Usually small samples are used to test corrosion resistance of a coating, it is sometimes difficult to make a coating on the small samples. When a cylindrical sample is used and the coating is formed on whole of the surface, three or four times coating processes are required. Thus an abacus bead type sample is proposed in this paper. In this sample, only two times coating processes are required to make coating on whole of the surface. In this paper, the high-temperature corrosion resistance of a novel coating made of Cr-based alloys for use in waste incineration plants is tested and compared with the coating made of a Ni-based alloy. Paper includes a German-language abstract.

High temperature protection requires full coating density, high adhesion, minor oxide inclusions, and preferably fine grains, which is not achievable in most thermal spray processes. High velocity oxygen-fuel (HVOF) thermal spray process has been applied extensively for making such coatings with the highest density and adhesion strength, but the existence of not or partially melted particles are usually observed in HVOF-formed coatings because of relative low flame temperature and short particle resident time in the process. This work has investigated the development of an innovative HVOF process using a liquid-state suspension/slurry containing small alloy powders. The advantages of using small particles in a HVOF process include uniform coating, less defective microstructure, higher cohesion and adhesion, full density, lower internal stress and higher deposition efficiency. Process investigations have proven the benefits for making alloy coatings with full density and high bond strength attributing to increased melting of the small particles and the very high kinetic energy of particles striking on the substrate. High temperature oxidation and hot corrosion tests at 800°C have demonstrated that the alloy coatings made by the novel process have superior properties to conventional counterpart coatings in terms of oxidation rates and corrosion penetration depths.

Activated Combustion HVAF Spraying (AC-HVAF) involves a jet of air-fuel combustion products to deposit coatings of metallic and carbide powders. In the process, spray particles are heated below their melting temperature while accelerated to velocity typically 700-850 m/s, forming a coating upon impact with a substrate. Extremely low oxygen content and high density are distinguished features of the AC-HVAF coatings, resulting in their excellent performance under conditions of severe wear and corrosion. Besides new level of coating quality, the AC-HVAF process demonstrates outstanding technological efficiency and spray rates 5-10 times exceeding those of the HVOF counterparts. The paper presents results on characterization of selected metallic and carbide coatings and describes their applications.

Sintered protective coatings are a proven technical solution against corrosion and erosion of tubes in biomass- and waste-fired boilers. The main field of application are tubes like superheater tubes or heat exchanger tubes in fluidized-bed installations corrosion endangered by the flue-gas stream. These sintered solutions complete other thermally sprayed coatings in those critical cases, where gas-dense overlays are needed due to corrosive damage. In those cases, where oxidation and erosion dominates, thermal spraying without sintering is remaining as primary solution. The development of Twin coatings out of sintered and non-sintered thermally sprayed alloys is showing a new and interesting alternative for tubes exposed to erosive-corrosive stresses under high temperature conditions.

In this study, Fe-Cr-Al and Fe-Cr-Al-B cored wires were produced and deposited on steel substrates by wire arc spraying. The microstructure, hardness, and high-temperature corrosion behavior of the cored-wire deposits were evaluated in comparison to Fe-Cr and commercial Fe-Cr-Al solid-wire coatings. All coating samples exhibited lamellar microstructures with oxide inclusions, the fewest being in the Fe-Cr-Al-B deposits. Microhardness was measured along coating cross-sections at various distances from the coating-substrate interface. The Fe-Cr coatings were the hardest, followed by the Fe-Cr-Al-B deposits. Thermogravimetric analysis was used to evaluate high-temperature corrosion behavior in a molten salt environment under cyclic conditions, with the Fe-Cr-Al-B cored-wire deposits performing the best.

In this study, several thermal spray coatings and reference materials were evaluated for potential use in biomass co-fired boilers. The coatings were applied to T92, A263, and X10Cr13 substrates by HVOF and wire-arc spraying using powder (IN625, FeCr, NiCr) and wire (NiCrTi) feedstocks. Coating samples were examined then tested for 5900 h in the superheater area of a fluidized bed boiler burning a mixture of wood, peat, and coal. The corrosion behavior of the coating and reference materials is reported in the paper and the underlying corrosion mechanisms are discussed.

