Test your knowledge: What are the main methods for controlling corrosion?

This post was last edited by JFTANG726 on 2010-1-26 at 20:09. Reference answer: The main methods for controlling corrosion are: 1. Proper selection of materials and design; 2. Adjusting the environment; 3. Adding corrosion inhibitors; 4. Cathodic protection; 5. Anodic protection; 6. Alloying; 7. Surface treatment; 8. Metal plating and coating; 9. Coatings; 10. Lining

This post was last edited by JFTANG726 on 2010-1-26 at 20:08. First, proper material selection and design are necessary; for this purpose, complete corrosion data is required. Second, the environment must be adjusted – if the various factors that cause corrosion in an environment can be eliminated, corrosion will cease or slow down. However, most environments cannot be controlled. However, the local environment can be adjusted without affecting the manufacturing process; for example, the air entering a sealed warehouse can have its moisture removed first, which helps prevent the metal components stored there from rusting. III. Addition of corrosion inhibitors: Adding a small amount of corrosion inhibitor to a corrosive environment can **slow down the corrosion of metals. Corrosion inhibitors can be classified into inorganic, organic, and weathering types, with variations among different types of corrosion inhibitors. IV. Cathodic protection: In a corrosion cell, the cathode is the electrode that accepts electrons to undergo a reduction reaction; only the anode experiences corrosion. Using this principle, a cathodic current can be introduced from the outside to the device that needs protection, with the entire surface of the device acting as the cathode. V. Anodic protection: For metals that can undergo passivation, by applying a current to the anode, the potential is maintained in the passivation region, thereby keeping the corrosion rate at very low levels. VI. Alloying: Metal alloys that can promote passivation are added to the base metal; when the addition amount reaches a certain proportion, materials with corrosion-resistant properties can be obtained. VII. Surface Treatment: Before coming into contact with the operating environment, metals are treated with passivators or film-forming agents, which result in the formation of a stable and dense passivation layer on the surface, thereby increasing their corrosion resistance. 8. Metal coatings and claddings: Thin layers of more corrosion-resistant metals can be used to protect the steel substrate. 9. Coatings: Organic paints are used to protect metal structures exposed to the atmosphere. 10. Linings: These are usually whole pieces of material, suitable for use inside equipment that is in contact with highly corrosive substances.

