Metal Materials Resistant to Hydrochloric Acid Corrosion
Hydrochloric acid is a typical non-oxidizing strong acid, which completely dissociates into H+ and Cl-. Most common metallic materials undergo severe activation corrosion in hydrochloric acid environments, with the corrosion rate significantly increasing as the concentration of hydrochloric acid and the ambient temperature rise. In hydrochloric acid mediums, the highly active chloride ions destroy the passivation layer on the surface of metallic materials, leading to general corrosion, pitting corrosion, and stress corrosion cracking in austenitic stainless steel. Selecting materials resistant to hydrochloric acid is generally more challenging than selecting those resistant to sulfuric acid. Although non-metallic materials may exhibit better corrosion resistance in hydrochloric acid mediums, metallic materials are superior in terms of mechanical processing, wear resistance, and thermal stability, making them more widely applicable. Only a few special metals, such as titanium, zirconium, and tantalum, as well as certain nickel-based and molybdenum-based alloys, exhibit sufficient corrosion resistance in hydrochloric acid environments.
Factors to Consider When Selecting Hydrochloric Acid-Resistant Materials
Choosing materials resistant to hydrochloric acid is a complex task that requires consideration of various factors. Different materials exhibit different levels of corrosion resistance in different environments. Therefore, when selecting hydrochloric acid-resistant metals, the first step is to evaluate the corrosion environment, including the acid's concentration, ambient temperature, and the presence of impurities. Schillmoller conducted systematic studies on hydrochloric acid-resistant materials and developed guidelines for their selection. These guidelines identify suitable materials for various concentration and temperature ranges, based on experimental corrosion data and practical industrial experience, without considering economic factors.
Common Hydrochloric Acid-Resistant Metal Materials
1. Nickel and Nickel-Based Alloys
In nickel-based alloys, Hastelloy is widely recognized as one of the best metal materials for resisting hydrochloric acid corrosion. To address the issue of chemical equipment being susceptible to hydrochloric acid corrosion, Hastelloy A alloy was developed early on, effectively solving the corrosion problem of hydrochloric acid at 70°C. Later, it was discovered that adding a significant amount of molybdenum (Mo) to nickel-based alloys could greatly improve their corrosion resistance in reducing acidic media. This led to the development of Hastelloy B alloy, which has the strongest hydrochloric acid corrosion resistance among Hastelloy alloys. It demonstrates excellent corrosion resistance to hydrochloric acid at any concentration below its boiling point.
Hastelloy B-2 alloy is a new type of nickel-molybdenum alloy developed based on Hastelloy B. It exhibits exceptional corrosion resistance in reducing media. At a temperature of 160°C and a hydrochloric acid concentration of 2%, its corrosion rate is <0.13~0.51 mm/a. It is commonly used for components in corrosion-resistant equipment under extremely harsh hydrochloric acid conditions.
Hastelloy B-3 alloy, developed in the 1990s, is a new nickel-based alloy (Ni-28Mo). In hydrochloric acid systems, it shows improved corrosion resistance and thermal stability against stress corrosion. Within the temperature range from room temperature to boiling point, Hastelloy B-3 alloy exhibits a corrosion rate of less than 0.5 mm/a in concentrated hydrochloric acid and dilute hydrochloric acid with low oxidizing properties, demonstrating excellent corrosion resistance. However, while B-3 alloy performs well in pure hydrochloric acid, it is unsuitable for environments with oxidizing ions or high levels of dissolved oxygen. When oxidizing ions such as Fe³⁺ are present, the Mo⁴⁺ passive film can be oxidized to Mo⁶⁺, leading to a sharp decline in corrosion resistance.
Hastelloy C alloy belongs to the Ni-Cr-Mo nickel-based alloy series, offering excellent corrosion resistance in both oxidizing and non-oxidizing acids. By controlling the carbon (<0.02%) and silicon (<0.08%) content in Hastelloy C alloy, Hastelloy C-276 alloy was developed. It is one of the most corrosion-resistant modern metallic materials, exhibiting excellent resistance under both oxidizing and reducing conditions, as well as in the presence of halide ions. As a result, it is widely used in extremely harsh corrosive environments.
2. Zirconium and Zirconium Alloys
Zirconium is a Group IV-B element in the periodic table, classified as a refractory metal, and possesses many excellent properties, such as a high melting point, low thermal expansion coefficient, superior mechanical performance, and excellent corrosion resistance. Zirconium is primarily used in nuclear reactors and chemical equipment. Due to its high reactivity, zirconium can react with its environment even at relatively low temperatures. When heated and exposed to air, zirconium forms a dense oxide layer on its surface, which gives zirconium and its alloys exceptional corrosion resistance. Zirconium and its alloys are highly resistant to most hydrohalic acids, especially pure hydrochloric acid, hydrobromic acid, hydroiodic acid, and hypochlorous acid. In atmospheric conditions, boiling temperatures, or even higher concentrations of hydrochloric acid, the corrosion rate remains below 0.13 mm/a. However, its high cost limits its broader application.
3. Tantalum and Tantalum Alloys
The best corrosion-resistant material in hydrochloric acid is tantalum and its alloys. Tantalum is entirely inert in hydrochloric acid across all concentrations, exhibiting outstanding corrosion resistance, making it suitable for long-term use in hydrochloric acid environments. Special corrosion test data indicate that at temperatures below 190°C and hydrochloric acid concentrations below 25%, the corrosion rate of tantalum materials is <0.025 mm/a. Tantalum and its alloys form an extremely thin, corrosion-resistant, and stable oxide layer on their surface, which accounts for their exceptional corrosion resistance. However, the high cost of tantalum alloys makes them uneconomical for manufacturing large-scale equipment or shells as a whole.
4. Titanium and Titanium Alloys
Titanium is a highly reactive metal with a low equilibrium potential, making it thermodynamically prone to corrosion in certain media. Pure titanium corrodes in hydrochloric acid concentrations above 5%. However, titanium's strong affinity for oxygen allows it to form a dense, highly inert oxide film on its surface in the presence of oxidizing substances such as copper chloride or iron chloride, which inhibits corrosion of titanium and its alloys. Adding a small amount of alloying element palladium (0.1%-0.2%) to titanium creates titanium-palladium alloys, significantly improving their resistance to hydrochloric acid corrosion. Studies show that titanium-palladium alloys exhibit a corrosion rate of approximately 0.1 mm/a in 5% hydrochloric acid at 208°C. At boiling temperatures in 5% hydrochloric acid, the corrosion rate is about 0.05-0.16 mm/a, making them a highly promising material for hydrochloric acid corrosion resistance.