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1. What is pitting corrosion? Pitting corrosion is also known as pitting and hole corrosion. Pitting can vary in size; generally, the depth of pitting is much greater than its diameter. Due to the heterogeneity in metal materials, such as defects, impurities, and solutes, when a medium contains certain active anions (such as Cl‑), these anions are first adsorbed at certain points on the metal surface, thereby damaging the passivation film on that metal surface. Once this passivation film is damaged and the metal lacks self-passivation ability, corrosion occurs on the metal surface. This is because metal from the body can easily leak out at the defects on the metal surface, putting it in an activated state, while the passivation layer remains in a passive state. This creates an active-passive corrosion cell. Since the area of the anode is much smaller than that of the cathode, the current density at the anode is very high; as a result, corrosion progresses deeper, and the metal surface is quickly corroded to form small holes. This phenomenon is known as pitting corrosion. In the statistics of corrosion failure types in the petroleum and chemical industries, pitting accounts for approximately 20% to 25%. In media with active anions that have poor flow, conditions for the accumulation and concentration of these active anions are easily created, thereby promoting the formation of pitting. Rough surfaces are more prone to pitting than smooth surfaces. A decrease in pH and an increase in temperature both increase the tendency for pitting. Oxidizing metal ions (such as Fe3+, Cu2+, Hg2+, etc.) can promote the formation of pitting. However, certain oxygen-containing anions (such as hydroxides, chromates, nitrates, and sulfates) can prevent pitting. Although the weight loss due to pitting is not significant, the corrosion rate is high because the anode area is very small. In severe cases, this can lead to perforations in the equipment, resulting in the leakage of large amounts of oil, water, and gas; sometimes it even causes serious accidents such as fires and explosions, posing a great risk. Pitting can exacerbate intergranular corrosion, stress corrosion, and corrosion fatigue; in many cases, pitting is the origin of these types of corrosion. 2. What is crevice corrosion? In an electrolyte, narrow crevices form between metal surfaces or between metal and non-metal surfaces. The movement of substances within these crevices is hindered, leading to the formation of concentration cells. This, in turn, causes localized corrosion; this type of corrosion is known as crevice corrosion. Crevice corrosion often occurs at the joints of flanges in equipment, as well as at the areas where gaskets, linings, and wraps overlap with metal. It can arise in different metals and in various corrosive environments, thereby causing serious disruptions to the proper operation of production equipment and even leading to damage. For titanium and titanium alloys, crevice corrosion is the most critical corrosion phenomenon to consider. In the medium, as the oxygen concentration increases, crevice corrosion also increases ； As the pH value decreases, the anodic dissolution rate increases, and so does the amount of crevice corrosion ； An increase in the concentration of reactive anions leads to higher susceptibility to crevice corrosion. However, an increase in certain oxygen-containing anions reduces the amount of crevice corrosion. 3. What is stress corrosion? The phenomenon of brittle cracking below the strength limit that occurs in materials when they are exposed to specific corrosive media and subjected to static tensile stress (including residual stresses caused by external loads, thermal stress, cold working, hot working, welding, etc., as well as wedge stresses resulting from the intrusion of corrosion products into cracks) is known as stress corrosion cracking. Stress corrosion cracking begins with the formation of tiny pits at the corrosion-sensitive areas of the metal, leading to slender cracks that propagate rapidly; this can result in severe damage in a short period of time. Stress corrosion cracking accounts for the highest proportion—up to 50%—among all types of corrosion failures in the petroleum and chemical industries. There are two basic conditions for the occurrence of stress corrosion: one is that the material has a certain sensitivity to stress corrosion cracking in the presence of the medium ； Second, there is a sufficiently high tensile stress. The stresses that lead to stress corrosion cracking can originate from operating stresses or from residual stresses generated during the manufacturing process. According to statistics, in stress corrosion cracking incidents, over 80% are caused by residual stresses, while those caused by working stresses account