studies on the properties of high-velocity oxy–fuel thermal

Materials Science, Vol. 41, No. 6, 2005

SCIENCE FOR PRODUCTION

STUDIES ON THE PROPERTIES OF HIGH-VELOCITY OXY–FUEL THERMALSPRAY COATINGS FOR HIGHER TEMPERATURE APPLICATIONS

T. S. Sidhu, S. Prakash, and R. D. Agrawal

Materials operating at high temperatures in corrosive media suffer erosion-corrosion wear, oxid-ation, and hot corrosion. Among various methods used for the protection of the surfaces againstdegradation, we can especially mention the technology of application of coatings by high-veloc-ity oxy-fuel spraying, which gives coatings which high strength and hardness, low (less than 1%)porosity, and high erosion-corrosion and wear resistances. The characteristics of the coatingsand their protective properties are presented. The role of some high-velocity oxy-fuel coatings inthe protection of metals and alloys against degradation at high temperatures in various media isdemonstrated.

Protective Coatings

In a wide variety of applications, materials have to operate under severe conditions such as erosion, corro-sion and oxidation at higher temperatures in hostile chemical environments. Surface modification of these com-ponents is necessary to protect them against the indicated types of degradation [1].

Only composite materials are able to meet such a demanding spectrum of requirements: the base materialprovides the necessary mechanical strength and coatings provide a way of extending the limits of use of materi-als at the upper end of their performance capabilities by allowing the mechanical properties of the substrate ma-terials to be maintained while protecting them against wear, erosion, or corrosion [2 – 3]. Even if the materialwithstands high temperatures without coating, coating enhances the life period of the material in this case. Themain advantages of coatings are summarized as follows [4, 5]:

— very high flexibility concerning alloy selection and optimization for specific resistance to corrosionenvironments and abrasion/erosion of particles;

— surface properties can be separated from the required mechanical properties of the structural compo-nent;

— coating systems (multi-layered or functionally graded) can be used by combining, e.g., good adhe-sion with optimized corrosion and erosion behavior;

— unique alloys and microstructures can be obtained by thermal spraying which are not possible withwrought materials; these include continuously graded composites and corrosion-resistant amorphousphases;

Metallurgical and Materials Engineering Department, Indian Institute of Technology Roorkee, Roorkee-247 667, India.

Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 6, pp. 80–95, November–December, 2005. Original article sub-mitted February 19, 2005.

1068–820X/05/4106–0805 © 2005 Springer Science+Business Media, Inc. 805

806 T. S. SIDHU, S. PRAKASH, AND R. D. AGRAWAL

— costs of the coating solution are normally significantly lower than those of highly alloyed bulk mate-rials; thermal spray coatings are of especial interest due to their cost/performance ratio;

— thermal-spray coatings additionally offer the possibility of on-site application and repair of compo-nents, given a sufficient accessibility for the sprayer and his equipment; however, thermal sprayingin the workshops is preferred, whenever possible, to achieve optimal results.

Table 1. General Property Criteria for Coating Systems for Elevated-Temperature Services [2]

Component system criteria / property Coating criteria / property

Aerodynamic property Smooth surface finish for coating must conformto the appearance of precision cast component

Mechanical strength and Coating must be resistant to all types of stress (impact, fatigue,microstructural stability creep, and thermal) to which system is exposed

System adhesion, bonding, Coating/substrate must be compatible without gross thermal or and interface stability structural mismatch; diffusion rates at the interface must be

minimum at operating temperatures just as the compositional changes, development of embrittling phases must be avoided

Surface resistance to erosion Coating composition must have sufficient reserve of all reactantand oxidation/hot corrosion constituents to meet scale reformation needs without marked

deterioration in the protection ability; coating must be ductile andmust develop uniform, adherent, and ductile scale at low rates

Use of Coatings at Higher Temperatures

Protective surface treatments are widely used at low temperature, the use of these at elevated temperature ismore recent. High-temperature applications are largely limited to the aerospace industry. An enormous chal-lenge exists to develop and apply these techniques to other high-temperature applications [6].

The demand for protective coatings has recently increased even for almost all types of superalloys, sincehigh-temperature corrosion problem become much more significant for these alloys with increasing operatingtemperatures of boilers, turbines, and heat engines. The necessities for higher performance and increased effici-ency have resulted in the progressive increase in their operation temperatures [7–9]. As a result, componentsoperating at high temperatures in plants of this sort are coated or surface treated [10].

Although superalloys were designed for higher temperature applications, protective coatings are also ap-plied to enhance their life for using in corrosive environments as they are not able to meet both the high-tempe-rature strength and high-temperature corrosion resistance simultaneously [11] (see Table 1).

Coating Techniques

There are many available coating-deposition techniques and choosing the best process depends on the func-

STUDIES ON THE PROPERTIES OF HIGH-VELOCITY OXY–FUEL THERMAL SPRAY COATINGS 807

tional requirements, adaptability of the coating material to the intended technique, the required level of adhesion(size, shape, and metallurgy of the substrate), availability and cost of the equipment (see Fig. 1) [12].

Fig. 1. Coating-deposition technologies [12].

