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In recent years, industries such as petroleum, chemicals, pharmaceuticals, and clean energy have developed rapidly, resulting in increasingly complex medium environments for materials and equipment, which places higher demands on the corrosion resistance of engineering materials. In engineering design, hydrochloric acid is a common corrosive medium. When it contains a certain amount of iron chloride (or oxidized salts such as copper chloride), its corrosivity toward chemical processing equipment is extremely high, leading to equipment damage and failure. Therefore, it is crucial to select materials resistant to hydrochloric acid corrosion appropriately. Against the backdrop of the advancement of clean energy strategies, the extraction of alumina from power plant fly ash has become a research hotspot. The related \"one-step acid dissolution\" process uses recyclable hydrochloric acid as the reaction medium; moreover, the erosion by silica particles during the flow of this medium renders the environment even more hostile, which further highlights the importance of selecting materials resistant to hydrochloric acid corrosion. 1 Corrosion properties of hydrochloric acid: Hydrochloric acid is a typical non-oxidizing strong acid that can completely dissociate into chloride ions. Common metal materials are prone to severe activation corrosion in hydrochloric acid environments, and the corrosion rate increases significantly as the concentration of hydrochloric acid and temperature rise. At the same time, the highly active chloride ions in hydrochloric acid can destroy the passivation layer on the surface of metal materials, leading to overall corrosion of these materials; many metals such as stainless steel may also suffer from pitting and stress corrosion cracking. In terms of the difficulty of material selection, choosing materials resistant to hydrochloric acid is more difficult than choosing those resistant to sulfuric acid. Although some non-metallic materials exhibit better corrosion resistance in hydrochloric acid media than metallic materials, metallic materials are more widely used due to their excellent comprehensive properties such as mechanical strength, wear resistance, and thermal stability. However, most common metal materials have poor resistance to hydrochloric acid corrosion; only a few special metals such as titanium, zirconium, and tantalum, as well as nickel-based and molybdenum-based alloys, can be used in hydrochloric acid environments. 2 Factors affecting the selection of acid-resistant materials: In the design of petroleum and chemical equipment, the choice of materials has a direct impact on the equipment’s performance, service life, safety, cost-effectiveness, and reliability. It is usually necessary to make this decision by taking into account the operating conditions (temperature, pressure, stress, wear, service life, etc.) as well as the surrounding environmental conditions (temperature, humidity, pollution, pH level, etc.). The factors affecting the resistance of metal materials to hydrochloric acid corrosion can be divided into internal and external factors. Internal factors are determined by the metal material itself, including alloying elements, impurities, surface condition, and internal stresses. For example, adding alloying elements with strong corrosion resistance can improve the alloy’s resistance to hydrochloric acid corrosion, but there is a critical amount to be added (according to Tammann’s law); it is necessary to add them in appropriate quantities ； External factors involve the physical properties, mechanical properties, cost-effectiveness, and supply lead time of the materials. These factors influence and restrict each other; therefore, when selecting metal materials resistant to hydrochloric acid corrosion, it is necessary to take all factors into consideration and choose a solution that is suitable for production, technologically advanced, economically reasonable, safe and reliable, as well as environmentally friendly. 3 Corrosion resistance of metal materials to hydrochloric acid: When selecting materials for use in hydrochloric acid environments, various factors must be taken into account, with the corrosion conditions of the material being the most important considerations (such as hydrochloric acid concentration, temperature, impurities, etc.). Relevant studies have produced a chart for selecting materials resistant to hydrochloric acid (Figure 1), which shows the equal corrosion lines at a corrosion rate of 0.05 mm/year under different hydrochloric acid concentrations and temperatures; these equal corrosion lines are typically used as the upper limit for alloy selection in design purposes. Figure 1. Selection of hydrochloric acid-resistant materials. As can be seen from the graph, the concentration-temperature diagram of hydrochloric acid is divided into five regions. The appropriate temperature and concentration ranges for each region are shown in the table below (the materials listed in the table are based on extensive corrosion test data and factory experience, with no consideration given to economic factors). Table 1 Acid-resistant metal materials. The materials listed in Table 1 are only applicable when the material is in concentrated hydrochloric acid at a static or low flow rate. If other impurities are present in hydrochloric acid, they can significantly alter the corrosion properties of the material; or if the high flow rate of the acid causes wear and corrosion of the material, then the materials listed in Table 1 may not be able to effectively prevent corrosion by hydrochloric acid. Furthermore, when selecting materials, we also need to conduct a comprehensive analysis and comparison of factors such as their physical properties, mechanical properties, cost-effectiveness, supply lead time, and operating conditions. 