for less than 20%. The stress corrosion process can generally be divided into three stages. The first stage is the incubation period, during which crack nucleation occurs as a result of the localization of the corrosion process and the action of tensile stresses ； The second stage is the period of corrosion crack growth; once cracks initiate, they expand under the combined effect of the corrosive medium and tensile stresses in the metal ； In the third stage, due to the local concentration of tensile stress, rapid crack growth leads to the failure of the part. When stress corrosion cracking occurs, there is no significant uniform corrosion, and even the amount of corrosion products is very small; sometimes it is difficult to detect them with the naked eye. Therefore, stress corrosion is a highly dangerous form of failure. Generally, an increase in the chloride concentration in the medium reduces the time required for stress corrosion cracking. The corrosive effects of different chlorides decrease in the order of ions such as Mg2+, Fe3+, Ca2+, Na1+, and Li1+. The temperature range for stress corrosion to occur is generally between 50°C and 300°C. Preventing stress corrosion requires measures aimed at both reducing corrosion and eliminating tensile stress. Mainly: First, we should try to avoid using materials that are sensitive to stress corrosion ； Second, when designing the equipment structure, it is necessary to strive for rationality, minimizing stress concentration and the accumulation of corrosive agents ； Third, when manufacturing processing equipment, attention should be paid to eliminating residual stresses. 4. What is corrosion fatigue? Corrosion fatigue occurs under the combined action of a corrosive medium and cyclic stress. This reduction in corrosion fatigue resistance caused by corrosive agents is known as corrosion fatigue. The stress value for fatigue failure is below the yield point; fatigue failure occurs only when the stress exceeds a certain critical cyclic stress value (the fatigue limit, or fatigue life). Corrosion fatigue, on the other hand, can cause failure even under very low stress levels, which makes it highly dangerous. The main factors affecting material corrosion fatigue include the stress alternation rate, medium temperature, medium composition, material size, processing, and heat treatment. Increasing the loading cycle rate, reducing the pH value of the medium, or raising the temperature of the medium will all lead to a decrease in corrosion fatigue strength. Stress concentration resulting from damage on the material surface or lower roughness reduces the fatigue limit, thereby decreasing the fatigue strength as well. 5. What is intergranular corrosion? Intergranular corrosion is a type of localized corrosion damage in which metal materials are corroded along the boundaries between their grains in a specific corrosive environment, resulting in a loss of cohesion between those grains. Equipment or parts affected by this type of corrosion may still appear intact and shiny on the outside, but since the bonds between the grains are broken, the material loses almost all of its strength; in severe cases, it loses its metallic sound and turns into powder upon gentle tapping. According to statistics, intergranular corrosion accounts for about 4% to 9% of cases of corrosion-related failures in petroleum and chemical equipment, and it mainly occurs in containers and heat exchangers welded from rolled materials. It is generally believed that the depletion of alloying elements at grain boundaries is the main cause of intergranular corrosion. Intergranular corrosion can be prevented by improving the purity of the material, removing harmful trace elements such as carbon, nitrogen, phosphorus, and silicon or adding small amounts of stabilizing elements (titanium, niobium) to control the carbides that precipitate at the grain boundaries, as well as by employing appropriate heat treatment procedures and processing techniques. 6. What is uniform corrosion? Uniform corrosion refers to corrosion that occurs at almost the same rate across the entire metal surface in contact with the environment. When using corrosion-resistant materials, resistance to uniform corrosion should be the primary criterion for evaluating their corrosion resistance, with resistance to local corrosion being considered only in special cases. 7. What is wear corrosion (erosion)? The material degradation process resulting from the combined action of wear and corrosion is called wear corrosion. Erosive wear can occur in fluid pipelines with high flow rates, as well as in pumps and pipelines carrying fluids with suspended abrasive particles. In some components subject to flow, such as the valve disc (head) and seat in high-pressure pressure relief valves, the impeller of centrifugal pumps, and the blades in fans, the relative flow velocity of the corrosive medium is very high. This causes the passivation film on the surface of corrosion-resistant metal materials to be unable to recover due to excessive mechanical erosion, resulting in a significantly increased rate of corrosion. If solid particles are present in the corrosive medium, it will **exacerbate wear corrosion**. 