Comparative characteristic and erosion behavior of NiCr coatings deposited by various high-velocity oxyfuel spray processes

The purpose of this study is to analyze and compare the mechanical properties and microstructure details at the interface of high-velocity oxyfuel (HVOF)-sprayed NiCr-coated boiler tube steels, namely ASTM-SA-210 grade A1, ASTM-SA213-T-11, and ASTM-SA213-T-22. Coatings were developed by two different techniques, and in these techniques liquefied petroleum gas was used as the fuel gas. First, the coatings were characterized by metallographic, scanning electron microscopy/energy-dispersive x-ray analysis, x-ray diffraction, surface roughness, and microhardness, and then were subjected to erosion testing. An attempt has been made to describe the transformations taking place during thermal spraying. It is concluded that the HVOF wire spraying process offers a technically viable and cost-effective alternative to HVOF powder spraying process for applications in an energy generation power plant with a point view of life enhancement and to minimize the tube failures because it gives a coating having better resistance to erosion.     

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.     

Comparative study of WC-cermet coatings sprayed via the HVOF and the HVAF Process

The high velocity air fuel (HVAF) system is a high-velocity combustion process that uses compressed air and kerosene for combustion. Two WC-cermet powders were sprayed by the HVAF and the high-velocity oxyfuel (HVOF) processes, using an AeroSpray gun (Browning Thermal Systems Inc., Enfield, New Hampshire) and a CDS-100 gun (Sulzer Plasma Technik, Wohlen, Switzerland) respectively. Several techniques, including x-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy, were used to characterize the microstructures and phase distribution of the powders and coatings. In addition, mechanical properties such as hardness and wear resistance (pin-on-disk) were investigated. A substantial amount of W₂C was found in the HVOF coatings, as well as a high concentration of tungsten in the binder phase, indicating that oxidation and dissolution processes change the composition and microstructure from powder to coating during spraying. This was in contrast to the HVAF coatings in which composition and microstructure were unchanged from that of the powder. Additionally, the wear resistance of the HVAF coatings was superior to that of the HVOF coatings.     

Comparative study on dry sliding wear and oxidation performance of HVOF and laser re-melted Al0.2CrFeNi(Co,Cu) alloys

Al_0.2CrFeNiCo and Al_0.2CrFeNiCu high entropy alloys were deposited with high velocity oxygen fuel (HVOF) on 316L substrate. Later, a laser re-melting (LR) process was applied to enhancing the coating microstructure. LR process effects on dry sliding wear and oxidation behaviors were investigated. The mixture of powders with free elements led to the formation of inner oxides in HVOF coatings. The oxide and porosity were eliminated using LR. After LR, FCC was the dominant phase in both alloys, while BCC, sigma and Cr₂O₃ phases were observed in Al_0.2CrFeNiCo alloy. The hardnesses of the Al_0.2CrFeNiCo and Al_0.2CrFeNiCu coatings after HVOF were HV 591 and HV 361, respectively. After LR, the hardnesses decreased to HV 259 and HV 270, respectively. Although HVOF coatings were most affected by increased load, they showed the highest wear resistance compared to other samples. The lowest wear resistance could be seen in the substrate. After the oxidation tests, HVOF coating layer was completely oxidized and also, the coating layer was delaminated from the substrate after 50 h oxidation due to its porous structure. LR coatings exhibited better oxidation performance. Al_0.2CrFeNiCo was dominantly composed of Cr₂O₃, exhibiting a slower-growing tendency at the end of the oxidation tests, while Al_0.2CrFeNiCu was composed of spinel phases.     

High-Temperature Exposure Studies of HVOF-Sprayed Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-25(NiCr)/(WC-Co) Coating

In this research, development of Cr₃C₂-25(NiCr) + 25%(WC-Co) composite coating was done and investigated. Cr₃C₂-25(NiCr) + 25%(WC-Co) composite powder [designated as HP2 powder] was prepared by mechanical mixing of [75Cr₃C₂-25(NiCr)] and [88WC-12Co] powders in the ratio of 75:25 by weight. The blended powders were used as feedstock to deposit composite coating on ASTM SA213-T22 substrate using High Velocity Oxy-Fuel (HVOF) spray process. High-temperature oxidation/corrosion behavior of the bare and coated boiler steels was investigated at 700 °C for 50 cycles in air, as well as, in Na₂SO₄-82%Fe₂(SO₄)₃ molten salt environment in the laboratory. Erosion-corrosion behavior was investigated in the actual boiler environment at 700 ± 10 °C under cyclic conditions for 1500 h. The weight-change technique was used to establish the kinetics of oxidation/corrosion/erosion-corrosion. X-ray diffraction, field emission-scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS), and EDS elemental mapping techniques were used to analyze the exposed samples. The uncoated boiler steel suffered from a catastrophic degradation in the form of intense spalling of the scale in all the environments. The oxidation/corrosion/erosion-corrosion resistance of the HVOF-sprayed HP2 coating was found to be better in comparison with standalone Cr₃C₂-25(NiCr) coating. A simultaneous formation of protective phases might have contributed the best properties to the coating.     

