Preparation and Anti-coking Property of SiO2/S Coating on HP40 Tube

SiO2/S coating was prepared on the inner surface of an HP40 tube using dimethyldisulfide and tetraethylorthosilicate by atmospheric pressure chemical vapor deposition (APCVD) to alleviate catalytic coking on the inner surface of radiant tube for ethylene production in petrochemical plants. The comparative coking experiments with the coated and uncoated HP40 tubes were carried out under the same cracking conditions. SiO2/S coating was compact and had excellent anti-coking property. The coke on the coated HP40 tube was about 22% of that on the uncoated HP40 tube, and only small granular coke was deposited on the coated HP40 tube. However, the filamentous coke formed on the uncoated HP40 tube. The thermal stability of SiO2/S coating was satisfactory at cracking temperature, and the anti-coking property of SiO2/S coating was still over 60% after 3 coking and decoking cycles.     

Cr/W/稀土氧化物涂层的抑制结焦性能

急冷锅炉(TLE)是乙烯热裂解装置的重要设备,在运行中急冷锅炉炉管内壁会有结焦现象,降低急冷锅炉传热系数并增加炉管压力降,严重影响传热效果,需要进行周期性清焦.为了能够有效地抑制结焦,在炉管内壁表面上制备了一层Cr/W/稀土Re_xO_y惰性涂层,借助乙烯裂解结焦模拟装置、金相显微镜、扫描电子显微镜(SEM)和透射电镜(TEM)等设备从试样结焦表面形貌特征和结焦量等方面对所开发的涂层进行了抑焦性能研究.实验结果表明,无涂层试样表面存在催化焦和颗粒状焦,结焦严重;而涂层试样抑焦效果明显,表面不存在催化结焦,且焦层与涂层表面黏附性差结合较弱容易脱落,其结焦抑制率可达79.1%.     

Preparation of Al2O3 coating on TiN coating by polymer-assisted deposition to improve oxidation resistance in coking inhibition applications

In order to inhibit the metal catalytic coking and improve oxidation resistance of single TiN coating, the TiN/Al₂O₃ double layer coatings were designed as a chemically inert coating for methylcyclohexane supercritical pyrolysis. Internal TiN coatings were prepared by atmospheric pressure chemical vapor deposition using TiCl₄–H₂–N₂ system. The external Al₂O₃ coatings with different thicknesses were prepared on the TiN surface by polymer-assisted deposition, and the coating with the most suitable thickness was further annealed at different temperatures of 600, 700, 800 and 900 °C. The morphology, elemental and phase composition of TiN/Al₂O₃ coatings were characterized by SEM, EDX and XRD respectively. The chemical state information of the coating elements was based on Ti 2p, Al 2p core level X-ray photoelectron spectroscopy (XPS) spectra. The results indicated that the external Al₂O₃ coating will partially peel off at 900 °C annealing temperature. The thermogravimetric analysis results indicated that all TiN/Al₂O₃ coatings show better oxidation resistance than single-layer TiN coating. The anti-coking test with methylcyclohexane supercritical pyrolysis showed that the TiN/Al₂O₃ coatings can effectively cover the metal catalytic sites and eliminate metal catalytic coking. However, the acid sites of external Al₂O₃ coating slightly promoted coking, so the anti-coking ratios of TiN/Al₂O₃ coatings were smaller than that of TiN. Thus, the addition of external Al₂O₃ coating can greatly improve the oxidation resistance of TiN coatings with little loss of coking resistance.     

