A Brief Discussion on Corrosion Prevention for Ultra-High-Temperature Flue Gas Ducts/Chimneys

For flue gas duct and chimney corrosion protection, if the primary concern is wet, hot, and acidic corrosion, this issue has already been effectively addressed in China. Over the course of development since 2000, the prevailing solution today is primarily the vinyl ester resin-based glass flake mortar lining. Of course, other solutions such as KPI linings and brick or tile linings still exist; however, for medium- and low-temperature applications as well as most general high-temperature scenarios, the glass flake mortar lining has gained widespread acceptance. Although it may not always be the absolutely perfect solution, when considering factors such as cost-effectiveness and service life, the vinyl ester resin-based glass flake mortar lining truly stands out as the optimal choice.

Engineering companies in China that specialize in FC applications are increasingly encountering more challenging issues: namely, the original flue ducts/chimneys of power plants and the original flue ducts/chimneys of boilers. Both of these components operate at exceptionally high temperatures—momentary peak temperatures exceed 200 degrees Celsius, and even long-term operating temperatures approach 180 degrees Celsius. Moreover, the substrates vary: some are made of steel, while others are constructed from concrete or reinforced concrete. During actual operation, these components often experience fluctuating temperatures, with sharp and frequent changes in the ambient operating environment.

In general, there are many engineering companies and material suppliers in China today working in this field, and the solutions they propose are highly diverse. However, there are virtually no successful cases that have undergone more than five years of actual operational experience in real-world environments. Below, Ouyang will analyze one by one the solutions currently being proposed in China’s anti-corrosion industry in this specific area, highlighting their respective advantages and disadvantages for the reference of those who need them.

Currently, the mainstream anti-corrosion solutions for ultra-high-temperature flue ducts and chimneys include: vinyl ester resin (VER) glass flake mortar, VER mortar-glass fiber reinforced plastic composites, VER mortar-inlaid brick and tile linings (such as Binggaode vitrified bricks, acid-resistant bricks, and acid-resistant ceramics), acid-resistant KPI mortar, KPI mortar-inlaid brick and tile linings, OM coatings, titanium alloy and Hastelloy alloys, and other alloy-based solutions. Although some new solutions have begun to appear on the market, there are almost no practical cases yet—those few cases that do exist have been in operation for less than two years: the Hunyuan body solution and the cyclic silicon polymer-metal hybrid solution.

