Common materials used in anti-corrosion engineering

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Commonly used materials in anti-corrosion engineering. I. Coatings (1) Functions of coatings 1. Protective function Coatings are essentially organic coatings. By using a mixture of organic polymer colloids applied to the surface of the object to be coated, a coating layer is formed that isolates the object from industrial air, moisture, sunlight, soil, and other corrosive substances outside, protecting it from erosion and thereby extending its service life. Painting the exterior of equipment and pipelines is one of the important corrosion prevention measures in petrochemical enterprises. Equipment, pipes, supports, platforms, railings, ladders, etc., made of carbon steel, low-alloy steel, or cast iron, should generally be painted for corrosion protection. Non-ferrous metals such as aluminum, copper, and lead, austenitic stainless steel, galvanized surfaces, and plastic surfaces are not painted. 2. Function of color markings: To enhance production management, facilitate operation and maintenance, promote safe production, and improve the appearance of the plant, various pipes and equipment in petrochemical facilities are painted with different colors as markings, to help people identify the fluids flowing within them. For example, water pipes are painted green, steam pipes are painted red, ammonia pipes are painted yellow, and air and oxygen pipes are painted blue, etc. Also, the surface color coding for petrochemical equipment includes painting towers, vessels, heat exchangers, reactors, and storage tanks in silver, while compressors and centrifuges are painted in light green. 3. Special functional effects: By applying coatings, it is possible to adjust the thermal and electrical conductivity of objects, provide electrical insulation, prevent the attachment of microorganisms, regulate the emission of sound waves as well as the reflection and absorption of light, and achieve properties such as luminescence and marker identification. (II) Basic components of coatings Coatings consist of paints and anti-corrosion coatings. Before the advent of synthetic resins, coatings made from plant oils and natural lacquers were commonly referred to as paint. Coatings prepared using synthetic resin as a raw material are known as anti-corrosion coatings. Generally, anti-corrosive coatings have better anti-corrosion properties than paints, and they are also more expensive. Paint generally consists of two components: non-volatile and volatile components. After being applied to a surface, the volatile components gradually evaporate, leaving behind the non-volatile components that form a dried film. Therefore, the film-forming substance of the non-volatile components is simply referred to as the solid content of the coating. Film-forming substances can be further divided into: ——primary film-forming substances ； ——Secondary film-forming substances ； ——Auxiliary film-forming substance. 1. Main film-forming substances: The main film-forming substances consist of oils and resins. (1) Oils and fats: Oils and fats are products of nature, derived from plant seeds and animal fats. The drying and curing reaction of oils is primarily due to the polymerization of oxygen in the air with the unsaturated double bonds in the oils. The properties of natural oils, particularly their corrosion resistance and anti-aging capabilities, are inferior to those of many synthetic resins; as a result, they are rarely used alone as anti-corrosion coatings. However, they can act as rust inhibitors for metals when combined with certain metal oxides or metal soaps. Therefore, oils can be used to modify various synthetic resins in order to produce matching anti-rust primers. (2) Resins: Resins are divided into two categories: natural resins and synthetic resins. Natural resins include raw lacquer, natural rubber, etc. Synthetic resins include epoxy resins, phenolic resins, furan resins, polyester resins, polyurethane resins, vinyl resins, vinyl chloride resins, fluorine-containing resins, and others. The resins mentioned above are all the main film-forming substances in common corrosion-resistant coatings. 2. Secondary film-forming substances: Pigments are the secondary film-forming substances in coatings, and they are basically classified into three categories: coloring pigments, rust-inhibiting pigments, and extender pigments (also known as fillers). (1) Pigments: They are primarily used to give coatings color, provide decorative effects, increase the thickness of the coating film, and enhance the durability of the coating. Common coloring pigments include zinc white, carbon black, zinc yellow, etc. (2) Rust-inhibiting pigments: mainly used in primers to provide rust prevention. Based on their anti-corrosion mechanisms, they can be classified as chemical anti-corrosion pigments, such as red lead, zinc chromate yellow, zinc powder, and zinc phosphate. These pigments exert their anti-corrosive effects in coatings through chemical or electrochemical processes ； Another category consists of physical rust-inhibiting pigments, such as aluminum powder, mica iron oxide, zinc oxide, graphite powder, etc. Their main function is to increase the density of the paint film, reduce its permeability, and prevent sunlight and moisture from