SiO2/S涂层与硫磷抑制剂对轻石脑油裂解结焦的影响

引言乙烯裂解炉辐射段炉管结焦是制约乙烯装置长周期运行的主要因素。裂解炉管的结焦会引起[1]:①管壁热阻增加,传热效率降低,裂解炉能耗增加;②炉管内径变小,流体压降增加,装置处理量减少;③炉管内壁渗碳,材料性能弱化;④周期性的清焦引起炉管热疲劳现象,而清焦过程中焦炭的     

Evaluation of the corrosion resistance of Ni-Co-B coatings in simulated PEMFC environment

The corrosion resistance behavior of Ni-Co-B coated carbon steel, Al 6061 alloy and 304 stainless steel was evaluated in simulated proton exchange membrane fuel cell (PEMFC) environment. The phase structure of the NiCoB based alloy was determined by Rietveld analysis. The PEMFC environment was constituted of 0.5 M H₂SO₄ at 60 °C and the evaluation techniques employed included potentiodynamic polarization, linear polarization resistance, open circuit potential measurements and electrochemical impedance spectroscopy. The results showed that in all cases the corrosion resistance of the Ni-Co-B coating was higher than that of the uncoated alloys; about two orders of magnitude with respect to carbon steel and an order of magnitude compared to 304 stainless steel. Except for the uncoated 304 type stainless steel, the polarization curves for the coated specimens did not exhibit a passive region but only anodic dissolution. The corrosion potential value, E_corr, was always nobler for the coated samples than for the uncoated specimens. This was true for the stainless steel in the passive region, but in the active state for the carbon steel and Al 6061 alloy. The corrosion of the underlying alloy occurred due to filtering of the solution through coating defects like microcracks, pinholes, etc. During the filtering process the E_corr value of the coating decreased slowly until it reached a steady state value, close to the E_corr value of the underlying alloy.     

Catalytic Fabrication of SiC/SiO2 coated graphite and its behaviour in Al2O3–C castable systems

SiC/SiO₂ coated graphite was prepared via a combined sol-gel coating and catalytic conversion route, using graphite flake and tetraethyl orthosilicate as the starting materials, and Fe(NO₃)₃·9H₂O as the catalyst precursor. X-ray diffraction analysis and microstructural examination revealed that a homogeneous coating comprising SiC and cristobalite (SiO₂) and covering the whole surface of graphite was formed. As prepared SiC/SiO₂ coated graphite exhibited better oxidation resistance and water wettability than its uncoated counterpart. Also, oxidation resistance and slag corrosion resistance of a model Al₂O₃–C castable using coated graphite as a carbon source were better than in the case of its counterpart using uncoated graphite.     

25Cr35NiNb合金表面Al-Si-Cr涂层抑制结焦性能

采用固体粉末包埋渗方法在25Cr35NiNb合金表面制备Al-Si-Cr涂层,通过乙烯裂解模拟装置中的结焦实验,对涂层抑制结焦性能进行分析评价。运用金相显微镜、扫描电子显微镜(SEM)和能谱仪(EDS)等方法分析了涂层显微组织及结焦层形貌特征。结果表明,采用粉末包埋共渗可以得到厚度在130 mm左右的Al-Si-Cr涂层,涂层具有多层结构且组织致密,外层为富铝硅层,内层为富铬硅层;Al-Si-Cr涂层具有良好的抗结焦性能,可有效抑制催化结焦,抑制结焦率达72.5%。     

Fabrication of Mn–Co Spinel Coatings on Crofer 22 APU Stainless Steel by Electrophoretic Deposition for Interconnect Applications in Solid Oxide Fuel Cells

This study investigates the microstructure, oxidation kinetics, and electrical behavior of Mn–Co spinel coating for interconnect applications in solid oxide fuel cells. A relatively dense, uniform, and well‐adherent Mn–Co (Mn_1.5Co_1.5O₄) spinel coating with good oxidation resistance and stable conductivity was successfully prepared on the surface of Crofer 22 APU stainless steel using electrophoretic deposition followed by sintering at 1150°C. During further thermal treatment at 800°C, the chromium oxide (Cr₂O₃) sublayer formed at the substrate/coating interface during sintering showed a very slow growth, and no chromium penetration was detected in the Mn–Co coating. The oxidation kinetics of the Mn–Co‐coated substrate obeyed the parabolic law with the a parabolic rate constant k_p of 5.20 × 10^?15 g²/cm⁴/s, which was 1–2 orders of magnitude lower than that of the uncoated Crofer 22 APU stainless steel substrate. For oxidation (at 800°C) times ≥50 h, the area‐specific resistance of the Mn–Co‐coated Crofer 22 APU substrate became ~17 mΩ·cm² and was almost constant after further oxidation.     