The design of anti-corrosion solutions for ultra-high-temperature flue ducts/chimneys hinges on three key considerations: corrosion resistance, impermeability, and resistance to delamination. Corrosion resistance is determined by the acid-resistance properties of the material itself, particularly its ability to withstand acidic conditions at high temperatures. Impermeability is primarily influenced by the thickness of the anti-corrosion coating and the density of the cured organic and inorganic components. Resistance to delamination depends on factors such as construction quality, the effectiveness of substrate preparation, the anti-corrosion material’s ability to withstand rapid temperature changes, and the performance of the anti-corrosion coating in resisting stress variations.
A high-temperature glass flake mortar solution
Here, we’re referring to a glass flake mortar with a high cross-linking density that can withstand temperatures of up to 180 degrees Celsius over the long term.
Advantages of the solution: excellent acid resistance, outstanding impermeability, convenient construction, low overall cost, and high cost-effectiveness.
Drawbacks (or challenges) of the proposed solution: The primer must also meet temperature resistance requirements of up to 180°C and exhibit a certain degree of flexibility—a challenge that the vinyl ester resin industry is currently struggling to overcome. The putty itself lacks sufficient toughness and tends to become brittle, especially under sudden temperature changes, where its adhesion to the substrate becomes poor and prone to delamination—this issue is particularly acute when the substrate preparation is inadequate. Moreover, during rapid temperature fluctuations, localized stresses can lead to stress concentration, which is a particularly difficult problem to address (indicating insufficient resistance to stress variations in this solution). The root cause of these drawbacks lies in the fact that, after curing, the linear thermal expansion coefficient of the FC putty’s final anti-corrosion layer differs from that of the substrate—this discrepancy becomes especially pronounced at extremely high temperatures. Under high-temperature conditions, the adhesion between the putty and the substrate cannot be reliably maintained. Furthermore, the polar bonds in vinyl ester resins, represented by hydroxyl groups, are not strong enough to provide the same level of bonding effectiveness as epoxy bonds. Additionally, the substrate preparation requirements are relatively stringent, yet in actual construction practices, substrate preparation is often carried out perfunctorily and without due diligence.
A grouting material capable of withstanding temperatures up to 180°C for extended periods can be readily obtained by using a vinyl ester resin of the phenolic type with a high cross-linking density, combined with glass flakes and other additives. It’s important to note that resins of this type generally have relatively high viscosities. Adding excessive amounts of styrene diluent during grouting preparation can lead to a brittle final cured product and reduced temperature resistance. Therefore, when preparing such grouting materials, one should neither rely excessively on adding styrene to improve workability nor simply reduce the content of inorganic fillers like glass flakes solely for ease of processing. Instead, a balanced approach is recommended: the resin content in the final grouting material should be somewhat higher than that typically found in conventional VER grouting materials. As long as the material meets the requirements for on-site construction, it’s best to avoid adding excessive styrene monomer.
High-temperature primer with a certain degree of flexibility—currently, there are very few manufacturers in China that can provide such primers, making it extremely difficult to find a perfect solution to this issue. At present, the vast majority of practices on the market involve adding highly cross-linked VER resins or similar resins to diluents containing other highly polar monomers—such as acrylic acid, methacrylic acid, and methyl methacrylate—that can also participate in cross-linking and curing, thereby creating a primer coat. After applying this primer coat, a small amount of powder (typically quartz powder or the like) is added to the primer, followed by another coat of very thin putty. Finally, a middle coat and top coat made of ultra-high-temperature putty are applied. In other words, the current approach does not involve using a specially formulated primer; instead, highly heat-resistant vinyl resins diluted with polar monomers are directly used as primers.
Of course, a silicone-composite, high-temperature-resistant vinyl resin primer has now emerged in China. Currently, however, it remains largely confined to pilot-scale trials in the R&D laboratories of various companies and has not yet been widely adopted in practical applications. The introduction of silicone resins can significantly enhance the temperature resistance of primers. New attempts have also been made to develop polyurethane-modified, high-temperature-resistant vinyl resin primers, but so far, no mature, large-scale applications have been reported.
To reduce the current coefficient of thermal expansion of vinyl ester resin glass flake putty designed for high-temperature resistance, making it more closely matched to the substrate while significantly enhancing the putty’s toughness and impact resistance, and effectively preventing issues such as delamination during rapid temperature changes and insufficient resistance to stress variations, some manufacturers in the market have begun experimenting with and using various modifiers to enhance existing VER putties. The main approaches include: 1. Adding powders of thermoplastic polymers, such as PET, PP, PE, and ABS. These materials act as low-shrinkage agents within the putty, reducing shrinkage while simultaneously improving the overall toughness of the coating. This approach also helps address the problem of inadequate resistance to rapid temperature changes and stress variations, which can otherwise lead to easy delamination. Several manufacturers originally engaged in coatings production are now actively pursuing this direction and have already brought these modified products to market; Ouyang has even met engineers from such manufacturers. 2. Incorporating certain high-temperature-resistant silicon-based substances or additives to elevate the overall temperature resistance rating of the putty coating. Ouyang has also encountered engineers from manufacturers working in this area. 3. Adding flakes or other materials with lower coefficients of thermal expansion—such as metallic flakes—to the putty. This not only increases the hardness and strength of the final coating but also brings its coefficient of thermal expansion closer to that of the inorganic or metallic substrate, thereby further addressing the issue of inadequate resistance to rapid temperature changes and stress variations, which can otherwise result in easy delamination. All three of these methods have already been widely adopted in practical applications across China.

B VER glass flake mortar + FRP fiberglass composite solution
This is also one of the more commonly used solutions today, and abroad—especially in Japan—it’s already a well-recognized approach for high-temperature resistance.
The advantage of this scheme over the pure clay-based approach lies in the fact that an FRP isolation layer has been added beneath the clay. Under favorable conditions, it is even possible to use a carbon-fiber-reinforced fiberglass isolation layer (1–2 mm thick), which effectively serves as a transition layer between the clay and the substrate. As a result, both the overall strength and impact resistance are significantly enhanced. While maintaining excellent resistance to temperature changes, acid resistance, and impermeability, this scheme shows some improvement over Scheme A in terms of resistance to sudden temperature fluctuations and stress variations. However, strictly speaking, this approach addresses only the symptoms rather than the root cause. Its drawbacks are identical to those mentioned earlier. If improvements are desired, the methods and underlying principles would remain the same as discussed above.

