Chloride ion corrosion mechanism and prevention measures

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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 layer 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, crevice corrosion, and intergranular corrosion. During the corrosion process, chloride ions not only accumulate in the pit areas 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 layer from being repaired. As corrosion progresses, the metal equipment develops branch-like cracks. Again, temperature has an inducing 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. Countermeasures and recommendations for preventing and controlling chloride ion corrosion of metal equipment. As is well known, chloride ions have highly stable chemical properties, and it is 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 steels have the best resistance to stress corrosion. In addition, it is necessary to optimize the materials used in metal equipment and improve its corrosion resistance; elements such as molybdenum, nitrogen, and silicon can be added to the metals, while also increasing the chromium content. 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 stress 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 using chromium-nickel stainless steel in chlorides containing dissolved oxygen, the mass fraction of oxygen should 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 corrosion control; this also facilitates the repassivation of damaged passivation films. In addition, external cathodic current protection can also be used to suppress pitting. Additionally, the mechanism of this agent involves binding with calcium ions in water or ferrous ions resulting from 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, increase investment in costs to further reduce the chloride ion 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 enables their wide range of applications. Anticorrosion coatings are highly advanced in technology, and they also provide excellent corrosion protection. 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 possesses 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 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; non-metallic materials can also be considered as appropriate to optimize economic benefits. 　　Anti-corrosion measures for conventional equipment 1. Anti-corrosion structural design of equipment Anti-corrosion structural design refers to the consideration given during the design phase to prevent corrosion of the equipment. It includes not only the design of individual devices but also the way in which these devices are installed together, as well as the layout of the piping systems – in other words, it relates to system design. There are mainly the following situations. 　　2. Avoid the formation of dead zones. Local residue 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 during shutdowns of the equipment. During design, efforts should be made to avoid dead corners and areas where fluid 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 selecting materials The materials used to manufacture chemical engineering machinery are mostly ordinary carbon structural steel. Their advantages include low cost, wide availability, good mechanical properties, and ease of processing. Under normal operating conditions, corrosion poses little threat to this type of steel; however, when used in environments with highly corrosive substances common in the chemical industry, its poor corrosion resistance makes it 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, including 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 cooling or heating ； 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 expected service life of the design, that is, the lifespan required to meet the needs of the entire production facility. Strive to ensure uniform degradation of the materials in all parts of the equipment or pipelines, and take into account economic factors such as material costs, construction costs, and maintenance costs in order to effectively reduce corrosion and extend the service life of the equipment. 　　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 through anodic passivation within a certain range of applied voltage, its surface is changed from a normal chemical state to a passivated state, thereby preventing the metal from corroding in certain acids, bases, or salts. Cathodic protection involves using the metal to be protected as the cathode. There are two methods for this: 1. Impressed current cathodic protection: An insoluble electrode is used as an auxiliary anode, which is placed in the electrolyte solution together 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 currently more effective corrosion control methods is the use of sacrificial anode protection, which involves cathodic protection of chemical engineering equipment. Currently, the commonly used materials for sacrificial anodes in our country are magnesium anodes, which include pure magnesium and Mg-Mn alloys, among others. 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 acts 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 fixation. 　　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 the 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 corrosion inhibitors most frequently; they are employed in all stages from oil drilling, development, and transportation to refining. In recent years, with the growing awareness of environmental protection among humans and the deepening concept of sustainable development, new requirements have been placed on the development and application of corrosion inhibitors. Research and development efforts now aim to produce environmentally friendly products that cause no 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 issues related to the corrosion prevention of chemical engineering equipment that need to be addressed; these require continuous efforts from researchers and actual producers to resolve. Applicable conditions of several stainless steels in chlorinated aqueous solutions: 1. Type 304 stainless steel – this is the cheapest and most widely used austenitic stainless steel (used in industrial equipment in industries such as food processing, chemicals, and nuclear energy). 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°C. Poor corrosion resistance in sulfuric acid and hydrochloric acid ； It is particularly sensitive to crevice corrosion caused by chlorinated media (such as cooling water). 2. The corrosion resistance and applications of 304L stainless steel are basically the same as those of 304 stainless steel. Due to its lower carbon content (≤0.03%), it exhibits better corrosion resistance (especially intergranular corrosion resistance, including in the weld area) and weldability, making it suitable for semi-welded or fully welded PHEs. 3. Type 316 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 solvents such as propylene glycol ； Dilute nitric acid at a temperature of ≤100℃ (concentration <20%), dilute phosphoric acid (concentration <30%), etc. However, it is not suitable for sulfuric acid. Due to its approximately 2% Mo content, its corrosion resistance in seawater and other chlorine-containing media is better than that of Type 304; thus, it can fully replace Type 304. 4. The corrosion resistance and applications of 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. 5. Stainless steel type 317 is suitable for applications that require a longer service life than that of 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 steel – this is an austenitic stainless steel that offers a good balance between price and corrosion resistance. It possesses better corrosion resistance than the materials mentioned earlier, and is 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 premium 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 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. 11, 31 alloys are improved versions of 904L (with increased Mo and N contents), as well as standard high-grade stainless steels 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 local 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 trend to replace C-22 alloys. However, for sulfuric acid with a concentration of ≥70%, its corrosion resistance is inferior to that of C-276. 14. Compared with C-2000, the chemical composition of the 59 alloy is basically the same, except that the Ni content is slightly higher (59%), there is less Fe, and it is free 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.