Selection of materials for resistance to high-temperature sulfuric acid corrosion and equipment manufacturing

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Selection of Materials Resistant to High-Temperature Sulfuric Acid Corrosion and Equipment Manufacturing – Zhang Weituo, Hefei General Machinery Research Institute, Ministry of Machinery. Preface: Sulfuric acid is a very important chemical product, and there are over 500 sulfuric acid production plants in China. At the same time, sulfuric acid is one of the basic chemical raw materials. In modern industry, it is widely used in various sectors; in fertilizer production, particularly in the manufacture of phosphate fertilizers, sulfuric acid accounts for about 40% of the total output of this substance. Additionally, it is extensively utilized as a reactant, dehydrating agent, solvent, and other types of raw material in industries such as petrochemicals, metallurgy, pharmaceuticals, textiles, and machinery. As a result, there are far more factories that use sulfuric acid than those that produce it. Whether it is the equipment used for producing sulfuric acid or the production facilities that utilize sulfuric acid during the manufacturing process, any device that comes into contact with sulfuric acid will face the problem of corrosion caused by it. In our country, the corrosion problem caused by sulfuric acid is a quite common and very important issue. Sulfuric acid is a complex oxyacid, and its corrosivity exhibits different forms of oxidative or reducing corrosion depending on the acid concentration. Therefore, when considering the corrosion caused by sulfuric acid, the concentration of sulfuric acid should be taken into account first, and secondly, temperature; as the temperature of sulfuric acid rises, its corrosiveness increases rapidly. Third, the impact of various impurities or other substances in sulfuric acid should be fully considered. Sulfuric acid used in industrial production is usually not pure sulfuric acid; different impurities or other substances, along with their concentrations, have varying effects on the corrosiveness of sulfuric acid. The presence of certain impurities can exacerbate the corrosion of certain metal materials by sulfuric acid, while other impurities may slow down such corrosion. Furthermore, the flow rate of sulfuric acid will also affect its corrosiveness to the material. In short, the corrosion caused by sulfuric acid in industrial production is variable, and there are a wide range of materials that can be used. However, at present, the materials with strong corrosivity and difficult material selection are mainly those used in high-temperature medium-concentration sulfuric acid environments as well as high-temperature high-concentration sulfuric acid environments. With the advances in metallurgical and materials science, over the past two decades, various high-alloy materials resistant to high-temperature sulfuric acid corrosion have been developed both domestically and internationally, and quite good application results have been achieved. This article will briefly introduce the corrosion resistance characteristics, application ranges, and key points for equipment manufacturing of several high-alloy materials currently in use at home and abroad for high-temperature sulfuric acid. Table 1 Chemical composition and mechanical properties of several corrosion-resistant materials for high-temperature sulfuric acid. Material grade, Element content (wt %), Mechanical properties: Ni, Cr, Fe, Si, Mo, Cu, C; σs (MPa) ≥ σb (MPa) ≥ δ (%) ≥. H-D-205 base: 2065, 2.5, 20.0, 3*, 377, 835, 6.5; H-B-2 base: 1*, 2*, 0.1*, 28/0.01*, 407, 902, 60; H-B-3 base: 1.5, 1.5, 0.1*, 28.5/0.01*, 400, 902, 57; H-C-276 base: 1.6, 5, 0.08*, 16/0.01*, 364, 785, 59; H-C-22 base: 22, 30, 0.08*, 13/0.01*, 358, 786, 62. Material grade, Element content (wt %), Mechanical properties: Zr+Hf, Hf, Fe+Cr, HNO; σs (MPa) ≥ σb (MPa) ≥ δ (%) ≥. R60702: ≥99.2, ≤4.5, ≤0.2, ≤0.005, ≤0.025, ≤0.16. 