C VER grouting mortar for brick and tile lining (including Binggaode foam vitrified bricks, acid-resistant bricks, acid-resistant ceramics, etc.)
The brick-and-plate lining solution is one of the more commonly adopted approaches in China, particularly among high-altitude anti-corrosion engineering companies in the Yancheng area—many of their projects fall into this category.

Brick lining is typically used only in environments with severe corrosion and heavy loads, where special requirements exist for temperature resistance, pressure bearing capacity, and wear resistance. Compared to fiberglass lining or clay mortar lining for corrosion protection, brick lining comes at a higher cost.
The advantages of brick and tile lining include its high temperature resistance (especially its resistance to rapid temperature changes), corrosion resistance, wear resistance, pressure-bearing capacity, and slow heat transfer. However, it suffers from insufficient toughness, poor impact resistance, and is prone to leakage if the grouting material is improperly selected. Additionally, improper use of bonding materials for the isolation layer can easily lead to the detachment of bricks and tiles.

The main raw materials for brick and slab linings fall into two categories: first, bricks and slabs; second, bonding materials.

Currently, the commonly used bricks and slabs include: acid-resistant ceramic materials (available in various sizes and specifications, including slabs and bricks); cast stone slabs (made from raw materials such as greenish-gray rock, basalt, and industrial slag); acid-resistant bricks of various sizes and specifications; natural acid-resistant stones (primarily granite); thermosetting resin-impregnated graphite materials; and water-glass-impregnated graphite materials.

Of course, when used in high-temperature flue ducts, the most common materials are acid-resistant carbon bricks, acid-resistant industrial ceramics, and Binggaode foam vitrified bricks (the latter being widely used for corrosion protection in high-temperature chimneys). Among the bonding materials, the primary ones are mortar pastes, which are mainly used for grouting, filling gaps, and bonding purposes; sometimes they are also directly employed as isolation layers. The main types include: water-glass mortar—also known as silicate mortar (KPI is currently the most widely used, with potassium water glass offering even better performance); phenolic mortar (not commonly used in practice); furan mortar (which must be used in conjunction with an epoxy primer); epoxy mortar (for high-temperature flue ducts, it generally uses organosilicone-modified epoxy resins); unsaturated resin mortar (widely used); and especially high-temperature-resistant, highly cross-linked density phenolic vinyl ester resin mortar (the most commonly used type). It’s important to note here that the selection of bonding materials is closely related to the final lining application environment—in terms of temperature resistance, acid resistance, alkali resistance, and other factors—and also depends on the substrate material itself, such as bonding strength, toughness, and shrinkage characteristics. In addition to considering performance at room temperature, it’s even more crucial to evaluate whether the bonding material can maintain excellent strength, toughness, acid-alkali corrosion resistance, and anti-permeation properties under high temperatures or at specific temperatures (after all, if the lining is intended for use at elevated temperatures, why else would one choose such a costly solution?). Regardless of the type of mortar paste, vacuum-dispersed mortars will always be significantly superior in quality compared to those hastily mixed on-site.

Let’s talk again about the setup of the isolation layer. In addition to grouting and joint filling, mortar often continues with the application of an isolation layer. The selection of the isolation layer is quite critical: if rapid heat transfer is required, most commonly metal materials are used for the isolation layer, though this approach comes with high costs. Rubber materials are also frequently chosen for isolation layers; even more common are fiberglass-reinforced plastics, which offer excellent adhesion and allow for a wide range of resin options. The surface of brick lining, while resembling the process of building walls, actually involves many intricate details that demand careful attention when examined closely. Particular care must be taken when laying bricks in special situations—such as corners or other non-planar areas.

