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Due to their small size and strong penetrating ability, chloride ions preferentially adsorb onto the passivation film, displacing oxygen atoms. They then combine with the cations in the passivation film to form soluble chlorides, resulting in the formation of small pits at specific points on the newly exposed base metal, which in turn causes corrosion of the equipment. The effect of chloride ions on metal corrosion is manifested in two main aspects: on one hand, they reduce the formation of a passive film on the surface of the material or accelerate the breakdown of this passive film, thereby promoting localized corrosion ； On the other hand, it reduces the solubility of sulfur dioxide and carbon dioxide in aqueous solutions, thereby alleviating material corrosion. Chloride ions can accelerate corrosion reactions in aqueous solutions over time; they are able to penetrate the protective film on the metal surface, leading to crevice corrosion and pitting. In particular, it can cause cracking in metals such as austenitic stainless steel, accelerating the likelihood of equipment becoming unusable in a short period of time. Therefore, it is essential to prevent chloride ions from corroding metal equipment. 1 Mechanism of chloride ions in aqueous solutions causing corrosion of metal equipment. Chloride ions, the hardness of aqueous solutions, and dissolved oxygen are all important factors that contribute to the corrosion of metal equipment. Among these, the impact of chloride ions is relatively minor, but it still cannot be ignored. The mechanism by which chloride ions corrode metals can be roughly divided into two types: the phase film theory and the adsorption theory. Although their views differ, the reaction processes are essentially the same. The higher the chloride ion concentration, the greater the electrical conductivity of the aqueous solution, the lower the resistance of the electrolyte, and the easier it is for chloride ions to reach the metal surface, accelerating the process of local corrosion. Especially in acidic environments, chloride ions form a layer of chlorides on the metal surface, replacing the protective iron carbonate film, which leads to pitting, stress corrosion, galvanic corrosion, and crevice corrosion. During the corrosion process, chloride ions not only accumulate in the pits but also in the areas surrounding them; this is the initial stage of corrosion formation. 2 Analysis of the factors affecting chloride-induced corrosion of metal equipment. Firstly, although metal equipment undergoes heat treatment to eliminate stresses during its manufacturing process, residual stresses still remain. Moreover, during the installation of the equipment, factors such as temperature and mechanical stress inevitably generate internal stresses in the equipment, which in turn allows chloride ions to accumulate easily in those areas and cause corrosion. Secondly, chloride ions act as a catalyst, causing stress corrosion cracking in metal equipment; the initial signs of this are corrosion cracks or grooves, which prevent the damaged passivation film from being repaired. As corrosion progresses, the metal equipment develops branch-like cracks. Once again, temperature has an influencing effect on chloride-induced corrosion. In the presence of both stress and chloride ions, corrosion is not significant at lower temperatures; as the temperature rises, corrosion cracking becomes more severe. Finally, the flow rate of water in the solution is also an important factor contributing to corrosion. When the water flow velocity is low, chloride ions do not diffuse easily, which creates favorable conditions for their accumulation and thus exacerbates the corrosive effects of these ions. 3 Measures and recommendations for preventing and controlling chloride-induced corrosion of metal equipment. It is well known that chlorides have highly stable chemical properties, making it difficult to reduce their concentration through chemical reactions using agents. Therefore, it is highly feasible and controllable to reduce the corrosion of metal equipment caused by chloride ions through methods such as equipment improvement and the use of corrosion inhibitors. 3.1 Proper material selection and stress control: On the one hand, it is necessary to use materials resistant to stress corrosion, mainly high-purity austenitic chromium-nickel steels, high-silicon austenitic chromium-nickel steels, high-chromium ferritic steels, and ferritic-austenitic duplex steels. Among them, ferrite-austenite duplex steel has the best resistance to stress corrosion. In addition, it is necessary to optimize the materials used in metal equipment and improve their corrosion resistance; elements such as molybdenum, nitrogen, and silicon can be added to the metals, while increasing the chromium content as well. On the other hand, during assembly, stress concentration should be minimized as much as possible, and the areas in contact with the medium should have the lowest possible residual stresses to prevent dents and scratches; welding procedures must be strictly followed. Strictly follow the operating procedures, and control process parameters such as raw material composition, flow rate, medium temperature, pressure, and pH value. Add a corrosion inhibitor within the limits permitted by the process conditions. When chromium-nickel stainless steel is used in chlorides containing oxygen, the mass fraction of oxygen must be reduced to the specified standard value. 