Corrosion problems of chemical processing equipment and anti-corrosion measures

Thread Content

Once chemical processing equipment is corroded, it not only experiences changes in color, appearance, and basic performance, which shortens its service life, but it also has adverse effects on the production of products by chemical companies, as well as on resource conservation and cost control. In severe cases, it can even lead to serious safety and environmental accidents. What are the common types of corrosion in chemical processing equipment, its causes, and what are the measures to prevent corrosion? Come and find out more. Classification of corrosion in chemical equipment: Corrosion is divided 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 degradation of a material due to purely physical effects, usually caused by dissolution or penetration, such as the dissolution of metal containers in molten metals, or the dissolution and penetration of these containers by high-temperature molten salts or alkalis. Chemical corrosion: Damage caused by a direct chemical reaction between a metal and a non-electrolyte; no electric current is generated during the corrosion process. 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. Causes and patterns of corrosion in chemical equipment: Many corrosive substances exist and are generated during chemical production processes, such as acids, bases, salts, water, oxygen, etc., which are the main causes of corrosion. The type of medium, its chemical composition, concentration, pH value, impurities, moisture content, and oxygen level are all external factors that cause corrosion. The faster the flow rate of the medium, the greater the likelihood of corrosion, as the flowing medium erodes the protective film, creating vortices, turbulence, and bubbles that lead to severe impact wear and cavitation corrosion. Improper material selection can lead to surface corrosion if the equipment’s surface comes into contact with corrosive substances and the equipment itself is not resistant to corrosion. The rougher the surface, the more prone it is to corrosion; the resulting symptoms include leakage, premature wear, damage, and noise. Uniform surface corrosion occurs in two forms: film-forming and film-free. Film-free corrosion is very dangerous, and the corrosion process proceeds at a certain rate; this is mainly caused by incorrect material selection. Film-forming corrosion: The passivation film usually has protective properties, but for materials used in certain devices, such as stainless steel, cobalt, and chromium alloys, the passivation film on their surfaces can be easily damaged due to friction at the ends. Under oxygen-deficient conditions, it is difficult for new films to form, which exacerbates galvanic corrosion. Lack of anti-corrosion measures or poor construction quality create an environment conducive to corrosion damage. Different environments require different materials. During the production of equipment, it is often not possible to balance material selection with resistance to environmental corrosion. Differences in temperature, concentration, and pressure lead to different material choices and varying levels of corrosion. Moreover, inadequate quality control during construction results in poor construction standards, making corrosion problems inevitable. Over-temperature and over-pressure during operation, inadequate equipment management, and a lack of attention are also among the causes of corrosion damage. Generally, the higher the temperature of the medium, the higher the pressure, and the faster the corrosion occurs, as corrosion is a chemical reaction; for every 10°C increase in temperature, the corrosion rate increases by 1 to 3 times. As the temperature rises, the diffusion rate increases, and at the same time the resistance of the electrolyte decreases, which accelerates the reaction in the corrosion cell. If there is severe friction at the device’s end face, excessive specific pressure, low surface finish, inadequate cooling, or poor surface lubrication, the frictional heat generated during operation of the device will accelerate corrosion. Corrosion prevention measures for chemical equipment: The current corrosion prevention techniques mainly include the development of corrosion-resistant materials, surface corrosion protection methods, and electrochemical protection. 1) Development of corrosion-resistant materials: Research into the development of corrosion-resistant materials is a key factor in the advancement of corrosion prevention technologies, and technological progress throughout 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 a substitute for steel. Currently under development are materials such as corrosion-resistant plastics, fiberglass-reinforced plastic, graphite, glass-lined materials, and engineering ceramics. 2) Surface corrosion prevention techniques: Among the existing methods of corrosion protection, the costs associated with surface corrosion-resistant coatings and metal surface treatment techniques account for around 87% of all corrosion prevention expenses. 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 efficiency. At the same time, the use of surface anti-corrosion techniques **improved the overall corrosion resistance of the 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 used. 3) Corrosion-resistant coatings: The development of corrosion-resistant coatings has always been a topic 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-prone coating systems, and fluoropolymer coatings, are currently the most widely studied coatings on an international level. 4) Surface engineering technology for equipment: The passivation of zinc coatings is a highly active field. The low-chromium or chromium-free colored passivations, black passivations, military-green passivations, and strong passivations introduced over the past decade – which involve composite passivation layers made of silicone resins or other resins – enable an improvement in the corrosion resistance of zinc coatings in marine and industrial atmospheric environments, as well as in industrial water, river, and stream environments. Electroless plating is a coating formation technique that utilizes metal salts and reducing agents to carry out a autocatalytic redox reaction in the same solution, 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 greater 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 technological 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. 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. 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. 5) Electrochemical corrosion prevention: Electrochemical protection refers to a corrosion prevention technique that uses an external electric 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 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 structures for sulfuric acid production, such as carbon steel storage tanks, various heat exchangers, sulfur trioxide generators, etc. Structures in ammonia water and ammonium salt solutions, carbonization towers, ammonia water storage tanks, etc. 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.