D Acid-Resistant KPI Mortar
The main component of KPI grouting material is potassium silicate, combined with other inorganic ingredients. Its advantages include excellent resistance to organic solvents—particularly pronounced at medium and low temperatures—and good temperature resistance (when additives such as titanium dioxide are included, the temperature resistance of KPI grouting can exceed 600°C; however, at such high temperatures, the bonding performance between KPI grouting and the substrate will also deteriorate significantly). It also has a low unit price. Its disadvantages, on the other hand, are that its mechanical strength and bonding performance are considerably inferior to those of FC grouting, fiberglass-reinforced plastics, and brick or tile linings; its acid resistance is weaker than that of VER grouting; and especially, its impermeability is much poorer than that of FC glass flake grouting. When exposed to wet flue gases, the situation becomes even more problematic (therefore, if KPI grouting is used, its application thickness must be greater than 10 mm).

The KPI mortar solution, once widely used in low-end anti-corrosion projects several years ago, is now becoming increasingly rare. Today, very few people still opt for KPI mortar to protect flue ducts against corrosion. This is because, after curing, the main components of KPI mortar are inorganic; thus, this solution does address, to some extent, issues related to temperature resistance and resistance to rapid temperature changes. However, it falls short in effectively tackling both corrosion prevention and impermeability.

E KPI grouting mortar for brick and tile lining
This solution, which several years ago appeared in some high-altitude flue and chimney anti-corrosion projects in Yancheng and Beijing, has become rarely used in recent years. Compared to Scheme C, the only difference is that VER mortar has been replaced with KPI mortar. Although the temperature resistance has been improved, the issues of impermeability and corrosion resistance remain unresolved. In fact, Scheme C originated from the early KPI mortar jointing technique.

F OM Coating
Some differences between OM coatings and vinyl ester resin glass flake putty (a type of FC):
1. Component Difference: OM is purely organic, whereas VER-FC is an organic-inorganic composite. After curing, the coefficient of thermal expansion (linear expansion coefficient) of VER-FC is closer to that of the substrate (firebricks, bricks, and metal substrates) compared to that of the purely organic OM coating. This difference determines many of the following performance characteristics.
2. Difference in bonding strength with bricks: VER-FC is significantly better than OM;
Bonding performance to metal substrates: VER-FC is significantly better than OM;
Especially after periodic fluctuations between high and low temperatures, the difference in bonding strength becomes even more pronounced.
The effective anti-corrosion thickness of 3 FC is significantly greater than that of OM, and its cost is also considerably higher. Before 2005, OM was widely used for corrosion protection inside flues and chimneys in China. However, since 2005—especially after companies such as SGL Wuhan and Prince Jingjiang in Japan introduced VER-FC technology into Chinese power plants equipped with wet flue gas desulfurization systems—the use of OM has steadily declined. Meanwhile, the number of application cases involving VER-FC has been increasing. It’s not that VER-FC has never encountered problems; indeed, there have been instances where issues did arise. Nevertheless, compared to OM, VER-FC or deeply processed VER-FC solutions are now more readily accepted by clients and engineering firms in the market (for example, using VER-FC mortar to grout brick-and-tile linings is a type of deep-processing application).
4. Excellent resistance to both temperature fluctuations and rapid temperature changes—VER-FC significantly outperforms OM.
5. In terms of impermeability, VER-FC significantly outperforms OM;
6. In terms of wear and erosion resistance, VER-FC significantly outperforms OM.
When it comes to flue gas duct corrosion protection, the following factors need to be considered: 1) acid resistance; 2) impermeability; 3) temperature resistance; 4) adhesion to the substrate; 5) wear resistance; and 6) resistance to thermal shock and stress changes. OM can hardly address any of these factors effectively, which is why OM has virtually withdrawn from the field of heavy-duty corrosion protection for flue gas ducts and chimneys.

In high-temperature chimneys, if the FGD system is not operating, the flue gas will directly enter the chimney, resulting in extremely high temperatures—especially at the inlet, where the temperature could easily exceed 200 degrees Celsius (at this point, of course, the gas is already dry). Under these conditions, OM is even less capable of addressing the key issues mentioned above, particularly in the case of chimneys with metal inner liners.

For high-temperature flue ducts and chimneys, the most commonly used solutions currently on the market are: glass-flake mortar, glass-flake mortar FRP composites, and glass-flake mortar with brick or tile lining applied via joint sealing. The greatest advantages of these three solutions all lie in their exceptional corrosion resistance, outstanding temperature tolerance, and excellent impermeability. If the environment involves only moderate fluctuations between high and low temperatures—with no drastic temperature changes, minimal stress variations (since excessive chimney sway can lead to severe stress concentrations)—then vinyl ester resin-based glass-flake mortar can indeed be considered a fairly perfect solution. However, when confronted with the aforementioned particularly harsh conditions—even if you were to add thermoplastic modifiers, silicone additives, metallic flakes, or even carbon-fiber reinforcement layers to the glass-flake mortar—such measures would only address the symptoms rather than the root cause, offering only marginal improvements. Under these circumstances, neither of these solutions can truly be regarded as flawless.

