Introduction to aluminized steel

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Aluminized steel and its applications in sulfuric acid plants by Zhang Hongwei from Changzhou Wujin Yangguang Aluminized Steel Co., Ltd. and the Nanjing Design Institute of Sinopec. Abstract: This article describes the microstructural characteristics of aluminized steel, focuses on its mechanical properties as well as its resistance to high-temperature oxidation and corrosion, and explains its processability. Practice has shown that the use of aluminized steel in sulfuric acid plants is successful and holds great potential for wider adoption. Keywords: aluminum infiltration ； Mechanical properties ； Resistance to high-temperature oxidation ； Corrosion resistance ； Sulfuric acid plant ； Application 1: Overview. With the continuous advancement of sulfuric acid production technology, the facilities used for its manufacture have become more diverse (such as those that use sulfur or smelting gases to produce sulfuric acid), and they are also becoming larger in scale. The use of new equipment and materials has raised the standard of sulfuric acid production facilities, laying a solid foundation for reducing costs and minimizing investment. The application of aluminized steel is one example. Aluminized steel is obtained by subjecting metals (including low-carbon steel, stainless steel, cast iron, etc.) to surface chemical heat treatment, which results in the formation of an aluminum-iron alloy layer 0.1–0.3 mm thick on the surface of the steel. This allows the operating temperature of the steel to be increased by 200–300°C compared to that of the original material. For low-carbon steel, this treatment also significantly improves its resistance to oxidation and corrosion, thereby extending its service life; in some cases, it can even replace heat-resistant steel or stainless steel. 2. Microstructure and properties of aluminized steel 2.1 Microstructure Hot aluminizing is a type of surface chemical heat treatment method in which an aluminum-iron alloy coating is formed on the surface of steel in molten aluminum. Aluminum hot-dip coatings can be divided into two types based on their production methods: immersion type and diffusion type. In the immersion type, after being heat-immersed, the workpiece is directly taken out of the molten aluminum and cooled in air; its surface is not smooth, often affected by aluminum adsorption and the solidification of excess molten aluminum, and its thickness is uneven. The typical structure of aluminized material by immersion method can be roughly divided into two layers: the outer layer is pure aluminum, with a chemical composition similar to that of the original aluminum melt; beneath this pure aluminum layer lies a compound layer formed by the mutual diffusion of aluminum and iron, which exhibits an irregular, tooth-like distribution on the steel substrate. In the diffusion type, the workpiece that has been heat-treated with aluminum immersion is placed in an air electric furnace and heated to 850–920°C for 3–4 hours. After that, it is taken out and cooled in air. During this diffusion annealing process, the aluminum on the surface melts and diffuses into the metal matrix; as a result, the pure aluminum layer disappears along with the tooth-like features. This leads to the formation of a surface with an aluminum-iron compound and solid solution crystal structure, which possesses properties such as high temperature resistance, oxidation resistance, and corrosion resistance. Numerous experiments and practices, both domestically and internationally, have shown that as long as the aluminum content on the surface of the aluminized layer is at least 7%, it can protect the metal substrate and slow down or even prevent oxidation and corrosion. Process flow for the immersion-type hot-dip aluminum coating process: degreasing--derusting--pre-plating--hot-dip aluminum coating--alignment--cleaning--inspection. Process flow for diffusion-type hot-dip aluminum coating: degreasing--derusting--pre-plating--hot-dip aluminum coating--alignment--cleaning--inspection--diffusion treatment--alignment--cleaning--inspection. The thickness and concentration of the aluminized layer are two main indicators for evaluating the quality of aluminization on workpieces. However, what is more critical is the aluminum concentration on the surface of the workpiece rather than its thickness; a thin aluminized layer with a high aluminum concentration can resist high-temperature oxidation and corrosion better than a thick aluminized layer with a low aluminum concentration. 