Review of Thermal Spray Coatings Perform in Protecting Boiler Steels against Corrosion at High Temperatures

Failure of boilers can cause huge economic loss to the power plants. In high temperature and aggressive working conditions erosion, hot corrosion and abrasions are most responsible factors for failure of boiler steels. Thermal spray coatings are the preferable method to minimize the cause of failures of the boiler steels due to these problems. Among different thermal spray techniques. By utilizing the HVOF process, it is possible to produce coatings with high micro-hardness and low porosity, making it an advanced and effective method that is currently undergoing rapid development. In this paper a review study regarding the performance of thermal spray coatings deposited on boiler steels against the hot corrosion has been presented. The outcomes of this research have the potential to assist in identifying the optimal coating combination and application technique to prevent the deterioration of boiler steels.


Introduction
Corrosion losses in India are estimated at about 6500 US dollars per year. 1 At high temperatures, corrosion can lead to hot corrosion, a process that accelerates oxidation. 2This occurs when the metal surface is covered with a salt layer.Numerous reactive species, such as ash content, moisture content, and alkali earth metal content, are produced as a result of fuel combustion, and these substances are highly corrosive in nature.Potassium, chlorine, sulfur, calcium, and silicon salts are formed as a result of rice husk combustion. 3Deposits of these salt species can accumulate on the surfaces of fireside components.The working temperature in boiler environments ranges from 500°C to 900°C. 4,5hen boiler components are subjected to high temperatures, a layer of oxide scale develops on their surfaces as a protective measure.The combination of salt species with the protective oxide layer leads to the dissolution of the oxide, leaving the component vulnerable to corrosion.The chlorine present in the environment influences the protective oxides by forming the gas phases of Cl2, HCl, NaCl, and KCl.7] Corrosion in boilers under a high-temperature environment is known as hot corrosion.This is induced by a thin film of fused salt deposits present in the environment.The two main types of hot corrosion are Type 1, also known as high-temperature hot corrosion, and Type 2, also known as low-temperature hot corrosion.Significant economic losses can be incurred as a result of hot corrosion, which may necessitate extra maintenance or forced outages of boiler materials. 8e considerable efforts have been done to reduce the economic losses caused due to corrosion attack.To mitigate the effects of hot corrosion, different strategies have been implemented, including incorporating corrosion inhibitors, using highchromium materials, and applying anti-corrosion coatings.Out of all the available methods, the application of coatings is considered to be the most efficient and effective approach as the use of inhibitors has proven to be inadequately successful, and incorporating materials with high chromium content would significantly escalate the expenses. 9he coating materials used in high temperature applications must have the characteristics to form a stable and the slow-growing surface oxide in order to provide good service behaviour. 10Various coating processes are used to improve the surface properties of various metals for different applications.The thermal spray coating process is a widely recognized method for enhancing surface wear resistance, with variations such as plasma spray, arc spray, detonation gun spray, and flame spray. 11is paper aimed to present the different coating deposition techniques, coatings, and their behaviour in high temperature conditions of boiler environments.

Types of Hot Corrosion
There are two types of hot corrosion that can occur at different temperature ranges.Low temperature hot corrosion, also known as Type II, happens at temperatures between 600-750°C, while high temperature hot corrosion, also known as Type I, occurs at temperatures between 800-950°C. 8arious factors, such as velocity, alloy composition, temperature cycles, and erosion process can impact both types of hot corrosion.Type I hot corrosion is caused by a fluxing process where the chemistry of sodium sulfate deposit is altered, allowing sulfur to penetrate the metal underneath.This penetration causes a reduction in the chromium content of the substrate metal, leading to oxidation and the formation of a porous oxide scale.In Type-II hot corrosion, sulfates of the base metal react with alkali metal, resulting in the creation of a eutectic with a low melting point that inhibits the development of protective oxides. 12Due to their lower melting point, Ni-based coatings are more prone to degradation in environments containing sulfur under Type-II hot corrosion.Type-II corrosion is characterized by pitting attacks and, under microscopic observation, shows minimal sulfide formation with no indications of chromium depletion.

