimum of <10 ppm. scanning electron microscopy, Auger surface analysis and Scanning Transmission Electron Microscopy were used to characterise the samples both before and after oxidation.Weight gain studies during oxidation showed that the high phosphorus containing alloy went into breakaway very quickly, after only 200 hours at 1300°C, while the Ti and Zr rich samples lasted for 1900 hours and 3300 hours respectively. In some cases, chromium or titanium rich precipitates were found along the alloy grain boundaries, often associated with carbon, while in other cases precipitates were found along the oxide metal interface. Although phosphorus was found at this interface in some of the samples, it was not always present, and may not be the only contributing factor to the premature breakaway failure of the oxides. A complete review of the microstructural evolution of these samples during oxidation will form the main topic of this paper.

Static and Cyclic Oxidation of Ti–44Al and Ti–44Al–xNb Alloys

Binary, Ti–44Al, and ternary, Ti–44Al–xNb (x=4, 8, and 11) alloys were subjected to static and cyclic oxidation in air up to a period of 1 week at 850 and 950°C. The oxidation behavior is characterized by plotting a graph between weight gain as a function of time or number of cycles. The binary alloy undergoes the most severe oxidation while Nb provides resistance to oxidation, which increases with an increase in concentration. Oxides were examined by SEM and EDX was used to perform chemical analysis. Alternate oxide layers rich in alumina and titania were found along with an aluminum-depleted zone underneath the oxide scale. Different microstructures were observed for the alloys subjected to static- and cyclic-heating modes. Defects like twins and stacking faults were generated after a static mode of heating, while transformation products at the grain boundaries dominate the microstructures for samples subjected to cyclic heating.

Growth and oxidation resistance of boron-modified and germanium-doped silicide diffusion coatings formed by the halide-activated pack cementation method

Niobium-base and titanium-base alloys and intermetallic compounds possess many attractive properties, but practical application at high temperature is limited by their rapid scaling and embrittlement by oxygen dissolution. Since alloying schemes have not provided an acceptable balance of oxidation resistance and mechanical properties, protective coatings are required. Procedures for codepositing either silicon and boron, or else silicon and germanium, in a single reaction-processing step by a fluoride-activated, pack cementation method have been developed. These B-modified and Ge-doped silicide coatings were grown on commercially pure titanium, Ti−20A1−22Nb, Ti−22Al−27Nb and Nb−40Ti−15A1 alloys by direct conversion of the base metal into a TiSi 2-base coating. For Nb-base alloys, a protective Ge-doped MoSi 2 coating was grown by the diffusive conversion of a Mo layer deposited by physical vapor deposition. These diffusion coatings have been characterized, and their performance was evaluated by cyclic oxidation in air. Microhardness profiles were used to demonstrate the absence of oxygen contamination for the coated substrates. Preliminary fatigue testing of coated substrates is reported.

Growth and oxidation resistance of boron-modified and germanium-doped silicide diffusion coatings formed by the halide-activated pack cementation method

Niobium-base and titanium-base alloys and intermetallic compounds possess many attractive properties, but practical application at high temperature is limited by their rapid scaling and embrittlement by oxygen dissolution. Since alloying schemes have not provided an acceptable balance of oxidation resistance and mechanical properties, protective coatings are required. Procedures for codepositing either silicon and boron, or else silicon and germanium, in a single reaction-processing step by a fluoride-activated, pack cementation method have been developed. These B-modified and Ge-doped silicide coatings were grown on commercially pure titanium, Ti−20A1−22Nb, Ti−22Al−27Nb and Nb−40Ti−15A1 alloys by direct conversion of the base metal into a TiSi2-base coating. For Nb-base alloys, a protective Ge-doped MoSi2 coating was grown by the diffusive conversion of a Mo layer deposited by physical vapor deposition. These diffusion coatings have been characterized, and their performance was evaluated by cyclic oxidation in air. Microhardness profiles were used to demonstrate the absence of oxygen contamination for the coated substrates. Preliminary fatigue testing of coated substrates is reported.