Research Progress in the Corrosion Mechanisms and Anticorrosion Technologies of Waste-to-Energy Plant Boilers

Abstract

High-temperature corrosion within waste incineration boilers leads to the thinning of their four-tube heating surfaces and frequent tube ruptures, posing a formidable challenge to the development of the waste-to-energy sector. This predicament critically constrains the advancement of China’s waste management and environmental protection sectors. This study focuses on elucidating high-temperature corrosion mechanisms and exploring coating protection methodologies relevant to waste boilers. For corrosion mechanisms, the study comprehensively reviews various factors such as the characteristics of high-temperature chlorine-induced corrosion, gaseous- and molten-chloride-induced corrosion, and sulfidation and multiphase-coupled corrosion; the influence of wall temperature on corrosion; and temperature effects on corrosion. Regarding coating protection technologies, this study traces the historical progression of various coating techniques, providing an overview of methods such as supersonic flame spraying, Inconel 625 surfacing, laser cladding, induction melting, thermosetting-reaction nanoceramic coating, and aluminizing. Special emphasis is placed on the mechanisms and principles of the widely adopted surfacing and induction melting techniques. Overall, the study ventures into the prevailing challenges and envisions the future trajectories of high-temperature anticorrosion mechanisms and coating protection technologies for China’s waste boiler sector.

1. Introduction

The introduction of municipal solid waste incineration technology in China dates back to the 1980s, with Shenzhen establishing the country’s first waste incineration plant in 1988. By 1992, Zhuhai had already begun constructing three waste incineration plants, each with a daily processing capacity of 2000 t, marking the onset of rapid progress in waste-to-energy (WtE) techniques within the country. By 2016, China boasted 249 WtE plants, collectively handling approximately 255.85 Mt/day. The installed capacity of these plants surged from 5.49 GWh in 2016 to 15.33 GWh in 2020, with an average annual compound growth rate of 29.27%. Projections anticipated this installed capacity to reach 17.29 GWh by 2021 [1].

Given the presence of chlorine, sulfur, and alkali metals in municipal solid waste, severe corrosion of the metal surfaces of the four tubes within waste incineration boilers—including water walls, superheaters, reheaters, and economizers—is inevitable during incineration [1]. These four tubes serve as critical heat-exchange components within the boiler system. Figure 1 depicts the distribution of these four tubes within a WtE boiler [2]. The water wall is the main heating part of the boiler, consisting of several rows of steel pipes distributed around the boiler furnace. Its interior consists of flowing water or steam, while the outside receives heat from the flames of the boiler furnace. Mainly absorbing the radiation heat from high-temperature combustion products in the furnace, the working fluid undergoes upward movement and evaporates upon heating. Superheater is a component in a boiler that further heats steam from saturation temperature to superheating temperature, also known as a steam superheater. A reheater is essentially a steam superheater that heats low-pressure steam that has undergone work and reaches a certain temperature. Economizer is a device installed at the lower part of the tail flue of a boiler to recover the waste heat from the exhaust gas. During the process of incineration power generation, the chlorine content in the waste has a direct impact on the emissions of hydrogen chloride and dioxins generated during the incineration process. The experimental results indicate that the concentrations of hydrogen chloride and dioxins in the flue gas increase with the increase in the proportion of waste in the fuel. Typically, when the protective coatings on these tubes are compromised by corrosion, leading to significant thinning of the tube walls, high-pressure steam can escape from the damaged regions, resulting in high-pressure leaks or even tube ruptures (as depicted in Figure 2). Not only will it cause serious safety hazards, but it will also have a significant impact on the electricity consumption rate of waste-to-energy generation. The electricity consumption rate of waste incineration is an important indicator for measuring the energy conversion efficiency of waste. It refers to the ratio of the amount of electricity consumed by waste incineration power plants during the process of processing waste and producing electricity to the amount of electricity generated. The calculation electricity consumption rate (%) = incineration plant electricity consumption (kWh)/power generation (kWh) × 100%.

High chlorine contents in waste are primarily responsible for the more aggravated corrosion of waste boilers compared to that of coal-fired boilers. According to research conducted by a team at Columbia University, USA, the chlorine content in coal is merely 0.1% (wt), whereas in waste, it is approximately 0.5% (wt). During waste incineration, the concentration of HCl gas within high-temperature flue gas can rise to 600 ppm [3]. Waste boilers, typically utilizing municipal solid waste as fuel, are characterized by the presence of high moisture contents; complex components; low calorific values; and various alkali metal chlorides, sulfides, and heavy metals. Furthermore, unlike coal combustion, waste combustion cannot achieve homogeneous burning, resulting in significant temperature fluctuations. These fluctuations cause frequent thermal expansion and contraction of tube walls, rendering them highly susceptible to thermal fatigue cracking and its propagation following corrosion-induced thinning [4]. The significantly higher content of volatile parts in fuels of the waste type in relation to coal, especially highly carbonized coal or anthracite, will strongly affect the temperature fluctuations mentioned in this paragraph, which are directly related to temperature fluctuations and, as mentioned, will have a huge impact on the thermal load of the boiler piping systems [4]. Consequently, effective high-temperature corrosion protection measures for waste boilers remain a bottleneck, severely impeding the development of the WtE sector and, thus, restricting the rapid advancement of China’s waste treatment and environmental protection industry [5]. Thus, elucidating the characteristics and mechanisms of chlorine-induced corrosion and developing effective anticorrosion strategies to maximize boiler tube lifespans represent pressing issues for scientists working in China’s WtE sector. Over the years, these scientists have dedicated strenuous efforts and made significant achievements. This paper offers an overview of the status of current research and progress on the mechanisms of high-temperature corrosion and coating protection technologies in WtE boilers, serving as a reference for scientists in the WtE technology industry.

2. Corrosion Mechanism of Waste Power Plant Boiler

2.1. Activated Oxidation Theory

Chlorine-induced corrosion within waste boilers is characterized by the formation of loose and porous oxide films on the outer surfaces of boiler tubes, exhibiting poor adhesion. This phenomenon occurs when oxide films formed in the presence of gaseous chlorides become more porous and less adhesive compared to films formed in chlorine-free environments, leading to compromised protective efficacy. During high-temperature corrosion, the chlorine concentration at the corrosion zone interface significantly exceeds that at the surface, creating a gradient of chlorine within the oxide film—featuring the highest concentration at the oxide film/metal interface and the lowest concentration at the outer oxide film. During the initial stages of chlorine-induced corrosion, the corrosion rate escalates rapidly but subsequently decelerates abruptly in later stages, a trend commonly evident in chloride-salt-film corrosion experiments and waste incineration boiler inspections. Figure 3 illustrates the well-established process of chloride-induced corrosion on the heated surfaces of waste boiler tubes [6].

