A review of thermal spray coatings for protection of steels from degradation in coal fired power plants
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Deepak Dhand
Deepak Dhand does a PhD in mechanical engineering and a research scholar at Punjabi University, Patiala, Punjab. He works as an assistant professor in Department of Mechanical Engineering at Guru Nanak Dev Engineering College, Ludhiana, Punjab, India. His research interests include hot and cold spray technologies. He has presented papers at national and international conferences.
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Parlad Kumar
Parlad Kumar has a PhD in mechanical engineering. He is an associate professor at Department of Mechanical Engineering, Punjabi University, Patiala, Punjab, India. His research interests include thermal spray techniques, manufacturing processes and rapid prototyping. He has published and presented a large number of research papers in national and international journals and conferences. Also, he has completed government funded research projects.
Jasmaninder Singh Grewal is a professor in Department of Production Engineering at Guru Nanak Dev Engineering College, Ludhiana, Punjab, India. He has vast research experience in the field of thermal spray technologies. He has contributed a large number of research papers to national and international journals and conferences.
Abstract
In coal fired power plants, the metallic surfaces such as boiler tubes, walls of combustion chambers and other parts degrade by corrosion, erosion and abrasion. It happens due to the hot gaseous environment, steam oxidation and presence of hard minerals and impurities in the coal. It is very important to protect these surfaces from material loss, otherwise it would lead to increased repair and maintenance cost along with decreased plant efficiency. In this paper, the role of thermal spray coatings has been studied for the protection of different steel grades exposed to such degrading conditions at high temperatures, in coal-based power plants. A comprehensive study has been done by analysing and comparing the work done by various researchers. Some recent advancements related to coating materials and modification in coating techniques have also been studied. This paper would be helpful for the researchers to get an idea for selecting an appropriate substrate material and coating material for industrial applications.
1 Introduction
Coal is most common source of energy and it is used worldwide. About 38% of world’s energy requirement for power generation is fulfilled by coal (British Petroleum 2018). The power generation plants are the main consumers of coal. The coal grinded to the powdered form and it is burned in the boilers to generate steam. The exhaust gases released from these coal fired thermal power plants consists of large variety of unwanted materials such as ash, silica, sulphur, chlorine and alkali-based contaminants. The fly ash particles entrapped in flue gases released by coal fired thermal power plants can cause serious corrosion–erosion problems in steels used in operational setups and structural elements of the plant. The deterioration of steel components inside the power plant may adversely affect their service life, safety and performance. It may also lead to unexpected failures of the power plant components. This degradation of steels in power plants is mainly due to the presence of elements such as silicon, sulphur, carbon and salts of sodium and potassium in coal used for combustion. The compounds of these elements may vaporise at high temperatures during combustion and get deposited on the surface of tubes and walls of other surrounding steel elements and get partially fused (Sidhu et al. 2005). The reducing environment inside the boiler lowers the ash fusion temperature and increases the mineral deposition. At high temperatures these volatile minerals result in products like NaOH, HCl, SO2, H2S and other particles which initiate the corrosion process on material surface surrounding the coal combustion process (Harb and Smith 1990). The extent of degradation depends upon variables like coal quality, erodent particle composition, gas particle flow characteristics, working temperature range and composition of boiler material/steel tubes (Levy 1993; Stack et al. 1993; Tomeczek et al. 2002). The main cause of material degradation in coal-based power plants is high-temperature corrosion of the inner surface of boiler or super-heater tubes and of the outside surfaces on fireside depending upon the ash composition (Pint 2013). The corrosion accelerates on the fireside or outer surfaces of boiler tubes due to the presence of contaminants such as acids (HCl, H2SO4), gases (O2, SO3, CO) and alkali salts (like Na2SO4, NaCl, etc.) in the surrounding environment (Kumar et al. 2018a). The other mode of degradation which works simultaneously with corrosion is erosion. The erosion of material surface is due to solid particles present in surroundings impacting the surface of the materials at high velocities and temperatures and at different impingement angles (Mishra et al. 2014). The erosion loss is highly responsible for severe material wastage in power generation industry. The fly ash particles present in exhaust gases consisting of SiO2, Al2O3 and Fe2O3, adds to the severity of erosion in fluidized bed boilers in which the bed of the boiler contains sand, limestone, etc. (Antonov et al. 2013). The free oxygen present inside the flue gases combines with constituents of fly ash to form oxide particles and these oxide-based erodent particles get deposited on the surface in large quantities and degrade the surface (Higuera et al. 2001). The coal and its ash may contain hard substances like quartz and pyrites. These are responsible for abrasive properties of coal and damages the coal handling machinery like rollers of the coal mills, surfaces of tubes and walls of boilers (Bandopadhyay et al. 2010). Another abrasive element present in coal is Al2O3 and it is responsible for abrasion in coal handling processes (Raask 1985).
In this way the accelerated degradation of metallic components in coal fired systems may be due to individual or combined effect of corrosion and erosion or abrasion. As these mechanisms are highly active at elevated temperatures, which may result in the early repair or replacement of metallic components thus increasing the maintenance cost of coal fired power plants. Therefore, it is very important to protect these metallic components from degradation at high temperatures.
There are several approaches by which surface degradation can be resisted. It can be done by modification of surface microstructure with the help of thermal/mechanical means; by changing the chemical composition of surface material; or by providing some protective coatings on the surfaces. Among these methods, the use of protective coatings is very common. In this review paper the literature related to protection of different types of steels with different coating materials and techniques have been discussed. Most of the studies compiled in this paper have been represented in the form of tables for their easy comparison and better understanding.
2 High temperature corrosion/oxidation mechanisms
The mechanisms responsible for degradation in steels are initially related to its area of application, operating temperature and service life. The principal degradation mechanisms at high temperature, in coal fired power plants are corrosion and oxidation. At elevated temperatures, these mechanisms become more complex. It is very important to understand these degradation mechanisms in different steels at high temperature for their proper control.
2.1 Corrosion
One form of corrosion take place on steel surface at high temperatures is known as hot corrosion. It is mainly governed by the factors like operating temperature, type of contaminants in the environment, moisture, etc. It accelerates due to the presence of various contaminants such as: acids (HCl, H2SO4), gases (O2, SO3, CO) and alkali salts (Na2SO4, NaCl, etc.) in the environment. The hot corrosion is generally classified into two categories i.e. type II hot corrosion and type I hot corrosion (Rapp and Zhang 1994).
Type II hot corrosion: It occurs below 750 °C. At temperatures 600 to 750 °C the solid Na2SO4 is converted to low melting point compounds such as Na2SO4–CoSO4 or Na2SO4–NiSO4 that prevents the formation of protective oxide layer and degrade the surface of alloy. Moreover, the Co or Ni present in the steel oxidize to form CoO and NiO and further react with SO3 at certain partial pressure to form sulphates like CoSO4 or NiSO4 (Luthra 1982; Pettit 2011; Rapp and Zhang 1994). This action on the surface of steels shows the formation of non-uniform pits. In these pits, there is extensive sulphidation and/or formation of non-protective and/or porous oxides.
Type I hot corrosion: This type of hot corrosion occurs between 750 and 950 °C. Low-grade fossil fuels used in energy generation systems contain compounds of sulphur and vanadium porphyrin. The compounds of sulphur react with the salts of sodium and potassium to form Na2SO4 and K2SO4 as shown in Equation (1).
Vanadium porphyrin transforms during combustion into V2O5. The V2O5 react with Na2SO4 to increase the acidity of melt as shown in Equation (2).
The sulphates vaporise and get deposited on steel surface in partially fused form. Thus, the compounds such as NiS2 (molten at 645 °C) and CoxSy (molten at 840 °C) are formed on steel surface. These molten salts of sulphur dissolve the protective oxides of Fe and Cr and increase the severity of corrosion above 800 °C (Uusitalo et al. 2002). These protective oxide scales are not formed on low alloys steels especially when partial pressure of oxygen is very low or sulphur, chlorine and hydrogen are present in the surroundings. This accelerates the degradation of steel by boosting the corrosion.
2.2 Oxidation
It is another form of corrosion in which the steels react with oxygen (or oxygen containing species) to generate surface oxide scale. The severe material degradation due to oxidation occurs at 950 °C and above. Different steels have different affinities for oxygen and the oxygen do not diffuse at the same rate in all steels. The oxide growth occurs by inward diffusion of oxygen to the scale-alloy interface. Some oxides grow in both inward and outward direction. Most of the steel alloys rely on the selective oxidation of an element (Al, Si, Cr) that forms a slower growing oxide for protection and that establishing and maintaining such a layer is the key to oxidation resistance. Consequently, the simple kinetic rate equations are often not followed and alloy compositions change in a complex way with time and temperature. The thickness of oxidation scale on pure metals initially depend on temperature, plastic properties, size and shape of metal subjected to oxidation. The scale formed is monophasic with outward diffusion of metal. With the increase in oxidation the monophasic scale get transformed into multilayer scale (Mrowec 1967). If impurities are present on the pure metal surface or the material is an alloy, the scale formed is highly porous in nature.
3 Steels and their degradation at high temperatures
Steels are used to make pressure vessels, heat exchangers, super heaters, re-heaters, boiler vessels, transportation ducts, chimneys, ID and FD fans, blades of turbines, etc. The steels used in coal fired power plants are of different grades depending upon the area of application. The steels which are subjected to high temperatures for longer durations are alloyed with high contents of nickel and chromium to ensure their strength, toughness, creep resistance and oxidation resistance. Table 1 represents the list and chemical compositions of some commonly used steels in coal-based power plants.
