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Types and definitions of corrosion: A certain stainless steel may exhibit good corrosion resistance in many media, but it may corrode in another medium due to its low chemical stability. Therefore, a single type of stainless steel cannot be resistant to corrosion in all media. Among its many industrial applications, stainless steel offers corrosion resistance that satisfies modern requirements. Based on practical experience, apart from mechanical failure, the corrosion of stainless steel mainly manifests as: a severe form of corrosion in stainless steel is local corrosion (i.e., stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue, and crevice corrosion). Failure cases caused by these local corrosions account for almost more than half of all failure cases. In fact, many failure accidents can be avoided through proper material selection. According to their mechanisms, metal corrosion can be divided into physical corrosion, chemical corrosion, and electrochemical corrosion. Metal corrosion in real-life and engineering applications is, in the vast majority of cases, electrochemical corrosion. Stress corrosion cracking (SCC): is a general term referring to the progressive failure of alloys under stress in corrosive environments due to the propagation of cracks. Stress corrosion cracking exhibits a brittle fracture morphology, but it can also occur in materials with high toughness. The necessary conditions for stress corrosion cracking are the presence of tensile stress (whether it is residual stress or applied stress, or both) and a specific corrosive environment. The formation and propagation of patterns are roughly perpendicular to the direction of tensile stress. The stress level that causes stress corrosion cracking is much lower than the stress level required for the material to fracture in the absence of a corrosive medium. At the microscale, cracks that pass through the grains are called transgranular cracks, while those that propagate along the grain boundaries are called intergranular cracks. When stress corrosion cracking progresses to a certain depth (at which point the stress on the cross-section of the material under load reaches its fracture stress in air), the material breaks in the usual manner, via normal cracks (in ductile materials, this usually occurs through the aggregation of microscopic defects). Therefore, the cross-section of a part that fails due to stress corrosion cracking will contain regions characteristic of stress corrosion cracking as well as \"ductile pit\" regions associated with the aggregation of pre-existing defects. Pitting corrosion: Pitting corrosion refers to a type of localized corrosion that occurs in scattered areas on the surface of metal materials, where most of the surface remains uncorroded or only shows slight corrosion. The size of the pits formed is usually less than 1.00 mm, while their depth often exceeds the diameter of the surface pores; in mild cases, shallow pits are formed, but in severe cases, perforations can occur. Intergranular corrosion: The grain boundaries are regions where grains with different crystallographic orientations are misaligned in a disordered manner; therefore, they serve as favorable areas for the segregation of various solute elements in steel or for the precipitation of metal compounds such as carbides and the δ-phase. Therefore, it is not surprising that in certain corrosive media, the grain boundaries may be corroded first. This type of corrosion is known as intergranular corrosion, and most metals and alloys can experience intergranular corrosion in certain corrosive environments. Intergranular corrosion is a type of selective corrosion damage. What sets it apart from ordinary selective corrosion is that the locality of the corrosion occurs at the microscopic scale, whereas it is not necessarily localized on a macroscopic level. Crevice corrosion refers to the formation of spot-like or ulcerated macroscopic corrosion pits in the gaps of metal components; it is a form of local corrosion that can occur in gaps where liquids remain stagnant or within shielded surfaces. Such gaps can form at the joints between metals or between metals and non-metals; for example, they form where there is contact with rivets, bolts, gaskets, valve seats, loose surface deposits, and marine organisms. General corrosion: It is a term used to describe corrosion that occurs in a relatively uniform manner across the entire surface of the alloy. When general corrosion occurs, the material gradually thins due to corrosion, and in extreme cases the material fails as a result of corrosion. Stainless steel may suffer from general corrosion in strong acids and strong bases. Failure issues caused by general corrosion are not too concerning, as this type of corrosion can usually be predicted through simple immersion tests or by consulting literature on corrosion. Uniform corrosion: refers to the phenomenon in which the entire metal surface in contact with the corrosive medium undergoes corrosion. Different criteria for corrosion resistance are required depending on the application; generally, they can be divided into two main categories: 1) Stainless steel refers to steel that is resistant to corrosion in atmospheric conditions and in mildly corrosive media. A corrosion rate of less than 0.01 mm/year is considered to indicate \"complete corrosion resistance\"" ； A corrosion rate of less than 0.1 mm/year is considered \"corrosion-resistant\". 