Protective Properties of High Temperature Oxide Films on Ni-based Superalloys in 3.5% NaCl Solution

The electrochemical behaviors of high temperature oxide film formed on the sputtered microcrystalline coating of M38 alloy （mc-M38） were investigated by potentiodynamic and electrochemical impedance spectroscopy （EIS） techniques in 3.5% NaCl solution. Mott-Schottky analysis was used to study the semi-conductive properties of the surface oxide. The results of the capacitance measurements showed that the oxide films on both the coating and the cast alloy were p-type semiconducting characteristics. Both the carrier density （Na）and the flat band potential （Efb） were obviously frequency-dependent, and the optimal frequency range was from 1000 to 1500 Hz. The oxidized coating exhibited higher protectivity than the oxidized cast alloy due to the lower carrier density compared with that of the oxidized cast alloy. The EIS data of the long-term immersing tests suggested that the oxide film served as an inner-barrier layer against chloride ions. The penetration of the aggressive ions into the surface oxide resulted in the decreased polarization resistance as a function of the immersion time.

Improving corrosion resistance of additively manufactured WE43 magnesium alloy by high temperature oxidation for biodegradable applications

Laser powder bed fusion(L-PBF)has been employed to additively manufacture WE43 magnesium(Mg)alloy biodegradable implants,but WE43 L-PBF samples exhibit excessively rapid corrosion.In this work,dense WE43 L-PBF samples were built with the relativity density reaching 99.9%.High temperature oxidation was performed on the L-PBF samples in circulating air via various heating temperatures and holding durations.The oxidation and diffusion at the elevated temperature generated a gradient structure composed of an oxide layer at the surface,a transition layer in the middle and the matrix.The oxide layer consisted of rare earth(RE)oxides,and became dense and thick with increasing the holding duration.The matrix was composed ofα-Mg,RE oxides and Mg_(24)RE_(5) precipitates.The precipitates almost disappeared in the transition layer.Enhanced passivation effect was observed in the samples treated by a suitable high temperature oxidation.The original L-PBF samples lost 40%weight after 3-day immersion in Hank’s solution,and broke into fragments after 7-day immersion.The casted and solution treated samples lost roughly half of the weight after 28-day immersion.The high temperature oxidation samples,which were heated at 525℃ for 8 h,kept the structural integrity,and lost only 6.88%weight after 28-day immersion.The substantially improved corrosion resistance was contributed to the gradient structure at the surface.On one hand,the outmost dense layer of RE oxides isolated the corrosive medium;on the other hand,the transition layer considerably inhibited the corrosion owing to the lack of precipitates.Overall,high temperature oxidation provides an efficient,economic and safe approach to inhibit the corrosion of WE43 L-PBF samples,and has promising prospects for future clinical applications.

Preparation and Characterization of Component Materials for Intermediate Temperature Solid Oxide Fuel Cell by Glycine-Nitrate Process

La1-xSrxGa1-y MgyO3-δ(LSGM) electrolyte, La1-xSrxCr1-y MnyO3-δ( LSCM ) anode and La1-xSrxFe1-y MnyO3-aaaaaaa(LSFM) cathode materials were all synthesized by glycine-nitrate process (GNP). The microstructure and characteristics of LSGM, LSCM and LSFM were tested via X-ray diffraction(XRD), scanning electron microcopy (SEM), A C impedance and four-probe direct current techniques. XRD shows that pure perovskite phase LSGM electrolyte and electrode (LSCM anode and LSFM cathode) materials were prepared after being sintered at 1400℃for 20 h and at 1000℃for 5 h, respectively. The max conductivities of LSGM (ionic conductivity), LSCM (total conductivity) and LSFM (total conductivity) materials are 0.02, 10, 16 S·cm-1 in the air below 850℃, respectively. The conductivity of LSCM becomes smaller when the atmosphere changes from air to pure hydrogen at the same temperature and it decreases with the temperature like metal. The porous and LSGM-based LSCM anode and LSFM cathode films were prepared by screen printing method, and the sintering temperatures for them were 1300 and 1250℃, respectively. LSGM and electrode (LSCM and LSFM) materials have good thermal and chemical compatibility.

