Corrosion and Protection of Chemical Equipment

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With economic development, the demand for chemical products continues to rise, and more and more production equipment is operating beyond its designed capacity. As a result, for chemical companies around the world, it has become an urgent issue to prevent equipment failures caused by corrosion, which could lead to losses. Many experts believe that material protection and anti-corrosion measures are essential for reducing maintenance costs and ensuring the safe and stable operation of factories. 　　 Corrosion damage is ubiquitous, and corrosion-related accidents occur frequently. Apart from the inherent spontaneous nature of corrosion itself, this is largely due to people’s underestimation of its hazards, insufficient awareness of the importance of corrosion protection, lack of adequate knowledge about corrosion and protection science, as well as the failure to implement anti-corrosion measures or the improper application of such measures. 　　 Corrosive media are common in chemical manufacturing enterprises, and the mechanical equipment used in these industries suffers from more severe corrosion than that in other sectors. In the industrial sphere, the mechanical equipment in chemical production facilities tends to corrode at a faster rate compared to equipment in other industries. By implementing proactive and effective anti-corrosion measures, taking into account aspects such as design, construction, and operational protection, it is possible to enhance the anti-corrosion capabilities and extend the lifespan of the equipment. Improving the corrosion resistance of chemical processing machinery and equipment holds great practical significance for today’s chemical-related manufacturing industries. 　　I. On Corrosion　　 1 Definition of Corrosion　Corrosion occurs when a material is worn away and damaged through chemical processes or actions. At a superficial level, corrosion occurs due to chemical and electrochemical reactions between the material and its environment, which result in a certain degree of damage to the material’s functions. After being corroded, chemical processing machinery and equipment can experience varying degrees of changes in their color, appearance, and mechanical properties, which leads to damage to such equipment as well as significant waste of energy and resources. This has a considerable impact on the production costs of chemical companies, resulting in financial losses for them. Therefore, adopting proactive and effective anti-corrosion measures to enhance the corrosion resistance of chemical machinery and equipment is a key issue facing the chemical industry today. 　　2 Classification of corrosion: Corrosion is the phenomenon in which a material is damaged as a result of interaction with its surrounding environment. 　　 Corrosion is classified into metal corrosion and non-metal corrosion based on the type of material. 　 Corrosion is classified into general corrosion and local corrosion based on surface morphology ； Local corrosion includes pitting corrosion, stress corrosion cracking, crevice corrosion, galvanic corrosion, wear corrosion, and so on ； 　　 Metal corrosion can be classified into physical corrosion, chemical corrosion, electrochemical corrosion, etc., based on its mechanism. 　　 Physical corrosion: The destruction of materials due to purely physical effects, generally caused by dissolution and penetration. Examples include the dissolution of containers made of molten metals, as well as the dissolution and penetration of high-temperature molten salts and molten alkalis into such containers. 　　Chemical corrosion: Damage caused by a direct chemical reaction between a metal and a non-electrolyte. The corrosion process is a pure oxidation-reduction reaction; the corrosive medium collides directly with the atoms on the metal surface to form corrosion products, and no electric current is generated during this reaction, in accordance with the laws of chemical kinetics. 　　Electrochemical corrosion: Damage caused by the electrochemical reaction between a metal and an electrolyte solution. During the reaction process, the anode loses electrons while the cathode gains electrons, along with the flow of electrons (current); this process follows the laws of electrochemical kinetics. 　　3 Hazards of Corrosion In the chemical industry, metals are the primary materials used for manufacturing equipment, and they suffer from corrosion due to frequent exposure to highly corrosive substances such as various acids, bases, salts, organic solvents, and corrosive gases. Corrosion not only causes significant losses to metal and alloy materials, affecting the service life of equipment, but also shortens the maintenance cycle of such equipment, increasing downtime and repair costs ； Corrosion exacerbates leaks and losses in equipment and pipelines, resulting in substantial losses of raw materials and finished products, affecting product quality, polluting the environment, and posing threats to human health ； Accidents such as equipment explosions and fires caused by corrosion lead to damage to the equipment and a halt in production, resulting in huge economic losses and even endangering human lives. 　　II. Corrosion prevention measures for chemical processing equipment The current corrosion prevention techniques mainly include: the development of corrosion-resistant materials, surface corrosion protection methods, corrosion inhibition techniques, and electrochemical protection. 　　