This post was last edited by JFTANG726 on 2010-1-26 20:08. The degradation of metal that occurs as a result of chemical and electrochemical reactions when the metal comes into contact with its surrounding medium is called metal corrosion. From a thermodynamic perspective, with the exception of a few precious metals such as Au and Pt, all metals have a tendency to transform into ions; in other words, metal corrosion is a spontaneous and widespread phenomenon. When metals are corroded, changes occur in their shape, color, and mechanical properties, leading to equipment damage, pipeline leaks, and product contamination. This can result in serious accidents such as fires or explosions, as well as severe waste of resources and energy, causing substantial losses to the national economy. It is estimated that in developed countries around the world, the economic losses caused by metal corrosion each year account for approximately 3.5% to 4.2% of their gross national product, which is more than the total losses resulting from various natural disasters such as fires, wind disasters, and others. Some estimates suggest that around 100 million tons of metal are lost worldwide each year due to corrosion! Therefore, studying the mechanisms of corrosion and taking protective measures are of great significance for economic development. There are many methods for preventing metal corrosion, including improving the properties of the metal itself, separating the metal to be protected from corrosive agents, treating the metal’s surface, altering the corrosive environment, and using electrochemical protection methods. 　　(1) Improving the nature of metals: Different material combinations are selected to form corrosion-resistant alloys based on their intended uses, or alloying elements are added to metals to enhance their corrosion resistance, thereby preventing or slowing down metal corrosion. For example, adding nickel to steel to create stainless steel can enhance its corrosion resistance. 　　(2) Forming a protective layer Covering the metal surface with various protective layers to separate the metal to be protected from corrosive substances is an effective method for preventing metal corrosion. The protective layers commonly used in industry fall into two main categories: non-metallic protective layers and metallic protective layers. They are achieved through chemical, physical, and electrochemical methods. 　　①Phosphating of metals: After steel products are degreased and rust removed, they are immersed in a phosphate solution with a specific composition; this results in the formation of a water-insoluble phosphate film on the surface of the metal. This process is known as phosphating. 　　The phosphating film is dark gray to black-gray in color, with a thickness generally ranging from 5μm to 20μm, and it exhibits good corrosion resistance in the atmosphere. The membrane has a microporous structure, giving it a strong adsorption capacity for paints and similar substances; when used as a base coat for paints, its corrosion resistance can be further improved. 　　②Oxidation treatment of metals: Steel products are placed in a mixed solution of NaOH and NaNO2 and heated; this causes a blue oxide film with a thickness of approximately 0.5μm to 1.5μm to form on their surface (the main component of this film is Fe3O4). This process serves to prevent corrosion of the steel, and it is known as bluing treatment. This oxide film possesses high elasticity and lubricity, without affecting the precision of the parts. Therefore, components for precision and optical instruments, such as spring steel, thin steel sheets, and fine steel wires, are commonly treated with blueing. 　　③Non-metallic coatings: Coatings made from non-metallic materials such as paint, plastic, enamel, and mineral oils, which are applied to metal surfaces to form a protective layer; these also serve to prevent corrosion. For example, ship hulls, carriages, buckets, etc. are often painted, car exteriors are frequently sprayed with paint, while guns and machines are usually coated with mineral oils. Spraying metal surfaces with plastics such as polyethylene, polyvinyl chloride, polyurethane, etc., yields better results than using paint. This plastic coating is dense and smooth, with bright colors, offering both corrosion protection and decorative functions. 　　Enamel is a glass-ceramic glaze with a high content of SiO2, and it possesses excellent corrosion resistance; therefore, as a corrosion-resistant non-metallic coating, it is widely used in industries such as petrochemicals, pharmaceuticals, and instrumentation, as well as in everyday household items. 　　④Metal protective coating: It is a protective coating formed by plating a metal onto the surface of another metal product that is to be protected. The preceding metal is often referred to as the coating metal. In addition to electroplating and chemical plating, methods for forming metal coatings include hot-dip plating, thermal spraying, immersion plating, and vacuum plating. 　　Hot-dip coating is a method in which metal parts are immersed in molten metal to obtain a metal coating; the metal used for this coating is a low-melting-point metal such as Zn, Sn, Pb, and Al. Hot-dip galvanizing is mainly used for steel pipes, steel plates, steel strips, and steel wires, and it is the most widely applied method ； Hot-dip tin plating is used for storage containers made of thin steel sheets and in food processing, among other applications ； Hot-dip lead is mainly used for chemical corrosion protection and cable coating ； Hot-dip aluminum coating is mainly used to prevent high-temperature oxidation of steel parts. 　　(3) Improving the corrosion environment 　Improving the environment is important for reducing and preventing corrosion. For example, reducing the concentration of corrosive agents, removing oxygen from these agents, and controlling environmental factors such as temperature and humidity can all help reduce and prevent metal corrosion. Metal corrosion can also be reduced and prevented by adding substances to the corrosive medium that can lower the corrosion rate (known as corrosion inhibitors). 　　(4) Electrochemical protection method The electrochemical protection method is a technique that utilizes electrochemical principles to take measures on metal equipment so that it functions as the cathode in a corrosion cell, thereby preventing or reducing metal corrosion. 　　①Sacrificial anode protection method: The sacrificial anode protection method involves using a metal or alloy with a lower electrode potential than that of the metal to be protected as an anode, which is then attached to the metal to be protected. This creates a corrosion cell, with the metal to be protected acting as the cathode and thus being protected. 　　Common materials used for sacrificial anodes include aluminum, zinc, and their alloys. This method is commonly used to protect the hulls of sea vessels, various metal equipment and components in seawater, as well as to prevent corrosion of large-scale installations such as oil storage tanks and oil pipelines. 　　②External current method: The metal to be protected and another additional electrode are used as the two poles of an electrolytic cell, with the metal to be protected acting as the cathode; under the influence of an external direct current, the cathode is protected. This method is mainly used to prevent corrosion of metal equipment in soil, seawater, and river water. 　　Although metal corrosion causes significant harm to production, the principles of corrosion can also be utilized to benefit production, leading to the development of corrosion processing techniques. For example, printed circuits are widely used in the electronics industry. Its manufacturing method and principle involve using photocopying to print the circuits on copper foil, and then corroding the copper areas not protected by the photosensitive adhesive with an iron trichloride solution, thereby obtaining a printed circuit board with clear lines. The reaction of ferric chloride in corroding copper is as follows: 2FeCl3 + Cu == 2FeCl2 + CuCl2. In addition, there are new techniques such as electrochemical etching and plasma etching, which are superior to the wet chemical etching method using ferric chloride, offering higher resolution.