8. What is hydrogen embrittlement? Once metal materials, especially titanium, absorb hydrogen, brittle hydrides are formed, which reduces their mechanical strength. In corrosive media, hydrogen released due to corrosion reactions in the metal, as well as hydrogen absorbed during the manufacturing process, are the main sources of hydrogen in the metal. The surface condition of metals has a significant impact on hydrogen absorption. Studies have shown that titanium surfaces with a ground finish absorb the most hydrogen, followed by surfaces in their original state, while surfaces that have undergone vacuum annealing or pickling are the least capable of absorbing hydrogen. Oxidizing titanium in the atmosphere can effectively prevent hydrogen absorption. 9. Definition of corrosion: (1) Corrosion refers to the process by which materials (including metals and non-metals) are damaged, degraded, or altered as a result of the action of their surrounding environment, resulting in the loss of their original properties. (2) Corrosion is the \"cancer\" of materials and equipment; it is the damage or deterioration of materials resulting from their reaction with the environment, representing a harmful interaction between a material and its surroundings. It generally refers to any form of damage other than simple mechanical destruction. 10. Hazards of corrosion in chemical plants: (1) It causes plant shutdowns or reduced production capacity, shortens the production cycle, and lowers the economic efficiency of the enterprise ； (2) Causing explosions, fires, or product leaks, leading to casualties and environmental pollution incidents ； (3) Corrosion products can cause accidents such as catalyst poisoning, affecting the further processing of products ； (4) Some corrosion products enter the product, causing it to contain impurities, have an unpleasant odor, or deteriorate, resulting in substandard product quality ； (5) When corrosion products deposit on the surface of heat exchange equipment to form scale, it reduces thermal efficiency and increases energy consumption ； (6) It reduces the service life of the equipment, increases maintenance costs and time, thereby raising the operating costs of the installation. 11. Mechanisms of metal corrosion: Metal corrosion is the degradation of metals under the chemical or electrochemical action of the surrounding medium, and often as a result of the combined effects of physical, mechanical, or biological factors. Based on the course of the corrosion process, metal corrosion can generally be divided into two categories: chemical corrosion and electrochemical corrosion. 12. Chemical corrosion: The corrosion that occurs when a metal undergoes chemical reactions in dry gases and non-electrolyte solutions is called chemical corrosion. The products of chemical corrosion are present on the surface of the metal, and no electric current is generated during the corrosion process. Depending on the properties of the products of chemical corrosion, there can be passivation or activation effects. A. If the compounds formed as a result of chemical corrosion are stable, that is, not prone to volatilization or dissolution, and have a dense structure that allows them to bond firmly with the metal substrate, then these corrosion products remain attached to the metal surface, providing protection for the substrate by preventing further corrosion – this phenomenon is known as \"passivation\". B. If the compounds formed as a result of chemical corrosion are unstable, that is, if they are volatile or soluble, or if they do not bind firmly to the metal, then these corrosion products will peel off layer by layer (oxidation scales fall into this category). Such corrosion products cannot protect the metal from further corrosion, and this effect is known as the \"activation effect\". 