Experimental Investigation of Nucleate Boiling on a Thermal Capacitive Heater Under Variable Gravity Conditions

In the present work the behavior of single vapor bubbles of FC-72, generated on a thermal capacitive heater element, has been investigated during microgravity. A newly developed heater design allows temperature measurements by highspeed infrared thermography on the backside of the heater surface at a distance of approx. 800 nm from the fluid/heater-interface. The employed heater was manufactured by Physical Vapor Deposition (PVD) of a chromium based layer for better emissivity (Slomski et al., Mater Sci Technol 41:161?C165, 2010) and a pure chromium heating layer supplying the energy required for bubble generation and sustainment by electrical heating. The thermal diffusivity of the employed Calcium Flouride (CaF) heater substrate is comparable to the thermal diffusivity of stainless steel, which makes this heater design very close to technical applications. The acquired transient temperature fields of the heater surface allow numerical determination of the local heat flux from the heater surface to the fluid. A local temperature drop and high heat fluxes have been observed in the vicinity of the 3-phase contact line. This effect has already been reported by former publications for thin stainless steel foil heaters (Stephan and Hammer, Int J Heat Mass Transfer 30:119?C125, 1994; Wagner et al., Int J Heat Mass Transfer 42:875?C883, 2006) and is also confirmed for heaters with significantly higher thermal capacities.     

掺银离子TiO2薄膜的结构及与不锈钢基板的界面反应

采用溶胶-凝胶法在不锈钢表面制备掺银的TiO2薄膜,研究氧化处理对不锈钢表面Ag/TiO2抗菌薄膜组成和性能的影响,利用X-射线衍射谱(XRD)、扫描电子显微镜(SEM)和X-射线光电子能谱(XPS)等对经氧化处理和未经处理的不锈钢表面Ag/TiO2薄膜进行比较,研究薄膜的结构、界面反应特征及机制.发现:未经氧化处理的不锈钢由于其中的铁原子进入薄膜与TiO2反应形成钛酸铁固溶体薄膜;经氧化处理后不锈钢表面形成一层比较致密的氧化铁层,阻止了不锈钢内部的铁原子扩散进入薄膜与TiO2反应,表面同时形成钛酸铁和锐钛矿型的TiO2薄膜;锐钛矿型的TiO2与银离子反应形成钛酸银,既保持了银离子的高杀菌性能(样品的6小时抗菌率达到100%),同时又不至使金属银形成而使材料变色.在未经氧化处理的不锈钢表面,掺银TiO2薄膜中的银离子转化为单质银,抗菌性能低.     

304不锈钢表面化学钝化膜对Sn粘附行为的影响

为了研究化学酸洗钝化在低熔点金属Sn与304不锈钢粘附过程中的作用,通过浸泡腐蚀实验分析了液态Sn与U型弯曲后的酸洗钝化不锈钢的交互作用行为,探讨了Sn粘附对不锈钢基体浸泡腐蚀性能的影响.实验结果表明:Sn与304不锈钢相互作用在界面处形成了片状(Fe,Cr)Sn2化合物冶金层,酸洗钝化处理改变了冶金结合,使液态Sn与304不锈钢界面成为直接物理接触;U型弯曲破坏了钝化膜的完整性,未能阻止Sn与不锈钢的界面冶金结合,但降低了界面化合物层的厚度.浸泡腐蚀实验结果表明,Sn粘附层促进了不锈钢基体腐蚀.     

Corrosion behaviour of nanocrystallines 304 stainless steel in simulated secondary side environment

Surface nanocrystallisation (SNC) was prepared on the top-surface layer of 304 stainless steel by ultrasonic surface rolling treatment (USRT). Meanwhile, martensite transformation also occurred during grain refinement, and the surface microhardness increased more than tripled after USRT. The corrosion resistance of nanocrystallines (NC) 304 stainless steel was notably better than that of untreated samples in simulated pressurised water reactor (PWR) secondary water by potentiodynamic polarisation curves, electrochemical impedance spectroscopy and Mott-Schottky curves. Meanwhile, the corrosion mechanism was discussed before and after USRT by X-ray photoelectron spectroscopy (XPS). The main reason is due to the formation of denser Cr(OH)₃ protective film and the reduction of the porous structure Fe₂O₃ content in the NC 304 stainless steel.     

