Fireside Corrosion of Superheater Materials in Coal/Biomass Co-fired Advanced Power Plants

A series of laboratory-based fireside corrosion exposures were conducted to assess the effect of such conditions on superheater/reheater materials at higher than conventional metal temperatures. Controlled atmosphere furnaces combined with the “deposit recoat” test method were used to generate the exposure conditions; the gaseous environment simulated that anticipated from air-firing 20 wt% cereal co-product mixed with a UK coal. The exposures were carried out at 600, 650 and 700 °C with four candidate materials: T92, HR3C and 347HFG steels; nickel-based alloy 625. After the exposures, the samples were examined by SEM/EDX to characterize the damage. Pre- and post-exposure dimensional metrology were used to quantify the metal damage in terms of metal loss distributions. For the austenitic steels, the combined deposit/gas/temperature exposure conditions enabled quantification of the characteristic ‘bell-shaped’ curves (of damage as a function of temperature) for fireside corrosion.     

Fireside Corrosion in Oxy-fuel Combustion of Coal

Oxy-fuel combustion is burning a fuel in oxygen rather than air for ease of capture of CO₂ from for reuse or sequestration. Corrosion issues associated with the environment change (replacement of much of the N₂ with CO₂ and higher sulfur levels) from air- to oxy-firing were examined. Alloys studied included model Fe–Cr alloys and commercial ferritic steels, austenitic steels, and nickel base superalloys. The corrosion behavior is described in terms of corrosion rates, scale morphologies, and scale/ash interactions for the different environmental conditions. Evidence was found for a threshold for severe attack between 10^?4 and 10^?3 atm of SO₃ at 700 °C.     

Corrosion Testing of Ni Alloy HVOF Coatings in High Temperature Environments for Biomass Applications

This paper reports the corrosion behavior of Ni alloy coatings deposited by high velocity oxyfuel spraying, and representative boiler substrate alloys in simulated high temperature biomass combustion conditions. Four commercially available oxidation resistant Ni alloy coating materials were selected: NiCrBSiFe, alloy 718, alloy 625, and alloy C-276. These were sprayed onto P91 substrates using a JP5000 spray system. The corrosion performance of the coatings varied when tested at ~525, 625, and 725 °C in K₂SO₄-KCl mixture and gaseous HCl-H₂O-O₂ containing environments. Alloy 625, NiCrBSiFe, and alloy 718 coatings performed better than alloy C-276 coating at 725 °C, which had very little corrosion resistance resulting in degradation similar to uncoated P91. Alloy 625 coatings provided good protection from corrosion at 725 °C, with the performance being comparable to wrought alloy 625, with significantly less attack of the substrate than uncoated P91. Alloy 625 performs best of these coating materials, with an overall ranking at 725 °C as follows: alloy 625 > NiCrBSiFe > alloy 718 ? alloy C-276. Although alloy C-276 coatings performed poorly in the corrosion test environment at 725 °C, at lower temperatures (i.e., below the eutectic temperature of the salt mixture) it outperformed the other coating types studied.     

Effect of Spray Parameters on the Corrosion Behavior of HVOF Sprayed WC-Co-Cr Coatings

WC-10Co-4Cr cermet coatings were deposited on the substrate of AISI 1045 steel by using high-velocity oxygen-fuel (HVOF) thermal spraying process. The Taguchi method including the signal-to-noise (S/N) ratio and the analysis of variance (ANOVA) was employed to optimize the porosity and, in turn, the corrosion resistance of the coatings. The spray parameters evaluated in this study were spray distance, oxygen flow, and kerosene flow. The results indicated that the important sequence of spray parameters on the porosity of the coatings was spray distance > oxygen flow > kerosene flow, and the spray distance was the only significant factor. The optimum spraying condition was 300 mm for the spray distance, 1900 scfh for the oxygen flow, and 6.0 gph for the kerosene flow. The results showed the significant influence of the microstructure on the corrosion resistance of the coatings. Potentiodynamic polarization and electrochemical impendence spectroscopy (EIS) results showed that the WC-10Co-4Cr cermet coating obtained by the optimum spraying condition with the lowest porosity exhibits the best corrosion resistance and seems to be an alternative to hard chromium coating.     

Wear Behavior of Plasma-Sprayed Al-Si/TiB2/h-BN Composite Coatings

In this study, mechanically alloyed Al-12Si/TiB₂/h-BN composite powder was deposited onto aluminum substrates by atmospheric plasma spraying. Wear performance of the coating was investigated with respect to the structural evolution of the composite powder coating. Non-lubricated ball-on-disk tests were used to examine the wear resistance of the coatings. The worn surfaces were examined using scanning electron microscopy and energy dispersive spectroscopy to elucidate the wear mechanisms operating at the sliding interface. It has been observed that TiB₂ and in situ formed AlN and Al₂O₃ phases in combination with h-BN solid lubricant strongly affect the wear performance of the coating.     

