Application of cast iron pipes in sulfuric acid engineering
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The 25th National \"Safety Production Month\" in 2026: Everyone should talk about safety and be able to handle emergencies; identify and address potential risks and hazards. -------------------------------------------------- In the entire process of sulfuric acid production, storage, transportation, and exhaust gas treatment in the chemical industry, corrosion, high temperatures, and medium impact represent the main challenges faced by pipeline systems. Compared to carbon steel, stainless steel, and plastic pipes, cast iron pipes are the preferred material for transporting concentrated sulfuric acid, serving as components for acid tanks, and forming part of process pipelines in sulfuric acid applications, thanks to their excellent resistance to concentrated sulfuric acid, superior structural stability, low cost, and long service life. However, most engineering failures, pipe leaks, and premature aging issues stem from misconceptions regarding the material properties, applicable limits, and design details of cast iron pipes. This article systematically analyzes the basic properties of cast iron pipeline foundations, the fitting systems, and industry standards, and explains the application logic in sulfuric acid projects through practical scenarios. It focuses on identifying the design challenges and common mistakes associated with pipelines for concentrated acids, providing professional guidance for the design, construction, and maintenance of chemical pipelines. I. Cast iron pipes specifically designed for sulfuric acid processing: Basic materials and key properties. The cast iron pipes used in sulfuric acid processing are not ordinary cast iron pipes for water supply and drainage; rather, they are corrosion-resistant cast iron materials that have been specially modified for this purpose. The main types include ordinary corrosion-resistant gray cast iron and high-silicon corrosion-resistant cast iron. There are significant differences between these two types in terms of material structure, corrosion resistance, and applicable scenarios, and these factors serve as the key considerations when selecting materials for such projects. 1.1 Classification of Basic Materials and Key Parameters (1) Ordinary corrosion-resistant gray cast iron: This material is based on flake graphite cast iron, with a carbon content of 2.5%~3.6%. It features a stable microstructure, excellent castability, and low cost, making it a commonly used pipe material in traditional sulfuric acid production projects. Its advantages include high stiffness, good compressive strength, and low tendency to deform, making it suitable for static transportation under normal pressure and temperature conditions ； The weaknesses are also quite evident; the internal continuous flake graphite structure can easily serve as pathways for corrosion, resulting in extremely limited corrosion resistance, with compatibility only possible in sulfuric acid solutions of specific concentrations. (2) High-silicon corrosion-resistant cast iron (the mainstream in the industry): A special type of cast iron developed specifically for highly corrosive environments. Its key characteristic is that the silicon content is controlled within the range of 14% to 18% (the main grades being STSi15 and STSi17). It is currently the primary pipe material used for transporting concentrated acids in sulfuric acid-related projects. Through modification with a high-silicon content, a dense and stable silicon dioxide passivation layer can spontaneously form on the pipe surface, effectively preventing the penetration and corrosion caused by sulfuric acid at the source. This completely resolves the problem of cracking and corrosion commonly encountered in ordinary gray cast iron. This material features high hardness, resistance to erosion, and tolerance to high-temperature acid attack; its corrosion rate can be kept below 0.13 mm/year, making it suitable for most applications involving the transport of concentrated sulfuric acid at high temperatures. Its only drawback is its high brittleness, as well as weak impact and vibration resistance, which requires more stringent conditions for installation and use. 1.2 Core corrosion resistance characteristics: Adaptation logic for sulfuric acid media. The corrosivity of sulfuric acid varies significantly depending on concentration, temperature, and flow rate; likewise, the corrosion resistance of cast iron pipes also depends heavily on the specific conditions. This is a key factor in selecting materials for sulfuric acid-related applications. ① In