2.2 Mechanical properties: After aluminum infiltration, the mechanical properties of the workpiece change little. For low-carbon steel, the tensile strength decreases slightly due to high-temperature diffusion annealing (by about 0.2–0.3 MPa). The aluminum-infused layer has a high hardness and good wear resistance. The high-temperature short-term strength is improved after aluminizing, and the creep value increases significantly. 2.3 Resistance to high-temperature oxidation: As we know, aluminum is a relatively reactive metal; in the atmosphere, its surface layer easily combines with oxygen to form an alumina film. This alumina film is stable, dense, pore-free, and continuous. It prevents direct contact between the metal matrix and high-temperature or corrosive media, thus avoiding oxidation. However, due to aluminum’s low melting point of just over 600 degrees, when aluminum is sprayed onto the surface of a metal, since the aluminum layer is applied as a coating and no aluminum-iron compound is formed, the aluminum layer tends to melt or peel off at high temperatures, resulting in poor performance. The aluminum-iron alloy formed after aluminizing has a melting point as high as 1135–1200°C. When the aluminum content on the surface of the aluminized layer exceeds 7%, its dense structure prevents the erosion by oxygen and other harmful substances, isolating the steel matrix from these substances and suppressing the diffusion of metal elements and reactions at the interface. Tests have shown that after aluminum infiltration of steel, its operating temperature can be increased by 200–300°C. However, as the operating temperature of the workpiece increases, high-temperature oxidation intensifies. The progression of high-temperature oxidation depends on the chemical reactions occurring at the two interfaces between the diffusion layer/oxidation film and the gas, and these chemical reactions are controlled by the composition of the oxidation film structure. At high temperatures, alumina formed due to diffusion consumes part of the aluminum, and at the same time some aluminum atoms evaporate. This leads to a decrease in the aluminum concentration on the surface of the treated layer, resulting in surface aluminum depletion. As a consequence, the oxidation medium can penetrate through the oxide film more easily, and the interfacial reactions accelerate. This causes the rate of oxidation and consumption of the aluminum-iron alloy in the treated layer to exceed its rate of formation, ultimately leading to a reduction or even disappearance of the aluminum-iron alloy. Therefore, the higher the operating temperature of the workpiece, the shorter its service life. Therefore, substrates with different compositions should be used in various application environments in order to form oxide films with different structures, thereby achieving the best performance. The temperatures at which certain steels begin to oxidize before and after aluminum infiltration are shown in Table 1. If the service time is within 1000–1500 hours, the temperature listed in the table can be multiplied by a coefficient of 0.9–0.85 to obtain the appropriate service temperature; if the service time exceeds 5000 hours, a coefficient of 0.85–0.8 should be used. Table 1 shows the temperatures at which oxidation begins before and after aluminizing certain steel grades. Base metal temperature (°C) / Aluminizing temperature (°C): Low-carbon steel – 570/1000; Cast iron – 600/900; Chromium-containing cast iron (Cr1.5) – 650/950; Cr13 – 750/1150; Cr18Ni9 – 950/1250; Cr25Ni20 – 1200/1300. The data on the oxidation weight gain rate of 10# steel samples after aluminizing, after being heated for 5 hours, are shown in Table 2. Table 2: Oxidation weight gain rate of samples after being heated at different temperatures for 5 hours (g/m2·h). Sample material and condition / Temperature (°C): 1050, 1125, 1200. For 10# steel before aluminizing: 210, 287, 334; for 10# steel after aluminizing: 3–5, 5–7, 10–25. The data on the oxidation rates of certain materials at different temperatures are shown in Tables 3 and 4. Table 3: Oxidation rate at 590°C for 1000 hours (g/m2·h). Material / Oxidation rate factor: Base metal after aluminizing – Low-carbon steel: 0.410/0.043/9.5; Mo1.0 steel: 0.353; Mo1.5 steel: 0.354/0.013/27.3; Cr2Mo0.5 steel: 0.24; Cr1.25Mo0.5 steel: 0.199/0.015/13.3; CrMo0.5 steel: 0.16. Table 4: Oxidation rates at 800°C for 1000 hours and at 900°C for 1000 hours (g/m2·h). Material / 800°C / 900°C: Low-carbon steel (tested for 100 hours) – 8.59; Low-carbon steel (boiling steel) after aluminizing – 0.048/0.1475; Low-carbon steel (killed steel) after aluminizing – 0.044/0.1117; Low-carbon steel (forged steel) after aluminizing – 0.037/0.139; 40Cr13 after aluminizing – 0.030/0.073; 40Cr25 after aluminizing – 0.030/0.068; 00Cr18Ni9 after aluminizing – 0.033/0.044; 40Cr18Ni9 –0.524/2.4. Corrosion resistance: In industrial production, metal corrosion is a very common phenomenon, and the losses caused by corrosion are quite significant. The corrosion resistance of aluminized steel actually relies on the electrochemical properties of aluminum to isolate the metal from the media that can cause corrosion, thereby achieving corrosion protection. 2.4.1 Resistance to corrosion by high-temperature sulfides Aluminized steel exhibits excellent resistance to corrosion caused by high-temperature sulfides. Tests have shown that at a temperature of 480°C and a hydrogen sulfide concentration of 6%, the corrosion resistance of low-carbon steel after aluminization is 29 times higher than that of steel that has not been aluminized, while it is 53 times higher for molybdenum-containing steel. In an operating environment at 650°C with a hydrogen sulfide concentration of 100%, the corrosion resistance of low-carbon steel after aluminizing increased by 290 times compared to that without aluminizing, while that of alloy steel and 0Cr18 Ni9 increased by 360 times. 2.4.2 Resistance to atmospheric corrosion: The principle of resistance to atmospheric corrosion is the same as that of resistance to high-temperature sulfide corrosion; the aluminum-iron alloy in the coating prevents the formation of a water film on the surface of the steel caused by humid air, thereby preventing electrochemical corrosion of the steel. According to foreign reports, the American company Amtec conducted exposure tests in Russia for 19 years. The results showed that the coating on the galvanized samples was damaged after 7 years, and 30% of them developed rust after 12 years; however, the aluminum-coated samples showed no changes even after 19 years. 2.5 Process Properties 2.5.1 Machinability The aluminum-impregnated layer is brittle, making machining difficult. Therefore, when using aluminum-impregnated steel, it is necessary to first machine the workpiece before carrying out the aluminum impregnation process; otherwise, not only will machining be difficult, but the dense layer on the surface of the impregnated layer will also be damaged, resulting in a reduced ability to resist corrosion and oxidation. 2.5.2 Plasticity: After aluminum infiltration, steel exhibits very poor plasticity. During cold bending tests on aluminum-infused steel pipes with dimensions of φ25x2.5, cracks appeared on the tensile side when the bend angle reached 60 degrees at a radius of R=20 ； When R=30 and the angle is bent to 90 degrees, cracks passing through the aluminized layer are observed on the tensile side ； When it is greater than 180 degrees, the crack extends into the matrix. The bending radius to avoid cracks should be at least 5 times the pipe diameter. 2.5.3 Weldability: When welding aluminized steel, if an ordinary stainless steel electrode is used to initiate arcing on the alumina surface layer, honeycomb-shaped pores or slag inclusions may appear in the weld joint. This is because aluminum oxidizes easily to form alumina during welding; alumina has a dense structure and a high melting point, and it covers the surface of the metal matrix, preventing the metal from melting. Additionally, alumina has a high density, which makes it prone to forming slag inclusions within the weld. It is generally necessary to remove the alumina layer from the surface of the components to be welded before welding, leaving only the aluminum-iron alloy layer, or to use specialized aluminized steel electrodes such as A312SL (Ö312 aluminized). A312SL is a stainless steel welding rod with a calcium-titanium type flux; proven through use in various projects, it ensures a smooth transition between the weld metal and the base material, effectively protects the aluminum-infused layer, and permits welding in all positions. It can be used with both AC and DC power during welding (65V), and the weld metal possesses corrosion resistance and high-temperature oxidation resistance comparable to those of aluminized steel. The specific values are shown in Tables 5, 6, and 7. Table 5: Main components of weld metal (%): C, Mn, Si, Cr, Ni, Mo, S, P