Basic knowledge of coating corrosion prevention

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Corrosion is a natural phenomenon, representing a return of materials from a high-energy state to a low-energy state; therefore, it is impossible to completely stop it. We can only control the corrosion rate by selecting appropriate materials and employing effective anti-corrosion measures, keeping it within acceptable and controllable limits. 1. Common methods for controlling corrosion include: 1.1 Proper material selection and structural design ； 1.2 Changing the environment (control and treatment of corrosive media, also known as process anti-corrosion measures) ； 1.3 Electrochemical protection. But we don’t have these three methods available under all conditions. In many cases, it is not possible to change the corrosive environment, nor do we have the conditions for electrochemical protection; structural choices alone cannot resolve the problem either. In such situations, choosing appropriate materials is often the most effective and convenient solution. The selection of anti-corrosion materials is sometimes influenced by the properties of the materials themselves; for example, when manufacturing equipment, metal materials with high strength, good temperature resistance, and excellent machinability are often required. However, metal materials often do not have corrosion resistance that meets the requirements, or materials with good overall properties are too expensive. At this point, people came up with a more effective method: covering the surface of the material that needed protection with a more corrosion-resistant material to form a protective layer. By leveraging the corrosion-resistant properties and barrier effect of this layer, the problem of corrosion could be addressed, and it was not necessary to invest more money in choosing materials with better overall performance in order to effectively control corrosion. 2. The protective effect of the anti-corrosion layer on the substrate is manifested in the following three aspects: 2.1 Isolation effect: It isolates the substrate from corrosive agents, thereby achieving anti-corrosion purposes ； 2.2 Corrosion inhibition: By reacting the internal components of the coating with the metal, a passivating or protective film is formed, thereby enhancing the protective effect of the anti-corrosion layer ； 2.3 Electrochemical protection: Adding active metal materials to the coating serves as a sacrificial anode to slow down corrosion. 3. The methods for corrosion protection using coating technologies include the following categories: 3.1 Coating-based corrosion protection ； 3.2 Glass fiber reinforced plastic lining and tape anti-corrosion ； 3.3 Rubber or plastic lining ； 3.4 Corrosion-resistant metal lining ； 3.5 Metal spray coatings, plating layers ； 3.6 Enameled or glass-lined. 4. The anti-corrosion layer shall possess the following properties: 4.1 Stable resistance to chemical media (including good water resistance) ； 4.2 It possesses sufficient mechanical properties such as impact resistance, bending resistance, and wear resistance ； 4.3 For corrosion protection under normal corrosion conditions, sufficient electrical insulation is required ； 4.4 Good adhesion and bonding strength to the substrate ； 4.5 It has good resistance to cathodic delamination, as well as decent resistance to aging and temperature effects ； 4.6 The coating shall not have an adverse effect on the base material and the media it comes into contact with, nor shall it pose any harm to humans or the environment during application ； 4.7 Good storage and construction stability ； 4.8 Easy to repair. Due to its advantages such as simple application, wide applicability (suitable for use in environments like the atmosphere, water bodies, general liquids, and soil for corrosion protection), ease of on-site application, suitability for large areas and equipment with complex shapes, relatively low overall cost for corrosion protection, short coating time, and the ability to achieve good corrosion protection results, this coating technology is the most widely used among various corrosion protection methods. However, coating anti-corrosion also has some shortcomings: it has a thin thickness, weak resistance to mechanical impact, a limited service life, restricted temperature tolerance, and is not suitable for highly corrosive environments. Therefore, today we will focus on introducing the relevant knowledge in this area. 5. Classification and characteristics of common anti-corrosion coatings 5.1 Definition and interpretation of coatings Definition: A series of liquid or solid materials applied to the surface of objects to form a solid coating layer that provides protection, decoration, or special functions such as insulation, corrosion resistance, or marking. Among them, liquid coatings account for the vast majority. Solid coatings are a new type of coating that has seen rapid development in recent years; they feature environmental friendliness, a short application time, and excellent performance, and their scope of use is continuously expanding. In liquid coatings, organic coatings hold an absolute advantage in the coating industry due to their excellent properties and good performance. Since the earliest paints were mostly made from plant oils and natural resins as their main ingredients, they were also called varnishes. However, with the advancement of technology, more and more coatings are synthesized directly from chemical raw materials; they have nothing to do with \"oil\" at all. Moreover, many of these coatings are water-soluble or solid forms, so the term \"paint\" can no longer represent all types of coatings. 