Recent Development of Corrosion Factors and Coating Applications in Biomass Firing Plants

: Due to global warming, biomass fuels are gradually being used to replace fossil fuels. However, high-temperature biomass corrosion is a crucial issue a ﬀ ecting its future application. In this article, di ﬀ erent factors a ﬀ ecting boiler performance are summarized from various studies to guide the optimization of boiler parameters in practical applications, such as corrosive components and boiler temperatures. Meanwhile, di ﬀ erent coating formation methods and materials are summarized to provide better protection strategies. The potential coating materials for future research are also discussed. The addition of other elements, such as Ti, Mo, and W, has the potential to accelerate the formation of oxide layers during high-temperature corrosion and directly slow down the corrosion rate. Future studies should focus on these elements containing materials.


Introduction
With the development of human society, fossil fuels have gradually become the main source of energy. Nowadays, 80% of the energy comes from the heat generated by burning coal and fuel [1]. The burning of fossil fuels results in a large amount of greenhouse gases, which gradually causes the greenhouse effect and endangers human survival. This urgently requires us to find another clean and pollution friendly way to obtain energy. Biomass combustion is a viable way by using carbon-neutral fuels, such as straws and trees, which also makes it the only renewable energy source that can be directly stored and transported [2]. What's more, Dermibas [3,4] reported that biomass-based power generation with appropriate composition, compared with pure coal power generation, could effectively reduce greenhouse gas emissions and pollution to land and water. In recent years, the demand for green environmental protection has gradually increased in many countries. The UK plans to reduce its greenhouse gas emissions to 20% of its 1990 emissions by 2050. The EU also targets at reducing CO 2 emissions by increasing the combustion efficiency of boilers. These initiatives have prompted the government to develop and support biomass burning projects [5][6][7].
Biomass fuels generally exist in three ways, solid fuels, liquid fuels, and gaseous fuels, such as wood products, biodiesel, and gases produced by the change of biomass. Compared with coal burning, these biomasses contain a higher content of inorganic impurities, such as potassium, chlorine, and sulfur, which will lead to problems, such as slagging in the boiler, deposition on the heating surface, chlorine corrosion on the heating surface at low temperature, and high-temperature corrosion of the superheated tube in the direct combustion process [8][9][10][11]. As shown in Figure 1, with the gradual decrease of temperature, metal chlorides in the flue gas are deposited on the wall of the superheater pipe [12]. Under the high temperature, the deposit causes serious corrosion to the pipe wall [1,13,14]. These processes reduce boiler steam temperature and increase the risk of boiler shutdown [15]. Meanwhile, with the development of boiler technology, the working temperature of the boiler will become higher, which can be seen from the experiments in many related literatures that have been carried out at about 650 • C [16,17]. As shown in Table 1 [5], the maximum combustion temperature of the boiler will gradually increase; this is a predictable improvement in boiler combustion technology. Figure 1. Examples of biomass-fired boilers: straw-fired vibrating-grate boiler [12]. Table 1. Biomass and co-firing plants in Denmark [5]. With the increase in the boiler combustion temperature and the presence of corrosion elements, intense corrosion in the furnace has made the traditional alloys no longer satisfactory. Therefore, a great deal of research has been devoted to the development of new materials and technologies to meet the high-temperature environment in the furnace. While these technologies are gaining popularity, there are still many things we need to learn in order to achieve longer service times and better combustion performance. In this paper, the first part briefly introduces the factors related to the corrosion in the boiler. In the second part, various coating materials used in high-temperature corrosive environments are listed. In the last part, the corrosion resistance of each material is summarized, and the future development of materials is expected.

Corrosion-Related Factors
Many factors, such as the composition of fuels and operating conditions, etc., can affect the severity of high-temperature corrosion in the boiler. These vital factors may simultaneously have a dramatic impact on the boiler, and it is quite difficult to introduce all the factors that affect the corrosion rate in detail. Thus, this section aims to briefly introduce some of the important factors and roughly explain how they affect the corrosion process.

