What If the Protection against Oxidation of Chromia-Forming Alloys Was Not Always Due to the Chromia Layer?

Chromia-forming alloys have good resistance to oxidizing agents such as O2, CO2, … It is accepted that the protection of these alloys is always due to the chromia layer formed at the surface of the alloys, which acts as a barrier between the oxidizing gases and the alloy substrates, forming a diffusion zone that limits the overall reaction rate and leads to parabolic kinetics. But this was not verified in the study devoted to Inconel®625 the oxidation in CO2 that was followed by TGA, with characterizations by XRD, EDS and FIB microscopy. Contrary to what was expected and accepted in similar studies on other chromia-forming alloys, it was shown that the diffusion step that governs the overall reaction rate is not located inside the chromia layer but inside the alloy, precisely inside a zone just beneath the interface alloy/chromia, this zone being depleted in chromium. The chromia layer, therefore, plays no kinetic role and does not directly protect the underlying alloy. This result was demonstrated using a simple test that consisted in removing the chromia layer from the surface of samples partially oxidized and then to continue the thermal treatment: insofar as the kinetics continued without any change in rate, this proved that this surface layer of oxide did not protect the substrate. Based on previous work on many chromia-forming alloys, the possibility of a similar reaction mechanism is discussed. If the chromia layer is not the source of protection for a number of chromia-forming alloys, as is suspected, this might have major consequences in terms of industrial applications.

Effect of SiC whiskers on the oxidation protective properties of SiC coatings for carbon/carbon composites

In order to effectively employ the unique high temperature mechanical properties of carbon/carbon composite substrates, SiC coatings reinforced by SiC whiskers were prepared by pack cementation method. The effect of SiC whiskers on the oxidation resistance properties of the single-layer coating and double-layer coating was investigated. SiC whiskers in the single-layer SiC coating have little effect on the anti-oxidation property but obviously improve the thermal shock property. The double-layer coating with inner-layer reinforced coating exhibits more perfect anti-oxidation ability than the double-layer coating with SiC inner-layer coating.

Influence of a Protective Coating Slurry on Enhancing the Descaling Ability and Oxidation Resistance of 9% Nickel Steel

A kind of ceramic slurry was prepared and sprayed onto the surface of 9Ni steel at room temperature. The coating layer will not only reduce the depth of the formed Ni-enriched entanglement at high temperature but also have an excellent ability to resist oxidation of the 9Ni steel. Compared to bare specimen, the depths of the entanglement of the coated 9Ni specimen could be successfully reduced by 74.1% and the oxidation loss be decreased by 62.3% by heating at 1 250 ℃ for 60 min. In addition, the coated specimen indicates no trace of oxide pegs. It proves that the coating has outstanding improvement to internal oxidation resistance. Some characterization methods such as metalloscopy, XRD, XPS, SEM and EDX have been used to reveal a possible protective mechanism. The result shows that the coating layer reacts with the iron oxide to form Mg Fe2O4 on the surface of the coated specimen, which could provide a smaller diffusion coefficient rate of Fe ion. The coating with a low cost and easy implementation is promisingly applicable in the slab-reheating process of the 9Ni steel.

Achieving oxidation protection effect for strips hot rolling via Al_(2)O_(3) nanofluid lubrication

It was discovered the application of Al_(2)O_(3) nanofluid as lubricant for steel hot rolling could synchronously achieve oxidation protection of strips surface.The underlying mechanism was investigated through hot rolling tests and molecular dynamics (MD) simulations.The employment of Al_(2)O_(3) nanoparticles contributed to significant enhancement in the lubrication performance of lubricant.The rolled strip exhibited the best surface topography that the roughness reached lowest with the sparsest surface defects.Besides,the oxide scale generated on steel surface was also thinner,and the ratio of Fe_(2)O_(3) among various iron oxides became lower.It was revealed the above oxidation protection effect of Al_(2)O_(3) nanofluid was attributed to the deposition of nanoparticles on metal surface during hot rolling.A protective layer in the thickness of about 193 nm was formed to prevent the direct contact between steel matrix and atmosphere,which was mainly composed of Al_(2)O_(3) and sintered organic molecules.MD simulations confirmed the diffusion of O_(2) and H_(2)O could be blocked by the Al_(2)O_(3) layer through physical absorption and penetration barrier effect.