conditions with concentrated sulfuric acid (concentration 80%~100%): Excellent adaptability. At normal and moderate temperatures, high-silicon cast iron can rapidly form a stable passivation layer in concentrated sulfuric acid, resulting in almost no corrosion ； Although ordinary gray cast iron can be used for a short period of time, its use over the long term can lead to acid seeping through the gaps in the graphite, causing internal corrosion and volume expansion; this ultimately results in pipe cracking and damage. Therefore, the use of ordinary gray cast iron pipes is strictly prohibited in applications involving concentrated acids. ② Conditions involving dilute sulfuric acid (concentration < 80%): Completely unsuitable. Dilute sulfuric acid lacks any passivation conditions; it continuously corrodes the interface between the cast iron matrix and graphite. The corrosion rate increases exponentially under such conditions. Neither ordinary gray cast iron nor high-silicon cast iron is suitable for long-term transportation of dilute sulfuric acid. Specialized piping materials such as plastic-lined pipes or fiberglass-reinforced plastic pipes must be used instead. ③ Operating conditions with fuming sulfuric acid: Its use is strictly prohibited. The free SO3 present in fuming sulfuric acid reacts with the silicon element in cast iron, forming expansive corrosion products that cause stress cracking inside the pipes; this represents a condition that should be avoided when using cast iron pipes. ④ Effect of flow rate and temperature: Cast iron pipes exhibit the best corrosion resistance under static, low-flow (＜0.5 m/s) conditions at normal temperature and in the presence of concentrated acid ； Excessively high flow rates and turbulent flow of the medium can erode and damage the passivation film, accelerating corrosion ； The higher the temperature, the poorer the stability of the passivation film, and the greater the risk of corrosion. 1.3 Advantages and disadvantages of composite material properties: Resistant to concentrated sulfuric acid corrosion, heat-resistant, aging-resistant, high strength, resistant to deformation, no risk of aging-related leakage, long service life (up to 8–10 years under standard operating conditions); cost is significantly lower than that of stainless steel and titanium pipes. Weaknesses: high brittleness of the material, poor impact resistance, sensitivity to vibration, and susceptibility to external compression ； Not resistant to dilute acids, not resistant to fuming sulfuric acid ； The pipeline is quite heavy, thus requiring high load-bearing capacity from supports and foundations. II. Introduction to the cast iron piping and fittings system for sulfuric acid plants: The cast iron piping system used in sulfuric acid plants is an integrated system, with pipes, fittings, joints, and seals all being specially designed for corrosion resistance; ordinary cast iron components used in water supply and drainage systems cannot be used interchangeably, as this will easily lead to leaks and corrosion-related failures. 2.1 Main pipe types 1: High-silicon corrosion-resistant cast iron straight pipes: These are the key materials used in projects; they are manufactured through centrifugal casting, resulting in dense pipe walls free from pores and with uniform thickness. Their sizes range from DN25 to DN300, making them suitable for use in acid-processing pipelines within factories as well as for the main inlet and outlet pipelines of storage tanks. 2. Corrosion-resistant cast iron shaped pipes: Customized for special operating conditions, including reducers, splices, and extended pipes, used for pipeline transitions and equipment connections. 2.2 Specialized Fittings: All fittings for sulfuric acid pipelines must be made of the same material as the pipeline itself and possess the same corrosion resistance level; mixed use of different materials is strictly prohibited. The key fittings include: 1. Elbow fittings: 90° and 45° elbows, which are used to adjust the direction of the pipeline, prevent erosion caused by sharp angles, and reduce corrosion due to turbulent flow of the medium ； 2. Branching and connecting fittings: tees and crosses, used for branching pipelines and connecting parallel process sections; reinforced, corrosion-resistant versions should be preferred ； 3. Connection transition fittings: socket pipes, straight-end pipes, flanged splices, suitable for various connection methods ； 4. Sealing fittings: pipe caps and plugs, used to seal the ends of pipelines and to seal spare connections. 