High Temperature Protection Ability of Oxide Scale Formed on Fe_3Al Enhanced by Surface-applied Y_2O_3 Thin Film

The Y_2O_3 thin film was applied on Fe-3Al intermetallic compound by electrodeposition and thermal decomposition. The cyclic oxidation of the Fe-3Al specimens with and without surfaceapplied Y_2O_3 thin film was carried out at 900 ℃ in air. The results show that the selective oxidation of Al in Fe-3Al was promoted, and both of the plasticity and the adhesion of the oxide scale formed on Fe3Al were improved and the high temperature oxidation resistance of Fe3Al was enhanced markedly.

In situ IR, pulse reaction and TPD-ITD study of catalytic performance of room-temperature carbon monoxide oxidation on supported gold catalysts

With in situ IR, two different CO adsorption bands were detected on various chemical state gold catalysts. One band is attributed to the linear CO on an oxidized gold catalyst(2100 cm -1 ), the other one is ascribed to the bridged CO on metallic gold (2085 cm -1 ). CO pulse reaction showed that Au/Fe 2O 3 catalyst had a room temperature activity even in the presence of moisture. The produced CO 2 was detained and more easily desorbed from supported gold catalyst than support oxide. TPD IDT results indicated that the O - 2 superoxide ions are the possible active oxygen species.

Status and prospects of intermediate temperature solid oxide fuel cells

Compared with conventional electric power generation systems, the solid oxide fuel cell （SOFC） has many advantages because of its unique features. High temperature SOFC has been successfully developed to its commercial applications, but it still faces many problems which hamper large-scale commercial applications of SOFC. To reduce the cost of SOFC, intermediate temperature solid oxide fuel cell （IT-SOFC） is presently under rapid development. The status of IT-SOFC was reviewed with emphasis on discussion of their component materials. 2008 University of Science and Technology Beijing. All rights reserved.

Direct decomposition of nitric oxide in low temperature over iron-based perovskite-type catalyst modified by Ru

Iron-based perovskite-type compounds modified by Ru were prepared through sol-gel process to study its catalytic activity of NOx direct decomposition at low temperature and evaluate the conversion of NO under the experimental conditions. The catalytic activity of La 0.9Ce 0.1Fe 0.8-nCo 0.2RunO3 （n=0.01,0.03,0.05,0.07,0.09）series for the NO, NO-CO two components, CO-HC-NO three components were also analyzed. The catalytic investigation evidenced that the presence of Ru is necessary for making highly activity in decomposition of nitric oxide even at low temperature（400 ℃）and La 0.9Ce 0.9Fe 0.75Co 0.2Ru 0.05O3 （n=0.05） has better activity in all the samples, the conversion of it is 58.5%. With the reducing gas（CO,C3H6）added into the gas, the catalyst displayed very high activity in decomposition of NO and the conversion of it is 80% and 92.5% separately.

Effect of Rare Earth Doping on the Tensile Properties of High Temperature Oxidation Coating

The failure process was characterized by complex diffusion of elements in the bonding layer,TGO growth and growth stress inside the coating.We studied the aluminum migration phenomenon of NiCoCrAlY and NiCoCrAlYHf coatings under high temperature oxidation,TGO growth characteristics,the microstructure and composition of the bonding layer,and integrates them into the description of the surface strain under coating tension.The experimental results show that the TGO growth rate of NiCoCrAIYHf coating is lower than that of NiCoCrAIY coating,and the formed TGO is thinner.After high temperature oxidation,the cracking time of NiCoCrAIY coating is advanced,while the cracking time of rare earth doped coating is delayed.The addition of rare earth elements can effectively inhibit the generation of spinel phase,improve the fracture toughness of TGO,refine the grains in the bonding layer,and increase the grain boundary strengthening by 29.1 MPa which is consistent with the experimental value.Therefore,the yield strength of the doped coating is improved and the crack time of the coating is delayed.