From the production point of view, chemical vapor deposition (CVD) from a pack, physical vapor deposi-tion (PVD), and thermal spraying (metal spraying) are in current use [13]. Since the CVD process is a nonline-off-site technique, proper masking and tooling are the major design considerations. Another shortcoming of theCVD process is the inclusion of pack particles in the coating, which can lead to its failure [14].

Thermally sprayed coatings often have superior properties, lower application cost, and less environmentalissues as and when compared to other industrially used coatings, such as CVD, PVD, and hard chromium plating[15]. It is reported that overlay coatings perform better than diffusion coatings at higher temperatures [16]. Fordepositing overlay coatings, thermal spray technologies are often considered [17].

Thermal Spray Techniques

In the early 1900s, Dr. Max Schoop, young Swiss inventor, invented thermal spraying after watching hisson playing with a toy cannon. Dr. Schoop observed that the hot lead shots projected out of the cannon stuck toalmost any surface, the result of which gave him the idea that if a metal could be melted and projected in aspray-like manner, then a surface could be built up with this material.


Fig. 2. Schematic diagram of a thermal spray process [19]: (1) materials, (2) gas/air, (3) energy, (4) thermal spray, (5) coated part;

the diamond-jet gun is used as an example [35].

The technology continued but expanded in the 1970s due to the development of thermal plasmas and theincreasing demand for high-temperature and wear-resistant materials and coating systems [18]. In thermalspraying, the initial coating material (materials in the form of rods, wires, or powder) is heated, generally to amolten state and projected onto a receiving surface, known as substrate, as shown in Fig. 2 [19].

The thermal spray processes used to deposit coatings for protection against high-temperature corrosion areas follows [5]: flame spraying with powder or wire, electric-arc wire spraying, plasma spraying, spray and fuse,high-velocity oxy-fuel (HVOF) spraying, and detonation gun.

The particle speed, flame temperature, and spray atmosphere are the main parameters differentiating vari-ous spraying techniques (see Table 2) [12, 19–20]. Coating porosity, bond strength, and oxide content are typi-cal properties influenced by the coating procedure. In 1988, METCO introduced the Diamond-Jet HVOF sys-tem, which is now the main topic of interest among the researchers and industrialists due to its superior proper-ties as compared to many other thermal spraying processes.

High-Velocity Oxy–Fuel Thermal Spraying

The HVOF processes belong to the family of thermal spraying technologies and are capable of producingcoatings with lower porosity, higher hardness, superior bond strength, and lower decarburization than manyother thermal spraying methods [1, 21–24]. In recent years, there has been a considerable growth in the use ofthis spraying process to deposit cermets, metallic and ceramic protective overlay coatings, which are typically

100–300 μm thick, onto the surfaces of engineering components to allow them to function under extreme condi-tions [21, 25]. Thermally sprayed coatings previously had limited usefulness as corrosion protection coatingsdue to the presence of interconnected porosity in the structure. However, HVOF coatings have recently gainedpopularity and are now extensively studied for their corrosion-resistant properties [26]. So far, several HVOFsprayed coatings have been subjected to corrosion testing in seawater, including cermets [27–29] and anticorro-sion alloys [30–32]. These studies make it possible to conclude that the HVOF method produce coatings withhigher corrosion resistance as compared with other spraying technologies.

The HVOF sprayed coatings have found wide application in marine, aircraft, automotive and other indus-tries. For reclaiming a wide range of petrochemical-process components, such as storage vessels, heat exchang-ers, pipe end fittings, and valves subjected to severe erosive, wear, and corrosive conditions, the Amoco OilCompany routinely employs the HVOF process by applying AISI-316-L and C-276 Hastalloy coatings [33].

The HVOF spraying techniques are predominantly used as wear, corrosion, and oxidation resistant barriersresulting in increased lifetimes as compared with the uncoated substrate components [34].


Table 2. Comparison of the Characteristics for Various Thermal Spraying Processes [12, 19 – 20]

Depositiontechnique

Heat source PropellantMaterialfeed type

Spray gun temp.

(°C)

Particlevelocitym / sec

Coatingmaterials

Relativebond

strength

Porositylevel

vol. %

Electric arc Arc between Air Wire 6000 240 Ductile Good 8 – 15electrodes materials

Plasma arc Plasma arc Inert gas Powder 16,000 120 – 600 Metallic, Very goodspraying ceramic, to excellent 2 – 5

plastic, andcompounds

Low- Plasma arc Inert gas Powder 16,000 900 Metallic, Excellent < 5pressure ceramic,plasma plastic, and

spraying compounds

Spray & — — Powder — — Fusible Excellent < 0.5fuse metals

Flame Oxyacetylene / Air Powder 3000 30–120 Metallic Fair 10 – 20spraying oxyhydrogen and ceramics

Detonation Oxygen / Detonation Powder 4500 800 Metallic, Excellent 0.1–1gun acetylene / shock ceramic,

spraying nitrogen gas waves plastic, anddetonation compounds

High- Oxypropylene / Combustion Powder / 3000 800 Metallic Excellent 0.1 – 2velocity hydrogen / jet wire and ceramicoxy-fuel propane /(HVOF) LPG

The HVOF Spraying Process

In the HVOF process (Fig. 3), the powder / wire material is melted and propelled at a high velocity towardthe surface with the use of oxygen and fuel gas mixtures. Propylene, propane, hydrogen, acetylene, methane,ethylene, crylene, SPRAL-29 kerosene, MAPP (methyleacetylene-propadiene-stabilized gas), LPG, etc. are usedas combustion fuels. The HVOP system consists of a spray gun, powder-feed unit, flow-meter unit, and an airand gas supply unit. The powder feed unit comprises a hopper assembly, air vibrator, feed-rate meter, and con-trol cabinet.


