Erosion-corrosion of thermal sprayed coatings in FBC boilers

Varieties of bed ash and fly ash were retrieved from operating fluidized bed combustor (FBC) boilers firing different fuels in North America and Europe. Using these ashes, the relative erosion-corrosion resistances of HVOF Cr₃C₂NiCr coating and several other thermal sprayed coatings were determined in an elevated temperature blast nozzle erosion tester. Test conditions attempted to simulate erosive conditions found at the refractory—waterwall interface and in the convection pass region in tubular heat exchangers of FBC boilers. Erosion-corrosion (E-C) wastage mechanisms of the structural metals (AISI 1018, ASTM SA213-T22) were discussed and compared with the E-C wastage of HVOF Cr₃C₂NiCr cermet coatings. The relatively different erosivities of ashes retrieved from North America and from Europe were also discussed.     

Erosion wear and mechanical properties of plasma-sprayed nickel- and iron-based coatings subjected to service conditions in boilers

The erosion wear and mechanical properties after exposure to simulated industrial service conditions in boilers of nickel-based, iron-based and chromium–nickel plasma-sprayed coatings on carbon steel and stainless steel have been obtained. These types of coatings are used as heat transfer and structural elements in boilers. Different tests simulating boiler service conditions under standard and extreme situations were carried out at 400, 600 and 800°C in a laboratory combustion unit. The influence of high temperature oxidation processes on the adherence, microhardness, microstructure, and wear erosion behaviour of both base materials and coatings have been evaluated.     

A comparative study of high-temperature erosion wear of plasma-sprayed NiCrBSiFe and WC–NiCrBSiFe coatings under simulated coal-fired boiler conditions

A comparative study was carried out of the behaviour of plasma sprayed NiCrBSiFe and WC–NiCrBSiFe alloys subjected to conditions which simulate a post-combustion gas atmosphere from a coal-fired boiler combustor. The study first evaluates the effects of thermal exposure at high temperatures on the microstructure of the coatings and on the adherence between substrate (austenitic stainless steel) and coatings. The oxidation rates of these coatings in atmospheres with 3–3.5% of free oxygen at 773 and 1073 K were then evaluated. The effect of WC on the low-velocity corrosion–erosion behaviour produced by the impact of fly ashes in the gas stream at high temperatures (773 and 1073 K) was assessed under impact angles of 30° and 90°. Finally, the eroded surfaces were analysed using scanning electron microscopy in order to characterize the ash embedment phenomena and the operating erosive micromechanisms.     

Solid Particle Erosion Behavior of WC Coating Obtained by Electrospark Technique and Detonation Spraying

Solid particle erosion is an important material degradation process. One way of improving the erosion resistance of a material is to suitably modify the surface. Electrospark deposition (ESD) is a well-known surface modification process. Operational simplicity, low capital cost, and low operational cost of the ESD process have made it attractive for high-technology areas in engineering industries. Tungsten carbide (WC) is considered a potential hard material for erosion-resistant application. This material can be deposited by ESD. The present investigation has been undertaken to evaluate the room-temperature erosion response of WC coating deposited by ESD and to compare the erosion behavior of this coating with that of detonation-sprayed WC-Co coating. WC coatings were deposited on mild steel (MS) and aluminum substrate by ESD. Similarly, WC-12% Co coatings were deposited on MS and Al by detonation spraying. The microstructural features and mechanical properties of these coatings were characterized using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction, and microhardness testing. The solid particle erosion rate was determined using an erosion test rig. The morphology of the eroded surfaces and the areas beneath the eroded surfaces were examined by means of SEM. The results showed that the WC coating by ESD improves erosion resistance. Although most coatings exhibit a ductile erosion response, WC coating by ESD on Al substrate exhibits a brittle erosion response. Material loss from ESD coating on Al occurs due to the joining of preexisting cracks and the removal of chunk of material.     