2). Corrosion-resistant steel refers to steel that can resist corrosion in various highly corrosive media. 2. Corrosion resistance of various stainless steels. 304 is a general-purpose stainless steel that is widely used in manufacturing equipment and components that require good overall properties (corrosion resistance and formability). 301 stainless steel exhibits significant work hardening upon deformation, and is used in various applications that require high strength. 302 stainless steel is essentially a variant of 304 stainless steel with a higher carbon content; cold rolling can be used to impart greater strength to it. 302B is a stainless steel with a high silicon content, and it possesses excellent resistance to high-temperature oxidation. 303 and 303Se are free-cutting stainless steels containing sulfur and selenium respectively, used in applications where free-cutting properties and high surface finish are primarily required. 303Se stainless steel is also used to manufacture components that require hot heading, as it exhibits good hot workability under such conditions. 304L is a variant of 304 stainless steel with a lower carbon content, used in applications that require welding. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone near the weld, and the precipitation of carbides can cause intergranular corrosion (weld erosion) in stainless steel under certain conditions. 304N is a nitrogen-containing stainless steel; nitrogen is added to enhance the strength of the steel. Stainless steels 305 and 384 contain higher levels of nickel, have a low rate of work hardening, and are suitable for various applications that require good cold formability. 308 stainless steel is used to make welding electrodes. Stainless steels 309, 310, 314, and 330 have relatively high nickel and chromium contents, which is intended to enhance the steel’s oxidation resistance and creep strength at high temperatures. 30S5 and 310S are variants of 309 and 310 stainless steels; the only difference is their lower carbon content, which is intended to minimize the formation of carbides in the area near the weld. Stainless steel 330 possesses particularly high resistance to carburization and thermal shock. Stainless steels 316 and 317 contain aluminum, which gives them significantly better resistance to pitting corrosion in marine and chemical industry environments compared to stainless steel 304. Among them, type 316 stainless steel has variants including low-carbon stainless steel 316L, high-strength nitrogen-containing stainless steel 316N, and free-cutting stainless steel 316F with a higher sulfur content. 321, 347, and 348 are stainless steels based on titanium, niobium plus tantalum, and niobium-stabilized respectively, and are suitable as welded components for use at high temperatures. 348 is a stainless steel suitable for the nuclear power industry, with certain limits on the combined content of tantalum and tungsten. Surface finish grade, characteristics, and applications: Original surface: NO.1 – a surface that has undergone heat treatment and pickling after hot rolling. It is generally used for cold-rolled materials, industrial tanks, chemical industry equipment, etc., with thicknesses ranging from 2.0 MM to 8.0 MM. Dull finish: NO.2D – material that has been cold-rolled followed by heat treatment and pickling; it is soft in texture and has a silver-white sheen on its surface. It is used for deep stamping processes, such as in the production of automotive components and water pipes. Matte finish: NO.2B – obtained by cold rolling, followed by heat treatment and pickling, and then precision rolling to give the surface a moderate level of brightness. Due to its smooth surface, it is easy to re-grind, allowing the surface to become even brighter; it has a wide range of applications, such as in tableware and building materials. After applying surface treatment to improve mechanical properties, it meets almost all applications. Coarse sand NO.3 is a product obtained by grinding with 100-120 grit grinding belts. It has a good gloss level and discontinuous coarse textures. Used in interior and exterior decoration materials for buildings, electrical products, and kitchen equipment, etc. Fine sand: NO.4 – a product obtained by grinding with grinding belts of grade 150-180. It has a better gloss, with discontinuous coarse grooves; the stripes are finer than those of NO.3. Used in bathtubs, interior and exterior decoration materials for buildings, electrical products, kitchen equipment, and food processing equipment, etc. #320 Products polished using 320-grade grinding belts. It has a better gloss, with discontinuous coarse textures; the stripes are finer than those of NO.4. Used in bathtubs, interior and exterior decoration materials for buildings, electrical