5. Insert ring (adjustable insert ring): A core adjusting fitting specifically designed for sulfuric acid plant high-silicon cast iron flanged pipelines; it consists of corrosion-resistant cast iron components tailored for use in pipelines carrying concentrated acids, with the same material as the main pipes and fittings (STSi17 high-silicon corrosion-resistant cast iron). It is an essential component for flanged pipeline connections. Unlike ordinary expansion joints, the primary function of the clamp ring is to precisely adjust the installation length of pipes and compensate for cumulative construction errors. This makes it suitable for cast iron pipes, which are rigid and cannot be bent or cut for fine-tuning. High-silicon cast iron pipes are prefabricated castings; they cannot be freely cut or resized on site. For pipelines joined over long distances using flanges, issues such as misalignment and dimensional discrepancies are quite common. The insert ring can compensate for length errors ranging from ±10 to 30 mm through its own structural design, thereby effectively resolving problems related to flange hole alignment and proper positioning of equipment interfaces. It can also compensate for minor thermal expansion and contraction of the pipes, reduce installation stress, prevent deformation of the flange sealing surfaces due to stress, and significantly improve the installation accuracy and sealing stability of acid pipelines. It is widely used in the main acid pipelines of sulfuric acid plants as well as in the pipeline connections at the inlets and outlets of storage tanks. 2.3 Connection and Sealing Systems (Engineering Critical) 1. Connection methods: Flange connection and socket connection are the most commonly used; welding is not employed (welding of cast iron can cause cracks and damage the corrosion-resistant layer) ； For long-distance pipelines, socket-type flexible joints are preferred, as they offer better seismic and displacement resistance. 2. Sealing components: The use of ordinary rubber gaskets is strictly prohibited. Acid-resistant PTFE gaskets, graphite gaskets, or acid-resistant rubber sealing rings must be used instead. These components are suitable for environments with highly corrosive concentrated sulfuric acid and moderate to high temperatures, thereby preventing gasket degradation and leakage.
3.1 Core **standards (GB)**: 1. GB/T 8491-2009 “High-silicon corrosion-resistant cast iron products” – This is the key national standard for special cast irons used in sulfuric acid-related applications. It specifies the chemical composition, mechanical properties, corrosion resistance indicators, and casting quality requirements for corrosion-resistant cast iron grades such as STSi15 and STSi17. It serves as the primary basis for selecting materials for concentrated acid pipelines and for their acceptance upon manufacture. 2. GB/T 13295-2019 \"Ductile iron pipes, fittings and accessories for water and gas pipelines\" – a general standard for cast iron pipelines that specifies pipeline dimensions, connection types, and mechanical properties. It can be used as a supplementary reference, but it is not suitable for the acceptance of main materials in highly corrosive sulfuric acid environments. 3. GB/T 43653-2024 “General specifications for delivery and acceptance of cast iron products” is the latest national standard for the acceptance of cast iron products. It stipulates the appearance quality, defect assessment, dimensional tolerances, and test methods for corrosion-resistant cast iron pipes. This standard serves as a crucial reference for the acceptance of such products upon delivery in engineering projects. 3.2 Industry Standards and Engineering Codes 1. T/CECS 823-2021 \"Technical Code for the Construction of Drainage Ductile Iron Pipes\": It specifies the general procedures for the construction, installation, pressure testing, and acceptance of cast iron pipes; it can be used as a reference for the civil engineering installation of sulfuric acid pipelines. 2. YB/T 4915-2021 \"Technical Specifications for the Evaluation of Green Design Products in Ductile Iron Pipes\": These specifications set requirements for the production quality and performance stability of ductile iron pipes, to facilitate the selection of high-quality pipes. 3.3 International reference standard ASTM A861-23 \"Standard Specification for High-Silicon Iron Pipes and Fittings\": This is an internationally recognized standard for acid-resistant cast iron pipes; it specifies the dimensions, tolerances, properties, and testing requirements for high-silicon cast iron pipes, and serves as an important reference for projects involving international cooperation as well as high-end sulfuric acid production facilities.