High temperature oxidation resistance and microstructure change of aluminized coating on copper substrate

The outermost coating with single phase Ni2Al3 was obtained on copper surface by electrodepositing nickel followed by slurry pack aluminizing at 800 °C for 12 h. The oxidation resistance and microstructure of the coating oxidized in ambient air at 1000 °C for 25-250 h were investigated using SEM, X-ray diffraction and optical microscope methods. The results show that the copper with single phase Ni2Al3 coating possesses the best high temperature oxidation resistance, and the mass gain of the coating is 1/15 that of pure copper and 1/2 that of nickel coating, respectively. The specimen surface after being oxidized for 25 h still comprises Ni2Al3 phase. However, when the time of oxidizing treatment increases to 50 h, the Ni Al phase is formed. It is also found that the Ni2Al3 phase completely turns into Ni Al phase after oxidizing treatment for 100 h and above. The Ni Al coating shows excellent high temperature oxidation resistance when oxidation time is 250 h.

High temperature oxidation behavior of Ti(Al,Si)_3 diffusion coating on γ-TiAl by cold spray

A Ti(Al,Si)3 diffusion coating was prepared on γ-TiAl alloy by cold sprayed Al?20Si alloy coating, followed by a heat-treatment. The isothermal and cyclic oxidation tests were conducted at 900 °C for 1000 h and 120 cycles to check the oxidation resistance of the coating. The microstructure and phase transformation of the coating before and after the oxidation were studied by SEM, XRD and EPMA. The results indicate that the diffusion coating shows good oxidation resistance. The mass gain of the diffusion coating is only a quarter of that of bare alloy. After oxidation, the diffusion coating is degraded into three layers: an inner TiAl2 layer, a two-phase intermediate layer composed of a Ti(Al,Si)3 matrix and Si-rich precipitates, and a porous layer because of the inter-diffusion between the coating and substrate.

Microstructure and high temperature oxidation resistance of Si-Y co-deposition coatings prepared on TiAl alloy by pack cementation process

In order to improve the high temperature oxidation resistance of TiAl alloy, Y modified silicide coatings were prepared by pack cementation process at 1030, 1080 and 1130 °C, respectively, for 5 h. The microstructures, phase constitutions and oxidation behavior of these coatings were studied. The results show that the coating prepared by co-depositing Si?Y at 1080 °C for 5 h has a multiple layer structure: a superficial zone consisting of Al-rich (Ti,Nb)5Si4 and (Ti,Nb)5Si3, an out layer consisting of (Ti,Nb)Si2, a middle layer consisting of (Ti,Nb)5Si4 and (Ti,Nb)5Si3, and aγ-TiAl inner layer. Co-deposition temperature imposes strong influences on the coating structure. The coating prepared by Si?Y co-depositing at 1080 °C for 5 h shows relatively good oxidation resistance at 1000 °C in air, and the oxidation rate constant of the coating is about two orders of magnitude lower than that of the bare TiAl alloy.

High temperature oxidation behavior of directionally solidified NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho eutectic alloy

The isothermal oxidation behavior of NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho directional eutectic alloy was investigated with the help of scanning electron microscopy and X-ray diffraction.The results revealed that a continuous Al2O3 scale was formed and owned excellent oxidation resistance in the temperature range of 900-1100°C.When the temperature was up to 1150°C,the continuous Al2O3 oxide film ruptured.Trace rare earth element Ho distributed uniformly in the alloy and relatively high level of Al in Cr（Mo）phase are beneficial to the formation of continuous and compact Al2O3 scale.During the oxidation,a phase transformation fromθ-Al2O3 toα-Al2O3 existed on the surface of oxidation film.It resulted in the abnormal oxidation mass gain happening when the alloy was oxidized at 1000°C or 1050°C.