products, kitchen equipment, and food processing equipment, etc. HAIRLINE: A product with a polished texture produced by continuous grinding with polishing belts of appropriate grain size, HL NO.4 (with fineness grades of 150-320). It is mainly used for architectural decoration, elevators, doors, panels of buildings, etc. Polished surface: A product obtained by performing bright annealing on BA after cold rolling, followed by leveling. It has an excellent surface gloss and a high reflectivity. Like a mirror-like surface. Used in home appliances, mirrors, kitchen equipment, decorative materials, etc. Product properties and uses: SUS304: It possesses good corrosion resistance, heat resistance, strength at low temperatures, and mechanical properties. It is suitable for hot working such as stamping and bending, does not experience hardening due to heat treatment, and is non-magnetic. It is widely used in household items (tableware of categories 1 and 2), cabinets, indoor piping, water heaters, boilers, bathtubs, auto parts, medical devices, building materials, the chemical industry, the food industry, agriculture, and ship components. SUS304L: An austenitic basic steel grade with the widest range of applications ； Excellent corrosion and heat resistance ； Excellent low-temperature strength and mechanical properties ； Single-phase austenitic structure, with no hardening due to heat treatment (non-magnetic, operating temperature: -196–800°C). SUS304Cu: An austenitic stainless steel with a basic composition of 17Cr-7Ni-2Cu ； It exhibits excellent formability, particularly good wire-drawing properties and resistance to aging cracks ； --Corrosion resistance is the same as that of 304. SUS316: It has excellent corrosion resistance and high-temperature strength, allowing it to be used under harsh conditions. It exhibits good work hardening properties and is non-magnetic. Suitable for seawater-related equipment, the chemical industry, dye production, papermaking, oxalic acid production, fertilizer manufacturing equipment, photography, the food industry, and coastal facilities. SUS316L: Mo (2-3%) is added to the steel, resulting in excellent corrosion resistance and high-temperature strength ； SUS316L has a lower carbon content than SUS316; therefore, its resistance to intergranular corrosion is better than that of SUS316 ； It has high high-temperature creep strength. It can be used under harsh conditions, has good work hardening properties, and is non-magnetic. Suitable for seawater-related equipment, the chemical industry, dye production, papermaking, oxalic acid production, fertilizer manufacturing equipment, photography, the food industry, and coastal facilities. SUS321: Ti is added to 304 steel, resulting in excellent resistance to intergranular corrosion ； Excellent high-temperature strength and high-temperature oxidation resistance ； It has high costs and inferior workability compared to SUS304. Heat-resistant materials, automotive and aircraft exhaust systems, boiler lids and pipes, chemical equipment, heat exchangers. SUH409H: It has good workability and weldability, as well as excellent oxidation resistance at high temperatures; it can withstand temperatures ranging from room temperature up to 575°C. Widely used in automotive exhaust systems. SUS409L: Controls the C and N contents in the steel, thus offering excellent weldability, formability, and corrosion resistance ； Containing 11% Cr, it is a ferritic stainless steel with a BCC structure at both high and room temperatures ； Due to the addition of Ti, it exhibits passive oxidation resistance and corrosion resistance below 750°C. SUS410: A martensitic steel grade with high strength and high hardness (magnetic) ； Poor corrosion resistance; not suitable for use in environments with severe corrosion ； It has a low carbon content, good workability, and its surface can be hardened through heat treatment. SUS420J2: A martensitic steel grade with high strength and high hardness (magnetic) ； Poor corrosion resistance, poor formability, good wear resistance ； Heat treatment can be applied to improve mechanical properties. It is widely used in manufacturing cutting tools, nozzles, valves, rulers, and tableware. SUS430: It has a low coefficient of thermal expansion, good formability, and excellent oxidation resistance; it is suitable for heat-resistant appliances, burners, home appliances, category 2 tableware, and kitchen sinks. Low price and good workability make it an ideal substitute for SUS304 ； It has good resistance to pitting corrosion; it is a typical ferritic stainless steel that does not harden through heat treatment. There are currently over 100 known chemical elements, and about twenty of these elements can be found in steel materials commonly used in industry. For the special series of steels known as stainless steel, which has been developed through years of effort to combat corrosion, there are more than a dozen elements that are commonly used. In addition to iron, the element that constitutes the base of steel, the elements that have the greatest impact on the properties and structure of stainless steel include carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, nitrogen, copper, cobalt, and others. Except for carbon, silicon, and nitrogen, these elements are all elements from the transition series in the periodic table of chemical elements. In fact, the stainless steels used in industry all contain several to a dozen different elements. When these elements coexist within the stainless steel matrix, their effects are much more complex than when they exist alone; in such cases, it is necessary to take into account not only the effects of each element individually but also their interactions with one another. Therefore, the structure of stainless steel is determined by the sum of the effects of all these elements. 