Papers by Keyword: Oxide Layer

Abstract: The reason why the failure protective properties of the oxide layer of T91 high temperature superheater tube were analyzed in this study. The microstructure of the oxide layer of T91 high temperature superheater tube was observed by scanning electron microscope (SEM) and the morphological features of it was also analyzed. The concentrations of alloy elements in the section of internal tube were quantitatively analyzed using Energy Dispersive System (EDS). The results showed that the oxide layer of T91 tube can be divided into three layers: inner layer, middle layer and outer layer. The inner layer was formed by chromium rich oxide with compact structure. The middle layer was made up by porous oxide with loose structure. The outer layer was identified as Fe₂O₃. When the content of dissolved oxygen in steam was excessive, the apparent peeling marks will be appeared in the oxide layer of T91 high temperature superheater tube and the distribution of alloy elements in the oxide layer will present obvious proliferation, migration and enrichment phenomenon. Two different mechanisms (steam oxidation mechanism and oxygen oxidation mechanism) will exercise different influences on the structure and protective properties of the oxide layer: when steam contained dissolved oxygen, the oxide layer will be peroxidated by steam and the structure of oxide layer will be broken; When the tube was over-temperature operating, the oxide layer will be oxidated by oxygen.

Abstract: In this paper, based on the analysis of cross-section of steam oxidation of Super304H sample at 600°C, thermal stress contours were obtained by using finite element analysis through establishing geometric model and refine meshing. After extracting the stress in the interface of oxide layer and analyzing the thermal stress, it can be found that there exists compressive and tensile stress in the interface between substrate and scale, which is affected by the slope of the interface.

Abstract: In order to study the states of oxide layer under different pre-dressing process conditions in ultrasonic vibration and electrolytic in-process dressing combined grinding. The effects of electrical parameters, wheel speed, ultrasonic vibration on the oxide layer are investigated with indicators of oxide layer surface morphology and thickness characteristics. The results indicate that the oxide layer thickness increases with the decrease of inter-electrode gap and the increase of duty ratio with pre-dressing time. The effect of ultrasonic vibration cause thinning of the oxide layer. Oxide layer surface morphology allowing processing requirements are obtained by duty cycle of 5μs: 5μs, electrode gap of 0.3mm, wheel speed of 3200 r/min.

Abstract: The oxide layer formed and removed on the surface of the grinding wheel has a significant influence on the grinding quality of ultrasonic-ELID multiplicate grinding. In this paper on the basis of the principles of electrochemical and ultrasonic vibration, the mechanisms of oxide layer formation and remotion were analyzed. Then, the predictive modeling of oxide layer thickness was proposed. The effects of main factors on the oxide film thickness were simulated and analyzed theoretically. The simulation results show that the layer thickness decreases with the decrease of the duty ratio or the grain volume ratio or the ultrasonic frequency ,and the increase of the electrode gap during the composite grinding process.

Abstract: As limited results were reported in terms of the evolution of sliding friction with growth of oxide layer in thickness during running-in, a pin-on-disc wear test was carried out in this study. 4.8Ni-1.5Cr cast iron as core layer and low carbon steel as outer layers, were thermo-mechanically processed via three different routes. For samples with lower hardness due to their predominantly austenitic or martensite retarded matrices, we found that initially rapid increase of thickness of oxide layer continually lowers the sliding friction. However, after the oxide layer was beyond a certain thickness, the sliding friction began to increase consecutively. After a fluctuation of friction caused by the break-down of oxide, a mild equilibrium wear with roughly constant friction followed.

Abstract: NiTi shape memory alloy was potentiostatically anodized in a molybdate electrolyte containing ammoniac ion to form a thick and uniform oxide layer. The oxide layer was characterized by SEM, XRD, EPMA, XPS and potentiodynamic polarization test. It was found that the as-prepared coating is a thick (approx. 40 μm), smooth and dense oxide layer, and is free of cracks and porous defects. Moreover, the surface analyses revealed that the anodic oxidation at 60 mV potential (SCE) reduces significantly the proportion of Ni in the outmost surface layer, and the atomic ratio of Ti and Ni elements is raised from 1:1 to 9.7:1. The as-prepared coating was primarily composed of noncrystalline TiO₂ on the outermost surface with a small quantity of Ni (OH)₂. Potentiodynamic polarization tests in Fusayama artificial saliva pH 6.2 demonstrated that the oxide layer presents a significant increase in breakdown potential due to titanium enrichment.

