Petrochemical equipment corrosion and protection

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Chapter 1, About Metal Corrosion Metals have many excellent properties, such as electrical conductivity, thermal conductivity, strength, toughness, plasticity, wear resistance, castability, etc. Metal materials are still the most important structural materials and are widely used in all aspects of production, life and scientific and technological work. Metal products are subject to various damages during the production and use process, such as mechanical wear, biological damage, corrosion, etc. 　　1. Definition of metal corrosion Metal corrosion is the damage or deterioration of metal caused by the action of the environment. There are other terms for corrosion of metals. The so-called environment refers to substances that come into contact with metals. For example, the naturally existing atmosphere, sea water, fresh water, soil, etc., as well as raw materials and products used in production and daily life. Due to the chemical or electrochemical interaction between these substances and metals, metal corrosion occurs. In many functional cases, there are also the effects of mechanical force, radiation, electric current, biology, etc. at the same time. The corroded part of the metal changes from a simple substance to a compound, causing rust, cracking, perforation, and brittleness. Therefore, in most cases, the process of metal corrosion is the reverse process of metallurgy. 　　2. There are many classification methods for metal corrosion. 　　(1) According to the corrosion process, there are mainly chemical corrosion and electrochemical corrosion. Chemical corrosion is damage caused by the direct chemical reaction between metal and environmental media. During the corrosion process, no current is generated. For example, corrosion of metals in high-temperature air or chlorine, corrosion of metals by non-electrolytes, etc. The medium that causes chemical corrosion of metals cannot conduct electricity. Electrochemical corrosion is the damage caused by the electrochemical reaction of metal in the electrolyte solution. During the corrosion process, electric current is generated. All media that cause electrochemical corrosion can conduct electricity. For example, the corrosion of metals in media such as acid, alkali, salt, soil, seawater, etc. The main difference between electrochemical corrosion and chemical corrosion is that it can be decomposed into two independent and simultaneous cathodic processes and anode processes, while chemical corrosion does not have this feature. Electrochemical corrosion is more common and prevalent than chemical corrosion. 　　(2) According to the form of metal corrosion damage and the distribution of corrosion areas, it is divided into general corrosion and local corrosion. General corrosion means that corrosion is distributed on the entire metal surface. General corrosion has uniform corrosion with the same degree of corrosion everywhere ； There is also non-uniform corrosion with different degrees of corrosion in different corrosion areas. The corrosion that occurs when cleaning steel and aluminum equipment with pickling liquid is generally uniform corrosion. Corrosion is mainly concentrated in certain areas of the metal surface, which is called localized corrosion. Although the amount of corrosion is not large, due to its high local corrosion rate, it can cause serious damage to the equipment or even explode. Therefore, it is more harmful. Metals can undergo different localized corrosions under different environmental conditions. For example, pitting corrosion, crevice corrosion, stress corrosion, intergranular corrosion, wear corrosion, etc. According to the corrosive environmental conditions, corrosion is divided into high temperature corrosion and normal temperature corrosion. ； Dry corrosion and wet corrosion, etc. The significance of corrosion and its relationship with enterprises Corrosion and protection is an independent discipline. It is a discipline that studies the corrosion process and corrosion control mechanism of structural materials and takes measures to extend the service life of structural materials. The correct selection of materials and protective measures are one of the main objectives of this subject. Corrosion and protection involve many disciplines. Therefore, it is also a highly applied technical science. Why is the science and technology of corrosion and protection attracting more and more attention from people? The main reason is that it has caused huge losses to mankind. According to relevant reports in the United States, it is estimated to be about 4% of the gross national product. Our country conducted a corrosion investigation on 10 chemical companies, and the loss due to corrosion accounted for 3.9% of the gross national product. For example, if a company has an annual output value of 10 billion, it will lose about 400 million due to corrosion. Corrosion includes two parts, one is unavoidable loss, and the other is loss that can be avoided by adopting protective measures. As our anti-corrosion science and technology workers, we must do this part of the work well. New materials and new technologies are constantly emerging, providing us with new ways to solve corrosion problems. Therefore, we must master the science and technology of corrosion and protection to serve the corrosion and management of equipment. (This lecture is different from ordinary books. This lecture is mainly based on the common corrosion conditions of petrochemical equipment, combined with some current effective protective measures. ) mainly talks about the following content: See Chapter 2 of the catalog. Corrosion and Protection of Chemical Industry