Comparison of the anti-coking performance of CVD TiN,TiO2 and TiC coatings for hydrocarbon fuel pyrolysis

Previous work has shown that both TiN and TiO₂ coatings can inhibit the metallic catalytic coking effectively, but both of them have their own shortage. In this work, TiC coating was prepared on the surface of SS304 tube using TiCl₄-CH₄-H₂ by CVD method. Its morphology, elemental composition, thickness and oxidation resistance were characterized by SEM, EDX, metalloscopy and TPO tests, respectively. The results show that CVD TiC coating is gray, homogeneous, and dense without cracks or holes. The TiC coating presents a cuboid particle structure with the sizes of about 1.0 µm for the cuboid crystals, and the Ti/C ratio close to 1:1, while the average thickness is about 11.62 µm. TPO results show that the TiC coating begins to react with O₂ and release CO₂ at about 810 °C. Compared with the TiN coating (The initial oxidation temperature of TiN is about 350 °C), the oxidation resistance of TiC coating is improved substantially. As a conclusion, the high oxidation resistance order is TiO₂ coating>TiC coating>TiN coating. Furthermore, the temperature programmed cracking of RP-3 Chinese jet fuel was employed to compare the anti-coking performance of TiN, TiO₂ and TiC coatings. The results show that each of TiN, TiO₂ and TiC coating has obvious anti-coking effect, and the anti-coking performance order is TiN coating=TiC coating>TiO₂ coating.     

Carbothermal synthesis of boron nitride coating on PAN carbon fiber

Boron nitride (BN) thin coating has been formed on the surface of chemically activated polyacrylonitrile (PAN) carbon fiber by dip coating method. Dip coating was carried out in saturated boric acid solution followed by nitridation at a temperature of 1200 °C in nitrogen at atmospheric pressure to produce BN coating. Chemical activation improved surface area of PAN fiber which favours in situ carbothermal reduction of boric acid. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) have shown the formation of boron nitride. The X-ray photoelectron spectroscopy reveals that the coating forms a composite layer of carbon, BN/BO_xN_y and some graphite like BCN with local structure of B–N–C and B(N–C)₃. The oxidation resistance of the coated fiber was significantly higher than uncoated carbon fiber. Tensile strength measurement reveals that the BN coated fiber maintained 90% of its original strength. As compared to chemical vapor deposition (CVD), this process is simple, non-hazardous and is expected to be cost effective.     

Addition methods of sulfur and sulfur/phosphorus-based compounds on coking behavior during thermal cracking of naphtha

Using naphtha as raw material for cracking, the effects of adding sulfides and sulfur/phosphorus compounds on the coking behavior of thermal cracking were investigated. The morphologies and structures of the pretreated HP40 alloy specimen and coke deposits were characterized by Raman spectroscopy, XRD, SEM and XPS. The results showed that the addition of phosphide into the sulfide additives led to the change in the structure of coke deposits under the condition of continuous addition of additives into the feed. The method with continuous addition of sulfur/phosphorus-based compounds showed excellent anti-coking properties. The surface pretreatment with sulfur or sulfur/phosphorus-based compounds made the increase of Fe content in the oxide film on the specimen. So anti-coking effect was limited when the pretreatment method was applied. The coking inhibition property of the application of surface pretreatment with sulfide/phosphide followed by continuous addition of sulfur/phosphorus-based compounds were similar to that of the application of continuous addition of sulfur/phosphorus-based compounds in the feed. However, the combination method showed the better anti-coking effect during the initial cracking process. All the addition methods led to the increased amounts of amorphous coke in the coke layer and the decreased graphitization degree of cokes. The results of the low H/C ratios showed the highly condensed structure of the coke deposits during thermal cracking process. The addition of sulfur and sulfur/phosphorus-based compounds could influence the dehydrogenation reaction to some extent during the coke formation by reducing catalytic coking.     

