Integrity of Co-Cr-C coated P92 steel for power plant pipework applications

The main limiting factor for P92 steel, in power plant at high temperature, is the increased oxidation damage on the inside surface which causes enhanced damage of components. Industry have attempted to address this problem by applying oxidation resistant coatings to the inside surface of the P92 pipework to prevent damage. Aluminide diffusion coatings have been a particular focus for research to date, however they have been found to have a number of detrimental effects on the creep properties and coating-substrate integrity. This paper introduces a Co-Cr-C type coating, composed of Cr₃C₂ particles electro-deposited within a cobalt matrix. On exposure to high temperature oxidation conditions the coating is shown to form a cobalt and chromium rich oxide which is slow growing, adherent and ideal for oxidation resistance. When applied to P92 substrate and exposed at service relevant temperatures the coated system retains its integrity and appears suitable for long term service. The coated P92 system is also shown to retain its integrity during high temperature creep testing and coating application does not have a negative effect on the mechanical properties of P92. Overall the Co-Cr-C coating has a number of superior properties compared to previously investigated coatings.     

Oxidation of MCrAlY coatings on Ni based superalloys

Abstract

Industrial gas turbine engines used for power generation generally employ Ni based superalloys for the turbine blades. The operating conditions for these blades are very arduous with high temperatures (>900°C) leading to oxidation and corrosion. Therefore in order to increase the service life of components, coatings are employed which allow the use of Ni based superalloys at higher temperatures and therefore more efficient engines. These systems are very complicated and in order to understand coating performance and service life, many modelling approaches have been utilised. However, there is still a lack of understanding of the thermally grown oxide (TGO) with regards to its chemistry, microstructure, adhesion strength and mechanical properties. Therefore a more detailed understanding of the TGO would be useful for both empirical and computational modelling. The effect of compositional changes in the MCrAlY bond coat and their effect on the TGO have been studied. Two different MCrAlY coatings have been examined after aging at representative operation temperatures. A number of analytical techniques have been used including, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray and electron backscattered diffraction. A dual beam scanning electron microscope has also been utilised for both TEM sample preparation and microstructural evaluation. These techniques will allow a better understanding of the microstructure of the TGO, ultimately leading to improved predictions of coating behaviour and service life.     

Comparison between predicted and actually observed in-service degradation of a nickel pigmented anodized aluminium absorber coating for solar DHW systems

The actual service degradation in optical performance of a nickel pigmented anodized aluminium absorber coating has been investigated in order to better validate predicted service life data from accelerated life testing. Samples from the coating taken from collectors used in solar DHW systems for time periods of 10 years or more were analysed for that purpose. The study, which was performed by the IEA Working Group: Materials in Solar Thermal Collectors, utilized results from a comprehensive joint case study on accelerated life testing previously performed in Task X of the IEA Solar Heating and Cooling Programme. It could be concluded from the present study that the agreement between degradation data determined for the absorber samples from the DHW systems and that from accelerated life testing from the Task X study was astonishingly good both from a quantitative and a qualitative point of view. For the anodized aluminium coating the results of the present study strongly point to the fact that the design of the solar collector with respect to airtightness is the most crucial factor in determining service life. The service life was defined as the period during which the optical performance is not less than 95% of its original value. The estimated service life is of the order of 30–40 years for the coating in an airtight solar collector with controlled ventilation of air, whereas in a non-airtight collector with essential uncontrolled ventilation of air, the corresponding life is around 5–10 years. The general conclusion from the study is that the accelerated life testing method as developed by the Task X group is an efficient tool in predicting expected service life of absorber coatings and is therefore to be recommended for qualification of durability of new kinds of absorber coatings.     