1) The effects and roles of various elements on the properties and microstructure of stainless steel. 1-1. The decisive role of chromium in stainless steel: There is only one element that determines the properties of stainless steel, and that is chromium; every type of stainless steel contains a certain amount of chromium. To date, there is no stainless steel that does not contain chromium. The fundamental reason why chromium becomes the key element determining the properties of stainless steel is that the addition of chromium as an alloying element to the steel causes the internal contradictory forces within it to develop in a direction that facilitates resistance to corrosion and degradation. This change can be explained by the following factors: ① Chromium raises the electrode potential of iron-based solid solutions; ② Chromium absorbs electrons from iron, thereby passivating it. Passivation is a phenomenon in which the corrosion resistance of metals and alloys increases due to the inhibition of anodic reactions. There are many theories explaining the passivation of metals and alloys, mainly the film theory, the adsorption theory, and the electron arrangement theory. 1-2. The dual role of carbon in stainless steel. Carbon is one of the main elements in industrial steel; the properties and microstructure of steel are largely determined by the amount of carbon present in it and its distribution pattern. In stainless steel, the influence of carbon is particularly significant. The effect of carbon on the microstructure of stainless steel is manifested in two main ways: firstly, carbon is an element that stabilizes austenite, and its effect is quite significant (about 30 times that of nickel); secondly, due to the strong affinity between carbon and chromium, they form a series of complex carbides together. Therefore, in terms of both strength and corrosion resistance, the role of carbon in stainless steel is contradictory. By understanding the patterns of this influence, we can choose stainless steels with different carbon contents based on various application requirements. For example, for the most widely used and basic type of stainless steel in industry – the five grades 0Crl3 to 4Cr13 – the standard chromium content is specified at 12–14%. This value is determined after taking into account the fact that carbon combines with chromium to form chromium carbide; the goal is to ensure that, after carbon and chromium combine to form chromium carbide, the chromium content in the solid solution does not drop below the minimum level of 11.7%. For these five steel grades, their strength and corrosion resistance vary due to differences in carbon content. The 0Cr13–2Crl3 steels have good corrosion resistance but lower strength compared to the 3Crl3 and 4Cr13 steels; they are commonly used for manufacturing structural components. The latter two steel grades, having a higher carbon content, exhibit high strength and are thus used for producing springs, cutting tools, and other parts that require high strength and wear resistance. For example, to overcome intergranular corrosion in 18-8 chromium-nickel stainless steel, the carbon content of the steel can be reduced to below 0.03%, or elements with a greater affinity for chromium and carbon (such as titanium or niobium) can be added to prevent the formation of chromium carbides. When high hardness and wear resistance are the primary requirements, the carbon content of the steel can be increased while simultaneously raising the chromium content, thereby meeting the needs for hardness and wear resistance while also maintaining a certain level of corrosion resistance. In industry, stainless steels such as 9Cr18 and 9Cr17MoVCo are used for bearings, measuring tools, and cutting tools; although their carbon content is as high as 0.85–0.95%, their elevated chromium content ensures that the required corrosion resistance is still maintained. Generally speaking, the carbon content of stainless steels used in industry today is relatively low; most stainless steels have a carbon content between 0.1% and 0.4%, while acid-resistant steels usually have a carbon content of 0.1% to 0.2%. Stainless steels with a carbon content of more than 0.4% account for only a small portion of all steel grades, as in most application scenarios, stainless steels are designed primarily for their corrosion resistance. Furthermore, the lower carbon content is also due to certain process requirements, such as ease of welding and cold deformation. 