Preparation and magnetic characterization of core–shell structure stainless steel/silica nanoparticles

SiO₂-coated martensite stainless steel nanoparticles were prepared using wire electrical explosion technique combined with sol–gel technique, and their structural and magnetic properties were studied. The coating silica on stainless steel nanoparticles was based on the use of silane coupling agent 3-mercaptopropyltrimethoxysilane (HS-(CH₂)₃Si(OCH₃)₃, MPTS) as a primer to render the stainless steel surface vitreophilic, thus rendering stainless steel surface compatible with silica. The control over the silica coating layer thickness can be achieved by varying the reaction time. For stainless steel nanoparticles, their saturation and remnant magnetizations decreased upon silica coating, and their saturation magnetizations obviously decreased with increasing the thickness of SiO₂ coating layer. These stainless steel/silica core–shell nanoparticles can be utilized as precursors for making property-tunable magnetic nanoparticles, thin films, and multilayered core–shell structure nanocomposites.     

Characterization of Al rich oxide layers on austenitic stainless steels by fluorescence spectroscopy

Abstract

An Al rich oxide passivation technique has been developed to improve the corrosion resistance of the stainless steel to ozone added ultra-pure water. Fluorescence spectroscopy was applied to the study on the selective oxidation of aluminum containing austenitic stainless steel in low oxygen atmosphere at 1,353K. It was found that in the thermal oxidation under low oxygen pressure, the minor alloying constitution of Al resulted in the formation of thin oxide layers. The frequency shifts of fluorescence spectra show that the compressive stresses exist in the oxide layers as a result of the difference of the thermal expansion coefficients between substrate steels and α-Al₂O₃, and depends on the thickness of the oxide layers. It is confirmed that pure α-Al₂O₃ protective layers grown on the stainless steels, which remain stable and attached to the stainless steels in ozone added ultra-pure water. These act as a diffusion barrier and protect the stainless steels from the metal dissolution.     

Evolution of the metal–oxide interface during the initial stage of the high temperature oxidation of ferritic stainless steels

Abstract

Two grades of ferritic stainless steel, a bi-stabilised Ti, Nb (AISI 441) and a stabilised Ti (AISI 439), were oxidised at 1060°C under the simulated process atmosphere for durations between 45 and 1800 s. Focused ion beam coupled with field emission gun and scanning electron microscopy was carried out to investigate the cross-section morphology of the oxide growing on ferritic stainless steels. Matrix protrusions localised at the metal – chromia interface through the silica layer are observed and the following mechanism for their formation is proposed. During the first step of oxidation, interface undulation, induced by growth stresses, in combination with silica precipitation at the metal – oxide interface lead to the formation of matrix protrusions in the chromia layer. For an increased oxidation time, due to the laterally silica growth matrix protrusions are trapped into the Cr₂O₃ layer as matrix inclusions.     

Development of a modified CVD coating process for the enhancement of the high temperature oxidation resistance of Cr2O3 and Al2O3 forming alloys

In this work, a new deposition method for coating Cr₂O₃ and Al₂O₃ forming alloys has been set up. This is a modified CVD deposition method based on the pyrolisis of an aerosol produced by an ultrasonic wave focussed on the surface of a solution containing a reactive element soluble salt that will later transform to an oxide. The deposition takes place in a reaction chamber maintained at 473 K. To test the efficiency of the deposition method, commercial AISI-304 stainless steel samples were coated directly on the mill finish surface and on a pre-formed oxide layer with lanthanum nitrate. Specimens were then oxidised at 1173 K in synthetic air atmosphere during 25 h in a thermobalance. Oxidation kinetics of coated samples was compared with that of untreated ones. The results show that coated samples improve their high temperature oxidation resistance by decreasing their parabolic rate constant by one order of magnitude. The deposition method developed proves to be an easy, cheap and efficient way of increasing the service temperature of conventional stainless steel.     