13. High-temperature oxidation and decarburization of metals. In petrochemical production, many machines and equipment operate under high-temperature conditions, such as ammonia synthesis towers, sulfuric acid oxidation furnaces, ethylene cracking furnaces, etc. High-temperature oxidation and decarburization of metals is a type of gas corrosion that occurs under high temperatures, and it is one of the common chemical corrosions in equipment corrosion. A. Generally, when the temperature of steel and cast iron exceeds 3000°C, a visible oxide scale appears on their surface. As the temperature rises, its oxidation rate also **increases**. B. When oxidized at temperatures below 570°C, the oxide formed as a result of oxidation does not contain FeO; its oxide layer is composed of Fe2O3 and Fe3O4. The oxide layers formed by these two oxides have a dense and stable structure; they adhere firmly to the surface of steel and do not easily come off, serving as a protective layer. Fe2O3 and Fe3O4 are types of passivating substances. Under normal circumstances, at temperatures above 570°C, there are three types of oxides formed on the surface of steel: Fe2O3, Fe3O4, and FeO. Their thickness ratio is approximately Fe2O3:Fe3O4:FeO = 1:10:100. The main component of the oxide layer is FeO; this layer has a loose structure and is prone to flaking, thus forming the commonly seen scale. The removal of this scale accelerates the corrosion of the steel. Therefore, in order to improve the high-temperature oxidation resistance of steel, it is necessary to find ways to prevent or reduce the formation of FeO on the surface of the steel. In the metallurgical industry, by adding appropriate amounts of alloying elements such as chromium, silicon, or aluminum to steel, it becomes resistant to oxidation. C. When the temperature is above 700°C, in addition to oxidation, decarburization also occurs in steel. The chemical reactions for decarburization are as follows: Fe3C + O2 → 3Fe + CO2; Fe3C + CO2 → 3Fe + 2CO; Fe3C + H2O → 3Fe + CO + H2. Decarburization of steel reduces its mechanical properties, particularly its surface hardness and fatigue resistance; therefore, equipment operating under high-temperature conditions must take these issues into account. 14. High-temperature and high-pressure hydrogen corrosion: In hydrocracking processes in chemical plants, the ammonia synthesis industry, and other chemical processes, it is common to encounter mixed gases in which hydrogen constitutes a significant proportion of the reaction medium. Moreover, these chemical reactions typically occur under high temperature and high pressure; for instance, the pressure in ammonia synthesis is usually around 31.4 MPa, while the temperature generally ranges from 470 to 500°C. At lower temperatures and pressures (temperature ≤ 200°C, pressure ≤ 4.9 MPa), hydrogen does not have a significant corrosive effect on ordinary carbon steel and low-alloy steel. However, under high temperature and pressure, they become corroded, resulting in a significant decrease in the material’s mechanical strength and plasticity, or even damage. This phenomenon is often referred to as “red corrosion” or “hydrogen embrittlement”. The hydrogen corrosion process of iron-carbon alloys under high temperature and pressure can be divided into a hydrogen embrittlement stage and a hydrogen erosion stage. The first stage is the hydrogen embrittlement stage. At this stage, hydrogen is adsorbed by the steel when in direct contact with it, and diffuses into the steel in an atomic state, dissolving in the ferrite to form a solid solution. However, at this stage, the hydrogen dissolved in the steel does not undergo any chemical reaction with the steel, nor does it alter its microstructure; no cracks can be observed under a microscope, and there is no significant change in the steel’s strength limit or yield limit. However, it reduces the plasticity of steel and significantly decreases its impact toughness value. This brittleness of steel is proportional to the amount of hydrogen dissolved in it. As long as the material is subjected to dehydrogenation treatment when it is in the hydrogen embrittlement stage, its properties can be restored to their original state. Therefore, this stage is reversible. The second stage is the hydrogen erosion stage. At this point, the hydrogen dissolved in the steel undergoes a chemical reaction with cementite in the steel, producing methane gas. This alters the structure of the steel. The chemical reaction can be represented by the equation: Fe3C + 2H2 --> 3Fe + CH4. This reaction typically occurs at grain boundaries. The generated methane gas tends to accumulate in the existing microscopic pores at these boundaries, creating localized high pressure. This leads to stress concentration and internal stresses, causing the grain boundaries to widen and resulting in the formation of larger cracks ； Or it may accumulate at defects such as inclusions on the surface of the steel, forming bubbles that reduce the mechanical properties of the steel. On the other hand, since the volume of cementite decreases as it is reduced to ferrite, corresponding structural stresses are generated within the steel due to these volume changes. These stresses, combined with the aforementioned internal stresses, contribute to crack propagation. The propagation of cracks, in turn, provides more favorable conditions for the diffusion and reaction of hydrogen and carbon. If this process continues repeatedly, it eventually