5.2 Basic Components of Paints Regardless of the type or properties of a paint, it is composed of three or four basic components, whose functions are shown in the table below: Basic Component Typical Examples Main Function Film-forming substance (also known as binder) Plant oils (such as tung oil), natural resins, various synthetic resins (such as epoxy resins, polyurethane resins) – These substances enable the paint to form a continuous solid film that adheres firmly to the substrate; they constitute the basis of the paint and determine its fundamental characteristics. Dispersing medium Volatile organic solvents (such as aromatics, esters, etc.), water – These substances help disperse the film-forming substance, resulting in a viscous liquid, and they contribute to improving the paint’s applicability and certain properties of the resulting film. The dispersion medium alone cannot form a film. Pigments and fillers (extenders) such as titanium white, chrome yellow, chrome green, iron red, talc, calcium sulfate, zinc powder, and aluminum powder are used to color the material and improve the properties of the coating, enhancing its protective, decorative, and anti-corrosive functions while also helping to reduce costs. Additives such as drying agents, levelers, anti-caking agents, curing agents, and plasticizers are used to improve the properties of coatings in terms of production, application, storage, and use. 5.3 Requirements for anti-corrosion coatings 5.3.1 Good adhesion is necessary; the adhesion of coatings, especially their wet adhesion, enables them to displace water and oxygen at the interface, thereby enhancing the protective capabilities of the coating layer. Therefore, coatings with good adhesion generally have better corrosion resistance. Anticorrosive coatings used as primers often require stronger adhesion. Therefore, epoxy and alkyd coatings are often used separately as primers. 5.3.2 Strong resistance to permeation: A high level of resistance to permeation can effectively prevent the penetration of corrosive agents. In addition to being related to the inherent properties of the coating resin itself, the resistance to penetration is often improved by adding fillers such as mica and iron oxide to enhance its performance. 5.3.3 Stability against corrosive media requires not only that the film-forming components of the coating be stable, but also that the additives and fillers incorporated be stable as well, so as to avoid chemical reactions or physical changes with the medium. 5.4 Classification Methods of Coatings There are thousands of different coating products, and various classification methods exist. Common classifications include: 5.4.1 By the presence of pigments: there are clear coats and colored coats. 5.4.2 By form: water-based coatings, solvent-based coatings, solid powder coatings, high-solid-content coatings. 5.4.3 By application: architectural paint, wood paint, marine paint, water tank paint, etc. 5.4.4 Classified by construction sequence: primer, putty, intermediate coat, topcoat, and sealant. 5.4.5 Classified by performance: insulating paint, anti-rust paint, anti-fouling paint, anti-corrosion paint, and fire-retardant paint. 5.4.6 Classified by the main film-forming substance of the coating: natural resin coatings, asphalt coatings, phenolic coatings, epoxy coatings, and polyurethane coatings. 5.5 Performance characteristics, applications, and selection principles of common anti-corrosion coatings 5.5.1 Depending on the main film-forming substance, anti-corrosion coatings vary in terms of their anti-corrosion performance, applicable conditions, application properties, service life, and price level. Therefore, it is necessary to have a certain understanding of the properties of various coatings before selecting one. The table below shows the properties and main applications of commonly used synthetic resin coatings:

| Main film-forming substance | Film properties | Main applications |
|--------------------------|-----------------|------------------|
| Alkyd resin | Glossy, weather-resistant, strong adhesion; soft texture, poor water and alkali resistance | Buildings, vehicles, machinery, ships, light industrial products |