Behaviour of 316 Ti stainless steel in deuterium oxide with chloride

Since tritiated water contains deuterium oxide, we require a better understanding of stainless steel corrosion in tritiated water and thus we have compared the behaviour of 316 Ti stainless steel in ²H₂O and H₂O with and without chloride. This was done by anodic polarization curves, cyclic voltammetry and electrochemical impedance spectroscopy. The corrosion potential of 316 Ti stainless steel in deuterium oxide changes and is related to pH modification due to the dissociation constant of this aqueous medium which shows the importance of pH in passivity. Without chloride, the insulating properties of the passive oxide layer depending on the pH and passive potentials are enhanced with ²H₂O. With deuterium oxide containing chloride at near neutral pH, the repassive potential is lower than that obtained with H₂O, consequently localized corrosion in grain boundaries and pit propagation, which lead to crevice corrosion, are greater. The critical pitting potential is in transpassivity indicating that pitting is less likely to occur. Comparison with and without Cl^- for the passive potentials near the corrosion potential, shows that although chloride reduces the insulation provided by the passive oxide layer it is still greater than that obtained with H₂O. This revised version was published online in November 2006 with corrections to the Cover Date.     

不锈钢/高硼不锈钢层状复合材料组织变化

为了解决高硼不锈钢材料变形难度大、易开裂、塑性低等问题,采用复合浇铸-热轧变形工艺,制备了以难变形金属高硼不锈钢为中间层的层状复合板,研究了复合材料在铸造、热轧和固溶处理各阶段的组织特点及芯层中硼化物的相组成规律.结果表明:含硼质量分数2%～2.5%的复合板芯层的铸态组织主要以共晶组织形式凝固,含硼较高的芯层铸态组织除以共晶组织形式凝固外,还形成大块的含有Cr2B和Fe2B的过共晶硼化物相;热变形使共晶硼化物发生破碎、细化,但过共晶硼化物的体积形状变化不大;固溶处理使覆层中细小的二次析出物明显减少,这有利于复合板的力学性能,但芯层中硼化物的形貌、数量及尺寸变化不大.     

Study of the characteristics of duplex stainless steel activated tungsten inert gas welds

The purpose of this study is to investigate the effects of the specific fluxes used in the tungsten inert gas (TIG) process on surface appearance, weld morphology, angular distortion, mechanical properties, and microstructures when welding 6 mm thick duplex stainless steel. This study applies a novel variant of the autogenous TIG welding, using oxide powders (TiO₂, MnO₂, SiO₂, MoO₃, and Cr₂O₃), to grade 2205 stainless steel through a thin layer of the flux to produce a bead-on-plate joint. Experimental results indicate that using SiO₂, MoO₃, and Cr₂O₃ fluxes leads to a significant increase in the penetration capability of TIG welds. The activated TIG process can increase the joint penetration and the weld depth-to-width ratio, and tends to reduce the angular distortion of grade 2205 stainless steel weldment. The welded joint also exhibited greater mechanical strength. These results suggest that the plasma column and the anode root are a mechanism for determining the morphology of activated TIG welds.     

Synthesis and electrochemical characterization of porous niobium oxide coated 316L SS for orthopedic applications

Niobium oxide was prepared using sol–gel process and coated on 316L stainless steel (SS) substrate via dip-coating technique. The surface characterization results after a thermal treatment revealed that the coated surface was porous, uniform and well crystalline on the substrate. The corrosion resistance and bioactivity of the porous niobium oxide coated 316L SS in simulated body fluid (SBF) solution was evaluated. The in vitro test revealed a layer of carbonate-containing apatite formation over the coated porous surface. The results indicated that the porous niobium oxide coated 316L SS exhibited a high corrosion resistance and an enhanced biocompatibility in SBF solution.     

