It's full of useful information! Analysis of the corrosion issues in vacuum pumps! If you are choosing a corrosion-resistant vacuum pump, this can serve as a reference! I...

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In the forum, friends from the industry often ask questions about the corrosion resistance of vacuum pumps. I have read all of them; just the web links for them fill up two pages. After carefully reading each page, I gained a great deal from it and learned a lot. We are a team specialized in researching corrosion issues related to vacuum pumps. With over 20 years of experience and practical knowledge, we aim to share some useful insights here today to help those who are facing problems caused by vacuum pump corrosion. It is said that only some problems can be solved, because there is no material in the world that is perfect. To claim that a certain material can resist all types of strong acids and bases, as well as solvents, and can also withstand high temperatures and wear, while having extremely high strength, is nothing but exaggeration. Therefore, taking into account the characteristics of various operating conditions, it is better to choose the right vacuum pump rather than the most expensive one; making a scientific and reasonable choice of a vacuum pump suitable for one’s own operating conditions is the key. Furthermore, there is a significant difference between theory and practice when it comes to corrosion resistance; the application scenarios vary from one user to another, and each manufacturer has its own advantages and perspectives. In short, we will conduct a fair and objective analysis, seek common ground while respecting differences, and allow various opinions to be expressed. If there are any mistakes in what is said below, please bear with us! If any colleagues in the industry have different opinions, please feel free to offer your corrections and advice! Over the nearly century-long development of human industry, vacuum pumps have become a highly mature technology and product. However, the shortening of their service life due to corrosion by acidic gases remains a serious problem that plagues this industry. ▲The impeller of the vacuum pump is damaged due to corrosion. So, why is it so difficult to make vacuum pumps resistant to corrosion? I. There are various types of corrosion, with chemical, electrochemical, and physical corrosion existing side by side. Firstly, vacuum pumps are mostly made of metal materials, and metals have a natural weakness in terms of corrosion resistance. As stated in the \"Corrosion Handbook,\" corrosion is damage to materials caused by environmental factors, and there are many different types of it. In a pump operating environment, metal materials are subject to various forms of corrosion that overlap in nature, including chemical, electrochemical, and physical corrosion – all of them. ▲The types of corrosion that metal materials used in pumps must withstand include physical corrosion, which mainly refers to cavitation; cavitation occurs in liquids flowing at high speeds in environments with pressure changes. When bubbles burst, shock wave pressures are generated that can tear metal particles apart and cause the equipment to deform. Regardless of the type of vacuum pump material you choose, if there is an error in the initial selection – such as choosing a vacuum pump with an excessive pumping capacity – or if improper operation occurs during use, cavitation will occur! In nature, pistol shrimp also use cavitation to easily break through shell armor in order to obtain food. II. High flow rates of the medium inside the pump accelerate corrosion. The \"Corrosion Handbook\" cites a classic example: when transporting 7% sulfuric acid through carbon steel pipes at flow rates of 0.0007–0.0026 m/s, the carbon steel pipes can withstand corrosion over a long period of time. However, when using carbon steel pumps to transport the same sulfuric acid, the pump’s service life is only 72 hours, as the flow rate in such pumps reaches as high as 23.08 m/s (1500 rpm), and high flow rates significantly accelerate the rate of corrosion! So, when you consult the Corrosion Handbook and find that a certain material has good acid resistance, this refers only to the corrosion rate under static immersion conditions; there is still a significant difference in the corrosion rate under the high-flow conditions of vacuum pumps! III. Rising temperature inside the pump accelerates corrosion. After a vacuum pump operates for an extended period, its mechanical movement generates heat, and the gas being pumped in also contributes to heat generation. In some cases, the working fluid inside the pump chamber releases heat due to vaporization; therefore, it is inevitable for the temperature of the vacuum pump to rise. Taking the corrosion of gray cast iron by sulfuric acid as an example, for every 10°C increase, the rate of corrosion of gray cast iron by sulfuric acid doubles. Even stainless steel 316L, which has excellent corrosion resistance, sees an increased corrosion rate when the temperature of hydrochloric acid rises; this is shown in the figure below. Fourthly, changes in concentration also affect the corrosion rate – at a constant temperature, changes in the concentration of the medium can either accelerate or slow down the corrosion process. The main reason for the reduction in corrosion resistance is that some metals form a passivation layer. On the other hand, relying on this passivation layer for corrosion protection has significant limitations; it must be within a certain concentration range, or