leads to complete decarburization of the steel, the formation of a network of cracks within it, a severe decline in its mechanical properties, and even destruction. Since high pressure facilitates the dissolution of hydrogen in steel, while high temperature increases the diffusion rate of hydrogen within the steel’s structure as well as the rate of decarburization, hydrogen corrosion in iron-carbon alloys intensifies as pressure and temperature rise. Generally, for iron-carbon alloys, there is a starting temperature and starting pressure at which hydrogen corrosion occurs; this serves as an indicator of the steel’s resistance to hydrogen corrosion. By reducing the carbon content in steel to prevent the precipitation of carbides (Fe3C), hydrogen corrosion can be effectively prevented. Furthermore, adding certain alloying elements to steel such as chromium, molybdenum, titanium, tungsten, vanadium, etc., allows these elements to form stable carbides with the carbon in the steel structure, making it less likely for the steel to react with hydrogen and thus preventing hydrogen corrosion. 15. Electrochemical corrosion: Electrochemical corrosion refers to the damage caused by electrochemical reactions between a metal and an electrolyte solution. Its characteristic is that an electric current is generated during the corrosion process. Under the action of water molecules, the metal in an electrolyte solution becomes ionized. When the binding force between metal ions and water molecules is greater than the binding force between metal ions and their electrons, the metal ions move from the metal surface into the electrolyte solution, resulting in electrochemical corrosion. 16. Function of a galvanic cell: The corrosion process of metals in an electrolyte solution is exactly similar to the electrochemical reaction process in a battery. Since iron has a lower potential than copper, it acts as the anode, while copper becomes the cathode. At the anode, iron is dissolved and electrons are released, namely Fe --> Fe2- + 2e; the released electrons can move to the cathode through external wires. At the cathode, the incoming electrons are absorbed by substances that can take in electrons; in this case, they are absorbed by the cations (H+) in the solution, resulting in the release of hydrogen gas, namely 2H++2e-->H2↑. During electrochemical corrosion, this type of corrosion cell is the most common. It arises because certain equipment may be made of two different metallic materials, or there may be different phases within the same material; additionally, variations in the physical and mechanical properties of various parts of the material, as well as uneven stress distribution, can result in differing potentials at different locations of the equipment material, thereby forming such corrosion cells. 17. Principles of microcells and electrochemical corrosion: As mentioned earlier, metal surface structures often contain different phases (such as ferrite and carbides in steel). In the same medium, two adjacent areas may have different potentials; since these areas are in direct contact, many local microcells are formed. The principle of microcells can be used to explain the chemical reaction process of electrochemical corrosion. ① Anodic reaction: Metal dissolution: Me --> Me++e ② Electron movement: Excess electrons at the anode move toward the cathode: e_anode --> e_cathode ③ Cathodic reaction: Electrons are absorbed by substances at the cathode: e_cathode + D --> (De). In the cathodic region, there are several different reaction scenarios depending on the substance that absorbs electrons. When metal ions with a higher potential are present in the solution (such as Cu2+), these ions absorb electrons and get reduced to metals: Me++e_cathode --> Me. When the solution is acidic and contains hydrogen ions H+, these H+ ions absorb electrons to form hydrogen atoms, which then combine to form hydrogen molecules: H++e --> H; H+ + H --> H2↑. When the solution is neutral, oxygen present in it absorbs electrons and combines with water to form OH- ions; the reaction is as follows: O2 + 4e + 4H2O --> 4OH-. During corrosion, metals are often not in solutions containing metal ions with a higher potential, and the cathodic reaction in which such ions absorb electrons does not occur frequently. The latter two chemical reactions are of greater significance, especially the second one, which is more common. It should be noted, however, that the cathodic reaction often involves the simultaneous presence of the latter two chemical reactions. 