Chloride gases are predominantly responsible for corroding several critical heated surfaces of waste incineration systems. Among these, both Cl₂ and HCl gases can lead to metal loss through the activated oxidation mechanism. This mechanism involves the initial oxidation of HCl to chlorine gas, which subsequently penetrates the oxide film through surface pores or cracks. At the metal/oxide film interface, chlorine reacts to form metal chlorides such as FeCl₂. The FeCl₂ species, possessing a high vapor pressure at 500 °C, volatilizes and diffuses outward at elevated temperatures. Subsequently, in regions with higher oxygen levels, it undergoes re-oxidation, regenerating both oxide and chlorine gases. Throughout this cycle, chlorine acts as a catalyst, persisting in an unconsumed state and undergoing continuous recycling, thus perpetuating metal corrosion [6].

2.2. Alkali-Metal-Chloride-Induced Corrosion

In addition to gaseous chlorides, solid-state salt chlorides, particularly those of alkali and heavy metals with inherently low melting points, also present significant corrosive threats. These salts typically amalgamate to form eutectic salt mixtures with even lower melting points, creating liquid phases on metal surfaces at elevated temperatures. These liquid phases aggressively interact with the metal-oxide layer, leading to profound corrosion and eventual tube wall failure [7]. Figure 4 illustrates the thermal corrosion process induced by molten KCl salt. During the initial stages of this thermal corrosion, a KCl salt film forms on the substrate surface, while the substrate material oxidizes to form an oxide film. During the middle stages of thermal corrosion, if the environmental temperature exceeds the melting point of the KCl salt film, Cl⁻ ions from the molten KCl infiltrate inward through grain boundaries and pores within the oxide film, initiating the electrochemical corrosion of the substrate material and forming metal chlorides. Conversely, if the environmental temperature remains below the melting point of the KCl salt film, KCl gradually reacts with oxides to form spinel phases and Cl₂. Some of this Cl₂ diffuses inward through grain boundaries and pores within the oxide film to undergo chlorine-activated oxidation, thus forming metal chlorides. Simultaneously, the KCl salt film and formed spinel phases create a low-melting-point eutectic melt, wherein the diffusion rates of Cl⁻ surpass those of Cl₂, thereby accelerating the low-temperature thermal corrosion process through electrochemical corrosion reactions with the substrate material. Ultimately, in the later stages of thermal corrosion, metal chlorides diffuse outward and react with O₂ to form non-protective oxide particles, releasing Cl₂ in the process. This Cl₂ then reacts again with metals and alloy elements to form metal chlorides, perpetuating continuous chlorination-oxidation processes within the substrate material and leading to the continuous consumption and eventual failure of metal components [8].

2.3. Sulfide-Induced Corrosion

In addition to chlorides, the sulfide contents of waste also contribute significantly to boiler corrosion. In oxidizing atmospheres, excess concentrations of SO₂ can convert sulfates into pyrosulfates, lowering the melting point of mixed salts and initiating molten-salt corrosion. Conversely, in reducing atmospheres, H₂S can directly react with metal-oxide films and the metal substrates of tube walls, leading to material degradation. Furthermore, molten Na₂SO₄ at elevated temperatures can compromise the structural integrity of metal-oxide films. Notably, the synergistic effect of sulfur and chlorine depends on ambient atmospheric conditions. These species may inhibit each other under oxidizing conditions, whereas their corrosive effects may be exacerbated under reducing conditions. Figure 5 illustrates a schematic depicting the molten-sulfate-induced thermal corrosion process.

As illustrated in Figure 5, in the initial stages of this thermal corrosion process, sulfates, formed during combustion, adhere to the metal substrate surface, while a protective oxide film concurrently develops on the substrate (Figure 5a). With thermal corrosion progress, these sulfates melt into a salt film covering the substrate surface, thus isolating the substrate from gaseous interactions (Figure 5b). The molten-salt film consistently penetrates and dissolves the protective oxide film through active sites such as cracks, pores, and grain-boundary defects, generating metal sulfides or metal sulfates (Figure 5c). In the later stages of thermal corrosion, the molten salt reacts with both the substrate material and oxide film, initiating an electrochemically induced uneven corrosion process and expediting material degradation (Figure 5d).

However, not all chlorides and sulfates can corrode metals. Alkali metals, being highly reactive, form salts capable of reacting with metal-oxide films, generating corrosive media such as Cl₂ or SO₂. These species also selectively react with certain alloying elements within tube wall components, altering the composition of the oxide film, deteriorating protective properties, and increasing metal susceptibility to chloride and sulfur attacks [9]. Empirical studies have demonstrated that both excessively high and low chlorine/sulfur ratios can cause severe corrosion. While the influence of alkali metal ions on corrosion may be limited, molten-salt corrosion induced by low-melting-point chlorides and sulfates significantly impacts the overall corrosion process [9,10].

It should be pointed out that among the three types of corrosion mechanisms mentioned above, namely “activation oxidation” mechanism, alkali metal chloride corrosion, and sulfide corrosion, the “activation oxidation” mechanism is the most representative, followed by sulfide corrosion, and the influence of alkali metal chloride on corrosion is relatively small. In fact, a Cl/S ratio that is too high or too low is most likely to cause severe corrosion.

2.4. Influence of Wall Temperature on Corrosion Rate

In the context of WtE incineration, understanding corrosion patterns within waste boiler tubes is essential for designing corrosion-resistant boiler systems and developing surface protection technologies for boilers. The curve depicted in Figure 6, widely recognized as a classic corrosion curve across the industry, illustrates the variation in the corrosion rates of waste furnace tubes with wall temperature [11].

