Advances in Corrosion-Resistant Thermal Spray Coatings for Renewable Energy Power Plants. Part I: Effect of Composition and Microstructure
Power generation from renewable resources has attracted increasing attention in recent years owing to the global implementation of clean energy policies. However, such power plants suffer from severe high-temperature corrosion of critical components such as water walls and superheater tubes. The corrosion is mainly triggered by aggressive gases like HCl, H2O, etc., often in combination with alkali and metal chlorides that are produced during fuel combustion. Employment of a dense defect-free adherent coating through thermal spray techniques is a promising approach to improving the performances of components as well as their lifetimes and, thus, significantly increasing the thermal/electrical efficiency of power plants. Notwithstanding the already widespread deployment of thermal spray coatings, a few intrinsic limitations, including the presence of pores and relatively weak intersplat bonding that lead to increased corrosion susceptibility, have restricted the benefits that can be derived from these coatings. Nonetheless, the field of thermal spraying has been continuously evolving, and concomitant advances have led to progressive improvements in coating quality; hence, a periodic critical assessment of our understanding of the efficacy of coatings in mitigating corrosion damage can be highly educative. The present paper seeks to comprehensively document the current state of the art, elaborating on the recent progress in thermal spray coatings for high-temperature corrosion applications, including the alloying effects, and the role of microstructural characteristics for understanding the behavior of corrosion-resistant coatings. In particular, this review comprises a substantive discussion on high-temperature corrosion mechanisms, novel coating compositions, and a succinct comparison of the corrosion-resistant coatings produced by diverse thermal spray techniques.
Keywordsarchitecture composition high-temperature corrosion microstructure renewable energy power plants thermal spray coatings
The Need for Coatings
to extend component life by increasing the corrosion resistance,
to improve functional and mechanical performances (such as creep as well as thermo-mechanical fatigue) by enabling higher operating temperatures,
possibility of refurbishment/repair, as well as
to reduce component cost by improving the functionality of a low-cost material with a protective coating.
Synopsis of the Review
This review aims to present a comprehensive state-of-the-art overview of the essential concepts in high-temperature corrosion-resistant coatings, particularly the coatings deposited by thermal spray techniques. The review takes a holistic approach to the relevant topics, at time being concise and critical, with a primary focus on the key developments, including the challenges and opportunities with thermal spray coatings. The review allows both fresh and skilled researchers entering the field to be informed of the various critical aspects of the field, and challenges, as well as their connection to high-temperature corrosion performance. The present paper is organized such that “Introduction” section is dedicated to introduction and need for coatings. “Corrosion in biomass/waste-fired boilers” section is devoted to a background on the power generation sector, mainly the renewable energy boiler industry. “Corrosion protection methods” section is dedicated to corrosion protection methods. In “Thermal spray coatings” section, the advances in thermal spray techniques, especially the newest methods, such as high-velocity air–fuel (HVAF) technique are discussed. The mature knowledge base of the conventionally used and recently developed thermal spray techniques can provide powerful synergistic benefits for deeper scientific understanding of advanced corrosion-resistant coatings. “High-temperature corrosion in thermal spray coatings” section is a summary of the role of the coating composition in high-temperature corrosive environments. This section covers the recent advances in the understanding of the corrosion mechanisms operating in high-temperature coatings and the effect of alloying. The focus is particularly on the high-temperature corrosion behavior of Ni- and Fe-based coatings, which are the most commonly used alloy systems in boiler applications.
Despite the growing interest in metallic coatings, a comprehensive treatment of the coatings from the experimental methodologies to the fundamentals and of their corrosion behavior in service environments is lacking. There are review papers on different aspects of coatings, including wear (Ref 10) or erosion-corrosion performance (Ref 11), hot corrosion (Ref 12), low-temperature corrosion (Ref 13) and corrosion in supercritical boilers (Ref 14). However, there are no comprehensive reviews on, for instance, the effect of coating microstructure and composition in high-temperature corrosive environments typical of renewable energy power plants. Furthermore, few reviews (Ref 15-17) adopt a comprehensive unbiased approach, in addition to the timely nature of capturing the surge in research activity in the past few years. Indeed, the previous and current research tendencies in the field of corrosion-resistant coatings are examined, recognizing the discernible knowledge gaps and at the same time seeking to identify the subsequent advancements and research directions in the field. To this end, this review has focused on high-temperature corrosion of metallic coatings and, as such, previously reviewed subjects such as ceramic coatings and high-temperature erosion/corrosion (Ref 11, 12, 18-22) are deliberately not covered.
Corrosion in Biomass/Waste-Fired Boilers
Biomass or waste fuels contain C, H, O, N, S, and Cl, major ash-forming elements (Al, Ca, Fe, K, Mg, Na, P, Si, and Ti), as well as minor ash-forming elements (As, Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Tl, V, and Zn). The content of each species depends on the types of biomass and waste used in a boiler (Ref 29). Summarizes the elemental constituents reported in different types of biomass and waste fuels, as well as in fly and bottom ashes.
During combustion in boilers, Cl contained in the biomass/waste fuels mainly forms gaseous HCl or/and alkali chlorides (e.g., KCl and NaCl) (Ref 30). Owing to the subsequent cooling of the flue gas in the boiler, a large fraction of Cl condenses as salts on the heat exchanger surfaces or the fly ash particles in the flue gas. The main consequences of the presence of Cl are the corrosive effect of chloride salts and HCl on the metal surfaces present in the boiler (Ref 31) and acidic pollutant emissions (e.g., HCl) (Ref 32). In short, the release of the relevant inorganic elements (e.g., Cl and K) not only leads to direct pollutant emissions (e.g., HCl) but also causes corrosion problems when they are deposited on the metal surfaces.
Corrosion of the boiler tubes can also occur due to the presence of molten phases (Ref 37), which are part of an exceptionally harmful corrosion mechanism in biomass and waste-fired boilers. This type of corrosion is due to the low-melting-point eutectic compounds formed between the alkali chlorides (NaCl and KCl) and other metal chlorides (FeCl2, NiCl2, CrCl2, etc.) (Ref 38). In other words, it should be considered that the hot gases that are typically formed from the incineration of the waste pass through several sections of the boiler. In the first section, the water wall tubes are located where heat is transferred to the tubes by radiation. The flue gas temperature ranges from ~ 650 to ~ 1000 °C. The temperature of the surface of the water wall tubes is typically ~ 250-300 °C. In the second section, the superheater, evaporator, and economizer bundles are located where heat is transferred to the tubes by convection. The flue gas temperature that passes through this section is ranged between approximately 150 and 650 °C. (Ref 39). The corrosion in the boiler is mainly caused by the interactions of chlorides, sulfates, polysulfates, and sulfides with iron or iron oxides on the tube surfaces and limits the steam temperature that can be achieved in waste-to-energy (WtE) plants, and accordingly, the electrical energy efficiency (Ref 40). These corrosive species originate from the waste that is incinerated and are condensed onto the colder tube surfaces as the temperature decreases further down the boiler.