Common steel grades used in coal-based thermal power plants.
| Steel grade | Composition (wt%) | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C | Cr | Ni | Mn | Mo | P | S | Si | Al | Cu | Fe | Others | |
| A213 T-91 | 0.08–0.12 | 8–9.5 | ≤0.4 | 0.3–0.6 | 0.85–1.05 | 0.02 max | 0.01 max | 0.2–0.5 | 0.015 | – | Balance (Bal) | V-0.18–0.25, Nb-0.06–0.1, N-0.03–0.07 |
| A213 T-22 | 0.05–0.15 | 1.9–2.6 | – | 0.3–0.6 | 0.87–1.13 | 0.025 | 0.025 | 0.5 | – | – | Bal | – |
| A213 T-11 | 0.09–0.15 | 2–2.5 | 0.3 | 0.3–0.6 | 0.9–1.10 | 0.025 | 0.015 | 0.5 | – | 0.3 | Bal | |
| A213 T-2 | 0.1–0.2 | 5–8.1 | – | 0.3–0.6 | 0.44–0.65 | 0.025 | 0.025 | 0.1–0.3 | – | – | Bal | – |
| 16Mo3 | 0.12–0.20 | 0.30 max | 0.30 max | 0.40–0.90 | 0.25–0.35 | 0.025 | 0.010 | 0.35 max | – | 0.3 | Bal | N-0.012 |
| A210 GrA1 | 0.27 | – | – | 0.93 | – | 0.035 | 0.04 | – | – | – | Bal | – |
| A 516 Gr70 | 0.3 | – | – | 0.85–1.20 | – | 0.035 | 0.035 | 0.15–0.40 | – | – | Bal | – |
| A312 TP 347 | 0.08 | 17–20 | 9–13 | 2.0 | – | 0.04 | 0.03 | 0.75 | – | – | Bal | Ta + Nb = 10xC, max. 1.0 |
| AISI 304 | 0.08 | 18–20 | 8–10.5 | 2.0 | – | 0.045 | 0.03 | 1.0 | – | – | Bal | – |
| AISI 309 | 0.2 | 22–24 | 12–15 | 2.0 | – | 0.045 | 0.03 | 0.75 | – | – | Bal | – |
| AISI 310 | 0.25 | 24–26 | 19–22 | 2.0 | – | 0.045 | 0.03 | 1.50 | – | – | Bal | – |
| AISI 316 | 0.08 | 16–18 | 11–14 | 2.0 | 2.0–3.0 | 0.04 | 0.03 | 0.75 | – | – | Bal | – |
| AISI 321 | 0.06–0.08 | 17–20 | 9–13 | 2.0 | – | 0.04 | 0.03 | 0.75 | – | – | Bal | – |
| AISI 410 | 0.15 | 11.5–13.5 | – | 1.0 | – | 0.040 | 0.030 | 1.0 | – | – | Bal | – |
| Incoloy 800 | 0.05 | 21 | 32.5 | 1.5 | – | – | – | 1.0 | 0.38 | – | 45.7 | Ti- 0.38 |
| Inconel 600 | 0.08 | 15.5 | 76.0 | 0.5 | – | – | – | – | 0.3 | 0.25 | 8.0 | – |
| Inconel 718 | 0.08 | 19 | 52.5 | 3.0 | 3.0 | – | – | 0.18 | 0.5 | 0.15 | 18.5 | Nb-5.13, Ti-0.9 |
The steels shown in Table 1 can be categorized as specially designed alloy steels, carbon steels, stainless steels and nickel super alloys. The stainless steels possess good mechanical strength at high temperatures. The performance of stainless steels is challenged by the moisture and deposit induced in the hot gaseous corrosive environment that leads to its oxidation. The conventional steels such as AISI 347, AISI 304, AISI 316, etc. suffer great oxidation attacks due to presence of oxygen, hydrogen and moisture in the surrounding environment, above 600 °C (Maziasz et al. 2007). The steel undergoes hot corrosion type II below 750 °C and type I at temperature above 850 °C with oxidation and decreasing mechanical strength by the time its temperature is raised to 900 °C. The composition of steels is an important factor in deciding the extent of degradation. The steels having high Cr content show improved oxidation resistance. Steels rich in Cr have high potential to form protective oxide scale of Cr2O3 (Peraldi and Pint 2004). Addition of Ni even in small amount help to increase the activity of Cr and decrease the Fe activity to form oxides. However, sometimes between 650 and 800 °C chemical failure of Cr protective scale may occur due to cracking and/or spalling of the scale and the top surface of the steel get degraded by oxidation–corrosion. In wet environment (steam), the inward diffusion of hydroxide ions towards the alloy surface, due to decomposition of H2O also restrict the Cr activity and this result into catastrophic oxidation of the base metal as shown in Equation (3).
The other deteriorating mechanism is acidic fluxing of protective layers of oxides of Al, Cr, Si, etc. which is also responsible for increasing the rate of high temperature corrosion.
The another type of steels apart from stainless steels which are highly used in high temperature applications are alloy steels such as T11, T22, T23, etc. The especially designed, alloy steels are made to perform in high temperature corrosion–oxidation conditions. None of the special alloy steels are fully capable to resist the corrosion at high temperatures. By addition of oxide formers such as Cr, Al, Si, etc. and other alloying elements these materials remain successful in retarding the corrosion rate, due to the formation of protective oxides scales on the surface of the material (Pint et al. 2006). Sometime, the content of oxide formers (i.e. Al, Si, Cr, etc.) fall below the critical value in alloy steels. It happen due to high sulphidation or chlorination of surrounding environment and the material undergo severe oxidation and corrosion at high temperatures. The alloy steels such as 15Mo3, T22 and T23 show formation of less protective or non-protective scales such as Fe2O3 (hematite), Fe3O4, (magnetite) and FeO (wustite) at temperatures higher than 600 °C and under steam oxidation. Alloy steels with low concentration of Cr and working at temperatures above 580 °C are covered predominantly with thick scales of such oxides. Below 580 °C, the formation of oxides of Fe is not possible as they are unstable. Hence, the steels exposed in steam environment below 580 °C represent much thinner oxide scale consisting Fe2O3 and Fe3O4 with diffused concentration of other elements such as Mn, S and Cr. In low-alloyed steels such as T23, Cr oxide layer offer poor corrosion resistance. In such steels, Cr directly reacts with C to form carbides such as Cr3C2, Cr7C3 and Cr23C6. Hence, Cr concentration decreases in bulk material due to formation of excessive carbides. This results into formation of poor corrosion resistance oxide scale. However, the steels (such as T91) with higher concentration of Cr in a matrix show better corrosion resistance under high-temperature exposure.
Super alloys are used for very high temperature applications. Different super alloys have their own temperature range of operation and they demonstrate different levels of protection from degradation at high temperature. This is due to the difference in their chemical compositions, alloying elements and microstructure. The super alloys of Fe, Ni and Co are made to perform in high temperature corrosion–oxidation conditions best up to 1200 °C. They rely on oxide scales to resist high temperature corrosion. The oxide scale of Al is brittle in nature and mainly responsible to counter the severe corrosion–oxidation attack up to 1200 °C, above this temperature it breaks down and undergo a catastrophic corrosion attack. This oxide scale is virtually immune to moisture effects and is more thermodynamically stable than Cr2O3 scale. The corroding species, at high temperatures reacts with the surface of the super alloy substrate and starts migrating through the interface. As the oxidation proceeds, elements such as Fe and Ni are oxidized at the top surface and provides protection up to some extent, thereafter, chromium gets oxidized and forms a continuous Cr2O3 layer below the top oxide layer. The super alloys, nimonic75 and incoloy800H during hot corrosion in the molten salt environment at 900 °C show spalling/sputtering right from the initial cycles which intensify with increase in temperature with a lot of corrosion products. The surface became rougher with progressive exposure of time with a uniform pitting throughout the surface. The predominant phases found on super alloys such as nimonic75, Inconel 718, Inconel 600, etc. are NiO, Cr2O3 and Fe2O3, whereas Incoloy800H shows Fe2O3, MnO and Cr2O3 as major phases. The presence of these elements at the surface decreases oxygen availability in the underlying alloy and favours the formation of most thermodynamically stable oxide Cr2O3. With the passage of time and rise in temperature the Cr2O3 layer fails by decomposition due to high temperature.
There are some steels which are used at temperatures below 550 °C in coal fired power plants and find its application in transportation ducts, ID and FD fans, chimneys, etc. Carbon steel is one of these and in accordance to their area of application it has maximum interaction with particles of water vapour, salts in solid form and gaseous environment consisting of CO2, Cl, O2, etc. This leads to the acidic environment which degrades the unprotected carbon steel by corrosion, acidic-oxidation, corrosion–erosion or by abrasion due to presence of ash particles and particles of silica and quartz in coal.
4 Thermal spray coatings
Thermal spray coatings represent a group of processes for controlling the high temperature degradation. In this method an extra protective layer of degradation resistant materials is coated on steel surface which helps to retard the material degradation by preventing/slowing down the corrosion–oxidation rate that leads to longer operational life of the component (Heath et al. 1997). In thermal spray coating technique, the protective materials such as ceramics, metallic, or some polymeric materials are coated on the substrate surface that is to be protected. The coating material is fed to a spray gun which heats and sprays it at a very high velocity onto the substrate surface. This material on solidification bonds to the substrate surface due to adhesion and diffusion. The major thermal spray processes include flame spray, electric arc deposition, detonation gun deposition, plasma spray coating, high velocity oxy-fuel coating, cold spray coating, etc. All these techniques have their own advantages and disadvantages and are selected on the basis of their suitability (like oxide formation, spray velocity, bond strength, etc.). Some of the important coating techniques and their method, suitability and limitations have been explained below.
4.1 Plasma spray process
The plasma spray process utilizes high temperature flame or plasma generated with help of arc or high frequency discharge and superimposed on the stream of gases to melt the particles of coating material. The coating materials such as metals and ceramics are introduced in the high velocity stream of gas and remain in it for very short period. These melted particles are directed towards the substrate surface where they solidify to form a coating and these particles get strongly bonded with each other on solidification. The plasma arc spray is mostly preferred for powder form coating materials as other forms of coating materials lead to high thermal losses. The conventional method of using this technique is to perform it in open atmospheric condition which is known as air plasma spraying. The other method is by providing controlled surrounding conditions like pressure, inert atmosphere in closed chamber, etc. to attain some desired qualities of coatings. This technique of plasma spraying is known as inert gas plasma spraying. Sometimes, plasma spray coating is performed at low pressures to avoid the interaction of plasma with any external matter. This type of coating is known as low pressure plasma spray. Further, if it is done at extremely low pressure near to vacuum, then it is known as vacuum plasma spray technique.