Hot Corrosion Behavior of HVOF Sprayed Coatings on ASTM SA213-T11 Steel

Cr₃C₂-NiCr, NiCr, WC-Co and Stellite-6 alloy coatings were sprayed on ASTM SA213-T11 steel using the HVOF process. Liquid petroleum gas was used as the fuel gas. Hot corrosion studies were conducted on the uncoated as well as HVOF sprayed specimens after exposure to molten salt at 900 °C under cyclic conditions. The thermo-gravimetric technique was used to establish the kinetics of corrosion. XRD, SEM/EDAX and EPMA techniques were used to analyze the corrosion products. All these overlay coatings showed a better resistance to hot corrosion as compared to that of uncoated steel. NiCr Coating was found to be most protective followed by the Cr₃C₂-NiCr coating. WC-Co coating was least effective to protect the substrate steel. It is concluded that the formation of Cr₂O_3, NiO, NiCr₂O₄, and CoO in the coatings may contribute to the development of a better hot-corrosion resistance. The uncoated steel suffered corrosion in the form of intense spalling and peeling of the scale, which may be due to the formation of unprotective Fe₂O₃ oxide scale.     

Increased Lifetime for Biomass and Waste to Energy Power Plant Boilers with HVOF Coatings: High Temperature Corrosion Testing Under Chlorine-Containing Molten Salt

Heat exchanger surfaces of waste to energy and biomass power plant boilers experience often severe corrosion due to very aggressive components in the used fuels. High velocity oxy-fuel (HVOF) coatings offer excellent protection for boiler tubes against high temperature corrosion due to their high density and good adherence to the substrate material. Several thermal spray coatings with high chromium content were sprayed with HVOF technique. Their mechanical properties and high temperature corrosion resistance were tested and analyzed. The coating materials included NiCr, IN625, Ni-21Cr-10W-9Mo-4Cu, and iron-based partly amorphous alloy SHS9172 (Fe-25Cr-15W-12Nb-6Mo). High temperature corrosion testing was performed in NaCl-KCl-Na₂SO₄ salt with controlled H₂O atmosphere at 575 and 625 °C. The corrosion test results of the coatings were compared to corrosion resistance of tube materials (X20, Alloy 263 and Sanicro 25).     

Microstructure and Pitting Corrosion Behavior of Plasma-Sprayed Fe-Si Nanocomposite Coating

In this study, Fe-Si nanoparticle composite coating (FSN) and Fe-Si microparticle composite coating (FSM) were prepared via atmospheric plasma spraying, and FSN was thermally treated under hydrogen atmosphere at 1120 °C for holding time of 2.5 h (TFSN). Under transmission electron microscopy, many unmelted nanoscale particles were observed in FSN, while no substantial particles were found in TFSN. On scanning electron microscopy analysis, pores and cracks were observed in FSM and FSN, while no defects were found in TFSN. Scanning electrochemical microscopy testing in 3.5 wt.% NaCl for 5 h revealed that FSM underwent severe pitting corrosion, FSN showed relatively minor pitting corrosion, and TFSN had no pitting corrosion.     

Microstructure and Wear Behavior of Conventional and Nanostructured Plasma-Sprayed WC-Co Coatings

WC-12%Co coatings were deposited by atmospheric plasma spraying using conventional and nanostructured powders and two secondary plasmogenous gases (He and H₂). Coating microstructure and phase composition were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray diffraction techniques (XRD) techniques. This study examined wear and friction properties of the coatings under dry friction conditions. SEM was used to analyze abraded surface microstructure. Coating microhardness and fracture toughness were also determined. All coatings displayed strong decarburization as a result of WC decomposition, which gave rise to the formation of secondary phases (W₂C and W). A very fine undissolved WC crystalline dispersion coexisted with these new phases. TEM observation confirmed that the matrix was predominantly amorphous and filled with block-type, frequently dislocated crystallites. Wear was observed to follow a three-body abrasive mechanism, since debris between the ball and the coating surface was detected. The main wear mechanism was based on subsurface cracking, owing to the arising debris. WC grain decomposition and dissolution were concluded to be critical factors in wear resistance. The level of decomposition and dissolution could be modified by changing the plasmogenous gas or feed powder grain size. The influence of the plasmogenous gas on wear resistance was greater than the influence of feedstock particle size.     