20737916. *: Maximum value. I. HASTELLOY D-205 alloy: The H-D-205 alloy is a high-Si nickel-based corrosion-resistant alloy developed on the Ni-20Cr system. In sulfuric acid environments, high-Si cast iron or high-silicon stainless steels (such as 1Cr17Ni20Si5, 1Cr18Ni11Si4AlTi) have long been used. However, the biggest drawback of these materials is their brittleness and poor deformability; not only is it difficult to produce forged and rolled parts, but the cast parts produced are also highly brittle, making machining, installation, and maintenance very challenging. D-205 alloy is a nickel-based alloy that possesses a face-centered cubic crystal structure in its solid solution state; as a result, it exhibits the good ductility and high deformation capacity characteristic of austenitic alloys. Due to the proper combination of various alloying elements in D-205 alloy, it is suitable over a wide range of sulfuric acid concentrations and temperatures. Figure 1 shows the corrosion rate curve of D-205 alloy in 10–90% sulfuric acid. Figure 2 shows a comparison of the corrosion resistance of high-Si stainless steel (1Cr17Ni20Si5) and D-205 alloy in sulfuric acid at different concentrations and temperatures. Figure 3 shows the corrosion rates of these two alloys in industrial sulfuric acid at 130°C. Figure 1 shows the corrosion rate of D-205 alloy in chemically pure sulfuric acid. Figure 2 compares the corrosion rates of D-205 alloy and 1 Cr17Ni20Si5 stainless steel in chemically pure sulfuric acid at 93°C. Figure 3 compares the corrosion rates of D-205 alloy and 1 Cr17Ni20Si5 stainless steel in industrial sulfuric acid at 130°C. Table 2 presents the uniform corrosion rates of D-205 alloy compared with other nickel-based and iron-based corrosion-resistant alloys under various test conditions. Table 2 Corrosion rate (MPY) under test conditions for various material grades: D-205, Cr17Ni-20Si5, 316L, 20Cb-36, 25C-276. Conditions: 65% HNO3, boiling – rates: 17, 289, 821, 848; 99% H2NO4 at 130°C – rates: 0.7, 1.1, 41, 141, 186, 9; 54% P2O5 at 116°C – rates: 55, 127, 201, 361, 222. According to ASTM G-28A, boiling – rates: 25, 192, 810, 242, 46; 40% formic acid + 0.4% H2SO4 at 70°C – rates: 915, 196, 115, 275, 463, 31; 65% HNO3 + 1.5% formic acid at 50°C – rates: 514, 1028, 736, 276, 167. It can be seen from the above data that the corrosion resistance of D-205 alloy in high-temperature concentrated sulfuric acid is superior not only to iron-based stainless steels but also to the nickel-based C-276 alloy. Furthermore, Hastelloy D-205 also exhibits good pitting and stress corrosion resistance in C1-containing environments. Due to its excellent resistance to sulfuric acid corrosion along with good formability, alloy D-205 is gradually being used abroad to replace high-silicon cast iron and high-silicon stainless steel. The most common application is the use of D-205 plate heat exchangers in place of cast iron, graphite, and C-276 heat exchangers. Table 3 Examples of the use of D-205 alloy in plate heat exchangers. Heat transfer area, operating environment, replaced material: 65 m2, 98.8% H2SO4, 105°C – anodized stainless steel shells and tubes; 10 m2, 98.5% H2SO4, 120°C – pilot tests; 200 m2, 98.5% H2SO4, 100°C – cast iron; 600 m2, 98.5% H2SO4, 105°C – cast iron; 160 m2, 9.8% H2SO4, 90°C – H-C-276; 190 m2, 98.5% H2SO4, 90°C – new plants. II. Hastelloy B-2, Hastelloy B-3 alloys: Both Hastelloy B-2 and Hastelloy B-3 belong to the Hastelloy B series of alloys. These alloy series are advanced Ni-Mo corrosion-resistant alloys developed to resist hydrochloric acid corrosion; they are the only nickel-based alloys that do not contain Cr. Hastelloy B series alloys exhibit excellent corrosion resistance in various reducing media, and can withstand the corrosion of hydrochloric acid at any temperature and concentration under normal pressure (see Figure 4); they are therefore often used in the distillation, concentration, and other processing operations involving hydrochloric acid. Hastelloy B series alloys exhibit excellent corrosion resistance in sulfuric acid (see Figure 5), and they maintain good corrosion resistance at all concentrations of sulfuric acid below 100°C. These alloy series are also resistant to corrosion by various non-oxidizing media such as hydrofluoric acid, phosphoric acid, and various organic acids like acetic acid and formic acid, as well as various chloride salts such as aluminum chloride, magnesium chloride, and antimony chloride. These alloy series also exhibit excellent resistance to pitting corrosion and stress corrosion. However, Hastelloy B series alloys are not resistant to corrosion by oxidizing media, such as nitric acid and