In chemical production processes, pipeline systems are the blood vessels connecting various production stages. The selection of pipeline materials directly affects production safety, operating costs, and equipment service life. In environments involving acids, alkalis, salts, organic solvents, and other complex media, combined with high temperature, high pressure, and corrosive conditions, choosing the appropriate piping material is a core competency every chemical engineer must master. This article systematically reviews commonly used pipeline materials in the chemical industry, from traditional cast iron to high-performance alloys and non-metallic materials, helping establish a clear and practical selection methodology.
Although modern chemical plants increasingly rely on advanced alloys and non-metallic materials, traditional materials such as cast iron, silicon iron, and carbon steel still hold irreplaceable positions in many conventional working conditions. Their mature manufacturing processes, low cost, and specific corrosion-resistant properties make them practical solutions in appropriate environments. Understanding their performance limits is the first step in mastering chemical pipeline selection.
Ordinary cast iron pipe is one of the oldest pipeline materials and remains widely used in specific scenarios. Its main advantages include good corrosion resistance to soil, acids, and alkalis, relatively low cost, and well-established installation techniques. It is commonly used in underground water mains, gas mains, and sewage pipelines in municipal and industrial infrastructure.
However, cast iron has clear disadvantages. It has relatively low strength and high brittleness, meaning it can crack under impact or internal pressure fluctuations. For safety reasons, it must not be used in high-pressure systems or for transporting toxic or explosive media. In chemical plants, cast iron pipes are typically limited to underground non-pressurized pipelines or low-pressure auxiliary systems.
For highly corrosive environments, silicon iron pipes offer a specialized solution. High-silicon iron pipes, typically containing 14–17% silicon, can resist corrosion from various strong acids, including sulfuric acid and nitric acid. The material has extremely high hardness, which contributes to corrosion resistance but also makes machining difficult. Cutting, welding, and mechanical processing are challenging, and the material is brittle, making it susceptible to fracture under vibration or impact.
Within the silicon iron family, chlorine-resistant silicon iron pipes are specifically developed for hydrochloric acid service. They can resist hydrochloric acid at various concentrations and temperatures, making them suitable for hydrochloric acid handling systems. However, due to brittleness, they should not be used in applications involving vibration or frequent mechanical stress.
Carbon steel pipes are the most widely used materials in industrial pipelines. They are favored for their high strength, low cost, and excellent weldability. However, their corrosion resistance is limited, and protective measures such as coatings or linings are usually required in chemical environments.
Carbon steel exhibits specific corrosion resistance in certain cases. At room temperature, concentrated sulfuric acid can form a protective passive film on the carbon steel surface. Carbon steel also demonstrates acceptable resistance to alkaline solutions such as sodium hydroxide at normal temperatures.
However, two major limitations must be emphasized. Carbon steel is unsuitable for chloride-containing environments and is highly sensitive to stress corrosion cracking. In chloride-rich media, it is prone to pitting and stress corrosion cracking, which are common failure modes in chemical plants. These risks must be carefully considered during material selection.
Austenitic stainless steels are the most widely used corrosion-resistant steels in the chemical industry. The 18-8 series, represented by 304 stainless steel and its variants, is particularly common. These materials offer excellent mechanical properties, ease of fabrication, and strong corrosion resistance in oxidizing environments.
Their major weakness is sensitivity to chloride ions (Cl⁻), which can lead to stress corrosion cracking. This failure mechanism is one of the primary causes of pipeline damage in chemical plants.
304 stainless steel contains approximately 18% chromium and 8% nickel, with carbon content below 0.08%. It provides good mechanical properties and can generally be used at temperatures below 650°C. It has good resistance to oxidizing acids, alkaline solutions, and many organic and inorganic acids.
In nitric acid solutions with concentrations up to 65% and temperatures below the boiling point, 304 stainless steel shows strong corrosion resistance, making it suitable for nitric acid transportation systems.
304L is the low-carbon version of 304, with carbon content not exceeding 0.03%. During welding, carbide precipitation in the heat-affected zone can cause intergranular corrosion. By reducing carbon content, 304L minimizes carbide formation and improves corrosion resistance of welded joints. It is preferred in welded systems exposed to intergranular corrosion risks.
321 stainless steel is a titanium-stabilized austenitic stainless steel based on 304. The addition of titanium improves resistance to intergranular corrosion and enhances high-temperature performance. It has excellent high-temperature stress rupture strength and creep resistance.