As illustrated in Figure 6, high-temperature corrosion initiates when wall temperatures exceed 320 °C. Within the 320–480 °C range, a mild corrosion zone is formed, where compounds such as FeCl₃ and basic sulfates begin to form gradually. Notably, both the molten and solid states of basic chlorates and sulfates exhibit increased corrosiveness owing to their inherently low melting points. Upon mixing, these species form eutectic salts with even lower melting points [12]. Thus, chlorates and sulfates covering the outer tube wall form a low-melting-point liquid eutectic salt with oxides produced from tube material oxidation. This configuration constitutes a four-phase three-interface system, comprising the tube wall, oxide film, molten-salt layer, and chlorine- and sulfur-containing flue gas. Collectively, these elements facilitate the thermal corrosion of the tube wall metal through electrochemical processes [13].

Notably, the most active high-temperature corrosion reactions occur within the 480–700 °C temperature range, primarily owing to the decomposition and melting of FeCl₃ and basic sulfates within this temperature interval. According to empirical studies [14], a chlorine concentration of ≥0.35% significantly accelerates the corrosion rate. Meanwhile, chlorine concentrations of 0.8% compromise the integrity of the metal surface oxide layer, and those beyond 2.0% disrupt the continuity of the oxide layer. Overall, within this temperature range, corrosion rates escalate rapidly with increasing wall temperatures, primarily influenced by the volatilization of chlorides. Below 500 °C, the corrosion rate demonstrates a parabolic relationship with time, owing to the formation of an iron oxide protective layer on the metal surface. Above 500 °C, severe corrosion initiates as HCl and Cl₂ chemically interact with metal oxides, thus weakening the metal-oxide layer. Ultimately, under reducing conditions, the corrosion rate on the outer walls of the tubes increases almost linearly with time, owing to the disappearance of the protective oxide film [15].

Molten-salt corrosion is predominantly related to the morphology of fly ash and its degree of adhesion to tube walls. During the process of waste incineration, the chemical composition of fly ash directly affects its deposition and scaling degree on the boiler tube wall. Specifically, the chemical composition of fly ash, such as alkali metals and sulfates, is closely related to the degree of scaling on boiler tube walls. These components are prone to melting at high temperatures, forming a molten liquid layer that then adsorbs onto the boiler tube wall, resulting in ash accumulation. As time goes by, the phenomenon of ash accumulation gradually worsens, forming coking, which seriously affects the heat transfer efficiency of boiler tubes. The corrosion rate curve within the wall-temperature range of 320–700 °C can be segmented into four distinct regions [16]. In the fly ash softening zone (300–480 °C), a gas-solid biphasic region is established. When the flue gas temperature surpasses the softening temperature of certain low-melting-point compounds within fly ash, the tube wall surface can accumulate substantial amounts of cohesive ash, which, however, lacks sufficient adhesion to firmly attach to the tube wall [16]. Consequently, wall corrosion in this region predominantly involves gas-solid two-phase flow erosion, with a relatively low corrosion rate. Meanwhile, in the fly ash melting zone (480–600 °C), a gas-solid-liquid triphasic region emerges. Here, the corrosion rate escalates dramatically, owing to the formation of a localized liquid phase at the ash-metal interface under high-temperature conditions. This scenario creates an electrochemical corrosion environment, leading to anodic dissolution of the substrate metal. Furthermore, the ash layer may exert stress impacts on the tube wall. During the corrosion stabilization period (600–700 °C), the outer wall of the tube and the inner layer of enveloping ash both undergo eutectic reactions. As reaction products accumulate, the corrosion rate gradually stabilizes and stops increasing. Finally, in the corrosion deceleration period (700–720 °C), corrosion reactions reach completion, and the corrosion rate rapidly declines, entering a low-reactivity stable phase upon reaching equilibrium [17].

2.5. Influence of Flue Gas Temperature on Corrosion Rate

Academic perspectives regarding the influence of flue gas temperature on corrosion rates within waste incinerators are diverse. In particular, some researchers posit that elevated flue gas temperatures significantly aggravate tube wall corrosion, attributing this to the direct impact of flue gas temperature on increasing wall temperatures. However, empirical data on boiler tube wall temperatures indicate a consistent correlation between corrosion rates and the temperature and pressure of the working fluid, regardless of fluctuations in the flue gas temperature. Meanwhile, others propose that during waste incineration, the flue gas temperature primarily influences the corrosion process by affecting the formation sequence and stability of corrosive media. Alternatively, another perspective suggests that the flue gas temperature influences corrosion rates by affecting ash deposition on tube walls. Specifically, when flue gas temperatures exceed the melting point of fly ash, alkali metal compounds undergo melting, forming liquid molten salts that adhere to the tube wall. This liquid-phase corrosion exacerbates the overall corrosion rate, representing the primary mechanism through which the flue gas temperature influences corrosion rates [18]. The functional relationship between chlorine-induced corrosion, flue gas temperature and wall temperature is discussed in [19]. Three regions delineating the susceptibility of tube panels to chlorine-induced corrosion are distinguished: a zone of minimal corrosion risk, wherein tubes are virtually free from corrosion; a corrosion zone, wherein tubes are highly susceptible to corrosion; and an intermediate transition area, where tube corrosion is likely.

Generally, as temperatures within waste incineration boilers increase, metal molecules within tube wall materials absorb more energy from the environment, resulting in a sharp increase in the number of activated molecules and a significant intensification of corrosion. At such elevated temperatures, the corrosion rate is predominantly dictated by the diffusion rate of corrosive agents within the composite corrosion film or corrosion products. As flue gas temperatures continue to increase, crystal growth within the polycrystalline composite corrosion film is promoted [20]. Notably, the oxides and sulfides within multi-metal composite corrosion films possess grain boundaries, and gaps at these boundaries expand with increasing temperature. These expanding gaps serve as conduits for the inward diffusion of S, O₂, and H₂S [21]. As sulfidation and oxidation continue, the interfaces at the alloy grain boundaries consistently recede, further enlarging the diffusion pathways [22]. When temperatures exceed 550 °C, new phases, such as iron carbides, precipitate between the grains, leading to Fe and Cr depletion along the grain-boundary regions. This depletion sets up a corrosion cell between the impoverished boundary zone and the grain body, resulting in intergranular corrosion and expediting the corrosion process [23].