By using plasma technique those materials can be easily coated which otherwise are difficult to coat such as refractory materials, as they require very high temperature for their decomposition to molten form during application on substrate (Heimann 2008). But at same time this technique is also flexible to coat the materials which can easily vaporise by heating at comparatively low temperatures like polymer coatings by controlling the process parameters. The coating quality obtained with plasma spray technique depends upon the parameters such as particle deposition velocity, impact angle of plasma jet, heat transfer to feedstock material, dwell time, nature of primary and secondary gases used and coating powder morphology (Sidhu et al. 2007a). The ceramic coatings obtained with this technique sometimes may lack in density and adhesion, this problem can be encountered by applying bond coat between main coating and substrate surface. The coating of such intermediate material used to fill the pores of main coating helps to improve the bond strength of coatings.
4.2 Electric arc spray
Electric arc spray is a preferred coating technique when the selection is based on thermal efficiency, cost of spraying and ease of maintenance. This process is very much similar to arc welding process. During electric arc spray process, an arc is generated between two consumable conductive electrodes/wires which melts the coating material and convert it into molten droplets. These droplets are blown away by high velocity carrier gas which further atomises the molten droplets. Therefore, this process provides high deposition rate and good coating thickness. With simplicity in application this coating technique has one major drawback of high oxidation rate of coatings. It may affect the properties of coating material by changing its composition due to oxidation. So this may reduce the coating quality especially while working at high temperatures. This process is sometimes carried out in such atmosphere that helps to minimize the oxidation of particles at high temperature. For example, nitrogen gas can be used as carrier gas as it helps to minimize oxidation and is beneficial in preventing the overheating of electrodes used for the production of electric arc.
The main process parameters that affect the performance of coating are: carrier gas, working temperature, gas pressure, flow velocity, etc. The high value of gas pressure increases gas flow velocity which decreases the time of flight to reduce the oxidation of liquid metal droplets. The high velocity of droplets reduces the porosity in the coating by decreasing the mean diameter of the droplets (Bolot et al. 2008). The low current of arc and low wire feed rate may lead to metal disintegration for very short range along with the melting of coating material (Abdulgader and Tillmann 2013). The properties of molten droplets of coating material and the characteristics of coating microstructure are highly influenced by spray velocity, gas pressure and nozzle design (Liao et al. 2005). In an another research it was observed that oxidation of the particles and porosity of the coating is related to turbulence in arc and it can be controlled by proper nozzle design (Watanabe et al. 1998).
4.3 Detonation spray coating
The detonation wave propelled coatings are generally considered when the coatings of high compressive strength, low porosity, hardness and wear resistance are required. It consists of specially designed detonation gun which utilizes combustible mixture for the generation of detonation wave which blow out the coating material particles at high temperature of range about 4000 °C and particle velocity of about 1200 m/s. The combustion mixtures which consist of oxygen and acetylene (or oxygen and hydrogen gases) are mixed in the detonation gun. The detonation gun is like a long tube where combustion mixture is prepared and ignited by a spark plug. The length of detonation gun is larger than a plasma spray gun and it provides more impact velocity to the heated particles at the end of every detonation wave.
Due to high pressure and spray velocity, detonation spray exhibits low porosity and high adhesive strength with substrate surface. The other important variables which effect the coating microstructure and properties are firing frequency, dwell time, composition of gases, gun geometry and its stand-off distance from substrate (Kadyrov and Kadyrov 1995).
4.4 High-velocity oxy fuel coating
The high velocity oxy-fuel (HVOF) coating process is similar to detonation spray technique but the difference between the two is that the HVOF process is continuous and steady state in nature. The combustible gases in high volume are introduced in combustion chamber after which they are made to pass through a confined nozzle accompanied by high volume of gas flow (nitrogen or argon) at high temperature. This results into generation of high velocity (1525–1825 m/s) of gases at the exit of nozzle. The combustion fuel used for this process may consist of hydrogen, acetylene, propane, propylene and kerosene. The HVOF process provides high particle velocity as compared to plasma spray process but the average temperature of the particles remains less as compared to plasma spray. HVOF process is generally used to deposit hard and dense cermet coatings of WC/Co, Cr2C3/NiCr, etc. During the application of HVOF coatings, the most frequently encountered problem is the oxidation of molten particles of coating materials. To control this oxidation, inert gases like helium and argon are used in surrounding environment of phase change of coating materials. Dobler et al. (2000) observed that the amount of oxidation at different regions mainly depends upon the nature of powder material used for coating. Several authors (Devi 2004; Trompetter et al. 2002) examined the Ni–20Cr coating for the oxide formation and found that a thin layer of oxide developed on coated surface resists the further oxidation. Further, it was also reported that the oxidation rate significantly depends upon the oxygen-fuel ratio. Some researchers have studied the wear behaviour of coatings obtained by HVOF process. The addition of hydroxyapatite and some other materials like nanocrystalline Al2O3 and Ni–Al2O3 composite materials particles can help to increase the mechanical strength of coating (Li et al. 2002; Sharma et al. 2019; Wu et al. 2018).
4.5 Cold gas dynamic spray (cold spray)
The cold spray technique is comparatively new and many experimental studies have been conducted in the areas of: physical modelling; gas dynamics; bonding mechanisms; improvement of equipment; economics of the process; and its development for commercial applications. Cold spray is a coating process in which the solid powder particles of size in the range of 1–100 µm in diameter are made to strike on a substrate at very high speed of the range 1500 m/s produced by a supersonic gas jet. The powder particles get deformed and adhere to the surface of substrate. In similar pattern, layer by layer formation of the coating is obtained on the surface of substrate (AlMangour 2018). The quality of the coating depends upon the powder, substrate type and process parameters such as gas pressure, gas temperature, particle velocity, stand-off distance, etc. The deposition efficiency of powders of pure metals like Ni, Al, Cu, etc. can be improved by the addition of ceramics such as Al2O3 or SiC (Sova et al. 2009).
5 Coating materials and their applications
The selection of coating material and coating technique is made according to the level of protection required for longlife operation, composition of base material, service temperature, area of application and major mode of degradation. Table 2 shows the application of some of the established coating materials, level of protection and most suitable coating technique recommended for such coatings. The issues related to substrate, service temperature and environmental conditions have also been discussed.
Coating materials and corresponding thermal spray techniques.
| Coating material | Features | Preferable coating techniques |
|---|---|---|
| Al2O3, M–Al2O3 (M = Ni, Al, Zn) |
|
High velocity oxy-fuel, plasma spray |
| Cr2O3 |
|
High velocity oxy-fuel, plasma spray |
| Al2O3–Cr2O3 |
|
Detonation spray, high velocity oxy-fuel, plasma spray |
| Cr3C2–NiCr |
|
Detonation spray, high velocity oxy-fuel, plasma spray |
| NiCr, NiCrX (X = Al, Si, B, Ti) |
|
Cold spray, electric arc spray, high velocity oxy-fuel, plasma spray |
| Ni-Al |
|
Electric arc spray, plasma spray |
| WC–Co–Cr, WC–Co, WC |
|
Cold spray, detonation spray, high velocity oxy-fuel, plasma spray |
| Fe, FeCr, FeX- (X = B, Si, Nb) |
|
Electric arc spray, high velocity oxy-fuel |
The coating materials represented in Table 2 is not an exhaustive list. Different researchers have tried different coating materials in different proportions on different substrates to evaluate their effectiveness against degradation of the base material. These combinations have been discussed in the next sections. It is also important to note that various thermal spray techniques can be used for coating of same material, but each coating technique has its own merits and demerits and difference in quality of coating. The coating techniques such as flame spray and arc spray are considered low grade but these are cost effective. These are used only when operating cost is main constraint, whereas high velocity oxy-fuel coating and plasma spray techniques are considered better and can be used to coat the materials having high melting temperature.
6 Effect of thermal spray coatings on different steels
6.1 ASTM A213 alloy steels
These steels are also known as modern high temperature, heat resistant grade materials and offer improved performance over conventional austenitic stainless steels due to their better creep resistance at elevated temperatures. They usually contain 0.5–1.15% of molybdenum for enhanced creep strength and 1–9.5% chromium for improved corrosion resistance. The ‘T’ designation indicates the application of this material in making tubes for boilers, super heaters and heat exchangers. These steels have low carbon content and micro-alloying of elements such as Titanium, Vanadium and Niobium help to develop strengthening precipitators and refinement of grains in steel microstructure. These steels are protected from degradation by using different types of coatings as done by some researchers, given in Table 3.
Coatings on ASTM A213 alloy steels.