An analytical model to estimate the time delay to reach spontaneous ignition considering heat loss in oil reservoirs

During air injection into an oil reservoir,an oxidation reaction generates some heat to raise the reservoir temperature.When the reservoir temperature reaches an ignition temperature,spontaneous ignition occurs.There is a time delay from the injection to ignition.There are mixed results regarding the feasibility of spontaneous ignition in real-field projects and in laboratory experiments.No analytical model is available in the literature to estimate the oxidation time required to reach spontaneous ignition with heat loss.This paper discusses the feasibility of spontaneous ignition from theoretical points and experimental and field project observations.An analytical model considering heat loss is proposed.Analytical models with and without heat loss investigate the factors that affect spontaneous ignition.Based on the discussion and investigations,we find that it is more difficult for spontaneous ignition to occur in laboratory experiments than in oil reservoirs;spontaneous ignition is strongly affected by the initial reservoir temperature,oil activity,and heat loss;spontaneous ignition is only possible when the initial reservoir temperature is high,the oil oxidation rate is high,and the heat loss is low.

Effects of Al and Mo on high temperature oxidation behavior of refractory high entropy alloys

Refractory high entropy alloys have superior mechanical properties at high temperatures, and the oxidation behavior of these alloys is very important. The present work investigated the high temperature oxidation behavior of three alloys with compositions of TiNbTa0.5Zr, TiNbTa0.5ZrAl and TiNbTa0.5ZrAlMo0.5, and the effects of alloying elements were discussed. Results indicated that the oxidation rates of the TiNbTa0.5Zr and TiNbTa0.5ZrAl alloys are controlled by diffusion, and obey the exponential rule. However, the oxidation rate of the TiNbTa0.5ZrAlMo0.5 alloy is controlled by interface reaction, and obeys the linear rule. The addition of Al leads to a better oxidation resistance by forming a protective oxide scale. However, the protection of Al-rich scale is weakened by the addition of Mo. Extensive pores and cracks occur in the oxide scale of the TiNbTa0.5ZrAlMo0.5 alloy, resulting in a significant decrease in oxidation resistance.

La-doped Pt/TiO_2 as an efficient catalyst for room temperature oxidation of low concentration HCHO

Catalytic oxidation of formaldehyde (HCHO) is the most efficient way to purify indoor air of HCHO pollutant. This work investigated rare earth La‐doped Pt/TiO2 for low concentration HCHO oxidation at room temperature. La‐doped Pt/TiO2 had a dramatically promoted catalytic performance for HCHO oxidation. The reasons for the La promotion effect were investigated by N2 adsorption, X‐raydiffraction, CO chemisorption, X‐ray photoelectron spectroscopy, transmission electron microscopy(TEM) and high‐angle annular dark field scanning TEM. The Pt nanoparticle size was reduced to 1.7nm from 2.2 nm after modification by La, which led to higher Pt dispersion, more exposed activesites and enhanced metal‐support interaction. Thus a superior activity for indoor low concentrationHCHO oxidation was obtained. Moreover, the La‐doped TiO2 can be wash‐coated on a cordieritemonolith so that very low amounts of Pt (0.01 wt%) can be used. The catalyst was evaluated in asimulated indoor HCHO elimination environment and displayed high purifying efficiency and stability.It can be potentially used as a commercial catalyst for indoor HCHO elimination.

Effect of Yttrium on High Temperature Oxidation Resistance ofa Directionally Solidified Superalloy

The effect of rare earth element yttrium on the high temperature oxidation resistance of a directionally solidified Ni-base superalloy was studied with scanning electron microscopy(SEM), energy dispersive spectrum(EDS)and X-ray diffraction(XRD)techniques. The results show that the oxidation resistance of the alloy is substantially improved by adding proper amount of yttrium.