1. Developing corrosion-resistant materials: The materials used to manufacture general mechanical equipment are mostly ordinary carbon steel, which is characterized by its low cost, easy availability, and suitability for processing. Under normal conditions of use, corrosion has little detrimental effect on it. However, in environments involving highly corrosive substances, such as those found in the chemical industry, its poor resistance to corrosion makes it prone to corrosion damage. Steel grades such as the commonly used Q235 have a corrosion rate of 0.5–1.0 mm/year in acidic and salt spray environments. Companies regularly apply anti-corrosive coatings to their equipment components; however, once scratches or localized peeling occur on the coating layer, electrochemical corrosion starts immediately and continues to spread, thereby reducing the service life of those components. As a result, chemical companies should avoid using such steel grades as raw materials for mechanical equipment. Instead, corrosion-resistant steels such as 16MnCu, 09MnCuPTi, and other common low-alloy steels should be used as the base materials for manufacturing equipment. Although this type of steel is more expensive than the former, its overall economic efficiency is much higher. Statistics show that the service life of equipment made from low-alloy steel is 2 to 3 times longer than that of equipment made from ordinary carbon steel, so its cost-performance ratio is much better. 　　The research and development of corrosion-resistant materials is a breakthrough for the advancement of anti-corrosion technologies; technological progress at all times in human history has been closely related to it. Corrosion-resistant materials are mainly divided into metal materials, polymer materials, and inorganic non-metallic materials. 　　Metal and alloy materials are the main components among structural materials, with steel playing a dominant role; however, steel’s corrosion resistance has limitations. The development and application of high-performance alloy and non-ferrous metal materials are advancing rapidly, addressing to some extent the problems of local corrosion and corrosion in special environments. Such as corrosion-resistant alloys with high molybdenum content, duplex stainless steels, high-purity ferritic stainless steels, nickel-based alloys, low-alloy steels, titanium and titanium alloys. Corrosion-resistant non-metallic materials are currently widely used in chemical production both domestically and internationally. Non-metallic materials possess excellent corrosion resistance, and their mechanical properties can be improved through methods such as reinforcement; in certain fields, there is a trend toward using them as substitutes for steel. Currently under development are materials such as corrosion-resistant plastics, fiberglass-reinforced plastic, graphite, glass-lined materials, and engineering ceramics. 2 Surface coatings: Among the existing methods of corrosion prevention, surface coatings and metal surface treatment techniques account for around 87% of all costs related to corrosion prevention. Adopting the correct surface anti-corrosion techniques is an essential way to extend the service life of equipment, reduce maintenance costs, and improve equipment management. At the same time, the application of surface anti-corrosion technology **improves the corrosion resistance of the overall material.** Common surface corrosion prevention techniques used in the chemical and petrochemical industries include coating, lining, plating, impregnation, as well as various high-tech methods developed in recent years; among these, coatings and linings are the most widely applied. 　　2.1 Corrosion-resistant coatings The development of corrosion-resistant coatings has always been a field of interest in research. In the chemical industry, such coatings are primarily used on the inner and outer surfaces of buildings, structures, equipment, and storage tanks, as well as on pipelines for transporting water, oil, and gas. According to statistics, improper surface treatment of the substrate accounts for about 75% of the cases where corrosion-resistant coatings are damaged; therefore, paying attention to the quality of surface treatment is an urgent priority. Several promising types of coatings, such as zinc-rich coatings, high-performance anti-corrosion coatings, high-temperature resistant coatings, ceramic coatings, rust-over-coating systems, and fluoropolymer coatings, are currently the most widely studied coatings on an international level. 　　2.2 Lining technology and composite pipes Lining technology utilizes materials with high strength (such as carbon steel, fiberglass-reinforced plastic, cast iron, etc.) as structural materials, while using materials with excellent corrosion resistance as the lining layer. Lining technologies include tight lining and loose lining, etc.; abroad, focus is placed on the development of tight lining technology. Lining technology is often used in chemical equipment, pipelines, etc. 　　2.3 Other surface engineering techniques Electroplating and electroless plating are used in chemical corrosion protection applications; the main types of electroplating include chromium plating, zinc plating, and nickel plating. The passivation of galvanized coatings is a highly active field. The low-chromium or chromium-free colored passivations, black passivations, military green passivations, and strong passivations (composite passivation layers containing silicone resins or other resins) that have been developed in the past decade enable an **improvement in the corrosion resistance of galvanized coatings in marine and industrial atmospheric environments, as well as in industrial water and river water environments. Electroless plating is a coating formation technique that uses metal salts and reducing agents in the same solution to undergo a self-catalytic redox reaction, thereby depositing a metal coating on a solid surface. Chemically deposited nickel-phosphorus alloys and ternary nickel-based alloys offer superior corrosion resistance compared to electroplated products, as well as a wider range of options; they are one of the surface treatment processes that is developing fastest at both domestic and international levels. 　　