Optimisation of HVOF thermal spray coatings for their implementation as MSWI superheater protectors

Abstract

The aim of this paper is to the study high velocity oxygen fuel thermal spray coatings as superheater tubes protectors against degradation problems in municipal solid waste incinerators. Ni based and Fe based high velocity oxygen fuel coatings have been sprayed through optimised spray parameters with the help of online monitoring technology. All these coatings have been tested under laboratory conditions simulating the boiler erosion and corrosion mechanisms. The influence of the spray parameters on the porosity and oxidation has been studied in order to optimise the coatings structure. Interconnected porosity and spray oxidation must be avoided to attain the best coating resistance under corrosion and erosion mechanisms identified on the municipal solid waste incinerators. Tribological and electrochemical coating properties have been determined under standardised tests. Different erosion tests have been carried out in order to determine coatings resistance. X-ray diffraction studies show the main formed phases under corrosion mechanisms for the different studied alloys. The Ni based Inconel 625 coatings have been reported as a good superheater tubes protector for its industrial application.     

Improvement of corrosion properties of porous alloy supports for solid oxide fuel cells

The development of protective coatings for porous metal supports is critical for sufficient life time for the fuel cells by enabling improved oxidation resistance, reduced chromium evaporation, and increased conductivity of the protective oxide scale. The oxidation of coated and non-coated substrates has been compared, and shows that it is possible to increase the oxidation resistance at 600 °C in air by a factor of 10 and in wet hydrogen by a factor of 1000, after vacuum coating of the porous metal supports by infiltration of a lanthanum–manganese–cobalt solution and fast curing in air at 900 °C. Chromium evaporation is also lowered by a factor of 10 in air at 600 °C. The experiments on pre-coated porous metal supports verify that the coating is well suited to use for metal supported fuel cells prepared by a low temperature fabrication route (below 1100 °C). An alternative coating procedure for coating of the metal supports after co-sintering of the anode and electrolyte has also been investigated and is well suited for the high-temperature fabrication route. For the high temperature fabrication route, the oxidation tests at 600 °C for 500 h in air and 100 h in wet hydrogen showed that post-coating is better than the pre-coating approach since the cell sintering steps has a detrimental effect on the pre-coated samples.     

The performance of solid oxide fuel cells with Mn–Co electroplated interconnect as cathode current collector

To add a coating on a metallic interconnect is one option to prevent Cr poisoning of the cathode and to retain high conductivity during solid oxide fuel cells (SOFC) operation. Electroplating of metals or alloys followed by oxidation offers a cost-effective method. In this study, pure Co and Mn/Co alloys formed by electrodeposition are used to protect the substrate, SUS 430. On-cell tests, using uncoated, cobalt-coated and MnCo-coated interconnects were conducted at 375 mA cm⁻² for 323, 500 and 820 h, respectively. The results show that cell power degrades at a rate of 33% in 320 h using an uncoated interconnect. Significant improvements are obtained for cell tests utilizing unoptimized coated interconnects with the degradation rate of 5% and 9% per 1000 h for cobalt and MnCo coatings, respectively. Based on the results from SEM and XRD studies, the advantages of both coatings are to successfully inhibit Cr diffusion to the scale surface. However, thin (∼2 μm) cobalt coating allows fast scale growth, while thicker cobalt coatings have the potential to fail due to mismatch in the coefficient of temperature expansion (CTE) between Co₃O₄ and the SUS 430 substrate. In spite of higher degradation rate for the MnCo coatings evaluated here, the addition of Mn into the cobalt coating not only aids in suppression of scale growth, but also reduces the CTE mismatch. Furthermore, no performance decay after two thermal cycles was observed. Finally, the cell degradation was observed to have a correlation with the cell cathode interlayer microstructure.     

