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This article cites material from **Chemical Engineering Forum**. Corrosion has always been one of the most troublesome hazards for chemical processing equipment; even a slight carelessness can lead to equipment damage, and in severe cases, it can result in accidents or even disasters. According to relevant statistics, about 60% of damage to chemical processing equipment is caused by corrosion; therefore, when selecting chemical pumps, it is essential to pay attention to the scientific choice of materials. There is a common misconception that stainless steel is a \"universal material\" that can be used in any medium or under any environmental conditions; this is very dangerous. Below are the key points for material selection regarding some commonly used chemical media: 1. Sulfuric acid – As one of the highly corrosive media, sulfuric acid is an important industrial raw material with a wide range of applications. Sulfuric acid at different concentrations and temperatures causes varying degrees of corrosion to materials. For concentrated sulfuric acid with a concentration of over 80% and a temperature below 80°C, carbon steel and cast iron exhibit good corrosion resistance; for example, the GBW type of concentrated sulfuric acid centrifugal pump. However, it is not suitable for sulfuric acid flowing at high speeds ； Common stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) also have limited utility in sulfuric acid media. Therefore, pump valves for transporting sulfuric acid are usually made of high-silicon cast iron (which is difficult to cast and machine) or high-alloy stainless steel (grade 20 alloy). Fluoroplastics exhibit good resistance to sulfuric acid, making the use of fluorinated-lined pumps (F46) a more economical option. For example: CQB-F type fluoroplastic alloy magnetic pump, PF high-corrosion-resistant centrifugal pump, FSB type fluoroplastic centrifugal pump, QBY pneumatic plastic diaphragm pump, IHF type fluoroplastic-lined chemical centrifugal pump. 2. Hydrochloric acid – The vast majority of metal materials are not resistant to corrosion by hydrochloric acid (including various stainless steel materials); iron with high molybdenum and silicon content can only be used in hydrochloric acid at temperatures below 50°C and concentrations of less than 30%. Unlike metal materials, the vast majority of non-metallic materials exhibit good corrosion resistance to hydrochloric acid; therefore, rubber-lined pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choices for transporting hydrochloric acid. For example: CQF46 type engineering plastic magnetic pump, IHF type fluoroplastic-lined chemical centrifugal pump, PF high-corrosion-resistant centrifugal pump, FSB type fluoroplastic centrifugal pump, QBY pneumatic plastic diaphragm pump. 3. Nitric acid – Most metals are rapidly corroded and damaged by nitric acid; stainless steel is the most widely used material resistant to nitric acid, offering good corrosion resistance against nitric acid of all concentrations at room temperature. It is worth noting that stainless steels containing molybdenum (such as 316, 316L) do not have better corrosion resistance to nitric acid than ordinary stainless steels (such as 304, 321); in some cases, their resistance is even lower. For high-temperature nitric acid, titanium and titanium alloy materials are commonly used. For example: IH-type chemical stainless steel centrifugal pump, ZXP stainless steel horizontal self-priming pump, ZWP-type stainless steel self-priming sewage pump, ISWH stainless steel chemical pipeline pump, IHG-type vertical chemical centrifugal pump, CQ-type stainless steel magnetic drive pump, ZCQ-type self-priming magnetic pump. 4. Acetic acid: It is one of the most corrosive organic acids; ordinary steel suffers severe corrosion in acetic acid at all concentrations and temperatures. Stainless steel is an excellent material resistant to acetic acid, and 316 stainless steel containing molybdenum can also be used in high-temperature and dilute acetic acid vapors. For demanding applications such as high-temperature, high-concentration acetic acid or environments containing other corrosive agents, high-alloy stainless steel or fluoroplastic pumps can be used. For example: IH-type chemical stainless steel centrifugal pump, ZXP stainless steel horizontal self-priming pump, ZWP-type stainless steel self-priming sewage pump, ISWH stainless steel chemical pipeline pump, IHG-type vertical chemical centrifugal pump, CQ-type stainless steel magnetically driven pump, ZCQ-type self-priming magnetic pump. 5. Alkalis (sodium hydroxide): Steel is widely used in sodium hydroxide solutions at temperatures below 80°C and concentrations up to 30%; many factories still use ordinary steel at temperatures of 100°C and concentrations up to 75%, as the increased corrosion rate is offset by cost advantages. Ordinary stainless steel does not have a significant advantage over cast iron in terms of corrosion resistance to alkaline solutions; it is not recommended to use stainless steel unless a small amount of iron is allowed to be present in the medium. For high-temperature alkaline solutions, titanium and titanium alloys or highly alloyed stainless steels are commonly used. 6. Ammonia (ammonium hydroxide): Most metals and non-metals suffer only mild corrosion in liquid ammonia and ammonia water (ammonium hydroxide); only copper and its alloys are not suitable for use. 7. Saltwater (seawater): Ordinary steel exhibits a low rate of corrosion in sodium chloride solutions as well as in seawater and brackish water; coating protection is generally required ； Various types of stainless steel also have very low uniform corrosion rates, but local corrosion may occur due to chloride ions; therefore, 316 stainless steel is generally the better choice. 