Owing to a large number of gaseous, solid, and even liquid compounds that are concurrently present in boilers, various competing corrosion reactions can occur simultaneously. Hence, the prediction of corrosion rates or the type of corrosion attack is challenging. Nevertheless, through a combination of simplified and controlled laboratory exposures, the significant features associated with the fireside corrosion of superheater tubes in full-scale power plants can be highlighted. Although S-containing compounds (Ref 41) may also have an impact on alloy performance, the main mechanisms associated with material failure involve the Cl-containing compounds present in the gaseous, liquid, or solid states, which are briefly explained below.
Corrosion Protection Methods
Modification of the Environment
However, the sulfation of condensed KCl particles on the surface of a superheater tube can have serious implications on corrosion attack, because HCl released close to the metal surface may serve as a source of Cl to propagate the corrosion. Moreover, the present understanding of the progression of the corrosion attack with time under such conditions is inconsistent (Ref 55). On the one hand, laboratory-scale studies with very low deposit coverages (0.01 mg/mm2) at 600 °C (Ref 56) have revealed that the conversion of KCl to the less corrosive K2SO4 is complete after 1 h of exposure; accordingly, the corrosion attack should reduce significantly afterward. On the other hand, by using a high deposit coverage (0.75 mg/mm2), laboratory-scale investigations at 560 °C under the same gas atmosphere have shown that the conversion of KCl to K2SO4 is below 10% after 72 h (Ref 57).
Organic carbon, C
Conventional and advanced materials utilized in boilers and their operating temperatures (Ref 76)
Temperature range, °C
DIN X20CrMoV121, 13CrMo44(T12), T91, HCM 12 (P122, SUS410J3TB), Super 304H, Sanicro 25, HR3C, TP347H(FG), Alloy 617, AC66, Alloy 625, A263
10CrMo910 (T22, SA213), 13CrMo44 (T12), HCM 12, Super 304H, TP347H, Alloy 617
ST35.8, DIN 15Mo3, SA201-Gr.A1
13CrMo44(T12), DIN 15Mo3, T22, T23, T24, HCM 12, Alloy 617
P91, P92 (NF616), P122, DIN X20CrMoV121, HCM 12, SAVE121, NF121, Alloy 617, A263
At present, the most economically and technically favored method for corrosion protection of boiler components is deploying an overlay coating. Coatings can be used to extend the limits of application of materials to their maximum performance capabilities by allowing the mechanical properties of the underlying materials to be preserved while protecting them in high-temperature corrosive environments (Ref 62, 63). A large number of coating processes, such as weld overlay, laser cladding, and thermal spraying, are available for providing surface protection. Although the coatings can increase the high-temperature corrosion resistance of a component, there are a few technical challenges in producing coatings that can meet the high-performance requirements.
Weld overlays are metallic coatings that are directly welded onto the substrate. The high-heat welding process results in the formation of a metallurgical bond with the base metal, essentially alloying the coating on the substrate at the interface. The result is a durable and almost non-porous coating that has good resistance to corrosion (Ref 48, 64). Furthermore, the application of Ni-based weld overlay coatings has been found to provide effective protection, and this practice has become widespread (Ref 65). However, a major concern regarding the weld overlay is that repeated applications at the same area may lead to embrittlement of the old overlay and cause cracks that propagate into the overlaid tube. Moreover, dilution of alloying elements such as Fe from base material to the weld overlay (for instance, Alloy 625 (Ref 66)) can occur, which reduces the corrosion resistance of the weld. Besides, the surface of the weld overlay coatings is typically rough, and uneven which can accelerate corrosion through easier sticking of slag or produce lower coating thickness areas that concentrate heat flux. The weld overlays can only be applied in high thicknesses (~ 2-3 mm) (Ref 21), which may increase the temperature drop across the layer. The lowered thermal conductivity of the weld overlay coatings due to their high thickness leads to an increase in the proportion of heat absorbed in the upper furnace and convection pass, resulting in a deviation from the design heat balance of the boiler, as well as the tendency of thermal fatigue cracking of the coated tubes.
Since weld overlays are typically applied in high thicknesses, substantial amounts of materials may be applied in a comparatively short time, so the process needs to be interrupted several times. The technical challenges associated with the weld overlay coatings mainly consist of varying corrosion resistance of the beads due to dilution of alloying elements, an increased risk of cracking due to a high level of residual stresses, and a mismatch in the CTE between the weld and substrate.
In laser cladding, a laser beam fuses a consumable onto the substrate surface to produce a dense uniform crack-free coating. During the deposition, the top layer of the substrate is melted to form a metallurgical bond, with some of it diluting the coating. The laser beam can be focused to a very small area and keeps the heat affected zone (HAZ) of the substrate very shallow. This minimizes the chance of cracking, distorting, or changing the metallurgy of the substrate. Additionally, the lower total heat input minimizes the dilution of the coating with material from the base material and prevents distortion of the substrate. Moreover, the laser beam can more rapidly treat larger surface areas. However, a big concern regarding the laser cladding is its high cost, which prevents the technique to be attractive in the boiler industry.
Thermal Spray Coatings
Upon perusal of early documents and the associated analysis, it can be inferred that most of the early applications were designed bottom-up. That is, the believers of the thermal spray technology pushed the applications upstream by demonstrating the value proposition of surface engineering. Corrosion and material reclamation were the primary drivers. A key demonstration of material reclamation was in the paper industry. The engineering components are generally large and, as such, the value propositions of surface engineering for both protection and reclamation were very high.
The thermal spray technology was then developed in many countries, especially Russia, Germany, and the USA, where the technology continued to be introduced to various applications over the years. The principle behind the thermal spray process during the early years of the 1920s to the 1950s was wire and powder flame spraying. Quinlan and Grobel (Ref 68) in 1937 were the first to introduce thermal spray coatings in boilers for high-temperature corrosion protection. The 1950s saw the emergence of DC atmospheric plasma spraying (APS) for application to refractory materials, notably ceramic oxides. It involved the use of the latent heat of ionized inert gas (plasma) to generate a heat source. The most common gas utilized to form the plasma was Ar, which was also referred to as the primary gas. The convergence of these developments led to the integration of thermal spraying as a process of choice in the 1970s and 1980s for component protection and performance enhancement in the aero-engine industry. It was in the early 1980s that Browning and Witfield (Ref 69) developed a new approach to spraying metal powders by using rocket engine technologies, which was known as high-velocity oxy-fuel (HVOF) spraying. The powder was partially melted in a jet stream by using straight (in the 1st and 2nd generations) or more advanced convergent-divergent (or so-called De Laval; in the 3rd generation) nozzles, and deposited on the substrate.