Preparation and hydrogen penetration performance of TiO2/TiCx composite coatings

Titanium carbide is a good candidate for tritium permeation barrier in a fusion reactor. However, its oxidation susceptibility and the mismatch between the ceramic coating and substrate are still a challenge. In this study, a promising candidate as a hydrogen permeation barrier, comprising a titanium-based ceramic TiO₂/TiC_x composite coating, was proposed. The preparation process of this TiO₂/TiC_x composite coating involves two steps of carbon ion implantation and oxidation under ultra-low oxygen partial pressure. According to the results, the optimal oxidation temperature for TiO₂ coating is 550 °C, with the increase of the oxidation temperature, the particles on the surface of the oxide layer become coarse and loosely arranged, and the protective performance of the oxide layer is greatly reduced. The hydrogen barrier permeation behavior of the composite coating in a fusion reactor was simulated via hydrogen plasma discharge environment, the results show that the hydrogen barrier permeation performance of the composite is significantly better than that of a single TiO₂ coating. In addition, the coatings treated with hydrogen plasma showed a certain self-repairing performance through the diffusion growth of the TiC_x layer. These findings illustrate a novel method for preparing composite coatings to restrain hydrogen permeation, providing insight into the development of hydrogen permeation barrier materials.     

Modeling and analysis of combustion assisted thermal spray processes

The combustion assisted thermal spray systems are being used to apply coatings to prevent surface degradation. They offer a highly attractive way to modify the surface properties of the substrate to extend the product life. In addition to the materials being sprayed, the quality of combustion assisted thermal spray coating depends greatly on the flow behavior of reacting gases and particle dynamics. The present study investigates the effect of gas phase and its interaction with particles through the nozzle of a thermal spray gun by developing a comprehensive mathematical model. The objective is to develop a predictive understanding of various design parameters of combustion assisted thermal spray systems. The model was developed by considering the conservation of mass, momentum and energy of reacting gases. The particle dynamics was decoupled from the gas phase dynamics since the particle loading in the spray process is very low. The developed model was employed to investigate the influence of various design parameters on the coating quality of thermal spray process.     

Improved oxidation resistance of a nanocrystalline chromite-coated ferritic stainless steel

An economical dip coating process was developed to synthesize uniform, crack-free, and adherent thin nanocrystalline LaCrO₃ films on a ferritic stainless steel substrate for the solid oxide fuel cell interconnect applications. LaCrO₃ perovskite phase was formed after annealing in air at 800 °C for 1 h for both the LaCrO₃ and La₂O₃ precursors. The effectiveness of the coating in improving the oxidation resistance of the alloy was demonstrated by both isothermal and cyclic oxidation tests. The LaCrO₃ coatings were found to cause a pronounced reduction in oxidation rate of the alloy, especially with low La-content precursors. The area-specific resistance of the oxide scales formed on the bare and coated alloy substrates was also evaluated and discussed.     

Controlling chromium vaporization from interconnects with nickel coatings in solid oxide devices

Vaporization of Cr-rich volatile species from interconnect materials is a major source of degradation that limits the lifetime of planar solid oxide devices (solid oxide fuel cells and solid oxide electrolysis cells) with metallic interconnects. Some metallic coatings (Ni, Co, and Cu) may significantly reduce the Cr release from interconnects and slow down the oxide scale growth on the steel substrate. To shed additional light upon the mechanisms of such protection and find a suitable coating material for ferritic stainless steel materials widely used for interconnects, we used a combination of first-principles calculations, thermodynamics, and diffusion modeling to investigate which factors determine the quality of the Ni metallic coatings. We found that Cr migration in Ni coatings is determined by a delicate combination of the nickel oxidation, Cr diffusion, and phase transformation processes. Although the formation of Cr₂O₃ is more exothermic than that of NiO, the kinetic rate of the chromia formation in the coating layer and its surface is significantly reduced by the low mobility of Cr in nickel oxide and in NiCr₂O₄ spinel. These results are in a good agreement with diffusion modeling for Cr diffusion through the Ni coating layer on the ferritic 441 steel substrate and available experimental data.     

