Moving Forward Every Day — April 6, 2010

Answer: The corrosion resistance of steel is related to its chemical composition, microstructure, and the structure of the equipment. Therefore, to improve the corrosion resistance of steel, measures can be taken from three aspects: 1. Raise the electrode potential. When two metals are connected to each other and placed in an electrolyte solution, a corrosion cell is formed between them due to their different electrode potentials. The metal with the lower electrode potential acts as the anode and will be corroded continuously ； The metal with a higher electrode potential acts as the cathode and will be protected. It can be seen that to improve the corrosion resistance of a metal, it is necessary to raise its electrode potential. To achieve this goal, a larger amount of alloying elements such as Cr and Ni are generally added to carbon steel. 2. To achieve a single-phase structure in the metal: The heterogeneity in the internal structure of metal materials can also lead to corrosion in electrolyte solutions, as it facilitates the formation of corrosion microcells. For example, in carbon steel, a corrosion cell often forms between the ferrite and cementite, as the electrode potential of cementite is higher, leading to corrosion of the steel. Therefore, from the perspective of microcell corrosion, a pearlitic structure is not corrosion-resistant, whereas steel with a structure of pure austenite or pure ferrite is relatively more corrosion-resistant. To achieve this goal, chromium and nickel are added to carbon steel simultaneously to form a single austenitic structure, thereby enhancing the steel’s corrosion resistance. 3. A well-designed equipment structure: Corrosion of chemical equipment is often related to its structure; an unreasonable structure typically leads to mechanical stresses, thermal stresses, liquid stagnation, and localized overheating – all of which can cause metal corrosion or exacerbate it. Therefore, designing a more reasonable device structure is often also an effective measure to reduce corrosion.

It is mainly related to the electrode potential of the metal, impurity content, and the external environment. Corrosion protection can be achieved by adding alloying elements, applying paint, etc.

The corrosion resistance of steel is related to its chemical composition, microstructure, and the structure of the equipment. Therefore, to improve the corrosion resistance of steel, measures can be taken from three aspects: 1. Raise the electrode potential. When two metals are connected to each other and placed in an electrolyte solution, a corrosion cell is formed between them due to their different electrode potentials. The metal with the lower electrode potential acts as the anode and will be corroded continuously ； The metal with a higher electrode potential acts as the cathode and will be protected. It can be seen that to improve the corrosion resistance of a metal, it is necessary to raise its electrode potential. To achieve this goal, a larger amount of alloying elements such as Cr and Ni are generally added to carbon steel. 2. To achieve a single-phase structure in the metal: The heterogeneity in the internal structure of metal materials can also lead to corrosion in electrolyte solutions, as it facilitates the formation of corrosion microcells. For example, in carbon steel, a corrosion cell often forms between the ferrite and cementite, as the electrode potential of cementite is higher, leading to corrosion of the steel. Therefore, from the perspective of microcell corrosion, a pearlitic structure is not corrosion-resistant, whereas steel with a structure of pure austenite or pure ferrite is relatively more corrosion-resistant. To achieve this goal, chromium and nickel are added to carbon steel simultaneously to form a single austenitic structure, thereby enhancing the steel’s corrosion resistance. 3. A well-designed equipment structure: Corrosion of chemical equipment is often related to its structure; an unreasonable structure typically leads to mechanical stresses, thermal stresses, liquid stagnation, and localized overheating – all of which can cause metal corrosion or exacerbate it. Therefore, designing a more reasonable device structure is often also an effective measure to reduce corrosion.

Stainless steel refers to a general term for steels that are iron-based alloys with a chromium content (by mass) of 13% or more. In addition to chromium being an essential alloying element in all types of stainless steel, small amounts of other alloying elements are also added to enhance the corrosion resistance of stainless steel in various environments, as well as its mechanical and processability properties. These are discussed as follows: 1. Chromium – Although the electrode potential of chromium is lower than that of iron, its tendency to form a passive layer makes it the most important corrosion-resistant alloying element in stainless steel. The chromium content in stainless steel generally must comply with Tamman’s law, that is, the atomic ratio of Cr to Fe is 1/8 or 2/8. The higher the chromium content, the better the corrosion resistance; however, it should not exceed 30%, as otherwise it will reduce the toughness of the steel. 2. Nickel: Nickel is an element that expands the austenite phase region; when added in sufficient quantities, it enables stainless steel to have a single-phase austenitic structure, thereby improving the steel’s plasticity as well as its properties related to processing and welding. Nickel can also improve the heat resistance of steel. 3. Molybdenum: Since molybdenum can act as a passivator in Cl-, it enhances the resistance of stainless steel to seawater corrosion. Additionally, the inclusion of molybdenum in stainless steel significantly improves its resistance to both general corrosion and localized corrosion. 4. Carbon: Carbon has a dual role in stainless steel; on the one hand, its presence helps to expand the austenite structure and increase the strength of the steel. On the other hand, an increased carbon content in the steel leads to the formation of carbides with chromium, namely chromium carbides, which reduces the chromium content in the solid solution. The presence of numerous microcells lowers the corrosion resistance of the steel. In particular, it reduces the resistance to intergranular corrosion, making the steel prone to such corrosion; therefore, the carbon content should be reduced in stainless steels for which corrosion resistance is a key requirement. Carbon content in most acid-resistant stainless steels