3.2 Reducing the chloride ion content in aqueous solutions: By adding corrosion inhibitors, the stability of the passivation film can be increased, thereby enabling control of corrosion; this also facilitates the re-passivation of damaged passivation films. In addition, external cathodic current protection can also be employed to suppress pitting. Additionally, the mechanism of this chemical agent involves binding with calcium ions in water or ferrous ions that are products of corrosion, to form compounds whose main component is polycalcium ferric phosphate. These compounds are deposited on the cathode surface through corrosion currents, thereby creating a protective film that prevents the metal from corroding. Finally, additional investment is made to further reduce the chloride content through methods such as ion exchange and reverse osmosis. 3.3 Use of inorganic anti-corrosion coatings: Inorganic anti-corrosion coatings can effectively prevent the corrosion of stainless steel caused by chloride ions. They possess high strength, high toughness, high temperature resistance, wear resistance, aging resistance, as well as resistance to acid, alkali, and salt corrosion; in addition, they have strong adhesion, which allows for their wide range of applications. Anti-corrosion coatings are highly advanced in technology and offer excellent anti-corrosion performance. A solution composed of specially formulated inorganic anti-corrosion and metal-passivating zinc powder exerts a strong ternary synergistic effect on the hydroxyl and alkyl groups attached to the silicon atoms. This solution has strong stability, which reduces the impact on the interior of polymers; the film formed is more dense, offering excellent anti-corrosion properties that protect metals from corrosion, and it also has strong adhesion. In summary, chloride ions have a significant impact on the corrosion of metal equipment. To ensure the stable and healthy development of production, companies need to pay more attention to the hazards caused by chloride ion corrosion, and take appropriate measures to control these ions based on the material used in the equipment, the manufacturing processes, and the operating conditions of the equipment. In addition, appropriate values for temperature, concentration, and pressure must be set to ensure that the chloride ion concentration remains below 25 ppm, thereby enabling the safe operation of metal equipment. Furthermore, where process conditions permit, appropriate corrosion inhibitors can be added or suitable new materials selected. Anti-corrosion measures for conventional equipment: 1. Design of anti-corrosion structures for equipment. Anti-corrosion structure design refers to considering how to prevent corrosion of equipment during the design phase; it includes not only the design of individual pieces of equipment but also the way in which they are installed together as well as the layout of piping systems – in other words, it involves system design. There are mainly the following situations. 2. Avoid the formation of dead zones. Local residues of liquid or accumulation of solid substances within the equipment can lead to concentration or buildup in those areas, causing corrosion; this phenomenon also occurs when the equipment is shut down. During design, efforts should be made to avoid dead corners and areas where liquid cannot be completely drained. 3. Avoid the formation of gaps Many devices are prone to having gaps, and areas where fluid flow is restricted are susceptible to gap corrosion; this phenomenon occurs in devices made of carbon steel, aluminum, stainless steel, low-alloy steel, and other materials. Once crevice corrosion occurs, it often leads to pitting and stress corrosion, causing greater damage; a good structural design is the best way to prevent crevice corrosion. The areas that often have problems are the sealing surfaces and the connection points. Since welding eliminates gaps at the joint, it is better than bolted connections. 4. Principles for material selection The materials used in the manufacture of chemical processing equipment are mostly ordinary carbon structural steel. These materials are characterized by their low cost, wide availability, good mechanical properties, and ease of processing. Under normal operating conditions, corrosion poses little threat to such steel; however, when used in environments with highly corrosive substances common in the chemical industry, their poor corrosion resistance makes them susceptible to corrosion damage. Therefore, the selection principles for corrosion-resistant metal materials are as follows: 1. The corrosion environment should be understood in as much detail as possible, such as the composition and concentration of the medium, as well as the operating temperature and pressure ； Is the liquid at rest or in motion? ； Stress state (including residual stress) ； Contact state of different materials ； Temperature changes, temperature cycle variations due to heating and cooling, as well as thermal shock and stress changes caused by sudden heating or cooling ； Environmental conditions that require special attention, such as high temperature, low temperature, high pressure, vacuum, impact loads, and alternating stresses ； 2 The type of equipment, its structure, and the requirements of the product must be taken into consideration ； 3 Consider the issue of the design’s expected service life, that is, the lifespan required to meet the needs of the entire production facility. Strive to ensure uniform degradation of the materials used in all parts of the equipment or pipelines, and take into account economic factors such as