Discussion on which component in steel provides resistance to which types of corrosion

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 stainless steel, a special category of steels that has been developed through people’s long-term efforts in combating corrosion, there are over a dozen elements that are most commonly used. Apart from iron, which is the basic element constituting steel, the elements that have the greatest influence on the properties and structure of stainless steel include: carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, nitrogen, copper, cobalt, etc. Except for carbon, silicon, and nitrogen, these elements are all elements from the transition series in the periodic table of chemical elements. 　　In reality, the stainless steels used in industry contain several or even a dozen elements simultaneously. When multiple elements coexist within a single stainless steel matrix, their effects become much more complex than when they exist individually. This is because, in such cases, one must consider not only the individual effects of each element but also their interactions with one another. Therefore, the microstructure of stainless steel is determined by the combined effects of all these elements. 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 causing passivation of iron. 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. 2. The dual role of carbon in stainless steel Carbon is one of the main elements in industrial steel; the properties and structure 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 five steel grades of 0Cr13 to 4Cr13—which are the most widely used and fundamental stainless steels in industry—the specified standard chromium content is 12–14%. This value was determined after taking into account the fact that carbon reacts with chromium to form chromium carbide. The purpose is to ensure that even after carbon and chromium combine to form chromium carbide, the chromium content in the solid solution does not fall below the minimum threshold 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, it is possible to increase the carbon content of the steel while simultaneously raising the chromium content, thereby meeting the demands 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 conditions, stainless steels are designed primarily for their corrosion resistance. Furthermore, a lower carbon content is also required for certain processing reasons, such as ease of welding and cold deformation. 3. The role of nickel in stainless steel is realized only when it works in combination 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. 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 steel driven by the expansion of the chemical industry, and given that nickel reserves are limited and concentrated in a few regions, a shortage of nickel has emerged globally in terms of supply and demand. Therefore, in the field of stainless steel and 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 context, 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, have been put into industrial use; some of them have successfully replaced the traditional 18-8 chromium-nickel stainless steels. 5. Titanium or niobium is added to stainless steel to prevent intergranular corrosion. 6. Molybdenum and copper can improve the corrosion resistance of certain stainless steels. 7. Effects of other elements on the properties and structure of stainless steel
The nine main elements mentioned above have a significant impact on the properties and structure of stainless steel. Besides these elements, there are also several other elements present in stainless steel. Some are common impurity elements, such as silicon, sulfur, phosphorus, etc., just like in ordinary steel. 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 a common impurity element in ordinary stainless steels. 　　Cobalt is not widely used as an alloying element in steel, 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, magnetic materials, 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 eutectics, but 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 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 some manganese-containing austenitic steels, the phosphorus content can reach 0.06% (e.g., in 2Crl3NiMn9 steel) or even 0.08% (e.g., in 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 are also common impurity elements in ordinary stainless steels. 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 kilograms/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; therefore, they are rarely used in practice as alloying elements for stainless steel. 　　Rare earth elements: Rare earth elements are used in stainless steel; currently, their main application is in improving process performance. 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, but the addition of rare earth elements enabled it to be rolled into various shaped products. Hope this helps you