Biomass Composition
As the main factor, biomass components greatly affect the high-temperature corrosion process. Table 2 shows the effect of various fuels on corrosion damage [18]. Compared with coal, the composition of biomass is more complex and diverse, which is the main cause of severe high-temperature corrosion. Much research has been done to explore the compositions of biomass and initiation of biomass corrosion [19]. Baxter et al. [10] summarized the combustion characteristics of biomass. The biomass components include cellulose, hemicellulose, lignin, water, small amounts of lipids, proteins, monosaccharides, starches, hydrocarbons, ash, and other ingredients. These components produce chlorides, sulfates, polysulfates, and sulfides after combustion, leading to the corrosion of metals and metal oxides in the boiler [20]. Vassilev et al. [11] reported that kinds of biomass have similar composition and a little difference between the mass. However, the change in constitutes results in a great difference in combustion products, which affects the rate of corrosion.
Chlorine cannot be ignored due to its severe corrosiveness and high content in biomass while researching the corrosion in the boiler [8,21,22]. There are three generally accepted corrosion mechanisms to explain how chlorine accelerates corrosion: (a) Active oxidation: the reaction between metal or metal oxides and gaseous chlorides leads to corrosion; (b) Electrochemical: chloride reacts with the metal to cause corrosion in the form of ions; (c) Molten-salt-based: corrosion reaction of molten metal chlorides and other chlorides with metals [23]. In addition, HCl can also accelerate the oxidation of metal coatings [24]. The difference in corrosion mechanism is determined by the actual situation of the boiler [25].
Chlorine is an important factor causing corrosion, but cations in biomass also have vital effects under biomass combustion. Lehmusto et al. [26] explained the effect of metal chloride ions on chromium.
The purpose of this experiment was to study the ease of reaction of different alkali metal chlorides with pure chromium, including BaCl 2 , CaCl 2 , KCl, LiCl, MgCl 2 , NaCl, PbCl 2 , and ZnCl 2 . The experimental results show that K, Na, and Pb have high activity, while other elements have no obvious corrosion. Moreover, the presence of Pb and Zn will change the melting point of the deposits [27][28][29]. In the process of combustion, Pb and Zn will be chlorinated to form metal chlorides with low melting points. PbCl 2 and ZnCl 2 will mix with other salts and reduce the melting point of the eutectic mixtures. Corrosion caused by salts with a low melting point can be explained by molten-salt-based [30].
S-containing compounds may also have an impact on alloy performance in biomass/waste-fired boilers, but this effect is relatively small. Moreover, the sulfur cycle technology can reduce corrosion and harmful gas emissions [31]. S is mainly found in flue gases, such as SO 2 , and a big amount of SO 2 will be converted to sulfates through 'sulfation reaction' [32]. The sulfate (K 2 SO 4 ), formed by the reaction, is believed to form an effective protective layer.