Abstract: The NiTi alloy (50.6 at.% Ni) passivated for 30 min at 130°C by autoclaving has been studied towards corrosion resistance in aqueous solutions of 3% NaCl, 0.1 M H₂SO₄, 1 M H₂SO₄ and HBSS. Structure and thickness of the passive layer (TiO2, rutile) were examined by X-ray reflectivity method and high resolution electron microscopy. Corrosion behavior of this oxide layer was investigated by open circuit potential method and polarization curves. It was found that the corrosion resistance of the passivated NiTi alloy is strongly dependent on the type of corrosive environment. The higher corrosion resistance of the tested samples was revealed in sulfate solutions as compared to chloride ones. The highest resistance to electrochemical corrosion of the NiTi alloy was observed in 0.1 M H2SO4 solution. Susceptibility to pitting corrosion of the tested samples was observed which increased with the concentration rise of chlorine anions in solution. Electrochemical tests for 316L stainless steel carried out under the same experimental conditions revealed a weaker corrosion resistance in all solutions as compared to the highly corrosion resistant NiTi alloy.

Abstract: The paper presents results of studies on the phase composition, crystallite sizes and lattice deformations of oxide layers formed during a long-term operation on X10CrMoVNb9-1 steel. Test specimens were taken from a live steam pipeline operated at 535°C for 70,000 hours. X-ray studies were carried out on the tube outside surface (on the flue gas side), then the layer’s surface was polished and the diffraction measurements repeated to reveal differences in the originated oxides layer. X-ray phase analysis was performed using a SEIFFERT 3003 T/T X-ray diffractometer, with a cobalt tube of λ_Co = 0.17902nm wavelength. crystallographic database were used for the phase identification.

Abstract: In this paper, we study the oxidation process during the heating of a titanium metallic surface by a Nd-YAG fiber pulsed laser beam under air environment. For this, we adopted an approach that considers a three-dimensional heat diffusion model coupled with an oxidation parabolic law (oxidation kinetics). The heat diffusion equation solved numerically, gives the temperature field. The oxide film growth is simulated by implementing a dynamic mesh technique. We developed computational procedures UDFs (User Defined Function) running interactively with the Fluent fluid dynamics software [ that implements the finite volume method. These UDFs are developed to insert the oxidation law, the temperature field, the specific boundary conditions and the mesh deformation into the calculation.

Abstract: 2 at.% and 4 at.% Mo were added to Co-9Al-9W-2Ta-0.02B alloy to replace W (hereafter referred to as the alloys of 2Mo and 4Mo, respectively; Mo-free alloy was referred to as the 0Mo alloy). The effects of Mo additions on the high temperature oxidation behavior of the alloys at 800 °C in air have been studied. The results indicated that, after oxidation in air at 800 °C for 100 h, the oxide film of the 0Mo alloy remained intact, but the cracking and spallation of the oxide film took place in the alloys of 2Mo and 4Mo. Oxidation kinetic curves revealed weight gain per unit area of the 0Mo alloy was 36.86 mg·cm^-2, which was lower than that of the alloys of 2Mo (65.16 mg·cm^-2) and 4Mo (48.54mg·cm^-2). These suggested that the 0Mo alloy displayed superior oxidation resistance compared to the alloys of 2Mo and 4Mo caused by the formation of volatile MoO₃ oxide, and sharp compressive stress formed in the outer layer during the oxidation. The oxide layer was composed of three layers of the Co₃O₄ + CoO outer layer, middle complex oxide layer containing Co, Al and W (Mo), inner Al₂O₃ layer and γ/Co₃W zone adhere to the γ/γ' substrate.