Synthesis of High Temperature Stable Carbon Coated Metal Nanoparticles in AlPO4 Based Matrix In Situ in Oxidative Atmosphere

In this work, we have reported a universal method for the synthesis of metal nanoparticles coated with graphite layer in AlPO₄ based matrix. As an example, graphitized carbon coated Ag, Pt, Cu and Ni nanoparticles were synthesized in the amorphous AlPO₄ based matrix. The metal nanoparticles were protected from oxidation up to very high temperatures due to the low oxygen diffusivity in AlPO₄ based matrix and carbon coating over the metal nanoparticles. The oxidation states of the Ag and Ni nanoparticles were detected with the help of X‐ray photoelectron spectroscopy. The synthesis technique followed very simple methodology. The entire processing including heat treatments at higher temperatures were carried out in oxidative atmosphere. The mechanism for the formation of metal particle in AlPO₄ based matrix has also been addressed. This approach can be a universal approach to achieve metal nanoparticles in AlPO₄ based matrix. Finally, catalyzing activities of the AlPO₄‐Cu nanocomposites in the oxidation of cyclohexane, AlPO₄‐Ni and AlPO₄‐Pt nanocomposites in the reduction in 4‐nitrophenol were successfully investigated.     

Sol–gel derived CeO2/α‐Al2O3 bilayer thin film as an anti‐coking barrier and its catalytic coke oxidation performance

A CeO₂/α‐Al₂O₃ bilayer was coated on a high temperature alloy (Incoloy 800H) by sol–gel dip‐coating and was evaluated for its potential as an anticoking barrier and coke oxidation catalyst. The bilayer effectively functioned as a barrier to metal surface catalyzed coking. The film prevented filamentous catalytic coking via blocking surface active metallic sites on the Incoloy substrate. Furthermore, the bilayer reduced the oxidation temperature of pyrolytic coke deposited on the film surface as compared to a bare oxidized Incoloy substrate, mostly owing to the oxidation catalytic activity of the CeO₂ layer. Finally, it is demonstrated that the presence of the α‐Al₂O₃ buffer layer is critically important to the overall performance. Without the α‐Al₂O₃ layer, a CeO₂ layer nearly completely lost both its barrier and oxidation catalytic functions. It is presumed that metallic species migrating from the substrate during high temperature treatments are responsible for the CeO₂ deactivation, likely by blocking catalytic sites on the CeO₂ surface. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4019–4026, 2018     

硫化物和硫/磷化合物的添加方式对石脑油热裂解结焦影响的研究

以石脑油为裂解原料，考察了硫化物和硫/磷化合物的添加方式对热裂解结焦行为的影响。采用Raman光谱、XRD、SEM和XPS等检测手段表征了HP40试样的氧化层和焦炭的形貌与结构。结果表明，原料连续注入硫化物条件下，磷化物的加入使得焦炭结构改变，显示出优异的抗结焦效果。硫化物和硫/磷化合物预处理导致氧化层中Fe含量升高，抑制结焦效果有限。硫/磷预处理与原料连续注入硫/磷化合物联合方式与原料连续注入硫/磷化合物方式的抗结焦效果接近，但前者在初期的抑制效果更明显。所有添加方式都会引起结焦层中无定形焦炭含量升高，焦炭的石墨化程度降低。热裂解焦炭缩合程度较高，硫化物和硫/磷化合物减少了催化结焦的生成，在一定程度上提高了焦层中氢含量。     

Oxidation Resistance of AlPO4 Bonded Ceramic Coating Formed on Titanium Alloy for High‐Temperature Applications