Operation Conditions
With the development of power plant technology, the temperature in the biomass combustion boiler is increasing. According to Table 2, the temperature of the biomass boiler has increased by 100 • C over the past two decades. The increase in the temperature of the superheater and water tube will change the corrosion mechanism and cause more severe corrosion.
The temperature in the boiler determines the alkali chloride melts, which means that the corrosion mechanism involved is different. The corrosion process includes chemical corrosion and electrochemical corrosion, as well as internal interactions and oxide dissolution. Liu et al. [33] accessed the influence of high temperature. They studied the effects of two different corrosion reagents, R1 (98.6 wt.% KCl and 1.4 wt.% NaCl) and R2 (95.5 wt.% KCl and 4.5 wt.% NaCl), on TP347H stainless steel (Fe-19.2Cr-12.75Ni-2Mn-1.4Nb-0.1C-0.09Si wt.%), C22 (Ni-21.3Cr-13.2Mo-3W-2.93Fe-2Co wt.%) alloy, and laser-cladding C22 coating under Cl-containing environments between 450 • C and 700 • C. At 450-600 • C, two kinds of corrosion reagents are generally not in a molten state. In this case, chlorine-induced corrosion plays a leading role and accelerates metal oxidation, which is, namely, active oxidation. At 650-750 • C, reagents are mostly melted, and electrochemical corrosion plays a dominant role, which are advantages for ionic transport. Finally, the corrosion rate increases and the metal is quickly consumed when the oxygen is sufficient.
Otsuka [34] proposed that the flue gas temperature has a greater influence on the deposition chemistry of the boiler tube, while the tube wall temperature has a smaller influence by calculating thermodynamic values. If the temperature of the tube wall goes beyond the critical point of melting, drastic corrosion would happen on the superheater tubes. If the fuel contains the right amount of Cl, K, S, and Na, they will cause more serious corrosion. It is worth noting that the temperature of the flue gas plays a crucial role in the formation of corrosive salts compared to the wall temperature. As the temperature of the flue gas rises, KCl and NaCl continuously condense on the superheater, but the tube wall temperature does not affect the deposition of chlorides.
The combustion improver also affects the corrosion process by changing the composition of the smoke produced during combustion. Burning in pure oxygen is more environmentally friendly than burning in air. Oxygen combustion is a new type of clean combustion technology by directly introducing oxygen and changing the combustion environment that can reduce the emission of harmful gases, such as NO x , and improve combustion efficiency. If the biomass burns in oxygen instead of the air, the mechanism of the corrosion process will also change. Since a large amount of NO x is not generated, CO 2 capture becomes simpler and cheaper [35].
At the same time, temperature fluctuations due to operation can also cause serious corrosion problems. Wu [25] investigated the effects of operating conditions on the corrosion resistance of coatings. Figure 2a,b show the operation conditions of two biomass boilers for one year. It is obvious that the Randers boiler has only stopped twice during this time, and the temperature is generally maintained within a certain range. Moreover, Maribo-Sakskøbing CHP (MSK) boiler has stopped intermittently 213 times, causing severe temperature fluctuations. Corresponding to the corrosion morphology of the Ni-coated tube (Figure 2c,d [25]), it can be seen that the Randers tube is only partially corroded, and the MSK tube is severely corroded. Part of the cause of such severe corrosion is the multiple thermal cycling of the boiler, which results in the coating being susceptible to cracking failure.

The Application of Corrosion-Resistant Coatings
This section is categorized by common coating preparation methods, including thermal spraying, metal infiltration, and laser cladding. Table 3 lists the coating preparation methods used by different authors in the biomass/co-fired boilers. Table 4 lists various materials studied by different authors in biomass corrosion-related environments. Table 3. List of preparation technology used by different authors in the biomass/co-fired boilers.

Thermal Spray
The thermal spray technique is a method in which a spray source is heated to a molten or semi-molten state by a heating source and spray onto a surface of a pretreated substrate to form a coating at a certain speed. As shown in Figure 3, thermal spray equipment generally consists of feed, heat source, and flow. The feedstock forms include powder, wire, solution, and suspension. High-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) thermal spray are often used in the production of corrosion-resistant coatings. The HVOF and the HVAF are characterized by the high speed, which can accelerate to speeds of up to 300-500 m/s or higher. These techniques all form a dense and uniform coating that helps improve the corrosion resistance of the coating. In 1937, thermal spray technology was first applied to high-temperature boilers [68]. In order to resist corrosion, thermal spraying coating requires high quality, i.e., low porosity and high bonding strength, and high corrosion resistance of coating materials. Oksa is committed to finding suitable coating materials and corresponding optimized spraying parameters [44,45].