It is suitable for applications exceeding 450°C, especially in chemical, petroleum, and coal industries where corrosion resistance and high-temperature strength are required.
316 stainless steel adds 2–3% molybdenum to the 304 composition, significantly improving corrosion resistance. It offers better pitting resistance, improved performance in marine and industrial atmospheres, and greater resistance to dilute sulfuric acid and chloride-containing solutions.
316L is the low-carbon version, offering enhanced resistance to intergranular corrosion. Although more expensive, 316L is widely used in chloride-containing environments and critical chemical applications.
High-alloy stainless steels contain higher nickel and chromium contents, often exceeding 20% chromium and approaching 30% nickel. These materials offer improved resistance to sulfuric acid and hydrochloric acid and are used in extreme corrosive conditions where standard stainless steels are inadequate.
When stainless steels cannot meet corrosion resistance requirements, nickel-based alloys become necessary. These materials use nickel as the primary component, with additions of copper, chromium, and molybdenum to enhance performance in aggressive environments.
Monel is a nickel-based alloy containing copper and small amounts of iron and manganese. It is widely used due to its excellent comprehensive corrosion resistance.
It performs exceptionally well in hydrofluoric acid and fluorine environments, where most stainless steels fail. It also resists hot concentrated alkalis and shows good resistance to seawater, neutral solutions, and organic compounds. Monel is generally resistant to stress corrosion cracking and offers good machinability. It can operate at temperatures up to 500°C and is often selected for chloride-containing environments and hydrofluoric acid systems.
Hastelloy is a high-performance nickel-based alloy series, including B, C, and G grades. It is designed for extremely corrosive environments where iron-based stainless steels and non-metallic materials cannot perform.
It provides resistance to wet chlorine, oxidizing chlorides, chloride salt solutions, sulfuric acid, and low- to medium-temperature hydrochloric acid. In high-temperature, high-concentration hydrochloric acid environments, Hastelloy is often the only feasible metallic solution.
Copper alloys used in the chemical industry include brass, tin bronze, aluminum bronze, and cupronickel. These materials offer excellent resistance to atmospheric and seawater corrosion due to the formation of protective surface films.
Brass is commonly used for valves and fittings in moderately corrosive environments. Tin bronze offers improved corrosion resistance compared to brass. Cupronickel is particularly suitable for seawater heat exchanger tubing due to its outstanding seawater resistance.
Titanium alloys are high-performance materials for chloride-containing environments. Titanium forms a dense TiO₂ oxide film in seawater and wet chlorine environments, providing exceptional corrosion resistance.
They are suitable for hydrochloric acid transport, chloride solution systems, seawater applications, and wet chlorine environments. Titanium offers high specific strength and low density but is expensive and requires strict welding procedures.
Non-metallic materials provide alternative solutions for corrosion control. They are immune to electrochemical corrosion and often provide cost-effective solutions in specific applications.
PVC pipes are resistant to most inorganic acids and salts but not to strong sulfuric acid, strong nitric acid, or most organic solvents. Their service temperature typically does not exceed 60°C.
PTFE pipes resist almost all chemicals except molten alkalis and fluorides. They can operate up to 250°C but have high thermal expansion, high cost, and processing challenges.
FRP pipes combine light weight, high strength, corrosion resistance, and good hydraulic performance. They are widely used in drainage systems, oil pipelines, and water supply networks.
Material selection should follow a structured process:
Clarify medium characteristics: chemical type, chloride presence, concentration, temperature.
Determine operating conditions: pressure, temperature, welding requirements, vibration, safety classification.
Evaluate economic factors: lifecycle cost, installation, maintenance, downtime.
Consider special environments: hydrofluoric acid (Monel), high-temperature hydrochloric acid (Hastelloy or titanium), chloride environments (316L or advanced alloys), seawater (titanium, cupronickel, or FRP).
Pipeline selection in the chemical industry is a systematic engineering task requiring balanced consideration of chemical compatibility, mechanical performance, operating conditions, and economic efficiency. From cast iron and carbon steel to stainless steels, nickel-based alloys, titanium alloys, and non-metallic materials, each has its proper application range. Understanding material limitations and establishing a logical selection framework is fundamental to ensuring safe, efficient, and economical chemical plant operation.