Performance of high-velocity oxyfuel-sprayed coatings on an fe-based superalloy in Na2SO4-60%V2O5 environment at 900 °C Part I: Characterization of the coatings

This article demonstrates the successful formulation of NiCrBSi, Cr₃C₂-NiCr, Ni-20Cr, and Stellite-6 coatings on an Fe-based superalloy by a high-velocity oxyfuel (HVOF) process for hot corrosion applications. The microstructure, porosity, coating thickness, phase formation, and microhardness properties of the coatings have been characterized using the combined techniques of optical microscopy, x-ray diffraction, scanning electron microscopy/energy-dispersive x-ray analysis. A microhardness tester was used to determine the hardness of the coatings. The coatings in general exhibit characteristic splat-like, layered morphologies due to the deposition and resolidification of successive molten or semimolten powder particles. The NiCrBSi, Cr₃C₂-NiCr, and Ni-20Cr coatings have shown a nickel-base face-centered cubic (fcc) structure as a principal phase, whereas Stellite-6 coating has an fcc Co-rich metallic matrix. Oxides/spinel oxides are formed in small fraction as intersplat lamellae or globules oriented parallel to the substrate surface. Coatings possess some unmelted/partially melted particles, inclusions, and porosity less than 2%. The microhardness of the coatings is found to be higher than the superalloys. The Cr₃C₂-NiCr coating has indicated a maximum microhardness of 990 Hv, while a Ni-20Cr coating has shown a minimum value of about 600 Hv. This article is focused on the characterization of HVOF coatings. The hot corrosion behavior of these coatings in a molten salt (Na₂SO₄-60%V₂O₅) environment at 900 °C under cyclic conditions is being presented as part II included in this issue.     

The Characteristics of Alumina Scales Formed on HVOF-Sprayed MCrAlY Coatings

HVOF MCrAlY (M=Ni, Co) coatings were isothermally oxidized in synthetic airbetween 850 and 1050°C for times up to 167 hr. During thermal spraying,aluminum and yttrium oxidized to form a fine oxide dispersion. The HVOFMCrAlY coatings exhibited a microstructure similar to ODS alloys. The finedispersion consisted of Al₂O₃ and aluminum–yttriumoxides. The oxidation experiments showed that the oxidation rate of HVOFcoatings was two times slower than the oxidation rate of VPS MCrAlYcoatings. The oxidation mechanism changed mainly in the transient-stage(no metastable modification of Al₂O₃ formed) and it wasassumed that the oxide dispersion hindered diffusion of various elementsfrom the bulk material during oxidation. The formation of the fine oxidedispersion also influenced the adherence of the oxide scale. Themicrostructures of the transient oxide scales were examined by X-raydiffraction (XRD) scanning electron microscopy (SEM), and transmissionelectron microscopy (TEM).     

Synthesis and oxidation behavior of nanocrystalline MCrAlY bond coatings

Thermal barrier coating systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO₂/Y₂O₃). In this work, the oxidation behavior of conventional and nanostructured high-velocity oxyfuel (HVOF) NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Freestanding bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at 1000 °C for different time periods to form the thermally grown oxide layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared with that of the conventional one. The observed behavior is a result of the formation of a continuous Al₂O₃ layer on the surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Elevated temperature oxidation behavior of HVOF sprayed TiB_2 cermet coating

Oxidation behaviors of Ni(Cr)-TiB2 coating deposited by HVOF technique were studied at 800,900 and 1 000℃in air. The microstructures of as-sprayed and oxidized coatings were characterized by X-ray diffractometry(XRD)and scanning electron microscopy(SEM).After oxidation at 800℃,a thin and adherent oxide layer was formed on the surface of the coating.With increasing exposure temperature,the thickness of the oxide layer increased;and at 1 000℃the oxide layer separated from the coating.Investigation of the kine...     