penetrating, thereby enhancing the rust-resistant properties of the coating. (3) Extending pigments: Also known as fillers, extending pigments serve to increase the thickness of the coating layer, thereby enhancing the coating’s weather resistance, impermeability, wear resistance, durability, and physical-mechanical strength. Commonly used ones include talcum powder, calcium carbonate, barium sulfate, mica powder, etc. 3. Film-forming auxiliaries Film-forming auxiliaries consist of a solvent and auxiliary materials. (1) Solvent. Also known as a thinner (or diluent, volatile component), it is a highly volatile liquid used to dilute and dissolve film-forming substances, thereby changing the consistency of the paint to make it easier to apply. Solvents affect various properties of coatings, such as application resistance, leveling property, film formation speed, flowability, drying time, gelling behavior, wettability, and performance at low temperatures. Common solvents include turpentine, alcohol, rosin oil, butanol, and various mixed solvents. To obtain a good coating, choosing and using solvents correctly is equally important. (2) Auxiliary materials. To improve the performance of the coating and meet the requirements of application, the following auxiliary materials are often added to the coating: Plasticizers: also known as toughening agents, they are used to overcome the brittleness of the paint film, increase its toughness, and enhance the plasticity of the coating. Commonly used ones include organic compounds (phthalate esters) and polymer compounds (polyurethane resins), etc. Drying accelerator: The main function of a drying accelerator is to accelerate the drying of the paint film and shorten the time required. Curing agent: Coatings in which resin is the main component for forming a film; some of these can form a film by drying at room temperature or by heating. However, some require the addition of substances such as acids, amines, and peroxides as curing agents that react with the resin to allow the coating to dry and form a film. The auxiliary materials also include some surfactants, mildew inhibitors, ultraviolet absorbers, anti-fouling agents, etc. (III) Principles for selecting coatings Choosing the right coating is of great importance for the quality and service life of the paint film. When selecting a coating, the following points should be taken into account: 1. Compatibility with the operating conditions of the surface to be coated When selecting a coating, it is first necessary to determine its applicable range and the environmental conditions in which it will be used. For applications such as outdoor steel structures, equipment frames, and pipe frameworks, coating is applied primarily to prevent rusting and ensure good durability in outdoor conditions ； If the outdoor environment is one subject to chemical atmospheric corrosion, the coating selected should take into account the corrosion resistance of its paint film ； When painting outdoor concrete surfaces, the adhesion strength and durability of the paint film should be taken into consideration. 2. Adapt to the material properties of the surface to be coated. The same coating can have different effects on objects made of different materials. For example, oil-based anti-rust coatings suitable for steel surfaces, when used on concrete surfaces, will undergo saponification reactions with the alkaline substances present in concrete, resulting in the coating peeling off. 3. Proper matching of primer and topcoat: The compatibility of coatings refers to the fact that when using putty, primer, topcoat, and sealant as a composite layer, it is essential to pay close attention to which topcoat is suitable for a given primer, whether there is any reaction between the primer and the topcoat, and the level of adhesion, among other things. If the compatibility of coatings is not understood and primers, intermediate coats, and topcoats are used arbitrarily, it can lead to quality issues such as delamination, peeling, precipitation, and flaking of the paint film. 4. Economic Reasonableness: When selecting coatings, attention should be paid to their price as well as their service life. Coatings with low cost, long service life, easy application, and low or no toxicity should be selected. (IV) Application methods of paints. When applying paints, it is necessary to pay attention to the surface preparation of the object to be painted on one hand; on the other hand, an appropriate application method must be selected based on the type and properties of the paint, as well as the material and shape of the object being painted. The common methods for applying coatings are shown in Table 1. Table 1 Common Methods for Applying Coatings Category Application Method Equipment and Tools Used Manual Tools Brush application method Roller application method Aerosol spraying method Scratching application method Various brushes, roller brushes, aerosol paint cans, scrapers Power Tools Air spraying method Airless spraying method Thermal spraying method Tumbler application method Various spray guns, air compressors, airless spraying devices, paint heaters, rollers Machine Equipment Dip coating method Roller coating method Centrifugal coating method Submersion coating method Electrostatic spraying method Dip coating machines, roller coating machines, centrifugal