4 Metal materials resistant to hydrochloric acid corrosion: Tantalum and tantalum alloys. Tantalum is the material with the best resistance to corrosion in hydrochloric acid; it remains inert in hydrochloric acids of various concentrations and can be used stably for long periods in such an environment. Its excellent corrosion resistance stems from the formation of an extremely thin, corrosion-resistant, and stable oxide film on its surface. Relevant corrosion test data show that in hydrochloric acid at temperatures below 190°C and concentrations below 25%, the corrosion rate of tantalum is less than 0.025 mm/year ； After 1000 hours of corrosion in a 20% hydrochloric acid solution at 95–100°C, the corrosion rate was only 0.001 mm/year, indicating extremely strong resistance to both uniform and localized corrosion. However, tantalum and its alloys are very expensive, making them uneconomical for use in large-scale equipment, which limits their widespread application. Nickel and nickel-based alloys have good ductility and moderate hardness, as well as good resistance to erosion by non-oxidizing acids. As an austenite-stabilizing element, nickel can increase the solid solubility of corrosion-resistant alloying elements such as chromium and molybdenum, facilitating the formation of various alloys ； Adding passivation elements to nickel-based alloys can improve the material’s corrosion resistance and thermodynamic stability. Hastelloy is an outstanding representative among nickel-based alloys for resistance to hydrochloric acid corrosion. Among them, Hastelloy B exhibits good corrosion resistance to hydrochloric acid at any concentration below its boiling point ； In hydrochloric acid at 160°C and a concentration of 2%, the corrosion rate of Hastelloy B-2 is less than 0.13–0.51 mm/year, making it suitable for extremely harsh hydrochloric acid corrosion environments ； Hastelloy C performs excellently in both oxidizing and non-oxidizing acids; its derivative, Hastelloy C-276, has even better corrosion resistance and is widely used in harsh corrosive environments. Zirconium and zirconium alloys belong to the category of refractory metals; they feature a high melting point, low coefficient of expansion, excellent mechanical properties, and good corrosion resistance. They are primarily used in nuclear reactors and chemical equipment. Zirconium has relatively reactive chemical properties; when heated and exposed to air, a dense oxide film forms on its surface, which endows it and its alloys with excellent corrosion resistance. Zirconium and its alloys exhibit strong resistance to hydrohalic acids such as hydrochloric acid; at atmospheric pressure, at boiling point, or at higher temperatures, the corrosion rate in hydrochloric acid of all concentrations is less than 0.13 mm/year. However, the high cost of zirconium limits its widespread use. Titanium and titanium alloys have high chemical reactivity; they tend to corrode thermodynamically in various media. Pure titanium corrodes in hydrochloric acid at concentrations of 5% or higher. However, titanium has a strong affinity for oxygen; in the presence of oxidizing substances such as copper chloride and iron chloride, a dense inert oxide film forms on its surface, preventing corrosion. However, titanium oxide films are prone to instability in reducing acids such as concentrated hydrochloric acid, and alloying elements need to be added to improve this. For example, the addition of 0.1%-0.2% palladium to form a titanium-palladium alloy significantly improves its resistance to hydrochloric acid corrosion; at 208°C in 5% hydrochloric acid, the corrosion rate is approximately 0.1 mm per year, making it a material with great potential for use in environments exposed to hydrochloric acid. 5 Outlook: With the development of industries such as petrochemicals, equipment will be exposed to harsh conditions involving higher temperatures, higher hydrochloric acid concentrations, and more solid particles, which places greater demands on the performance of materials resistant to hydrochloric acid corrosion. In the future, research on tantalum could focus on two areas: one is to explore composite technologies involving tantalum and other metals, in order to reduce costs while maintaining corrosion resistance; for example, by using coating techniques to apply tantalum to the surfaces of low-cost metals, thereby balancing performance and cost efficiency ； Second, optimize the processing technology of tantalum to improve its feasibility in the manufacturing of components for large-scale equipment and expand its application areas. Statement: This article was originally published on the WeChat official account [Tantalum and Niobium New Technology Services and Applications]