18. Conditions for electrochemical corrosion: From the aforementioned process and principles of electrochemical corrosion, it can be seen that the electrochemical corrosion process consists of three components: the anodic reaction process, electron flow, and the cathodic reaction process. None of these components can be missing. The step with the greatest resistance determines the speed of the entire corrosion process. It can also be seen that for electrochemical corrosion to occur, the following three conditions must be met: ① There must be areas on the same metal with different potentials, or there must be a potential difference between different metals ； ② The cathode and anode are connected to each other ； ③ The anode and cathode are in a mutually connected electrolyte solution. 19. Factors affecting metal corrosion: The factors influencing the corrosion of metal products include atmospheric humidity, temperature, oxygen, and pollutants in the atmosphere ； The material properties of the metal itself, its surface condition, etc., also affect its corrosion. 20. Current anti-corrosion methods in engineering: (1) Metal and non-metal coating methods – Metal coatings: metal linings, metal platings, composite metal sheets, etc.; Non-metal coatings: linings, enamel, glass lining, paints, combined coatings, etc. (2) Electrochemical protection, including cathodic protection and anodic protection. (3) Anti-corrosion structural design. (4) Medium treatment. 21. Anti-corrosion method using paint coatings – Paint coatings are currently mainly used as protective layers to prevent atmospheric corrosion on the outer surfaces of equipment, pipelines, and buildings, or to protect the inner surfaces of equipment in less harsh corrosive environments. The main types of coatings are: A. Oil-based coatings with drying oils as the main component; B. Resin-based coatings with synthetic or natural resins as the main component. 1. Main components of coatings and their functions: Components include film-forming substances, pigments, thinners, and curing agents. 22. Selection of coatings: Based on the specific corrosive environment, selection should be made considering corrosion resistance, impermeability, adhesive strength, and cost. Anti-corrosion coating for inner walls: good resistance to corrosion by chemical agents and permeation, such as resin-based coatings. Primer: It provides good adhesion to the surface to be bonded, and the coating itself does not cause corrosion of the material being protected. Rusted primer (stable type, conversion type): It can reduce the amount of rust removal work and improve construction efficiency. Topcoat: Outdoor use: alkyd enamel, epoxy enamel, vinyl chloride paint, vinyl chloride-vinylidene chloride compounds, etc. Indoors: vinyl chloride paint, vinyl chloride-vinylidene chloride paint, epoxy enamel paint, polyurethane paint. Varnish: It seals off certain defects in the topcoat. 23. Rust removal: To carry out anti-corrosion treatment on steel, the surface of the steel is usually pre-treated first, that is, rust is removed. Rust removal is the first and most important step in anti-corrosion treatment. 24. Rust removal grade: Steel is classified according to its degree of rusting as A, B, C, or D: A. A steel surface that is completely covered with scale with almost no rust present. B. The surface of steel that has rusted, with some of the oxide scale having peeled off. C. Steel surface whose oxide scale has peeled off due to rusting or can be scraped off, with minor pitting. D. Steel surfaces whose oxide scale has completely peeled off due to rusting, and where pitting has occurred widely. 25. Common rust removal methods: a. Rust removal using manual or power tools (rust removal grades St2, St3). b. Blast or projectile descaling (descaling grades Sa1, Sa2, Sa2.5, Sa3). c. Flame rust removal (rust removal grade F1). d. Chemical rust removal (rust removal grade Pi). Methods a and b for rust removal are the most common and widely used rust removal methods in on-site construction. 26. Rust removal methods: (1) Manual or power tool rust removal: For manual treatment, tools such as hammers, scrapers, shovels, wire brushes, and gauze (paper) can be used. Power tools can use electric brush wheels, electric grinding wheels, or various types of rust removers. When using manual or power tools, tools and methods that may damage or deform the metal surface must not be employed. Rust removal using manual or power tools is flexible in application but has low efficiency; it is generally used for rust removal at weld locations within devices. (2) Jet or blast rust removal: The main equipment used for jet rust removal is an air compressor and a sand tank. The rust removal principle is as follows: The air compressor generates compressed air, which is then sent to a gas storage tank for pressure stabilization and temporary storage. After that, a precision filter removes any oil mist or moisture from the compressed air, and finally the air is fed into the sandblasting machine. A sandblaster uses the pressure of compressed air to shoot abrasives at high speed onto the metal surface. Through