Ni-Based Coatings
Ni-based coatings generally have high corrosion resistance due to the high corrosion resistance of the Ni element itself. At the same time, Ni-based coatings have a thermal expansion coefficient similar to that of steel and also have a high melting point; it is often used to produce high-temperature corrosion-resistant coatings by thermal spraying of HVOF and HVAF. This kind of coating is uniform and continuous, and the porosity is small and dense. According to the data in Table 4, the NiAl coating exhibits different corrosion resistance under different testing conditions. Under the simulated biomass boiler atmosphere of 550 • C, HVOF-sprayed Ni5Al coating has exhibited poor corrosion resistance and failed for 672 h [60]. However, under an air or simulated atmosphere of 600 • C, the Ni5Al coating has relatively low corrosion weight changes and corrosion product layer thickness [58,61,62]. With the increase in aluminum content, the corrosion resistance of NiAl(Ni31Al) coating has been improved under the condition in which the temperature is higher than 600 • C [24,49]. As shown in Figure 4, under the same conditions, they all show better corrosion resistance than NiCr coatings. (a) Weight gain of the exposed Ni21Cr and NiAl coatings in 5% O 2 + 500 ppm HCl + N 2 up to 168 h at 600 • C with and without KCl deposit [62]. (b) The thickness of the formed scale (average of 10 measurements) and mass gain of the samples exposed to KCl deposit at 600 • C for 168 h [58].
Generally, NiAl coating can form protective Al 2 O 3 layers on the surface. It should be noted that the formation of the Al 2 O 3 layer in the corrosion environment is independent of Al content and is related to the integrity of the coating. As shown in Figure 5a, a continuous layer of Al 2 O 3 is formed on the surface of the Ni5Al coating. As shown in Figure 5b, in a similar environment, Al 2 O 3 is not formed on the surface of the Ni31Al coating. Instead, Al 2 O 3 is formed at the interface between the substrate and the coating. Alumina can be classified into α-Al 2 O 3 , β-Al 2 O 3 , θ-Al 2 O 3 , and δ-Al 2 O 3 , according to the crystal structure, among which α-Al 2 O 3 has the best corrosion resistance [69]. However, the formation of α-Al 2 O 3 needs a temperature higher than 900 • C and cannot be directly converted into the biomass boiler (the maximum operating temperature is about 600 • C). At the same time, the long-term service performance of NiAl coating needs to be studied due to the peeling of Ni5Al coating at 550 • C after 672 h exposure [60]. Because of the good corrosion resistance of Ni element and low porosity of the coating, many authors have studied the corrosion resistance of Ni-based coatings by trying to change alloy composition in high-temperature corrosive environments. As seen from Table 4, the Ni-based coating materials frequently used in experiments in the past decade are NiAl, NiCr, NiCrAlY, and Alloy 625. Under high-temperature conditions, aluminum or chromium in Ni-based coating can form a protective oxide film and slow down the corrosion rate significantly.
Due to the good corrosion resistance of Ni-Cr alloy, many authors try to deposit NiCr material on the tube wall to form a protective coating. According to Table 4, it can be seen that a large number of NiCr coating materials have been studied and put into actual environmental trials. According to the chromium content, they can be divided into chromium content < 20 wt.%, 20 wt.% < chromium content < 40 wt.%, and chromium content > 40 wt.%.
In the high-temperature and chlorine-containing corrosion environment, the corrosion resistance of IN625 (Ni < 21.5Cr < 9Mo < 3.7Nb + Ta < 2.5Fe < 0.2Si < 0.1Mn wt.%) coating is at a medium level, and many experiments have been carried out to test its corrosion resistance under different test conditions [47,50]. Therefore, it can be used as a reference material to study the corrosion resistance of a material.
Materials with a chromium content of < 20wt.% mainly include C276, NiCrBSiFe, and Alloy 718, which were investigated by Paul et al. at different temperatures in KCl-45%K 2 SO 4 mixture and gaseous HCl-H 2 O-O 2 -containing environments [53]. As shown in Figure 6, the coatings sprayed by HVOF with low chromium content are relatively poor, except for C-276 (Ni-15.55Cr-16.48Mo-5.09Fe-3.81W wt.%), at 525 • C, and IN625 has shown the best corrosion resistance at 625 and 725 • C. Materials with a chromium content between 20 and 40 wt.% can be divided into NiCr, NiCrAlY, NiCrMo, and NiCrMoW, according to their composition. Both Ni20Cr coating sprayed by HVOF and Ni21Cr coating sprayed by HVAF have shown good corrosion resistance in different environments at 600 • C for 168 h, and the thickness of the corresponding corrosion product is relatively small between 1 and 22 µm. In ambient air, Ni21Cr coating sprayed by HVAF shows similar corrosion properties as Ni5Al and NiCrAlY coatings [62], while it has performed poorly in mixed atmospheres [58]. NiCrAlY material has high oxidation resistance, and its thermal expansion coefficient is close to that of the base Fe alloy materials [40], which is widely used in coating materials. In Hussain's research [47,48], the corrosion resistance of NiCrAlY coating is similar to IN625 coating.
Molybdenum is believed as the potential choice to improve the corrosion resistance of materials, but further research is needed in high-temperature corrosive environments [70]. Ni21Cr9Mo coating is the coating material with the largest corrosion weight change and the highest thickness of corrosion product in Jafari's experiment [62] (larger than NiCrAlY), but in Sadeghimeresht's experiment [46], the corrosion resistance of Ni21Cr9Mo coating containing SiO 2 is better than NiCrAlY. Therefore, further research is needed to clearly study the high-temperature corrosion performance of NiCrMo coating.
With the increase of chromium content, the corrosion resistance of NiCr (>45Cr) coating is significantly improved, which is obviously better than IN625 coating [44,47,48,51,56]. The increase in Cr makes chromium oxide easier to form and can be replenished in time, even if it is consumed. In the actual boilers, NiCr (>45Cr) coatings also have been widely used and have relatively good results. As shown in Figure 7, Oksa found that Ni45Cr, alloy 625 (IN625), SHS9172, and Diam4006 coatings show good corrosion resistance to NaCl, Na 2 SO 4 , KCl molten deposits with a temperature of 575 and 625 • C [50]. It is worth noting that Diam4006, sprayed with HVOF DJ gun, shows good corrosion resistance at both corrosion temperatures.