Fig. 3. Schematic cross section of a diamond-jet spray gun [35]: (1) powder with nitrogen carrier gas, (2) oxy-propylene, (3) com-

pressed air, (4) gun head, (5) air envelope, (6) shock diamonds due to supersonic spread of the particles, (7) balanced oxy-fuel flame, (8) combustion zone, (9) molten powder particles, (10) sprayed material, (11) substrate or forming die.

The desired powder is fed from the powder feed unit by means of a carrier gas to the gun, where combus-tion occurs. The amount of powder required for deposition can be regulated by using the powder feed-rate me-ter. In the combustion zone, the powder material enters the flame, where it becomes molten or semimolten, de-pending on the melting temperature and the feed rate of the material. The flame temperature for the HVOF pro-cess is about 3000°C [20]. The molten or semimolten particles are then propelled out of the gun nozzle at veryhigh velocities toward the target/substrate, where the material is deposited. Powder particles, typically within the

range 10–63 μm, attain velocities of 300–800 msec– 1 at the substrate to be coated [12, 22, 36–38].

The quality of the coatings significantly depends on the velocity and temperature of the powder particlesimpinging upon the substrate surface, which, in turn, is associated with the gas pressure developed in the com-bustion chamber. In the HVOF spray systems of the first and second generations (Continuous Detonation Spray-ing, Top Gun, Jet-Kote, and Diamond Jet), combustion occurs under pressures of 3–5 bar and the flame attains asupersonic velocity in the process of expansion at the exit of the nozzle. These systems produce comparable par-ticle velocities with the standard spray parameters and the same fuel gases and powders. Thus, in the course ofspraying of a WC–17% Co powder in these systems by using particles whose sizes were distributed within the

range – 45 + 10 μm and propane as fuel, the resulting particle velocities were about 450 m / sec [39]. The HVOF systems of the third generation (Diamond Jet Hybrid 2600 and 2700, JP-5000, OSU Carbide

Jet, and TOP Gun K) operate at higher combustion pressures within the range of 6–10 bar. These systems per-mit higher particle velocities and higher spray rates. Thus, for the WC–Co powder, the velocities are about 600–650 m / sec and the spray rates increase up to 10 kg / h or even up to 18 kg / h (in the JP-5000 system) without anydeterioration of the coating quality [39].

Advantages of the HVOF System

The HVOF process is designed around producing high velocities of spray particles and this contributes tothe advantages of the HVOF spraying over the other processes of thermal spraying in terms of particle condition[40–41] including:

— more uniform and efficient particle heating due to the high turbulence experienced by the particles in-side the combustion chamber;

— much shorter exposure time in flight due to the high particle velocities;


Fig. 4. Characteristics of the HVOF and standard plasma-process coatings [42]: (I) hardness, (II) porosity: (III) oxide content, (IV)

bond strength, (V) maximum thickness.

Table 3. Benefits of Using HVOF Coatings [19]

Coating benefit Main causes of this benefit

Higher density (lower porosity) Higher impact energy

Improved corrosion barrier Lower porosity

Higher hardness ratings Better bonding, less degradation

Improved wear resistance Harder and tougher coating

Higher bond and cohesive strengths Improved particle bonding

Lower oxide content Less in-flight time of exposure to air

Lower content of unmelted particles Better particle heating

Greater chemistry and phase retention Reduced time at higher temperatures

Thicker coatings (per pass and total) Lower residual stress

Smoother as-sprayed surfaces Higher impact energies

— lower surface oxidation due to the short particle exposure time as compared to other thermal sprayingtechniques;

— reduced mixing with ambient air once the jet and particles leave the gun;

— lower ultimate particle temperatures as compared to other processes, such as plasma or arc guns, asthese processes operate at temperatures of 16,000 and 6000°C, unlike 3000°C in the HVOF (oxy-gen/propylene mixture) process.

Stokes [19] summarized the causes of producing of high-quality coatings by the HVOF process as shown inTable 3. The HVOF coatings have better qualities as compared with those produced by using the standard plas-ma-spraying process as indicated in Fig. 4 [42].


Fig. 5. Schematic diagram of a spherical particle impinged upon a flat surface [19]: (1) rim, (2) core (& 0.1 d high), (3) splat, (4) sub-

strate, (5) spherical particle prior to impact [19].

(a) (b)

Fig. 6. Two morphological forms of lamellae splashed on the substrate: (a) pancake, (b) flower; (1) crack, (2) deformed substrate,

(3) corona, (4) substrate, (5) lamella; (I) top view, (II) X-section view [43].

Splat Formation and Building Up an HVOF Coating

Depending on the melting temperatures of the particle relative to the flame temperature, the particle may bemolten, semimolten, or solid when it impacts a substrate or a precoated surface. The state of the particle as itleaves the combustion zone affects the final microstructure of the coating [1].