Characterization and solid particle erosion behavior of plasma sprayed alumina and calcia-stabilized zirconia coatings on Al-6061 substrate

Repeated impact by solid particles causes erosion and degradation of engineering components. In internal combustion engines, during combustion, hot gases are generated in large quantity which causes erosion of cylinder, combustion chamber, exhaust system, etc. In this work, two types of plasma sprayed coating systems were developed on Al-6061 substrate. For each system, a systematic microstructural study was carried out to understand changes occurred after spraying. Mechanical properties like density, adhesion strength and hardness of coatings were determined. A solid particle erosion test was conducted on coating systems according to ASTM G-76-02 and results were correlated with the microstructural and subsequent mechanical property change. It was observed that volume erosion is more at 45° angle of impact and shows that behavior is in between ductile and brittle. This work also discusses the mechanism involved in erosion wear of plasma sprayed coating systems.     

Studies on erosion behaviour of plasma sprayed coatings on a Ni-based superalloy

In the present investigation NiCrAlY, Ni-20Cr and Ni₃Al metallic coatings were deposited on a Ni-based superalloy (18.5Fe-19Cr-0.15Cu-0.5Al-3.05Mo-0.18Mn-0.9Ti-0.18S-0.04C-5.13 (Ta + Cb)-balance Ni). NiCrAlY was used as bond coat in all the cases. Erosion studies were conducted on uncoated as well as plasma spray coated superalloy specimens at room temperature. The erosion experiments were carried out using an air-jet erosion test rig at a velocity of 40 m/s and impingement angles of 30 and 90°. Silica sand particles of size ranging between 150 and 212 μm were used as erodent. The coatings have been characterised by scanning electron microscope (SEM), optical microscope, microhardness tester and X-ray diffractometer (XRD). Scanning electron microscope (SEM), equipped with an energy dispersive X-ray analyser (EDAX) was used to analyse the eroded surfaces. Possible erosion mechanisms are discussed. The phases revealed by XRD of the coatings have shown the formation of solid solutions. Out of the three plasma sprayed coatings, the Ni₃Al coating gave the lowest erosion rate regardless of the impact angle, and the Ni-20Cr coating gave the highest erosion rate.     

Laser Surface Melting Cr–Ni Coatings, in the Erosive-Corrosive Atmosphere of Boilers

This article summarizes the results of a research on laser-treated Cr–Ni-based coatings under the erosive-corrosive environment of steam boilers containing fly ash and a 3–3.5% volume of free oxygen at 500 and 800 °C. The total erosion-corrosion process has been measured by sample weighing. Eroded layers were examined by scanning electron microscopy with an energy-dispersive spectrometer. Results were compared with erosion results for AISI 304 and two Cr–Ni plasma-sprayed coatings taken from previous papers. The significant superior erosion-corrosion resistance of laser-treated coatings compared to that of the plasma-sprayed coatings is considered to be due to a soft and cohesive coating without pores and the formation of a thin oxidized protective film.     

Solid Particle Erosion Behavior of WC-CoCr Nanostructured Coating

In the present study, solid particle erosion resistance of high-velocity oxy-fuel (HVOF)-sprayed WC-CoCr coatings was evaluated. Erosion testing was conducted using alumina (Al₂O₃) powder as the erodent with three different impact angles (30, 60, and 90°) and impact velocity was kept constant. The coatings were deposited using two different powders; one was composed of conventional WC particles and second one contained nanoscale particles mixed with CoCr binder material. Erosion testing was carried out at room temperature using an air-jet erosion test setup. The effect of varying impact angles was studied and discussed with the help of scanning electron microscopy images of worn surfaces of coatings. The results showed that coating properties like microhardness and fracture toughness have a strong influence on the erosion behavior. During erosion testing, material was removed by fracturing and pullout of WC grains from the binder matrix. The morphology of the eroded surface also showed cutting, lip, and groove formation in the binder matrix caused by the repetitive impacts of erodent particles. It was observed that coating with nano-WC grains exhibited higher erosion resistance compared to conventional coating.     