Like HVOF, the developments of high-velocity air fuel (HVAF) spraying started by Browning (Ref 70) and involved the concept of “Hypervelocity Impact Fusion.” According to this original HVAF concept, powder is injected into a nozzle and entrained by a hot supersonic gas stream, the temperature of which is below the melting point of the powder material (the melting point of the binder phase in the case of hard metals). The solid powder particles are heated and accelerated to extreme velocities by using a De Laval nozzle. Upon impact with the substrate, the kinetic energy of the particles is converted into thermal energy, which further heats the particles and melts or deforms them sufficiently to form a part of the coating. Several gun models have been developed for this process. After the first improvement of the HVAF gun design through the introduction of a permeable burner block into an internal combustion chamber, further progress was made by introducing a catalytic member in the internal burner. Hybrid coatings using two different types of feedstock materials (a coarse powder and a fine structured suspension) have been shown as a promising approach to obtain coatings with enhanced functional performance (Ref 71-74). The spraying processes will be individually discussed in detail in the subsequent sections.
Although all thermally sprayed coatings typically contain splats, there are huge differences among the coatings in terms of the nature of splat boundaries (like oxide pick-up, which is usually observed at such boundaries), level of porosity, and residual stress. While the formation of the in situ oxides leads to depletion of the scale-forming elements (e.g., Al, Cr, etc.), the exact effect of such oxides on corrosion mechanism of the coatings is still unclear. Moreover, the intersplat bonding and interconnected pores can act as short-circuit diffusion paths for corrosive agents (Ref 75). Therefore, the microstructure and composition of the coatings should be tailored to ensure their high capability in various applications; for instance, high protection in harsh corrosive conditions.
Flame Spray Technique
Electric Arc Spray (Wire Arc) Technique
In this process, the coating material is melted not by the application of heat from external sources, such as a flame but by producing a controlled electric arc between two consumable electrode wires. The wires have compositions close to that of the desired coating. The arc melts the wire at the tip. A compressed air or inert gas stream fragments the molten material and propels the droplets toward the substrate. A schematic of the electric spraying system is shown in Fig. 13(c). Greater bonding strengths, lower porosities, and higher spray rates are attained by using this process rather than by using the flame spray technique. However, it produces ozone, arc light, and fumes, which have detrimental effects on human health and environment. Fantozzi et al. (Ref 44) exposed an arc spray Alloy 625 coating to ambient air at 550 °C under KCl salt deposit for 168 h. The coating performed rather good and it was found that the higher degree of melting of the particles during the spraying process was perhaps beneficial in reducing the interconnected porosity and acting as a barrier to the penetration of Cl. Although the dense non-porous oxides formed at the splat boundaries might act as a barrier, their effect on the corrosion resistances of the coatings remains unclear. Highly alloyed Fe-based coatings like FeCrAlBY sprayed by this process were also shown to be satisfactory in reducing high-temperature corrosion (Ref 79), which was mainly due to their chemical compositions rather than their microstructures. While both flame and arc spray processes are cost effective in boiler applications, the in situ oxides and pores formed in the coatings limit their use in high-temperature applications.
Atmospheric Plasma Spray (APS) Technique
Hussain et al. (Ref 84) studied four different Ni- and Fe-based compositions (Alloy 625, NiCr, FeCrAl, and NiCrAlY) of alloys that were sprayed by using the APS process and exposed in advanced fossil fuel plants to address the fireside corrosion involving coal/biomass-derived flue gases. The exposure was performed in a furnace under controlled atmosphere for 1000 h at 650 °C under a coal ash deposit (containing Na2SO4, K2SO4, and Fe2O3). While the NiCr coating performed better than the others (with the order from the best to the worst coating being NiCr > FeCrAl > Alloy 625 > NiCrAlY), all the coatings suffered from a significant corrosion attack. The high level of porosity and the poor intersplat bonding allowed the corrosion to progress rapidly in the coatings. In another work, Singh et al. (Ref 85) investigated an APS NiCrAlY coating that was exposed to air at 900 °C under cyclic conditions (each cycle: 1 h heating followed by 20 min cooling). While the coating showed fairly good adherence to the substrate and developed oxide scales that protected the substrate from oxidation, the main reason for the coating failure was reported to be the pores and poor intersplat bonding.
Hong et al. (Ref 92) studied HVOF-sprayed NiCrBSiWFeCoC coatings. An amorphous phase and nanoclusters were obtained in the coating, and the major crystalline phases were Cr23C6, Cr7C3, Ni3B, WC, and solid solution γ-Ni. The formation of the amorphous phase was attributed to the high cooling rates of the molten droplets and the multicomponent alloy system of the feedstock powder. The coating was recommended for corrosion applications owing to the presence of the amorphous phase and low porosity.
Ma et al. (Ref 93) developed a new HVOF process “LS-HVOF,” where a liquid state suspension/slurry containing small metallic particles was used to deposit an ultrafine-grained alloy coating; see Fig. 13(g). The LS-HVOF NiCrBSi coating obtained exhibited a superior corrosion performance compared to the conventional HVOF. The LS-HVOF coating also displayed a nano-grained microstructure with a high percentage of melted splats, low defects, and good uniformity. The oxidation test carried out in the air and the hot corrosion test in Na2SO4-20%NaCl, both performed at 800 °C, revealed that the LS-HVOF coating was more resistant than the conventional HVOF owing to a higher degree of melting and a higher velocity of the small particles. Therefore, it was shown that the LS-HVOF technique is very promising for fabricating superior coatings in many advanced industrial applications, such as ultra-supercritical boiler components.
Cold Spray Technique
As the name implies, the cold spray process produces coatings at relatively lower temperatures (< 1000 °C) compared to the above-mentioned processes. The kinetic energy rather than the thermal energy imparted to the feedstock material drives the formation of the coating. The coating material, in the form of powders with particle sizes ranging between 1 and 50 µm, is injected into the nozzle by using a carrier gas; see Fig. 13(h). The gas, typically air, nitrogen, helium, or their mixtures, may be resistively heated to increase the propulsion velocity. However, the temperature is always kept well below the melting point of the coating material (Ref 94). The technique can be used in corrosion protection applications where the absence of process-induced oxides may offer improved performance. Due to its low temperatures, the cold spray process is an alternative method for spraying nanostructured materials as there is no particle melting and all the nanostructure is kept intact during the process (Ref 95). Despite several advantages of the process including high deposition efficiency, lower oxide content of metallic coatings, and the absence of oxidation, phase change, decomposition, grain growth, as well as post-coating component distortion, the cost of the process might be a major drawback for its application in boilers.
