Metal corrosion

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Metal corrosion is the most harmful spontaneous process in industry; therefore, the development and application of corrosion-resistant alloys hold great social significance and economic value. From military applications to civilian uses, and from oil and gas fields to areas such as nuclear power, photovoltaics, and hydrogen energy, an increasing number of corrosion-resistant alloy products are demonstrating their excellence in a wide range of application scenarios. Introduction: How much do you know about “corrosion-resistant metals”? The ability of metal materials to resist erosion by corrosive media is known as the corrosion resistance of the metal. Pure metals with high corrosion resistance usually possess at least one of the following properties: · Metals with high thermodynamic stability. It can generally be determined using its standard electrode potential; those with a more positive value exhibit higher stability, while those with a more negative value have lower stability. Precious metals with good corrosion resistance, such as Pt, Au, Ag, Cu, etc., belong to this category. · Metals that are easy to passivate. Many metals can form a dense protective oxide film in oxidizing media; this phenomenon is known as passivation. Among metals, those that are most easily passivated include Ti, Zr, Ta, Nb, Cr, Al, etc. · Metals whose surface energy results in the formation of insoluble and highly protective corrosion product films. This situation occurs only when metals are in specific corrosive media, such as Pb and Al in H2SO4 solutions, Fe in H3PO4, Mo in hydrochloric acid solutions, and Zn in the atmosphere. Corrosion-resistant alloy materials mainly include iron-based alloys (corrosion-resistant stainless steels), nickel-based alloys (Ni-Cr alloys, Ni-Cr-Mo alloys, Ni-Cu alloys, etc.), and active metals. It should be noted that although corrosion-resistant alloys have the ability to resist erosion by media, they cannot be used in fluorine-containing environments. 1 Corrosion-resistant stainless steel mainly refers to the ordinary 300 series stainless steel, 304, which is resistant to corrosion caused by the atmosphere or seawater. The most common corrosion-resistant alloys include: Hastelloy 316L, 317L, etc ； Austenitic stainless steels with strong corrosion resistance: 904L, 254SMO ； Duplex steels such as 2205 and 2507, as well as corrosion-resistant alloys containing CU such as alloy 20. 2 Nickel-based corrosion-resistant alloys: The main property of nickel-based corrosion-resistant alloys is their strong resistance to corrosion by liquid media. Since metal NI itself has a face-centered cubic structure, its crystallographic stability enables it to accommodate more alloying elements such as CR and MO than FE, thereby allowing it to resist various environmental conditions. At the same time, nickel itself possesses a certain degree of corrosion resistance, especially against stress corrosion caused by chloride ions. The nickel content in the alloy is usually less than 65%; elements such as chromium, copper, molybdenum, and tungsten are added to meet the requirements of various applications with different chemical properties. The addition of these elements also enhances nickel’s corrosion resistance in different environments. Nickel-based corrosion-resistant alloys are corrosion-resistant materials with excellent overall properties; they are capable of dealing with severe engineering corrosion problems that other metallic and non-metallic materials cannot handle. At the current level of technology and stage of engineering development, they are worth promoting widely. At the same time, due to the different elements added to nickel-based corrosion-resistant alloys, various types of alloys are formed; therefore, it is necessary to pay attention to the properties of the medium and the working environment during use. 3 Active metals are those with excellent corrosion resistance, including TI, ZR, TA, etc., among which TI is the most typical representative. Titanium has a wide range of applications, mainly in corrosive environments where stainless steel cannot be used. Principle of titanium’s corrosion resistance: In an oxidizing atmosphere, a dense oxide film is formed to provide protection; therefore, it generally cannot be used in corrosive environments with high reactivity or high sealing properties. Meanwhile, the operating temperature of titanium materials is generally below 300 degrees Celsius. It is particularly important to note that no active metals can be used in fluorine-containing environments.