The HVOF coatings have lenticular or lamellar grain structure resulting from rapid solidification of smallglobules flattened upon striking a colder surface at high velocities (Fig. 5). Initially, the particle is melted andpropelled out from the gun in the form of a sphere. Then, in its first contact with the substrate, the impact createsa shock wave inside the lamella and in the substrate. Postimpact forms the particle into a “pancake” shape la-mella or splat (associated with moderate particle velocities and moderate heat contents) shown in Fig. 6a or a flo-wer type lamella (connected with an elevated velocity of the particle and an elevated heat content) as shown inFig. 6b when the process of solidification takes place [43]. When a molten droplet arrives at the surface at a highvelocity, the process of spreading of the splat is restrained by surface tension. Otherwise, the extremities of thesplat either become weak and break off or form small spherical drops [19].


Fig. 7. Cross section of the lamella splat depicted in diagram (a) is shown in diagrams (b, c) representing possible microstructures of

the lamellae: (1) layers of lamella, (2) substrate, (3) lamella split, (4) formed columns, (5) brick-wall effect [19].

The surface roughness of the substrate also affects the splat formation and the loss of adherence occurs if aninadequate amount of surface roughness is present whilst spraying [44 – 45].

Building Up a Coating

A coating is the buildup of individual particles striking the substrate. In one pass of the spray gun, a layerof 5–15 lamellae in thickness is formed depending on the processing parameters, such as the powder feed rate,spray distance, particle size, and linear speed of the torch. Hence, several passes of the spray gun are required tobuild up a coating across the workpiece. Meanwhile, the layer deposited by the first pass may be subjected tooxidation (for highly oxidizable materials) and cooling. On the second pass, the temperature of the first layer(which may be partially solidified) cools the second layer due to the difference in temperatures between the firstand second layers. The final coating may comprise 5 –200 passes of the deposited material. Afterward, thecoating is allowed to cool down to room temperature. During this period, thermal stresses are generated oftenleading to crack formation in the coating or separation from the substrate [19].

Once a molten or semimolten particle strikes the substrate or previously deposited material, solidificationstarts and a columnar structure of the deposit is formed as shown in Figs. 7a, b [19] or changes into a brick-wall-type structure depicted in Fig. 7c [1, 19], where a low rate of heat removal is experienced between the particleand the adjoint material interface due to substrate oxidation and/or surface roughness. In either case, the solidifi-cation inside each lamella is repeated as the coating is built up to the required thickness.

Physical and Mechanical Properties of the Coatings

In comparison with the other thermal spraying processes, the HVOF coatings have low roughness, highbond strengths, higher hardness, lower porosity, low oxide content, and higher thickness [46–47]. The overallresult is a coating with reproducible characteristics and high erosion, corrosion, and wear resistance. Due to highvelocity and high impact of the sprayed powder particles, the coatings produced by the HVOF spraying processare less porous and have higher bond strength than the coatings produced by the other methods, such as plasmaspraying, flame spraying, and electric arc spraying [48 – 50].

The high kinetic energy of the particles in the HVOF process deforms the particles in a plastic state ratherthan in a molten state. As a result, the oxidation of spray metal during flight and flattening is relatively weaker,since oxidation occurs only by a relatively slow diffusion mechanism.


Fig. 8. Schematic diagram of adhesion of a particle to a substrate asperity [19]: (1) asperities, (2) particle in flight, (3) particle mech-

anically locked to the asperities of the substrate, (4) substrate.

Despite the plastic state, the high kinetic energy of particles still allows flattening by deformation and leadsto a dense and pore-free coating with low oxygen content. This characteristic of the HVOF process is of highimportance for spraying mechanically alloyed materials [50 – 53].

Hamatani et al. [54] reported that the HVOF sprayed carbide dispersed Ni-based alloy (Cr3 C2 – NiCr) can

have a hardness of 1150 HV and an adhesion strength of 200 MPa. Further, by using Ni Cr of 20 wt. % and asmaller primary powder size, the adhesion strength of Cr3

C2 – Ni Cr coating can be improved to 250 MPa. Inthe case of Ni Cr < 20 wt. %, due to the high carbide rate, the strength of the coating decreases. On the otherhand, if Ni Cr > 20 wt. %, then the softer particles could give a decrease in adhesion strength. Further, in thecase of using smaller particles, the relatively extensive surface area causing effective kinetic momentum and heattransfer from the gas flame to the particles attributes superior acceleration and heat, which results in better adhe-sion strength of the coatings.

Microstructural Properties. The detailed microstructural examination of HVOF sprayed powders showsthat these coatings exhibit characteristic splatlike layered morphologies due to the deposition and resolidificationof molten or semimolten powder particles. During HVOF spraying, powder particles contain, generally speak-ing, three separate zones: fully melted regions, partially melted zones, and the unmelted core. However, the re-lative proportion formed in an individual powder depends on its particle size, trajectory of motion through thegun, the gas dynamics (velocity/temperature) of the thermal-spray gun, and the type of the gun [55–58].