1-3. The role of nickel in stainless steel is realized only when it combines with chromium. Nickel is an excellent corrosion-resistant material and an important alloying element for alloy steels. Nickel is an element that promotes the formation of austenite in steel; however, to achieve a pure austenitic structure in low-carbon nickel steel, the nickel content must reach 24% ； Only when the nickel content is 27% does it cause a significant change in the corrosion resistance of the steel in certain media. Therefore, nickel alone cannot form stainless steel. However, when nickel and chromium are present together in stainless steel, nickel-containing stainless steel possesses many valuable properties. Based on the above, it can be seen that the role of nickel as an alloying element in stainless steel is to alter the structure of high-chromium steel, thereby improving certain aspects of the stainless steel’s corrosion resistance and processability. 1-4. Manganese and nitrogen can replace nickel in chromium-nickel stainless steels. Although chromium-nickel austenitic steels have many advantages, in recent decades, due to the extensive development and use of nickel-based heat-resistant alloys and heat-resistant steels containing less than 20% nickel, as well as the growing demand for stainless steels driven by the expansion of the chemical industry, and considering that nickel reserves are limited and concentrated in a few regions, a mismatch between supply and demand for nickel has emerged worldwide. Therefore, in the field of stainless steel as well as many other alloys (such as steels used for large castings and forgings, tool steels, heat-resistant steels, etc.), especially where nickel is relatively scarce, extensive scientific research and practical applications have been carried out to reduce the use of nickel and replace it with other elements. In this regard, manganese and nitrogen are commonly used to substitute for nickel in stainless steel and heat-resistant steels. Manganese has a similar effect on austenite as nickel. To be more precise, the role of manganese is not to form austenite, but rather to reduce the critical quenching speed of steel, increase the stability of austenite during cooling, inhibit its decomposition, and allow the austenite formed at high temperatures to be retained at room temperature. Manganese has little effect on improving the corrosion resistance of steel; even when the manganese content in steel varies from 0 to 10.4%, it does not cause any significant change in the steel’s corrosion resistance in air and acids. This is because manganese has little effect on raising the electrode potential of iron-based solid solutions, and the protective effect of the resulting oxide film is also low; as a result, although there are austenitic steels alloyed with manganese used in industry (such as 40Mn18Cr4, 50Mn18Cr4WN, ZGMn13 steel, etc.), they cannot be used as stainless steels. Manganese’s effect in stabilizing austenite in steel is about half that of nickel; that is, 2% nitrogen also plays a role in stabilizing austenite, and its effect is even greater than that of nickel. For example, to obtain an austenitic structure at room temperature in steel containing 18% chromium, low-nickel stainless steels that use manganese and nitrogen in place of nickel, as well as chromium-manganese-nitrogen stainless steels without nickel, are now in use in industry; some of them have successfully replaced the traditional 18-8 chromium-nickel stainless steels. 1-5. Titanium or niobium is added to stainless steel to prevent intergranular corrosion. 1-6. Molybdenum and copper can improve the corrosion resistance of certain stainless steels. 1-7. Effects of other elements on the properties and microstructure of stainless steel. In addition to the nine main elements discussed above, which have a significant impact on the properties and microstructure of stainless steel, there are also other elements present in stainless steel. Some are common impurity elements present in ordinary steel, such as silicon, sulfur, phosphorus, etc. Others are added for specific purposes, such as cobalt, boron, selenium, rare earth elements, etc. In terms of the main property of stainless steel, which is its corrosion resistance, these elements are not significant compared to the nine elements that have been discussed. Nevertheless, they cannot be completely ignored, as they also affect the properties and microstructure of stainless steel. Silicon is an element that forms ferrite, and it is often considered an impurity element in misleading advertisements for ordinary stainless steels. Cobalt is not widely used in steel as an alloying element, due to its high cost and its more important applications in other areas such as high-speed steels, cemented carbides, cobalt-based heat-resistant alloys, magnets, or hard magnetic alloys. Cobalt is not often added as an alloying element in ordinary stainless steels. In commonly used stainless steels such as 9Crl7MoVCo steel (which contains 1.2–1.8% cobalt), the addition of cobalt is not intended to improve corrosion