coating machines, submersion coating machines, electrostatic spraying devices (V) Requirements for Coating Application 1. The coating should come with a quality certification from the manufacturer ； 2. Before painting, rust, slag, burrs, oil, water, and other contaminants on the surface of the object to be painted must be removed, so as to achieve the required rust removal level on the substrate ； 3. The primer for various types of anti-corrosion coatings should be applied by brushing, and the surface to be coated must be dry. The next coat of paint can be applied only after the previous coat has dried. A simple way to determine whether the paint film is fully dry is to press it firmly with your finger; if no fingerprint appears, then it is dry ； 4. When the quality inspection of equipment and pipeline welds is unsatisfactory, painting shall not be applied to the welds. Before applying the primer, special areas such as assembly marks, weld grooves, and threads should be protected to prevent them from being painted ； 5. Pressure vessels must be painted only after all heat treatment has been completed and the hydrostatic test has passed ； 6. Non-ferrous metal pipes, stainless steel pipes, galvanized steel pipes, as well as galvanized iron and aluminum sheets as protective coatings, should not be painted ； 7. Painting work should be carried out at an ambient temperature of 15–30°C; it should not be done outdoors in windy, sandy, rainy, or snowy conditions ； 8. Parts of the pipeline that are difficult to paint after installation should be painted in advance ； 9. The type and color of the coating, the number of application layers, and the markings shall comply with the provisions of the design documents ； 10. The quality of the coating shall meet the following requirements: (1) The coating shall be uniform, with consistent color ； (2) The paint film should be firmly attached, without defects such as peeling, wrinkles, bubbles, or pinholes ； (3) The coating should be intact, without any damage or sagging ； (4) The coating thickness shall comply with the specifications in the design documents ； (5) When applying the color rings, the spacing should be even and the width consistent. (VI) Painting procedure The general painting procedure is as follows: rust removal → applying the first coat of primer (by brushing or spraying) → inspection and touch-ups (applying putty) → applying the second coat of primer (by brushing or spraying) → inspection and touch-ups → (applying 1–2 coats of intermediate coat) → applying the topcoat (1–2 coats) → inspection and repairs → curing → (thermal curing treatment) → finished product. Note: ① The bracketed values in this procedure should be determined according to the design requirements. ②This procedure is applicable to single-component coatings; for two-component or multi-component coatings, a coating formulation step must also be added. (VII) Commonly used anti-corrosion coatings
Table 2: Commonly used anti-corrosion coatings

| Category | Model | Name | Main applications | Consumption per layer (g/m²) |
|----------|-------|------|-------------------|------------------------------|
| Oil-based paints | Y00-1 | Clear oil | Used to thin thick paints and prepare red lead anti-rust paint; can also be applied directly on metal or wood surfaces for anti-corrosion and rust prevention. | – |
| | Y53-1 | Red lead oil-based anti-rust paint | Used as a primer for anti-corrosion treatment on steel surfaces. | ≤100 |
| | Y02-1 | Thick paints in various colors | Suitable for construction projects where high-quality finish is not required; can also serve as a primer for wooden items. | 80 |
| | Y03-1 | Oil-based enamel paints in various colors | Used for protective and decorative purposes on outdoor metal, wooden objects, and buildings. White: ≤70; other colors: ≤60 |
| Alkyd paints | C01-1 | Alkyd varnish | Applicable to indoor and outdoor metal and wood surfaces; can also be used as a topcoat over alkyd enamel paints. | 35 |
| | C04-42 | Alkyd enamel paints in various colors | Used for coating outdoor metal surfaces. | 100 |
| | C06-1 | Iron-red alkyd primer | Serves as a primer for metal surfaces. | 90 |
| Asphalt paints | L01-6 | Asphalt paint | Provides moisture resistance, water resistance, and anti-corrosion protection on metal surfaces. | 268 |
| | L04-2 | Aluminum powder asphalt enamel | Used as a topcoat on indoor and outdoor steel surfaces. | 80 |
| | L44-1 | Aluminum powder asphalt ship-bottom paint | Prevents rust on steel surfaces submerged in saltwater or freshwater. | ≤145 |
| Phenolic resin paints | F53-1 | Red lead phenolic anti-rust paint | Acts as a primer for rust prevention on steel surfaces. | ≤100 |
| | Fo4-1 | Phenolic enamel paints in various colors | Used for interior and exterior decoration of metal and wood surfaces. | 97 |
| | F03-1 | Phenolic blending paints in various colors | Suitable for interior and exterior application on metal and wood surfaces; best used indoors. | 60–70 |
| | F50-1 | Phenolic acid-resistant paints in various colors | Protects metal surfaces exposed to acidic gases from corrosion. | 70 |
| Silicone paints | W61-25 | Silver-white silicone heat-resistant paint | Applied to steel surfaces of high-temperature equipment such as engine casings, steel chimneys, exhaust pipes, ovens, furnaces, and heating pipe casings; serves as a heat-resistant and anti-corrosion coating. | 20–35 |