Analysis of High-temperature Boiler Tube Failure for T91 ＆ T22

IN2x600MW units of one power plant,the type oboiler is sub-critical,primary reheat,natural circulationbalanced draft,single drum and semi-open coal burnerrating evaporation1745t/h.Its superheater waconsisting of64panels of tubes with12loops eachpanel.The raw materials of tubes was chosen oSA213-T91medium-chrome martensitic heat resistansteel for high temperature part of superheater outletand SA213-T12&SA213-T22low-alloy heat resistansteel for low temperature part.The design temperaturof t…     

Cyclic oxidation behavior of some plasma-sprayed coatings in Na2SO4-60%V2O5 environment

Cyclic oxidation behavior of plasma-sprayed NiCrAlY, Ni-20Cr, Ni₃Al, and Stellite-6 coatings was investigated in an aggressive environment of Na₂SO₄-60%V₂O₅ by thermogravimetric techniques for 50 cycles. These coatings were deposited on a Ni-base superalloy, namely Superni 600; 10Fe-15.5Cr-0.5Mn-0.2C-Bal Ni (wt.%). X-ray diffraction, scanning electron microscopy/energy dispersive x-ray (SEM/EDX), and electron probe microanalyzer (EPMA) techniques were used to analyze the oxidation products. The uncoated superalloy suffered accelerated oxidation in the form of intense spallation of its oxide scale. After deposition of the NiCrAlY coating, the superalloy showed a minimum mass gain, whereas after application of the Stellite-6 coating, a maximum mass gain was observed among the coatings studied. All of the coatings were found to be useful in reducing the spallation of the substrate superalloy. Moreover, the coatings were successful in maintaining continuous surface contact with the base superalloy during the cyclic oxidation. The phases revealed for the oxidized coatings were mainly the oxides of chromium and/or aluminum and the spinels containing nickel-chromium/cobalt-chromium/nickel-aluminum mixed oxides, which are reported to be protective against high-temperature oxidation/hot corrosion.     

Hot corrosion resistance of high-velocity oxyfuel sprayed coatings on a nickel-base superalloy in molten salt environment

No alloy is immune to hot corrosion attack indefinitely. Coatings can extend the lives of substrate materials used at higher temperatures in corrosive environments by forming protective oxides layers that are reasonably effective for long-term applications. This article is concerned with studying the performance of high-velocity oxyfuel (HVOF) sprayed NiCrBSi, Cr₃C₂−NiCr, Ni−20Cr, and Stellite-6 coatings on a nickel-base superalloy at 900 °C in the molten salt (Na₂SO₄-60% V₂O₅) environment under cyclic oxidation conditions. The thermogravimetric technique was used to establish kinetics of corrosion. Optical microscope, x-ray diffraction, scanning electron microscopy/electron dispersive analysis by x-ray (SEM/EDAX), and electron probe microanalysis (EPMA) techniques were used to characterize the as-sprayed coatings and corrosion products. The bare superalloy suffered somewhat accelerated corrosion in the given environmental conditions. whereas hot corrosion resistance of all the coated superalloys was found to be better. Among the coating studied, Ni−20Cr coated superalloy imparted maximum hot corrosion resistance, whereas Stellite-6 coated indicated minimum resistance. The hot corrosion resistance of all the coatings may be attributed to the formation of oxides and spinels of nickel, chromium, or cobalt.     

Mechanical and microstructural properties of HVOF sprayed WC–Co and Cr3C2–NiCr coatings on the boiler tube steels using LPG as the fuel gas

The characteristics and the phase composition of high velocity oxy fuel (HVOF) sprayed WC–Co and Cr₃C₂–NiCr coatings applied onto boiler tube steel substrates using liquid petroleum gas (LPG) as the fuel gas, have been evaluated. The coatings were examined using metallography, SEM/EDAX and XRD techniques. An attempt has been made to describe the transformations that take place during HVOF spraying. The purpose of this study was to compare the microstructure, porosity, surface roughness and microhardness of HVOF sprayed WC–Co, Cr₃C₂–NiCr coatings deposited on boiler steels. The paper will also investigate if there is an application of the deposit in energy generation plants with a view to enhance the life of the component and reduce tube failures in such power plants.     