resistance but rather to increase hardness, as these stainless steels are primarily used for manufacturing cutting tools for slicing machines, scissors, and surgical blades. Boron: Adding 0.005% boron to high-chromium ferritic stainless steel Crl7Mo2Ti improves its corrosion resistance in boiling 65% acetic acid. Adding a trace amount of boron (0.0006–0.0007%) can improve the hot plasticity of austenitic stainless steels. A small amount of boron increases the tendency for hot cracks to occur during the welding of austenitic steel due to the formation of low-melting-point eutectics; however, when a higher amount of boron is present (0.5–0.6%), it can actually prevent the occurrence of hot cracks. This is because when it contains 0.5–0.6% boron, an austenite-boride two-phase structure is formed, which lowers the melting point of the weld. When the solidification temperature of the molten pool is lower than that of the semi-molten zone, the tensile stress generated during the cooling of the base material is borne by the weld metal in its liquid-solid state; this prevents cracks from forming. Even if cracks do appear in the area near the weld, they can be filled in by the molten metal in its liquid-solid state. Boron-containing chromium-nickel austenitic stainless steels have special applications in the nuclear industry. Phosphorus: It is an impurity element in ordinary stainless steels, but its harmful effect in austenitic stainless steels is not as significant as in ordinary steels; therefore, its concentration can be allowed to be higher. Some sources suggest a level of up to 0.06%, which facilitates control during manufacturing. In certain manganese-containing austenitic steels, the phosphorus content can reach 0.06% (such as 2Crl3NiMn9 steel) or even 0.08% (such as Cr14Mnl4Ni steel). Taking advantage of phosphorus’s strengthening effect on steel, it is also added as an alloying element in age-hardening stainless steels; examples include PH17‑10P steel (containing 0.25% phosphorus) and PH‑HNM steel (containing 0.30% phosphorus). Sulfur and selenium: These are also common impurity elements in ordinary stainless steel. However, adding 0.2–0.4% sulfur to stainless steel can improve its machinability, and selenium has a similar effect. Sulfur and selenium improve the machinability of stainless steel by reducing its toughness; for example, the impact value of typical 18‑8 chromium-nickel stainless steel can reach 30 kg/cm2. The impact value of 18-8 steel containing 0.31% sulfur (0.084% C, 18.15% Cr, 9.25% Ni) is 1.8 kg/cm² ； Contains 0. The impact value of 18-8 steel containing 22% selenium (0.094% C, 18.4% Cr, 9% Ni) is 3.24 kg/cm². Both sulfur and selenium reduce the corrosion resistance of stainless steel, so they are rarely used in practice as alloying elements for stainless steel. Rare earth elements: Rare earth elements are used in stainless steel, primarily to improve its processability. By adding a small amount of rare earth elements to Crl7Ti steel and Cr17Mo2Ti steel, the bubbles caused by hydrogen in the ingots can be eliminated, and the cracks in the billets can be reduced. Adding 0.02–0.5% of rare earth elements (cerium-lanthanum alloys) to austenitic and austenitic-ferritic stainless steels can significantly improve their forgeability. There was once an austenitic steel containing 19.5% chromium, 23% nickel, as well as molybdenum, copper, and manganese; due to its poor properties during hot working, it could only be manufactured in cast form in the past, but the addition of rare earth elements enabled it to be rolled into various shaped products. 2) Classification of stainless steels based on their microstructural properties and general characteristics of each type. Based on chemical composition (mainly chromium content) and application, stainless steels are divided into two categories: stainless steels and acid-resistant steels. In industry, stainless steels are also classified according to the type of matrix structure of the steel after cooling it from high temperatures (900–1100 degrees), and this classification is based on the effects of carbon and alloying elements on the structure of stainless steels, as discussed above. Stainless steels used in industry can be classified into three major categories based on their microstructural organization: ferritic stainless steels, martensitic stainless steels, and austenitic stainless steels. The characteristics of these three types of stainless steel can be summarized (as shown in the table below). It should be noted, however, that not all martensitic stainless steels are unweldable; rather, certain conditions impose restrictions on them, such as the need for preheating before welding and high-temperature tempering after welding, which makes the welding process more complex. In actual production, some martensitic stainless steels such as 1Cr13, 2Cr13 are quite often welded to steel grades like 45. Put simply, stainless steel is steel that does not rust easily; in fact, some types of stainless steel possess both rust resistance and acid resistance (corrosion resistance). The rust resistance and corrosion resistance of stainless steel