Corrosion Behavior of the Coatings. Dense coatings provide better corrosion resistance than the porouscoatings because porosities can do harm to the persistent corrosion resistance of the coating [27, 59 – 61]. In theHVOF process, the powder particles are propelled out of the gun nozzle at high velocities toward the substrate.Due to high velocities and high impacts of sprayed powder particles, the coatings produced by the HVOF spray-ing process are very dense. Numerous research workers have reported that the HVOF coatings have higher cor-rosion resistance than the other thermal sprayed coatings [28, 30, 60 – 67].

Adhesion of the Coatings. The HVOF coatings have higher bond strengths than the most part of the coat-ings obtained by the other processes of thermal spraying due to the high kinetic energy attained by the impingingparticles [68]. The bond strength observed in the HVOF system is higher than the strength attained in the plasmathermal spraying process by 25% [19].

The adhesion strength of the impinging particle to the substrate depends on the mechanical and metallurgi-cal–chemical mechanisms. An impinging particle flattens, conforms to a suitably prepared surface, and thenmechanically locks itself to the receptive asperities as shown in Fig. 8. Subsequently, diffusion or alloying mayoccur leading to the formation of intermetallic compounds. This is known as the metallurgical–chemical mech-anism of adhesion [19, 69].


Elevated bond strength depends not only on the applied deposition process but also on the purity (absenceof oxides and other foreign elements), adequate roughness, and adhesion to the substrate surface parallel withplastic deformation exhibited by the particle on the substrate surface. Cohesive strength depends on the rough-ness, temperature difference, and bond/cohesive strength of the previously coated surface. Coating propertiesare also governed by the splat layering which, in turn, depends on the particle parameters at the impact, the shapeand topology of the already deposited layers, the ability of the flattening particle to accommodate their pores, as-perities, etc., and finally, on the temperature at the time of impact [70].

The full melting of spray particles does not contribute to the increase in the adhesion of HVOF metalliccoatings. On the other hand, the deposition of partially melted large particles contributes to the substantial im-provement of adhesion strength to more than 76 MPa, i.e., twice as large as for the coating deposited with com-pletely melted particles [71].

Protective Role of HVOF Coatings at Elevated Temperatures

The thermal spray coatings used at higher temperatures must be dense enough so that protective oxides canform inside voids and fill them and thick enough to postpone the diffusion of corrosive species to the substratematerial till the formation of protective oxides inside the coating [72]. Guilemany et al. [73] also reported thatthicker coatings provide better resistance against corrosion.

Splats formed in HVOF coatings are very flat, which is the desired structure of coatings intended for opera-tion in corrosive environments at higher temperatures. Corrosive species mostly propagate along the splatboundaries. In the HVOF coatings, the distance from the coating surface to the coating/substrate interface alongthe splat boundaries is very long [74].

Alloys and coatings designed to resist oxidizing environments at high temperatures should be able to deve-lop a surface oxide layer, which is thermodynamically stable, slowly growing, and adherent [75]. The perfor-mances of some HVOF-sprayed coatings used at higher temperatures (up to 1000°C) in corrosive environmentshave been reviewed.

Cr3 C2 – Ni Cr Coating. The HVOF-sprayed Cr3

C2 – Ni Cr coatings are mainly used at temperatures be-tween 600 and 900°C in steam turbine blades or boiler tubes for power generation [59, 76] due to their behav-ior against aggressive corrosion and erosive–abrasive atmospheres and their thermal stability at high tempera-tures [77]. In view of of their good resistance to high-temperature corrosion and erosion, these coatings are con-sidered as protective coatings for heat exchanger pipes and fluidized bed combustors [78 – 79]. The HVOF-sprayed Cr3

C2 – Ni Cr coatings have high erosion, corrosion, and wear resistance properties [64, 80 – 84]. Vuoristo, et al [80] reported that plasma spraying, detonation gun spraying, and high velocity oxy-fuel

spraying methods are mostly used to apply chromium carbide coatings to work as wear-resistant coatings againstabrasion and erosion in corrosive environments at high temperatures up to 900°C. The room temperature abra-sion and erosion wear resistance of

Cr3

C2 + 25% Ni Cr

coatings applied by these methods was compared and

the high-velocity combustion process was reported to have highest wear resistance. The corrosion resistance of HVOF-sprayed Cr3

C2 – Ni Cr coatings protecting the heat exchanger pipes ofrecuperators used in steel mills was evaluated. It was found that these pipes corroded via oxidation, sulfidation,and molten salt corrosion. Three kinds of corrosion tests under cyclic conditions were carried out: high-tempera-ture oxidation tests in air, cyclic oxidation tests in an SO2-containing environment, and a molten-salt corrosiontest. High-temperature oxidation tests were carried out in a box furnace for 50 h at 1000°C. Another high-tem-perature corrosion test was performed in a N2 – 4.8% O2 – 0.2% SO2 atmosphere at 1000°C for 100 h. The

molten-salt corrosion test of the coating was carried out at 700°C for 20 h in a sodium sulfate ( Na2 SO4 ) and

iron sulfate Fe2 ( SO4 )3 salt environment carrying an O2 – 0.5% SO2 gas mixture. In this experiment, a plati-


num catalyst was used for the fast transformation of SO2 into SO3 . It was found that Cr3

C2 – NiCr coatingsexhibited excellent corrosion resistance in the molten salt as well as in the oxidation environment. The HVOF-sprayed Cr3

C2 – Ni Cr coating was recommended as a promising coating for heat exchanger pipes suffering themolten-salt corrosion attack [85].