| S. no. | Substrate | Coating materials | Coating techniques | Findings | References |
|---|---|---|---|---|---|
| 1 | T91 | 50Ni–50Cr | Plasma spray | In this research two layered coating was tried with the aim to fill the pores that were produced by lower coating. For this purpose, a coating of Al was used on main coating of 50Ni–50Cr. It was observed that due to steam oxidation at 600–750 °C, Al diffuses to Ni and formed Ni–Al intermetallic. The two-layer coating exhibited an excellent performance against steam oxidation till 3000 h of test. However there was an internal oxidation in top layer. | Sundarajan et al. (2005) |
| 2 | T91 | 80Ni–20Cr, 75Cr3C2–25NiCr | High velocity oxy-fuel spray | Two different types of coatings were done, using 80Ni–20Cr and 75Cr3C2–25(Ni–20Cr). These coatings were exposed to a Na2SO4–60%V2O5 solution at 750 °C under cyclic conditions to check effectiveness against hot corrosion. Both coatings were found suitable. The weight gained by 80Ni–20Cr and 75Cr3C2–25(Ni–20Cr) coating was 89 and 72% less than the bare steel respectively. The oxides and spines of Ni–Cr were found responsible for prevention of hot corrosion. | Chatha et al. (2012) |
| 3 | T91 | NiCrAlY | High velocity oxy-fuel spray | The Ni-based alloy coating was investigated for its oxidation behaviour at 900 °C, for 50 cycles. It was observed that the coating remained almost unaffected to oxidation losses at high temperature. This supported the suitability of the coating material in high temperature degrading conditions due to oxidation. It was due to the presence of oxides of Ni, Cr and Al in large amounts, as indicated by XRD and EDS analysis of oxidation-tested coated samples. | Singh et al. (2017a) |
| 4 | T91 | Fe alloy | Plasma spray | In this study, the coating of chromium- and nickel-based Fe alloy was evaluated for hot corrosion in simulated environment of 70% Na2SO4 and 30% K2SO4 molten salt at 700 °C, for 84 h. Initially the coating showed sharp rise in corrosion rate from 0 to 12 h of exposure and became stable after 12 h, till end. This was due to the resistance offered by protective oxides of Ni, Cr and Ni–Cr. This research showed that Fe–alloy coating with high content of Ni and Cr can be a better option for the protection of steel in corrosive environments. | Jiang et al. (2018) |
| 5 | T91, T22 | 83WC −17Co, 86WC–10Co–4Cr | High velocity oxy-fuel spray | Two WC-based coatings were made on two different tube grade boiler steels. These coatings were studied and compared for cyclic corrosion resistance at 900 °C with uncoated samples for 1000 h in actual boiler environment. The performance of 86WC–10Co–4Cr coating on T-22 and T-91 proved better than 83WC–17Co coating. This was due to the formation of α-WC and β-W2C compounds in 86WC–10Co–4Cr coating. These compounds helped in protecting the 86WC–10Co–4Cr coating from degradation at high temperatures. The presence of Cr content in coating material was found to be beneficial in corrosion protection. | Sidhu et al. (2017) |
| 6 | T22 | 75Cr3C2–25NiCr | Plasma spray | The research was focused in direction to check the candidature of 75Cr3C2–25NiCr coating to ensure protection of steel from corrosion, in an open air oxidizing environment at 700 °C, for 72 h. The coating provided 50.4% more corrosion resistance than uncoated material. This proved the effectiveness of 75Cr3C2–25NiCr coating in preventing the high temperature corrosion. The post analysis of the corroded coated specimen by X-ray mapping revealed the presence of oxides of Ni and Cr in the scale and coating, that helped to restrict the formation of non-protective ferrous oxide layer. | Ghosh et al. (2014) |
| 7 | T22 | 75Cr3C2–25NiCr | Detonation spray | The 75Cr3C2–25NiCr coating applied by detonation spray technique was studied for its oxidation–erosion behaviour in actual degrading environment at 700 °C for 1500 h. The coated samples were placed in the path of exhaust gases such that it made 90° impingement angle with the hot gases. The study showed that 75Cr3C2–25NiCr coating was 78.34% more effective in resisting the high temperature erosion than uncoated samples. On the other hand, the coating also proved exceptionally better (i.e. 103%) in resisting oxidation at high temperature. So this coating was highly recommended for its application in high temperature degrading environment due to oxidation. | Kaur et al. (2011) |
| 8 | T22 | Cr3C2–XNiCr (where X = 65, 80, 90, 100%) | High velocity oxy-fuel spray | In this study, Cr3C2–XNiCr coating was tried by varying the Ni–Cr content in four combinations of 65, 80, 90 and 100%. The motive of research was to find the best coating combination ratio that can provide maximum resistance to cyclic corrosion at elevated temperatures. The different coatings were tested in simulated corrosive environment of 40% Na2SO4–60% V2O5 molten salt maintained at 900 °C for 50 h. The 10Cr3C2–90NiCr coating was found best among the other combinations in providing protection from corrosion. This was due to excessive formation of protective oxides and carbides of Ni and Cr in scale as compared to other coating materials. | Kaur et al. (2010) |
| 9 | T22 | Cr2O3 | Plasma spray | The plasma sprayed chromium oxide coating was evaluated for corrosion resistance in simulated boiler environment of molten salt 60% Na2SO4–40% V2O5 at 850 °C for 25 h. The results of study indicated the linear increase in corrosion with time of exposure to corrosive media, both in coated and uncoated samples. However the coated sample was found 21.83% more efficient than bare steel in protection from corrosion. | Singh et al. (2017b) |
| 10 | T22, Superfer 800H | 50Cr2O3–50Al2O3 | Detonation spray | The ceramics are known for their high temperature resistance and hardness. In this study, the ceramic-based 50Cr2O3–50Al2O3 coating was tested on two different steel grades for corrosion resistance. The coated samples were examined at 900 °C in 40% Na2SO4–60% V2O5 corrosive environment for 50 h. After exposure to the corrosive environment, the 50Cr2O3–50Al2O3 coated T-22 sample showed 97% reduction in corrosion wear whereas Superfer 800H sample resulted only 19% decline in corrosion rate as compared to the corrosion resistance offered by the bare material. The decrease in performance of coating in super alloy might be due to the mass gained by it after 25th cycle due to fluxing of Al oxides. | Rani et al. (2017) |
| 11 | T22 | WC–12Co, Ni–20Cr | Detonation spray | The coating WC–12Co and Ni–20Cr was applied on T-22 steel with the aim to evaluate its oxidation behaviour at high temperatures. Each uncoated and coated sample was placed in silicon wire tube furnace at 900 °C for 50 h. Both the coatings were found helpful in resisting the degradation due to oxidation but the Ni–20Cr coating was found almost 100% efficient as compared to uncoated sample. The decomposition of WC–12Co coating into non protective α-CO phase had significantly decreased its performance. | Kumar et al. (2018b) |
| 12 | T22 | 80Ni–20Cr, 50Ni–50Cr | Cold spray | Two Ni–Cr-based coatings with variable composition were studied and compared for their cyclic oxidation behaviour on T-22 steel. The coated and uncoated samples were tested in tube furnace of silicon carbide for 50 h at 900 °C, individually. The resistance to oxidation offered by 50Ni–50Cr coating was most among the other coated and uncoated samples. This indicated that the coatings with high content of Ni and Cr provide better protection from oxidation. The presence of Cr2O3 and NiCr2O4 phases was the key factors for better performance of coating. Moreover the scale formed after oxidation of coated samples was dense and adherent to surface. | Bala et al. (2009) |
| 13 | T22, SA-516 | Ni–20Cr, Ni–5Al | Electric arc spray | In this research Ni-based coatings, Ni–20Cr and Ni–5Al were evaluated and compared for their behaviour in high temperature oxidation conditions. The samples were tested in silicon carbide tube furnace for 50 cycles, at 900 °C. Both the coatings were found dense and adherent, without any spallation of scale during oxidation. The performance of Ni–20Cr was found better than the other coating. This might be due to high micro hardness offered by Ni–20Cr coating on both the steels as compared to the other coating and bare steel. | Kumar et al. (2019) |
| 14 | T22, T11, SA210–GrA1 | 75Cr3C2–20Ni5Cr, 88WC–12Co | High velocity oxy-fuel spray | In this study the erosion characteristics of 75Cr3C2–20Ni5Cr and 88WC–12Co coatings made on three different steel grades were studied. The samples during testing were maintained at 250 °C and sand was used as an erodent. The sand was made to strike on coatings at the 30° and 90° angles. It was found that the T-22 coated samples performed slightly better than coated T-11 samples whereas 88WC–12Co coating was found to be more erosion resistant as compared to 75Cr3C2–20Ni5Cr coating and bare steel. | Sidhu et al. (2006a) |
| 15 | T22, T11, SA210–GrA1 | NiCrFeSiB | High velocity oxy-fuel spray | The high temperature oxidation behaviour of Ni-based NiCrFeSiB alloy powder coating was investigated by applying it on three boiler steels. The coated and uncoated samples were laboratory tested in SiC tube furnace for 50 h at 900 °C. The NiCrFeSiB coating was found highly effective in controlling oxidation at high temperatures. The coated T-11 sample showed minimum oxidation rate as compared to other coated and uncoated samples. The presence of SiO2 along with Cr2O3 in protective glossy layer formed on coating surface is also reported by researchers. Below this protective layer, no oxidation of the coating material took place. | Ramesh et al. (2010a) |
| 16 | T22, T11 SA210–GrA1 |
Ni3Al, NiCrAlY | Plasma spray | Two coating materials, Ni3Al and NiCrAlY were used for protection of different boiler steels in actual degrading environment at 755 °C for 1000 h. Before applying Ni3Al coating, the laser melted NiCrAlY coating was done on substrate surface to close surface pores and smoothen its structural deformations. The results obtained after experimentation showed that the protection provided by Ni3Al coating was almost negligible as compared to uncoated steels. This might be due to significant decrease in micro hardness of both coatings after laser re-melting of NiCrAlY coating. Also the Ni3Al coating represented poor adherence and high porosity through which O anion penetrated through coating towards substrate surface and the formation of NiO fraction in scale cracked the coating. | Sidhu et al. (2004) |
| 17 | T22, T11, SA210–GrA1 | Ni–20Cr, Stellite-6 | High velocity oxy-fuel spray | The coatings of Ni–20Cr and Stellite-6 were made on T-22, T-11 and SA-210 Gr A1 steel to evaluate their erosion resistance behaviour. The coated and uncoated specimens were maintained at 250 °C and erodent made to strike them at 30° and 90°. The results obtained after testing showed that the coatings are not much effective in resisting surface erosion. This might be due to high porosity of coatings obtained by HVOF technique. | Sidhu et al. (2007b) |
| 18 | T11 | Cr3C2–NiCr, WC–Co, Ni–Cr and Stellite-6 | High velocity oxy-fuel spray | Four different coatings were made on T-11 steel to compare their ability to counter corrosion at high temperatures. The experiments were conducted in corrosive 40% Na2SO4 and 60% V2O5 salts environment at 900 °C for 50 h. All the coatings were found better to protect the steel from corrosion but Ni–Cr coating followed by Cr3C2–NiCr coating showed highly impressive performance whereas WC–Co provided least resistance to corrosion. | Sidhu et al. (2007c) |
| 19 | T2 | FeCrSiB, FeCr, NiCrTi | Electric arc spray | In this study, the performance of FeCrSiB coating developed by adding Si and B to FeCr, was compared with FeCr and NiCrTi coatings. The corrosion studies were carried out in 75% Na2SO4–25% K2SO4 molten salt at 650 °C for 200 h for each sample. It was found that the presence of Si and B in coating material helped to decrease the affinity of oxide formation of molten particles. Therefore the thin oxide layer was found in FeCrSiB corroded coating as compared to other coatings but the corrosion resistance offered by it is far better than other coated and uncoated samples. | Li et al. (2015) |
| 20 | T2 | NiCrTi, Fe–X Cr (X = 15, 20, 25, 30, 35 and 40 wt%) | Electric arc spray | The study was conducted by varying the chromium content in FeCr coating to check its impact on oxidation behaviour at 650 °C on T-2 steel. After 200 h of exposure to oxidation conditions, it was found that the oxidation resistance of coating increased with increase in Cr content. Therefore, Fe–40Cr exhibited highest oxidation resistance among other coatings. Its level of protection was found equivalent to the protection provided by NiCrTi coatings when tested in same conditions. | Li et al. (2014) |
Table 3 shows that a variety of coating materials can be used to protect the ASTM A213 alloy steels from high temperature corrosion. However, the Ni–Cr-based coatings are more effective in resisting the degradation due to hot corrosion (type II). The Ni–Cr coating material taken in 50–50 ratio has shown good results. The Ni–Cr coatings applied in combination with CrC such as Cr3C2–NiCr, are also found effective in protection from corrosion. Chromium is one of the main constituents of coatings which helps to prevent the corrosion. This is due to the formation of protective oxides of Cr at high temperatures. The Ni–Cr coatings alloyed with Al and/or Y becomes more effective against high temperature oxidation. In such coatings the additional protective oxide of Al is also formed but Cr2O3 kinetically establishes much quicker than Al2O3. The addition of Y or Zr in Ni–Al-based coatings helps to reduce spallation and improve adherence of protective oxides of Al (Pint and Hobbs 1994). In order to control the oxidation and avoid the penetration of oxides through the coating thickness, two different protective layers can also be applied. For this Ni–Al-based coatings proved helpful when applied in-between the outer protective coating and substrate surface. The bond coat between the protective coating and substrate helps to provide the better protection from degradation by ensuring strong bond between them.
