Corrosion resistance of ferrous metals and their alloys

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Corrosion resistance of metals and their alloys: The most important chemical property of metal materials is their corrosion resistance. The ability of a material to resist corrosion and damage caused by the surrounding medium is known as its corrosion resistance. Corrosion resistance is not an inherent and constant property of a material; it changes depending on the operating conditions of that material. I. Corrosion resistance of carbon steel and ordinary cast iron 1. Corrosion resistance: It has no corrosion resistance in neutral media such as fresh water, atmosphere, soil, and seawater ； It exhibits good corrosion resistance in various media such as dry gases and organic solvents ； In low-concentration alkaline solutions as well as in media such as concentrated sulfuric acid and concentrated hydrofluoric acid, a stable film can form on the surfaces of carbon steel and ordinary cast iron, thereby providing corrosion resistance. 2. Factors affecting corrosion resistance: medium composition ； Composition and microstructure of iron-carbon alloys ； Differences in steel heat treatment conditions. II. Corrosion-resistant alloy cast iron: Adding certain alloying elements to cast iron can improve its corrosion resistance in certain media. Adding elements such as silicon, chromium, and aluminum can enable the formation of a continuous, dense, and strong surface film on the surface of cast iron ； Adding nickel yields austenitic cast iron with excellent resistance to corrosion in alkaline media ； Adding rare earth elements and magnesium can spheroidize graphite, thereby improving the mechanical and process properties of high-silicon cast iron. However, after cleaning with a low concentration of alkali, the surface easily reaches conditions conducive to rusting, resulting in pitting rust. III. Corrosion-resistant low-alloy steels: Corrosion-resistant low-alloy steels generally refer to alloys in which the total amount of alloying elements added to carbon steel is less than about 3%. Different types and amounts of alloying elements added have different effects. Based on their corrosion resistance in different media, corrosion-resistant low-alloy steels can be classified into the following categories: (1) Steels resistant to atmospheric corrosion ； (2) Sea-water corrosion-resistant steel grades ；    (3) Hydrogen sulfide-resistant steel grades. IV. Corrosion resistance of stainless steel: Stainless steel is an alloy steel with high levels of chromium and nickel. Alloy steels resistant to atmospheric corrosion are generally referred to as stainless steel, while alloy steels resistant to corrosion in acids and other highly corrosive media are called acid-resistant steel. Generally, the aforementioned stainless steels and acid-resistant steels are collectively referred to as stainless acid-resistant steels, or simply as stainless steel. 1. Chromium 13 stainless steel: The grades for chromium 13 stainless steel (martensitic, ferritic) are shown in the table below: China, United States, United Kingdom, West Germany, Japan, Soviet Union – 1Cr13 (martensitic); AISI 403, 410; En 56BW-Nr, 4021. SUS 21, 22Э > K22Cr13 (martensitic); AISI 420; En 56CW-Nr, 4034. SUS 23Э > K21Cr13 (ferritic); AISI 405; En 56AW-Nr, 4001, 4024. SUS 38. This type of stainless steel contains 12–14% chromium, which is approximately the minimum amount of chromium required for stainless steel (Cr > 12%). It can be hardened through heat treatment, resulting in a magnetic martensitic structure; hence it is commonly known as martensitic stainless steel. Low-carbon grades include ferritic stainless steel. Its corrosion resistance is similar to that of other stainless steels. Capable of withstanding corrosion in atmospheric conditions, water, nitric acid, alkalis, salts, organic acids, organic compounds, and other oxidizing environments ； It is not resistant to corrosion by non-oxidizing acids (such as sulfuric acid, hydrochloric acid, etc.). Pitting and stress corrosion cracking can occur in solutions containing halide ions. Since it contains no nickel and has a low chromium content, its general corrosion resistance is lower than that of chromium 17 ferritic and chromium 18 nickel 9 austenitic stainless steels. It has high strength and hardness but low toughness, and is commonly used in components that require corrosion resistance, strength, and hardness all at once, such as valve parts and ball bearings. It is generally not used in chemical equipment such as tanks and pipes. 2. Chromium 17 stainless steel: The grades of chromium 17 stainless steel (ferritic) are listed in the table below: China, United States, United Kingdom, West Germany, Japan, former Soviet Union – Cr17Ti, AISI 430, En 60W-Nr, 4016, 4510; SUS 24Э, K645. Chromium 17 stainless steel contains 17–27% chromium, and its grain structure is primarily magnetic ferrite; hence it is commonly referred to as ferritic stainless steel. It cannot be hardened by heat treatment, but can be hardened through cold working. Its corrosion resistance is similar to that of other stainless steels; it has excellent resistance to the atmospheric conditions, as well as to nitric acid and other oxidizing environments. It is also resistant to corrosion caused by alkalis, salts, water, organic acids, and other organic compounds. It is not resistant to corrosion by non-oxidizing acids (such as sulfuric acid, hydrochloric acid, etc.). Its corrosion resistance is better than that of martensitic stainless steels but lower than that of austenitic stainless steels; however, it outperforms austenitic stainless steels in terms of resistance to stress corrosion cracking. It is mainly used for various components resistant to atmospheric corrosion in industries such as automobiles and construction, as well as household appliances; it can also be used for equipment in the nitric acid industry (such as storage tanks). However, its welding and machining properties are much inferior to those of chromium 18 nickel 9 steel, which limits its use in the chemical industry. This steel grade is available in several variants, including those with titanium, those without titanium, and low-carbon versions; their corrosion resistance is generally similar, with the low-carbon titanium-containing steel showing greater resistance to intergranular corrosion. 