Elevated temperature erosion–corrosion of thermal sprayed coatings in chlorine containing environments

A series of hot erosion and erosion–corrosion (E–C) tests was carried out on thermal sprayed coatings, diffusion coatings and boiler steels using a burner-rig type elevated temperature E–C tester in order to evaluate the possibility to utilise thermal sprayed coatings in shielding of boiler components. Test conditions simulated the E–C conditions in the superheater section of a circulating fluidised bed combustor (CFBC). Carbide containing HVOF coatings performed well in erosion tests, as expected. Also diffusion coatings and nickel-based, high-chromium HVOF coatings performed well. In E–C tests in presence of chlorine, nickel-based HVOF coatings performed the best, whereas carbide containing HVOF coatings and diffusion coatings wore away.     

Erosive wear of hardfaced Fe–Cr–C alloys at elevated temperature

Many engineering components are subjected to erosive wear at elevated temperature. As erosive wear at elevated temperature is governed by the synergistic effect of erosive wear and oxidation, it is possible to modify surfaces of the components in order to achieve improved performances. In view of the above, two different types of hardfacing alloys of Fe–Cr–C were designed incorporating Nb, Mo and B to ensure improved performances at elevated temperature. In order to achieve the above objective, mild steel was hardfaced with these alloys under optimised gas metal arc welding (GMAW) condition. The microstructures of the hardfaced coating was characterised with the help of optical microscopy (OM) and scanning electron microscopy (SEM). The mechanical properties of these coatings were obtained by means of micro indenter. Erosive wear of these coatings was evaluated for four different temperatures, for two different impact angles and at one impact velocity. The morphologies and the transverse sections of the worn surfaces are examined with SEM. The erosive wear of these coatings were compared with conventional M2 tool steel. Results indicate that erosion rate of these coatings increases with increase of test temperature and impact angles. Among various coatings, Fe–Cr–C coating containing higher amount of Nb, Mo and B exhibits best erosion resistance particularly at elevated temperature.     

Effect of oxidation on erosive wear behaviour of boiler steels

High efficiency of thermal processes in energy applications is available at high temperatures. The ashes and other components in fluidized-bed boilers act as an erodent particles provoking the competition and interaction between the oxidation and erosion processes. The aim of the study was to investigate the effect of oxidation on the erosive behaviour of boiler steels. Tests were performed using specially developed device allowing testing in either oxidizing or protected atmosphere in a cyclic mode. Several parameters are proposed and the assessment is done on their applicability to describe the erosion–oxidation phenomena under different impact angles. It is shown that under specific conditions oxide scales provide improved wear resistance for some steels, particularly austenitic ones, that enables reduced material losses.     

锅炉水冷壁管的复合涂层防护技术研究

针对燃煤电站锅炉水冷壁管吹灰冲蚀严重,导致泄漏、爆管等而被迫停炉检修的情况,根据水冷壁管的形状、工况条件和使用环境状况,对电弧喷涂工艺以及对Ni基高Cr丝材所制成的涂层的微观组织结构、硬度、涂层结合性能以及耐冲蚀磨损性能等进行了深入的研究,在此基础上,确定了以电弧喷涂NiAl丝材作为结合底层,高Cr材料作为功能层,并辅以高温封孔的复合涂层制备技术,以提高锅炉运行的可靠性和经济性。     

Influence of WC size and HVOF process on erosion wear performance of WC-10Co4Cr coatings