Mechanism of Hot corrosion
High-temperature hot corrosion starts with the breakdown of the protective oxide layer, which then allows the molten salt to directly attack the underlying substrate material.This breakdown can result from erosion, erosion-corrosion, thermal stresses, and chemical reactions.Two mechanisms have been proposed for the propagation stages of Type-I hot corrosion, namely sulphidation-oxidation and salt fluxing. 14Figure 1 shows the schematic illustration of reactive/corrosive environment surrounds the biomass fired boiler.At first, the salt fluxing mechanism was suggested by Goebel and Pettit et al., which implies that the molten salt may cause the surface oxide layer to lose its protective efficiency due to fluxing of this layer.This can be either basic type or acidic type of fluxing.Basic fluxing occurs when oxides combine with oxygen to form anions, whereas acidic fluxing occurs when oxides decompose into cations and oxygen.Acidic fluxing is more severe than basic fluxing because it leads to a more intense oxidation reaction, especially when the oxidation activity in the molten salt is significantly reduced.On the contrary to basic fluxing, acidic fluxing has the ability to sustain itself, as the degree of salt displacement from stoichiometry does not intensify with the advancement of the reaction. 15

Various Stages in Hot Corrosion
The hot corrosion mechanism of boiler steels typically involves several stages leading up to component failure: Stage I (Incubation Period) In the beginning phase, the reaction follows a pattern comparable to typical oxidation reactions.
Stage II (Initiation Stage) At this point, corrosion is accelerated compared to the incubation stage.

Stage III (Propagation Stage) During this stage, corrosion
occurs at a very rapid rate.
After Stage III, the components ultimately fail. 16ermal Spray Coatings Thermal spray coatings encompass various techniques aimed at managing the detrimental effects of high temperatures.This approach involves applying an additional layer of materials that are resistant to degradation onto the surface of steel.This layer serves to impede the rate of corrosionoxidation, thereby slowing down or preventing material degradation, which can result in a prolonged operational lifespan for the component.The process of thermal spray coating entails the application of safeguarding substances, like ceramics, metals, or specific polymeric materials, onto the surface of the underlying substrate that necessitates protection.. [17][18] The spray gun receives the coating material and expels it onto the substrate surface at a high velocity after heating it.The material adheres to the substrate surface through a combination of adhesion and diffusion upon solidification.Flame spray, electric arc deposition, plasma spray coating, high velocity oxy-fuel coating, detonation gun deposition, and cold spray coating are among the thermal spray processes that exist.Each of these methods has its advantages and disadvantages.To select the appropriate thermal spray process, the bond strength, spray velocity, oxide formation, etc. factors among others are taken into consideration.The suitability of each available technique is determined by these factors.19