Recently, two relatively new corrosion-resistant solutions have emerged in China, designed specifically for environments involving ultra-high-temperature, wet flue gases with frequent fluctuations in stress and temperature—solutions that you may have already heard about: Hybrid Corrosion-Resistant Coating and Hybrid Polymer Corrosion Protection. Let’s start with the former. At present, both of these solutions have been applied in real-world cases for too short a period to draw any definitive conclusions; thus, we can only discuss them at this stage.

G Huyuan Body Program
Hunyuan Body: A term coined by a manufacturer in Foshan, Guangzhou—quite fitting, actually. To understand exactly what a Hunyuan Body structure is, just take a look at their website. The principle behind the Hunyuan Body is:
Concrete base:
The first component—penetrating into the concrete matrix—is what Xinglu himself refers to as a reducing agent. What it’s called doesn’t really matter. Let’s take a closer look at its main ingredients: low-molecular-weight epoxy resin, an active diluent, and an epoxy curing agent of the T31 type that cures at room temperature. There may also be water-based varieties of epoxy resin, since their documentation claims they can be applied to wet substrates and even underwater.
Second layer: Reinforcing layer. Epoxy (main component) + high-temperature-resistant silicone resin + paraffin wax + curing agent;
Third layer: Repair coat. Epoxy + reactive diluent + quartz powder + T31-type curing agent;
Fourth layer: Toughening layer. Epoxy resin + polysiloxane additive + aliphatic epoxy curing agent + diluent;
Fifth layer: Glaze layer. A resin—primarily composed of polymerized vegetable oil—plus a curing agent and an inert diluent (such as benzene, xylene, esters, or alcohols).
Among them, the fourth step can be reinforced using fiber fabric. The key question regarding this solution is:
1) The active diluent should penetrate into the substrate, while the one on top is non-active. This is also the primary reason why the final coating can only achieve a thickness of 1.5 mm or less. If the top corrosion-resistant coating contains too much active diluent, it will ultimately lead to a decline in the epoxy coating’s heat resistance and corrosion resistance.
2) To ensure that the material can penetrate deeply into the substrate, the molecules of the substance must have an extremely low molecular weight and very low viscosity, and they must also be capable of reacting within the substrate to bond it together even more firmly. Without the addition of reactive diluents, it would be impossible to achieve such excellent penetrability; the viscosity would inevitably become very high. It is more likely that epoxy resins of type E51 or similar will be used, as they offer better adhesion. Of course, other compounds containing epoxy bonds could also be employed—these would have even lower molecular weights—but their adhesion and curing behavior would be harder to control.
3) The reinforcing layer uses resin with added silicone resin, which enhances heat resistance.
4) The repair layer contains inorganic materials such as quartz powder, which helps the glass flakes in the putty.
5) The toughening layer should be the primary anti-corrosion layer (although their documentation claims that the substrate penetrated by the coating itself can provide corrosion resistance, in reality, so many diluents have been added to it, and since the main resin is epoxy, it’s simply impossible for this layer to achieve such outstanding heat and corrosion resistance).
The merit of the Hunyuan Body scheme lies in:
1) Between the substrate and the anti-corrosion layer, we have indeed adopted the principle currently used in ground anti-corrosion projects by introducing a penetration layer. This approach is similar to the method employed by some flooring companies today: when the water-to-sand ratio is too high and the concrete substrate becomes sandy and unstable, they use so-called concrete substrate repair agents to treat the substrate. Indeed, this practice significantly improves the quality of the substrate.
2) Next, apply the repair layer and reinforcement layer. Actually, the principle is similar to applying putty in flooring work—both aim to create an intermediate layer with better transition properties between the anti-corrosion layer and the substrate, thereby enhancing adhesion to the substrate.