the impact and grinding action of these abrasives on the metal surface, impurities and oxide layers are removed, increasing the roughness of the metal surface. This enhances the adhesion of paint, resulting in superior corrosion resistance for the paint. The abrasives used for jet rust removal include quartz sand, river sand, sea sand, and emery ； When using abrasives, they must be dry and clean, with a moisture content not exceeding 1%. Jet rust removal is highly effective; proper measures should be taken during installation to prevent dust from spreading. The main equipment used for shot blasting to remove rust is a shot blasting machine, with a crane used to assist in the process. The principle of shot blasting for rust removal is: high-speed steel sand and steel pellets are propelled by a blast gun and impact the surface of the steel, thereby removing impurities and oxide scale from its surface. The abrasive used for shot blasting to remove rust is steel shot. The microstructure of steel shot consists of tempered troostite and tempered martensite; its density is generally 7.4–7.85 Kg/cm3, and the desired hardness range is HRC 40–50. The hardness of steel shot is proportional to the cleaning speed, but inversely proportional to its lifespan. Therefore, it has high hardness, fast cleaning speed, is fragile, has a short lifespan, and high consumption. The hardness is too low, resulting in poor cleaning performance and failure to meet quality requirements. Jet rust removal achieves good rust-removal results, offers high efficiency in operation, requires less labor, but involves higher costs for equipment. 26. Painting: After the steel surface has been properly treated, the primer should be applied within 4 hours. If required, a intermediate coat and a topcoat can be applied separately. The main function of the primer is to prevent corrosion and improve the adhesion of the intermediate coat or topcoat. The intermediate coat enhances the adhesion between coats, and it serves to seal and protect the primer. The topcoat serves both decorative and aesthetic purposes, while also protecting the entire coating. The most common methods for applying paint are spraying and rolling. Spraying offers fast construction speeds and high efficiency, resulting in a uniform paint film thickness, but it requires a large amount of paint ； Roll coating uses less paint and causes less pollution to the surrounding environment; therefore, it is commonly used for applying paint in various installations. After the painting is completed, the coating surface should be smooth and have a uniform color, with no signs of missed painting, rust, bubbles, sagging, wrinkling, undercutting, peeling, or cracking. 27. Function of insulation: Insulation is a measure taken to reduce heat loss of the medium inside equipment and pipelines, or to prevent burns to people and ensure stable operation, by minimizing heat conduction. Insulation is usually the final construction step in a device, and the quality of this insulation work plays an important role in the production of the device. 28. Insulation structure: The insulation structure generally consists of an inner layer and an outer layer; the inner layer serves as the heat-insulating layer, while the outer layer acts as a protective layer. Common materials used for insulation layers include calcium silicate, aluminum silicate, and ultra-fine glass wool. Common materials for protective layers include aluminum alloy sheets, stainless steel sheets, and galvanized iron sheets. 29. Main performance requirements for insulation materials: (1) The density of rigid insulation materials and their products should not exceed 300 kg/m3, while the density of semi-rigid and soft insulation materials and their products should not exceed 200 kg/m3. (2) The compressive strength of rigid insulation products shall not be less than 0.4 MPa. (3) The pH value of thermal insulation materials and their products should not be less than 8. (4) The moisture content of the insulation material shall not exceed 7.5% (on a mass basis), the water resistance shall be no less than 95%, and the rebound rate of soft insulation materials shall be no less than 90%. (5) The protective layer material should possess fire-resistant, moisture-resistant, non-flammable properties, as well as resistance to atmospheric corrosion; it should also have stable chemical properties and not corrode the insulation layer. 