Spraying: Spraying is a technical process that uses flame, plasma, or arc spraying to apply metal, alloy, inorganic, or organic corrosion- and wear-resistant surface layers to materials and products. 　　Chemical heat treatment, particularly aluminum and chromium infiltration, has been widely used in the corrosion protection of chemical processing equipment over the past decade or so. 　　Phosphating, as an important pretreatment process for coating, has been in use for many years. Over the past decade or so, alkali metal ternary low-temperature or room-temperature phosphating has brought about a qualitative improvement in phosphating technology. 　　3 Electrochemical protection Electrochemical protection refers to a corrosion prevention technique that uses an external current to alter the corrosion potential of metals (including alloys), thereby reducing their corrosion rate. Electrochemical protection can be divided into cathodic protection and anodic protection. Electrochemical protection technology has attracted widespread attention and application in the field of chemical corrosion prevention; it is an effective, economical, and practical method for preventing corrosion. 　　Cathodic protection involves passing sufficient cathodic current onto the metal surface to make the metal potential more negative and reduce the rate of metal dissolution. The structural shape of the equipment to be protected should generally not be too complex; in devices with complex structures, the current density is high near the auxiliary anode and low away from it, resulting in insufficient protective current. It even fails to provide protection, resulting in the so-called “shielding effect”. Cathodic protection is mainly used for metal structures in water and soil, but it generally has to be applied in environments with simple equipment structures and low corrosivity of the medium. In addition to preventing general uniform corrosion, cathodic protection can also prevent pitting, intergranular corrosion, impact corrosion, selective corrosion, and other types of corrosion in certain materials. 　　Anodic protection involves connecting the metal component to be protected to the positive pole of an external direct current power supply, thereby polarizing the metal component at a certain potential in the electrolyte solution. This allows a stable passive state to be established and maintained, which in turn suppresses anodic dissolution and significantly reduces the corrosion rate, thus protecting the equipment. Anodic protection cannot be used for metals that do not have passivation characteristics. It is mainly used in: 1. Structures in sulfuric acid production, such as carbon steel storage tanks, various heat exchangers, sulfur trioxide generators, etc. 2. Structures in ammonia water and ammonium salt solutions, such as carbonization towers and ammonia water storage tanks. 　　In strongly oxidizing media, anodic protection should be considered first ； When both anodic protection and cathodic protection are available and their protective effects are similar, cathodic protection should be given priority ； If hydrogen embrittlement cannot be ignored, cathodic protection must be used. 　　4 Buffers Corrosion inhibitors are substances that, at very low concentrations, can **reduce the corrosion rate** of metals in corrosive media. This substance can be a compound or a composite material composed of several compounds. In the chemical industry, corrosion inhibitors are primarily used to prevent corrosion in industrial cooling water systems, as well as during the chemical cleaning (pickling) of chemical processing equipment, pipelines, boilers, etc. There is also a small amount used for process-related corrosion prevention during the production process. 　　Corrosion inhibitors for industrial cooling water systems: In chemical production processes, a large amount of industrial cooling water is used, and aeration introduces substantial amounts of dissolved oxygen into this water, leading to severe corrosion. Since corrosion also deteriorates water quality and consumes valuable water resources, it is evident that the use of corrosion inhibitors to prevent corrosion brings significant economic and social benefits. 　　Corrosion inhibitors for chemical cleaning: Chemical equipment, pipelines, boilers, etc. develop fouling on their surfaces due to chemical, physical, or biological effects (such as scale, rust, oil deposits, and biological growth). The process of removing these fouling substances using chemical methods in order to restore the surfaces to their original condition is known as chemical cleaning. 　　In summary, in the production processes of chemical enterprises, chemical machinery is essential equipment for ensuring normal operations of such enterprises. The corrosion protection issues faced by chemical machinery have also come to receive widespread attention in the fields related to chemical production. Striving to improve the corrosion resistance of chemical processing machinery and equipment, to extend their service life as much as possible, and to reduce the production costs of chemical companies have become key challenges in the field of chemical manufacturing today.