| Amino resin | Glossy, good light and color retention, hard; requires heat curing | Vehicles, machinery, mechanical products |
| Vinyl chloride resin | Weather-resistant, corrosion-resistant, water- and oil-resistant, flame-retardant; poor adhesion, poor temperature resistance, low solid content, thin film thickness | Chemical equipment |
| Vinyl resin | Light color, good flexibility, corrosion-resistant; low solid content, thin film thickness | Cement, chemical equipment |
| Acrylic resin | Glossy, light color, good light and color retention, strong weather resistance | Building exteriors, cars, machinery, light industrial products |
| Polyester | Good flexibility, wear-resistant; difficult to apply | Wood products |
| Epoxy resin | Durable, wear-resistant, excellent adhesion, corrosion-resistant; dark color, poor weather resistance | Chemical equipment |
| Polyurethane resin | Durable, corrosion-resistant, good insulation; dark color, poor weather resistance | Aircraft, ships, chemical equipment, wood products |
| Silicone resin | Weather-resistant, high-temperature resistant, good insulation; poor adhesion, resistant to gasoline, brittle | Coating of electrical materials, heat-resistant equipment |
| Inorganic polymers | High-temperature resistant, flame-retardant | Ships, bridges |

5.5.2 Each type of coating has different performance characteristics, which in turn determine the environments in which it can be used. The table below provides a recommended guide for selecting coatings in common corrosive environments in the petrochemical industry. Recommended types of coatings for corrosive medium environments, along with recommended thicknesses in μm and examples of their use in practice:
For general atmospheric conditions: Phenolic resin coatings, asphalt coatings, alkyd resin coatings – thickness ≥120 μm. Suitable for equipment in power plant boiler rooms, oil refining facilities, fertilizer production plants, and other dry indoor environments.

For chemical industry environments: Asphalt coatings, vinyl chloride coatings, vinyl resin coatings, epoxy resin coatings, polyurethane coatings, polyester coatings, inorganic zinc-rich coatings, chlorosulfonated polyethylene coatings, neoprene coatings, highly chlorinated polyethylene – thickness ≥150 μm. Appropriate for various production units in oil refining, fertilizer production, textile manufacturing, and chemical plants.

In the presence of acidic gases and acid mist: Vinyl chloride coatings, vinyl resin coatings, epoxy resin coatings (used only as primers and intermediate coats), chlorosulfonated polyethylene coatings, neoprene coatings, polyurethane coatings, special cyanate-based coatings – thickness ≥200 μm.

For fiber oxidation processes and melamine production in chemical plants, as well as in power plant water treatment systems: Vinyl chloride coatings, vinyl resin coatings, epoxy resin coatings, silicone coatings, chlorosulfonated polyethylene coatings, neoprene coatings – thickness ≥150 μm.

In environments with acids, alkalis, and salts: Phenolic resin coatings, vinyl chloride coatings, vinyl resin coatings, epoxy resin coatings, polyurethane coatings, inorganic zinc-rich coatings, chlorosulfonated polyethylene coatings, neoprene coatings, special cyanate-based coatings – thickness ≥250 μm.

For oil refinery wastewater: Oil-resistant coatings such as epoxy resin coatings and polyurethane coatings (anti-static variants must be used when applied to the inner walls of storage tanks for refined oil) – thickness ≥250 μm.

For the inner walls of oil storage tanks in oil refining: Water- and moisture-resistant coatings such as phenolic resin coatings, asphalt coatings, vinyl chloride coatings, vinyl resin coatings, epoxy resin coatings, polyurethane coatings (including special cyanate-based coatings), chlorosulfonated polyethylene coatings, neoprene coatings, SBS waterproof membranes (for roofs), chloroprene waterproof membranes (for roofs), chlorosulfonated polyethylene, epoxy, polyurethane, phenolic coatings – thickness ≥200 μm ； Asphalt paint ≥300 ； For SBS waterproofing membranes, those with a polyester substrate and a sand-coated surface layer having a thickness of more than 4 mm should be selected. Circulating water systems, cooling towers, ponds, and factory rooftops: When the temperature tolerance is ≤100°C, use phenolic resin coatings, alkyd resin coatings, epoxy resin coatings, polyurethane coatings, or inorganic zinc-rich coatings. For various types of equipment and pipelines when the temperature is between 100°C and 200°C, use epoxy resin coatings or inorganic zinc-rich coatings. For heating furnaces, equipment, flue gas pipes, and chimneys when the temperature is above 200°C, use silicone coatings with a temperature tolerance of ≥60°C. For gasification furnaces, high-temperature reactors, and regenerators, use specialized coatings such as TH847, DH22, and JST-2L. For heat exchangers, use coatings with a temperature tolerance of 110–160°C. For carbon