In the present study, based on the velocity and temperature measurements of in-flight particles and parameter optimization, multimodal and conventional WC-10Co4Cr cermet coatings were sprayed by high velocity oxygen gas fuel spraying (HVOGF) and high velocity oxygen liquid fuel spraying (HVOLF). The coatings’ structure, porosity, microhardness and fracture toughness were investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) etc. Furthermore, erosion resistance of the coatings to solid sand was tested, followed by the exploration of the material failure mechanisms. Results show that the WC-10Co4Cr coatings deposited by HVOLF are mainly composed of WC with no obvious decarburization and amorphous CoCr binder. The mechanical properties of the coatings deposited by HVOLF are much more superior to those deposited by HVOGF. Multimodal WC-10Co4Cr coating deposited by HVOLF possesses the highest microhardness and fracture toughness, the lowest porosity and the most excellent resistance to sand solid erosion wear, which was enhanced by 15 and 40% than that of HVOLF conventional coatings at 30° and 90° impact angles. The improvement is even greater in comparison with multimodal coating deposited by HVOGF. These results have provided important reference for WC-CoCr anti-erosion coating design and optimization of high velocity oxygen fuel (HVOF) process.     

Improvement in wear resistance of plasma sprayed yttria stabilized zirconia coating using nanostructured powder

Plasma sprayed yttria stabilized zirconia coatings were prepared using nanostructured and conventional powders with optimized process parameters for the highest deposition efficiency, the smallest porosity and the highest microhardness. The tribological properties of these coatings against 100C6 steel were then tested with a ball-on-disc arrangement. Results showed that although the friction coefficients of the coatings sprayed using the nanostructured powder were slightly different from those of the coatings sprayed using the conventional powder, the former coatings were more wear resistant than the latter coatings. The wear mechanisms of all the coatings were explained in terms of adhesion-induced spallation and micro-fracturing of lamellae. The improvement in wear resistance of the coatings sprayed using the nanostructured powder could be mainly ascribed to the decrease of micrometer-sized defects such as pores and interlamellar and intralamellar cracks in the coatings.     

Tribological behavior of plasma sprayed Cr2O3-3%TiO2 coatings

Tribological behaviors of plasma-sprayed conventional and nanostructured Cr₂O₃-3%TiO₂ ceramic coatings (i.e., CC3T and NC3T, respectively) using pin on disc type dry sliding and pot type slurry erosion test were investigated in the present work. The experimental results indicated that there were two main wear mechanisms, plastic smearing and adhesive tearing, in the worn coatings under dry sliding. Plastic smearing corresponded to a lower average friction coefficient and wear rate, while adhesive tearing corresponded to higher values. The erosive environment selected for the slurry erosion experiments include 10, 20 and 30% of SiO₂ slurry concentrations in water with particle size 75-106 μm. The main damage mechanism observed in all the coatings submitted to slurry erosion were the formation and propagation of brittle cracks resulting in the detachment of coating surface material. Microstructural investigation was also carried to investigate the wear and erosion mechanism of the coatings using FE-SEM and EDS analysis. Properties like microhardness and porosity were also investigated for these coatings. Tribological performance of NC3T was better as compared to CC3T as observed in the present work.     

Experimental investigations of mechanical properties and slurry erosion behaviour of high velocity oxy fuel and plasma sprayed Cr2O3–50%Al2O3 coatings on CA6NM turbine steel under hydro accelerated conditions

Slurry erosion behaviour of HVOF (High Velocity Oxy Fuel) and plasma sprayed coatings on CA6NM hydraulic turbine steel has been investigated at different levels of various parameters. The Cr₂O₃–50%Al₂O₃ composite powder was prepared and deposited on CA6NM steel samples to get the uniform thickness coatings. The surface roughness, porosity and microhardness of as-coated samples were measured. The as-coated samples were subjected to SEM/EDS analysis to evaluate the surface microstructure of the developed coatings. Erosion tests were performed on self made erosion test rig under hydro accelerated conditions. The study reveals that the velocity, impact angle and slurry concentration were the most significant parameters, influencing the erosion rate of these coatings. The average particle size had least affect on the erosion rate. HVOF-coated samples showed better corrosion resistance as compared to plasma-coated samples due to high hardness of HVOF-coated CA6NM samples.     