chromic acid, as well as oxidizing salts like ferric trichloride and copper dichloride. In reducing acids, attention must also be paid to the presence of oxidizing salts. As shown in Figures 5 and 6, when oxidizing salts such as ferric trichloride are present in trace amounts in hydrochloric acid or sulfuric acid, it **increases the corrosion of Hastelloy B series alloys; even dissolved oxygen can increase the corrosion rate of Hastelloy B series alloys in hydrochloric acid. Figure 4: Equal corrosion diagram of Hastelloy B-2 in hydrochloric acid solution. Figure 5: Equal corrosion diagram of Hastelloy B-2 in sulfuric acid solution. Figure 6: Equal corrosion diagram of Hastelloy B-2 in hydrochloric acid containing 100 ppm Cl-. Although Hastelloy B-series alloys exhibit excellent corrosion resistance in reducing media, there are still significant differences in their corrosion resistance among Hastelloy B, Hastelloy B-2, and Hastelloy B-3. After welding, Hastelloy B alloy suffers from severe intergranular corrosion in the weld and heat-affected zone due to the extensive precipitation of various carbides and intermetallic phases at the grain boundaries. Therefore, Hastelloy B alloy must undergo solution treatment after welding to be used effectively. Currently, Hastelloy B alloy is no longer produced in profile form; it is only available as cast products. To address the severe intergranular corrosion problem after welding of Hastelloy B, Hastelloy B-2 alloy with low carbon, low silicon, and low iron content was developed. The as-welded state of Hastelloy B-2 alloy exhibits excellent intergranular corrosion resistance, allowing it to be used directly in this state. However, years of industrial application have shown that: (1) the intergranular corrosion resistance of Hastelloy B-2 in its post-welded state is sometimes at a critical level; therefore, some large-scale Hastelloy B-2 equipment still requires overall heat treatment after fabrication. (2) Hastelloy B-2 does not have good thermal stability. During the production of its raw materials, as well as during the overall heat treatment of the equipment made from this alloy and throughout its service life, there have been numerous instances of cracking in the material. To improve the thermal stability of Hastelloy B-2, Hastelloy B-3 was developed abroad at the end of the 1980s. Through the adjustment of alloying elements, Hastelloy B-3 maintains good post-weld intergranular corrosion resistance, as well as excellent thermal stability. Table 4 Corrosion rates of Hastelloy B-2, Hastelloy B-3, 316L, and Monel 400 alloys in various boiling acid solutions. Material grade, Test conditions, Uniform corrosion rate, mpy: H—B—3, H—B—2, 316L, Monel 400. 50% acetic acid, boiling: 0.2, 0.4, 0.2; 40% formic acid, boiling: 0.5, 0.7, 4, 12.1; 55% phosphoric acid, boiling: 3.0, 6, 18, 4.5; 50% sulfuric acid, boiling: 1.7, 1.2; >20,000: 18, 5. 20% hydrochloric acid, boiling: 12, 15; >20,000: 15, 87. Stress corrosion tests conducted in 60% boiling sulfuric acid showed that Hastelloy B-3 has better resistance to stress corrosion than Hastelloy B-2. In recent years, large-scale chemical processing equipment made of Hastelloy B-3 has been put into use in the United States, Germany, Taiwan, and other places. From a development perspective, Hastelloy B-2 may be replaced by Hastelloy B-3. III. Hastelloy C-276 and Hastelloy C-22 are alloys in the Hastelloy C series; they are nickel-chromium-molybdenum corrosion-resistant alloys with a single-phase face-centered cubic crystal structure. It exhibits excellent corrosion resistance in both oxidizing and reducing media. For example, it shows its unique corrosion resistance in oxidizing acids containing F- and Cl- ions, in reducing acids in the presence of oxygen or oxidants, in mixed acids of oxidizing and reducing acids, as well as in wet chlorine and aqueous solutions containing chlorine gas. Therefore, the Hastelloy C series is the most widely used, versatile, and common corrosion-resistant alloy among nickel-based alloys. Due to the precipitation of large amounts of carbides and intermetallic phases along the grain boundaries after welding, Hastelloy C is highly susceptible to intergranular corrosion; as a result, deformed products made from Hastelloy C were quickly phased out, and currently only cast products of Hastelloy