钼黑太阳能选择性吸收涂层的研制

利用NH_4Cl、(NH_4)_6Mo_7O_(24)·4H_2O的水溶液为原料液在经过预处理后的铝或铝合金基片上进行化学沉积,制备钼黑太阳光谱选择性涂层。实验考察了反应时间、反应温度、反应溶液的pH值和表面预处理方式等对涂层太阳吸收率α的影响,得到了制备钼黑涂层的优化条件,并对涂层表面进行了XPS及SEM表征。实验结果表明,涂层由Mo_xO_y组成,Mo的化学态介于 4～ 6之间;铝基片制得的涂层的吸收率明显高于铝合金基片;反应温度和反应时间对于涂层的吸收率影响较为明显;基片经碱性预处理后涂层的太阳吸收率优于酸性预处理。     

SiCrOxNy选择性吸收涂层制备及性能研究

采用直流及中频反应溅射在铜基底上沉积SiCrO_xN_y光谱选择性吸收涂层。对该涂层的光学性能进行表征,其吸收比为0.938,80℃发射比为0.07。经300℃,200 h热处理后,吸收比无明显变化,发射比小幅升高;俄歇电子能谱(AES)分析显示,界面处元素扩散和Cu基底氧化是涂层光学性能下降的主要原因。在35℃下进行5%盐雾腐蚀试验,腐蚀初期涂层发射比迅速升高,腐蚀36 h后吸收比衰减加速,涂层表面开始出现剥落现象;致密的SiO2减反射层对增强涂层耐盐雾腐蚀性能有明显效果。     

LaCrO3-based coatings deposited by high-energy micro-arc alloying process on a ferritic stainless steel interconnect material

Currently used ferritic stainless steel interconnects are unsuitable for practical applications in solid oxide fuel cells operated at intermediate temperatures due to chromium volatility, poisoning of the cathode material, rapidly decreasing electrical conductivity and a low oxidation resistance. To overcome these problems, a novel, simple and cost-effective high-energy micro-arc alloying (HEMAA) process is proposed to prepare LaCrO₃-based coatings for the type 430 stainless steel interconnects. However, it is much difficult to deposit an oxide coating by HEMAA than a metallic coating due to the high brittleness of oxide electrodes for deposition. Therefore, a Cr-alloying layer is firstly obtained on the alloy surface by HEMAA using a Cr electrode rod, followed by a LaCrO₃-based coating using an electrode rod of LaCrO₃-20 wt.%Ni, with a metallurgical bonding between the coating and the substrate. The preliminary oxidation tests at 850 °C in air indicate that the LaCrO₃-based coatings showed a three-layered microstructure with a NiFe₂O₄ outer layer, a thick LaCrO₃ sub-layer and a thin Cr₂O₃-rich inner layer, which thereby possesses an excellent protectiveness to the substrate alloy and a low electrical contact resistance.     

Comparison of solid oxide fuel cell anode coatings prepared from different feedstock powders by atmospheric plasma spray method

Atmospheric plasma spray (APS) deposition of a high-performance anode coating, which is essential for obtaining high power density from a solid oxide fuel cell (SOFC), is developed. A conventional, micron-sized, nickel-coated graphite – yttria stabilized zirconia (YSZ) – graphite blend feedstock leads to a non-uniform layered coating microstructure due to the difference in the physical and thermo-physical properties of the components. In this research, new types of feedstock material received from a spray-drying method, which includes nano-components of NiO and YSZ (300 nm), are used. The microstructure and mechanical properties of a coating containing a nano composite that is prepared from spray-dried powders are evaluated and compared with those of a coating prepared from blended powder feedstock. The coating microstructures are characterized for uniformity, mechanical properties and electrical conductivity. The coatings prepared from spray-dried powders are better as they provide larger three-phase boundaries for hydrogen oxidation and are expected to have lower polarization losses in SOFC anode applications than those of coatings prepared from blended feedstock.     

Use of La2O3 with 8YSZ as thermal barrier coating and its effect on thermal cycle behavior,microstructure, mechanical properties and performance of diesel engine operated by hydrogen-algae biodiesel blend