Fe-Based Coatings
Fe-based metal coatings also exhibit good corrosion resistance in various experiments. Fe-Cr and Fe-Al coatings are often used for corrosion studies, and both exhibit good corrosion resistance, as shown in Table 4. Many researchers have improved the corrosion resistance of Fe-based alloys by adjusting alloy composition and adding elements. The composition of the Fe-Cr-Al stable phase has been investigated. Orlicka et al. [67] conducted a series of experiments to study a range of Fe-Cr-Al coatings with Physical vapor deposition (PVD) by co-sputtering (two kinds of powder: Cr 99.95 wt.% and Fe30Al). Several stable compositions were determined at 550 • C by the presence or absence of an HCl atmosphere, which generally ranges from 50-96 at.% Cr, 3-29 at.% Fe, 2-22 at.% Al. As the Fe-Al content in the coating increases, the microstructure crack increases, which means the release of internal residual stress. Based on these experimental foundations, suitable compositions are selected within this range for further simulated combustion atmosphere and wall deposition experiments.
The iron-based coating still seems to protect the substrate from chloride corrosion at temperatures greater than 700 • C. Hussain et al. [55] investigated the corrosion resistance of FeCrAl coatings by HVOF in the high-temperature environment of biomass co-firing boiler. The experimental results show that the FeCrAl coating has good corrosion resistance at 700 • C through the formation of a composite protective layer of Cr and Al oxide. However, the corrosion is exacerbated at a higher temperature, and a hermetic protective layer is not formed under the oxide layer to prevent the penetration of S.
The corrosion resistance of Fe-based coatings has been verified in actual boiler environments. Oksa et al. [32] conducted a long-term real environmental corrosion test on two kinds of Fe-based alloy coatings in a fluidized bed boiler. The Fe-based coating is applied to the pipe, and the flue gas environment is produced by a boiler that burns peat and wood products. Compared to St35.8 carbon steel, the Fe-based alloy coating provides excellent protection during exposure, especially for Fe27Cr coatings with a high Cr content.
Meanwhile, Fe-based amorphous alloy coatings exhibit much higher corrosion resistance than stainless steel materials. The amorphous phase of the coating protects the coating from corrosion due to the rapid formation of a dense, uniform, and stable high purity passivation film upon solidification. Many related documents mention the role of the amorphous phase of the Fe-based coating in improving the corrosion resistance of the coatings [42,[50][51][52], such as SHS9172 (Fe < 25Cr < 15W < 12Nb < 6Mo < 4C < 3Mn < 2Si wt.%) coatings, which can also obtain good high-temperature corrosion resistance by optimizing parameters of HVOF thermal spraying technology [50].
Furthermore, if the base material is a carbon steel metal, the bond strength of the Fe-based amorphous coating to the substrate is higher than that of the Ni-based amorphous coating so that the coating is less likely to peel off. However, Table 2 shows that more and more researchers are showing more interest in Ni-based coatings than Fe-based coatings. According to the previous mechanism, it can also be found that Ni is more difficult to be chlorinated and oxidized than Fe, meaning that it has better corrosion resistance.