Wang and Luer [86] reported that HVOF-sprayed Cr3 C2 – Ni Cr coatings are used in elevated-temperature

service environments including fluidized bed boilers, coal-fired boilers, and municipal waste incinerators, inlight of the excellent corrosion and oxidation resistance of the nickel chromium alloy and reasonable wear resis-tance of chromium carbides at temperatures of up to 900°C. They investigated the erosion-corrosion (oxidation)behavior of the HVOF Cr3

C2 – Ni Cr coatings at elevated temperature. It was found that the erosion-oxidationresistance of the HVOF Cr3

C2 – Ni Cr coating was higher than the resistance of the substrate (1018 steel) andother thermal-spray coatings including FeCr Si B (Armacor M), Ni-base, Cr2

O3 – 6Si O2 – 4Al2 O3 (Rokide C),

Cr2 O3 –12Si O2 – 2Al2

O3 – 4Mg O (Rokide MBC), and WC – Ni Cr Co (SMI 712). The high erosion-oxidationresistance of the HVOF Cr3

C2 – Ni Cr coating was explained by its low porosity, fine-grained structure, and thehomogeneous distribution of hard carbides/oxides which form a skeletal network inside the ductile and corro-sion-resistant metal binder.

The performance of HVOF-sprayed Cr3 C2 – Ni Cr coatings on a Fe-based superalloy in a Na2 SO4 – 60%

V2O5 environment at 900°C under cyclic conditions were evaluated by Sidhu, et al. [87]. The thermogravimet-ric technique was used to establish the kinetics of corrosion. It was found that the mass gain of the coated super-alloy was less than 1 / 5 of the mass gain of the uncoated superalloy. The corrosion resistance of Cr3

C2 – Ni Cr-coated alloys was reported to be due to the formation of phases like Cr2

O3 , Ni O, and Ni Cr2

O4 .

Nickel-Chromium Coatings. Nickel-chromium alloys are used as coatings to deal with oxidation and cor-rosive environments at high temperatures. When nickel is alloyed with chromium, this element oxidizes to theprotective surface oxide

Cr2

O3 at rates which could make it suitable for use up to about

1473°K

[88]. The fast-

er the formation of the surface oxide layer, the better the offered protection [89]. There is an increasing interestin the deposition of Ni-based metallic alloys for protection against corrosion. Ni-based coatings are used in ap-plications, where wear resistance combined with oxidation or hot-corrosion resistance is required [90].

The high resistance of high-chromium nickel-chromium alloys to high-temperature oxidation and corrosionmakes them widely used as welded and thermally sprayed coatings in fossil fuel-fired boilers, waste incinerationboilers, and electric furnaces [91]. Modern thermal-spray processes, such as HVOF, are often applied to deposithigh-chromium nickel coatings onto the outer surfaces of various parts of the boilers, e.g., tubes, to prevent thepenetration of hot gases, molten ashes, and liquids to the less noble carbon-steel boiler tubes [92].

Sundararanjan, et al [93] evaluated the steam-oxidation resistance of the HVOF and air-plasma-sprayed Ni–20Cr metallic coatings on 9Cr–1Mo-type steel at four steam temperatures within the range 600–750°C. The re-sults showed that the thick and dense HVOF coating exhibited better steam oxidation resistance than the thin andporous APS coating. The formation of continuous protective oxide scale of Cr2

O3 was observed on the sur-faces of the HVOF coatings, despite their low concentration in the coating material. The diffusion of nickel fromthe coatings to the substrate and iron from the substrate to the coatings for longer exposures to steam oxidationwas also noticed. The diffusion rates of Ni and Fe were found to be almost similar. The diffusion of iron led tothe formation of

Fe2

O3 scale which was suggested to be the cause of nonprotectiveness of the coatings for long-

er exposure times. In the case of Ni – 50

Cr

coatings, the HVOF coating formed a complete

Cr2

O3 healing layer,

which prevents the growth of scale. In addition, in the presence of an APS coating, the initiation of scale wasalso observed on the interface of the coating and the substrate. At the same time, in the HVOF coating, it wasabsent [94].

The high-temperature corrosion resistance of Ni – 20Cr,

Ni – 50

Cr

and

Cr

coated boiler tubes in the actual

refuse incineration plant, as well as in the laboratory tests, was evaluated by Yamada, et al. [95]. It was observedthat the detonation-sprayed

N i – 50Cr

coating exhibited the highest corrosion resistance in laboratory tests at


873°K followed by HVOF among the detonation-gun-sprayed, plasma-sprayed, and HVOF-sprayed coatings.The detonation-sprayed Ni – 50Cr coated tubes performed very well for seven years of testing in the actual plantwithout any problems and were expected to have a longer life. The authors also conclude that the Incolloy-825and Inconel-625 with high content of nickel possess higher corrosion resistance than stainless steels.