The HVAF process is a relatively new technology in the family of thermal spray processes receiving increasing attention during the last decade in several applications including power plants. In this process, compressed air and fuel gas (propane, propylene, or natural gas) are pre-mixed before entering the combustion chamber, where ignition occurs with the help of an electric spark plug (Fig. 13i). When the catalytic ceramic wall of the chamber is heated above the auto-ignition temperature of the mixture (shortly after the process starts), it provides further ignition and promotes combustion, thus taking over the role of the spark plug (Ref 96). Powder is injected into the process axially by using an injector through the hot back wall, with N2 as the carrier gas. The operating cost is low owing to the use of air in combustion. A fuel (such as propane) is also added between the first and second nozzles of the chamber to further increase the in-flight particle velocity and control the temperature of the particles (Ref 97). The type of fuel, nozzle features (length, diameter), amount of gas injected into both the primary (input chamber) and secondary (in-die) processes are of high importance in controlling the combustion process. The temperature and velocity of the in-flight particles are also influenced by the feedstock material characteristics (chemical composition, particle size/distribution, and morphology), which in turn will influence the properties and performance of the coating. The HVAF process produces a jet stream with a high velocity, which accelerates the injected powder particles to velocity over 1100-1200 m/s (Ref 98). This high velocity of particles establishes a good adhesion strength between the coating and the substrate. Moreover, compressive stresses are formed in the coatings due to peening (hammering effect) at such high particle velocities (Ref 77). The temperature of the HVAF flame is reported to be less than 1950 °C, which results in the in-flight particles being heated to around 1500 °C (Ref 98), depending upon the thermo-physical properties of the material being sprayed. The comparatively low process temperature, combined with low residence time, enables the spraying of materials that are inherently sensitive to high temperature and oxidation, such as materials comprising Cr or/and Al (Ref 67). Therefore, the protective scale-forming elements such as Cr or/and Al are not depleted during the spraying process but preserved for oxidation protection. The low heat input during the process also leads to a negligible oxide content in the coating. Under typical spraying conditions, the total oxygen content in HVAF coatings can be kept below 1 wt.% for most materials, particularly Ni-based coatings (Ref 99). The coatings have been shown to yield high bond strengths that are sometimes much higher than what can be determined by the most common adhesion test method ASTM C633 (Ref 100). The feedstock material characteristics (such as optimal particle size with narrow distribution) combined with the optimized process parameters increases the DE. The few studies performed so far on HVAF-sprayed coatings show that the coatings possess excellent oxidation behavior in high-temperature environments (Ref 101-103). However, the high-temperature corrosion mechanisms in HVAF-sprayed coatings exposed to a given corrosive environment need to be further studied.
Advances in Thermal Spray Processes for Power Plant Application
Comparison of thermal spray processes
Jet temperature ( °C)
Jet velocities (m/s)
Gas flow (slpm)
Air, N2, Ar
Ar, He, H2, N2
CH4, C3H6, H2 + O2
Power input (kW)
Particle temperature ( °C)
Particle velocities (m/s)
Feed rate (g/min)
Density range (%)
Bond strength (MPa)
Moderate to high
Moderate to coarse
Moderate to low
Hardness (WC coating) (HRC)
High-Temperature Corrosion in Thermal Spray Coatings
Several prior efforts have focused on the oxidation behavior of coatings deposited by the commonly used thermal spraying techniques such as APS and HVOF (Ref 47, 121-126). However, the coating features associated with the above techniques, such as splat boundaries, pores, and in situ formed oxides (Ref 75), lead to the formation of a discontinuous oxide scale, which motivates to seek even more efficient protective barriers. The high amount of interconnected pores, as well as poor splat cohesion due to the oxides and/or voids formed at the splat boundaries in the coatings, have an adverse effect on the oxidation behavior of the coatings (Ref 127). Previous studies, while claiming that the HVOF-sprayed coatings could provide oxidation protection (Ref 47, 84), showed that oxidation occurred through the splat boundaries and pores of the coating. Cr or/and Al cannot be uniformly supplied to the surface to form a uniform protective oxide scale if such coating features are present within the diffusion paths. Therefore, a highly cohesive, pore-free, and less in situ oxidized coatings could provide superior performance in high-temperature corrosive environments.
There are a few microstructural characteristics, such as splat boundaries, pores, and surface roughness (Ref 128) that directly affect the corrosion performance of thermal spray coatings. Apart from the microstructure, the chemistry of the coatings is important to achieve high corrosion resistance. Therefore, in the next section, the effect of these features on the high-temperature corrosion behavior of the coatings, especially in boiler environments, is discussed.
Role of Coating Microstructure
The observation of high-temperature corrosion damage in thermal spray coatings suggests that attack at the splat boundaries is the major corrosion mechanism, because most likely, they represent short-circuit diffusion paths for O or Cl; see Fig. 21(b). Indeed, the most severe corrosion failure takes place along with the boundaries of rounded, unmelted particles and, more generally, along with the splat boundaries (Ref 129). These areas are important in corrosion protection for several reasons. First, poor splat cohesion (lack of intimate contact between splats) causes the splat boundaries to act as corrosion sites; thus, passivation is prevented and the corrosion processes can be triggered (Ref 130, 131). Secondly, in situ oxidation of the feedstock particles (occurring either in the gas jet or just after particle impact) leads to the presence of oxides around the splats. This oxidation depletes the protective scale-forming elements like Cr or Al from the coating, which may favor the onset of selective corrosion (Ref 132, 133). The presence of elements like Cr and Al is crucial for the coating to provide passivation and corrosion protection (Ref 134-136). However, owing to their in situ oxidation, their effective content in the coating is reduced and the protection is limited. For instance, Rezakhani (Ref 137) investigated two arc-sprayed (FeCrAl and Tafaloy 45CT) and two HVOF-sprayed (50Ni-50Cr and Cr3C2-NiCr) coatings exposed to 550 and 650 °C for 192 h with a synthetic ash mixture of 70%V2O5-20%Na2SO4-10%NaCl. The coatings were mainly attacked through the oxides and voids present at the splat boundaries; see Fig. 21(c). FeCrAl and 50Ni-50Cr were prone to spalling. The Cr3C2-NiCr coating remained almost intact as its splat boundaries were less affected. In most HVAF coatings, cohesion among the splats is excellent, which makes it very difficult for the corrosive species to readily diffuse along with the splat boundaries and find direct paths to the substrate (Ref 138).