HIGH-TEMPERATURE OXIDATION OF FGH96 P/M SUPERALLOY

High temperature oxidation behaviors of FGH96 P/M superaUoy have been studied in air at temperatures ranging from 600 to 1000℃. By means of isothermal oxidation testing, X-ray diffraction, SEM （scanning electron microscopy）, and EDS （energy dispersive X-ray spectroscopy） analyses, the oxidation kinetics as well as the composition and morphology of scales were investigated. Thermodynamic calculations were used to explain the oxidation mechanism. The results showed that as the oxidation temperature increased, the oxidation rate, the scale thickness, and scale spallation increased. FGH96 P/M superalloy exhibits good oxidation resistance at temperature below 800℃. The oxidation kinetics follows an approximately parabolic rate law, and the oxide layer was mainly composed of Cr2O3 TiO2 and a little amount of NiCr2O4. The oxidation is controlled by the transmission of chromium. titanium, and oxygen through the oxide scale.

An experimental study of the effect of ionic liquids on the low temperature oxidation of coal

Fourier transform infrared spectroscopy(FTIR) and constant heating rate experiments were performed to study the low temperature oxidation of coal treated by an ionic liquid,1-allyl-3-methylimidazolium chloride.The inerting effect of the ionic liquid toward the low temperature oxidation process is discussed.The results show that:(1) The hydroxyl content associated with hydrogen bonds,the aliphatic methyl content,the methylene group content,and the ether oxygen bond content are reduced in the treated coal.At the same time the content of aromatic C@C bonds is constant but these chemical bonds weaken and some substituted aromatic hydrocarbon content increases while other types decrease.This demonstrates that(AMIm)Cl dissolves and destroys the coal surface microstructure;(2) The oxygen consumption of the treated coal is less than what is seen in raw coal.The CO,CO 2,C 2 H 4,and C 2 H 6 content from the treated coal is reduced compared to the untreated coal;(3) The apparent activation energy for the oxidizing reaction is different in the treated and raw coals.Micro-structural changes and macroscopic gas production allow us to conclude that(AMIm)Cl can effectively inhibit low temperature oxidation of coal.

Improvement of thermally grown oxide layer in thermal barrier coating systems with nano alumina as third layer

A thermally grown oxide （TGO） layer is formed at the interface of bond coat/top coat. The TGO growth during thermal exposure in air plays an important role in the spallation of the ceramic layer from the bond coat. High temperature oxidation resistance of four types of atmospheric plasma sprayed TBCs was investigated. These coatings were oxidized at 1000 °C for 24, 48 and 120 h in a normal electric furnace under air atmosphere. Microstructural characterization showed that the growth of the TGO layer in nano NiCrAlY/YSZ/nano Al2O3 coating is much lower than in other coatings. Moreover, EDS and XRD analyses revealed the formation of Ni（Cr,Al）2O4 mixed oxides （as spinel） and NiO onto the Al2O3 （TGO） layer. The formation of detrimental mixed oxides （spinels） on the Al2O3 （TGO） layer of nano NiCrAlY/YSZ/nano Al2O3 coating is much lower compared to that of other coatings after 120 h of high temperature oxidation at 1000 °C.

Protective Effect of the Sputtered TiAlCrAg Coating on the High-Temperature Oxidation and Hot Corrosion Resistance of Ti-Al-Nb Alloy

The effect of a sputtered Ti-48AI-8Cr-2Ag (at. pct) coating on the oxidation resistance of the cast Ti-46.5AI-5Nb (at. pct) alloy was investigated in air at 1000-1100℃. Hot corrosion in molten 75 wt pct Na2SO4+25 wt pct K2SO4 was investigated at 900℃. The scale on the cast TiAINb tends to spall in air, while the scale on coating is very adherent. The sputtered Ti-48AI-8Cr-2Ag coating remarkably improved high temperature oxidation resistance of the cast Ti-46.5AI-5Nb alloy because of the formation of an adherent Al2O3 scale. Due to the inward diffusion of Cr, Kirkendall voids were found at the coating/substrate interface. TiAICrAg coating provided excellent hot corrosion resistance for TiAINb alloy in molten 75 wt pct Na2SO4+25 wt pct K2S04 at 900℃ due to the formation of a continuous Al_2O_3 scale.