The High-Temperature Wear and Oxidation Behavior of CrC-Based HVOF Coatings

Three commercially available chromium carbide-based powders with different kinds of matrix (Cr₃C₂-25%NiCr; Cr₃C₂-25%CoNiCrAlY and Cr₃C₂-50%NiCrMoNb) were deposited by an HVOF JP-5000 spraying gun, evaluated and compared. The influence of heat treatment on the microstructure and properties, as well as the oxidation resistance in a hot steam environment (p = 24 MPa; T = 609 °C), was evaluated by SEM and XRD with respect to their potential application in the steam power industry. The sliding wear resistance measured at room and elevated (T = 600 °C) temperatures according to ASTM G-133. For all three kinds of chromium carbide-based coatings, the precipitation of secondary carbides from the supersaturated matrix was observed during the heat treatment. For Cr₃C₂-25%NiCr coating annealed in hot steam environment as well as for Cr₃C₂-25%CoNiCrAlY coating in both environments, the inner carbide oxidation was recorded. The sliding wear resistance was found equal at room temperature, regardless of the matrix composition and content, while at elevated temperatures, the higher wear was measured, varying in dependence on the matrix composition and content. The chromium carbide-based coating with modified matrix composition Cr₃C₂-50%NiCrMoNb is suitable to replace the Cr₃C₂-25%NiCr coating in a hot steam environment to eliminate the risk of failure caused by inner carbide oxidation.     

超音速火焰喷涂制备的WC-WCoB涂层在熔锌中的腐蚀行为及其耐蚀机理

目的揭示新型WC-WCoB涂层在锌液中的腐蚀行为及耐蚀机理,从而提高WC基涂层的耐熔锌腐蚀性能。方法以WC、Co和WB粉末为原料,结合离心喷雾干燥和真空热处理,制备得到具有高球形度、结构致密的WC-WCoB热喷涂粉末喂料,并利用超音速火焰喷涂工艺进行涂层的制备。将涂层浸泡于熔融锌液中不同时间,观察其截面组织,以评价涂层的耐熔锌腐蚀性能,并通过X射线衍射仪、热重/差热分析仪、扫描电子显微镜、能谱分析仪对涂层进行结构、性能表征。结果 WCoB相与熔锌间不发生化学反应,制备的WC-WCoB涂层在锌液中浸泡达600 h时,仍未观察到Zn向涂层内的扩散,但在锌液中氧的缓慢作用下,涂层边缘处易产生微裂纹并逐步向内扩展,最终导致涂层材料逐层剥落。WC-WCo B涂层在腐蚀600 h后,完好区域面积占试验涂层总面积的56.3%。结论在传统WC-Co涂层中添加一定量的WB,可使Co相完全转化为WCoB相,与目前广泛使用的WC-η涂层相比,该研究制备的WC-WCoB涂层具有更突出的抗氧化性能,使其在锌液中由于氧化引起的裂纹形成扩展速率显著降低,宏观上表现出更强的耐熔锌腐蚀性能。     

Effects of Laser Re-melting on the Corrosion Properties of HVOF Coatings

HVOF coating of Inconel 625 powder on carbon steel is carried out. Laser melting of the resulting coating is realized to improve coating structural integrity. Morphological and microstructural changes are examined in the coating prior and after laser treatment process using scanning electron microscopy, energy dispersive spectroscopy, and x-ray diffraction (XRD). The residual stress developed is measured on the surface vicinity of the laser-treated coating using the XRD technique. The corrosion resistance of the laser-treated and untreated coating surfaces is measured, incorporating the potentiodynamic tests in 0.5 M NaCl aqueous solution. It is found that laser treatment reduces the pores and produces cellular structures with different sizes and orientations in the coating. Laser-controlled melting improves the corrosion resistance of the coating surface.     

Structural and Thermal Behavior of Fe-Cr-Mo-P-B-C-Si Amorphous and Nanocrystalline HVOF Coatings

The microstructure, thermal behavior, and mechanical properties of amorphous/nanocrystalline 70Fe-15Cr-4Mo-5P-4B-1C-1Si (wt.%) coatings produced by high velocity oxy fuel (HVOF) spraying of mechanically alloyed powders were investigated by x-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Thermal stability of samples was investigated using differential scanning calorimetry (DSC). The results show that by adjusting the HVOF parameters especially fuel/oxygen ratio and proper selection of powder composition, the desired microstructure with different amount of amorphous and nanocrystalline phases and therefore with different mechanical properties could be obtained.     

The Tribological Behavior of Plasma-Sprayed Al-Si Composite Coatings Reinforced with Nanodiamond

Al-Si composite coatings reinforced with 0?vol.%, 0.5?vol.%, and 2?vol.% nanodiamond were synthesized by plasma spraying. The effect of the addition of nanodiamond on the microstructure, hardness, and tribological performance of the composite coatings is investigated. The addition of 2?vol.% nanodiamond results in 45% improvement in the wear resistance of Al-Si coating. Al-Si coating with 0.5?vol.% nanodiamond exhibited lower coefficient of friction (0.45) with a 12% improvement in the wear resistance. Plasma-sprayed AlSi coatings with nanodiamond have excellent potential as wear-resistant coatings in automotive applications.     