This post was last edited by JFTANG726 on 2010-1-26 at 20:08. First, proper material selection and design are necessary; for this purpose, complete corrosion data is required. Second, the environment must be adjusted – if the various factors that cause corrosion in an environment can be eliminated, corrosion will cease or slow down. However, most environments cannot be controlled. However, the local environment can be adjusted without affecting the manufacturing process; for example, the air entering a sealed warehouse can have its moisture removed first, which helps prevent the metal components stored there from rusting. III. Addition of corrosion inhibitors: Adding a small amount of corrosion inhibitor to a corrosive environment can **slow down the corrosion of metals. Corrosion inhibitors can be classified into inorganic, organic, and weathering types, with variations among different types of corrosion inhibitors. IV. Cathodic protection: In a corrosion cell, the cathode is the electrode that accepts electrons to undergo a reduction reaction; only the anode experiences corrosion. Using this principle, a cathodic current can be introduced from the outside to the device that needs protection, with the entire surface of the device acting as the cathode. V. Anodic protection: For metals that can undergo passivation, by applying a current to the anode, the potential is maintained in the passivation region, thereby keeping the corrosion rate at very low levels. VI. Alloying: Metal alloys that can promote passivation are added to the base metal; when the addition amount reaches a certain proportion, materials with corrosion-resistant properties can be obtained. VII. Surface Treatment: Before coming into contact with the operating environment, metals are treated with passivators or film-forming agents, which result in the formation of a stable and dense passivation layer on the surface, thereby increasing their corrosion resistance. 8. Metal coatings and claddings: Thin layers of more corrosion-resistant metals can be used to protect the steel substrate. 9. Coatings: Organic paints are used to protect metal structures exposed to the atmosphere. 10. Linings: These are usually whole pieces of material, suitable for use inside equipment that is in contact with highly corrosive substances.

This post was last edited by mopeizhi on 2010-2-7 at 14:04. Corrosion inhibitors are additives used in corrosive media to prevent metal corrosion. They require only small amounts, do not alter the properties of the corrosive medium, do not need special dosing equipment, and no treatment of the equipment surface is required. Therefore, the use of corrosion inhibitors is a cost-effective and adaptable metal protection measure. In open-loop cooling water systems, common corrosion inhibitors include silicates, molybdates, zinc salts, phosphates, polyphosphates, organic polyphosphonic acids, mercaptobenzothiazole (MBT), benzotriazole (BTA) and thiazolotriazole (TTA), as well as ferrous sulfate. To reduce the pressure of environmental eutrophication, there is a growing tendency to use these latter organic phosphonates and low-phosphorus corrosion inhibitors. ⑵ Raising the pH value of the circulating water increases the tendency for an oxidizing protective film to form on the metal surface, facilitating passivation and thus helping to control equipment corrosion. An open-loop circulating cooling water system typically raises the pH value through aeration in the cooling tower; when CO2 in the water and in the air reaches equilibrium, the pH of the water is 8. Around 5. Raising the pH value of circulating water inevitably leads to some problems: an increased tendency for scaling in the circulating water ； The corrosion rate of the equipment has decreased, but it still does not meet the requirements ； Some commonly used corrosion inhibitors have lost their effectiveness. At present, this can be addressed by adding composite corrosion inhibitors developed specifically for alkaline cooling water treatment, such as: polyphosphate-zinc salt-phosphonate-dispersant, polyphosphate-normal phosphate-phosphonate-terpolymer, organic polyphosphonic acid-polymer dispersant-oxazole, polyol phosphate ester-acrylic polymer, HEDP-PMA, etc. The combined formulations of these water treatment agents can exert a combined effect of scale removal and corrosion prevention; due to synergistic or additive effects, their performance is more significant than that of single agents used alone, and this represents the development trend in corrosion inhibitors. ⑶ Heat exchangers made of corrosion-resistant materials, such as those made of polypropylene or graphite-modified polypropylene, are used, but they are rarely employed due to their poor heat exchange efficiency. ⑷ Coating with anti-corrosion paint provides shielding, corrosion inhibition, cathodic protection, and pH buffering effects through the anti-corrosion paint, thereby protecting the equipment from corrosion

This post was last edited by mopeizhi on 2010-2-7 at 14:05: 1. Choose the appropriate material, 2. Line the pipes, 3. Apply paint or galvanize, 4. Keep it clean