steel cooling coil tubes in factories, use asphalt glass cloth, coal tar asphalt glass cloth, epoxy coal tar glass cloth, polyethylene tape, or epoxy powder coatings with a thickness of 2–7 mm. For various types of buried pipelines, use asphalt glass cloth, coal tar asphalt glass cloth, epoxy coal tar glass cloth, polyethylene tape, or epoxy powder coatings. For equipment in rooms and other dry environments, use chemical-resistant asphalt coatings, vinyl chloride coatings, vinyl resin coatings, epoxy resin coatings, polyurethane coatings, polyester coatings, inorganic zinc-rich coatings, chlorosulfonated polyethylene coatings, chlorinated rubber coatings, or highly chlorinated polyethylene with a temperature tolerance of ≥150°C. For production units in oil refining, fertilizer, and chemical industries, where there are acidic gases and acid mists, use vinyl chloride coatings, vinyl resin coatings, epoxy resin coatings (only for primers and intermediate coats), chlorosulfonated polyethylene coatings, chlorinated rubber coatings, polyurethane, or special cyanate-based coatings with a temperature tolerance of ≥200°C. For fiber oxidation processes and melamine production in chemical plants, as well as in power plant water treatment facilities in alkaline environments, use vinyl chloride coatings, vinyl resin coatings, epoxy resin coatings, silicone coatings, chlorosulfonated polyethylene coatings, chlorinated rubber coatings, with a temperature tolerance of ≥150°C. For fertilizer synthesis and urea production, use phenolic resin coatings, vinyl chloride coatings, vinyl resin coatings, epoxy resin coatings, polyurethane coatings, inorganic zinc-rich coatings, chlorosulfonated polyethylene coatings, chlorinated rubber coatings, or special cyanate-based coatings with a temperature tolerance of ≥250°C. For oil refining wastewater, use oil-resistant coatings such as epoxy resin coatings and polyurethane coatings (antistatic varieties must be used for the inner walls of storage tanks for refined oil). For the inner walls of oil storage tanks in oil refining, use water- and moisture-resistant coatings such as phenolic resin coatings, asphalt coatings, vinyl chloride coatings, vinyl resin coatings, epoxy resin coatings, polyurethane coatings (including special cyanate-based coatings), chlorosulfonated polyethylene coatings, chlorinated rubber coatings, SBS waterproof membranes (for roofs), and chloroprene waterproof membranes (for roofs). Use chlorosulfonated polyethylene, epoxy, polyurethane, and phenolic coatings with a temperature tolerance of ≥200°C ； Asphalt paint ≥300 ； For SBS waterproofing membranes, those with a polyester substrate and a sand-coated surface layer having a thickness of more than 4 mm should be selected. For circulating water systems, cooling towers, ponds, and factory rooftops, when the temperature tolerance is ≤100°C: phenolic resin coatings, alkyd resin coatings, epoxy resin coatings, polyurethane coatings, and inorganic zinc-rich coatings are suitable. For various types of equipment and pipelines, when the temperature range is 100°C–200°C: epoxy resin coatings and inorganic zinc-rich coatings are appropriate. For heating furnaces, equipment, flue gas pipes, and chimneys, when the temperature exceeds 200°C: silicone coatings are suitable. For gasification furnaces, high-temperature reactors, and regenerators, special coatings such as TH847, DH22, and JST-2L are used. Special coatings for heat exchangers are suitable for temperatures ranging from 110°C to 160°C. For carbon steel tube bundles in factories, as well as for buried pipelines, materials such as asphalt glass cloth, coal tar asphalt glass cloth, epoxy coal tar glass cloth, polyethylene tape, and epoxy powder coatings are used; the thickness of these coatings ranges from 2 mm to 7 mm. When selecting anti-corrosion coatings, it is necessary to take into account the operating environment and construction conditions. For example, when protecting buried pipelines from corrosion, asphalt-based coatings with excellent water resistance should be given priority ； In highly corrosive environments, anti-corrosion coatings with high viscosity and a large film thickness per application should be used ； Volatile, fast-drying coatings such as perchloroethylene and nitro paints are not suitable for spraying applications ； Chlorosulfonated polyethylene is not suitable for application using the back-and-forth rolling method. 5.5.4 When selecting anti-corrosion coatings, their compatibility must also be considered. There should be good compatibility between the primer and the substrate; moreover, no mutual dissolution or undercutting should occur between the primer and the topcoat. For example: on concrete surfaces, chlorosulfonated polyethylene can be used as both a primer and a topcoat, but it is not suitable as a primer on steel surfaces ； Epoxy tree paint can be used as a primer in most cases, but it is not suitable as a topcoat in outdoor environments, as epoxy coatings have poor resistance to ultraviolet rays ； Coatings with good corrosion resistance but poor adhesion (such as vinyl chloride) can generally only be used as topcoats; for primers, coatings with excellent adhesion must be chosen (epoxy and alkyd types). 