material costs, construction costs, and maintenance costs to effectively reduce corrosion and extend the equipment’s service life. 5. Electrochemical protection method The electrochemical protection method plays a very important role in preventing corrosion of chemical processing machinery and equipment. Its main principle is to utilize the electrochemical principles of galvanic cells in order to eliminate the galvanic cell reactions that cause electrochemical corrosion of metals, thereby protecting them. Electrochemical protection is divided into two main categories: anodic protection and cathodic protection. Anodic protection involves using the metal to be protected as an anode, and applying an external voltage within a certain range to induce anodic passivation, thereby changing its surface state from a normal chemical state to a passivated state and thus preventing the metal from corroding in certain acids, bases, or salts. Cathodic protection involves using the metal to be protected as a cathode, and there are two methods for this: 1. Cathodic protection with external current, which involves using an insoluble electrode as a auxiliary anode, placed in the electrolyte solution along with the cathode. When an external DC power supply is connected, a large number of electrons are forced to flow toward the metal cathode being protected (such as steel equipment), where they accumulate. This prevents or suppresses the oxidation of steel, in which electrons are lost, thereby protecting it. 2 The cathodic protection method using sacrificial anodes involves connecting a metal (such as zinc) or alloy with a stronger reducing power than iron to steel products. When electrochemical corrosion occurs, this active metal acts as the negative electrode of a microcell and is corroded, thereby protecting the steel equipment. Among electrochemical protection methods, one of the more effective corrosion control techniques at present is the use of sacrificial anodes to provide cathodic protection for chemical processing machinery and equipment. The commonly used materials for sacrificial anodes in our country at present are magnesium anodes, which include pure magnesium as well as Mg-Mn alloys. Among them, the standard electrode potential of zinc is negative relative to iron; therefore, under certain medium conditions, when these two metals come into contact with each other, a microcell is formed. In this microcell, zinc acts as the anode while iron functions as the cathode. Zinc corrodes in the medium, whereas iron is protected. In actual use, the sacrificial anode can be welded to the components of chemical machinery and equipment as a method for installing it; bolts can also be used for fixing it. 6. Corrosion inhibitors Corrosion inhibitors are chemical substances that, when added to a corrosion inhibition system at an appropriate concentration, can significantly reduce the rate of metal corrosion with little effect on the concentration of the corrosive agents. The amount of corrosion inhibitor used is very small; although it cannot change the metal’s tendency to corrode in a given medium, it can form a protective film on the metal surface, thereby slowing down the rate of corrosion and suppressing it. Compared with other corrosion prevention methods, corrosion inhibitors are characterized by their ease of use, cost-effectiveness, and effectiveness. They are widely applied in industrial sectors such as petrochemicals, machinery manufacturing, and transportation, and have become an indispensable important protection measure in certain industrial processes, being included in the production procedures or operating guidelines. The oil industry is one of the sectors that uses the most corrosion inhibitors, with these agents being needed in everything from oil drilling and development to transportation and refining. In recent years, as human awareness of environmental protection has increased and the concept of sustainable development has gained prominence, new requirements have been placed on the development and use of corrosion inhibitors. Research and development efforts are focused on creating inhibitors that are environmentally friendly, meaning they do not cause harm to the environment, while also meeting performance and economic objectives. 7. Corrosion inhibitors represent the future direction for their development. In summary, the corrosion protection of chemical processing equipment is a very practical issue. Therefore, there are still many pressing issues related to the corrosion prevention of chemical processing equipment that need to be addressed by researchers and manufacturers. Applicable conditions of several stainless steels in chlorinated aqueous solutions 1 304 stainless steel: This is the cheapest and most widely used austenitic stainless steel (used in industrial equipment for food, chemicals, nuclear energy, etc.). Suitable for general organic and inorganic media. For example, nitric acid with a concentration of <30%, temperature ≤100°C, or a concentration of ≥30% and temperature <50°C ； Carbonic acid, ammonia water, and alcohols at various concentrations at temperatures ≤100℃. Poor corrosion resistance in sulfuric acid and hydrochloric acid ； It is particularly sensitive to crevice corrosion caused by chlorinated media (such as cooling water). The corrosion resistance and applications of 2304L stainless steel are essentially the same as those of 304 stainless steel. Due to its lower carbon content (≤0.03%), it exhibits better corrosion resistance (especially against intergranular corrosion, including in the weld area) and