Coatings Tested under Aggressive Environments by Various Researchers
The comprehensive review of recent research papers focussing on the studies about the high temperature corrosion behaviour of different coatings in the reactive environments is presented in Table 1 as follows:  67 13-T22 and 8 wt-% of CNT environment at high temperature of 900°C.
The following outcomes were discussed in these studies: Reddy et al., 20 24 25 concluded that the use of nano coatings provided greater protection compared to micro coatings.This is due to the formation of oxides of boron resulting from the full melting of nano particles of boron carbide within the inner splats, in combination with oxides of chromium, nickel, aluminum, and a nickel/chromium spinel.Lone et al., 26 revealed that cerium concentration in the cerium oxide-coated Superni-75 samples increases when the weight variation per unit area decreases.
The conclusion drawn was that the presence of a cerium oxide layer on the surface impedes the movement of ions by reducing their short circuit diffusion.Bala et al. 27 and Mittal et al., 28 found that the uncoated steels experienced severe spallation in the form of oxide scales, likely due to the formation of unprotective Fe 2 O 3 oxide scales.In contrast, the Ni-50Cr coated steels exhibited reduced weight gains and maintained their oxide scales until the end of the experiment.The studies also revealed that the protective oxides present on the coated specimens mainly comprised of Chromium and Nickle, as well as their spinel.Nithin et al., 29 concluded that CoCrAlY + Al 2 O 3 + YSZ (C1) has effective resistance to corrosion than CoCrAlY + CeO 2 (C2) deposited on boiler steels, because of the formation of thermodynamically stable α-Al 2 O 3 in C1 and outward growth of superficial irregular cracks CeVO 4 in C2.Singh et al., 32 indicated that Ni-20Cr-TiC-Re coating offered the better corrosion resistance among different coatings of Ni-20Cr, Ni-20Cr-TiC.Jafari and Sadeghi 33 and Sadeghimeresht et al., 34 found that KCl can cause significant corrosion to coatings that contain chromium, due to frequent chlorination-oxidation processes that destroy the protective chromium layer and lead to the loss of Cr through the formation of chromate.Eklund et al., 35 concluded that NiAl coating exhibited excellent resistance to corrosion and effectively blocked the diffusion of all corrosive substances, including chlorine and oxygen, through the coating.Sundaresan et al., 36 concluded that detonation sprayed coatings performed better than plasma sprayed coating.In a study, Singh et al., 38 revealed that TiO 2 functions as a protective barrier against corroding agents by occupying any gaps or fractures in the coating's microstructure.Wang et al., 39 found that NiCrB coatings have greater corrosion resistance in comparison to NiCrTi coatings as a result of boron's inclination towards producing small oxide particles, ultimately augmenting the safeguarding of oxide layers.Abuwarada et al., 40 showed that the presence of a stable Al 2 O 3 oxide scale on the CoNiCrAlY coating reduced chloride penetration and prevented the degradation of the protective oxide scale through the formation of chromates.Bhatia et al., 41 indicated that Mo and Nb exhibited a proclivity for outward diffusion from the substrate to the coating.Zhou et al., 43 indicated that the incorporation of multiple alloying elements into the Cr 3 C 2 -NiCrMoNbAl coating led to a significant reduction in the corrosion rate compared to the Cr 3 C 2 -NiCr coating.Bala et al., 44 revealed higher microhardness of the Ni-50Cr coating than Ni-20Cr was identified as the reason behind better resistance to oxidation and erosion.Sadeghi and Joshi 45 found that the HVAF coating showed better high-temperature corrosion as compared to HVOF coating due to the formation of slightly denser protective oxides.A significant finding of the study was that the lower hardness of the HVOF coating, as compared to the HVAF coating, resulted in more substantial removal of the former during the erosion test, due to the impact of particles on the coating.Kaushal et al., 48 highlighted that the denser structure of the D-gun sprayed coating provided superior corrosion resistance in comparison to the cold spray and HVOF spray coatings.Kai et al. 49 showed that the nanostructured NiCrC coating facilitated an increased rate of grain boundary diffusion, leading to the development of a denser oxide scale with a higher growth rate.This helped to stop the depletion of chromium at the substrate/scale interface.Rani et al., 53 revealed that by utilizing the detonation gun coating technique, a uniform and strongly adherent coating with a dense microstructure was achieved.Thakare et al., 54 suggested that the creation of Cr 3 C 2 could be attributed to the enhancement of the steel substrate's corrosion resistance.Abu-warda et al., 56 found that the NiCr coatings tend to protect the substrate but undergo severe corrosion attacks by the chlorides.In addition to it, the formation of potassium chromate and penetration of chlorides as major causes of the attack.Matikainen et al., 57 discussed the benefits of using HVAF, such as the formation of more durable coatings with improved toughness and a more uniform coating structure.This is attributed to the higher particle velocities and temperature control achieved during the HVAF process.Chatha et al., 58 62 concluded that the substrate of Superni 600 and Superni 718 displayed the formation of a thick and dense oxide scale, whereas the corroded coated substrate of Superco 605 showed porous and loose oxide.Goyal et al., 63 noticed the lowest corrosion rate with value of 28.49 mpy was observed for Cr 3 C 2 -20NiCr -2 wt-%CNT coating on T22 steel.Sidhu et al., 64 listed the performance of coating in the order of 83WC-17Co coated T91 <83WC-17Co-coated T22 < 93(WC-Cr 3 C 2 )-7Ni coated T22 < 93(WC-Cr 3 C 2 )-7Ni-coated T91.In another study, Sidhu et al., 65 found that the presence of a thin layer of tungsten carbides, nickel oxides and chromium oxides is responsible for the coating performance on steel alloys made up of 93% WC-Cr 3 C 2 and 7% Ni.. Goyal et al., 66 67 found that even distribution of CNTs within the coating matrix and In addition to it, the protective nature of chromium oxides found on the surface scale exhibited superior corrosion resistance.