In this present work, the effect of lanthanum oxides (La₂O₃) on the thermal cycle behavior of TBC coatings and mechanical properties such as adhesion strength and microhardness of 8% Yttria Stabilized Zirconia (8YSZ) TBCs were investigated. CoNiCrAlY and aluminium alloy (Al–13%Si) were used as bond coat and substrate materials. 8YSZ and different wt % of La₂O₃ (10, 20, and 30%) top coatings were applied using the atmospheric plasma spray (APS) method. The thermal cycling test for TBC coated samples were conducted at 800 °C in the electric furnace. The XRD pattern shows that the La₂O₃ doped 8YSZ material transformed to cubic pyrochloric structured La₂Zr₂O₇ during thermal cycling. Further, the Taguchi-based grey relation analysis (GRA) method was applied to optimize the TBC coating parameters to achieve better mechanical properties such as adhesion strength and microhardness. And the optimized La₂O₃/8YSZ TBC coating was coated on CRDI engine combustion chamber components. The engine was tested with microalgae biodiesel and hydrogen, and the results were promising for the TBC-coated engine. The engine performance increased while using La₂O₃/8YSZ coated components, and the emissions from engine exhaust gas such as CO, HC, and smoke reduced considerably. It was found that there was no separation crack and spallation of the coating layer in the microstructure. Ultimately, the microstructural analysis of the optimized TBC coated piston sample after 50 h of running in the diesel engine confirmed that the developed coating had a superior thermal insulation effect and longer life.     

The effect of coating crystallization and substrate impurities on magnetron sputtered doped LaCrO3 coatings for metallic solid oxide fuel cell interconnects

For IT-SOFC metallic interconnects, surface coating is effective for reducing Cr poisoning of the cathode and controlling scale growth. In this work, LaCrO₃ and doped LaCrO₃ coatings were deposited by magnetron sputtering on SS446 and Crofer 22 APU substrates. The crystallization process was studied by means of X-ray Diffraction (XRD) during the annealing of the sputter coated samples in ambient and reducing environments. The formation of intermediate phases when annealed in air, LaCrO₄ and La₂CrO₆, results in vacancy formation upon subsequent transformation to the LaCrO₃ phase and thus a decreased oxidation resistance. While the avoidance of an intermediate phase change when the coatings are initially annealed in a reducing environment leads to dense and compact coatings. This confirmed both by XRD and by scanning electron microscopy (SEM) of coating cross-sections. Crofer 22 APU alloys with various silicon and aluminum levels are deposited with doped LaCrO₃ coating to study substrate impurity effects on coating properties. It was found that silicon content in the substrates leads to increased ASR of the coatings. In addition, long term annealing in air shows that aluminum impurities in the substrate can lead to the formation of alumina at substrate grain boundaries, which in turn leads to enhanced Mn migration at the grain boundaries. Increased manganese concentrations at the film/grain boundary interface in coated samples produces larger than normal amounts of (Mn,Cr)₃O₄ spinel in these regions, which cracks the coating and reduces the ASR value due to extra electronic conduction path. A similar mechanism is not observed in a low Al/Si alloy.     

Life expectancy prediction and application properties of novel polyurethane based thickness sensitive and thickness insensitive spectrally selective paint coatings for solar absorbers

A novel Thickness Sensitive Spectrally Selective (TSSS PU B: a_s=0.90, e_T=0.20) paint coating on aluminium substrate was prepared from commercially available polyurethane binder (Binder B) (HELIOS TBLUS, SI) and black pigment (spinel (Mn–Fe)), in combination with trisilanol polyhedral oligomeric silsesquioxane (POSS), which served as pigment dispersant. Polyurethane resin binder B was selected because of its higher thermal stability (determined from thermogravimetric measurements (TG)) than polyurethane resin binder A, which has previously been used for making Thickness Insensitive Spectrally Selective (TISS PU A) coatings (a_s=0.90, e_T=0.38) deposited on copper absorbers (Kuni?, 2009 [36]).Thermal degradation of the TSSS PU B and TISS PU A coatings, both deposited on aluminium substrates, was studied by following, as close as possible, the methodology worked out within TASK 10 of the IEA's Solar and Heating Programme. Thermal load tests were performed in the temperature range from 170 to 200 °C at various time intervals (1, 6, 10, 15, 21 days). Degradation of the coatings was assessed using a variety of degradation indicators: changes of solar absorptance and thermal emittance determined from the hemispherical IR and VIS/NIR spectra, intensity changes of selected vibrational modes attributed to the polymeric backbone and ester and urethane linkages and combined with peel-off tests used as adhesion and cohesion indicators. The results revealed that degradation of the polyurethane resin binder was attributable to the breaking of the urethane linkages, also shown from the AFM and XPS spectra measurements. For the TISS PU A coating, the life expectancy was estimated to be 22.77 years (activation energy (E_a)=163.2 kJ/mol, T_eff=113.4 °C) while for the TSSS PU B coatings, it was at least 25.96 years (activation energy (E_a)=96 kJ/mol, T_eff=102 °C).     