OXIDATION RESISTANCE AND SELF-SEALING PROPERTY OF CERAMIC COATINGS FOR C/C COMPOSITES

The oxidation behavior and damage mechanism of modifiers and compounds of many types of ceramic coatings were investigated experimentally. A MoSi 2/SiC coating was produced by infiltration process. The oxidation behavior of the coated C/C composites was studied at various temperatures below 1650℃. The oxidation results showed that the MoSi 2/SiC coating for thermal protection of C/C composites has high oxidation resistance at temperature up to 1650℃. In the present work, a new model of an oxidation protective, self sealing multi layer coating system was proposed for C/C composites. The multi layer coating possessing the self sealing property was obtained by pack cementation and infiltration process. The protection coating system for C/C composites consists of an inner layer of SiC and an outer layer of porous refractory oxides filled by modified SiO 2 glass. Isothermal and cyclic thermal oxidation tests showed the multi layer coating was capable of protecting the C/C composites in an oxidizing atmosphere at temperature up to 1800℃.

Preparation and Microstructure of a Si-Mo Fused Slurry Coating on Carbon/Carbon Composites for Oxidation Protection

A coating of composition Si-40Mo (wt pct) was prepared by fused slurry coating method on the two-dimensional carbon/carbon (2D-C/C) composite to improve oxidation resistance. In the procedure of the fabrication, pure St slurry inner layer in the pre-coating was necessary to apply because of infiltration of liquid Si into the substrate during the sintering. The coating consists of Si continuous phase and MoSi2 particles. In addition, the infiltration of Si into the substrate and the SiC reaction layer between the coating and the C/C composite were observed. Oxidation behavior of coated and uncoated C/C composites was studied in cyclic mode. The oxidation resistance and the thermal shock resistance of the Si-Mo fused slurry coating were quite excellent at 1370℃.

Neuroprotective effects of ganoderma lucidum polysaccharides against oxidative stress-induced neuronal apoptosis

Ganoderma lucidum polysaccharides have protective effects against apoptosis in neurons exposed to ischemia/reperfusion injury, but the mechanisms are unclear. The goal of this study was to investigate the underlying mechanisms of the effects of ganoderma lucidum polysaccharides against oxidative stress-induced neuronal apoptosis. Hydrogen peroxide（H_2O_2） was used to induce apoptosis in cultured cerebellar granule cells. In these cells, ganoderma lucidum polysaccharides remarkably suppressed H_2O_2-induced apoptosis, decreased expression of caspase-3, Bax and Bim and increased that of Bcl-2. These findings suggested that ganoderma lucidum polysaccharides regulate expression of apoptosis-associated proteins, inhibit oxidative stress-induced neuronal apoptosis and, therefore, have significant neuroprotective effects.

SiC/Si-W-Mo coating for protection of C/C composites at 1873 K

In order to prevent carbon/carbon composites from oxidation at 1873 K, an efficient oxidation protective SiC/Si-W-Mo coating was prepared by a two-step pack cementation technique. The microstructures and the phase composition of the as-received multi-coating were examined by scanning electron microscopy （SEM） and X-ray diffraction （XRD）. It is seen that the compact multi-coating is composed of α-SiC, Si, and （WxMO1-x）Si2. Oxidation test shows that, after oxidation at 1873 K in air for 102 h and thermal cycling between 1873 K and room temperature for 10 times, the weight loss of the SiC/Si-W-Mo coated C/C composites is only 0.26%. The invalidation of the multi-coating is attributed to the formation of penetrable cracks in the coating. 2008 University of Science and Technology Beijing. All rights reserved.