3. Chromium 18 nickel 9 stainless steel: The grades for chromium 18 nickel 9 stainless steel (austenitic type) are listed in the table below: China, United States, United Kingdom, West Germany, Japan, former Soviet Union – 0Cr18Ni9, 304, 304L; 58E4, 301, 430; 627, 28ЭИ8; 421. Cr18Ni9, 302; 58A, 430; 039, 40ЭЯ11. Cr18Ni9TiNb, 321 (347); 58B (F), 4541 (4550); 29ЭЯT. Cr25Ni20, 310, 314; 42. Cr18Mn8Ni5, 202. Chromium 18 nickel 9 stainless steel (austenitic type) refers to steel grades that contain 11% or more chromium, along with nickel as well. It easily passes into a passive state in oxidizing environments at room temperature (such as the atmosphere, water, strongly oxidizing acids, etc.), forming a highly protective film on its surface composed mainly of chromium oxide (Cr2O3). Its corrosion rate is extremely low, which is why it is called \"stainless steel\". However, when the temperature increases or the oxidizing capacity of the environment decreases, it shifts from a passive state to an active state, resulting in a significant increase in corrosion. Various types of stainless steel exhibit good corrosion resistance to organic acids, organic compounds, alkalis, neutral solutions, and various gases. It suffers severe corrosion in non-oxidizing acids (sulfuric acid, hydrochloric acid, etc.). Corrosion of stainless steel equipment is often localized; at the boundaries between the passive and active states, pitting may occur in salt solutions containing halide ions. In solutions containing stress-corrosion-sensitive ions (such as Cl‑, OH‑, etc.), the stressed areas (such as those near welds) may experience dangerous stress-corrosion cracking. Intergranular corrosion also tends to occur in the sensitized zones on both sides of the weld. Chromium-nickel steel exceeds plain chromium steel in both corrosion resistance and mechanical properties. The addition of nickel promotes the formation of an austenitic structure, resulting in better mechanical properties, particularly an increase in toughness. It also expands the passivation range, making it easier for the material to be passivated. Chromium 18 nickel 9 steel is the most widely used type of stainless steel. It has no magnetism and cannot be hardened through heat treatment, but like ferritic stainless steels, it can be hardened through cold working. Varieties with a small addition of titanium or niobium have higher resistance to intergranular corrosion and are also known as \"stabilized steels\". The higher the chromium and nickel content, the greater the corrosion resistance and heat resistance; for example, steel with 25% chromium and 20% nickel possesses resistance to high-temperature oxidation. In addition, nickel-saving or nickel-free steels using manganese in place of nickel (chromium-manganese-nitrogen steels) have also been produced. All of the above grades have roughly the same corrosion resistance. Only the chromium-nickel-molybdenum steel containing 2–4% molybdenum exhibits better resistance to non-oxidizing acids, halides, and other such media, and is less prone to pitting. Due to these significant differences, it is listed separately. 4. Chromium 18 nickel 12 molybdenum (titanium) stainless steel: The grades of austenitic chromium 18 nickel 12 molybdenum (titanium) stainless steel are listed in the table below: China – USA AISI; UK – En; West Germany – DIN; Japan – SUS; Former Soviet Union – ЭИ. Cr18Ni12Mo2~3: 316, 316L; 58H, 58J; 4436, 4570-1; 4573; 32, 33, 35; 400-1. Cr18Ni12Mo2Ti: 317; 4449. Cr26Mo1E-Brite: 26-1. This type of stainless steel is similar to ordinary chromium-nickel stainless steels; however, due to the addition of 2–4% molybdenum, it offers superior properties in many aspects. Its corrosion resistance in non-oxidizing acids, hot organic acids, and chlorides is much better than that of chromium-nickel stainless steels, and its resistance to pitting corrosion is also higher. Stainless steels containing titanium or niobium exhibit strong resistance to intergranular corrosion. The corrosion resistance of molybdenum-containing chromium-manganese-nitrogen (nickel) steel is essentially similar to that of the base steel grade, therefore it can be used as a substitute for this steel grade. Chromium 26 molybdenum 1 microcarbon ferritic stainless steel (E-Brite 26-1) has corrosion resistance similar to that of this steel grade; like other nickel-free ferritic steels, it offers advantages in terms of resistance to pitting and stress corrosion cracking. Moreover, due to its low levels of carbon and nitrogen, it is also resistant to intergranular corrosion. It can be used as a nickel-saving substitute for this steel grade. 5. Chromium 20, Nickel 22–30 stainless steel – Alloy 20; chromium 20, nickel 22–30 stainless steel. Its foreign product names are Durime 20 (for cast products), Carpehter 20 (for forged products), and Worthite. The grades are listed in the table below: Grade 20 is an alloyed nickel-chromium-molybdenum heat-resistant stainless steel, C6. Due to their high chromium and nickel content, these alloys possess greater corrosion resistance than ordinary stainless steels. It can be used to handle sulfuric acid, nitric acid, phosphoric acid, mixed acids, sulfurous acid, organic acids, alkalis, salt solutions, hydrogen sulfide, etc. At certain concentrations, it can also be used at high temperatures. However, it is not resistant to the corrosion of concentrated or hot hydrochloric acid, as well as wet fluorine, chlorine, bromine, iodine, aqua regia, etc. Due to their high cost, they are generally used in environments with severe corrosion, as well as in components that suffer from significant abrasion under high temperatures and high speeds, such as valves and pumps. Due to slight differences in alloy composition, steel grades of different types may exhibit varying corrosion resistance in certain environments, and this should be taken into consideration when using them.