Erosion behavior of thermal sprayed,recycled polymer and ethylene–methacrylic acid composite coatings

Polymer consumption is increasing and the recycling rate is 30–40 wt.%. Thus any process or application that uses recycled plastic residue will be looked upon with favor. It has been demonstrated that post-consumer commingled polymer, or PCCP, coatings can be produced by thermal spraying. Furthermore, polymeric coatings are widely used as protective coatings against solid particle erosion. Therefore, in this paper the erosion behavior of thermal spray coatings that have some PCCP material is investigated. The coatings were produced using a low velocity combustion thermal spray process and a PCCP mixed with different levels of virgin ethylene–methacrylic acid co-polymer (EMAA). The erosion rates using 50 μm alumina were determined at impact angles of 30° and 90°. The wear features were analyzed by scanning electron microscopy and profilometry. The results exhibited brittle wear features, consistent with the relationship between erosion rates and mechanical properties of the polymers. However, a decrease in erosion rate with an increase in impact angle, from 30° to 90°, indicates ductile behavior during erosion.     

Triboperformance Analysis of Coatings of LD Slag Premixed with TiO2 Using Experimental Design and ANN

This article reports on the analysis of triboperformance in regard to the erosion wear of a new class of coatings by an integrated implementation of Taguchi's experimental design and artificial neural networks (ANNs). Plasma-sprayed coatings of LD slag premixed with TiO₂ in different weight proportions are deposited on metal substrates at various input power levels of the plasma torch. Solid particle erosion trials, as per ASTM G 76 test standards, are conducted on the coating samples following a well-planned experimental schedule based on Taguchi's design of experiments. An air jet–type erosion test rig capable of creating reproducible erosive wear situations is used. Significant process parameters predominantly influencing the rate of erosion are identified. The study reveals that the impact velocity is the most significant among various factors influencing the wear rate of these coatings. A prediction model based on an ANN is proposed to predict the erosion performance of these coatings under a wide range of erosive wear conditions. This model is based on the database obtained from the experiments and involves training, testing, and prediction protocols. This work shows that an ANN model helps to save time and resources that are required for a large number of experimental trials and successfully predicts the erosion wear rate of the coatings both within and beyond the experimental domain.     

Wear Behavior of Thermally Sprayed Coatings under Different Loading Conditions

Wear behavior of three kinds of thermally sprayed coatings with similar hardness have been investigated under steady-state and dynamic loading tests. The steady-state loading tests were conducted on a reciprocating sliding device and the dynamic loading tests were conducted with a single-pendulum scratching device. Experimental results show that the wear mechanisms of the coatings under steady-state sliding friction testing are microcutting and microploughing, whereas the material losses under the dynamic impact scratch testing are mainly due to split cutting and fracture. Tribo-oxidization in the sliding process was found to have an influence on the wear behaviors of the thermally sprayed coatings. The results also indicated that wear resistance of thermally sprayed coatings can be correlated to hardness, plasticity, toughness, and cohesion. As far as the coatings of similar hardness were concerned, the wear resistance under steady-state loading was mainly due to the cohesion of the laminar structure of the coatings and the wear resistance under dynamic loading was mainly due to the toughness and deformation compatibility of the coatings.     

The erosive wear behavior of basalt based glass and glass–ceramic coatings

In this study, the effects of impact angle and SiC reinforcement on the erosion wear behavior of basalt base glass and glass–ceramic coatings were investigated. Basalt powders with 0–50% SiC additions were coated on AISI 1040 steel by atmospheric plasma spray coating technique. Controlled heat treatment was applied to amorphous basalt base coatings for glass–ceramic transformation. Erosion tests were realized by using corundum media at the different impact angles and velocities. Wear test results showed that the SiC addition in the basalt based coatings resulted in increase erosive wear resistance of glass and glass–ceramic coatings.