The most common source of coating porosity is unmelted particles. Depending on the particle temperature, the arriving droplets may cover the full range, from a fully molten liquid to an entirely unmolten solid state. The liquid particles (droplets) flow easily and fill most of the voids. On the other hand, the solid or partially melted particles need to have a high enough impact velocity to be able to be plastically deformed upon impact. Thus, it can be seen that higher particle velocities lead to greater particle deformation, and therefore, better closure of the voids. The solid particles may either rebound from the solid surface or get trapped in the coatings due to the subsequently arriving particles if the impact velocity is low. These undeformed particles are not well bonded, nor are they in intimate contact with the underlying splat, which creates voids.
If closed pores exist in a coating, the roles of (a) porosity itself and (b) the chemistry around the pores on the corrosion resistance of the coating should be taken into close consideration (Ref 145). During a long-term corrosion reaction, especially when a molten salt is present, the chemical/electrochemical reaction occurring in the region surrounding the pores makes the isolated pores grow and eventually interconnect with the nearby pores. In this way, open channels can be formed, which facilitate the corrosive medium in reaching the coating/substrate interface (Ref 147). This effect can be explained based on the amount of porosity available in the coatings: the higher the pore content, the easier is the interconnection of isolated pores.
By increasing the particle temperature, the in-flight particles become more prone to oxidation. The particles are plastically deformed at high temperatures, which generally results in a less porous coating. The melting state of the in-flight particles and their oxidation are influenced by the dwell time, heat transfer, and interactions of the particles upstream with the flame and downstream with ambient air (Ref 150). The level of intersplat bonding and oxidation has an impact on coating performance (Ref 151).
Porosities of different HVAF-sprayed Ni-based coatings (Ref 186)
Pore content, vol.%
0.2 ± 0.07
0.1 ± 0.02
0.4 ± 0.10
1.5 ± 0.30
0.2 ± 0.06
Properties of coatings obtained using HVAF process compared with those of the corresponding HVOF and APS coatings reported in different studies
0.6 ± 0.1
1.3 ± 0.2 (Ref 186)
3.2 (Ref 206)
2.88-4.42 (Ref 205)
391 ± 19 (HV0.3) (Ref 186)
298 (HV0.2) (Ref 207)
284 (HV0.2) (Ref 207)
486 ± 26 (HV0.3) (Ref 186)
350-400 (HV0.3) (Ref 208)
300-375 (HV0.1) (Ref 209)
Role of Coating Composition
Under ordinary oxidation conditions, it is mainly three alloying elements that are capable of forming a protective oxide scale: Al, Cr, and Si. In some cases, Fe or even Ni oxides at lower temperatures can provide a certain level of protection through scale formation. Under oxidizing-chloridizing conditions, the situation is much more complex, since not only the scale-forming alloying elements have to be regarded but also a severe type of internal selective attack can occur, depending on the alloying elements present in the material. Therefore, detailed knowledge of the behavior of different alloying elements in commercial materials, in particular, coatings, is important.
The aim of this section is to investigate the role of several elements that are currently used in thermal spray coatings under process atmospheres of the type mentioned above. A further aim of this section is to determine how a protective coating can be developed. The composition of this coating should be based on the results obtained from an investigation of the effect of the different alloying elements.
The alloying elements discussed in this review are mainly Al, Cr, Fe, Ni, Mo, and Si. Furthermore, the influence of the rare earth elements present in the coating has been discussed. The most important elements for forming protective oxide layers at high temperatures are Cr, Al, and Si. Other alloying elements, e.g., Mo, can also influence the high-temperature corrosion behavior in Cl-containing environments (Ref 155-157).
Cr is a commonly used alloying element in materials for high-temperature applications (< 850 °C) that can form a stable oxide (Cr2O3) with hexagonal corundum structure. Cr is also a typical alloying element in many thermal spray powder chemistries such as Ni21Cr, Ni49Cr, and MCrAlY (M = Ni, Co, or/and Fe) to provide high-temperature corrosion protection (Ref 75, 78, 167). The diffusion of a corrosive species through Cr2O3 is rather slow. Therefore, it is considered as a protective oxide. As long as the oxide scale well adheres to the underlying alloy and the growth rate of the scale is controlled by the diffusion of ions through the scale, the oxidation rate follows a parabolic trend (Ref 168). Figure 27(a) shows the ternary phase diagram of the Cr-O-Cl system at 600 °C taken from HSC Chemistry 6.0 (Outotec, Espoo, Finland), which indicates that the formation of various corrosion products including oxides like Cr2O3 and chlorides like CrCl2 and CrCl3 depends on pO2 and pCl2.
Similar to Cr, Al is also widely used in thermal spray powder composition, particularly for high-temperature corrosion applications. Al forms only one thermodynamically stable oxide (α-Al2O3), which has a hexagonal corundum structure. The α-Al2O3 phase has excellent protective properties with a lower growth rate than the other oxides found on engineering alloys. However, during the transient oxidation of Al, other crystal structures such as θ-Al2O3, γ-Al2O3, and δ-Al2O3 may also form (Ref 169). It should be considered that α-Al2O3 is unable to form in the temperature range typical of boiler applications (400-700 °C). Since the transient oxides are less protective, it is recommended that Al2O3-forming alloys are pre-oxidized in controlled atmospheres to ensure the formation of the protective α-Al2O3. However, the formation of such a layer involves a few technical challenges. The α-Al2O3 layer is highly sensitive to failures, especially those caused by scratching during handling. Moreover, if the pre-oxidation is performed inside an actual boiler, as the typical temperature for α-Al2O3 formation is typically above 900 °C, the other temperature-sensitive materials inside the boiler will be damaged. Therefore, great attention needs to be paid to such a post-processing step. In general, in water-containing atmospheres, Al2O3 has no volatile form and is not sensitive to moisture.
A high addition of protective scale-forming elements such as Al is usually recommended for high-temperature applications; however, the mechanical properties may be impaired by the high Al content. Interestingly, if Cr is added to an Al2O3-forming alloy, the level of Al needed for forming a protective oxide layer is lowered (Ref 170). This is commonly referred to as the third element effect, but the exact mechanism is still under debate. Figure 27(b) shows a ternary phase diagram of the Al-O-Cl system at 600 °C. The formation of Al2O3 is expected at high pO2 and low pCl2 (see Fig. 27b). However, the formation of AlCl3 could be anticipated at low pO2 and high pCl2. A vast majority of thermal spray efforts to combat high-temperature corrosion have sought to utilize Al2O3-forming alloys, with materials such as Ni5Al, and Ni30Al being among the most widely used compositions (Ref 91, 109).