3. Anticorrosion Technology for Waste Power Plant Boilers

3.1. Substrate Materials for Tubes

Currently, tubes within WtE incineration boilers primarily use traditional metallic substrate materials, mostly low-alloy steels, with a few employing high-temperature alloying materials such as stainless steel and Ni- and Co-based alloys [24]. Notably, materials such as 20G, 12Cr1MoVG, 15CrMoG, TP347H, and TP310S are preferred for high-parameter waste boiler water walls and superheaters.

Table 1. Performance of five commonly used tubing materials in waste incinerators.

Tubing Materials	Performance Characteristics	Applicable Locations	Thermal Conductivity (W/m k)	Temperature Resistance (K)	Element Content (%)
20G	Exhibits higher levels of temperature resistance compared to ordinary carbon steel tubes, with superior plasticity and toughness.	Water walls	45–50	<450	C: 0.17–0.23 Si: 0.17–0.37 Mn: 0.35–0.65
15CrMoG	Demonstrates commendable overall performance below 500 °C; however, mechanical properties gradually diminish with increasing temperature.	Water walls or medium- and low-temperature superheaters	150–165	<500	C: 0.12–0.18 Si: 0.17–0.37 Mn: 0.40–0.70
12Cr1MoVG	Demonstrates better high-temperature performance than 15CrMoG, particularly in terms of its oxidation resistance.	High- and medium-temperature superheaters	150–160	<550	C: 0.08~0.15 Si: 0.17~0.37 Mn: 0.40~0.70
TP310S	Exhibits excellent oxidation resistance, corrosion resistance, acid and alkali resistance, and high-temperature performance, but undergoes softening at temperatures above 800 °C.	Medium- and high-temperature superheaters	14–19	<700	Ni: 19.00–22.00 Cr: 24.00–26.00 S ≤ 1.50 Mn ≤ 2.00
TP347H	Demonstrates superior high-temperature performance compared to TP310S, with strong resistance to high-temperature acid and intergranular corrosion.	High-temperature superheaters	14–19	<750	C ≤ 0.08 Si ≤ 1.00 Mn ≤ 2.00

summarizes the performance and cost-effectiveness of these five tubing materials. Typically, the thermal conductivity of materials varies with temperature. The thermal conductivities listed in Table 1 denote the corresponding average values across the operational temperature range. The term “temperature resistance” represents the maximum temperature that the material can withstand under long-term service conditions [25].

From the perspective of anticorrosion designs, the water walls of waste heat boilers are often prioritized as they are utilized more frequently compared to superheaters and thus determine the primary costs of boiler tubing. Hence, low-cost substrate materials meeting service performance requirements are often selected for the water walls of waste heat boilers. Among the potential candidates, 20G stands out for its high strength at ambient and medium–high temperatures, good weldability, suitability for long-term service at wall temperatures below 450 °C, and cost-effectiveness. Notably, to mitigate corrosion at high flue gas temperatures, protective coatings are often applied to heating surfaces, making 20G a popular substrate material for water walls domestically [26]. For medium- and low-temperature superheaters with wall temperatures below 500 °C, characterized by relatively mild corrosion, either 20G or 15CrMOG can be selected as the substrate materials. Conversely, for high-temperature superheaters with wall temperatures exceeding 500 °C, experiencing faster corrosion rates, materials with better high-temperature corrosion resistance, such as 15CrMOG or 12Cr1MoVG, are typically utilized. Although these low-alloy steels, 15CrMOG or 12Cr1MoVG, exhibit similar properties, 12Cr1MoVG performs better at temperatures exceeding 500 °C. Furthermore, 310S and TP347H stainless steels are viable options for superheater materials. While both specimens demonstrate commendable high-temperature anticorrosive properties, 310S undergoes softening as flue temperatures approach 800 °C, leading to a corresponding decline in its wear and corrosion resistance. Comparatively, TP347H demonstrates better high-temperature performance; however, its service temperature must not exceed 800 °C [27].

3.2. Protective Coating Technology

While developing and utilizing new alloy materials can help boost the corrosion and wear resistance of boilers in high-temperature environments, their high costs and limited overall performance restrict their applications to specific areas. Alternatively, coating protection technology, which involves the formation of high-temperature protective coatings on boiler tube heating surfaces, offers a viable solution. This technique not only considerably reduces the likelihood of safety incidents caused by tube ruptures and leaks but also enhances power generation efficiency by retarding ash accumulation on heating surfaces, thereby reducing the carbon emissions of power generation boilers [28].

Protective coatings must predominantly satisfy the following four design requirements: (1) The intrinsic structure of the coating must be dense, with minimal internal pores, cracks, and other defects. (2) The thermal expansion coefficients of the coating and substrate must closely match to minimize thermal stress-induced delamination. (3) A high bond strength must form between the coating and substrate. (4) To achieve the high-bond-strength and low-porosity requirements, the coating preparation process must allow mutual diffusion between the coating and substrate. These specifications can be met by adopting design strategies such as creating a melt pool facilitated by electromagnetic stirring effects, similar to that in surfacing, or by subjecting the substrate and coating to pre/post-treatments, such as low-temperature brazing [29].

3.2.1. Coating Materials

The various alloy materials used in anticorrosion coatings differ in their strengthening and modification effects on the coatings, with potential mechanisms including solid-solution, second-phase, and grain-boundary strengthening [30]. Among these, commonly used high-temperature corrosion-resistant alloy coatings are predominantly Ni-based, comprising specific ratios of Ni, Cr, Mo, Fe, B, Si, and other alloying elements, with Ni and Cr being the most crucial. Ni, the major component of Ni-based alloys, is preferred for high-temperature anticorrosion coatings owing to its remarkable wettability with other alloys, good corrosion resistance, and a comparable thermal expansion coefficient to the substrate. The proportion of each element in the coating material is roughly as follows: Ni60%–70%, Cr17%–18%, Mo11%–13%, Cu1.7%–2%, B1.7%–2%, Si2.5%–3%, and Fe3%–5%. The strengthening mechanisms of Ni-based alloys include austenitic solid-solution strengthening through Fe, Cr, and Mo addition; lattice strengthening through Al and Ti addition; and grain-boundary strengthening through Co, B, and Zr addition. Furthermore, the added Cr can significantly influence coating performance by enhancing its strength and toughness. In summary, when designing protective coatings for WtE power station boiler tubes, different alloying elements must be selected and combined according to the aforementioned three strengthening principles to completely leverage the high-temperature anticorrosion performance of the coatings [31].