This post was last edited by mopeizhi on 2010-2-7 14:06. Metal Corrosion Control Techniques (1) Proper Material Selection and Design. Choosing materials with a low corrosion rate in specific environments, low cost, and good performance, based on the corrosion data of metal materials, is a common and simple method for controlling corrosion; it enables equipment to have an economical and reasonable service life. Since the structure of the equipment often has an impact on corrosion, proper design is also important. Furthermore, those responsible for material selection also need to have certain knowledge of corrosion and anti-corrosion measures in order to address material selection issues more effectively. 　　(2) Adjusting the medium environment 　　If the various factors that cause corrosion in metal materials and equipment within their operating environment can be eliminated, corrosion will cease or slow down. However, most environmental conditions are uncontrollable; for example, moisture in the atmosphere and soil, as well as oxygen in seawater, cannot be removed. The production process cannot be altered arbitrarily; in such cases, the local environment can be adjusted. For example, deoxygenation of the water entering the boiler (by adding deoxidizing agents such as Na2SO3 and N2H4) can help protect the boiler tubes from corrosion ； Remove the moisture from the air entering the sealed warehouse first, to prevent the metal components from rusting during storage ； Adding alkalis or acids to water regularly to adjust the pH to an optimal range (usually near neutral) can prevent scaling and perforation of heat exchangers and other equipment caused by cooling water ； Alkalis or ammonia are also often added in the petroleum refining process to keep the production fluids neutral to slightly alkaline. 　　(3) Addition of corrosion inhibitors Adding a small amount of corrosion inhibitors to media that may cause metal corrosion can **slow down the process of metal corrosion**. Corrosion inhibitors can be divided into three categories: inorganic corrosion inhibitors, organic corrosion inhibitors, and gas-phase corrosion inhibitors. 　　Some inorganic corrosion inhibitors slow down the anodic process; they are known as anodic-type corrosion inhibitors, such as oxidants that promote anodic passivation (chromates, nitrites, Fe3+) or agents that form anodic films (alkalis, phosphates, silicates, benzoates) ； Another category of inorganic corrosion inhibitors is those that promote cathodic polarization; they are known as cathodic corrosion inhibitors. Examples include Ca2+, Zn2+, Mg2+, Cu2+, Cd2+, Mn2+, Ni2+, etc. These ions can react with OH- produced during cathodic reactions to form insoluble hydroxides, which cover the cathode surface in a thick layer. This prevents oxygen from diffusing to the cathode, thereby increasing concentration polarization. There are also mixed-type corrosion inhibitors that simultaneously inhibit both the anodic and cathodic processes. Impurities in some solutions, such as compounds of S, Se, As, Sb, Bi, etc., can inhibit the hydrogen evolution process at the cathode, increase cathodic polarization, and thereby slow down corrosion. The amount of corrosion inhibitor generally needs to be determined through testing first. 　　Organic corrosion inhibitors belong to the adsorptive type of corrosion inhibitors; they adsorb on the metal surface to form an invisible film several molecules thick. Generally, they inhibit both anodic and cathodic reactions, but the degree of inhibition varies. Commonly used varieties include organic compounds containing N, S, O, and P, such as amines, heterocyclic compounds, long-chain fatty acid compounds, thioamides, aldehydes, organophosphates, etc. The adsorption types of slow-release agents include electrostatic adsorption and chemical adsorption. Electrostatic adsorbents include aniline and its substitutes, pyridine, butylamine, benzoic acid and its substitutes such as benzenesulfonic acid ； Chemical adsorbents include nitrogen and sulfur heterocyclic compounds ； Some compounds exhibit both electrostatic and chemical adsorption effects. Furthermore, some chelating agents can form a thin layer of metal-organic compounds on the metal surface. In recent years, organic corrosion inhibitors have developed rapidly and are widely used. However, the use of these inhibitors also has drawbacks, such as the potential to contaminate products and causing adverse effects on the production process. 　　Vapor-phase corrosion inhibitors are mostly highly volatile substances and also belong to the category of adsorption-type corrosion inhibitors. Its vapor decomposes water in the atmosphere to produce effective corrosion-inhibiting groups, which adhere to the metal surface and reduce corrosion; it is generally used for the protection, storage, and transportation of metal components. It must be used to seal packages, and it can also be used for protection in the compartments of ocean oil tankers. Common effective vapor corrosion inhibitors include alicyclic amines and aromatic amines ； Polymeramine ； Nitrite and thiourea mixture ； Urotropine and ethanolamine ； ***and **, etc. 　　(4) Cathodic protection: In the process of electrochemical corrosion of metals, the cathode of a microcell is the electrode that accepts electrons and undergoes reduction reactions, while the anode is the electrode that loses electrons and undergoes oxidation reactions; it is only the anode where corrosion occurs. Cathodic protection involves using the metal that needs to be protected as the cathode of a corrosion cell (the positive electrode of a galvanic cell) or as the cathode of an electrolytic cell, thereby preventing it from corroding. The former is called the sacrificial anode method, while the latter is called the impressed current method. The sacrificial anode method involves connecting a metal or its alloy with a more negative (more reactive) electrode potential to the equipment that needs protection. For example, steel equipment can be connected to a Zn, Mg, or Al alloy; in the galvanic cell thus formed, these materials act as the anodes and corrode, while the metal equipment serves as the cathode and is thus protected. This sacrificial anode must be replaced periodically. The impressed current method involves connecting a current-guiding electrode (such as graphite, platinum, or materials coated with ruthenium, titanium, high-silicon iron, scrap steel, etc.) to the system as the anode. When an external cathodic current is applied, the local cathodic current and the local anodic current become equal in magnitude but opposite in direction, thus canceling each other out; this stops metal corrosion and achieves the purpose of protecting the equipment. Cathodic protection is widely used for metal structures and devices in soil and seawater, such as pipelines, cables, sea vessels, port facility docks, drilling platforms, reservoir gates, oil and gas wells, etc. To reduce current input and extend service life, cathodic protection is generally used in combination with metal surface coating methods; it is an economical, simple, and effective method for metal corrosion prevention.