weldability, making it suitable for semi-welded or fully welded PHEs. Type 3316 stainless steel is suitable for general organic and inorganic media. For example, natural cooling water, cooling tower water, softened water ； carbonic acid ； Acetic acid and caustic alkali solutions with a concentration of <50% ； Alcohols and propylene glycol as solvents ； Dilute nitric acid at a temperature ≤100°C (concentration <20%), dilute phosphoric acid (concentration <30%), etc. However, it is not suitable for sulfuric acid. Due to its approximately 2% Mo content, it has better corrosion resistance in seawater and other chlorinated media than type 304, and can fully replace type 304. The corrosion resistance and applications of 4 316L stainless steel are basically the same as those of 316 stainless steel. Due to its lower carbon content (≤0.03%), it exhibits better weldability and corrosion resistance after welding, making it suitable for semi-welded or fully welded PHEs. Stainless steel type 5317 is suitable for applications that require a longer service life than type 316. Due to slightly higher contents of Cr, Mo, and Ni elements compared to type 316, it exhibits better resistance to crevice corrosion, pitting, and stress corrosion. 6 AISI 904L or SUS 890L stainless steels are cost-effective austenitic stainless steels that balance price and corrosion resistance; they offer better corrosion resistance than the aforementioned materials, making them particularly suitable for use in acids such as sulfuric acid and phosphoric acid, as well as halides (containing Cl— and F—). Due to its high contents of Cr, Ni, and Mo, it exhibits excellent resistance to stress corrosion, pitting, and crevice corrosion. 7 Avesta 254 SMO is an ultra-low carbon high-grade stainless steel that represents an improvement over type 316 through an increased Mo content; it offers excellent resistance to chloride pitting and crevice corrosion, making it suitable for use in saline solutions, inorganic acids, and other media where type 316 is not appropriate. 8 Avesta 654 SMO is a super-low carbon high-grade stainless steel with chromium, nickel, molybdenum, and nitrogen contents that are all higher than those of 254 SMO; it exhibits better resistance to chloride corrosion than 254 SMO and can be used in cold seawater. 9 RS-2 (OCr20Ni26Mo3Cu3Si2Nb) stainless steel is a domestically produced Cr–Ni–Mo–Cu stainless steel. Its resistance to pitting and crevice corrosion is equivalent to that of type 316, while its resistance to stress corrosion is better. It can be used with concentrated sulfuric acid (concentration 90–98%) at temperatures below 80 °C, with an annual corrosion rate of ≤0.04 mm/a. 10 Incoloy 825(S) is a high-grade stainless steel composed of Ni (40%)–Cr (22%)–Mo (3%). Incoloy is a registered trademark of Thennational Nickel Co. Suitable for sulfuric acid of various concentrations at low temperatures ； It exhibits good corrosion resistance in caustic alkali solutions (such as NaOH) with concentrations of 50% to 70%, and does not suffer from stress corrosion cracking. However, it is very sensitive to crevice corrosion caused by chlorides. Furthermore, its stamping performance is also poor, so it is not a commonly used material for sheets. The 11 31 alloy is an improved version of 904L (with increased Mo and N contents), as well as a standard high-grade stainless steel containing 6% Mo (31% Ni-27% Cr-6.5% Mo-32% Fe). Better corrosion resistance in many media than 904L ； In sulfuric acid at concentrations of 20% to 80% and temperatures of 60°C to 100°C, its corrosion resistance even exceeds that of C-276. 12 33 alloy is a fully austenitized chromium-based high-grade stainless steel, whose corrosion resistance can compare with that of certain Ni-Cr-Mo alloys such as Inconel 625. It exhibits good resistance to localized corrosion and stress corrosion cracking in acidic and alkaline media, including nitric acid and mixtures of nitric acid and hydrofluoric acid ； Its corrosion resistance in concentrated nitric acid is much better than that of 304L. For example, it is suitable for sulfuric acid with a concentration of over 96%–99%, a temperature of ≤150°C, and a sulfur dioxide content of less than 200 mg/L ； Hot seawater ； Highly corrosive boiling solutions with a concentration of ≤50% ； Phosphoric acid with a concentration of ≤85% and a temperature of ≤150°C, etc. However, it is not suitable for reducing media (such as dilute sulfuric acid). The price is roughly the same as that of C-276. 13 C-2000 alloy is a nickel-based alloy developed in the 1990s; its price is similar to that of C-276, and it is one of the materials with the best corrosion resistance among those listed. In media such as sulfuric acid at concentrations below moderate levels, dilute hydrochloric acid, phosphoric acid at concentrations of ≤50% at boiling temperature, and hot chlorides, its corrosion resistance is better than that of C-276 and C-22, showing a tendency to replace C-22 alloys. However, for sulfuric acid with a concentration of ≥70%, its corrosion resistance is inferior to that of C-276. The chemical composition of the 14 59 alloy is essentially the same as that of C-2000, with the exception of a slightly higher Ni content (59%), lower Fe levels, and the absence of Cu and W. It is currently the nickel-based alloy with the best corrosion resistance, thermal stability, formability, and weldability. Since its commercialization in 1990, it has been widely used in sulfuric acid, hydrochloric acid, hydrofluoric acid, as well as many media containing chlorine, oxygen, and at low pH levels.