Aluminizing
The diffusion coating is a high-temperature diffusion and corrosion resistance element (such as aluminum, chromium, etc.), which is subjected to high-temperature diffusion and permeation process, reacts with the surface of the metal alloy, and forms alloying resistance to high-temperature oxidation and high-temperature corrosion on the surface of the metal alloy. Figure 8 shows the mechanism diagram of metal penetration in the laboratory. The sample is buried in the medium and placed in the crucible under the protection of inert gas. Coatings of different compositions and thicknesses are formed by changing the medium and temperature. A layer or intermetallic compound significantly enhances the coating of metal alloys against high-temperature oxidation and high-temperature corrosion resistance, which is also currently the most widely used high-temperature protective coating. It is a coating prepared by thermal diffusion technology, combined with the base alloy to deal with gold, so the adhesion of the coating is great, and the coating is tightly bonded to the substrate. Many tests have been carried out, and it has been found that aluminum compounds have the best corrosion resistance compared to chromium compounds. Therefore, Kiamehr [66] conducted tests on Fe 1−x Al, Fe 2 Al 5 , and Ni 2 Al 3 coatings at 600 • C for 168 h in static lab air and found that all three coatings exhibited good corrosion resistance without KCl deposition. However, under the deposition of KCl salt, local corrosion occurred on the surface of Fe 1−x Al coating, and selective corrosion of Fe 2 Al 5 led to the removal of Al. Only Ni 2 Al 3 coatings were not found to have significant corrosion and Al depletion. Vokal et al. [16] researched the excellent performance of Fe 2 Al 5 coating formed by diffusion aluminide with KCl/K 2 SO 4 deposits at 650 • C for 300 h, and Kiamehr et al. [66] investigated that Ni 2 Al 3 showed no sign of attack in the environment of static lab air with a KCl deposit at 600 • C for 168 h.
Wu et al. [65] conducted a long-term corrosion exposure test for 6757 h on TP347H steel tubes coated with Ni and Ni 2 Al 3 by aluminizing. They found that Ni-coated pipes do not have corrosion resistance in biomass burning power plants, and Ni 2 Al 3 coatings provide a protective layer at a rate that slows corrosion. It is worth noting that the experiments were conducted in the actual biomass-fired boiler environment, which means that the corrosive environment is even worse. Although the relevant experimental parameters are more complicated and cannot be accurately simulated in the experiment, such as unstable temperatures, undulating airflow, and increasing deposit thickness, the data obtained after the test is more authentic and is useful for determining the future experimental direction.
Metal infiltration can be directly applied not only to ordinary base metal materials but also to thermal spray coating to form multi-layered anti-corrosion coatings. Bellucci et al. [64] studied the properties of spraying FeCr on Grade 91 steel and pack coating it with Al. Because of the high thermodynamic stability of Al oxides in such high-temperature corrosive environments, Al can be used as a beneficial coating element in advanced biomass co-firing power generation systems. Coated Grade 91 steel has good corrosion resistance in an oxy-fuel biomass co-firing environment. This composite coating formed on Grade 91 steel prevents corrosion penetration and protects the matrix material. FeCr alloys work as an intermediate layer to prevent the coating from falling off because both of them have the property of conducting heat rapidly and a similar thermal expansion coefficient with ordinary carbon steel substrates. The oxide film formed on the surface serves as a protective barrier to protect the substrate by packing the coating technique. Because the diffusion infiltration technique can be performed directly on the surface of the sample, it can form appropriate thickness and low porosity by thermal spraying and does not form new interfaces that can cause shedding.
According to Table 4, experiments by different authors show that the Ni-Al phase coating formed by aluminizing has better corrosion resistance than Fe-Al coating.