The nickel–chromium-based coatings lack in high temperature hardness and may fail due to erosive wear at high temperatures. Therefore, the outer protective layer of Al2O3, Cr2O3, or WC-based coatings can be used to resist the material loss due to erosion–abrasion wear by particles in the surrounding environment.
To apply these coatings generally high velocity oxy-fuel, Plasma or detonation spray techniques are used due to their higher temperature working range; high particle flow velocity and low coating porosity. Moreover, in high velocity oxy-fuel or plasma technique the oxidation of molten coating particles is minimum while escaping through the spray gun. The cost effective electric arc spray technique is generally not used for Ni–Cr coatings due to formation of large amount of oxides during the coating process. However, this oxidation can be reduced by adding some alloying elements like Si and B.
6.2 Stainless steels
The stainless steels are classified into different categories like ferritic, austenitic, martensitic, etc. on the basis of their microstructure. These are conventionally used at high temperatures. The low and medium carbon martensitic steels like type 410 are used in steam and gas turbines. The austenitic stainless steels usually possess good high temperature strength. The 300 series steels contain large content of nickel (upto 35%) and chromium (16–26%) for better corrosion and oxidation resistance at high temperature. Some of the studies related to application of thermal spray coatings on stainless steels to enhance their protection during high temperature operation are shown in Table 4.
Coatings on stainless steels.
| S. no. | Substrate | Coating materials | Coating techniques | Findings | References |
|---|---|---|---|---|---|
| 1 | AISI 304 | 60NiCrSiB–40Al2O3 | Plasma spray | The addition of Al2O3 in NiCrSiB alloy enhanced its micro hardness by approximately 10%. This composition was coated on AISI 304 steel to study its erosion resistance behaviour at 450 °C. The erodent i.e. alumina, was made to strike on coated and uncoated samples at 30° and 90° angles. The erosion resistance offered by coating was found 2.5 times better at 30° angle and 1.5 times at 90° angle of impingement as compared to uncoated samples. | Praveen et al. (2015) |
| 2 | AISI 304, AISI 310 | Ni–Cr, NiCrMoAlFe | Plasma spray | The study was conducted to compare and evaluate the level of resistance offered by two Ni–Cr-based coatings made on AISI 304, 310 steels in chemically degrading environment. The coated and uncoated samples were examined at 900 °C in salt environment of K2SO4, KCl and natural pyrites for 24 h to check the effect of oxidation, sulphidation and chlorination. The results showed that the coated samples were highly effective in resisting the chemical attack of K, Cl and S elements than pre-oxidized bare AISI 310 steel. The study also reported that higher the chromium content in coating, more would be the resistance from chemical attack. | Mayoral et al. (2006) |
| 3 | AISI 304L | Al particles | Electric arc spray | The research was conducted to understand the splat morphology and coating properties of Al coating made on AISI 304L during the temperature range of 25–450 °C. The study was conducted by varying the impact velocity and substrate temperature during the coating process. After studying the splats formed below 215 °C of substrate temperature at 143 m/s and 109 m/s impact velocities it was found that a large number of voids and pores were formed between coating and substrate surface with very irregular and discontinuous formation. These splats were transformed into disc shaped flat splats having porosity less than 5% at temperatures above 250 °C. It was also found that the deposition efficiency and adhesion strength increased with increase in substrate temperature. | Abedini et al. (2006) |
| 4 | AISI 304 | Ni–Al | High velocity oxy-fuel spray | The corrosion behaviour of coating was studied for 250 h in ash environment consisting of 10% KCl salt; O2, N2 and HCl gases at 700 °C. The corrosion became severe with growing of Al2O3 at interface of NiAl-steel from edges to centre. It was suggested to avoid direct exposure of chlorine/oxygen gases which significantly depleted Al. The Ni–Al coating was not found much effective in resisting corrosion at high temperatures. | Bai et al. (2018) |
| 5 | AISI 304 | NiCrBSi-X (75Cr3C2–25NiCr) [X = 40%, 85%] | High velocity oxy-fuel spray | The coatings obtained by the combination of two established hot corrosion resistance coatings, NiCrBSi and 75Cr3C2–25NiCr were investigated for their erosion resistance during cavitation. For this, two coatings having 40 and 85% of 75Cr3C2–25NiCr in NiCrBSi were evaluated for 21 h. The results indicated that the erosion resistance offered by both the coatings was better than bare steel. It was found that the performance of NiCrBSi–85%(75Cr3C2–25NiCr) coating was best among all samples. This might be due to its high micro hardness and Cr content. | Yang et al. (2016) |
| 6 | AISI 309 | Ni–20Cr, Ni3Al | Plasma spray | The steel grade (i.e. AISI 309) selected in this study is extensively used in making coal handling structures and therefore it is highly prone to sliding wear losses. The coatings of Ni–20Cr and Ni3Al were tried on AISI 309 steel to reduce its sliding wear loss. The wear was examined at load of 30 and 50 N on pin-on-disk setup. Both the coatings exhibited approximately equal and exceptionally high (i.e. 96%) reduction in wear loss. | Kaur et al. (2009b) |
| 7 | AISI 310S | FeCrBSiMn | Electric arc spray | High temperature corrosion resistance was evaluated for coatings of FeCrBSiMn alloy on AISI 310S steel. The specimens were tested in Na2SO4–82% Fe2(SO4)3 molten salt, at 900 °C for 50 h. The coating exhibited excellent corrosion resistance as compared to bare steel. It might be due to presence of oxides of Cr and high coating density. The Fe-based coating shown good resistance to corrosion as compared to bare steel. | Shukla et al. (2014) |
| 8 | AISI 316 | WC–12Co, WC–10Co–4Cr, Cr3C2–25NiCr | High velocity oxy-fuel spray | Coatings of WC–12Co, WC–10Co–4Cr and Cr3C2–25NiCr were compared under erosion and abrasion wear at room temperature. Coatings were subjected to solid particle erosion and two-body abrasion in laboratory conditions. It was found that the erodent flow velocity, erodent-coating hardness ratio and fracture toughness of erodent and coating surface are some of the key parameters which affected the erosion rate. In the case of abrasion, the load applied, properties of abrasive particles and surface properties were the influencing factors. It was found that WC–12Co and WC–10Co–4Cr coatings exhibited better erosion and abrasion resistance as compared to Cr3C2–25NiCr coatings. | Vashishtha et al. (2017) |
| 9 | AISI 321 | Ni3Al | Plasma spray | The coatings of Ni3Al were made on AISI 321 steel and the coated specimens were heated at different temperatures from 500 to 800 °C for 10, 30, 60 and 100 h. These coated samples were evaluated for their oxidation and erosive wear behaviour at high temperature. It was found that with increase in temperature above 500 °C and exposure time more than 100 h, the internal oxidation of coating also increased due to increased NiO phase. | Mehmood et al. (2016) |
| 10 | AISI 347 | NiCrTi, NiCrMo | Electric arc spray | The corrosion resistance of NiCrTi and NiCrMo coatings were studied in mixed salt environment of molten 40% K2SO4, 40% Na2SO4, 10% NaCl and 10% KCl for 157 h at 700 °C. Both coatings exhibited excellent resistance to corrosion in chlorine gas environment. The minor alloying of Ti and Mo elements in Ni–Cr coating helped to improve the microstructure of coatings. | Qin et al. (2017) |
| 11 | AISI 347H | Ni–20Cr | Detonation spray | The performance of Ni–20Cr coating on AISI 347H steel was evaluated in high temperature corrosive environment of 40% Na2SO4–60% V2O5 molten salt at 900 °C for 50 h of exposure. The coating was found highly helpful in resisting the corrosion than bare steel due the formation of oxide layers of Ni and Cr in scale. During corrosion test, there was no crack formation or delamination of coating which showed better coating quality. | Kaushal et al. (2011) |
| 12 | AISI 347H | Cr3C2–NiCr | High velocity oxy-fuel spray | The Cr3C2–NiCr coated AISI 347H boiler steel was investigated at 700 °C to evaluate its oxidation and hot corrosion resistance behaviour in Na2SO4–82Fe2(SO4)3 molten salt environment at 700 °C for 50 h. The Cr3C2–NiCr coating was found effective in resisting high temperature oxidation and corrosion. The Cr3C2–NiCr coating was 15% better against oxidation and 53% better against corrosion resistance when compared to uncoated samples. | Kaur et al. (2009a) |
| 13 | AISI 410 | Cr2O3 | Plasma spray | The change in erosion wear with stand-off distance of spraying nozzle in Cr2O3 coated samples was studied in this research. During experimentation, silica sand was made to strike the coated and uncoated samples at 60° angle from 10, 20, 30 and 40 mm stand-off distances. All the coated samples were found better than uncoated sample in terms of erosion resistance, due to their high micro hardness. The material loss by erosion was found maximum at 30 mm stand-off distance. The coating of Cr2O3 was found helpful in resisting wear. | Rao et al. (2016) |
The stainless steels mainly used in coal fired power plants are of 300 series such as AISI 304, AISI 310, AISI 309, AISI 316, etc. These steels can be protected from oxidation, sulphidation and chlorination by thermal spray coating of protective alloy materials. The steels which lack in high temperature hardness are coated with oxides of Al or Cr in combination with other compositions of coatings (Praveen et al. 2015). It has been observed that Ni–Al-based coatings made by either high velocity oxy-fuel or plasma spray technique are not very effective in resisting corrosion–oxidation at high temperatures (Bai et al. 2018; Mehmood et al. 2016). This is due to excessive formation of NiO phase in coatings during high temperature exposures. The oxide formation propagates from outer surface of coating to the substrate surface along the coating thickness and it leads to degradation of coating. To counter this, if the thickness of the coating is increased then Ni–Al-based coating may get peel off due to weak bonding between particles of Ni and Al. The stainless steels with high hardness such as AISI 316, AISI 347, etc. are coated with Ni- or Fe-based coatings along with alloys of Mo, Ti, Si, B, Al, Co, Cr, etc. to attain the desired level of corrosion–oxidation protection and microstructural refinement of coatings (Mayoral et al. 2006; Qin et al. 2017; Yang et al. 2016).