5.5.5 The selection of coating types should also take comprehensive economic factors into account. Sometimes, low-cost coatings with a short lifespan are not economical, as the cost of carrying out re-coating procedures in certain situations can be much higher than the difference in coating costs. Generally, when designing anti-corrosion coatings for bridges, transmission towers, large-scale structures, etc., a lifespan of 10 to 20 years or more is required, while for ordinary chemical equipment, 5 years or more is sufficient. In highly corrosive environments, it is difficult to achieve a long service life for conventional anti-corrosion coatings, mainly due to insufficient technical reliability and high economic costs. 5.6 Key points for controlling paint application: Once the type of paint is determined, the quality of the coating is the critical factor that determines its corrosion resistance and lifespan. Among the various factors that affect coating quality, surface treatment of the substrate and control of conditions during the coating application process (including ratio, thickness, pinholes, temperature, and humidity) account for 80% of the impact on coating quality. Therefore, to control the corrosion resistance and lifespan of coatings, it is essential to pay close attention to the following key aspects: 5.6.1 Surface treatment before coating application. The purpose of surface treatment is to ensure that the substrate surface achieves the necessary level of cleanliness, along with an appropriate degree of porosity and roughness. Reaching the necessary level of cleanliness involves removing rust, oxide scale, oil, dust, water, and other contaminants from the surface of the substrate, in order to ensure proper adhesion between the coating and the substrate and to prevent impurities from causing pores and pinholes. A certain degree of porosity and roughness can improve the adhesion of the coating. However, excessive roughness can lead to insufficient local thickness, making pinholes more likely to occur, and it also increases the amount of paint used. Surface treatment methods include mechanical cleaning (including sandblasting, high-pressure water jetting, power tools, hand tools, etc.), chemical cleaning (acid washing, alkali washing), solvent cleaning, electrochemical treatment, flame cleaning, etc. A choice should be made based on different circumstances. In daily construction, mechanical cleaning is the most widely used method, with sandblasting yielding the best results. Current domestic surface treatment standards have established grades for mechanical cleaning, including four levels for tool-based rust removal: St0, St1, St2, and St3, as well as five levels for spray rust removal: Sa0, Sa1, Sa2, Sa2.5, and Sa3. Generally, the corrosion resistance of steel structures should reach at least St3 level, and sometimes it needs to reach Sa2.5 level. The internal anti-corrosion treatment of storage tanks, as well as the use of rubber linings, fiberglass-reinforced plastics, metal spraying, aluminum infiltration, etc., should meet the Sa2.5 and Sa3 standards. The surface of the treated substrate must be approved by both the construction party and the user party before further construction can proceed. The primer must be applied within 8 hours after successful general acceptance. A surface that has been properly treated must have a certain level of roughness, and it must be free of water, oil, and any loose old coatings. 5.6.2 Selection of a suitable coating method: Common methods for applying anti-corrosion coatings include brushing, rolling, dipping, pouring, air spraying, and airless spraying. Each method has its own characteristics and scope of application, and the choice should be made based on the site conditions (including temperature, humidity, and wind speed) as well as the type of coating. Generally, brushing and rolling are suitable for anti-corrosion applications under most conditions. Brush coating is relatively flexible and suitable for various complex areas, while roller coating allows for a faster application speed. Dip coating and spray coating are mainly suitable for the corrosion protection of workpieces and small equipment in workshops. Spraying is suitable for large-area applications, but it results in high material loss; when used on vertical surfaces, the thickness is uneven and the material tends to flow. 5.6.3 Environmental requirements for applying anti-corrosion coatings: Most coating applications require a temperature of above 5°C. Too low a temperature makes drying and curing difficult, and the adhesion of the paint film is poor as well. Construction cannot be carried out in conditions of wind, rain, snow, or when humidity is above 70%. Furthermore, the surface of the treated and qualified substrate must be coated with primer promptly; otherwise, contamination or moisture exposure will **affect the anti-corrosion effect**. 