Laser Cladding
Laser cladding can simultaneously melt the coating material and a thin layer of the substrate surface and rapidly solidify to form a surface coating that is metallurgically bonded to the substrate. Figure 9 shows a schematic diagram of laser cladding. Laser cladding provides good corrosion and oxidation resistance of the coating, which is dense and uniform. The coating formed by laser cladding has better corrosion resistance than the coating formed by thermal spraying due to its non-porous morphology. Song et al. [54] studied the corrosion resistance of NiCr coatings formed by HVOGF, HVOLF, and laser coating techniques under normal experimental conditions. Coatings produced by laser coating technology exhibit good corrosion resistance with or without KCl deposits. Reddy et al. [52] reported the same experimental results. Fe-Cr-B coating produced by laser coating has better adhesion performance than the coating formed by HVOF with KCl deposition at 700 • C for 250 h. Although the coating produced by HVOF has good corrosion resistance, the separation occurs at the interface. The adhesion of the coating is a major concern in actual production runs. For such Fe-based coatings, the coating produced by laser coating is not easily peeled off.
The coating formed by laser cladding is not only better than the coating formed by thermal spraying but also better than the alloy of the same material. Liu et al. [33] conducted a series of experiments to compare the property of TP347H, C22 alloy, and laser-cladding C22 coating. They found that laser-cladding C22 coating has the best corrosion resistance in high-temperature in two kinds of alkali chloride reagents environments. Although C22 alloy and laser-cladding C22 coating have the same ingredients and corrosion outcome, C22 coating has better corrosion resistance. Laser cladding forms a subtle structure, which makes Cr and Mo easier to form oxide and dissolute in the molten chlorides. The undissolved NiO, forming a framework together with the refined nanostructures, gives the coating the best corrosion resistance. The same experimental results also appeared in the literature of Li et al. [63], who reported that the corrosion resistance of C22 coating formed by laser cladding is higher than that of TP347H stainless steel and C22 bar due to the formation of protected Cr 2 O 3 and the fine grain strengthening effect during the laser cladding.
However, there are relatively few experiments related to laser cladding. More materials that perform well in thermal spraying technology can be studied through laser cladding technology.