The corrosion resistance of steel is related to its chemical composition, microstructure, and the structure of the equipment. Therefore, to improve the corrosion resistance of steel, measures can be taken from three aspects: 1. Raise the electrode potential. When two metals are connected to each other and placed in an electrolyte solution, a corrosion cell is formed between them due to their different electrode potentials. The metal with the lower electrode potential acts as the anode and will be corroded continuously ； The metal with a higher electrode potential acts as the cathode and will be protected. It can be seen that to improve the corrosion resistance of a metal, it is necessary to raise its electrode potential. To achieve this goal, a larger amount of alloying elements such as Cr and Ni are generally added to carbon steel. 2. To achieve a single-phase structure in the metal: The heterogeneity in the internal structure of metal materials can also lead to corrosion in electrolyte solutions, as it facilitates the formation of corrosion microcells. For example, in carbon steel, a corrosion cell often forms between the ferrite and cementite, as the electrode potential of cementite is higher, leading to corrosion of the steel. Therefore, from the perspective of microcell corrosion, a pearlitic structure is not corrosion-resistant, whereas steel with a structure of pure austenite or pure ferrite is relatively more corrosion-resistant. To achieve this goal, chromium and nickel are added to carbon steel simultaneously to form a single austenitic structure, thereby enhancing the steel’s corrosion resistance. 3. A well-designed equipment structure: Corrosion of chemical equipment is often related to its structure; an unreasonable structure typically leads to mechanical stresses, thermal stresses, liquid stagnation, and localized overheating – all of which can cause metal corrosion or exacerbate it. Therefore, designing a more reasonable device structure is often also an effective measure to reduce corrosion.

