【Prized Discussion】How should metal corrosion be classified? There are three classification methods. What are they?

1. Chemical corrosion: refers to the corrosion that occurs as a result of a chemical reaction taking place when an oxidizing agent comes into contact with the metal surface. 2. Biological corrosion: refers to corrosion caused by the metabolic activities of various microorganisms. 3. Electrochemical corrosion: is corrosion that results from electrochemical reactions occurring in metals

Further in-depth discussion can be carried out from three aspects: the reaction mechanism of corrosion, the form of corrosion, and the corrosion environment. :lol

There are many classification methods for metal corrosion, which are usually based on the corrosion mechanism, the form of corrosion damage, and the corrosion environment. From the perspective of corrosion mechanisms, metal corrosion can be divided into two main categories: chemical corrosion and electrochemical corrosion. Based on the form of corrosion damage, metal corrosion is divided into two main categories: general corrosion and local corrosion. Classified by the corrosive environment: chemical medium corrosion, atmospheric corrosion, high-temperature corrosion, seawater corrosion, soil corrosion, and so on. The forms of local corrosion damage include the following: (1) pitting corrosion, (2) stress corrosion cracking, (3) intergranular corrosion, (4) crevice corrosion, (5) galvanic corrosion, (6) hydrogen corrosion, and (7) other forms of local corrosion, such as selective corrosion, cavitation corrosion, corrosion fatigue, etc

1. Chemical corrosion 2. Biological corrosion 3. Electrochemical corrosion.     Chemical corrosion refers to the corrosion that occurs as a result of a chemical reaction taking place when an oxidizing agent comes into contact with a metal surface.     Biological corrosion refers to corrosion caused by the life activities of various microorganisms.     Electrochemical corrosion refers to corrosion that occurs as a result of electrochemical reactions. Electrochemical corrosion is the most common and severe type of corrosion; therefore, studying it is of great significance!

There are three classification methods for metal corrosion. 1. Classified by corrosion reaction mechanism: there is chemical corrosion and electrochemical corrosion. 2. Classification by corrosion form: There is uniform corrosion and localized corrosion. In local corrosion, it is further divided into pitting corrosion, crevice corrosion, galvanic corrosion, erosion, stress corrosion cracking, intergranular corrosion, abradion, selective corrosion, etc., depending on their morphology and causes of formation. 3. Classification by corrosion environment: There is soil corrosion, atmospheric corrosion, seawater corrosion, high-temperature corrosion, naphthenic acid corrosion, polysulfuric acid corrosion, etc.

Based on the characteristics of the reactions, metal corrosion can be divided into 1. chemical corrosion, 2. biological corrosion, and 3. electrochemical corrosion.