5.6.4 Requirements for primer adhesion: In key anti-corrosion projects, it is often necessary to test the adhesion of the primer. Generally, there are the laboratory circle-drawing method and the on-site grid-drawing method. The on-site grid method better reflects the actual corrosion resistance quality. The method involves using a set of specialized grid-cutting blades to make two cuts vertically, ensuring that the paint film is completely pierced, and then using a soft brush to gently remove the pieces of paint film that have been cut off from the small squares thus created. The more small squares are retained, the better the adhesion. 5.6.5 Number of coating layers: During the application process, the coating is inevitably prone to pinholes and defects. Therefore, when the total thickness is the same, the coating obtained through multi-layer application tends to be more dense. Anti-corrosion coatings used in critical environments generally require no less than 5 application layers. When managing construction, it is common to encounter situations where the construction party applies the coating twice before notifying the client’s staff to conduct an inspection. It’s difficult to determine at this point whether all areas have been brushed twice enough. A relatively simple control method is to require that each application by the construction team use a different color, which makes it easy to identify where the required number of applications has not been carried out. Another approach is to require the construction party to apply the coating only once per application, and to make small marks in hidden areas and on parts where it’s easy to miss applying the coating; if those marks are still visible, then the construction party should naturally be punished. 5.6.6 Coating thickness: Generally speaking, anti-corrosion coatings must reach a certain thickness in order to provide effective anti-corrosion protection. The basic requirement is that the total thickness of the paint film be at least 2 times the roughness. Furthermore, a thicker thickness is required in highly corrosive environments. Generally, under atmospheric corrosion conditions, the total thickness of the paint film should be greater than 150 μm, while in a liquid-phase environment it is usually around 250–300 μm. However, the coating thickness is also related to the type of paint; epoxy paints with a thickness of 110 μm have better impermeability properties than asphalt-based paints with a thickness of 1 mm. After the anti-corrosion treatment is completed, a thickness test must be carried out using instruments. Human senses are unable to determine whether the coating thickness meets the standard. Construction companies often use a wet film thickness gauge to accurately control the final thickness. There is a certain quantity requirement for conducting the final thickness test. The testing ratio is as follows: test points are selected uniformly at random, with one test point taken every 20 meters along the pipeline, and no less than 3 tests must be conducted on each pipeline ； One measurement point should be taken for every 10 square meters of area, with no less than 3 measurement points per device. The acceptable standard for the thickness of the anti-corrosion coating is that 90% or more of the measured points should reach the specified thickness (the specified thickness is determined according to the design drawings or in accordance with the requirements in the appendix to these rules; the thickness of any remaining old coating must be removed). The minimum thickness of those points that do not reach the specified thickness shall not be less than 80% of the specified value. Those that fail to meet the standard requirements must undergo additional testing, and any deficiencies must be remedied by repainting. If more than 20% of the test points do not reach the specified thickness, they must all be repainted and retested. 5.6.7 Detection of pinholes. For some critical anti-corrosion coatings, pinhole testing is also required to ensure the reliability of the anti-corrosion layer. For example, pinhole testing is necessary for the corrosion protection of heat exchanger coatings. There are two methods: the saline conductivity method and the high-voltage spark test method. The testing voltage must be determined based on the coating thickness. 5.6.8 When applying anti-corrosion coatings, there must be a certain interval between each layer. Generally, the next layer can be applied once the underlying coating has dried on the surface. For coatings such as chlorosulfonated polyethylene and special cyanic cement, it is required to wait until they are completely dry before applying the next layer. 