| | W61-22 | Silicone heat-resistant paints in various colors | Used as heat-resistant coatings on surfaces of high-temperature equipment and pipelines. | – |
| | W61-24 | Olive-green silicone heat-resistant paint | Ideal for coating steel parts that must dry at normal temperatures while still being resistant to high temperatures. | – |
| Polyvinyl chloride paints | G01-5 | Polyvinyl chloride paint | Used as an anti-corrosion coating on chemical processing equipment and pipelines. | – |
| | G06-4 | Iron-red polyvinyl chloride primer | Functions as a primer for chemical anti-corrosion treatments and for metal surfaces. | 130–200 |
| | G52-1 | Polyvinyl chloride anti-corrosion paints in various colors | Specifically designed for chemical anti-corrosion; protects against chemical atmospheres, acids, and alkalis. | 125–175; maximum: 200 |
| Chlorosulfonated polyethylene paints | J52-81 | Chlorosulfonated polyethylene anti-corrosion primer | Widely used as an anti-corrosion primer for metal surfaces; can also be applied to cement surfaces. | 200–250 |
| | J52 | Chlorosulfonated polyethylene intermediate coat | Functions as an intermediate coating for metal structures, equipment, pipelines, and cement surfaces. | 170–300 |
| | J52-61 | Chlorosulfonated polyethylene anti-corrosion topcoat | Extensively used for protecting industrial buildings, cement walls, steel structures, equipment, pipelines, oil pipelines, and tank trucks from chemical atmospheric corrosion. | 150–200 |
| Epoxy resin paints | H01-2 | Epoxy-phenolic varnish | Applied to prevent corrosion on the inner walls of chemical pipelines, tanks, and vessels. | 60–70 |
| | H01-4 | Epoxy-asphalt varnish | Provides moisture resistance and chemical corrosion protection for underground and underwater pipelines, sluice gates, and storage tanks. | 100 |
| | H52-3 | Epoxy anti-corrosion paints in various colors | Used to protect metal structures and equipment from chemical corrosion. | – |
| Polyurethane paints | S01-2 | Polyurethane varnish | Used in urea granulation towers; can also serve as an anti-corrosion coating for metals. | 100 |
| | SQS52-1 | Polyurethane anti-corrosion paint | Effective for preventing corrosion and moisture damage on chemical plants, equipment, and underground pipelines. | – |
| | Polyurethane-asphalt varnish | Suitable for use in marine, petroleum, and chemical environments. | 100 |
| | Polyurethane-asphalt primer | Provides heat resistance and anti-corrosion protection for high-temperature equipment and pipelines. | 125 |
| Other paints | E06-1 | Inorganic zinc-rich primer | Applied to black metal equipment and pipelines to provide oil resistance, water resistance, and protection against atmospheric corrosion; also used as a heat-resistant anti-corrosion coating on chimneys, etc. | 80–100 |

II. Spraying (coating) materials
Based on the type of material used, these can be categorized into two types: metal spraying and non-metal spraying (powder coating). (1) Metal spraying 1. Principle of the spraying process Metal spraying involves melting metal wire or metal powder materials at high temperatures, and then using a stream of compressed air or inert gas to turn them into a mist, which is then rapidly sprayed onto the surface of the substrate to form a metal coating; this process is known as metal spraying. Its working principle is as follows: The metal material used in melt spraying is melted at high temperatures and then sprayed in the form of mist. These tiny mist particles are in a semi-molten state at the moment they are ejected. As these semi-molten metal particles continuously collide with the surface of an object, they immediately deform, flatten out, and then cool down rapidly, adhering firmly to the surface of the metal object and thus forming a metal coating. Based on the different melting processes of melt-blown materials, it is further divided into flame spraying, air jet spraying, arc spraying, plasma spraying, etc. 2. Spraying applications Spraying is a surface engineering technique characterized by high quality, efficiency, low consumption, low pollution, and ease of production. It can endow the substrate surface with precise, anti-friction, wear-resistant, corrosion-resistant, high-temperature resistant, fatigue-resistant, radiation-proof, conductive, magnetic, and various other special properties. The coating thickness is generally several micrometers to several millimeters, which is only a few hundredths to a few tenths of the size of the substrate; yet this results in a coating layer that possesses higher wear resistance, corrosion resistance, and heat resistance compared to the substrate material. Therefore, even if high-performance, expensive and rare elements are used for the coating material, it will not significantly increase costs, resulting in good economic benefits. Due to its excellent corrosion resistance and mechanical wear resistance, metal coatings are widely used in domestic and international corrosion protection and machinery industries, such as in the repair and processing of mechanical wear on crankshafts and components with complex structures. Currently, zinc spraying and aluminum spraying processes are widely used in projects such as petrochemical plants, power stations, reservoirs, and sluices to achieve corrosion protection. 