3) The substances that permeate blend seamlessly with the original substrate, resulting in a stronger bond. At the same time, this creates a thicker transition layer with a longer gradient between the substrate and the coating. This transition layer effectively bridges the significant difference in linear thermal expansion coefficients between the substrate and the coating. As a result, this transition effect significantly reduces the likelihood of poor adhesion between the coating and the substrate when subjected to sudden temperature changes or stress variations.
4) Unlike current solutions such as clay mortars, brick lining, fiberglass, OM, and KPI, the Huyuan body approach takes a different path and addresses the issue of delamination and peeling between the final anti-corrosion layer and the substrate from another angle and perspective—specifically, by placing greater emphasis on substrate preparation. (Their reduction layers and repair layers are, in essence, just a disguised form of substrate treatment—a broad-spectrum concept of substrate preparation.) Indeed, in current high-temperature flue gas corrosion protection practices, construction companies rarely engage in such thorough substrate treatment; typically, they merely sandblast the surface to create a rough texture. Compared to the Huyuan body approach, substrate treatment under this broader definition truly yields entirely different results.
Problems with the Hunyuan Body Scheme:
1) Regarding the selection of epoxy resin types, based on the construction feasibility of this scheme, the lower layer uses resins with lower equivalent weights, while the upper layer employs resins with higher equivalent weights—ranging from phenolic epoxy resins all the way to high-equivalent-weight, multi-functional epoxies. By adopting this approach, we ensure that the lower layer achieves sufficient penetration, while the upper layer guarantees the hardness, strength, and impermeability of the final cured coating. Using T31 phenolic amine-based curing agents and aliphatic curing agents for curing allows for stepwise polymerization; thus, the crosslink density can be effectively controlled. However, the resulting cured product cannot simultaneously achieve both excellent heat resistance and outstanding strength and corrosion resistance—particularly in environments involving high temperatures and strong acids. Although the addition of silicone resin does enhance heat resistance, it still fails to improve corrosion resistance. Therefore, compared to VER’s glass flake mortar, this proposed solution will inevitably fall short in terms of its actual dynamic performance in both corrosion resistance and heat resistance.
2) Using epoxy resin, the final substrate adhesion performance and overall impact resistance are undoubtedly excellent. Moreover, a transition layer is formed between the epoxy and the substrate. Indeed, the impact resistance and stress-strain characteristics of this approach represent its greatest advantages. However, although reinforcing the surface with fiber fabric and incorporating diluents into the formulation is not entirely unacceptable, the diluent layer must be extremely thin. If the diluent fails to fully evaporate, it will significantly compromise the final heat and corrosion resistance. On the other hand, if the diluent does evaporate—as in the film-forming mechanism of acrylic coatings using xylene as a solvent—a distinction between wet-film thickness and dry-film thickness will emerge. It’s impossible for all the solvent to completely evaporate; unlike free-radical curing, where styrene solvent can participate in cross-linking reactions, the non-reactive diluents used here, which don’t evaporate during application, will inevitably expand or cause other adverse effects—such as swelling of the epoxy resin—once they’re sealed beneath the subsequent glaze layer under high-temperature conditions in the future.
3) As for wear resistance, the glaze layer is made exceptionally smooth and doesn't easily adhere to it—of course, that’s ideal. However, in reality, the ultimate corrosion resistance largely depends on the hardness of the anti-corrosion layer. That’s why, in certain situations, people add silica fume; surely they don’t do so without reason—there must be a good rationale behind its inclusion. But if we use an inactive diluent here, adding any powders or flakes will significantly affect the diluent’s flow and spreading. Therefore, only materials like continuous-fiber fabrics with slight gaps can be used.