6.3 Carbon steels
The carbon steels are highly used in fabrication of pressure vessels due its versatile mechanical properties and low cost. They are successfully employed in non-corrosive environments subjected to thermal cyclic load between the temperature ranges 350 and 450 °C. At temperatures above 450 °C, the steels start weakening and its operational life may decrease. The carbon steels are commonly used in fabrication of superheaters, reheaters, economizers, pipes, etc. Some studies related to application of thermal spray coatings on carbon steels are shown in Table 5.
Coatings on carbon steels.
| S. no. | Substrate | Coating material | Coating technique | Findings | References |
|---|---|---|---|---|---|
| 1 | A210 GrA1 | WC–Co/NiCrFeSiB | High velocity oxy-fuel spray | The study was conducted to evaluate the performance of NiCrFeSiB–(WC–Co) coating in air jet erosion conditions at 30° and 90° impingement angles. The NiCrFeSiB–WC–Co coating demonstrated both ductile and brittle deformations during testing but it was found less effective in providing erosion protection than uncoated steels. The uncoated samples showed better resistance to erosion due to high hardness ratio of silica sand to substrate material. | Ramesh et al. (2010b) |
| 2 | A516 Gr70 | Ni–20Cr, Ni–20Cr–24TiC, Ni–20Cr–24TiC–1Re | Cold spray | NiCr, NiCrTiC and NiCrTiCRe coatings were made on A516 Gr70 and evaluated for their corrosion resistance ability at 700 °C in actual environment for 1500 h. All the coatings were found effective in providing resistance to corrosion. The oxides of Ni–Cr were found responsible for protection from corrosion at high temperatures. The performance of Ni–20Cr–24TiC–1Re coating was found best, followed by NiCrTiC and NiCr coating. | Bala et al. (2017) |
| 3 | A204 16Mo3 | Ni–30Cr, Al2O3 | Plasma spray | In this study, corrosion resistance behaviour of coated and uncoated evaporator tubes was compared in actual environment at 450 °C. The improvement in corrosion protection after addition of Al2O3 in Ni–30Cr coating was also investigated during the research. The coated tubes offered better resistance to corrosion than uncoated tubes. The Ni–30Cr coating with Al2O3 outer protective layer lasted for one more year than Ni–30Cr coating without Al2O3 outer layer and failed at last due to cracking of scale and degradation of coating through its pores. | Łabanowski and Ćwiek (2010) |
| 4 | AISI 1020 | NiCrTi (Ni–43Cr–0.3Ti) | Electric arc spray | The NiCrTi coating behaviour was studied in moderately high temperature corrosion environment along with cyclic oxidation for 550–750 °C range. At 650 °C the corrosion rate of coating was found 1.7 times more than oxidation. The oxidation was reduced due to formation of oxides of Ni and Cr. The oxides of Ni and Cr were dissolved due to presence of sulphates of molten salts at high temperature and this reduced the coating protection from oxidation. | Guo et al. (2015) |
| 5 | AISI 1020 | FeCrNiAlBSi/Cr3C2 | Electric arc spray | The coating of FeCrNiAlBSi/Cr3C2 was made on AISI 1020 to evaluate its oxidation behaviour in atmospheric conditions at 550, 650 and 750 °C for 200 h. The study revealed that the coated samples performed much better than uncoated samples. The coated samples recorded severe rise in oxidation with rise in temperature from 550 to 750 °C. It might be due to the transformation of protective oxides of Cr formed at 550/650 °C to un-protective and unstable oxides of Fe–Cr at 750 °C and above. | Cheng et al. (2019) |
| 6 | AISI 1020 | Ni–X Cr (X = 30, 45 and 50 wt%) | Electric arc spray | The Ni–Cr coating was investigated to understand the variation in its oxidation resistance property at 650 °C, by varying its Cr content. For this, three Ni–Cr coatings were applied with 30, 45 ,and 50% Cr in them. The coated samples were found highly effective in providing protection from oxidation at high temperatures. It was found that by increasing the Cr content in Ni–Cr coating from 30 to 50%, the high temperature oxidation resistance of coatings was increased by 43.5%. | Long et al. (2019) |
| 7 | AISI 1020 | 1.5Cr8Ti2Al, Cr13Al5B5, Cr13Al5B5Y | Electric arc spray | Coatings of 1.5Cr8Ti2Al, Cr13Al5B5 and Cr13Al5B5Y were applied on AISI 1020 steel for comparison of their corrosion resistance performance at 700 °C and abrasion wear behaviour at 20 °C. Both the coatings were found helpful in increasing abrasive wear and corrosion resistances of the substrate. The addition of Yattrium (Y) in Cr13Al5B5 increased its adhesion strength by 1.2 times due to formation of complex oxides. | Yury et al. (2018) |
| 8 | AISI 1020 | NiCrBSiFeC | High velocity oxy-fuel spray | The research was conducted to analyse the corrosion resistance of coated and uncoated samples in 1 M copper chloride and acetic acid solution. The study was also made to understand its abrasion wear behaviour at 35, 250 and 350 °C temperatures. The coating showed three to six times improvement in abrasion wear resistance and 3.5 times improved corrosion resistance as compared to uncoated material. | Prince et al. (2012) |
| 9 | AISI 1020 | WC–12Co, Cr3C2–NiCr, WC–CrC–Ni | High velocity oxy-fuel spray | Three coatings were applied on AISI 1020 by HVOF technique to investigate and compare their erosion behaviour with uncoated samples at 310 °C. Fly ash was used as erodent in the study to simulate the real erosive conditions as in case of economizer tubes. All coatings exhibited better protection from erosion as compared to uncoated samples. The sequence of erosion wear resistance offered by the coatings was: WC–12Co > WC–CrC–Ni > Cr3C2–NiCr. | Vicenzi et al. (2006) |
| 10 | IS 2060 | Fe-alloy | Electric arc spray | Three commercially available ferrous-based coatings were studied for their erosion resistance performance at room temperature, 300, 400, 500 and 600 °C temperatures and 30° and 90° erosion angles. It was found that all the coated samples provided better erosion resistance at 30° than uncoated samples but at 90°, FeBCrMnSi-coated sample recorded maximum material loss due to erosion. The coating of FeBAlSiC was found best amongst other coatings at 30° and 90° impingement angles. | Venugopal et al. (2008) |
| 11 | ASTM A36 | WC–12Co, Al2O3–13TiO2 | Plasma spray | Two cermet coatings, WC–12Co and Al2O3–13TiO2 were applied on ASTM A36 alloy to evaluate its wear resistance at room temperature. The study was conducted using pin-on-disc setup at 40, 50 and 60 N loads. Both coatings were found effective in resisting wear as compared to uncoated alloy. The coating of WC–12Co was found most effective and helped to reduce wear by 57% whereas Al2O3–13TiO2 coating was found 3.6 times better than uncoated alloy. | Panwar et al. (2019) |
Unlike other previously discussed steels, the carbon steels cannot be used at high or very high temperature applications. These steels can work only up to moderately high temperatures i.e. 550 °C without any deformation. Therefore, the thermal spray techniques which operate at very high temperature are not recommended for applying the protective coatings on these steels. The thermal spray techniques such as cold spray, electric arc spray or low temperature high velocity oxy-fuel spray can be applied under such conditions. These techniques spray coating materials in semi-molten or molten form. This, sometimes generate unstable oxides of constituent elements such as Cr, Si, etc. which decomposes at high temperatures. In case of Ni–Cr coatings, the weak oxides of Ni and Cr get dissolved in presence of sulphates and other salts of working environment and it reduces the coating protection from oxidation (Guo et al. 2015). It can be controlled by increasing the coating spray temperature so that coating material is applied in fully molten form and unstable oxide formation is avoided.
The coatings of hard materials such as Al2O3, WC, etc. made with electric arc spray or cold spray techniques, on carbon steels fail early due to cracking or degradation through pores. This is due to high porosity of coating or/and excessive oxide formation during coating deposition. Such problems can be addressed by selecting high velocity spray coating, by preheating the substrate surface and by avoiding oxidation of molten particles in the passage during the spray of coating.