CYP2C8-derived Epoxyeicosatrienoic Acids Decrease Oxidative Stress-induced Endothelial Apoptosis in Development of Atherosclerosis:Role of Nrf2 Activation

The aim of the present study is to investigate how cytochrome P450 enzymes（CYP） 2C8-derived epoxyeicosatrienoic acids（EETs） regulate the nuclear factor erythroid 2-related factor 2（Nrf2） signaling pathway and protect against oxidative stress-induced endothelial injuries in the development and progression of atherosclerosis. In this study,cultured human umbilical vein endothelial cells（HUVECs） were transfected with CYP2C8 or pretreated with exogenous EETs（1 μmol/L） before TNF-α（20 ng/m L） stimulation. Apoptosis and intracellular ROS production were determined by flow cytometry. The expression levels of ROS-associated NAD（P）H subunits gp91 and p47,the anti-oxidative enzyme catalase（CAT）,Nrf2,heme oxygenase-1（HO-1） and endothelial nitric oxide synthase（e NOS） were detected by Western blotting. The results showed that CYP2C8-derived EETs decreased apoptosis of HUVECs treated with TNF-α. Pretreatment with 11,12-EET also significantly blocked TNF-α-induced ROS production. In addition,11,12-EET decreased oxidative stress-induced apoptosis. Furthermore,the ability of 11,12-EET to protect cells against TNF-α-induced apoptosis via oxidative stress was abrogated by transient transfection with Nrf2-specific small interfering RNA（si RNA）. In conclusion,CYP2C8-derived EETs prevented TNF-α-induced HUVECs apoptosis via inhibition of oxidative stress associated with the Nrf2 signaling.

Hyperbaric oxygen preconditioning improves postoperative cognitive dysfunction by reducing oxidant stress and inflammation

Postoperative cognitive dysfunction is a crucial public health issue that has been increasingly studied in efforts to reduce symptoms or prevent its occurrence. However, effective advances remain lacking. Hyperbaric oxygen preconditioning has proved to protect vital organs, such as the heart, liver, and brain. Recently, it has been introduced and widely studied in the prevention of postoperative cognitive dysfunction, with promising results. However, the neuroprotective mechanisms underlying this phenomenon remain controversial. This review summarizes and highlights the definition and application of hyperbaric oxygen preconditioning, the perniciousness and pathogenetic mechanism underlying postoperative cognitive dysfunction, and the effects that hyperbaric oxygen preconditioning has on postoperative cognitive dysfunction. Finally, we conclude that hyperbaric oxygen preconditioning is an effective and feasible method to prevent, alleviate, and improve postoperative cognitive dysfunction, and that its mechanism of action is very complex, involving the stimulation of endogenous antioxidant and anti-inflammation defense systems.

Corrosion of candidate materials for supercritical water-cooled reactor

Supercritical water reactor(SCWR) was proposed as a GenerationⅣconcept for building large capacity nuclear power plants.Comparing with the present GenerationⅡandⅢlight water reactors,SCWR possesses great advantages of 10%higher efficiency,simpler system design,better sustainability,and so on. However,the selection of materials for fuel cladding and reactor internals of SCWR is facing a great challenge. Corrosion in supercritical steam is of the first important issue to be solved to meet the stringent requirement of the reactor internal components.Corrosion screening tests were conducted on candidate materials for nuclear fuel cladding and reactor internals of supercritical water reactor(SCWR) in static and re-circulating autoclave at the temperatures of 550,600 and 650℃,pressure of about 25 MPa,deaerated or saturated dissolved hydrogen(STP). Nickel base alloy type Hastelloy C276,austenitic stainless steels type 304NG,AL-6XN,HR3C.NF709 and SAVE 25,ferritic/martensitic(F/M) steel type P92,P122 and 410,and oxide dispersion strengthened steel MA 956,are tested.This paper presents corrosion rate,and focuses on the formation and breakdown of corrosion oxide film,and proposes the future trend for the development of SCWR internal structure materials.

Preparation of Air-Stable Iron-Oxide-Coated Metallic Iron Nanoparticles

The α-Fe nanoparticles with oxide shell were prepared by the complete reduction of iron oxide ones with hydrogen, followed by the selective surface oxidation as a thin layer. As-treated α-Fe nanoparticles preserved their saturation magnetization and metal α-Fe phase for at least 80 days in the air. In comparison, the unstabilized α-Fe nanoparticles protected only by their silica shell were oxidized instantly in the air and the saturation magnetization of the unstabilized α-Fe nanoparticles was decreased drastically. Since the stabilization procedure was carried out under dry conditions using silica-coated iron oxide nanoparticles as precursors, it could be applied to nanoparticles of various sizes and shapes to obtain a stable α-Fe phase.