Hybrid structure of steel substrate
The primer applied during substrate treatment can react with rust to form a barrier layer, significantly reducing the need for sandblasting to remove rust. The other toughening layers and glaze layers remain as described above. Analysis:
1) Although resins offer significantly better toughness and impact resistance compared to other solutions, there’s still a substantial difference in the linear thermal expansion coefficients between inorganic and metallic materials—the ratio by which these materials contract due to thermal expansion and contraction. Moreover, since the solution contains diluents, it’s extremely challenging to rely solely on coatings with such a high organic content to address this issue.
2) The material exhibits excellent resilience, and even adding some thermoplastic powder to it can indeed enhance its shape-memory effect under varying temperatures and stresses. However, whether this approach can fundamentally solve the problem remains theoretically inconclusive.
3) If thermosetting resins such as epoxy are used, based on the currently common epoxy, unsaturated, vinyl, phenolic, furan, bismaleimide, melamine, polyurethane, silicone, polysulfone, and thermoplastic polymers (including most general-purpose plastics, engineering plastics, and fluoropolymers), adopting this film-forming mechanism can ensure corrosion resistance at room temperature. However, at high temperatures, relying solely on epoxy resin makes it difficult to simultaneously achieve both heavy-duty corrosion protection and excellent temperature resistance.
The proposal of the "Hunyuan Body" approach is excellent—indeed, it offers professionals in the heavy-duty anti-corrosion industry a new direction worthy of further consideration. Beyond just focusing on heavy-duty corrosion resistance and heat resistance, we can take a broader perspective by examining substrate treatment from a more generalized standpoint. The goal is to strike an optimal balance among the following key attributes: heavy-duty corrosion resistance, heat resistance, resistance to rapid temperature changes, impact resistance, and stress resistance. To achieve this harmony, heavy-duty anti-corrosion engineers and construction personnel must collaborate more closely and share their expertise openly and effectively.
H Siloxane Polymer Hybridization Scheme
Also known as APC hybrid polymer coating, it is an organic-inorganic hybrid polymeric material—a corrosion-resistant coating with a highly cross-linked, three-dimensional spatial structure.
Advantages: High temperature resistance, excellent corrosion resistance, good wear resistance, high flexibility, flame retardant properties, excellent impermeability when made thick, good resistance to rapid temperature changes, good resistance to stress variations, and superior aging resistance compared to organic solutions (though inferior to titanium alloys).
Disadvantages: The impermeability performance and process cost cannot be optimized simultaneously; the adhesion performance to the substrate needs further improvement; and the construction process and costs are too high.
　　Currently, there are application cases of APC hybrid polymer coatings in power plant flue ducts and chimneys in the United States—but none domestically.
The key points of this plan:
1) In terms of organic components, unlike conventional anti-corrosion coatings, typical organic components can be applied and formed at room temperature. Most of these components contain hydroxyl or ester bonds—for example, epoxy resins, vinyl ester resins, and phenolic resins. (Of course, it’s worth noting that some solvent-based, non-curing film-forming coatings do not contain either hydroxyl or ester bonds.) In contrast, the main organic components in hybrid polymer structures almost completely lack hydroxyl and ester bonds; instead, they primarily rely on ether bonds to link numerous functional groups capable of participating in crosslinking or polymerization reactions. Currently, this type of cyclic-silicone polymer remains at the laboratory stage in China (being developed by the Chinese Academy of Sciences), with only small-scale production underway. The main organic component used in Chemline 784 from APC Corporation in the U.S. is precisely this same type of material.
2) Unlike the approach taken by the Huan Yuan body, this solution draws more heavily on the general substrate treatment principles applied to metal substrates, aiming to reduce the linear thermal expansion coefficient of the coating to a level that is close to that of the metal substrate itself.
3) To achieve the effect described in 2), in addition to organic components, this formulation also incorporates inorganic ingredients such as stainless steel flakes, quartz powder, silicon carbide ceramic powder, modified carbon fibers (which exhibit superior interfacial adhesion performance compared to conventional carbon fibers), and titanium dioxide.
4) The curing agent combines alicyclic amine and aromatic amine to ensure the final coating’s thermal resistance and crosslinking density.
5) By controlling the content of inorganic components, curing agents, diluents, and other additives, it is possible to produce coating materials that closely resemble substrates made of concrete, reinforced concrete, or metal.
Key advantages:
1) From the perspective of the linear thermal expansion coefficient, making the thermal expansion and contraction behavior of the anti-corrosion coating structure more closely aligned with that of the substrate is the fundamental approach for composite-material corrosion protection (combining organic and inorganic components). However, this particular approach goes even further: whereas conventional methods merely add glass flakes and powdered materials to the mastic, this innovative solution not only introduces titanium dioxide, ceramic powder, and carbon fibers but even incorporates metallic flakes. Although this approach significantly increases costs, it is foreseeable that it will undoubtedly help address the issue of the linear thermal expansion coefficient. Currently, the most common practice involves adding small amounts of abrasives such as silicon carbide or quartz sand to reduce the overall structural layer’s linear thermal expansion coefficient; however, the use of metallic flakes is relatively rare.
2) The introduction of boron fibers and carbon fibers will inevitably increase costs, but this is also aimed at reducing the overall linear thermal expansion coefficient of the coating.
Feasibility Question:
1) Solvent-based formulations contain solvents such as cyclohexanone, alcohols, and even petroleum ether and naphthalene derivatives. These solvents evaporate during film formation. While they pose no problem when applied to fiber fabrics, they shouldn't have much difficulty penetrating metal flakes either. However, when these solvents come into contact with titanium dioxide—especially ceramic powders in higher concentrations—their evaporation is significantly hindered. That’s precisely why glass flakes aren’t added; adding glass flakes would only exacerbate the solvent-evaporation issue.
2) How can we prevent the settling and accumulation of metallic flakes?
3) If the coating isn't applied thick enough, how can we ensure its impermeability? And if we have to apply it at least 5–10 mm thick each time, the cost would be astronomical! 4) The solvent needs to evaporate, yet we’re aiming for a thicker coating—this poses a challenge in terms of construction methods and application intervals.
5) The reason why it’s difficult to make the mixed-phase body thicker is precisely due to the issue of solvent evaporation (of course, cost is also a factor). So how can this approach ensure that, while making the body thicker, most of the solvent oil is still able to evaporate?
6) Cyclosiloxane polymers, such as Chemline 784, are extremely expensive and must be made 5–10 mm thick. As a result, the cost of this solution would undoubtedly be astonishingly high. If the price tag is really that steep, why not just make a one-time investment in titanium alloy or Hastelloy instead?