6.4 Nickel-, iron- and cobalt-based super alloys
Nickel-, iron- and cobalt-based super alloys are used for temperature range of 540 °C and above due to their excellent creep resistance, better thermal fatigue resistance, high strength and ability to withstand in high temperature degradation. These alloys are widely used in turbine blades, rotating parts, fixtures exposed to high temperatures, structural components such as piping pump and fan, hardware, recuperators used to recover waste heat, etc. Table 6 shows the key findings of some studies related to super alloys. In this table, super alloys superni 800, superni 600 and superni 718 are equivalent to Incoloy 800, Inconel 600 and Inconel 718 respectively.
Coatings on super alloys.
| S. no. | Substrate | Coating materials | Coating technique | Findings | References |
|---|---|---|---|---|---|
| 1 | Alloy 80A | Cr3C2–25NiCr, NiCrMoNb | Plasma spray | The corrosion resistance of Cr3C2–25NiCr, and NiCrMoNb coatings were studied in simulated air and molten salt environment of 40% Na2SO4 and 60% V2O5 salts at 900 °C for 50 h. The results obtained after experimentation showed that both the coatings were not much effective in resisting corrosion. In molten salt corrosive environment, Cr3C2–25NiCr coating proved better against corrosion resistance than NiCrMoNb coated and uncoated alloy. | Sreenivasulu et al. (2018) |
| 2 | Inconel 718 | Cr3C2–NiCr | Detonation spray | The Cr3C2–NiCr coating made on Inconel 718 super alloy was investigated for its corrosion resistance in molten salt environment of 75% Na2SO4 and 25% NaCl at 900 °C for 50 h. The study showed that the coating was highly effective in resisting corrosion and surface deformation as compared to uncoated sample which witnessed severe sputtering/spallation with increase in exposure time. The corrosion rate in coated samples were found 90% less than in uncoated samples | Saladi et al. (2014) |
| 3 | Inconel 718 | NiCoCrAlYTa | High velocity oxy-fuel spray | The coating of NiCoCrAlYTa alloy was studied and compared for its corrosion resistance behaviour on Inconel 718 when exposed to molten salt environment of V2O5 and mixture of salts 50% Na2SO4 and 50% V2O5 in temperature range of 800–1000 °C. The coating provided better protection from corrosion in V2O5 environment than mixed salt environment. This might be due to increase in hardness of coating material exhibited by formation of oxides of different elements formed during decomposition of V2O5 salt. | Liu et al. (2016) |
| 4 | Superni 718 | Ni3Al, Ni–20Cr and NiCrAlY | Plasma spray | Coatings of Ni3Al, Ni–20Cr and NiCrAlY were made on superni 718. Intermediate coating of NiCrAlY was applied between substrate and other two protective coatings to improve their bond strength. The coated and uncoated samples were examined for their erosion resistance at 30° and 90° impingement angles of silica erodent. The Ni3Al coated samples offered lowest erosion rate amongst other coated samples at both the impingement angles. This might be due to low porosity of the coating which resisted the penetration of erodent particles through it. | Mishra et al. (2006) |
| 5 | Superni 75,600, 718, Superfer 800H | Ni–20Cr, NiCrAlY | Plasma spray | The study was done to understand the erosion–corrosion behaviour of Ni–20Cr and NiCrAlY coatings at 540 °C, in actual boiler environment for 1000 h. The coating NiCrAlY was applied between Ni–20Cr coating and substrate surface to improve the bond strength and decrease porosity of the coating. The corrosion–erosion rate of Ni–20Cr coated Superfer 800H was found highest whereas it was least for coated superni 718 sample | Mishra et al. (2014) |
| 6 | Superni 75, 718, Superfer 800H | Cr3C2–25NiCr | Detonation spray | The high temperature corrosion resistance of Cr3C2–25NiCr coating was studied and evaluated on super alloys in 75% Na2SO4 and 25% K2SO4 molten salt corrosive environments for 100 h at 900 °C. The performance of all the coated samples were found better than uncoated samples. The coated samples of superni 718, superni 75 and Superfer 800H were 50, 30 and 20% respectively, better in corrosion resistance than uncoated samples. | Kamal et al. (2009) |
| 7 | Superni 75, 600, SuperCo 605 | Cr3C2–25NiCr | Detonation spray | The study compared the hot corrosion resistance performance of Cr3C2–25NiCr coated super alloys of nickel and cobalt in actual environment. The results showed that uncoated superco 605 undergone delamination whereas coated samples of both the substrates remained unaffected to such deformations. Better corrosion resistance was demonstrated by coatings when exposed to 1050 °C for 1000 h. Chromium oxide and NiCr2O3 were main phases formed during exposure. | Mudgal et al. (2017) |
| 8 | Superfer 800 H | Ni3Al, Ni–20Cr, Stellite-6, NiCrAlY | Plasma spray | Coatings of Ni3Al, Ni–20Cr, Stellite-6 and NiCrAlY were deposited on Superfer 800H. The coatings were studied for their corrosion resistance behaviour in 40% Na2SO4–60% V2O5 molten salt at 900 °C for 50 h. These coatings were proved highly effective in protecting the super alloy from corrosion. NiCrAlY coating exhibited 90% whereas Ni–20Cr, Ni3Al and Stellite-6 recorded 80, 66 and 50% improvement in corrosion resistance respectively when compared with uncoated material. | Singh et al. (2005) |
| 9 | Superfer 800 | Ni–5Al, NiCrAl | High velocity oxy-fuel spray | Two nickel-based coatings with and without Cr content were compared for their corrosion resistance in actual super heater zone of coal fired boiler for 1000 h. NiCrAl coating was found more effective in resisting corrosion than Ni–5Al coating. The NiCrAl coating was found more effective in providing corrosion protection than Ni–5Al coating. It might be due to excessive formation protective oxides of Ni and Cr in top layer of NiCrAl. | Mahesh et al. (2010) |
| 10 | Superfer 800H | NiCoCrAlY | Cold spray | The hot corrosion behaviour of NiCoCrAlY coating deposited on Superfer 800H was investigated in actual environment for 1000 h. The coated samples exhibited 76% better protection from corrosion as compared to bare material. The formation of Al2O3 and Cr2O3 protective oxides in coated samples at high temperature helped to ensure better protection from corrosion. | Kalsi et al. (2014b) |
| 11 | Superni 75 | NiCrBSi, Stellite-6 | High velocity oxy-fuel spray | The performance of NiCrBSi and stellite-6 coatings made on superni 75 alloy were evaluated and compared for their corrosion resistance property. The study was conducted in actual boiler environment for 1000 h. It was found that both the coatings were effective in resisting corrosion at 900 °C. The coating of stellite-6 was proved better than NiCrBSi due to the formation of dense chromium oxide layer. | Sidhu et al. (2006b) |
| 12 | Superni 600 | Cr3C2–25NiCr, Ni–20Cr | High velocity oxy-fuel spray | Cr3C2–25NiCr and Ni–20Cr coatings made by HVOF technique on superni 600 alloy were studied and compared for corrosion in molten salt environment of 40% Na2SO4–60% V2O5 at 900 °C for 50 h. Both the coatings were found effective in resisting corrosion at high temperatures. The Ni–20Cr coating showed 60% whereas Cr3C2–NiCr coating offered 45% better corrosion protection than offered by bare material. | Sidhu et al. (2006c) |
| 13 | Superni-600 | NiCrAlY alloy | Cold spray | The coating of NiCrAlY alloy was made on superni- 600 alloy and studied for corrosion resistance in actual high temperature oxidizing environment for 1000 h. The coating was found 36% more helpful in reducing degradation than bare material. The dense Al2O3 and Cr2O3 oxides formed during oxidation restricted the penetration of corrosive particles across the coating and enhanced its protection from corrosion. | Kalsi et al. (2014a) |
The super alloys already possess good oxidation resistant properties but to enhance their operational life, the protective coatings are done. The coatings provide extra protection from corrosion–oxidation degradation. The alloys of Ni, Fe or Cr materials are used as coating materials for the protection of super alloys from high temperature corrosion. At high temperature, due to oxidation these materials are converted into different protective phases such as CrO, NiCr2O3, Cr2O3, Al2O3, etc. (Mudgal et al. 2017). However, there may be a problem of interdiffusion between the elements of coating and substrate materials due to their similar nature or chemical affinity. Due to interdiffusion, if the content of protective elements in coating drops below a critical value, then the coating may fail. These coatings are found to be effective for working below 1000 °C temperature.
The ceramic coating such as Al2O3, Cr2O3, TiO2, SiO2, etc. are recommended for the protection of super alloys from high temperature degradation due to corrosion–erosion. However, the ceramic coatings have some limitations. Cr2O3 coating is effective upto 800 °C and become volatile above this temperature and SiO2 becomes volatile in the presence of H2O (Pint et al. 2006). If the super alloys used as structural component, turbine blade or pump component that continuously comes in contact of high temperature, then in such cases ceramic coatings are found beneficial.
These coatings require high temperature coating techniques and therefore, high velocity oxy-fuel spray or plasma spray methods are preferred. The cold spray technique is also suitable for applying coating of Ni-based alloys because it helps to form dense oxides of Al and Cr during oxidation which restrict the further degradation of coating.
7 Recent advancements in thermal spray processes
Many researchers are working to improve the quality of coatings. Vast research is going on in different directions to get the desired results after coating. Researchers are working on pre-treatment of feedstock materials by alloying them with non-conventional materials, by using nanoparticles or by improving the coating setups like changing the design of nozzles, inert gas environment, pre-heating the substrate material, etc. The research is also going on for post-treatment of coatings by applying heat treatment techniques, laser assisted remelting of coatings to decrease the porosity. Some of the recent advancements in this field are discussed in Table 7.