Corrosion refers to the physico-chemical interaction between a metal and its environment, which causes changes in the metal’s properties and results in damage to the metal, the environment, and the systems they form. Corrosion types can be divided into wet corrosion and dry corrosion. Wet corrosion refers to the corrosion of metals in the presence of water, while dry corrosion refers to corrosion in dry gases in the absence of liquid water. Due to the widespread presence of water in the atmosphere and the frequent handling of various aqueous solutions in chemical manufacturing, wet corrosion is the most common form. However, the hazards caused by dry corrosion during high-temperature operations cannot be ignored either. Wet corrosion: The corrosion of metals in aqueous solutions is an electrochemical reaction. A corrosion cell is formed on the metal surface, with an anode area and a cathode area separated from each other; the metal loses electrons in the solution and turns into positively charged ions, which is an oxidation process, namely the anodic process. Meanwhile, on the metal surface in contact with the aqueous solution, electrons have many opportunities to be neutralized by a substance present in the solution; the process of neutralizing electrons is a reduction process, that is, a cathodic process. Common cathodic processes include the reduction of oxygen, hydrogen evolution, the reduction of oxidants, and the deposition of precious metals. As the corrosion process progresses, in most cases the cathodic or anodic processes are hindered and slow down; this phenomenon is known as polarization, and the corrosion of the metal slows down as a result of polarization. Dry corrosion generally refers to corrosion that occurs in high-temperature gases, with high-temperature oxidation being the most common form. In high-temperature gases, an oxide film forms on the metal surface; the properties of this film and its growth patterns determine the metal’s corrosion resistance. The growth patterns of membranes can be divided into linear, parabolic, and logarithmic patterns. Oxidation following a linear pattern is the most dangerous, as the metal loss increases at a constant rate over time. The pattern observed for parabolas and logarithms is that the oxidation rate decreases as the film thickness increases, which makes it relatively safe; for example, aluminum follows a logarithmic pattern during oxidation at room temperature, and film growth stops after a few days, granting it good resistance to atmospheric oxidation. The forms of corrosion can be divided into uniform corrosion and local corrosion. In chemical production, the latter poses more severe hazards. Uniform corrosion occurs over the entire or most of the metal surface; it is also known as general corrosion. In most cases, a protective film of corrosion products forms on the metal surface, slowing down corrosion. Some metals, such as steel, dissolve rapidly in hydrochloric acid without forming a film. The average corrosion rate (i.e., the amount of material thickness lost per year in millimeters) is commonly used as a measure of the degree of uniform corrosion, and it also serves as a guideline for material selection. Generally, an annual corrosion rate of less than 1–1.5 mm is considered acceptable, ensuring a reasonable service life. Local corrosion: Corrosion occurs only in localized areas on the metal surface. Its harmfulness is much greater than that of uniform corrosion; it accounts for about 70% of all corrosion-related damages to chemical processing equipment. Moreover, it can be sudden and catastrophic, leading to accidents such as explosions and fires. Corrosion prevention measures: The main methods for protecting metals from corrosion are: ① Altering the internal structure of the metal. For example, chromium and nickel are added to ordinary steel to create stainless steel. ②Cover the metal surface with a protective layer. For example, painting metal surfaces, electroplating, or using chemical methods to form a dense, corrosion-resistant oxide film. ③Electrochemical protection method. Since elemental metals cannot gain electrons, by making the metal to be protected the cathode—the pole where the reduction reaction occurs—in an electrochemical cell, it is possible to eliminate the galvanic cell reaction that causes electrochemical corrosion of the metal. The specific methods include: a. Cathodic protection using an external current. By using an electrolytic device, the metal to be protected is connected to the negative pole of the power supply, while an inert electrode serves as the anode; as long as the applied voltage is strong enough, the protected metal can be prevented from corroding. b. Cathodic protection using a sacrificial anode. Using a galvanic cell setup, the metal to be protected is combined with another metal that is more prone to losing electrons to form a new galvanic cell. When a galvanic cell reaction occurs, the base metal acts as the positive electrode (i.e., the cathode) and is protected, while the active metal added externally serves as the negative electrode (i.e., the anode) and is the one that corrodes. In addition, methods such as adding corrosion inhibitors are also used to slow down or prevent metal corrosion.

Corrosion requires an oxidizing agent, while electrochemical corrosion needs an electrolyte; the steel must be in electrical contact with the electrolyte and act as the anode in a galvanic cell system. Therefore, the method to improve steel corrosion is to prevent oxidants from coming into contact with the steel, as well as to ensure that it remains in contact with electrolytes or to place it in a cathodic position within the system. Based on the above method, the first step is surface treatment; plating or spraying the surface of the steel is done to prevent contact with oxidizers and electrolytes. The purpose of adding sacrificial anodes or external current is to keep the steel in a negative polarity within the system. Additionally, adding a certain amount of other elements to steel can improve the formation and density of the passive film on its surface, thereby enhancing its corrosion resistance; examples include various stainless steels and corrosion-resistant alloys.

Water-based rust inhibitors can be used. The key to improving the corrosion resistance of steel lies in its composition and density; a dense oxide layer on the surface of the steel provides some level of corrosion protection. This layer should be very thin and dense – if it is too thick, it will become porous and thus ineffective. High-temperature water cooling should be considered for greater density.

Relevant factors: 1. Composition of steel materials; 2. Environment in which the material is used – temperature, pressure, flow rate, ion content, stress conditions, surface roughness, etc. Methods to improve corrosion resistance: 1. Determine the appropriate environment for use based on the properties of the material; 2. Heat treatment; 3. Surface coating; 4. Electrochemical protection

It is related to the material of the steel and the medium in which it is found (i.e., the environment of use), as well as to the crystal structure within the steel. Steel with better manufacturing techniques has a more compact crystal structure, which significantly improves its corrosion resistance. Stress corrosion is also an important factor affecting steel; therefore, material shaping and post-treatment processes have a significant impact on the material’s corrosion resistance. When different types of steel are used together without any barrier layers between them, microcells can form, thereby accelerating corrosion. To improve the corrosion resistance of steel, it is necessary to choose the appropriate type of steel for the given environment. When different types of steel are combined, barrier layers should be used, and anti-corrosion techniques such as sacrificial anode protection, corrosion inhibitors, spraying, electroplating, etc., should be employed, along with regular maintenance.