3.2.2. Coating Preparation Methods

Since the late 20th century, China has been exploring WtE technologies, learning from several developed European countries and the United States. The consequent evolution of boiler anticorrosion technology, from thermal spraying to surfacing, low-temperature surfacing, laser cladding, thermosetting-reaction nanoceramic coating, and aluminizing, has significantly advanced WtE technology. These methods have also evolved in accordance with the changes in the calorific values of waste and boiler parameters at different historical development stages [32].

(1)

Thermal spraying technology

In the mid-to-late 1990s, when the WtE technology in China was still in its nascent stages, anticorrosion measures predominantly adopted thermal spraying techniques, including flame, arc, and plasma spraying [33]. These thermal spraying techniques primarily differ in terms of their flame temperature and particle flight velocity, which directly influence the degree of powder melting, as well as the coating’s bonding strength and porosity. Thermal spraying technologies commonly employed in the field of boiler protection include arc, plasma, flame, high-velocity oxygen fuel, and cold spraying.

Table 2. Performance comparison of thermal spraying technologies commonly employed for corrosion protection in waste boilers [34].

Thermal Spray Technology (THSP)	Spraying Temperature/K	Spraying Velocity/m s−1	Porosity/%	Bond Strength/MPa	Coating Lifespan/Years
Arc spraying (AS)	>6000	50–100	10–20	25–30	1–3
Plasma spraying (PS)	10,000–15,000	300–500	5–10	20–50	2–3
Flame spraying (FST)	3000	100–200	10–15	20–30	1–2
High-velocity oxygen fuel (HVOF) spraying	2500–5500	1000–1200	<2	50–70	>5
Cold spraying	0–700	300–1200	<5	>50	>3

compares the performance of these technologies.

After 2000, however, with improvements in the calorific values of waste and boiler parameters, these thermal spraying technologies began demonstrating weaknesses, such as the formation of high-porosity and low-bond-strength coatings, leading to the reduced service lives of tubes. Consequently, they were gradually replaced by subsequently developed technologies. Among these, the high-velocity oxygen fuel (HVOF) technology stands out in terms of its performance, as indicated in Table 2 [34].

(2)

High-velocity oxygen fuel technology

Since its development and maturation following the year 2000, HVOF spraying stands out among other thermal spraying methodologies owing to its high jet velocity, facilitating the formation of coatings with high densities, low oxidation levels, and higher bond strengths. As delineated in Table 2, the service lives of medium-temperature and medium-pressure waste incineration boilers can extend up to 5 years, a notable enhancement compared to the 1–3 years typically offered by other thermal spraying methods. Consequently, HVOF spraying continues to find applications across both domestic and international markets. However, its widespread adoption is significantly constrained by factors such as low powder deposition rates, high process costs, substantial equipment investment, frequent wear part replacement, high gas consumption, and stringent powder quality requirements, particularly in cost-sensitive domestic markets [35,36].

(3)

Inconel 625 surfacing welding technology

After 2002, the United States and several European nations began extensively adopting Inconel 625 surfacing welding technology to protect power station boiler water walls against corrosion. By approximately 2005, this technology was also introduced in China, marking a significant advancement in its high-temperature corrosion protection strategies for boilers [37].

Inconel 625, a Ni-based superalloy subjected to solid-solution strengthening using Mo and Nb, is renowned for its exceptional corrosion and oxidation resistance, as well as its remarkable tensile and fatigue strength at elevated temperatures. However, the application of Inconel 625 alloy surfacing poses challenges, particularly owing to high dilution rates that adversely affect the corrosion resistance of alloys under high-temperature conditions. To reduce these dilution rates, the surfacing thickness must exceed 2.5 mm, which, in turn, results in low efficiency and high costs. Furthermore, the construction process places stringent requirements on welding equipment and techniques, demanding meticulous control of the heat input to prevent burn-through or deformation of the tubing material [38].

The dilution rate plays a critical role in determining the corrosion resistance of materials subjected to surfacing processes. Notably, achieving metallurgical bonding during Inconel 625 alloy surfacing welding requires a fusion depth that inherently results in a high dilution rate. Furthermore, excessive transfer of Fe species from the substrate to the surfacing layer can compromise its corrosion resistance. To reduce the dilution rate, welding techniques with low heat input are essential. Cold metal transfer surfacing, for instance, offers a lower heat input compared to traditional methods and is suitable for surfacing boiler water walls. Furthermore, the suboptimal flowability of molten Inconel 625 alloys, which hampers their spread and wetting capabilities, does not improve with increasing heat input from the surfacing power source; instead, it adversely impacts the performance of the surfacing layer and leads to high dilution rates [39].

At temperatures exceeding 420 °C, the corrosion resistance of Inconel 625 alloy surfacing welding layers notably decreases, with the corrosion rate nearly doubling for every 50 °C increase in temperature [40]. Below 550 °C, the corrosion rate is predominantly dictated by the chemical reaction rate associated with composite corrosion film formation. Above 550 °C, the chemical reaction rate increases, and diffusion processes accelerate, owing to Cr and Fe deposition at the grain boundaries on the surface of the surfacing layer, along with residual carbon elements, forming compounds such as Cr₂C₃ and Fe₃C. Notably, the depletion of Cr and Fe species from these grain boundaries widens interstitial spaces, weakens grain-boundary strength, and creates Cr- and Fe-depleted zones that are prone to preferential corrosion owing to insufficient corrosion resistance, ultimately leading to intergranular corrosion. This electrochemical self-corrosion effect, combined with the dual corrosion effects of external corrosive agents, accelerates the corrosion rate from 0.05–0.1 μm/h to 0.15–0.2 μm/h, predominantly driven by the diffusion of various corrosive elements within the corrosion film [41].

(4)

Induction welding technology

In induction welding, also referred to as remelting, coatings are subjected to heating after spraying to induce remelting, thereby enhancing their properties. Common remelting techniques include flame, laser, plasma, electron beam, and furnace remelting. Among these, the laser, electron beam, and plasma remelting technologies offer high-energy densities but are often restricted in their application owing to high equipment costs and complex operation and maintenance requirements. Meanwhile, flame remelting potentially leads to significant thermal distortion and deformation of the workpiece, and furnace remelting is unsuitable for large components such as water walls. Conversely, high-frequency induction remelting offers several advantages, including rapid heating speeds, high-power densities, precise control over heating parameters (time, temperature, and depth of heating), and a clean working environment. These attributes make this technique particularly suitable for remelting coatings formed on large components such as boiler water walls [42].