Recent advancements in thermal spray coating techniques.
| References | Coating technique | Substrate | Coating materials | Description |
|---|---|---|---|---|
| Kim et al. (2005) | Cold spray | AISI 304 | WC–12Co (nano-structured) | The study was conducted to explore the possibility of performing nano-structured WC–12Co coating on AISI 304 steel by cold spray technique. It was necessary to restrict the transformation of WC phase in WC–12Co coatings during decomposition on exposure to high temperatures which generally happens with HVOF and plasma spray techniques. The research showed that it was possible to apply nano-structured coatings with cold spray technique. It not only helped to avoid phase transformation but also reduced the coating porosity. It was also found that the increase in percentage of Co in coating powder from 12 to 17% may improve its ductility. |
| Sharma et al. (2015) | High velocity oxy-fuel spray | AISI 304 | NiCrBSi | The research was done to investigate the effect of using boron nano-particles in coating material and impact of laser treatment done after coating, to understand the changes in microstructure and wear behaviour of coating. It was found that laser assisted HVOF coating helped to refine the microstructure of coating due to formation of nano-borides of chromium and nickel. The laser treatment of coating not only helped to reduce its coefficient of friction but also improved its wear resistance significantly. |
| Kumar et al. (2015) | Cold spray | SA516 Gr 70 | Ni–20Cr (nano-structured) | The nano-crystalline Ni–20Cr coating was made on SA 516 Gr 70 steel to understand its erosion–corrosion resistance behaviour in actual conditions at 740 °C for 1500 h. The study showed that coating possessed high density with good bond strength. The hardness of coated substrate was 2.5 times more than its conventional counterpart. The nano-structured coating was found 100% effective in reducing thickness loss of substrate in actual degrading environment. |
| Shi et al. (2016) | Plasma spray | Q235 | FeCr (nano-composite) | The investigation was made to compare the corrosion resistance behaviour of conventional FeCr coating and FeCr coating containing Cr nano-particles. It was found that FeCr coating with Cr nano-particles performed better than conventional FeCr coating. The study revealed that with the increase in exposure time the pores in nano-structured coating was enlarged and cracks were also developed, so to overcome this problem heat treatment was done in hydrogen environment by heating the coated samples up to 800 °C. This refined the grains of nano-structured coatings which further helped to decrease the corrosion rate to large extent. |
| Grewal et al. (2016) | Plasma spray | AISI 304 | TiAlN (nano-structured) | The study was done to understand the difference in mechanical and structural characteristics of conventional and nano-structured TiAlN coatings made at 200 and 500 °C. In nano-structured coating, porosity was found less than 0.5% whereas in conventional it was in range 2.45–4.10%. The surface roughness of nano-structured coatings was in range 3.60–3.74 nm which was very low as compared to the range 10.25–15.45 μm in conventional TiAlN coatings. But conventional coating provide more micro hardness than nano-structured coatings. |
| Thakur et al. (2017) | High velocity oxy-fuel spray | AISI 304 | WC–10Co–4Cr | In this study the effect of addition of multi walled carbon nanotubes (MWCNTs) in nano-structured WC–CoCr coating was studied for its erosion resistant properties. It was found that the addition of MWCNTs helped to improve the fracture toughness by 58% when compared with without MWCNTs nano-coating. It also decreased the porosity of coating by 33%. The MWCNTs-induced nano-structured coatings were found 50 and 66 % better than without MWCNTs nano-structured coating and conventional coating respectively at 30° erosion angle. |
| Ding et al. (2017) | High velocity oxy-fuel spray | AISI 304 | WC–10Co–4Cr | The study was conducted compare the characteristics and abrasive wear performance of WC–10Co–4Cr coating material composed of multiple size particles from micron, submicron to nano in WC (MWC) and nano-structured WC (NWC). It was found that MWC coatings offered low porosity, 12% higher fracture toughness and 36% better abrasive wear resistance as compared to NWC coatings. |
| Tang et al. (2018) | Cold spray | AISI 304 | Ti–WC | The effects of post spray heat treatment of Ti–WC coating on mechanical and wear resistance characteristics were studied through this research. Heat treatment of coating was done at 550 and 650 °C. It was found that post spray heat treatment of coating increased its density and hardness from 68 to 146% due to formation of carbides but its abrasive wear resistance remained almost unaffected. |
| Fals et al. (2019) | Flame spray | AISI 304L | NbC | The surface modification of NbC coating done with laser technique in controlled argon atmosphere was evaluated for high temperature erosion wear. It was found that laser treatment homogenized the coating structure and improved its bond with substrate surface. It was also revealed during experimentation that laser modified coatings recorded 35% improvement in abrasion wear resistance at 90° erosion angle whereas at 30° erosion angle the coating was proved 12% less effective than the uncoated samples. |
| Babu et al. (2019) | Detonation spray | AISI 316L | Ni, WC powders | The study was done to understand the erosion behaviour of post spray heat treated WC–10Co–4Cr and Ni coatings by microwave technology. The post spray microwave assisted heat treatment was done to homogenize the coating microstructure by eliminating splat boundaries and pores. It helped to increase the erosion resistance of the modified coatings by 46%. Although there was decrease in hardness by 15% in WC-based coatings but its fracture toughness was also improved by 67%. |
| Qin et al. (2019) | High velocity oxy-fuel spray | AISI 316L | Cr3C2–NiCr | The laser processing of Cr3C2–NiCr coating was done to refine its microstructure and evaluate the improvement in structural properties of coating. After laser treatment columnar dendrites, equi-axed dendrites and fine grains were found in coating microstructure across thickness. The study showed that post spray heat treatment helped to reduce the coefficient of friction by 66% and increased its bond strength and coating hardness from 3.5 to 8.21 GPa as compared to untreated coating. |
The recent research is going on to improve the performance of the coating materials by using nano-structured coatings. The coating characteristics of nano-structured coatings are better as compared to conventional coatings due to their lesser porosity. Plasma spray as well as cold spray technology can be used for obtaining the nano-structured coatings of WC–Co, Ni–Cr, Al, etc. The nano-structured coatings obtained with plasma spray technology produce microstructural homogeneity with less than 1% porosity and better erosion resistance (Grewal et al. 2014).
The recently developed high pressure cold spray technique is the another advancement in the field of thermal spray coatings. It is more effective as compared to present low pressure cold spray technique due to its capability to coat materials like Ti–WC which are otherwise difficult to coat with current low pressure technique. It is possible due to high impact velocity of the spray during the high pressure coating. High pressure cold spray helps to reduce the porosity in the coatings and provide no time for unstable oxide formation (Tang et al. 2018).
Another method to improve the coatings is the post coating heat treatments. The treatments may be laser assisted, microwave assisted, or with torch flame, etc. This heat treatment is found to be helpful in refining the microstructure of coatings. In this method the re-melting of the coated layers is done to close the pores to control the oxide formation and erosion at high temperatures.
8 Conclusions
After studying the work done by various researchers in the field of thermal spray coatings to protect the boiler steels from high temperature degradation, it has been found that different types of thermal spray coatings can be successfully used for different types of working environments. However, the success and life of coating would largely depend upon the coating method, coating material, substrate and working environment.
The studies revealed that most commonly used coatings materials are Ni, WC, Al or Cr elements. Some of the established protective compounds of these elements are Ni–Cr, WC–Co and Cr2C3-based or tailor made by researchers as per requirement or demand of application. During high temperature oxidation–corrosion, the formation of protective carbides of Cr and oxides of Ni, Cr, Al, etc. helps to restrict the coating from further degradation by resisting high temperature penetration of corrosive species across the coating thickness. Such claims have been confirmed by the XRD analysis of different coatings done after undergoing cyclic exposure to the deteriorating environment.
In the recent years, modifications in the coating techniques have been tried. Some of these modifications are: laser remelting of applied protective layer to decrease porosity and to increase homogenity in microstructure; or heat treatment to improve the bond strength and deposition efficiency of coatings during the process. In addition to this, nano-structured coatings have been developed to achieve good surface finish of the coating with very less porosity. Cold spray coating techniques have also been developed to eliminate the high temperature oxidation problems. However, further research is required for its application in coating of cermets and alloys of other protective elements such as Ti and W.
About the authors
Deepak Dhand does a PhD in mechanical engineering and a research scholar at Punjabi University, Patiala, Punjab. He works as an assistant professor in Department of Mechanical Engineering at Guru Nanak Dev Engineering College, Ludhiana, Punjab, India. His research interests include hot and cold spray technologies. He has presented papers at national and international conferences.
Parlad Kumar has a PhD in mechanical engineering. He is an associate professor at Department of Mechanical Engineering, Punjabi University, Patiala, Punjab, India. His research interests include thermal spray techniques, manufacturing processes and rapid prototyping. He has published and presented a large number of research papers in national and international journals and conferences. Also, he has completed government funded research projects.
Jasmaninder Singh Grewal is a professor in Department of Production Engineering at Guru Nanak Dev Engineering College, Ludhiana, Punjab, India. He has vast research experience in the field of thermal spray technologies. He has contributed a large number of research papers to national and international journals and conferences.
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Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
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Research funding: None declared.
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Conflicts of interest: The authors declare no conflicts of interest regarding this article.
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Articles in the same Issue
- Frontmatter
- In this issue
- Reviews
- Investigations of the intrinsic corrosion and hydrogen susceptibility of metals and alloys using density functional theory
- Integration of green nanotechnology with silica for corrosion inhibition
- Analysis of internal corrosion of supercritical CO2 pipeline
- A review of thermal spray coatings for protection of steels from degradation in coal fired power plants
- Original articles
- The importance of phase composition for corrosion resistance and thermal oxidation behavior of gas-borided layer produced on nickel alloy
- Tribological behavior of stainless steel in sulfuric acid in the presence of Thymus zygis subsp. gracilis essential oil: experimental and quantum chemical studies
Articles in the same Issue
- Frontmatter
- In this issue
- Reviews
- Investigations of the intrinsic corrosion and hydrogen susceptibility of metals and alloys using density functional theory
- Integration of green nanotechnology with silica for corrosion inhibition
- Analysis of internal corrosion of supercritical CO2 pipeline
- A review of thermal spray coatings for protection of steels from degradation in coal fired power plants
- Original articles
- The importance of phase composition for corrosion resistance and thermal oxidation behavior of gas-borided layer produced on nickel alloy
- Tribological behavior of stainless steel in sulfuric acid in the presence of Thymus zygis subsp. gracilis essential oil: experimental and quantum chemical studies