3. Characteristics of spraying: (1) It does not alter the physical and chemical properties of the object being sprayed: A metal coating represents a combination of mechanical properties. Under normal operating conditions, for melt-spun metal materials, the time from melting to the formation of the coating is extremely short, and the cooling rate is very fast; as a result, the surface temperature of the object being sprayed remains around 80°C. At this temperature, the shape of the metal structure and its microstructure do not change. Therefore, the metal spraying process is an ideal technique for mechanical processing and the repair of mechanical wear on complex structures. (2) Melt-blown materials are not restricted by weldability: Melt-blown materials can be metallic or non-metallic, and the objects to be coated can also be metallic or non-metallic; thus, using the spraying process can help save a large amount of precious metals or metals in general ； Moreover, it has a low cost and short cycle for repairing mechanical wear, and improves wear resistance. It can improve the corrosion resistance of stationary equipment or metal structures. (3) Porosity of the coating: The metal particles in the metal coating are not only arranged irregularly but there are also gaps between them. This is because, during the spraying process, as the molten metal particles are ejected onto the surface of the object, their surfaces oxidize in contact with air to form oxides or nitrides, resulting in the porosity of the coating. The porosity of the coating affects its impermeability; however, the porous structure allows for the retention of oil, which in turn improves its mechanical wear resistance. (4) The utilization rate of melt-blown material is low: During the spraying process, the melt-blown material melts and is sprayed in the form of a mist onto the surface of the object to be coated; inevitably, some of the particles splash off and are lost, resulting in losses. The utilization rate of gas-sprayed metal is generally 70% to 90%. 4. Common spraying materials: The metal materials commonly used for spraying include zinc, aluminum, tin, copper, stainless steel, etc. (II) Powder coating: Plastic spraying technology has become a popular field in China nowadays, with its applications in corrosion prevention and decoration gradually expanding. 1. Principle of powder coating The working principle of plastic powder flame spraying technology is: compressed air is used to eject the plastic powder through the central tube of the spray gun ； Compressed air for cooling is sprayed around the plastic powder to form an inner curtain (air ring) ； On the outermost layer is the flame formed by the combustible gas. In this way, the heating flame heats the plastic powder to a molten state through compressed air, thereby forming a coating. The key to plastic powder flame spraying is the control of the heating degree of the plastic powder. When plastic powder is heated, either overheating or poor melting can affect the quality of the coating and its adhesion strength. The flame used for plastic spraying typically consists of a fuel gas (such as acetylene, propane, or liquefied petroleum gas) and an oxidizing gas (such as oxygen or air); the heat generated by combustion is used to heat the plastic powder until it melts, after which it is sprayed onto the workpiece using compressed air. 2. Characteristics and applications of powder coating: (1) The prominent advantages of plastic spraying include good chemical stability, excellent corrosion resistance to most acids, bases, and organic solvents, good anti-friction and lubricating properties, electrical insulation, shock absorption, low specific gravity, and light weight. Plastic powder flame spraying technology has a history of over 30 years abroad. Although it has been in use in China for a relatively short time, it has been applied in various important engineering fields and achieved excellent results. (2) The difficulty in applying plastic powder flame spraying technology to large steel structures lies in the fact that the substrate needs to be preheated to over 100°C during spraying, which results in low spraying efficiency and limits its application. According to information from the United States and Germany, when using EVA (ethylene-vinyl acetate copolymer) special plastic powder for spraying, the substrate can be sprayed directly without preheating. This can greatly simplify the process of spraying large steel structures. However, the price of this powder is high, which limits its widespread use. To ensure a good adhesion between the plastic coating and the substrate without the need for preheating, N.K.K. sprays a bonding underlayer on the substrate before applying the plastic powder; the thickness of this underlayer is no more than 0.3 mm. Once the bonding layer has adhered to the substrate, the plastic coating is then applied. Choosing the right bonding coating is key. Japan has achieved success in using this preheating-free plastic spraying technology, which is applied in the chemical, petroleum, and offshore steel structure industries, where its service life reaches 40 years. 3. Common powder coating materials Common powder coating materials include thermoplastic and thermosetting materials such as polyethylene, polyvinyl chloride, epoxy resin, and nylon.