In 2005, the Nagasaki Shipyard and Machinery Works facility of Mitsubishi Heavy Industries collaborated with Dai-ichi High Frequency Co., Ltd., to pioneer the development and implementation of induction welding technology for the high-temperature corrosion protection of water walls within WtE power station boilers (as illustrated in Figure 7), marking its global debut. In 2011, Taiwan’s Asakusa Waste-to-Energy Company became the first organization outside Japan to adopt this technology, rapidly expanding its use across the island’s WtE plants [43]. Since 2017, domestic advancements in induction welding technology have succeeded in preparing coatings suitable for heat-exposed surfaces such as water walls and superheaters. Consequently, this technology has proven to be highly effective across various high-temperature corrosion environments of waste boilers, not just matching the performance and service life specifications of surfacing technology, but also significantly surpassing it in terms of preparation efficiency and cost. Owing to these attributes, induction welding technology has garnered widespread industry recognition, with promising market prospects [44].

High-frequency induction coils are generally employed in induction welding to supply energy to the overlay layer, precisely melting the pre-deposited coating layer. Notably, the diffusion mechanism of Ni-based self-fluxing alloy coatings essentially involves mutual diffusion between Ni species in the coating and Fe species in the substrate. While the substrate itself does not melt, the remelted alloy powder wets the substrate surface, indicating that the surface temperature of the substrate exceeds the melting point of the Ni-based self-fluxing alloy powder, which is approximately 1050 °C. At this temperature, Fe and Ni species, being homologous elements with identical electronic structures, readily undergo mutual diffusion. Furthermore, these Fe and Ni species completely dissolve in both solid and liquid states, forming a solid-solution diffusion layer with a certain thickness, thus facilitating the crystallization of single-phase solid solutions. Additionally, the composition transition curve at the interface does not manifest as a steep vertical line but spans a few micrometers, indicating the formation of a narrow blending region at the interface between the overlay layer and substrate. This creates a diffusion transfer layer between the coating and substrate, thus forming a micro-metallurgical bond between the two [44].

(5)

Laser cladding technology

Since 2010, the technique of applying a coating to the surface of a substrate followed by laser cladding has garnered significant research attention, although interest in this method has diminished in recent years. Notably, laser cladding technology offers numerous advantages over traditional cladding techniques, such as a minimal thermal impact on the base material and a lower dilution rate. Furthermore, the high cooling rate associated with laser cladding often results in fine and uniform grain structures within the cladding layer, promoting the thermal corrosion resistance of the cladding layer, as depicted in Figure 8, which illustrates the laser cladding of a superheater. However, laser cladding also presents shortcomings, including the requirement of large equipment, stringent construction precision requirements, low production efficiency per equipment unit, high investment costs, and difficulties in deformation control. Consequently, the practical applications of this technique remain primarily restricted to research purposes, with limited adoption in mass production [45].

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Thermally cured nanoceramic coating technology

Since 2012, high-temperature-resistant nanoceramic coatings have advanced rapidly. These nanoceramic coatings predominantly comprise inorganic binders, various functional fillers, additives, and deionized water. During the application process of these coatings, a coating layer is initially cold sprayed onto the substrate surface using a spray gun. Subsequently, during the boiler start-up and temperature ramp-up phase, high temperatures resulting from waste incineration induce an in situ thermochemical curing reaction within the coating, creating a smooth and dense ceramic layer. Nanoceramic coatings are renowned for their excellent wear and high-temperature corrosion resistance. Furthermore, these coatings enhance the tidiness of the heat-exposed surfaces of boilers and reduce their surface energy, thereby imparting good antifouling and antislagging properties [46]. However, the limited thicknesses of these coatings (0.1–0.2 mm) lead to patchy peeling after approximately two years of use. This delamination necessitates boiler shutdowns for sandblasting and original coating removal before reapplication, thus increasing operational costs. This limitation has predominantly constrained the widespread adoption of this method to date.

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Aluminizing coating technology

Aluminizing coating technology, a primitive technique with long-standing applications in various corrosion protection fields such as ships and bridges, initially encountered skepticism owing to the high-temperature corrosion resistance of Al coatings. Only after 2013 did researchers begin exploring its potential applications in boiler corrosion protection. Surprisingly, the research outcomes revealed that aluminized coatings effectively mitigate high-temperature corrosion in boilers. Furthermore, this method is not only straightforward and cost-effective but also delivers exceptional cost performance [47]. Traditional methods for aluminized coating preparation include hot-dip, slurry, thermal spraying, and powder pack cementation. Recent advancements have tailored aluminizing processes specifically for thermal power plant boiler tubes. These innovations include leveraging external electric fields to accelerate aluminization and facilitating the batch aluminization of irregular elongated tubes [48]. Despite these strides, the application of this technique in waste boiler corrosion protection requires further sophistication, given its relatively recent exploration [49].

4. Challenges and Prospects for Future Research

4.1. Corrosion Mechanisms

Recent research on corrosion mechanisms within waste boilers has predominantly focused on chlorine-induced corrosion and the inhibitory effects of sulfur on chlorine-induced corrosion. Although the corrosive effects of sulfur and alkali metal salts on biomass incinerators have been extensively investigated, similar research in the field of waste incineration remains inadequate. Furthermore, an increasing number of researchers have recognized the complexity of the actual flue gas environment within waste incinerators, where corrosion often results from the synergistic action of three primary corrosive agents: Cl, S, and alkali metals. Thus, a comprehensive understanding of corrosion mechanisms cannot be achieved by independently focusing on a single type of corrosion. Consequently, to fundamentally mitigate corrosion effects within waste incineration systems, future research efforts must focus on addressing integrated corrosion problems involving Cl, S, and alkali metals [50].