High Power Diode Laser-Treated HP-HVOF and Twin Wire Arc-Sprayed Coatings for Fossil Fuel Power Plants

This article deals with high power diode laser (HPDL) surface modification of twin wire arc-sprayed (TWAS) and high pressure high velocity oxy-fuel (HP-HVOF) coatings to combat solid particle erosion occurring in fossil fuel power plants. To overcome solid particle impact wear above 673 K, Cr₃C₂-NiCr-, Cr₃C₂-CoNiCrAlY-, and WC-CrC-Ni-based HVOF coatings are used. WC-CoCr-based HVOF coatings are generally used below 673 K. Twin wire arc (TWA) spraying of Tafa 140 MXC and SHS 7170 cored wires is used for a wide range of applications for a temperature up to 1073 K. Laser surface modification of high chromium stainless steels for steam valve components and LPST blades is carried out regularly. TWA spraying using SHS 7170 cored wire, HP-HVOF coating using WC-CoCr powder, Ti6Al4V alloy, and high chromium stainless steels (X20Cr13, AISI 410, X10CrNiMoV1222, 13Cr4Ni, 17Cr4Ni) were selected in the present study. Using robotically controlled parameters, HPDL surface treatments of TWAS-coated high strength X10CrNiMoV1222 stainless steel and HP-HVOF-coated AISI 410 stainless steel samples were carried out and these were compared with HPDL-treated high chromium stainless steels and titanium alloy for high energy particle impact wear (HEPIW) resistance. The HPDL surface treatment of the coatings has improved the HEPIW resistance manifold. The improvement in HPDL-treated stainless steels and titanium alloys is marginal and it is not comparable with that of HPDL-treated coatings. These coatings were also compared with “as-sprayed” coatings for fracture toughness, microhardness, microstructure, and phase analyses. The HEPIW resistance has a strong relationship with the product of fracture toughness and microhardness of the HPDL-treated HP-HVOF and TWAS SHS 7170 coatings. This development opens up a possibility of using HPDL surface treatments in specialized areas where the problem of HEPIW is very severe. The HEPIW resistance of HPDL-treated high chromium stainless steels and titanium alloys, HPDL-treated TWAS SHS 7170 and HP-HVOF coatings, and their micrographs and X-ray diffraction analysis is reported in this article.     

Performance of high-velocity oxyfuel-sprayed coatings on an Fe-based superalloy in Na2SO4-60%V2O5 environment at 900 °C part II: Hot corrosion behavior of the coatings

NiCrBSi, Cr₃C₂-NiCr, Ni-20Cr, and Stellite-6 coatings were deposited on an Fe-based superalloy by the high-velocity oxyfuel (HVOF) thermal spray process. The hot corrosion behavior of the coatings in an aggressive environment of Na₂SO₄-60%V₂O₅ at 900 °C under cyclic conditions was studied. The thermogravimetric technique was used to establish the kinetics of corrosion. X-ray diffraction, scanning electron microscopy/energy-dispersive x-ray and electron probe microanalysis techniques were used to analyze the corrosion products. Hot corrosion resistances of all the coatings were found to be better than the uncoated superalloy. The Ni-20Cr coating was found to be the most protective, followed by Cr₃C₂-NiCr coatings. The Ni-20Cr coating had reduced the mass gain by 90% of that gained by the uncoated superalloy. The hot corrosion resistance shown by the Cr₃C₂-NiCr coating was slightly better compared with the NiCrBSi coating; however, both of the coatings performed better than the Stellite-6 coating. The Stellite-6 coating was the least effective among the coatings studied, but it was still successful in decreasing the mass gain to about one fourth compared with the uncoated superalloy. The formation of oxides and spinels of nickel, chromium, or cobalt may be contributing to the development of hot corrosion resistance in the coatings. This article focuses on the hot corrosion behavior of HVOF coatings. The characterization of these coatings has been presented in part I included in this issue.     

Ultrafine Particulate Dispersed High-Temperature Coatings by Hybrid Spray Process

Oxide dispersion strengthened alloys (ODS), although not commonly used in coating applications, have long been used for high-temperature structural applications due to their superior creep properties. In this paper, we present the design, synthesis, and characterization of a new class of functionally engineered high-temperature coatings in which ultrafine oxide particulates are dispersed in the matrix alloy to achieve superior creep resistance along with improved high-temperature corrosion and erosion resistance. These coatings were fabricated using a novel technique called “hybrid spray process”. Hybrid spray technique combines arc spray and high-velocity oxy fuel (HVOF) spray processes; the metallic matrix alloys are fused by the wire arcing component of the process, whereas the ultrafine particles are synthesized in-flight by the HVOF component from liquid precursors. These particulate dispersed high-temperature composite coatings were fabricated using liquid precursors for SiO₂, Cr₂O₃, Al₂O₃, and wire feed stock of 55/45 NiCr, in one step. The coatings were then characterized using electron microscopy (SEM/TEM) and thermogravimetric analysis (TGA). High-temperature erosion, oxidation, and corrosion performance of these coatings were also evaluated and compared with 304 stainless steel, arc sprayed NiCr coatings as well as Alloy 625 overlay cladding. The hybrid spray process produced dense coatings with uniform dispersion of the ultrafine oxide particles. Further, these coatings also demonstrated superior corrosion, erosion, and oxidation resistance; SiO₂ particulate dispersion being most effective in terms of high-temperature corrosion resistance.     