Influence of ion implantation on the microstructure of oxide scales formed on a 20Cr/25Ni/Nb-stabilized stainless steel in carbon dioxide at 825° C

Specimens of a stainless steel (20%Cr, 25%Ni stabilized with niobium and also containing 0.9% Mn and 0.6% Si) implanted with lanthanum to a dose of 10¹⁷ ion cm^–2 , together with unimplanted specimens, have been oxidized in carbon dioxide at 825° C for times up to 9735 h. Transverse sections through the oxide scales formed on the respective specimens have been studied by analytical electron microscopy. After this exposure the scale on the unimplanted 20/25/Nb stainless steel consists of an outer, large-grained, spinel layer, a middle fine-grained Cr₂O₃ layer and an inner, discontinuous silicon rich, niobium and chromium bearing, amorphous layer. The main effects of the lanthanum implantation are to improve oxidation resistance and maintain scale adherence during thermal cycling. The microstructural changes in the scale formed on the lanthanum implanted 20/25/Nb steel include finer Cr₂O₃ oxide grains and an intermediate region between the outer spinel and inner Cr₂O₃ layers comprised of both oxides. The lanthanum concentrates in this region and appears to act as a marker due to its low diffusivity. Mechanisms of scale development on the 20/25/Nb stainless Red and the influence of lanthanum implantation are discussed.     

Study of the reaction of lanthanum nitrate with metal oxides present in the scale formed at high temperatures on stainless steel

The reaction of lanthanum nitrate with metallic oxides that can be present in the oxide scales formed at elevated temperatures on the surface of stainless steel has been investigated as model systems of the processes occurring during the oxidation of lanthanum coated stainless steel. X-ray diffraction (XRD) experiments have shown that LaCrO₃ and LaFeO₃ are the most stable compounds. XRD, chemical analysis and thermogravimetric experiments have demonstrated that in the case of Cr₂O₃, nitrate anions are able to oxidize Cr(III) to Cr(VI) resulting in a precursor phase of perovskite structure that influences the corrosion inhibition of stainless steel at high temperatures.     

The corrosion of INCONEL alloy 740 in simulated environments for pulverized coal-fired boiler

The corrosion of a new nickel base superalloy, INCONEL alloy 740, has been studied at 550 and 700 °C on exposure to the synthetic coal ash/flue gas environments by means of XRD, SEM, and EDX. Low temperature hot corrosion of the new alloy occurred at two temperatures. The corrosion started to form the thin Cr₂O₃ scale on the alloy at 550 °C and developed as pitting attack resulted from sulfidation. The frontal attack at 700 °C consisted of two successive stages in which the corrosion mechanism started from the sulfidation and ended up in the fluxing of oxide. The compact and protective Cr₂O₃ scale formed and the internal sulfidation took place during the initial stage. The severe hot corrosion occurred due to the presence of the molten CoSO₄ during the propagation stage. The loose and porous outer layer and the compact inner layer consisted of spinels and oxides, respectively. The sulfides of Cr, Ti, and Nb formed on the front of oxide scale and in Cr-depletion zone. The rapid degradation of corrosion resistance of the alloy can be attributed to the dissolution of both cobalt and cobalt oxide on the surface. The alloy of 25% Cr exhibited better resistance to coal ash/flue gas corrosion as compared to the alloy of 23% Cr in the present case.     

A study on the diffusion kinetics of borides on boronized Cr-based steels

In the present study, kinetics of borides formed on AISI H13 hot work tool and AISI 304 stainless steels have been investigated. Boronizing treatment was carried out in slurry salt bath consisting of borax, boric acid and ferrosilicon at temperature range of 1073–1223 K for 3, 5 and 7 h. X-ray diffraction analysis of boride layers on the surface of steels revealed various peaks of FeB, Fe₂B, CrB and Ni₃B. Metallographic studies revealed that the boride layer has a flat and smooth morphology in the 304 steel while H13 steel was a ragged morphology. Depending on temperature and layer thickness, the activation energies of boron in 304 and H13 steels were found to be 253.35 and 244.37 kJ mol⁻¹, respectively.     