AlPO₄ based coatings were prepared on Ti‐6Al‐2Zr‐1Mo‐1V titanium alloy using aluminum phosphate as a binder and Al₂O₃/Cr₂O₃ based mixing particles as the fillers. The microstructure, phase and chemical composition of the coatings were analyzed by SEM, XRD and EDS techniques. The high temperature infrared emissivity values of coated and uncoated titanium samples were tested. The results show that the coating had a higher infrared emissivity value (>0.8) than titanium substrate (0.15–0.3) in the wide wavelength range of 5–20 mm, which is attributed to the uniform dispersion of high emissivity Al₂O₃ and Cr₂O₃ particles in the AlPO₄ binder matrix. The coated titanium samples exhibited excellent oxidation resistance performance with significantly decreased oxidation rates at 600 and 800°C. The mass gain of the coated sample kept at a low and stable constant of 0.15 mg/cm², significantly lower than that of titanium substrate (0.54 mg/cm²) when oxidized at 600°C up to 100 h.     

Novel bi-layered nanostructured SiO2/Ag-FHAp coating on biodegradable magnesium alloy for biomedical applications

In this study, a novel bi-layered nanostructured silica (SiO₂)/ silver-doped fluorohydroxyapatite (Ag-FHAp) coating was deposited on biodegradable Mg-1.2Ca-4.5Zn alloy via physical vapor deposition (PVD) combined with electrodeposition (ED). The nano-SiO₂ underlayer had a compact columnar microstructure with thickness of around 1 µm while the Ag-FHAp overlayer presented large plate-like crystals accompanied with small rounded particles with thickness about 10 µm. Potentiodynamic polarization test exhibited that the double layer SiO₂/Ag-FHAp coated Mg alloy has superior corrosion resistance compared to uncoated and single layer SiO₂ coated samples. Contact angle measurement showed that Ag-FHAp coating over nano-SiO₂ layers significantly increased surface wettability which is favorable for the attachment of cells. Cytotoxicity tests indicated that the nanostructured SiO₂/Ag-FHAp coating enabled higher cell viability compared to nano-SiO₂ coating and uncoated samples. In addition, bi-layer and single-layer coatings considerably improved the ability of cell attachment than that of the uncoated samples. The cell viability of coated and uncoated samples increased with increasing incubation time. The double layer SiO₂/Ag-FHAp coated biodegradable Mg alloy possessed high corrosion resistance and cytocompatibility and can be considered as a promising material for implant applications.     

Achieving thermally stable and anti-hydrolytic Sr2Si5N8:Eu2+ phosphor via a nanoscale carbon deposition strategy

Chemical stability of phosphors is critical to the efficiency and lifetime of the white light-emitting diodes. Therefore, many strategies have been adopted to improve the stability of phosphors. However, it is still lack of report on the improvement of thermal stability and hydrolysis resistance of phosphors by a single layer coating. Due to the high transmittance and high chemical inertness of graphene, it was coated on the surface of Sr₂Si₅N₈:Eu²⁺ phosphor by chemical vapor deposition, aiming to improve its thermal stability and hydrolysis resistance. The chemical composition and microstructure of the coating were characterized and analyzed. A nanoscale carbon layer was attached on the surface of Sr₂Si₅N₈:Eu²⁺ phosphor particles in an amorphous state. In coated Sr₂Si₅N₈:Eu²⁺ phosphor, the oxidation degree of Eu²⁺ to Eu³⁺ was significantly suppressed. At the same time, the surface of Sr₂Si₅N₈:Eu²⁺ particle turned from hydrophilic to hydrophobic after carbon coating, and consequently the hydrolysis resistance of Sr₂Si₅N₈:Eu²⁺ phosphor was greatly improved. After tests at 85 °C and 85% humidity for 200 h, the carbon coated Sr₂Si₅N₈:Eu²⁺ phosphor still maintained about 95% of its initial luminous intensity as compared with 35% of the uncoated. By observing the in-situ microstructure evolution of coated phosphor in air-water vapor environment, remained presence of the carbon layer even at 500 °C explained the excellent chemical stability of carbon coated Sr₂Si₅N₈:Eu²⁺ phosphor in complex environment. These results indicate that a nanoscale carbon layer can be used to provide superior thermal stability and hydrolysis resistance of (oxy) nitrides phosphors.     