A multiscale model is being built in order to better understand and predict high-temperature corrosion and erosion properties of thermal spray coatings and materials in general. The approach uses molecular dynamics to predict diffusion kinetics, constrained free energy to determine reactions, and FEA to simulate structure. To obtain oxidation behavior data for validation, surface polished bulk materials and thermal spray coatings were exposed to various temperatures and exposure times. Oxidation depth and diffusion were assessed by optical emission spectroscopy and cross-sectional SEM examination and surface oxidation in grain and lamellae boundaries was characterized by 3D profilometry and SEM-EDS. Rough validation of the model was achieved using indentation test data, and a more complete validation will be done when high-temperature erosion test results are available.

In this investigation, Ni-20Cr alloy powder was deposited on SA 516 boiler steel by cold spraying. The oxidation kinetics of both coated and uncoated samples, evaluated in molten salt, followed a parabolic rate. The rate constant of the Ni-20Cr coated steel was much lower than that of the bare boiler steel. X-ray diffraction, SEM-EDX, and X-ray mapping were used to analyze the oxidation products of the coated and uncoated boiler steel. The uncoated steel exhibited intense spalling and peeling of its oxide scale, which may be due to the formation of Fe 2 O 3 oxides. The Ni-20Cr coating reduced the weight gain of the steel by more than 87% possibly due to the formation of nickel and chromium oxides.

This study evaluates the hot corrosion behavior of NiCoCrAlY, NiCoCrAlYHfSi, NiCoCrAlTaReY, and CoCrAlYTaCSi coatings on 1.4923 stainless steel, applied by high-pressure HVOF spraying. All coatings were cycled in in an environment of Na 2 SO 4 and 82% Fe 2 (SO 4 ) 3 at 690 °C. Each cycle consisted of 1 h of heating in a silicon carbide tube furnace followed by 20 min of cooling. Weight change measurements were performed after each cycle to track corrosion kinetics, and SEM and EDS analysis were employed to analyze the corrosion mechanism. CoCrAlYTaCSi showed the best corrosion resistance of the coatings tested.

Oxides are chemically stable and wear resistant materials. Because of these properties, they are often applied as protective coatings in harsh environments. However, their chemical and mechanical stability at high temperature in chlorine containing environments is uncharted. These conditions are present in waste-to-energy and biomass boilers in which the currently available metallic and metal matrix composite coatings provide unsatisfactory protection. To be effective in these conditions the coatings should be chemically inert, erosion resistant and act as environmental barriers. For this purpose, this research studies the corrosion behavior and microstructural features of HVOF and APS-sprayed Al 2 O 3 -, Cr 2 O 3 -, TiO 2 -based coatings. Their chemical stability was evaluated by high temperature corrosion testing of self-standing coatings under KCl salt deposit at 550, 650 and 720 °C for the duration of 72 h.

A series of Ni-based cored wires with different boron contents were designed to prepare corrosion-resistant coatings by two-roll wire-arc spraying. These coatings were evaluated for their potential to provide added protection and reduced maintenance for applications in waste-to-energy (WTE) plants. The as-deposited coatings, which primarily are composed of nanocrystalline particles, exhibit uniform and dense layered structures with porosity of about 3%. The investigators selected thermo-gravimetric techniques to evaluate the high-temperature corrosion behavior of the coatings in molten salt environment (Na 2 SO 4 -10 wt% NaCl) at 800°C. The coated surfaces exhibited significantly reduced corrosion rates in comparison to those of the SA 213-T 2 substrate during all tests. These results were due to the formation in the coatings of composite surface oxide films, including Cr 2 O 3 and NiCr 2 O 4 , which serve to prevent the diffusion or penetration of corrosive species. Furthermore, the boron content appears to have a significant influence on the corrosion behavior of the designed coatings: the coating with the best performance had 16 at. % B added. The wire-arc sprayed Ni-based coatings could be an effective and economical treatment to prevent corrosion and extend the lifetime of super-heater tubes in WTE plants.