To date, the majority of corrosion research has been conducted in laboratory settings, as field experiments are hampered by the difficulty encountered in precisely controlling various parameters owing to the presence of numerous influencing factors. Traditional corrosion studies typically adopt gravimetric methods to compute corrosion rates, either through weight gain or weight loss measurements. However, these methods only provide information on the overall mass change in the sample, largely disregarding localized corrosion. Furthermore, the combustion of complex solid fuels, such as municipal solid waste, involves fluctuations in temperature, flue gas composition, and flue gas velocity, all of which can contribute to tube wall failure—a factor often neglected in contemporary research. Moreover, the impact of localized stress on the corrosion of heat-exposed surfaces within waste incineration boilers has received little attention. These scientific issues require further in-depth investigation, leading to significant breakthroughs [51].

Notably, the corrosion rate curve depicted in Figure 6 is plotted using experimental results obtained abroad during the mid-to-late 1980s. Since then, the materials, processes, and technologies relevant to waste incineration boiler tubes have evolved considerably. Furthermore, waste incineration sources and systems are known to vary significantly across countries. Although the curve displayed in Figure 6 serves as a qualitative reference for studying and analyzing the corrosion rates of waste incineration equipment, it must not be rigidly adhered to. This is particularly true for the corrosion behaviors of high-parameter boilers currently under rapid development in China, which are likely to differ from previously reported ones, suggesting that different boiler parameters may result in distinct corrosion curves [52]. Thus, in conclusion, domestic scholars within the field are expected to soon develop a high-fidelity experimental platform to examine the performance of high-temperature anticorrosion coatings for waste power stations. Furthermore, they must establish corrosion rate curves tailored to China’s conditions for reference by industry professionals.

4.2. Protective Coating Technology

In China, the most commonly employed methods to protect waste incineration boilers from corrosion predominantly include surfacing, accounting for approximately 70% of the total, followed by induction welding, accounting for 20%. The remaining 10% includes HVOF, laser cladding, and high-temperature ceramic coating. This trend indicates the dominance of surfacing and induction technologies.

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Technological advancements in coating materials

Coating materials: To meet the requirements of high-temperature chlorine-induced corrosion resistance, the predominant material systems currently in use include high-Cr content FeCr and NiCr alloys. Remarkably, both systems exhibit significant advantages in terms of chlorine-induced corrosion resistance and demonstrate thermal expansion coefficients similar to those of commonly used carbon steel tubes, which are known to substantially reduce the likelihood of coating detachment. NiCr-based coatings outperform Fe-based ones in terms of their chlorine-induced corrosion resistance, as the chlorides of Ni and Cr formed during various types of corrosive chlorine attacks are less volatile than those of Fe. Owing to this, chlorine circulation within the chlorine cycle reduces and consequently decelerates the corrosion process. However, Fe-based coatings demonstrate improved corrosion resistance in environments containing HCl at flue temperatures above 600 °C, particularly those containing silicon. This improvement is attributed to the formation of SiO₂ precipitates within pores and microcracks, which seal the coating [53]. Future developments must focus on addressing fly-ash-induced erosion during waste incineration. This can be achieved by developing NiCr-based coatings enhanced with elements such as W and Mo to increase coating hardness and boost high-temperature performance, thereby achieving excellent corrosion and wear resistance. Furthermore, the use of high-temperature fiber-reinforced, rare-earth-enhanced, high-entropy, and smart materials in waste boilers must be examined to further enhance the high-temperature anticorrosion capabilities of coatings.

Surfacing technology: Although Inconel 625 alloy surfacing technology has significantly contributed toward the development of the boiler corrosion protection industry, it has prominent limitations. These include its low production efficiency, high cost, and high dilution rates. Furthermore, the corrosion protection performance of the cladding layer is significantly affected by the service temperature. For instance, at operating temperatures exceeding 540 °C, the corrosion rate of the cladding layer increases rapidly [53]. Consequently, developing more suitable high-performance, low-cost, low-dilution, and long-life surfacing technologies is crucial [54].

Induction welding technology: During the high-frequency remelting process of water wall tube banks, inconsistent separations between various points along the tube coating surface and upper surface of the coil cause the top of the tube, lying closest to the coil surface, to melt first. Given that this tube top lies approximately 25 mm from the tube base, it develops a localized liquid phase that gradually flows outward by the time the base and fins begin melting [34]. This results in grain coarsening on the upper surface of the tube wall coating owing to prolonged heating, a reduction in surface hardness, and the formation of a transition layer comprising martensite and residual austenite, leading to microstructure embrittlement. Meanwhile, inadequate remelting at the tube base may compromise the equipment service life. Recently, Jiangsu Ke Huan New Material Co., Ltd., developed a dual-heat-source preheating and cooperative remelting patent technology to comprehensively resolve this issue [43]. Furthermore, in 2019, an integrated technique combining water wall induction remelting with supersonic plasma spraying of metal–ceramic composite coatings was developed, signifying a domestic breakthrough. During this process, after the induction remelting of the Ni-based self-fluxing alloy on the water wall heating surface, a layer of NiCr–Cr₂C₃/YSZ/Al₂O₃ + TiO₂ metal–ceramic coating was applied using supersonic plasma spraying technology. High-velocity ceramic particles are likely to partially penetrate the soft underlayer, creating an anchoring effect that enhances the bond strength between the coating and remelted base layer to a metallurgical level [55].

Aluminizing technology: In 2016, a research group led by Shen Mingli at the Institute of Metals, Chinese Academy of Sciences, utilized an eddy current electric field to expedite the aluminizing process. Specifically, the electric eddy current induced by the electric field enabled electromigration effects, facilitating ultrafast aluminization on stainless steel surfaces. The speed of the aluminizing process was then compared using direct, pulsed direct, and alternating currents, as illustrated in Figure 9 [56]. The results revealed that the self-induced eddy currents generated by pulsed and alternating currents significantly accelerated the formation of the aluminized layer.

Experimental results demonstrate that alternating the current enhances the coupling between electromigration forces and chemical potential gradients, realizing a synergistic effect that accelerates aluminization. Once the Al melts, it infiltrates into the substrate’s surface layer, forming an aluminized layer with a thickness of approximately 30 μm. Concurrently, the high temperatures generated by eddy currents cause the Al alloy coating to remelt, enhancing its bond strength and significantly reducing its porosity. This ultrafast aluminization process reduces traditional aluminization durations from several hours to mere minutes, offering a feasible strategy for the high-speed aluminization of large, irregularly shaped workpieces, such as water walls [57].