in the case of certain acids—for example, an increase in sulfuric acid concentration enhances the corrosion resistance of gray cast iron, while an increase in hydrochloric acid concentration reduces the corrosion resistance of stainless steel 316L. As shown in the figure below: V. Complexity of operating conditions. What was mentioned earlier are just some of the basic challenges related to corrosion resistance. These represent only the basic factors such as various types of corrosion, high flow rates, and temperatures and concentrations that accelerate corrosion. In practical applications, however, we often find that the gases to be pumped contain different substances, and the operating conditions vary from one chemical manufacturing facility to another—each with its own unique characteristics. The diagram below reflects my feelings when I first started working on designing solutions. The problem is that sometimes pharmaceutical factories need to change their production processes; they may produce one product this year and switch to another next year, resulting in significant changes in operating conditions. Vacuum pumps with limited corrosion resistance often cannot be used in these new conditions. Sometimes, managers (owners) require pumps that are resistant to acids, alkalis, organic solvents, as well as being wear-resistant, inexpensive, durable, easy to maintain, and capable of adapting to future changes in operating conditions. What kind of perfect requirements are those? Therefore, our main task over the past few months was to collect all the patents related to corrosion-resistant vacuum pumps in China. After in-depth research and study, we discovered that the technologies used to make vacuum pumps resistant to corrosion can be divided into several categories: lining, coating, and plating. First, the characteristics of lining, coating, and plating technologies: The basic idea behind these technologies is similar to that of Iron Man – giving a body with limited durability a layer of corrosion-resistant armor. Using ceramic as a lining, coating as a topcoat, and Hastelloy as an infiltrated layer – this process is widely applied in liquid ring pumps, screw pumps, reciprocating pumps, and Roots pumps. The lining, coating, and plating technologies went through a period of diverse approaches and experimentation. In the end, practice proved that this type of armor-like corrosion-resistant technology is indeed effective; however, it has certain limitations, namely a short protection duration and a limited margin of safety against corrosion. Some customers report that the protective layer is worn out after half a year, requiring them to return to the factory for re-coating; however, other customers need continuous and stable production, and therefore prefer a longer corrosion resistance lifespan. II. Technical features of superalloy materials: The corrosion resistance technology of superalloys involves the use of superalloys such as titanium alloys, Hastelloy, and duplex steels to create superalloy vacuum pumps, which are widely used in liquid ring pumps, screw pumps, Roots pumps, and steam ejectors. Its feature is its high corrosion resistance and safety margin; it’s entirely made of corrosion-resistant superalloys, just like Transformers, with pure gold throughout. Therefore, it’s meant for the wealthy only! ! ! Titanium is used in liquid ring vacuum pumps. This super alloy is expensive to acquire, and its processing requires special techniques; welding must be carried out in an inert gas environment. It is generally used for pumping high-concentration chlorine gas (for lower concentrations, fiberglass vacuum pumps can be considered). As stated in a post by an expert on a forum, titanium materials are widely used in industries such as chlor-alkali production, chlorate manufacturing, and hydrochloric acid production, thanks to their strong corrosion resistance, stable chemical properties – as they hardly react with any substances – and excellent mechanical properties. But it cannot be used in dry chlorine environments. Under dry chlorine conditions, even at temperatures below 0 degrees Celsius, chlorine and titanium metal undergo a vigorous chemical reaction to form titanium tetrachloride; this compound then decomposes into titanium dichloride. When the reaction is intense, combustion of the titanium metal occurs. In the chlor-alkali sector, there have been incidents in the past where the tubes in chlorine heat exchangers were burned out. Interestingly, when the water content in chlorine gas is higher than 0.5%, no reaction occurs between the two, and titanium exhibits considerable stability; this is why titanium is widely used in the chlor-alkali, chlorate, and hydrochloric acid industries. Moreover, titanium is lightweight and strong; the wall thickness of titanium equipment used is usually much smaller than that of materials such as steel and stainless steel. It is also widely used in aerospace applications. Attempts have also been made to use duplex steel in corrosion-resistant liquid ring pumps and screw pumps. However, based on market feedback and actual usage, it has been found that although duplex steel offers better corrosion resistance than stainless steel, its service life is only a few months when exposed to hydrogen chloride corrosion, resulting in high maintenance costs over time. Hastelloy is widely used in steam jet vacuum pumps, with European and American companies such as those in Germany being the main users. Steam ejectors do not have mechanical components, which ensures stable vacuum levels; yet they require continuous