There are a sizeable number of reports on the influence of pCl2 and pO2 on Fe as a bulk material or coating. The main motivation for using Fe-based alloys is that they are rather cheaper and more environmentally friendly compared to many other alloying elements (Ref 171-173). In an oxidizing-chloridizing environment, Fe oxides are formed that are rather porous, allowing the outward diffusion of Fe chlorides. Figure 27(c) shows the predominance diagram of the Fe-O-Cl system. There is a new category of Fe-based alloys called “Fe-based metallic glasses” which has been less explored. Fe-based bulk metallic glasses with a disordered and defect-free structure are emerging as high-performance materials for high-temperature corrosive applications. However, poor plastic deformation after yielding and no work hardening during room temperature deformation significantly limit their potential for application as structural materials (Ref 174-176). Such drawbacks with bulk glassy alloys make Fe-based amorphous coatings more suited for industrial applications in aggressive environments (Ref 177-180). There has recently been an upsurge in research interest on Fe-based amorphous coatings deposited by thermal spraying techniques. Thermal spraying leads to exceptional properties of the Fe-based coatings, particularly in terms of high corrosion and wear resistances (Ref 181-185).
There are a few other reasons why Ni is selected over Fe. Ni has good high-temperature creep properties, which are required for load-bearing components at high temperatures. The formation of NiO is slower than those of Fe-rich oxides, implying that oxide breakaway occurs slower. Ni-based alloys require less Cr compared to Fe-based alloys to display the same high-temperature corrosion behavior. The diffusion of Ni in Cr2O3 is slower, which highly favors the delaying of the breakaway oxidation. The vapor pressure of FeCl2 is much higher than that of NiCl2 at a given temperature, implying that NiCl2 can be found in the solid state, while FeCl2 has already vaporized.
Figure 27(d) shows the predominance diagram of the Ni-O-Cl system. Both NiO and NiCl2 can be expected to form in a typical boiler condition. The addition of alloying elements or the formation of a protective oxide scale on the coating surface, which are discussed in the previous sections, may alter the amounts of Cl2 and O2 available for the formation of NiO and NiCl2.
Low concentrations of Si are often used in high-temperature alloys. A small level of Si can also be found in thermal spray powder compositions to provide the high-temperature corrosion resistance by the formation of a protective SiO2 layer on the coating surface. A small level of Si can also facilitate formation of other protective oxide scales such as Al-rich or Cr-rich oxides (Ref 187). If Cr is also available in the composition, the high O affinity of Si leads to the formation of a SiO2 layer beneath the chromia scale. Different crystal structures of SiO2 are possible, depending on the specific conditions prevailing (Ref 188). For instance, amorphous SiO2 is frequently encountered but crystalline phases may also form. SiO2 has excellent protective properties. However, the Si concentration should be kept below 2-3 wt.% to avoid embrittlement of the material. Poor oxide adhesion has also been reported in the case of SiO2-forming alloys under thermal cycling conditions (Ref 189). Figure 27(e) shows a ternary phase diagram of the Si-O-Cl system at 600 °C, which suggests that the formation of SiO2 is thermodynamically more favored than SiCl4 in O-Cl-containing environments.
Rare Earth Elements (RE) or Dispersed Oxides (RExOy)
Grain boundary segregation of RE, leading to a “site blocking” effect: REs may interrupt the outward diffusion of Cr and block the fast diffusion paths of Cr3+.
Dynamic segregation of REs at the oxide grain boundaries: The RE sergeants at the oxide grain boundaries are not static. They can be transported outward along with the grain boundaries, driven by the O2 potential (i.e., the chemical activity of O) gradient across a growing alumina scale and their high affinities for O.
Segregation of RE at the alloy/oxide interface: RE atoms segregate to the scale/metal interface and pin the climb of misfit dislocations, required for scale growth.
Incorporation of RE as an oxide (RExOy) in the oxide scale: After internal oxidation, RExOy is decomposed to produce ions (REn+) that can segregate to the grain boundaries of the oxide scale to suppress the outward diffusion of Cr3+.
There have been several efforts to enhance the corrosion resistance of thermal spray coatings by incorporating certain O-active elements (like SiO2, Al2O3, or Y2O3) into the coating to fabricate composite structures (Ref 193, 194, 195). It has already been reported that the high-temperature corrosion performance can be improved with uniformly distributed oxide dispersoids, which act as nucleation sites and facilitate the eventual formation of a continuous dense and protective oxide scale on the surface (Ref 196). However, this proposed mechanism is debated and has been rejected by others (Ref 197), as no difference can be seen between the oxidation behavior of a dispersoid-containing alloy and that of a free-dispersoid alloy during the early stages of oxidation exposure. Therefore, it is proposed that the dispersoids alter the microstructure of the formed oxide by reducing the grain size, which promotes the inward diffusion of O2− rather than the outward diffusion of Cr3+ through the oxide grain boundaries (Ref 197). The dispersed oxides can lead to excellent scale adherence to the substrate material, based on a mechanism known as the “pegging effect” or “mechanical keying” that has been frequently reported by various authors (Ref 198, 199). Oxide scale protrusions into the underlying alloy were also found, which pinned the scale to the substrate. The dispersed oxides in the alloy were also reported to act as the preferred sites for the nucleation of vacancies, which are thereby prevented from accumulating at the oxide scale/metal interface (Ref 200).
Final Remarks on Thermodynamic Considerations
Considering a coating consisting of Ni, Fe, and Cr, the plausible oxide layers are outer Ni or Fe-rich scales and inner Cr-rich scale. In general, solid NiCl2 is less stable than other chlorides (owing to its less negative Gibbs free energy). It is also difficult for volatile NiCl2 to form, as it has a low vapor pressure but, when it forms, it should be considered that NiCl2 is transported away (due to its high diffusivity and the high pO2 required to form the oxide) to a great extent, unoxidized by the gas flow.