IV. Practical application scenarios of cast iron pipes in sulfuric acid projects: Considering the conditions throughout the entire process of sulfuric acid production, storage, and transportation, cast iron pipes (specifically high-silicon corrosion-resistant cast iron) are only suitable for use in environments with normal temperature, moderate temperature, normal pressure, and low-velocity concentrated sulfuric acid. Their main application areas are well-defined, and their use outside these limits is strictly prohibited. 4.1 Core application scenarios 1: Concentrated sulfuric acid product delivery pipelines: For short-distance transportation of 80%~98% concentrated sulfuric acid within workshops, as well as for pipelines used for feeding and discharging from storage tanks. These applications feature stable operating conditions and controllable flow rates, making them the most typical use cases for cast iron pipes. 2. Piping associated with sulfuric acid storage tanks: acid discharge pipes at the bottom of the tanks, reflux pipes at the top, vent pipes, and drain pipes. These pipelines operate under static conditions most of the time, posing a low risk of corrosion; thus, they are suitable for high-silicon cast iron pipes. 3. Auxiliary pipelines for the dry absorption process: The auxiliary pipelines for concentrated acid at normal temperature and the circulation branches in the sulfuric acid production unit allow stable operation by avoiding high-speed erosion and extreme high-temperature conditions. 4. Waste concentrated acid recovery pipeline: A pipeline for collecting and transporting waste concentrated sulfuric acid that meets the concentration standards; the medium composition remains stable, with no dilution by dilute acid occurring in this scenario. 4.2 Absolutely Prohibited Scenarios (Engineering Red Lines) 1. All pipelines for transporting dilute sulfuric acid, mixed acids, and acidic wastewater ； 2. Process pipelines for fuming sulfuric acid and SO3-containing media ； 3. Pump outlet main pipeline subjected to high-speed erosion, high-pressure impact, and frequent vibration ； 4. High-temperature concentrated sulfuric acid pipelines with temperatures exceeding 120°C.
V. Challenges in the design of acid-resistant cast iron pipelines and reminders of common engineering mistakes: The brittle nature of high-silicon cast iron pipelines, their limitations in terms of suitability for certain operating conditions, and the special requirements for corrosion protection result in a very low tolerance for design errors. Most problems related to leakage, fracture, and failure in such pipelines stem from oversight in the design details. The following are the key challenges and common mistakes that designers must pay close attention to. 5.1 Common mistakes in material selection: Confusing ordinary cast iron with corrosion-resistant cast iron. Frequent errors include, in an effort to reduce costs, using ordinary gray cast iron or ordinary ductile iron in place of high-silicon corrosion-resistant cast iron, or using pipe fittings made of different materials together. Consequences: Ordinary cast iron cannot resist the long-term penetrating corrosion caused by concentrated sulfuric acid; pipe wall cracking and acid leakage occur within 3 to 6 months, leading to safety accidents. Key design points: For acid pipelines, high-silicon corrosion-resistant cast iron (STSi17 is preferred) should be used 100% throughout the pipeline. The pipes, fittings, and flanges must all be of the same material; mixed materials are strictly prohibited. The drawings must clearly specify the material grade and the applicable standards. 5.2 Challenges in adapting to operating conditions: Frequent errors due to improper control of flow rate, temperature, and pressure; the flow rate of the medium is not controlled, with pipeline flow rates exceeding 0.5 m/s ； No material upgrade was made for high-temperature operating conditions ； Normal-pressure cast iron pipes are still used under high-pressure conditions. Consequences: High-speed fluid flow erodes and damages the passivation film, resulting in localized erosion-corrosion ； High temperatures and pressures exacerbate material fatigue, leading to brittle fracture of pipes and leakage at joints. Design key point: The design flow velocity for sulfuric acid pipelines must be strictly controlled between 0.2 and 0.5 m/s ； When the medium temperature is >100°C, thicken the tube wall and optimize the shock absorption design of the support ； Cast iron pipes are only suitable for normal pressure conditions; cast iron should not be used for high-pressure concentrated acid pipelines. 