This post was last edited by mopeizhi on 2010-2-7 14:07

This post was last edited by mopeizhi on 2010-2-7 at 14:08. First, proper material selection and design are necessary; for this purpose, complete corrosion data is required. Second, the environment must be adjusted – if the various factors that cause corrosion in an environment can be eliminated, corrosion will cease or slow down. However, most environments cannot be controlled. However, the local environment can be adjusted without affecting the manufacturing process; for example, the air entering a sealed warehouse can have its moisture removed first, which helps prevent the metal components stored there from rusting. III. Addition of corrosion inhibitors: Adding a small amount of corrosion inhibitor to a corrosive environment can **slow down the corrosion of metals. Corrosion inhibitors can be classified into inorganic, organic, and weathering types, with variations among different types of corrosion inhibitors. IV. Cathodic protection: In a corrosion cell, the cathode is the electrode that accepts electrons to undergo a reduction reaction; only the anode experiences corrosion. Using this principle, a cathodic current can be introduced from the outside to the device that needs protection, with the entire surface of the device acting as the cathode. V. Anodic protection: For metals that can undergo passivation, by applying a current to the anode, the potential is maintained in the passivation region, thereby keeping the corrosion rate at very low levels. VI. Alloying: Metal alloys that can promote passivation are added to the base metal; when the addition amount reaches a certain proportion, materials with corrosion-resistant properties can be obtained. VII. Surface Treatment: Before coming into contact with the operating environment, metals are treated with passivators or film-forming agents, which result in the formation of a stable and dense passivation layer on the surface, thereby increasing their corrosion resistance. 8. Metal coatings and claddings: Thin layers of more corrosion-resistant metals can be used to protect the steel substrate. 9. Coatings: Organic paints are used to protect metal structures exposed to the atmosphere. 10. Linings: These are usually whole pieces of material, suitable for use inside equipment that is in contact with highly corrosive substances.

This post was last edited by mopeizhi on 2010-2-7 at 14:10. First, proper material selection and design are necessary; for this purpose, complete corrosion data is required. Second, the environment must be adjusted – if the various factors that cause corrosion in an environment can be eliminated, corrosion will cease or slow down. However, most environments cannot be controlled. However, the local environment can be adjusted without affecting the manufacturing process; for example, the air entering a sealed warehouse can have its moisture removed first, which helps prevent the metal components stored there from rusting. III. Addition of corrosion inhibitors: Adding a small amount of corrosion inhibitor to a corrosive environment can **slow down the corrosion of metals. Corrosion inhibitors can be classified into inorganic, organic, and weathering types, with variations among different types of corrosion inhibitors. IV. Cathodic protection: In a corrosion cell, the cathode is the electrode that accepts electrons to undergo a reduction reaction; only the anode experiences corrosion. Using this principle, a cathodic current can be introduced from the outside to the device that needs protection, with the entire surface of the device acting as the cathode. V. Anodic protection: For metals that can undergo passivation, by applying a current to the anode, the potential is maintained in the passivation region, thereby keeping the corrosion rate at very low levels. VI. Alloying: Metal alloys that can promote passivation are added to the base metal; when the addition amount reaches a certain proportion, materials with corrosion-resistant properties can be obtained. VII. Surface Treatment: Before coming into contact with the operating environment, metals are treated with passivators or film-forming agents, which result in the formation of a stable and dense passivation layer on the surface, thereby increasing their corrosion resistance. 8. Metal coatings and claddings: Thin layers of more corrosion-resistant metals can be used to protect the steel substrate. 9. Coatings: Organic paints are used to protect metal structures exposed to the atmosphere. 10. Linings: These are usually whole pieces of material, suitable for use inside equipment that is in contact with highly corrosive substances.