The power generation industry is progressively shifting toward the use of renewable energies derived from sources such as biomass and waste to suppress emission of environmental pollutants like CO2, and increase the electrical/thermal efficiency. However, such power plants suffer from severe high-temperature corrosion of critical load-bearing components such as water walls and superheater tubes. Thermal spray processes are now increasingly used in these sectors, particularly for protecting components with an ideal combination of bulk and surface properties that cannot be easily accomplished using conventional alloys. At the same time, there are several synergistic factors responsible for making thermal spray coatings increasingly attractive to be deployed for imparting economical and long-lasting protection against corrosion damage in harsh operating environments. These include the ability to deposit high-performance coatings reliably by virtue of continuing improvements of the computational hardware and software, development of more robust yet affordable equipment, improvements in automation, the enhanced knowledge base with respect to both the metallurgy and degradation of thermal sprayed coatings, a larger portfolio of available coating materials in terms of both chemical composition and powder size, advances in coatings testing and characterization, improved skills/expertise in coating processes so higher quality coatings. However, there also continues to be an incessant demand for further improvements in the performance of the coatings to impart extended durability. In the present review, various thermal spray techniques have been assessed for corrosion protection applications. Despite significant improvements in the spraying techniques to achieve dense and defect-free microstructures for corrosion protection applications, several scientific and technological issues still affect the quality and cost of thermal spray coatings. These challenges include the presence of inherent features in the coatings, such as pores, splat boundaries, coating/substrate adhesion, intersplat bonding (cohesion), surface quality, and residual stresses. More importantly, the applicability of the selected thermal spray process inside the boiler for coating and repair needs to be always considered. While deposition of a dense and adherent Ni-based coating containing protective scale-forming elements such as Al or/and Cr using more advanced thermal spray techniques like high-velocity air–fuel (HVAF) has been shown as a promising approach to extend the component’s lifetime, hence increase the thermal/electrical efficiency of the boilers, the inherent features of the thermal spray coatings and coating/substrate bonding need to be thoroughly checked to enable wider commercial adaptation of the coatings in power plant applications.
The complex interaction of in-flight particles with supersonic flame in the thermal spray process, and formation of a complex deposit encumber understanding the inter-relation of process, microstructure, properties, and corrosion performance of the coatings under simplified laboratory-scale corrosion studies and complex field exposures. An additional challenge is the development of an in-depth understanding of high-temperature corrosion mechanisms of coatings due to their intricate microstructures. A better understanding of the thermal spray processes will be important to avoid the common defects and to ultimately tailor the composition, microstructure, and properties of thermal spray coatings based on scientific principles. Furthermore, greater market penetration of thermal spray coatings will require an increased level of standardization and control to achieve repeatable processes that can produce coatings with consistent properties. Developing a better metallurgical knowledge base of thermal spray coatings will require sustained research and development of the thermal spray processes and the structure and properties of thermal spray coatings over the coming decades.
Challenges also remain in improving the productivities of thermal spray processes, enhancing the performances of thermal spray coatings, lowering the high cost of producing quality metal powders, and making the equipment more affordable. Many unique technical capabilities such as spraying in small-diameter pipes, onsite spraying of boiler components, and repairing the worn coatings provide unprecedented business opportunities for thermal spraying. A more comprehensive understanding of the thermal spray processes and the innovations in these areas are needed for greater technological adaptation of the thermal spray processes.
Open access funding provided by University West. Financial support of the Knowledge Foundation for the SCoPe project (RUN 20160201) and Västra Götalandsregionen (VGR) for the PROSAM project (RUN 2016-01489) are highly acknowledged.
All data included in this study are available upon request by contact with the corresponding author.
- 2.G. Boyle, Renewable Energy Power for a Sustainable Future, 2nd ed., Oxford University Press, Oxford, 2004Google Scholar
- 10.P. Fauchais, and A. Vardelle, Thermal sprayed coatings used against corrosion and corrosive wear, Adv. Plasma Spray Appl., 2012.Google Scholar
- 12.D. Mudgal, S. Singh, and S. Prakash, “Corrosion problems in incinerators and biomass-fuel-fired boilers,” International Journal of Corrosion, 2014, 2014.Google Scholar
- 17.H. Singh, T. S. Sidhu, and S. B. S. Kalsi, “Effect of Nano Coatings On Waste to Energy (WTE) Plant: A Review,” i-Manager’s Journal on Electronics Engineering, 20101, 1, p 1Google Scholar
- 19.J. Mehta, V.K. Mittal, and P. Gupta, Role of Thermal Spray Coatings on Wear, Erosion and Corrosion Behavior: A Review, J. Appl. Sci. Eng., 2017, 20(4), p 445-452Google Scholar
- 20.S. Amin, and H. Panchal, A Review on Thermal Spray Coating Processes, Transfer, 2016 2, p 4Google Scholar
- 22.E. Sadeghi, S. Raman, and S. Joshi, “High Temperature Erosion-Corrosion of HVAF-Sprayed Ni-based Coatings”, in 8th Rencontres Internationales sur la Projection Thermique, Limoges, France, 2017Google Scholar
- 27.J. Koppejan, and S. Van Loo, The Handbook of Biomass Combustion and Co-firing. Routledge, London, 2012.Google Scholar
- 41.T. Sharobem and M. J. Castaldi, The effect of SO2/HCl ratio on superheater high temperature corrosion, presented at the 20th Annual North American Waste-to-Energy Conference, NAWTEC 2012, 2012, 23-27Google Scholar