The high-temperature corrosion behavior of HVOF-sprayed Ni / Cr coatings on ferritic and austenitic boilersteels was studied in an oxidizing atmosphere of 500 vppm HCl, 3% O2

, 14% CO2 , 20% H2

O, and argon asbalance for 1000 h . It was discovered that the Ni – 49Cr – 2Si coating performed well because no corrosionproducts were detected on the coating surface. No internal attack or attack on the substrate through this coatingwas observed. At the same time, the Ni – 57Cr Mo Si B and Ni – 21Cr – 9Mo Fe coatings proved to be poor in re-sisting high-temperature oxidation as the substrates were attacked by corrosive species through the voids and ox-ides [74].

The high-temperature behavior of the same HVOF coatings in the presence of a salt environment of 40% Na2 SO4 – 40% K2 SO4 – 10NaCl – 10KCl was again investigated in two environments, namely, in an oxidizingenvironment of N2 – 20H2

O – 14CO2 –3O2 – 500 vppm HCl and a reducing environment of N2 – 20H2 O – 5CO –

0.06H2 S – 500 vppm HCl . It was found that corrosion was more severe in oxidizing environments than in re-

ducing environments. Active oxidation was responsible for the accelerated corrosion in oxidizing environments.The coatings were prone to chlorine attack in both atmospheres through the interconnected oxide network on thesplat boundaries. The Ni – 57CrMoSi B coating was the only material forming a protective oxide layer. At thesame time, under reducing conditions, the materials with high chromium content were found to be able to form aprotective layer containing chromium, sulfur, and sodium. The corrosion resistance of this layer increased withits chromium content. The corrosion resistance of nickel-based high chromium coating materials was found sat-isfactory in the testing conditions [96].

Lianyong, et al [97] reported that oxidation, sulfidation, and molten-salt corrosion are the main high-tempe-rature corrosion problems for the water-wall tubes of boilers used in thermal power plants, where the most severecorrosion occurs in molten-salt corrosive environments. They studied the corrosion resistance of high-velocityelectric-arc-sprayed (HVAS) coatings. By comparison, they recommended Ni Cr (Ni – 45Cr – 4Ti) as a promis-ing alloy coating for the water-wall tubes, which can resist the molten-salt corrosion attack. They found that thecorrosion resistances of Ni Cr and Fe Cr Al coatings were much better than the corrosion resistance of 20 g steel.The Ni Cr coatings showed the best anticorrosion properties and had a slightly lower porosity than Fe Cr Al coat-ings. The authors also concluded that the corrosion resistance of the coatings was mainly determined by thechromium content, whereas the microstructure of the coating is as important as the chemical composition of thematerial.

The HVOF-sprayed Ni – 20Cr wire coatings have a dense and nearly uniform lamellar microstructure withporosity less than 1% and hardness varying within the range 600–630 Hv [58]. Sidhu et at. [87] examined theperformance of this coating on a Fe-based superalloy in a Na2 SO4 – 60% V2 O5 environment at 900°C undercyclic conditions. The thermogravimetric technique was used to establish the kinetics of corrosion. The authorsreported that the Ni – 20Cr wire coating showed high corrosion-resistance behavior and successfully reduced themass gain by 90% as compared with the gain observed for bare superalloys. The corrosion resistance of thiscoating may be caused by the presence of nickel and chromium oxides and their spinels.

Ni Cr B Si Coatings. The largely employed Ni-based powder belongs to the Ni–B–Si system with the ad-dition of other alloying elements. The addition of chromium improves the oxidation and corrosion resistance atelevated temperatures and increases the hardness of the coating by the formation of hard phases. Boron de-creases the melting temperature and contributes to the formation of hard phases. Silicon is added to increase theself-fluxing properties. Carbon produces hard carbides with elevated hardness, thus improving the wear resis-tance of the coatings [98 – 99].

The microstructural characterization of the HVOF-sprayed Ni Cr BC coating applied to low-carbon mild


steel shows that the coatings has layered morphologies due to the deposition and solidification of successive

molten or semimolten splats. The microhardness of the coatings is found to be ∼ 6.0 GPa [55]. Cha, et al. [100] studied the high-temperature corrosion behavior of HVOF-sprayed Ni – 20Cr, Ni Cr BSi

and Cr3 C2 – Ni Cr coatings applied to 15Mo3 high-temperature steel in a corrosive atmosphere of HCl – H2

O –

O2 – N2 at 500°C. They revealed that the Ni – 20Cr coatings are more corrosion resistant than the Ni Cr BSi andCr3

C2 – Ni Cr coatings. Further, they discovered that the Cr3 C2 – Ni Cr coatings exhibited slightly better corro-

sion-resistant behavior than the Ni Cr BSi coatings under similar conditions. It was also shown that the HVOF-sprayed Ni – 20Cr, Ni Cr BSi and Cr3

C2 – Ni Cr coatings are resistant to corrosion at higher temperatures. Cha and Wolpert [101] again studied the high-temperature erosion and corrosion behavior of the HVOF-

sprayed coatings on 15Mo3-based materials in the environment containing HCl,

H2

O, O2

, and

Ni

at a tempera-

ture of 500°C for 168 h. The coatings were fabricated from Colmonoy-62 Ni-based (Ni

Cr

Fe

BSi) and T800

Co-based materials. The authors reported that T800 Co-based coatings showed poor corrosion resistance ascompared to Colmonoy-62 Ni-based coatings. They further indicated that Colmonoy-62 applied by the HVOFand APS processes exhibited almost the same corrosion rate. However, the HVOF process produced less oxides,especially along the fusion line, and the coalescence of oxides was stronger for the AP process.