are due to the formation of a chromium-rich oxide film (passivation film) on its surface. This resistance to rust and corrosion is relative. Tests have shown that in weak media such as air and water, as well as in oxidizing media such as nitric acid, the corrosion resistance of steel increases with the increase in the chromium content in the steel. When the chromium content reaches a certain percentage, there is a sudden change in the steel’s corrosion resistance – it goes from being prone to rusting to less prone to rusting, and from being non-corrosion-resistant to corrosion-resistant. There are many ways to classify stainless steel. Classified by their microstructure at room temperature, there are martensitic, austenitic, ferritic, and duplex stainless steels ； Classified by their main chemical components, they can be basically divided into two major categories: chromium stainless steels and chromium-nickel stainless steels ； Classified by application, there are nitric acid-resistant stainless steels, sulfuric acid-resistant stainless steels, seawater-resistant stainless steels, and so on. Classified by corrosion resistance type, they can be divided into pitting-resistant stainless steels, stress-corrosion-resistant stainless steels, intergranular corrosion-resistant stainless steels, and so on ； Classified by functional characteristics, they can be further divided into non-magnetic stainless steel, free-cutting stainless steel, low-temperature stainless steel, high-strength stainless steel, and so on. Due to its excellent corrosion resistance, formability, compatibility, and strength and toughness over a wide temperature range, stainless steel is widely used in industries such as heavy industry, light industry, household goods manufacturing, and building decoration. Austenitic stainless steel: Stainless steel that has an austenitic structure at room temperature. When steel contains about 18% Cr, 8%~10% Ni, and about 0.1% C, it has a stable austenitic structure. Austenitic chromium-nickel stainless steels include the well-known 18Cr-8Ni steel, as well as high Cr-Ni series steels that were developed by increasing the contents of Cr and Ni and adding elements such as Mo, Cu, Si, Nb, and Ti. Austenitic stainless steels are non-magnetic and possess high toughness and ductility, but they have low strength; they cannot be strengthened through phase transformation and can only be strengthened through cold working. When elements such as S, Ca, Se, and Te are added, it exhibits good machinability. In addition to resisting corrosion in oxidizing acidic media, such steels can also withstand corrosion by sulfuric acid, phosphoric acid, as well as formic acid, acetic acid, urea, etc., if they contain elements such as Mo and Cu. If the carbon content in such steels is below 0.03% or if they contain Ti and Ni, their resistance to intergranular corrosion can be significantly improved. Austenitic stainless steels with high silicon content exhibit good corrosion resistance in concentrated nitric acid. Due to their comprehensive and excellent overall properties, austenitic stainless steels are widely used in various industries. Ferritic stainless steel: A stainless steel that is primarily composed of a ferritic structure in its operating condition. It has a chromium content of 11% to 30% and a body-centered cubic crystal structure. These types of steel generally do not contain nickel; sometimes they also contain small amounts of elements such as Mo, Ti, Nb, etc. They feature a high thermal conductivity, a low coefficient of expansion, good oxidation resistance, and excellent resistance to stress corrosion. They are often used to manufacture components that can withstand corrosion caused by the atmosphere, water vapor, water, and oxidizing acids. These types of steel have disadvantages such as poor plasticity and a significant reduction in plasticity and corrosion resistance after welding, which limits their application. The use of secondary refining technologies (AOD or VOD) enables the reduction of interstitial elements such as carbon and nitrogen, which is why these types of steel are widely used. Austenite-ferrite duplex stainless steel: is a type of stainless steel in which austenite and ferrite structures each make up about half of it. At low carbon levels, the Cr content ranges from 18% to 28%, while the Ni content ranges from 3% to 10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel possesses the characteristics of both austenitic and ferritic stainless steels. Compared to ferritic steels, it has higher plasticity and toughness, no room-temperature brittleness, and significantly improved resistance to intergranular corrosion as well as weldability. At the same time, it retains the brittleness at 475°C and high thermal conductivity associated with ferritic stainless steels, as well as superplasticity. Compared to austenitic stainless steels, it has higher strength, as well as significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel exhibits excellent pitting resistance and is also a nickel-saving stainless steel. This post was last edited by The wise are not confused on 2009-3-2 15:32]