- 60.M. Fukuda, “22 - Advanced USC technology development in Japan,” in Materials for Ultra-Supercritical and Advanced Ultra-Supercritical Power Plants, A. Di Gianfrancesco, Ed. Woodhead Publishing, 2017, 733-754.Google Scholar
- 63.A. Hjörnhede, Erosion - Corrosion Resistance and Adhesion of Laser and Thermally Deposited Coatings in Fluidised Beds, 2004Google Scholar
- 65.B. Backer, S. Kizer, and R. Zhang, “Recent experience with materials utilized to resist coal ash corrosion”, presented at the CORROSION/2011, NACE, Houston, TX, USA, 2011Google Scholar
- 67.J. R. Davis, Handbook of Thermal Spray Technology. ASM International, 2004.Google Scholar
- 68.F. B. Quinlan and L. P. Grobel, “Treatment of metals,” US2303869A, 01-Dec-1942.Google Scholar
- 76.M. Oksa, “Nickel- and iron-based HVOF thermal spray coatings for high temperature corrosion protection in biomass-fired power plant boilers: Dissertation,” 2015.Google Scholar
- 79.Y. Korobov, S. Nevezhin, and M. Filippov, “Study of high velocity arc sprayed heat resistant coatings from FeCrAlBY cored wire. Thermal spray. Fostering a sustainable world for a better life,” in of the Int. Thermal Spray Conf. and Exposition ITSC 2016 (10-13 May, 2016, Shanghai, China), 2016, 852-856.Google Scholar
- 81.C. Moreau, P. Gougeon, M. Lamontagne, V. Lacasse, G. Vaudreuil, P. Cielo, “On-Line Control of the Plasma Spraying Process by Monitoring the Temperature, Velocity, and Trajectory of in-Flight Particles,” (1994).Google Scholar
- 85.H. Singh, D. Puri, and S. Prakash, High Temperature Oxidation Behaviour of Plasma Sprayed NiCrAlY Coatings on Ni-Based Superalloys in Air, Trans. Indian Inst. Metals, 2005, 59, p 215-227Google Scholar
- 96.A. Verstak and V. Baranovski, Activated Combustion HVAF Coatings for Protection Against Wear and High Temperature Corrosion, Presented at the International Thermal Spray Conference and Exposition, ITSC 2003, Orlando, Florida, 2003Google Scholar
- 97.C. Lyphout and S. Björklund, Internal Diameter HVAF Spraying for Wear and Corrosion Applications, J. Therm. Spray Technol., 2015, 24, p 235-243Google Scholar
- 113.R. Jafari, E. Sadeghi, T. Shahrabi Farahani, N. Markocsan, S. Joshi, High Temperature Corrosion Mechanisms of HVAF-Sprayed Ni-Based Coatings Exposed to Alkali-Sulphate and Chloride Mixture Environments. 8th Rencontres Internationales sur la Projection Thermique, Limoges, France (2017).Google Scholar
- 114.R. Jafari, E. Sadeghimeresht, T. Shahrabi Farahani, N. Markocsan, S. V. Joshi, KCI-induced corrosion behavior of HVAF-sprayed Ni-based coatings in ambient air. Presented at the International Thermal Spray Conference and Exposition, ITSC 2017; Dusseldorf; Germany; 7 June 2017 through 9 June 2017, 2017, vol. 2, pp 946-950 (2017).Google Scholar
- 115.P.L. Fauchais, J.V.R. Heberlein, and M.I. Boulos, Industrial Applications of Thermal Spraying Technology. Thermal Spray Fundamentals, Springer, New York, 2014, p 1401-1566Google Scholar
- 120.K.A. Unocic et al., High-Temperature Behavior of Oxide Dispersion Strengthening CoNiCrAlY, Mater. High Temp., 2017, 0(0), p 1-12Google Scholar
- 123.M. Oksa, J. Metsäjoki, and J. Kärki, Thermal Spray Coatings for High-Temperature Corrosion Protection in Biomass Co-Fired Boilers, J. Therm. Spray Technol., 2014, 24(1-2), p 194-205Google Scholar
- 128.P. Zhang et al., Isothermal Oxidation Behavior of HVAF-Sprayed NiCoCrAlY Coatings: Effect of Surface Treatment, DIVA, pp 456-461 (2017).Google Scholar
- 153.E. Sadeghi, N. Markocsan, T. Hussain, M. Huhtakangas, S. Joshi, Effect of SiO2 Dispersion on Chlorine-Induced High-Temperature Corrosion of High-Velocity Air-Fuel Sprayed NiCrMo Coating, Corrosion, pp 984-1000 (2018).Google Scholar
- 157.E. Guerin, E. Sadeghi, N. Markocsan, S. Joshi, Role of Chemistry on Oxidation Behavior of Various Ni-based HVAF-Sprayed Coatings in Simulated Boiler Environments, 8th Rencontres Internationales sur la Projection Thermique, Limoges, France (2017)Google Scholar
- 167.E. Sadeghimeresht, J. Eklund, J. Phother Simon, J. Lyske, N. Markocsan, S. V. Joshi, Oxidation behaviour of HVAF-sprayed NiCr coating in moisture-laden environment. Presented at the nternational Thermal Spray Conference and Exposition, ITSC 2017; Dusseldorf; Germany; 7 June 2017 through 9 June 2017, 2017, vol. 2, pp 644-646.Google Scholar
- 169.N. Israelsson, High Temperature Oxidation and Chlorination of FeCrAl alloys. Doctoral thesis, Chalmers University of Technology (2014).Google Scholar
- 171.E. Sadeghimeresht, N. Markocsan, P. Nylén, HVAF thermal spray Fe-based coating: An environmentally acceptable alternative to cobalt-based coating. Presented at the EUROCORR 2015,EUROPEAN CORROSION CONGRESS 6-10 September 2015 Graz / Austria The annual event of the European Federation of Corrosion (2015)Google Scholar
- 172.E. Sadeghimeresht, N. Markocsan, P. Nylén, S. Dizdar, Corrosion behavior of high-chromium Fe-based coatings produced by HVAF thermal spraying technique. Presented at the 7th Rencontres Internationales sur la Projection Thermique. 9th to 11th December 2015-Limoges, France (2015)Google Scholar
- 186.E. Sadeghimeresht, Ni-Based Coatings for High Temperature Corrosion Protection, DIVA (2018).Google Scholar
- 189.P. Viklund, High Temperature Corrosion During Waste Incineration: Characterisation, Causes and Prevention of Chlorine-Induced Corrosion, KTH Royal Institute of Technology, Stockholm, 2011Google Scholar
- 193.K.A. Unocic et al., High-Temperature Behavior of Oxide Dispersion Strengthening CoNiCrAlY, Mater. High Temp., 2017, 1(1), p 1-12Google Scholar
- 198.T. Huang, J. Bergholz, G. Mauer, R. Vassen, D. Naumenko, and W.J. Quadakkers, Effect of Test Atmosphere Composition on High-Temperature Oxidation Behaviour of CoNiCrAlY Coatings Produced from Conventional and ODS Powders, Mater. High Temp., 2017, 0(0), p 1-11Google Scholar
- 210.G. Marland, T. Boden, R.J. Andres, National CO2 Emissions from Fossil-Fuel Burning, Cem. Manu. Gas Flaring., 1751, 54, p. 2005Google Scholar
- 211.R. Narayan, An introduction to metallic corrosion and its prevention. Mohan Primlani for Oxford & IBH Publishing Company, 1983.Google Scholar
- 213.K. Veijonen, P. Vainikka, T. Järvinen, and E. Alakangas, “Biomass co-firing: an efficient way to reduce greenhouse gas emissions - Altener programme - Energy - European Commission,” European Bioenergy Networks, 2000. [Online]. Available: /energy/en/studies/biomass-co-firing-efficient-way-reduce-greenhouse-gas-emissions-%E2%80%93-altener-programme. [Accessed: 15-Aug-2018].Google Scholar
- 216.“Oerlicon metco.” .Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.