5.7 Common Defects in Anti-corrosion Coatings and Corresponding Remedies Causes of common defects and treatment measures Poor adhesion, not meeting requirements ① Improper surface treatment method ； ②The surface treatment quality does not meet the requirements ； ③Improper rust removal methods result in surface polishing ； ④Apply on the contaminated surface on wet surfaces ； ⑤The painting interval is too short. ①Use reliable rust removal methods such as sandblasting, replace manual rust removal with mechanical tools, and employ coarse sandpaper and hard wire brushes ； ②Clarify the rust removal grade and the acceptance standards to the construction workers ； ③Choose different rust removal methods depending on the substrate, and use degreasing cleaners if necessary ； ④Properly extend the interval between applications. Slow curing speed or no curing① Insufficient amount of curing agent used ； ②The curing agent has expired or become ineffective ； ③The temperature and humidity during construction were not within the allowable range. ①Replace the specialized curing agent or check its effectiveness ； ②Check whether the amount of curing agent is appropriate for the on-site temperature ； ③Proceed with construction only after the ambient temperature meets the requirements. Coating delamination and bubbling: ① The base coat has not fully cured, or the upper and lower coats are not compatible ； ②The paint bucket is wet, or the surface of the base layer is damp ； ③The diluent is too strong. ①Apply the next layer only after the primer has fully cured ； ②Remove water and moisture to ensure dryness ； ③Choose the appropriate diluent. The coating is uneven, with small bumps and inconsistent gloss; the coating mixture was not mixed thoroughly enough. Stir thoroughly; the coating thickness is uneven and the painting marks are visible. ①The painting method was not chosen properly ； ②Too much force was applied when applying it ； ③The thickness of a single layer is too large during construction. ①To ensure a uniform coating, the coating method should be selected in the order of spraying, brushing, rolling, and dipping ； ②Reduce the application force or dilute the paint appropriately ； ③Check whether the construction sequence is reasonable; it is necessary to ensure that painting is done from top to bottom, and that corners and recesses are painted separately first. The application direction between the two coats of paint should be perpendicular. Cracking and wrinkling on the coating surface ① Poor compatibility among the base coat, intermediate coat, and top coat during painting ； ②Too short a painting interval ； ③It was painted too thickly or exposed to sunlight after painting. ①Change the type of coating and select an appropriate supporting solution ； ②Properly extend the application interval ； ③Appropriately increase the diluent ； ④Protect from direct sunlight. Exposed bottom or sagging; oil stains on the surface① ； ②The previous coating does not match the next one ； ③The coating is too thick, the paint is too thin, or the spray gun is not used properly. ①Remove oil stains using a solvent or degreaser ； ②Check the compatibility of the coatings ； ③Properly dilute with a diluent or inspect the spray gun ； ④Stir thoroughly to prevent over-dilution. The paint becomes cloudy① due to an inappropriate choice of thinner ； ②Hygroscopic, contains moisture ； ③The reaction is incomplete during the manufacturing process. ①Replace the solvent ； ②Filter ； ③Back-reaction. Rust appears some time after painting ① Inadequate rust removal ； ②The primer was not applied in time after rust removal ； ③The thickness is insufficient. ①Thoroughly remove rust to meet the specified standards ； ②Apply the primer promptly after rust removal is approved ； ③Ensure the thickness and select a paint type with good corrosion resistance. 6. Service life of coatings: Generally speaking, coatings have a certain service life for corrosion protection. Factors affecting it include design service life, type of coating, rust removal method, thickness, and operating environment. The factors that generally have a significant impact on the service life include the type and quality of the coating, control during application (primarily proper rust removal, correct mixing ratios, absence of oil, water, and dust, as well as an appropriate thickness), and factors such as temperature, humidity, environmental corrosivity, and ultraviolet radiation during subsequent use. Generally, the corrosion resistance period for outdoor steel structures should be at least three years, with most lasting even longer than that. For structures such as power transmission towers that are difficult to modify after being put into use, the design corrosion protection period should generally be at least 10 to 15 years. The anti-corrosion coating inside the tank is generally required to last for more than 6 years. The service life of the anti-corrosion coating for buried pipelines is generally required to be over 30 years.