Atmosphere Section 1. Corrosion of Metal Equipment by Chemical Industry Atmosphere Corrosion of metal under natural atmospheric conditions is called atmospheric corrosion. The number of metal surfaces exposed to the atmosphere is large, and the metal loss caused is also large. For example, about 70% of metal components in petrochemical plants work under atmospheric conditions. Atmospheric corrosion has caused serious damage to many metal structures. Common steel platforms, electrical appliances, instruments and other materials are severely corroded. It can be seen that atmospheric corrosion is both common and serious in petroleum and petrochemical production. The atmosphere contains water vapor. When the water vapor content is large or the temperature decreases, it will condense on the metal surface to form a water film. It is especially easy to form a water film in the depressions on the metal surface or where solid particles accumulate. This water film can act as an electrolyte by dissolving gases and other impurities in the air, making metal prone to chemical corrosion. Because the composition of the industrial atmosphere is relatively complex, the ambient temperature and humidity are different, and the corrosion of equipment and metal structures is different. For example, the air humidity around the wet air cooler in the production device is high, and the combined effect of harmful impurities causes severe corrosion on the surface of the equipment. Coatings applied to the surface of equipment, metal frames, etc., such as: Due to exposure to wind and sun, the surface of phenolic paint, alkyd paint, etc. will be powdered, cracked, and peeled off after about a year of use, and will lose its function. Section 2. Corrosion of metals (steel and iron) in chemical atmosphere. Since iron has the tendency to naturally form iron oxides, it is highly active in many environments. Because of this, it also has certain corrosion resistance. Sometimes it reacts with oxidation in the air to form a protective oxide film on the surface. This film can prevent rust in the air with a relative humidity of 99%. However, the presence of 0.01% SO2 will destroy the film effect and allow corrosion to continue. Generally, in the chemical atmosphere, the corrosion rate of ferrous metals increases with time. This is because the accumulation of contaminated corrosive agents makes the corrosive environment more serious. Section 3. Analysis of corrosion causes 1. Damage to the coating surface SO2, SO3 and CO2 in the industrial atmosphere dissolve in rain or humid air to generate sulfuric acid and carbonic acid, which adhere to the surface of equipment and metal frames. Due to the action of acid, the coating is corroded and destroyed. Low molecular weight polymers have larger porosity, and it is easier for water molecules to pass through the coating surface and reach the interface between the coating and the substrate, which reduces the bonding strength of the coating and causes the coating to peel off or bulge. 2. Corrosion of metal under coating Corrosion of metal under coating is caused by electrochemical effects. Oxygen at the cathode has a depolarizing effect, and the reaction is as follows: O2 + H2 + 2e = 2OH – Therefore, the solution in the bubble under the coating is alkaline, also called an alkaline bubble. At this time, the pH value of the cathode can be as high as 13 or more. Once the interface forms a highly alkaline state, alkaline dissolution of the base oxide film and alkaline decomposition of the coating will further occur. The following reaction occurs at the anode: Fe = Fe2+ + 2eFe2+ reacts with oxygen, water and OH– to produce corrosion products such as Fe(OH)2, Fe(OH)3, Fe2O3· At this time, the solution inside the bubble is acidic, so it is called an acidic bubble, and the pH value inside the bubble is only 2-4. Therefore, judging from the cathode and anode reactions produced at the peeling part of the paint film, the OH– ions generated by the cathode reaction increase the interface pH value, causing the hydrolysis of Fe2+ ions.: Fe2+ ​​+ 2H2O = Fe(OH)2 + 2H+ at this time reduces the interface pH value, thereby accelerating the anodic reaction (metal corrosion), expanding the corrosion area, and the scope of paint film peeling. The paint painted on the surface of some equipment will crack and fall off in less than half a year, causing the equipment to be corroded. Chapter 3. Corrosion and Protection of Storage Tanks 1. Inner Wall of Oil Tanks 1) Material Selection Basis Through the investigation of the corrosion situation of oil tanks, the inner wall of the gasoline tank was first anti-corrosion. In the early 1990s, anti-corrosion coatings were generally protected by coatings with good corrosion resistance, such as epoxy resin paint or polyurethane paint, which effectively protected the oil tanks. But these coatings have high insulation properties. Due to the friction between the oil flow and the tank channel and tank wall during transportation, static electricity is generated, which increases the static voltage in the tank and can easily generate static sparks and cause the oil tank to explode. Therefore, the anti-corrosion coating for the inner wall of the oil tank must not only have good corrosion resistance, but also have antistatic properties. At present, "epoxy glass flake antistatic coating" is used more frequently in our country. This coating is composed of primer and topcoat. In terms of anti-corrosion, its main performance is as follows: 1. The main component of the primer is silicone-rich zinc paint, which mainly performs electrochemical protection and chemical protection in anti-corrosion. ⑴.Electrochemical protection: Organic zinc-rich coatings contain a large amount (more than 70%) of ultra-fine metallic zinc particles, which are connected to each other in the coating. Moreover, metallic zinc is in close contact with the metal matrix. Therefore, many microbatteries are created when an electrolyte is present (such as water, solution). Since the electrode potential of zinc (-0.75V) is lower than that of iron (-0.44V), according to the electrochemical principle, zinc powder is continuously consumed to protect the cathode iron. That is, when zinc-iron comes into contact, under rust conditions (water, solution), zinc is first oxidized to form zinc hydroxide and zinc oxide, and further absorbs carbon dioxide in the air to form zinc carbonate. Due to this protective effect, organic zinc-rich coatings can protect steel and prevent rust spots from spreading even when rust spots occur. Such as galvanized iron corrosion. ⑵.Chemical protection: Metal zinc is chemically active and easily reacts with other substances. Especially in humid air or solutions, various double salts and insoluble compounds are quickly generated. If zinc is oxidized, alkaline substances such as zinc hydroxide, zinc oxide, and zinc carbonate (referred to as white rust) are generated. These substances are easy to expand in volume, blocking the gaps, cracks and holes in the coating film, blocking the intrusion of oxygen, air and other electrolytes, acting as a physical isolation, preventing zinc and iron from being oxidized, thereby improving the stability of the coating. At the same time, these insoluble compounds firmly cover the surface of the coating, protecting the coating and preventing the continued dissolution of zinc. The organic zinc-rich coating has extremely excellent anti-rust properties, and at the same time, the coating does not pollute the oil. In addition, the material has good bonding force with the metal base surface, and the dry film thickness is about 40 microns when applied once. Due to the large porosity of the material, the topcoat can easily penetrate and increase the bonding force between the primer and the topcoat. The antistatic properties of this material are shown in Table 1. 2. Topcoat: It is mainly composed of modified epoxy resin, scaly conductive material, glass flake filler, and thixotropic agent. The properties of this material are as follows: Epoxy resin has good corrosion resistance. The cured epoxy resin system contains stable benzene rings, ether bonds, and aliphatic hydroxyl groups, so it has good resistance to certain solvents, dilute acids, and alkalis. In the lipid layer of scales, extremely thin glass scales are basically arranged in parallel and overlapping. When the thickness of the anti-corrosion layer is 1 mm, there are hundreds of layers of glass scales arranged in parallel, which effectively prevents the penetration of corrosive media, so the ability to resist the penetration of corrosive media is particularly strong. At the same time, because the glass flakes exist discontinuously in the resin, the shrinkage force * * Reduced, the crack resistance of the coating is also good. This makes the structure of the flake resin coating fundamentally different from that of traditional anti-corrosion coatings, so its corrosion resistance and impact resistance are very good. Table 1 Paint Static Electricity Index Items * * Standard primer topcoat paint film surface resistivity (Ω) ‹1082.3×1062.9×106 Paint film volume resistivity (Ω·m) ‹1086.2×1061.0×106 The above data shows that the resistivity is much lower than * * Issue relevant electrostatic safety standards. 3. It is more reasonable from theory to practice to use a combination of zinc-rich and epoxy coatings as the anti-corrosion layer. The specific performance is as follows: ⑴.When the coating has antistatic requirements, the paint film is required to have a certain conductivity. Judging from the antistatic coatings currently produced in my country, resin is basically used as the base and topcoat. Since the resin itself is an insulator, if the paint film is required to have a certain conductivity, a certain amount of conductive material (such as carbon black) must be added to the primer. Although the antistatic index meets the requirements, the bonding force between the primer and the metal surface is not good, and cracking or delamination is easy to occur. ⑵.When zinc-rich paint is used as primer and topcoat, although it has good corrosion resistance, the zinc powder of zinc-rich paint is easily oxidized into alkali and salt over time, so the conductivity of the paint film decreases and cannot reach * * Relevant electrostatic safety standards issued. ⑶.When using zinc-rich primer as the primer and epoxy as the topcoat, the problem of poor adhesion when using other materials as the primer can be overcome. At the same time, epoxy paint is used as the topcoat to avoid the shortcomings of oxidation and reduced conductivity of zinc-rich paint. 2). Usage: This material was used on the inner wall of a gasoline tank in a refinery in 1995. The situation after anti-corrosion is shown in Table 2.: Table 2 Comparison of the situation before and after application of the paint before anti-corrosion and after anti-corrosion The inner wall and top of the tank were severely corroded, with local perforations on the top of the tank, many rust products, and the surface was pitted. After more than 8 years of use, the anti-corrosion layer on the surface is intact and has not fallen off, and the scratch inspection still maintains good adhesion. Regarding the corrosion and protection of the outer surface of the tank, we can refer to the anti-corrosion of the steel structure. However, the anti-corrosion requirements for the metal surface of the insulated tank bottom corners are relatively strict.