Microstructure and Creep Behavior of Plasma-Sprayed Yttria Stabilized Zirconia Thermal Barrier Coatings

The purpose of this study was to determine the creep/sintering characteristics of thermally sprayed zirconia coatings and attempt to understand the influence of microstructure on the creep resistance of deposits. The major modification, compared with more typical practice, was employment of a new powder feedstock with agglomerated sub-micron size particles (Nanox), which is compared to one of the best commercially available powders (HOSP). Thick plasma-sprayed coatings were prepared and their physical and mechanical properties were characterized. Creep/sintering experiments were then conducted to investigate the response of the materials when exposed to high temperatures under load. The results showed that it could be possible to correlate the splat thickness to the creep behavior of the coatings.     

Microstructure and Thermal Cycling Behavior of Air Plasma-Sprayed YSZ/LaMgAl11O19 Composite Coatings

Yttria-stabilized zirconia (YSZ) based composite coatings with the addition of LaMgAl₁₁O₁₉ (LaMA) as the secondary phase, were prepared by air plasma spraying in order to improve the performances of the traditional YSZ coating. Results indicate that the newly developed composite coating shows increased vertical crack density with the enhancement of the LaMA content during thermal cycling process, which results in increased strain tolerance and service lifetime. However, such composite coatings about 200 ??m thick, exhibit inferior thermal cycling lifetimes with respect to the typical YSZ coating for surface temperatures above 1400 °C. The presence of amorphous LaMA phase in the composite coating results in increased thermal conductivity and a relative thin top coat leading to a reduced thermal insulation efficiency. These are believed to be responsible for the premature degradation of bond coat and final top coat spallation failure. Such an investigation gives useful guidelines to develop advanced composite coatings based on YSZ/LaMA systems.     

Analysis of Corrosion Behavior and Surface Properties of Plasma-Sprayed HA/Ta Coating on CoCr Alloy

The cobalt–chromium (CoCr) alloys have been extensively used as implants, especially in total joint replacements and in odontology, due to their superior mechanical properties and wear resistance in vivo. However, the excessive release of the ‘Co’ and ‘Cr’ ions from CoCr implants can lead to adverse health issues, such as hypersensitivity and inflammatory reactions. The present study aimed to improve the corrosion resistance of a medical grade CoCr alloy (ASTM F-1537) plasma-sprayed with tantalum (Ta)-reinforced hydroxyapatite (HA) coating. The weight percent (wt.%) of Ta content in HA coating was varied at three levels, i.e., 10, 20, and 30%. In vitro corrosion behavior was investigated by electrochemical measurements in Ringer’s solution along with surface properties analysis. The results revealed an increase in surface hardness value with an incremental increase in Ta content in the HA coating. The surface of HA as well as Ta-reinforced HA coatings possessed adequate roughness and demonstrated hydrophilic nature. With the Ta reinforcement in HA coating, the E_corr values shifted toward nobler potentials and I_corr values declined noticeably which indicated an increase in corrosion resistance of the surface. The results of the study indicate that the proposed Ta reinforcement in HA is potentially important for CoCr bio-implant applications.     

超音速火焰喷涂Ni-CeO2复合涂层的数值模拟及耐磨耐腐蚀性能

超音速火焰喷涂因其制备的涂层具有优异性能而被航空航天、石油化工等领域广泛使用,其工艺参数较为复杂且对涂层质量具有重要影响,但对其制备Ni基涂层的工艺参数选择及涂层性能研究相对较少。采用数值模拟的方法对超音速火焰喷涂Ni基涂层进行模拟,并对焰流与粒子特性进行分析;利用模拟指导试验,在316L不锈钢基体上成功制备Ni-CeO₂复合涂层;对复合涂层组织形貌及耐磨耐腐蚀性能做进一步研究。研究结果表明：当氧气煤油比等于3,注入颗粒粒径在20～80μm时,喷涂工艺最优;在添加CeO₂后,复合涂层的耐磨性能耐腐蚀性能均得到提升,且当CeO₂含量为1 wt.%时,涂层硬度最大,摩擦因数最低,其摩擦因数相较于基体降低了39.8%,相较于Ni基涂层降低了22.2%,其耐磨性能相较于Ni基涂层提升了62.5%。探究了超音速火焰喷涂工艺参数对喷涂系统状态的影响,分析了添加CeO₂在复合涂层中的作用,对超音速火焰喷涂Ni-CeO₂复合涂层具有引领与推动作用。