5.3 Difficulties in structural design: Insufficient protection for brittle materials; frequent errors: No reinforcement at stress concentration points such as pipe elbows and tees ； No expansion joints are installed in the long pipeline ； The pump outlet and equipment interfaces lack shock absorption treatment. Consequences: High-silicon cast iron is highly brittle and unable to withstand vibrations as well as stresses caused by thermal expansion and contraction; it is prone to cracking and leaking at joints and shaped fittings. Sudden pressure spikes at the pump outlet are a common cause of pipe failure. Design key point: Use thickened fittings at stress concentration areas ； For long-distance pipelines, flexible socket joints and expansion joints are installed to counteract thermal displacement ； Buffer pipes and shock-absorbing brackets must be installed at the pump outlet; it is prohibited to connect cast iron pipes directly to the pump body ； Pipelines must not be left suspended for excessive lengths; additional supports should be installed ； When flanging long pipelines together, corrosion-resistant cast iron gaskets must be used to eliminate stress accumulated during installation and avoid the risk of cracking due to rigid jointing. 5.4 Errors are common in sealing and connection design: improper selection of gaskets and interfaces; frequent mistakes occur when ordinary rubber gaskets or metal gaskets are used ； The flange bolts are not tightened evenly ； For convenience, welded connections are used. Consequence: Ordinary gaskets have poor resistance to acid corrosion, and they fail due to short-term aging and leakage ； Welding destroys the passivation layer of cast iron, causing weld cracks that pose long-term risks ； Uneven stress on the bolts caused uneven wear of the flange, leading to acid leakage. Design key points: Uniform use of PTFE gaskets and flexible graphite acid-resistant gaskets ； Flange and socket connections shall be used throughout; welding is prohibited ； High-silicon cast iron collars are used to fine-tune the pipeline length and correct installation errors, ensuring precise alignment of the flanges ； Specify the requirements for diagonal tightening of flange bolts; the drawings shall indicate the parameters of the sealing material, the specifications of the gaskets, and their installation locations. 5.5 Challenges in heating and corrosion protection design: Local overheating and corrosion – a common issue; in the design for antifreeze heating of concentrated sulfuric acid, the heating wire is in direct contact with the outer wall of the pipe, without any protective barrier. Consequence: Excessively high local temperatures damage the passivation layer of the pipes, leading to localized accelerated corrosion; over time, this results in pipe wall perforation and leakage. Design key points: Leave an isolation gap between the heat tracing wire and the outer wall of the pipe, and use indirect heat tracing ； The insulation layer covers everything completely, preventing localized overheating and large temperature differences ； The outer wall is coated with a specialized anti-corrosion coating to prevent both atmospheric corrosion and corrosion caused by surrounding media. 5.6 The design reserved for operation and maintenance is prone to errors: frequent issues occur with regard to maintenance, venting, and waste discharge; the pipeline design is overly simplified, with no vent valves, waste discharge ports, or maintenance joints provided. Consequences: After the pipeline is shut down, the residual concentrated acid cannot be completely drained or cleaned; this remaining acid stays in place and corrodes the pipe walls, accelerating the aging of the pipeline. Moreover, it becomes difficult to carry out repairs promptly after a failure occurs. Design key points: Install venting at the high point and drainage at the low point of the concentrated acid pipeline ； Removable splices are added to long-distance pipelines to meet the needs of subsequent cleaning, maintenance, and replacement. VI. Conclusion: Cast iron pipes (high-silicon corrosion-resistant cast iron) represent a cost-effective choice for applications involving concentrated sulfuric acid at normal temperature, low flow rates, and normal pressure. Thanks to their excellent resistance to concentrated acid corrosion, structural stability, and low cost, they have become an essential material for sulfuric acid processing projects. However, its limitations in terms of corrosion resistance applications, the brittleness of its material, and its high sensitivity to operating conditions mean that it is by no means a universal anti-corrosion pipe material. In engineering design, it is essential to strictly adhere to national and industry standards. By following the four principles of \"selecting the right materials, defining appropriate operating conditions, optimizing the structure, and ensuring strict control over details\", common issues such as the use of incompatible materials, operating conditions that exceed acceptable limits, and flaws in structural design can be avoided. This approach helps to prevent risks related to leakage, failure due to fractures, and corrosion, thereby ensuring the long-term safe and stable operation of sulfuric acid pipeline systems.
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