Highly corrosion and wear resistant thermally sprayed chromium carbide (Cr 3 C 2 ) based cermets coatings are nowadays a potential highly durable solution to allow traditional fluidised bed combustors (FBC) to be operated with ecological waste and biomass fuels. However, the heat input of thermal spraying processes causes carbide dissolution in the metal binder. This alters the coating structure and forms carbon saturated amorphous and nanocrystalline metastable areas, which can affect the behaviour of the materials under the corrosive chlorides containing environment of the flue gases. This study analyses the effect of carbide dissolution in the metal matrix of MMC coatings and its effect on the onset of chlorine induced high temperature corrosion. Four Cr 3 C 2 -NiCrMoNb coatings were thermally sprayed with high-velocity air-fuel (HVAF) and high-velocity oxygen-fuel (HVOF) spray processes in order to obtain microstructures with increasing amount of carbide dissolution in the metal matrix. The specimens were heat treated in an inert argon atmosphere at 700°C for 5 hours to induce secondary carbide precipitation. As-sprayed and heat-treated self-standing coatings were covered with KCl and their corrosion resistance was investigated with thermogravimetric analysis (TGA) at 550°C for 4 hours. High carbon dissolution in the metal matrix appeared to be a detrimental factor in the initial stage of corrosion. The microstructural changes induced by the heat treatment hindered the corrosion onset in the coatings. Moreover, an optimal amount of oxides and melting degree seemed beneficial.

Ti-Al and Al-Cr metallic coatings were deposited on Superfer800H (Fe-based superalloy) through a plasma spray process. Then the gas nitriding of the coatings was done in the lab and the parameters were optimized after conducting several trials on plasma sprayed coated specimens. Characterization and high temperature corrosion behaviour of coatings after exposure to air and molten salt at 900°C were studied under cyclic conditions. Techniques like XRD, SEM/EDAX and EPMA analysis have been used for characterization of the coatings and to analyze the oxide scale. Both the coatings have successfully protected the substrate and were effective in decreasing the corrosion rate when subjected to cyclic oxidation at 900°C for 50 cycles in air and molten salt. The coatings subjected to cyclic oxidation in air have shown relatively high weight gains in the early cycles of the exposure. Uncoated Superfer800H (Fe-based superalloy) showed very poor resistance to hot corrosion in molten salt environment.

Well known that high temperature corrosion resistance of thermal barrier coatings (TBC) may depend not only on chemical composition, thickness, the rate of formation of TGO layer, but also on the presence of post treatment. In this work we studied the influence of presence of post treatment on properties of TBC particularly on high temperature corrosion. TBC system were sprayed on Ni-based substrate and consists of HVOF NiCo-based bond coat (BC) and APS yttria-stabilized zirconia (YSZ) top coat. Thermal cycling test in the presence of corrosive environment was used to determine effect of post treatment presence. Before and after Thermal cycling test cross-section of specimens were studied by SEM and EDX analyzes. Preliminary results demonstrated that high temperature corrosion resistance of TBC with APS-top layer and HVOF-bond layer can be improved by applying post treatment.

This paper summarizes the results of high-temperature corrosion and erosion tests conducted on a wide range of coating materials, including Cr 3 C 2 -NiCr, Cr 3 C 2 -CoNiCrAlY, TiMoCN-Ni, Stellite 6, NiCrBSi, and Hastelloy C-276. All coatings were deposited on stainless steel substrates by HVOF spraying, and after high-temperature testing, were evaluated by means of SEM and EDX analysis. Of the coating materials evaluated, Hastelloy C-276 provided the best protection against high-temperature corrosion. It also exhibited the highest erosion resistance at a particle impact angle of 90°, but at the sharpest impact angle of 15°, Cr 3 C 2 -NiCr coatings were found to be the most erosion resistant, likely due to the strong bonding of carbide particles in matrix. NiCrBSi coatings, on the other hand, exhibited the highest values of volume loss.