steam consumption. To meet corrosion resistance requirements, Hastelloy and titanium alloys are used for the venturi tubes and condensers, making the overall cost quite high. Such systems are commonly used by top foreign companies and are suitable for pumping acidic corrosive gases with viscous properties, as shown in the figure below. III. Characteristics of composite materials: Composite materials have an inherent advantage in terms of acid resistance, as they are not affected by chemical or electrochemical corrosion and generally do not react with acids. However, they are not resistant to alkalis. Some composite materials can also be dissolved by organic solvents, leading to issues such as swelling, shrinkage, weight loss, weight gain, or loss of strength. Therefore, the selection of composite materials requires careful consideration. In addition, there is another challenge, which is the strength of composite materials ； For example, the tensile strength of polypropylene materials ranges from 20 M to 30 MPa, while its bending strength ranges from 25 M to 50 MPa. If PP is to be used in engineering structural components, it must be reinforced with glass fiber. GF-reinforced modified PP: The mechanical properties of PP products reinforced with glass fibers can be improved by a factor of two or even more. Specifically, the tensile strength ranged from 65 MPa to 90 MPa, while the flexural strength ranged from 70 MPa to 120 MPa. However, this is far from enough. It should be noted that the tensile strength of metals used in pumps ranges between 200 and 550 MPa. Therefore, it took us over twenty years to improve the strength of a fiber-reinforced plastic three times; this material is a composite of glass fibers and phenolic resin, and its enhanced strength reached 315 MPa. This value is higher than that of gray cast iron used in pumps, but lower than that of stainless steel used in pumps, making it suitable for the normal operation of vacuum pumps. After addressing the issue of material strength, we conducted numerous immersion tests, subjecting the materials to long-term immersion in various acids and organic solvents such as hydrochloric acid, sulfuric acid, nitric acid, benzyl alcohol, toluene, p-xylene, ethyl chloroacetate, benzyl chloride, and furan. After that, we tested the changes in the materials’ strength to determine whether phenomena such as swelling or loss of strength occurred ； Once the material meets the requirements regarding acid resistance and strength, the next step is to maximize the use of composite materials in the construction of the vacuum pump. We have managed to achieve a situation in which over 90% of the components are made from the same material. The advantage of this approach is that since all components are made from the same material, the stress on them is consistent, which improves the fit between various components and simplifies the manufacturing process. It also enables the vacuum pump to have optimal corrosion-resistant safety features, as shown in the figure below. PS: Thank you to all our forum members for their support. I would like to add two more points: First, if the operating conditions involve bromine or hydrofluoric acid, our products cannot withstand their corrosive effects. One of our clients in Taizhou tried all kinds of vacuum pumps available on the market, but none of them worked. In the end, with no other options left, they contacted a design institute to modify the operating conditions from the outset, thereby avoiding such corrosive environments ； Second, some people might say that there are also water jet pumps, which can be used for many years without any problems and have excellent corrosion resistance. That’s true indeed. However, vacuum pumps vary in terms of efficiency, and if we conduct a comparative analysis using 11KW vacuum pumps, we will find that different types of vacuum pumps cover the entire market demand from various perspectives. There are both dry pumps and liquid ring pumps, allowing for use in conditions with or without moisture present ； There are screw pumps that require a clean air source, as well as water jet and liquid ring pumps that are not sensitive to dust and can handle various conditions without issue ； While screw pumps occupy less space and achieve high vacuum levels, there are also roots pumps combined with liquid ring pumps, reciprocating pumps, or water jets to achieve high vacuum, but these require more space; moreover, their prices vary as well ； Therefore, it is unrealistic to rely on just one type of vacuum pump to meet the entire market demand for corrosion-resistant vacuum pumps; each type of pump exists for a reason, and there is a market demand for it ； Corrosion-resistant vacuum pumps are also designed and optimized based on traditional vacuum pumps; they cannot exist outside these constraints or beyond these boundaries ； On the one hand, water jetting is restricted by **environmental policies; many industrial parks prohibit its use. On the other hand, the working principle of water jetting involves a venturi structure that mixes steam with water. After the water jet pump operates for one hour in winter, the temperature of the liquid in the tank rises to around 35°C, and the vacuum level drops rapidly to around 0.07–0.08 MPa, a decrease that can be observed with the naked eye ； In summer, the process is even faster: after just 20-30 minutes of operation, the vacuum level drops rapidly. Considering the long, hot summers in southern China, this is difficult to accept for those customers who require a high vacuum level over extended periods of time.