Sol–gel derived (La0.8M0.2)CrO3 (MCa,Sr) coating layer on stainless-steel substrate for use as a separator in intermediate-temperature solid oxide fuel cell

A ceramic coating technique is applied to reduce the voltage drop caused by oxidation of the metallic separator (SUS444) in intermediate-temperature (IT) solid oxide fuel cell (SOFCs) systems. Precursor solutions for (La, Ca)CrO₃ (LCC) and (La, Sr)CrO₃ (LSC) coatings are prepared by adding nitric acid and ethylene glycol into an aqueous solution of lanthanum, strontium (or calcium) and chromium nitrates. Dried LCC and LSC gel films are heat-treated at 400–800 °C after dip-coating on the SUS444 substrate. XRD and Fourier-transform infrared (FT-IR) analysis is used to examine the crystallization behaviour and chemical structure of the precursor solution. The oxidation behaviour of the coated SUS444 substrate is compared with an uncoated SUS444 substrate. The oxidation of the SUS444 is inhibited by the LCC and LSC thin film layers.     

Internal polymeric coating materials for preventing pipeline hydrogen embrittlement and a theoretical model of hydrogen diffusion through coated steel

This work develops a theoretical analysis of the coating permeability necessary for use as internal coatings of transmission pipelines to prevent hydrogen embrittlement. Internal coating materials suitable to be applied in situ on existing steel pipelines are also evaluated. Twelve different commercially available coatings; crosslinked poly (vinyl alcohol) (PVA), poly (vinyl chloride) and bisphenol A diglycidyl ether (DGEBA)/polyetheramine (D-400) epoxy coatings prepared in-house were tested. Films fabricated from two commercial epoxies had hydrogen permeability of 0.40 Barrer and 0.35 Barrer respectively, which show potential as coating materials. A hydrogen permeability of 0.0084 Barrer was achieved with a crosslinked poly (vinyl alcohol) coating, indicating that this material shows the highest potential of all coatings tested. Unsteady-state hydrogen diffusion through coated steel was then modeled to evaluate the effect of the coating film in reducing hydrogen embrittlement. The result shows that with a 2 mm PVA coating, hydrogen permeation inside the coating will take seven years to reach equilibrium and the final hydrogen concentration on the steel surface will be 44% lower than that without a coating. Greater protection can be provided if coatings can be developed with lower hydrogen permeability.     

Hydrogen permeation properties of CrxCy@Cr2O3/Al2O3 composite coating derived from selective oxidation of a CrC alloy and atomic layer deposition

Metal oxides and carbides are promising tritium permeation barrier coatings for fusion reactors. However, the thermomechanical mismatch between the coating and substrate poses a threat to their interface's integrity during fabrication and operation. To address this issue, a metallic interlayer coating was introduced followed by selective oxidation in which a compact and uniform CrC amorphous alloy coating was successfully deposited on the stainless steel substrate by pulsed electrochemical deposition. A new composite coating of Cr_xC_y@Cr₂O₃/Al₂O₃ was formed by subsequent controlled oxidation conversion and atomic layer deposition. The phase, morphology, chemical state and defects of the films were analyzed and compared both before and after hydrogen exposure at 300 °C. The results show that this new kind of composite coating, based on the principles of grain boundary pinning of chromic oxide with carbide and defect healing of alumina, can remarkably improve the hydrogen permeation barrier performance of these materials.