There are various methods for classifying metal corrosion. (1) Based on the corrosion process, there are mainly chemical corrosion and electrochemical corrosion. Chemical corrosion is damage resulting from a direct chemical reaction between a metal and an environmental medium, with no electric current generated during the corrosion process. For example, the corrosion of metals in hot air or chlorine gas, and the corrosion of metals by non-electrolytes. Media that cause metal chemical corrosion are non-conductive. Electrochemical corrosion is damage to metals caused by electrochemical reactions in an electrolyte solution, with an electric current being generated during the corrosion process. Any medium that causes electrochemical corrosion is conductive. For example, the corrosion of metals in media such as acids, alkalis, salts, soil, and seawater. The main difference between electrochemical corrosion and chemical corrosion is that the former can be divided into two independent yet simultaneous cathodic and anodic processes, whereas chemical corrosion does not have this characteristic. Electrochemical corrosion is more common and widespread than chemical corrosion. (2) Based on the pattern of metal corrosion damage and the distribution of the corroded areas, it is divided into general corrosion and local corrosion. Uniform corrosion refers to corrosion that is distributed over the entire surface of the metal. General corrosion is uniform corrosion in which the degree of corrosion is the same throughout ； There is also non-uniform corrosion, in which the degree of corrosion varies across different areas. The corrosion that occurs when cleaning steel and aluminum equipment with acid wash solutions is generally uniform corrosion. Corrosion that occurs mainly in certain areas on the metal surface is known as local corrosion. Although the extent of corrosion in this type is relatively small, its high local corrosion rate can cause severe damage to equipment, or even explosions; thus, its hazards are greater. Metals can suffer from different types of localized corrosion under various environmental conditions. Such as pitting corrosion, crevice corrosion, stress corrosion, intergranular corrosion, wear corrosion, etc. Corrosion is also classified into high-temperature corrosion and room-temperature corrosion based on the environmental conditions of corrosion ； Dry corrosion and wet corrosion, etc. When metal comes into contact with its surrounding medium, the degradation caused by chemical and electrochemical reactions 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 metal is corroded, changes occur in its 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 disasters each year (such as fires, storms, and earthquakes). Some estimates even suggest that around 100 million tons of metal worldwide are lost due to corrosion, wear and tear each year! Therefore, studying corrosion mechanisms 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 employing electrochemical protection. (1) Improving the nature of metals: Different material combinations are selected to form corrosion-resistant alloys based on specific applications, 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 by chemical, physical, and electrochemical methods. ①Phosphating of metals: After degreasing and derusting steel products, they are immersed in a phosphate solution with a specific composition. This process forms a water-insoluble phosphate film on the metal surface; this process is known as phosphating. The phosphating film is dark gray to black-gray in color, with a thickness generally ranging from 5 μm to 20 μm, 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. ②Metal oxidation treatment involves adding steel products to a mixed solution of NaOH and NaNO2 and then heating them; this results in the formation of a blue oxide layer on their surface with a thickness of approximately 0.5 μm to 1.5 μm (the main component of which is Fe3O4). This process serves to prevent corrosion of the steel, and it is known as bluing treatment. This oxide film has great elasticity and lubricity, without affecting the precision of the parts. Therefore, components of 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, hulls, carriages, buckets, etc. are often coated with varnish; automobile bodies are often painted; guns and machines are often coated with mineral oils, etc. Spraying metal surfaces with plastics (such as polyethylene, polyvinyl chloride, polyurethane, etc.) yields better results than painting. This plastic coating is dense and smooth. It has vibrant colors and serves both anti-corrosion and decorative purposes. Enamel is a glass-ceramic glaze with a high content of SiO2, and it possesses excellent corrosion resistance; as such, it is used as a corrosion-resistant non-metallic coating in industries such as petrochemicals, pharmaceuticals, and instrumentation, as well as in everyday household items. ④Metal protective layer: 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 plating metal. Apart from electroplating and electroless plating, the formation of metal coatings also involves methods such as hot-dip coating, thermal spraying, diffusion coating, and vacuum coating. Hot-dip coating is a method in which metal parts are immersed in molten metal to obtain a metallic 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 aluminizing is mainly used for preventing high-temperature oxidation of steel parts, etc. (3) Improving the corrosion environment: Improving the environment is of great significance for reducing and preventing corrosion. For example, reducing the concentration of corrosive agents, removing oxygen from the medium, and controlling environmental 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: This method involves using a metal or alloy with a lower electrode potential than the metal to be protected as the anode. This anode is attached to the metal to be protected, thereby forming a corrosion cell; the metal to be protected acts as the cathode and thus gets 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. ②In the impressed current method, the metal to be protected and another auxiliary electrode serve as the two poles of an electrolytic cell; the metal to be protected acts as the cathode, which is thus protected under the action of an applied direct current. This method is mainly used to prevent the 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 circuit patterns onto copper foil. Subsequently, the copper not covered by the photosensitive resin is etched using ferric chloride solution, thereby producing a printed circuit board with clearly defined lines. The reaction of iron trichloride 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 iron trichloride for copper etching; they offer higher resolution.

1. Chemical corrosion: refers to the corrosion that occurs as a result of a chemical reaction taking place when an oxidizing agent comes into contact with the metal surface. 2. Biological corrosion: refers to corrosion caused by the metabolic activities of various microorganisms. 3. Electrochemical corrosion: is corrosion that results from electrochemical reactions occurring in metals

There are three classification methods for metal corrosion: 1. Classification by corrosion mechanism: chemical corrosion； Electrochemical corrosion. 2. Classification by form of corrosion damage: uniform corrosion ； Local corrosion. In local corrosion, it is further divided into pitting corrosion, crevice corrosion, galvanic corrosion, erosion, stress corrosion, intergranular corrosion, abradative corrosion, selective corrosion, etc., depending on its morphology and causes of formation. 3. Classification by corrosion environment: soil corrosion, atmospheric corrosion, seawater corrosion, wet corrosion, high-temperature corrosion, corrosion by chemical media, precipitation corrosion, and so on.