Miguel, et al. [102] studied the wear resistance and the mechanisms of wear of Ni Cr BSi coatings obtainedby spray and fuse and as-sprayed coatings obtained by atmospheric plasma spraying (APS) and HVOF spraying.They found that both the HVOF-sprayed and sprayed-and-fused coatings performed well, whereas the plasmasprayed coatings showed the worst sliding wear resistance.

Electrochemical tests were used to evaluate the corrosion resistance of HVOF-sprayed Ni

Cr

BSi

coatings

on the steel substrate. It was found that HVOF-sprayed Ni

Cr

BSi coatings have excellent corrosion resistance in

alkali solutions because their surface can be kept in the self-passivation condition. It was reported that the ef-fects of porosities on the early corrosion of the coating are not serious unless there are penetrating porosities.However, the presence of porosities can do harm to the persistent corrosion resistance of the coating [60].

Cobalt-Based Coatings. Cobalt-based alloys containing chromium, tungsten, and carbon, are known as“Stellite” alloys. One of the common Co-based alloys in use is Stellite-6 [56]. In Stellite-6 alloys, Cr providesoxidation and corrosion resistance, as well as strength by the formation of M7

C3 and M23 C6 carbides. Refrac-

tory metals, such as Mo and W, are known to be solid-solution hardening elements and also contribute tostrength via precipitation hardening by forming MC and M6

C carbides and intermetallic phases, such asCo3

(Mo, W). Further, the alloying addition of Ni, C, and Fe promotes the stability of the fcc structure of Co-

rich matrix, which is stable at high temperatures up to the melting point (1495°C), whilst Cr, Mo, and W tendto stabilize at low temperatures in the hexagonal close-packed (hcp) crystal structure, which is stable at tempera-

tures below 417°C. Stellite-6 has high hardness at high temperatures, high corrosion resistance, and high wearresistance [103–104]. However, there is a very little discussion in the literature on the performance of HVOF-sprayed Stellite-6 coatings at elevated temperatures [105].

Kong, et al. [56] studied the microstructural characterization of HVOF-sprayed Stellite-6 coatings depositedon low-carbon mild steel. It was revealed that the microstructure of the as-sprayed coatings comprised, in gen-

eral, a microcrystalline fcc Co-based metallic matrix with some regions accounting for ∼ 20% of the exhibitedpartially melted dendritic structure. A small fraction of unmelted feedstock powder particles was also present inthe coating. It was also reported that the

M7

C3 carbide phase was not present in the coatings, although it was

present in the original feedstock powder. The authors suggested that its formation in the coatings might havebeen suppressed by high cooling rates occurring in the course of splat solidification.

The high-temperature erosion and corrosion were studied for thermally sprayed coatings on 15Mo3- and13CrMo44-based materials. The coatings made of Ni-based materials, like Colmonoy-62 (Ni Cr Fe BSi) and Ni Cr BSi / WC, and T800 Co-based material were fabricated by thermal-spray processes, such as HVOF, flame


spraying and sintering (FS/sinter) and APS. The corrosion studies in the environment containing HCl, H2 O,

O2 , and N2 at a temperature of 500°C indicated that Colmonoy-62 applied by the HVOF and APS processesrevealed the same corrosion rates. The HVOF-sprayed T800 coating experienced the highest corrosion rate. TheNi Cr BSi / WC coating formed by the FS/sinter method proved to be a fair combination of erosion and corrosionresistance [101].

Sidhu, et al. [87] investigated the performance of HVOF-sprayed Stellite-6 coatings on a Fe-based superal-loy in the Na2 SO4 – 60% V2 O5 environment at 900°C under cyclic conditions. The authors demonstrated thatthe Stellite-6 coating was successful in decreasing the mass gain to about one-fourth as compared with the un-coated superalloy. The protection exhibited by this coating was reported to be due to the formation of chromiumoxides and chromium and cobalt spinels.

CONCLUSIONS

The HVOF coatings have higher bond strengths than the most part of the coatings obtained by the otherprocesses of thermal spraying. Moreover, they are higher than for the plasma-spray coatings by 25%. The bet-ter adhesion strength of the HVOF coatings is attributed to the better mechanical interlocking of the sprayeddroplets with the substrate (as well as with each other) due to the high kinetic energy experienced by the imping-ing particles. Corrosive species mostly propagate along the splat boundaries. Due to the high velocities and highimpacts of sprayed powder particles, the HVOF coatings are very dense and the splats formed in this case arevery flat. Thus, in the HVOF coatings, the distance from the coating surface to the coating/substrate interfacealong the splat boundaries is very long. This is the desired structure if the coatings are intended for operation incorrosive environments at higher temperatures. Hence, the HVOF coatings can perform very well in corrosiveenvironments. The studies performed by some researchers revealed the protective behavior of HVOF coatings athigher temperatures. However, the formation of oxides during HVOF spraying may affect the performance ofthe coatings in corrosive environments. Therefore, high-temperature oxidation and hot-corrosion behavior of theHVOF coatings need to be further investigated to understand the behavior of these coatings at higher tempera-tures in various corrosive environments.

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