Scuffing resistance of plasma and HVOF sprayed WC12Co and Cr3C2-25(Ni20Cr) coatings

The objective of this study is to characterize the tribological properties of plasma and HVOF sprayed WC12 Co and Cr₃C₂ 25(Ni20Cr) coatings. The coefficient of friction and scuffing resistance of coatings were found using the tests of ball-on-disc and Falex routines. The chemical composition of coatings was characterized using energy dispersive spectrometry (EDS) performed on the cross-sections of the coatings. The surface of deposits after measuring of friction coefficient was observed using scanning electron microscope (SEM). It was found out that the following properties of the coatings influence the scuffing process: (i) hardness; (ii) friction coefficient; (iii) method of spraying; and (iv) coatings' morphology. The WC12Co coating sprayed using HVOF method showed the best scuffing resistance and the most homogeneous structure.     

Aluminium depletion in NiCrAlY bond coatings by hot corrosion as a function of projection system

Three different projection system are used to prepare NiCrAlY bond coats over metallic substrates: atmospheric plasma spray (APS), high velocity oxyfuel (HVOF) and high frequency pulse detonation (HFPD). These coatings were tested in hot corrosion experiments with sprayed Na₂SO₄ at 1000 °C for 20 and 100 h experiments in air. Coatings surface composition after thermal treatment was characterised by XRD and SEM. Cross section of coatings were analysed by SEM-EDX. A relationship between microstructural characteristics of initial coatings and final performance in hot corrosion was found in terms of porosity percentage: plasma sprayed coatings present higher percentage of porosity compared to HVOF and HFPD projection systems for the same composition and Al is heavily consumed in interparticle oxidation. This Al depletion in turn involves intrinsic chemical failure and surface layer is comprised by a porous spinel of mixed oxides. On the other hand, high energy projection systems produce dense coatings allowing the Al migration to external alumina layer, particularly in the case of HVOF coating.     

Oxidation and Hot Corrosion Resistance of HVOF WC-NiCrFeSiB Coating on Ni- and Fe-based Superalloys at 800 °C

The present work evaluates the oxidation and hot corrosion resistance of high velocity oxy-fuel (HVOF) sprayed WC-NiCrFeSiB coating deposited on Ni-based superalloy (Superni 75) and Fe-based superalloy (Superfer 800H). The coated as well as uncoated specimens were exposed to air and molten salt (Na₂SO₄-25% NaCl) environment at 800 °C under cyclic conditions. The thermogravimetric technique was used to establish the kinetics of corrosion. The corrosion products were characterized using the combined techniques of x-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe micro analyser (EPMA). The WC-NiCrFeSiB coating provides necessary resistance against oxidation and hot corrosion to both the nickel and iron-based superalloys in the given environmental conditions at 800 °C. The oxides of active elements of the coatings, formed in the surface scale as well as at the boundaries of nickel and tungsten rich splats, have contributed for the oxidation and hot corrosion resistance of WC-NiCrFeSiB coatings, as these oxides act as barriers for the diffusion/penetration of the corrosive species through the coatings. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Slurry Erosive Wear Evaluation of HVOF-Spray Cr2O3 Coating on Some Turbine Steels

In this study, Cr₂O₃ coatings were deposited on CF8M and CA6NM turbine steels by high-velocity oxy-fuel (HVOF)-spray process and analyzed with regard to their performance under slurry erosion conditions. High Speed Erosion Test Rig was used for slurry erosion tests, and the effects of three parameters, namely, average particle size, speed (rpm), and slurry concentration on slurry erosion of these materials were investigated. SEM micrographs on the surface of samples, before and after slurry erosion tests, were taken to study the erosion mechanism. For the uncoated steels, CA6NM steel showed better erosion resistance in comparison with CF8M steel. The HVOF-sprayed Cr₂O₃-coated CF8M and CA6NM steels showed better slurry erosion resistance in comparison with their uncoated counterparts. It may be due to the higher hardness as a result of HVOF-sprayed Cr₂O₃ coating in comparison with the uncoated CF8M and CA6NM steels.