Incorporation of zirconia nanoparticles into coatings formed on aluminium by AC plasma electrolytic oxidation

Composite ceramic coatings were formed on aluminium by AC plasma electrolytic oxidation (PEO) using Na₆P₆O₁₈ or Na₂SiO₃ · 5H₂O/KOH electrolytes with monoclinic zirconia nanoparticles in suspension. The coatings grown in Na₂SiO₃ · 5H₂O/KOH electrolyte revealed γ-Al₂O₃ and amorphous phase; α-Al₂O₃ and AlPO₄ were additionally produced with the Na₆P₆O₁₈ electrolyte. Higher temperature zirconia phases, possibly tetragonal and orthorhombic, in addition to the monoclinic phase, were indicative of elevated temperatures at sites of microdischarges. Further, local melting resulted in zirconium-rich dendrites in the coating formed in silicate electrolyte. Zirconium was mainly located in the relatively compact, outer layer of the coating, constituting ∼70–90% of the coating thickness. Nanoparticles appeared to be incorporated at the coating surface and following transport to the interface regions between the inner and outer layers along short-circuit paths through the outer coating.     

Electrochemical performance of pyrolytic carbon-coated natural graphite spheres

Natural graphite (NG) spheres were coated by pyrolytic carbon from the thermal decomposition of C₂H₂/Ar at 950 °C in a fluidized bed reactor. Scanning electron microscopy and secondary electron focused ion beam (FIB) images clearly showed that a pyrolytic carbon layer with a thickness of ∼250 nm was uniformly deposited on the surface of the NG spheres. Electrochemical performance measurements for the original and coated NG spheres as anode materials of a lithium-ion battery indicated that the first coulombic efficiency and cyclability were significantly improved in the coated sample. The reasons for this were investigated by analyzing structural characteristics, specific surface area, pore size distribution, and solid electrolyte interphase (SEI) film. Using a FIB workstation, we demonstrated, by cross-section imaging of a coated NG sphere that had experienced five electrochemical cycles, that the SEI film formed on the non-graphitic pyrolytic carbon surface became thinner (60-150 nm) and more uniform in composition compared with that on the surface of uncoated NG spheres; and the formation of an “internal SEI film” inside the NG spheres was also remarkably suppressed due to the uniform coating of pyrolytic carbon.     

Control of aluminum phosphate coating on mullite fibers by surface modification with polyethylenimine

Aluminum phosphate (AlPO₄) is a promising oxidation-resistant and weak interface for ceramic-matrix composites. In this research, AlPO₄ coating was deposited on mullite fibers by an improved liquid-phase method based on electrostatic attraction. A cationic polyelectrolyte, polyethylenimine (PEI), was used for surface modification of mullite fibers. The formation process, phase evolution and microstructure of the coating were studied. The zeta potential of AlPO₄ particles, PEI-adsorbed AlPO₄ particles, and PEI-adsorbed mullite particles was characterized to find the proper pH value for improving electrostatic attraction. The obtained AlPO₄ coating was porous and continuous, whose thickness could be controlled by multiple coating cycles. The relatively low calcination temperature (600 or 1000 °C) was a useful heat treatment method to develop bonding between coating and fiber as well as reduce the fiber strength degradation. The phase transformations of AlPO₄ have little volume change, and cristobalite AlPO₄ is thermal compatible with mullite. Therefore, the coating structure was preserved after calcining at 1200 °C. The technique is also applicable for other fibers contained mullite phase to fabricate high-performance AlPO₄ coated mullite/mullite composites.