Mechanism of the protective effects of noninvasive limbs preconditioning on myocardial ischemia-reperfusion injury

Background This study aimed at assessing the effect of noninvasive limb preconditioning on myocardial infarct size, and determining whether nitric oxide and neurogenic pathway play an important role in the mechanism of acute remote ischemic preconditioning （IPC）.Methods Forty Wistar rats were randomly divided into four experimental groups. In Group I , the rats underwent 30-minute occlusion of the left anterior descending coronary artery, and 120-minute reperfusion. In Group PL, the rats underwent four cycles of 5-minute occlusion and reperfusion of both hind limbs using a tourniquet before the experiment was continued as in Group I. In Group PL-N and Group PL-,, we administered L-nitro-arginine methyl ester （L-NAME） 10 mg/kg or hexamethonium chloride 20 mg,/kg intravenously, 10 minutes before IPC. Infarct size as a percentage of the area at risk was determined by triphenyhetrazolium chloride staining.Results There were no statistically significant differences in mean arterial pressure and heart rate among these groups at any time point during the experiment （ P〉0. 05 ）. The myocardial infarct size （IS） was decreased significantly in Group PL and Group PL-U compared with Group I , and the IS/AAR was 34. 5%± 7.6%, 35.9%±8.6% and58.5%±8.5%, respectively （P〈0.05）. The IS/AAR was 49.1%±6.5% in Group PEN, and there was no significant difference compared with Group I （P〉0. 05 ）.Conclusions Noninvasive limb IPC is effective in protecting the myocardium from ischemia reperfusion injury. Nitric oxide plays an important role in the mechanism of acute remote IPC, in which the neurogenic pathway is not involved.

Plasma electrolytic oxidation of the magnesium alloy MA8 in electrolytes containing TiN nanoparticles

The formation of protective multifunctional coatings on magnesium alloy MA8 using plasma electrolyt- ic oxidation （PEO） in an electrolytic system containing nanosized particles of titanium nitride was investigated. Electrochemical and mechanical properties of the obtained layers were examined. It was established that microhardness of the coating with the nanoparticle concentration of 3 gl-1 increased twofold （4.2 ± 0.5 GPa）, while wear resistance decreased （4.97 × 10-6 mm3 N-1 m-1）, as compared to re- spective values for the PEO-coating formed in the electrolyte without nanoparticles （2.1 ± 0.3 GPa, 1.12 × 10.5 mm3 N-1 m-1）.

A review on molten sulfate salts induced hot corrosion

Hot corrosion has been extensively observed in various high temperature applications,which might cause large economic losses.To deepen the insight and understanding of hot corrosion,herein,we provide a detailed discussion on hot corrosion induced by molten sulfate salt,in which Na_(2)SO_(4)is the main chemical reactant.The hot corrosion is triggered and sustained by the negative solubility gradient of protective oxide from the oxide/salt interface to salt/gas interface.In this work,we reviewed several key factors,including temperature,gas composition,molten salt composition,alloy element and external stress,influencing the hot corrosion of alloy beneath the molten salt.The application of anti-corrosion coating seems to be the most effective and commercial mothed to mitigate hot corrosion.Therefore,the progress of the development of effective coatings,e.g.,Ni-Cr coatings,aluminide coatings,silicide coatings and MCrAlY(X)coatings,has also been summarized.

Preparation of spherical Y_2SiO_5 powders for thermal-spray coating

Yttrium silicate, for its high oxidation resistance, is an important candidate for protective coating for carbon-fiber-reinforced composites at temperatures above 1600 ℃. A novel method, consisting of coprecipitation, spray-drying, heat-treatment and plasma-densification, is developed to prepare Y2SiO5 powders for thermal-spraying. The composition, morphology and flowability of the synthesized Y2SiO5 powders are investigated by XRD, SEM and Hall Flowmeter, respectively. The results show that the synthesized Y2SiO5 powders are nearly spherical with high purity. The apparent density and flowability of the Y2SiO5 powders are 1.87 g/cm^3 and 37 s/50 g, respectively, which lead to a high deposition efficiency of up to 80700 for atmospheric plasma spraying.