C are still in use. Hastelloy C-276 is an improved version of Hastelloy C with lower carbon and silicon content. It exhibits excellent intergranular corrosion resistance in its as-welded state, allowing it to be used directly after welding. Hastelloy C-276 also shows good corrosion resistance in hydrochloric acid and sulfuric acid (see Figures 7, 8, and 9). Figure 7 Equichorography of Hastelloy C-276 in sulfuric acid and sulfuric acid + 200 ppm Cl- solution. Figure 8: Equichorography of Hastelloy C-276 in hydrochloric acid solution. Figure 9: Equichorography of Hastelloy C-276 in hydrochloric acid solution containing saturated oxygen. Hastelloy-276 has high levels of both chromium and molybdenum, which makes it a material that can be used as a viable option in many corrosive environments. For example, in coolers used to cool 98% H2SO4, Hastelloy C-276 is not the best corrosion-resistant material for 98% sulfuric acid, but it is the best material for resisting corrosion on the cooling water side; it can use contaminated water, alkalized water, or seawater as a coolant. However, Hastelloy C-276 does not have good thermal stability; material cracking and intergranular corrosion can occur easily during the production of raw materials (such as forging and rolling), as well as during equipment manufacturing and operation. Therefore, Hastelloy C-4 and Hastelloy C-22 alloys were subsequently developed. Hastelloy C-4 possesses excellent high-temperature stability, but its corrosion resistance in strongly reducing media or in chloride-containing environments is lower than that of Hastelloy C-276. Hastelloy C-22 is a alloy with superior performance among the Hastelloy C series alloys. It possesses excellent thermal stability, which not only improves its hot working properties such as forging and rolling, but also confers high resistance to intergranular corrosion after welding. Laboratory test data and practical applications have shown that the weld metal of Hastelloy C-22 has higher corrosion resistance than that of Hastelloy C-276. Hastelloy C-22 has good performance experience in a solution of 80°C, 20% HF + 64% H2SO4, as well as in an acid cleaning solution of 20% HF + 20% HCl + 40 mg/L Fe. From the perspective of comprehensive corrosion resistance and overall performance, Hastelloy C-22 is superior to Hastelloy C-276 and Hastelloy C-4. IV. Zr Metal: Zirconium exhibits excellent corrosion resistance in many highly corrosive media. Abundant amounts of zirconium are used each year in the chemical industry, for example in hot concentrated hydrochloric acid, sulfuric acid, high-temperature nitric acid, and other highly corrosive process media. Zirconium-based towers, heat exchangers, pumps, valves, etc., have replaced traditional brick-built towers, graphite and lead heat exchangers, as well as high-silicon cast iron pumps. Zirconium-based equipment **increases production efficiency, reduces maintenance costs, and improves product purity.** In the 1980s, the United States spent over 100 tons of zirconium per year on the chemical industry. Zirconium is a reactive metal; its standard electrode potential at 25°C is -1.53 V, which is more negative than the standard electrode potentials of iron (-0.036 V) and copper (0.0336 V). According to chemical thermodynamics, zirconium is less stable than iron and copper in corrosive environments, meaning it is more susceptible to corrosion. However, zirconium has a high affinity for oxygen; when exposed to an oxygen-containing environment (such as room-temperature air or water), a dense oxide film with strong adhesion forms on the surface of zirconium. Below 300°C, this film can self-heal to protect the base metal from chemical or mechanical damage. Therefore, the corrosion resistance of zirconium depends on the density of its oxide film and the ability of this oxide film to repair itself in corrosive media. Obviously, the purity of the zirconium material itself has a significant impact on its corrosion resistance. Test data show that the corrosion rate of high-purity zirconium in hydrochloric acid at high temperatures can be 6–7 times higher than that of low-purity zirconium; moreover, low-purity zirconium suffers from intergranular corrosion. The accumulation of impurity elements in the weld area, due to various reasons, is the fundamental cause of intergranular