8. Alcohols, alkanes, esters, and ethers. Common alcohol-based media include methanol, ethanol, ethylene glycol, propanol, etc.; alkanes include propane, butane, etc.; ester-based media comprise various methyl esters, ethyl esters, etc.; ether-based media include dimethyl ether, diethyl ether, butyl ether, etc. These substances generally have no corrosive properties, and most common materials can be used with them. When making a selection, it is necessary to consider the properties of the medium and relevant requirements in order to make a proper choice. It is also worth noting that alcohols, esters, and ethers are soluble in various rubbers, which helps to avoid mistakes when selecting sealing materials. There are many other media that cannot be introduced one by one here. In short, when selecting materials, one must not act casually or blindly; it is advisable to consult relevant information and draw on proven experience. Cooling issues: The transportation of high-temperature media places higher demands on the pump’s structure, materials, and auxiliary systems. Below, we discuss the cooling requirements associated with different temperature levels, as well as the types of pumps suitable for use in such applications: 1. For media with temperatures below 120°C, a dedicated cooling system is usually not required; instead, the medium itself is used for lubrication and cooling. For stainless steel chemical pumps such as ISWH and shielded chemical pumps like DFLWPH, the protection class of the shielded motor at temperatures above 90°C should be Class H ； The ISH chemical pumps and IH-type chemical pumps can achieve an upper temperature limit of 140°C to 160°C thanks to their suspension structure℃ ； The IHF fluorinated lining pump can reach a maximum operating temperature of 200℃ ； Only the CQB type magnetically driven pump is designed for use at temperatures not exceeding 100°C. It is worth noting that media that are prone to crystallization or contain particles should be equipped with a sealing surface flushing pipeline (interfaces are provided during design). 2. For media with temperatures above 120°C and below 300°C, a cooling chamber is generally required on the pump cover, and the sealing chamber should also be connected to coolant (a double-end face mechanical seal is necessary). When it is not allowed for coolant to penetrate into the medium, the medium itself should be cooled before being fed in (this can be achieved using a simple heat exchanger). 3. For high-temperature media above 300°C, not only the pump head section but also the suspension bearing chamber requires a cooling system. The pump structure is generally of a central support type, and metal diaphragm seals are preferred for mechanical sealing, although they are expensive (their cost is more than 10 times that of ordinary mechanical seals). Sealing issues: Zero leakage is the perpetual goal for chemical processing equipment, and it is precisely this requirement that has led to the increasing use of magnetic pumps and shielded pumps. However, there is still a long way to go before true leaklessness can be achieved, such as the lifespan issues of magnetic pump isolation sleeves and shielded pump shielding sleeves, the pitting problem of materials, and the reliability of static seals, among others. Here is a brief introduction to some basic aspects of sealing: 1. Sealing types. For static sealing, there are generally only two types: gaskets and seals, with O-rings being the most widely used type of seal ； For dynamic sealing, chemical pumps rarely use packing seals; mechanical seals are preferred. Mechanical seals come in single-face and double-face types, as well as balanced and unbalanced versions. Balanced mechanical seals are suitable for sealing high-pressure media (typically those with pressures greater than 1.0 MPa). Double-face mechanical seals are used for media that are high-temperature, prone to crystallization, viscous, contain particles, or are toxic and volatile. An isolation fluid must be injected into the sealing chamber using double-face mechanical seals, with its pressure generally being 0.07–0.1 MPa higher than that of the medium. 2. Sealing materials: Fluororubber is generally used as the static sealing material for chemical pumps, while polytetrafluoroethylene is employed only in special cases ； The material selection for the stationary and rotating rings of mechanical seals is quite important. It’s not true that one combination of cemented carbide materials is always the best; high costs are one issue, but it’s also unreasonable if there is no difference in hardness between the two materials. Therefore, it’s best to choose materials based on the characteristics of the medium being handled. The viscosity of the medium has a significant impact on the performance of the pump. As viscosity increases, the pump’s head curve declines; both the head and flow rate at optimal operating conditions decrease, while power consumption increases, resulting in reduced efficiency. The parameters for standard samples represent the performance when transporting clean water; adjustments are necessary when transporting viscous media (the correction factors for different viscosities can be found in the relevant conversion tables). For the transfer of slurries, pastes, and viscous liquids with high viscosity, it is recommended to use screw pumps.