Materials and Technique Selection Strategy
In summary, the three different coating generation techniques have different advantages and can all be used to protect the substrate in different situations. According to Table 3, thermal spray technology is the primary means of coating formation. Thermal spray coatings exhibit good high-temperature corrosion resistance not only in experimental results but also in practical applications. However, some problems remain regarding thermal spray coating applications. (1) The coating may have defects, such as holes and crack; (2) the bond strength between the coating and the substrate may be too low, especially after exposure; (3) the efficiency of the spray powder deposition is low. Most of the problems can be solved by adjusting spray parameters and substrate surface treatment processes.
Laser cladding techniques have similar advantages to thermal spray techniques. The coating material is not limited, even if the material is ceramic, which is attributed to the high-temperature heat source. Moreover, the coating strength produced by the laser cladding technique is higher than that of the thermal spraying technique because the coating material placed on the surface and a thin layer of the substrate surface is simultaneously melted. However, some disadvantages make it difficult to be widely used. On the one hand, the heat generated by the high-temperature source can melt the coating material and the surface material of the substrate, but excessive heat may change the phase and overall structure of the matrix material. On the other hand, the residual stress on the surface is high, which affects the service life of the coating.
In the above two technologies, the Ni-based and the Fe-based coating materials are widely used. Many authors have studied the addition of various elements to improve high-temperature corrosion resistance, such as Al, Cr, Ti, Mo, and W, which can improve the coating corrosion resistance or accelerate the formation of protective oxide layers in most of the researches. However, the latest research shows that Ni-based alloys composed of chromium may not provide sufficient protection to keep the substrate away from chloride ions [71][72][73][74]. On the one hand, in the high-temperature chlorine-containing environment, the active oxidation process will make the surface chromium oxide protective layer useless [22,[75][76][77]. On the other hand, if the deposit contains KCl, in a certain high-temperature environment, the protective chromium oxide layer reacts with KCl to form potassium chromate and loses its protection [78][79][80][81]. Therefore, many studies are still needed to find suitable coating materials that provide adequate protection, such as Ni-Al and Ni-Ti, which have good corrosion resistance in some articles and may show better potential for future applications. In addition, the newly developed high-entropy alloy coatings with attractive and unique properties are also a potential choice for future coating development [82].
Unlike other thermal spray techniques, metal penetration can produce a denser and superior coating quality than other coating forming techniques. Through this technique, a metal compound layer is formed on the surface of the metal, which has good corrosion resistance and does not cause coating peeling due to problems, such as coating bonding strength. However, metal infiltration techniques are unable to form coatings quickly and are complicated to operate because the process of infiltration is very slow and needs to be done at high temperatures.

Conclusions
With the development of technology and the continuous improvement of the thermal efficiency of boilers, not only the elements contained in the fuel but also the rise in temperature will increase the corrosion inside the boiler. The harsh environment within the boiler urgently requires the research and development of boiler materials. Many related experiments have verified that the application of coating technology can improve the corrosion resistance of the boiler. In this article, the main factors affecting corrosion are listed, and the corresponding corrosion-resistant coating materials are also exhibited. The main contents are summarized as follows: 1.
Elements, such as K, Cl, S, in biomass fuels will seriously reduce the service life of superheater.
In particular, Cl ions will penetrate the material and form loose oxides on the surface, which will destroy the material. However, these elements are unavoidable during the combustion process. As technology advances, the temperature inside the boiler will continue to increase, which will inevitably lead to more severe corrosion. Not only that but also the magnitude of the temperature changes and the concentration of the combustion oxidizer plays a very important role.

2.
Coatings formed by thermal spray technology (HVAF and HVOF), laser cladding technology, and aluminizing technology provide protection. Under high-temperature corrosion, chromium and aluminum in Ni-based or Fe-based coatings form an oxide film to retard corrosion, which has been verified in experimental and practical applications. 3.
The increase of chromium content can significantly enhance the corrosion resistance of the coating in a certain range. The high-temperature corrosion resistance of NiAl coating is better than that of NiCr coating formed by thermal spraying, but its long-term service performance and actual boiler service performance need to be further studied. The addition of other elements, such as Ti, Mo, and W, may accelerate the formation of oxide layers during high-temperature corrosion.

4.
The coating formed by aluminizing has low porosity and high substrate bonding strength.
Depending on the substrate material and process parameters, aluminizing technology can form different coatings on the substrate surface, including Ni-Al and Fe-Al. Ni-Al coating exhibits better corrosion resistance than Fe-Al coating.

5.
Laser cladding technology can not only form a coating that is consistent with the density of the alloy but also refine the surface grains of the coating. However, there are few relevant experimental data, and many materials can be studied.
The use of coatings is expected to increase the efficiency and life of the boiler, which will help to improve economic efficiency. However, many problems still need to be faced; for example, the coating is prone to cracking and flaking. These problems require improved performance of the coating through the use of new spray techniques and materials. There will be more new research results in the future, which are expected to improve the service of the coating.

Conflicts of Interest:
The authors declare no conflict of interest.