corrosion in the weld areas of zirconium equipment. Figure 10: Equichorography of zinc in hydrochloric acid. Figure 11: Equichorography of zinc in sulfuric acid. According to Figures 10 and 11, zinc exhibits excellent corrosion resistance in high-temperature concentrated hydrochloric acid as well as in high-temperature sulfuric acid. Zirconium is already widely used in the hydrochloric acid industry, but when certain oxidizing metal ions such as Fe+3 are present in the hydrochloric acid, the corrosion potential of zirconium becomes polarized above its pitting potential, leading to pitting and stress corrosion of zirconium. Therefore, when zirconium equipment is operating in a hydrochloric acid medium, the presence of certain oxidizing metal ions is not allowed. At the same time, certain oxidizing metal ions also affect the corrosion resistance of zirconium in sulfuric acid; however, when the sulfuric acid concentration is below 40%, zirconium can tolerate a large amount of such oxidizing ions. Therefore, zirconium is often used in acid washing equipment for stainless steel. In summary, as can be seen from the brief introduction to various materials above, all of the aforementioned materials can be used in high-temperature sulfuric acid, but they each have their own applicable ranges within sulfuric acid. At the same time, it must be noted that the test data from laboratories serve only as a guide. Due to the complexity of the process media used in chemical manufacturing, it is necessary to carefully consider the combined effects of various factors based on the specific characteristics of those media before determining whether the selected materials are appropriate. This is especially true for relatively expensive materials such as zirconium and Hastelloy; mistakes in material selection can result in significant direct and indirect economic losses. V. Manufacturing of Hastelloy and zirconium equipment: With the continuous development of the petroleum, chemical, and fertilizer industries, as well as the increasing complexity of the process parameters, the corrosivity of the process media has also risen. To meet the demands of these more corrosive media, and to ensure that production processes can operate safely, efficiently, and over extended periods of time, more and more advanced alloys such as Hastelloy, zirconium, and zirconium-based metals are being used in large-scale chemical equipment. Since these materials are used in harsh, highly corrosive environments, ensuring that the purchased materials possess the required corrosion resistance, and preventing a degradation of their original corrosion-resistant properties during the manufacturing process of the equipment – that is, maintaining their original corrosion resistance even after processes such as cold and hot deformation as well as welding – is the second most important task after material selection. As is well known, the codes and standards established internationally or domestically for the manufacturing of materials and equipment (such as ASME codes, GB150 codes, etc.) are principle-based and general in nature. Therefore, before purchasing and processing a selected material, it is necessary to establish specific technical requirements for material procurement and equipment manufacturing, based on the operating conditions of the equipment and the properties of the material, and to append these requirements to the ASME codes and GB150 standards. Whether these technical requirements are well-defined, rigorous, and accurate depends on the degree of understanding that the formulators have regarding the operating conditions of the equipment, the properties of the materials, as well as the impact of various processing techniques such as cold and hot working and welding on the material properties. Hastelloy and zirconium metal have very strict requirements regarding their chemical composition as well as various processing techniques such as forging, rolling, cold and hot deformation, welding, and heat treatment. Deviations in the chemical composition or inappropriate processing methods can affect the ultimate performance of these materials in terms of corrosion resistance. At present, China is unable to supply industrial-scale raw material products of Hastelloy and zirconium metal for chemical processing equipment; these advanced