30. Insulation layer installation: (1) When installing insulation layers on vertical equipment, storage tanks, and vertical pipes, insulation supports or retaining rings should be provided. (2) At the disconnect points of the insulation layers around the valves and flanges of equipment and pipelines, a distance sufficient for bolt removal should be provided. (3) When the thickness of the insulation layer is greater than 100 mm, if a single type of insulating material is used, it should be applied in two or more layers, with the thickness of each layer being as similar as possible ； If heterogeneous insulation materials are used, the thickness of each material shall comply with the specifications in the design documents. (4) During the installation of the insulation layer, the joints in the same layer should be staggered, and the joints between the inner and outer layers should be overlapped, with the overlap length should not be less than 50 mm. (5) The joint width of the insulation layer shall not exceed 5 mm; if it is greater than 5 mm, it should be filled tightly with material of identical properties. (6) The longitudinal joints of the insulation layer in horizontal pipes shall not be located within a 45° range on either side of the vertical centerline of the pipe. For large-diameter pipes, when multiple rigid molded insulation products are used, the location of the longitudinal joints in the insulation layer is not subject to this restriction, but it should be offset from the vertical centerline of the pipe. (7) The insulation layer should be tied using binding materials, with the tying spacing generally not exceeding 300 mm. For multi-layer insulation installation, it should be bundled layer by layer. (8) The insulation work for vertical equipment or vertical pipes should start from the supports and be carried out from bottom to top. (9) For the insulation layer at pipe bends, prefabricated products are preferred; in the absence of such prefabricated insulation products, the straight pipe sections should be cut into segments to facilitate installation. (10) When rigid thermal insulation materials are used for equipment or pipelines, appropriate expansion joints should be provided. The expansion joint should first be cleared of impurities and hard particles, and then filled with a soft thermal insulation material having a similar thermal conductivity. 31. Construction of the protective layer: (1) Once the insulation layer has been constructed and passed inspection, the construction of the protective layer should be carried out promptly. Before applying the metal protective layer, ensure that the surface of the thermal insulation layer is smooth and dry. (2) The amount of metal protective coating to be used for the straight sections of equipment and pipelines should be determined through on-site measurement; it is not advisable to pull the material too tightly during measurement, and there should be a margin of 30–50 mm in the perimeter after measurement. For the metal sheet after blanking, one edge on each of the transverse and vertical sides should be pressed to form raised ribs in the required direction. . (3) The circumferential joints of the metal protective layers of equipment and pipelines should preferably be joined by lapping or interlocking, while the longitudinal joints should preferably be joined by lapping or interlocking. . (4) Except for the movable joints, the lap joints of the metal protective coatings on equipment and pipelines shall be fastened using core pull rivets or self-tapping screws; the spacing between them shall be 150–200 mm, with no fewer than 5 such joints per 1 m length. (5) When the metal protective layer used for insulating equipment and pipelines encounters obstacles, appropriate holes should be made, with the deviation of the hole openings not exceeding 3 mm. The gaps at the openings should be sealed. (6) The joints between the segments of the equipment head should preferably be lapped joints. If they are butt joints, then raised ribs must be pressed out along the entire upper edge to cover the lower edge, and then secured with self-tapping screws. (7) The length of the metal protective layer cut for square equipment or pipes should not exceed 1 m, and it should be pressed into a diamond pattern along the diagonals. During installation, the patterns should be aligned to form a single unit. (8) The circumferential joints of the metal protective layer in horizontal pipes shall overlap along the slope of the pipe, and their longitudinal joints shall not be located within a 45° range on either side of the vertical center line above the horizontal center line; the joint openings shall face downward. The longitudinal seams can form a continuous straight line or be offset from each other, but they must always remain parallel to the axis of the pipe. (9) The metal protective layer for the insulation of vertical equipment and vertical pipes shall be applied from bottom to top, with the upper end overlapping the lower end. For vertical pipes with a slope greater than 45°, their metal protective coating should also be fixed in sections to the supports. (10) The thermal insulation metal protective layer at pipe elbows should be installed using a shrimp-shaped protective layer.