Conclusions
Researchers have reached the conclusion that various thermal spray coating processes can easily coat any material onto a substrate material, such as boiler steels.Among different thermal spray techniques.With its advanced and effective capabilities, the HVOF process is increasingly being used to produce high-quality coatings that exhibit high micro-hardness and low porosity.Surface modification techniques are crucial in various industries to avoid boiler failures.These techniques enhance the durability of machine parts, leading to longer lifespan and reduced replacement expenses.This results in high thickness coatings that are dense, strongly adhered to the material of substrate, and exhibit significantly greater micro-hardness as compared to the base steels.The valuable insights presented by this study can aid in selecting the optimal coating method and combination to prevent failure of boiler steels.

Table 1 : Tabular representation of various behavioural studies of coatings under aggressive environments S. Author Substrate Coating Coating Method Environment No.
23ated that Cr 2 O 3 has less solubility in highly acidic molten salt mixture of Na 2 SO 4 -60%V 2 O 5 .According to their study, the presence of chromium oxide formed around the Cobalt and Nickel-rich splats present in the pores could have acted as barriers against the penetration and diffusion of corrosive species, blocking their passages.Dolekar et al.,21demonstrated that, in comparison to the conventional YSZ TBC system, the YSZ/Gd 2 Zr 2 O 7 layer demonstrated better performance in terms of hot corrosion and oxidation.This was due to the pre-sacrificial layer of Gd 2 Zr 2 O 7 in YSZ/Gd 2 Zr 2 O 7 content, which lessen the damage to the YSZ layer.Singh et al.,22in their paper, defined that the subsurface inter-splats in the composite coating of Ni-22Cr-10Al-1Y-SiC (N) deposited on T22 by HVOF process exhibited favourable results against hot corrosion due to the presence of SiO 2 .In another study, Singh et al.,23arrived at the BHAGRIA et al., Mat.Sci.Res.India, Vol.20(Special Issue), pg.01-12 (2023) conclusion of the formation of silicon dioxide accompanied by oxides of Nickle, Aluminium, and Chromium which have proved to be more resistive to corrosion than the formation of B 2 O 3 with these oxides.Singh et al., 61vealed that the pores in the Cr 3 C 2 -NiCr coating became blocked due to the rapid formation of oxides at the boundaries of the coating splats and within the open pores during heat treatment.This prevented corrosive substances from flowing inward towards the substrate.Mudgal et al.,59concluded that the presence of Cr 2 O 3 on the surface may be the reason behind corrosion resistance against the aggressive surroundings on Cr 3 C 2 -(NiCr) coating and the splat boundaries.During corrosion, the inclusion of Zr in the coating powder led to a decrease in the oxidation rate and enhanced the ability of the oxide scale to adhere to the surface of the coating.Mudgal et al.,60conducted concluded that Cr 3 C 2 -25(NiCr)+0.4%CeO 2 coating consists CeS on the surface scale and CeO 2 along the splat boundaries other than Cr 2 O 3 and spinels, The presence of CeO 2 at the splat boundaries restricted the flow of reacting species which decreses the corrosion rate.Mudgal et al.,61pointed out that the Co-based alloy experienced a greater increase in weight compared to the Ni-based alloy because the scale on the Co-based alloy was not protective in nature.Ahuja et al.,