EXAFS investigation of low temperature nitrided stainless steel

Low temperature nitrided stainless steel AISI 316 flakes were investigated with EXAFS and X-ray diffraction analysis. The stainless steel flakes were transformed into a mixture of nitrogen expanded austenite and nitride phases. Two treatments were carried out yielding different overall nitrogen contents: (1) nitriding in pure NH₃ and (2) nitriding in pure NH₃ followed by reduction in H₂. The majority of the Cr atoms in the stainless steel after treatment 1 and 2 was associated with a nitrogen–chromium bond distance comparable to that of the chemical compound CrN. The possibility of the occurrence of mixed substitutional–interstitial atom clusters or coherent nitride platelets in nitrogen-expanded austenite is discussed.

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Degradation behavior of a commercial 13Cr ferritic stainless steel (SS405) exposed to an ambient air atmosphere for IT-SOFC interconnect applications

The objective of this work was to verify the feasibility of using low Cr ferritic stainless steels to improve IT-SOFC interconnect properties in terms of electrical conductivity and Cr evaporation resistance by taking advantage that low Cr ferritic stainless steels may form conductive and moisture-stable oxide layers of chromite spinels. In this context, surface degradation of a commercial 13Cr ferritic stainless steel (SS405) was studied at 700 °C in ambient air (containing ca. 3% H₂O by volume) for up to nearly 5000 h. The results showed a slow, nearly linear oxidation rate indicating that the oxide scale could not act as a fully protective barrier during a prolonged exposure at 700 °C. Oxidation-induced degradation was mainly due to the effects of Cr depletion in the subscale matrix region and internal oxidation attack. However, analysis of the oxide scale after long-term exposure revealed the presence of an interesting example of all-spinel structure consisting of outer layer of iron-substituted Mn–Cr spinel particles on the top of an inner and continuous Fe–Cr spinel layer. Since SS405 steel contains also Al and Si as minor additions, the distribution of these elements in the spinel oxide scale was studied and found to be rather different. Whereas Si was present exclusively in the inner oxide regions, Al migrated also to the outermost part of the oxide scale. Possible explanations for this unexpected Al surface enrichment are discussed.     

Pits with coloured halos formed on 1Cr18Ni9Ti stainless steel surface after ennoblement in seawater

Micropits surrounded by coloured halos were observed under incident-light microscope on 1Cr18Ni9Ti stainless steel surface after “ennoblement” in seawater. Ennoblement has been attributed to biofilm formation on stainless steel surface in seawater. In this study, the environment in biofilm for ennoblement was simulated by adding H₂O₂ into seawater at concentrations that were reported to detect in marine biofilm [N. Washizu, Y. Katada, T. Kodama, Corros. Sci.46 (2004)1291.]. H₂O₂ increased the passivity of stainless steel in seawater, but this passivation was not uniform. The probability of pitting corrosion was increased after ennoblement. Equal thickness interference on the deposition film around the pits was believed to be the reason for the coloured fringes. It is conceivable that haloed pits on the stainless steel surface are the characteristic morphological indications for pitting corrosion formed under ennoblement condition in seawater. Examinations on the microbial and structural effects of the biofilm were not included in this study.     

Effects of atomic layer deposited HfO2 compact layer on the performance of dye-sensitized solar cells

The performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of photogenerated electrons from the photoelectrode back into liquid electrolyte. An atomic layer deposited (ALD) hafnium oxide (HfO₂) ultra thin compact layer was grown on the surface of the transparent conducting oxide (TCO) and its effects on the DSSC performance were studied with dark and illuminated current-voltage and electrochemical impedance spectroscopy measurements. It was found that this compact layer was effectively blocking the back-reaction of electrons from TCO to the liquid electrolyte, resulting in the overall photoconversion efficiency being enhanced by 66% compared to a DSSC with a conventional sol-gel processed TiO₂ compact layer. Reasons for the improved photovoltaic performance were attributed to passivation of the TCO surface, better electronic quality of the compact layer material and the higher compactness, shown by atomic force microscopic images, obtained from gas-based deposition methods. Also, an increased short-circuit current density suggests that the interfacial resistance for the injection of electrons from the porous nanoparticle network to TCO was reduced. Further, the theory of electron recombination at the TCO/compact layer/electrolyte interface was developed and used to explain the improved DSSC performance with an ALD HfO₂ compact layer.