Cr 3 C 2 -25%NiCr, Stellite 6, NiCrBSi and Hastelloy C-276 coatings were deposited on substrate material P91 by HP/HVOF (High Pressure / High Velocity Oxygen Fuel) thermal spraying technology. The resistance against high temperature corrosion was evaluated exposition of coatings to corrosive-aggressive environment in the form of molten salts mixture with composition of 60% V 2 O 5 and 40% Na 2 SO 4 at temperature of 750 °C. Further, coatings were exposed to cyclic conditions. After the corrosion tests, all coatings were analyzed using scanning electron microscope (SEM), and analysis of elemental composition (EDX). Alloys-based coatings showed very similar corrosion mechanism in the selected aggressive environment and the same can be stated about cermet coatings. The obtained results prove that HVOF deposited coatings can replace current surface protection of components in power equipment such as nitriding.

This paper reports the corrosion behaviour of coatings deposited by high velocity oxy-fuel (HVOF) 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 K 2 SO 4 - KCl and gaseous HCl-H 2 O-O 2 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 outperforms the other coating types studied.

Detonation-gun spray technology is a novel coating deposition process which is capable of achieving very high gas and particle velocities approaching 4-5 times the speed of sound. This process provides the possibility of producing high hardness coatings with significant adherence strength. In the present study, this technique has been used to deposit WC-Co coatings on T22 boiler steel. Investigations on the oxidation performance of detonation-sprayed WC-Co coatings in air and in molten salt Na 2 SO 4 -82Fe 2 (SO 4 ) 3 at 700°C under cyclic conditions have been carried out. The thermogravimetric technique was used to establish the kinetics of corrosion. The uncoated boiler steel suffered a catastrophic corrosion in the form of intense spalling of its oxide scale during air, as well as, molten salt induced oxidation. The WC-Co coated specimens showed lesser overall weight gains in comparison to their uncoated counterparts in both the environments. The oxidation kinetics for the coated specimens followed nearly the parabolic rate law. The overall weight gain has been found to be higher in the case of air oxidation as compared to that in salt environment for all the cases. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) techniques were used to analyse the corrosion products, which indicated the formation of W and O as main elements in the oxide scales of the coated steel in both the environments.

There are numerous industrial applications where materials are subjected to simultaneous high temperature oxidation/corrosion and wear (such as erosion). This combination often leads to accelerated degradation. Specific industries include: chemical, waste incineration, power generation and paper/pulp with typical applications including boilers and cyclones. Previous studies have established wrought material compositions and microstructures which can resist these environments. In searching for cost-effective industrial solutions, surface coating via thermal spray becomes attractive. However, the microstructural complexity of such coatings can make the simple extrapolation of bulk material behavior to these coatings dangerous. If these coatings are to be used more widely, a greater understanding of their high temperature erosion/corrosion behavior and the influence of coating process is required. A range of Fe-and Ni-based alloys and thermal spray techniques were studied under various high temperature erosion/corrosion conditions. The critical erosion parameters of impact angle (30° to 90°) and temperature (up to 550°C in air) have been studied in an atmospheric fluidized bed test rig environment, using Al2O3 erodent at a typical impact velocity of 4m/sec and conventional high temperature erosion test equipment. The important microstructural and mechanical features of the coatings and the effect of the thermal spray process are discussed in terms of their high temperature degradation mechanisms.

In the glass manufacturing, stainless-steel parts are used in characteristic corrosive reduction atmosphere at high temperature. The reactive products, that are induced by volatilization from the stainless steel surface, cause defects of the glass in this usage. Some thermal spraying coating of thermal-resistant alloys is tested in this study for the protection to the volatilization. It is clarified that, the Ni-Cr-Al-Y film, containing plate-like metal oxide dispersions, shows an outstanding effect as followings; 1) The diffusion rate of Fe from a stainless substrate in this coating film is decreased by the dispersions, and Fe is not detected at surface layer of the coating after a long-term exposure test at high temperature. 2) The reaction between Sn and this coating is not active relatively can, and the further inactivation can be possible by pre-existing heat-treatment in oxidant atmosphere. 3) Both effect (decreasing of Fe diffusion rate and inactivation with Sn) are induced by plate-like Al 2 O 3 particles. These particles are generated by oxidation of Al-content in the coating material.