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Proactive development of high-temperature corrosion protection technologies

At present, WtE plants in China urgently require transformation and rapid advancement. These plants exhibit varying characteristics in terms of their scale, quality, and corporate demand benefits, owing to factors such as urban size and regional development levels. Consequently, developing new coating technologies with superior protective performance, extended service life, and more competitive production efficiency and preparation costs presents a significant challenge for scientists and engineers in this industry.

Onsite preparation techniques for protective coatings: Onsite coating preparation following the installation of tubes within boilers ensures both coating performance and economic efficiency. Since 2008, some Western countries have begun using automated arc welding technology to protect water walls in power station boilers from corrosion [58]. However, China lacks domestic technology capable of onsite coating preparation for boilers. Current domestic protective technologies encounter challenges, such as the cumbersome size of HVOF equipment, which hinders mobility. Meanwhile, despite the advantages of high-speed arc spraying, it suffers from short service lives, necessitating the development of more economical and reliable welding wires and construction processes with better controllable porosity rates. Furthermore, high-temperature ceramic coating technology, although suitable for onsite application, is restricted in its practical use, owing to the limited thicknesses of coatings and their brief lifespan.

Ni–Al coating technology: Following the advancement of the aluminizing coating technology, the preparation of Ni–Al-based coatings for boiler superheaters by Dr. Wu Duoli of Yangzhou University and Professor John Hald of the Technical University of Denmark marks a significant achievement in the realm of high-temperature protective coatings for boiler heating surfaces. Through a two-step process involving Watts Ni plating and low-temperature aluminizing, a Ni₂Al₃ coating was prepared on the surfaces of austenitic stainless steel tubes in a biomass power plant. Subsequently, these Ni₂Al₃-coated tubes were welded into the superheater tubes of a biomass power plant, where steam temperatures and corrosion rates are known to be the highest, and were operated for 7100 h. Experimental results revealed that the Ni₂Al₃ coating effectively protected the substrate from high-temperature corrosion in the real power plant environment, with corrosion localized to specific areas. Hence, this technology holds promising prospects for corrosion protection in WtE boiler applications [59].

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Addressing challenges encountered by high-parameter boilers

The enhancement of primary steam parameters has emerged as a developmental direction for WtE boilers worldwide. In high-parameter WtE boilers, the elevated temperatures of the heating surfaces of tube walls correspondingly increase the temperatures of the corrosion reaction and ash deposition layers, thus aggravating corrosion reactions. Furthermore, the high operational pressures characteristic of high-parameter conditions subject the tube walls to significant tensile stresses, thus accelerating the growth of corrosion cracks and reducing the service lives of heating surfaces. Meanwhile, the medium temperatures typically observed in the water walls and superheaters of high-parameter waste heat boilers are 35 °C–85 °C higher than those in medium-parameter waste heat boilers. Consequently, high-temperature corrosion effects restrict the development of WtE boilers with higher parameters. Hence, developing corrosion protection technologies suited to high-parameter conditions presents greater challenges for scientists and engineers in the field, requiring accelerated efforts [60].

5. Conclusions

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Typical chlorine-induced corrosion mechanisms in waste incinerators can be classified into gaseous corrosion and molten-salt corrosion, which coexist and mutually enforce each other, thus accelerating the overall chlorine-induced corrosion process within waste boilers. Chloride gases, particularly Cl₂ and HCl, are primarily responsible for corrosion in waste boilers, causing metal material loss through active oxidation. Solid-salt chlorides, particularly alkali metal chlorides and heavy metal chlorides, also contribute to this corrosion, owing to their relatively low melting points and their ability to form eutectic salts featuring even lower melting points. The study of the chlorine/sulfur ratio could be an important aspect of further research and development of this topic. Furthermore, both excessively high and low chlorine/sulfur ratios can lead to severe corrosion, primarily owing to the significant impact of molten-salt corrosion triggered by low-melting-point chlorides and sulfates.

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The correlation between corrosion and boiler tube wall temperatures indicates that high-temperature corrosion zones begin forming at wall temperatures of >320 °C. Within the 320–480 °C range, mild corrosion effects are prevalent, owing to the gradual formation of FeCl₃ and basic sulfates. The most active high-temperature corrosion reactions occur within the 480–700 °C range, primarily owing to the decomposition and melting of FeCl₃ and basic sulfates. Meanwhile, academic opinions regarding the impact of flue gas temperature on corrosion rate vary widely. One perspective suggests that high flue gas temperatures are primarily responsible for tube wall corrosion, mainly affecting water walls and superheaters by raising wall temperatures. Meanwhile, another perspective suggests that the magnitude of the impact varies across temperature ranges, as flue gas temperatures fundamentally influence the formation sequence and stability of corrosive media. Finally, based on years of engineering practical experience in operating waste boilers, the effect of flue gas temperature on wall temperature is believed to be limited, with the working fluid temperature predominantly influencing the wall temperature.

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Corrosion protection technologies for waste boilers include various methods such as thermal spraying, surfacing, induction welding, laser cladding, thermoset reaction nanoceramic coating, and aluminizing coating. Conventional thermal spraying is less frequently adopted owing to the high porosity and low bond strength of the resulting coatings, while HVOF is somewhat more prevalent. Laser cladding remains in the research stage, and thermoset reaction nanoceramic coatings are limited in their applications owing to thin coating layers and brief service lives. Research on aluminizing coatings for waste boilers is relatively recent, and the technology is yet to completely mature. At present, surfacing and induction welding are predominantly used for waste incineration boiler protection in China.

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To impart systems with resistance against high-temperature chlorine-induced corrosion, high-Cr content FeCr and NiCr alloys are adopted as the primary material systems. Notably, improving existing coating corrosion protection technologies primarily involves developing more suitable high-performance, low-cost, and low-dilution-rate surfacing materials and processes to address the uneven heating resulting from high-frequency remelting in water wall tube arrays. Existing research reveals that superfast aluminizing and similar processes can be implemented on stainless steel surfaces by inducing electromigration effects through electric eddy currents. In the future, the development of new technologies for high-temperature corrosion protection primarily focuses on introducing new coating technologies with excellent protective performance and longer service life, along with competitive production efficiency and lower preparation costs; onsite preparation technologies for protective boiler coatings; novel Ni–Al coating technologies; and strategies to actively address the challenges posed by the rapid development of high-parameter boilers.