I Titanium alloy inner cylinder, sprayed titanium alloy, Hastelloy
The advantages of these solutions are that they simultaneously address resistance to temperature, resistance to rapid temperature changes, resistance to stress variations, and (partial) corrosion resistance.
The main drawback is the high cost. While in Europe and the U.S., there are indeed quite a few cases of directly using alloys to manufacture ultra-high-temperature flues and chimneys, at present, such applications in China can still be counted on one’s fingers.
At high temperatures, titanium alloys exhibit poorer resistance to moist acid mist compared to Hastelloy. This is precisely why some people no longer use titanium alloys today.
Ouyang himself has very little knowledge of metallic materials and is hesitant to make rash comments. Even less familiar with alloy materials, he feels it’s inappropriate for him to elaborate further here. He hopes that friends working on alloy corrosion resistance will take this opportunity to share their views extensively.

In summary, how can we simultaneously address the challenging issues of heavy-duty corrosion resistance, heat resistance, resistance to rapid temperature changes, impact resistance, and stress resistance? Whether it’s the previous OM, KPI, or coating solutions, or the vinyl ester resin glass flake mortar solution—currently used by 95% of the market—or even the “Hunyuan” system, each approach can only partially tackle some of these challenges and none can perfectly address them all. No single solution should be outright dismissed, nor should any one be blindly trusted as a panacea. Therefore, Ouyang suggests that the client owner, the engineering and technical team, and the material supplier should engage in more open communication and sit down together to comprehensively evaluate material selection based on the client’s actual operating conditions. Moreover, they should draw inspiration from different approaches, particularly those involving advanced substrate treatment and efforts to align the linear thermal expansion coefficient more closely with that of the substrate. By applying these ideas to the current VER vinyl ester resin glass flake mortar solution, we might achieve unexpected results. Specifically, we could explore the following avenues:
First, in the broad sense of substrate treatment, for concrete substrates, efforts should be made on the primer, on the penetrant, and on the overall substrate treatment itself.
Second, by reducing the linear thermal expansion coefficient of the cured existing clay coating to make it closer to that of the substrate, we can appropriately add materials such as carbon fibers, ceramic powders, quartz powders, and metallic flakes to the existing clay coating—while ensuring process feasibility.
Third, to further reduce the shrinkage of the existing clay and improve its toughness and impact resistance, while ensuring the feasibility of the curing process, we can appropriately add low-shrinkage agents—such as thermoplastic plastic powders—to the existing clay.
Fourth, further enhance the temperature resistance of the existing putty. While maintaining its corrosion-resistant performance, use a vinyl ester resin with a high cross-linking density and exceptional high-temperature resistance. Additionally, consider incorporating high-temperature-resistant silicone resins or additives, especially for primer resins.