alloy materials generally have to be imported from abroad. The procurement of imported materials and the re-inspection of these materials after they arrive are the first key factors determining the service life of the equipment. Hastelloy has two major drawbacks: its inherent poor thermal stability and sensitivity to intergranular corrosion. Hastelloy from different manufacturers and with different batch numbers can exhibit significant variations in corrosion resistance. Therefore, it is necessary to establish well-defined, reasonable, and accurate technical requirements when purchasing imported materials, and to conduct tests on their chemical composition, thermal stability, and intergranular corrosion, in order to ensure that the purchased materials possess excellent corrosion resistance. Secondly, in the equipment manufacturing process, issues such as welding procedure qualification, the establishment of welding technical specifications, the determination of heat treatment protocols, and the restoration of material properties after cold and hot deformation are all key control points. At the heart of these key control points lies the inspection for intergranular corrosion. Zirconium (ASTM R 60702) is a highly pure form of metallic zirconium, and its resistance properties depend on the purity of the material. In the manufacturing process of zirconium equipment, the key remains ensuring the purity of the zirconium material and the welds, especially the purity of the zirconium welds. In short, for Hastelloy and zirconium metals, it is only by fully understanding the properties of these materials during the procurement of raw materials and in the design and manufacturing of equipment, by establishing comprehensive, accurate, and rigorous technical specifications, and by implementing these specifications correctly, that equipment with excellent corrosion resistance and meeting the design requirements can be produced. VI. Development of Hastelloy castings: Over the past decade, Hastelloy B and Hastelloy C castings in China have been increasingly used in various industrial applications. Generally, the service life of cast products made from the same alloy grade is shorter than that of their forged or rolled counterparts; as a result, cast products require more frequent replacement. Moreover, castings of the same alloy can often exhibit significant differences in their performance. Hastelloy possesses excellent corrosion resistance, but Hastelloy castings can also exhibit the situation where castings of the same alloy do not always provide the same level of corrosion resistance. Since Hastelloy castings are more sensitive to carbides and intermetallic compounds, precipitation tends to occur at the grain boundaries; these precipitates along the grain boundaries can easily form pathways for corrosion. Additionally, due to the poor fluidity of Hastelloy resulting from the coexistence of its solid and liquid phases, its products are prone to porosity and defects. According to foreign statistics, approximately 50% of the early failures in Hastelloy castings are caused by intergranular corrosion. In China, most Hastelloy cast products are manufactured in small factories, where the production processes are not easily standardized and the inspection methods are insufficient. As a result, the quality of these products varies greatly, with their service life differing by 5 to 10 times. Most manufacturers provide Hastelloy castings without either chemical composition or mechanical property and corrosion test data, and even the alloy grade is not exact. Therefore, to ensure that Hastelloy castings possess stable and reliable corrosion resistance as well as mechanical properties, the casting production process must have a comprehensive set of technical requirements to guarantee the quality of the castings. However, the provisions on Hastelloy castings in ASTM and ASME standards are general in nature and based on principles; they do not cover the casting techniques for Hastelloy or the methods for testing the corrosion resistance of these alloys. Therefore, to obtain Hastelloy castings with stable and reliable quality, it is necessary to establish a comprehensive set of casting technical requirements and testing methods, and to adopt new grades of Hastelloy castings based on foreign standards. November 1998