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Almost all industrial processes involving liquids use pumps at some stage. From deep-sea oil and gas extraction to DNA sequencing, pumps are needed to carry out a variety of production tasks. However, regardless of the pump’s design or size, the key to every application is reliability and efficiency. Minimizing downtime and operating costs is crucial for modern industry. For users who employ large industrial pumps, the specific challenges they face include two main points: first, they usually operate in harsh environmental conditions; second, they need to maintain the pump’s excellent performance while constantly being subjected to the threats of corrosion and abrasion. As knowledge of these production processes and the technologies used to address the challenges in them increases, users can implement more cost-effective pump repair procedures. Corrosion generally refers to the chemical reaction that occurs between the surface of a component and the reactive fluid flowing through the pump. Corrosion can be divided into two main categories: general or uniform corrosion, and local corrosion, the latter including pitting and crevice corrosion. Non-stainless steel materials are primarily affected by uniform corrosion, whereas metals that form an oxide layer adhering to the surface of the object and thereby passivating it are prone to local corrosion. Flow-accelerated corrosion Flow-accelerated corrosion (FAC) refers to the removal of the protective oxide layer on the metal surface. The speed of this process is influenced by the oxygen content, flow rate, and to a certain extent, the chlorine content. The calcium layer formed due to the high carbonate hardness of water can weaken or even prevent FAC. Coating technology improves the performance and durability of pumps. The effect of oxygen can be seen from the following example: in water with an oxygen content of less than 20 ppb (parts per billion) and a flow rate of about 15 meters per second, the corrosion rate is typically around 0.01 millimeters per year. However, when the oxygen content increases, the corrosion rate rises to several millimeters per year, which poses a significant challenge to industrial production processes. Fortunately, FAC only causes real problems with low-carbon steel and cast iron. Increasing the chromium content or using stainless steel will greatly eliminate the vulnerability caused by flow-accelerated corrosion. Erosion: When used to transfer liquids containing abrasive substances such as sand, the pump suffers severe erosion, especially in areas with high flow velocities. This phenomenon is common in the oil and gas industry. In such applications, injection pumps are typically used to force water back into the oil field, thereby maintaining the pressure required to lift the oil to the surface. Sand particles carried in the water flow can cause wear on the pump, and high operating pressures exacerbate the erosion problem. From a purely design perspective, in order to minimize erosion, pump manufacturers in this field have two options available: • reduce the flow velocity at every point within the pump, or • design the pump in such a way that the flow velocity of the liquid through the tightly fitted clearance areas is low. In cases where abrasion-corrosion is significant, the best solution is to use special coatings. However, in most cases, the specifications required by the application limit the implementation of any of the above solutions. In these applications, a solution that has proven effective in practice is to apply a coating with high wear and corrosion resistance to the selected areas of the pump. Abrasion-corrosion: In conditions where both abrasion and corrosion are present, the mechanism of degradation can become very complex, and it depends on the type of substrate and the chemical properties of the fluid. Corrosion can cause a weakly adherent oxide layer to form on the surface of the substrate, making it more susceptible to abrasion. Alternatively, corrosion may damage the passivation layer, leading to the activation of the material’s surface and thus accelerating corrosion. In such cases, surface protection mechanisms are often the best and only option. Cavitation is the most common problem on pump impellers; it is caused by pressure differences and can occur in either the pump body or the impeller. When the local pressure drops below the saturation pressure of the fluid being pumped, the sudden pressure drop in the fluid causes the liquid to flash and turn into vapor. Cavitation damage is prevented by altering the characteristics of the pump system. Any bubbles formed due to pressure drop will pass over the impeller blades as the fluid flows. When bubbles enter a region where the local pressure is higher than the saturation pressure, they burst suddenly, generating shock waves; over time, this phenomenon can cause significant damage to the impeller and/or the pump casing. In most cases, it is better to prevent cavitation from occurring rather than trying to reduce its impact on pumping equipment. This goal is usually achieved by adopting one of the following three measures: • Increasing the suction head • Lowering the fluid temperature • Reducing the required NPSHR. In cases where cavitation cannot be avoided, or when the pumping system is subject to internal circulation or excessive turbulence, it may be necessary to review the pump’s design, or to use custom coating systems to minimize any potential damage to the pump. Appropriate material selection is crucial for pump manufacturers; it involves using the most suitable base materials in pump manufacturing to reduce corrosion issues. In some applications, carbon steel or cast iron materials are preferred due to cost considerations, and the corrosion rate for these applications can be estimated with high accuracy. Based on the acceptable annual corrosion rate, the service life of the pump can be estimated and incorporated into the maintenance costs of the application (WeChat official account: Pump Manager). When cavitation is inevitable, a custom coating system should be used. If the expected rate of corrosion is not acceptable, then the pump material must be upgraded to stainless steel, which increases costs. If this option involving increased costs is not acceptable, an alternative is to customize the use of advanced coatings based on the characteristics of each application. If stainless steel is chosen as the base material for a particular application, the expected service life of the pump can be significantly extended, and in some cases it can even be indefinite. However, only by selecting the appropriate stainless steel grade for a specific application, carefully manufacturing the pump, and using it in accordance with the required fluid specifications can one ensure that the pump has a sufficiently long service life. Once particles are introduced into the fluid, special care is required. Under such conditions, even stainless steel is prone to corrosion, as the passivation layer is damaged, the substrate becomes activated, and it then begins to corrode. The passivation layer can usually be re-established, but if the chloride content is too high or the pH value is too low, the material may remain active and corrosion will continue. Another common cause of corrosion in stainless steel pumps is the stagnant condition resulting from process interruptions or intermittent operation. Another threat to stainless steel is chlorine, which is mainly used to prevent biological growth in pumps or connecting pipes. A low concentration of chlorine of around 2 ppm has almost no effect on stainless steel, but it is important to understand how and where chlorine is introduced into the water flow in order to avoid damaging the protective layer due to high concentrations in certain areas. Compared to the cheaper carbon steel materials, accidental corrosion easily undermines the positive expectations regarding the improved durability of stainless steel. When it comes to protective coatings, it is crucial to clarify the following two issues: first, whether applying a protective coating can indeed improve the performance and service life of pumps; second, whether the cost of the coating is indeed lower than the cost of upgrading the materials. In most cases, pump manufacturers strive to meet the requirements of the production process by using materials that are most suitable for that application, reserving the use of coatings as a fallback solution. Polymer coatings such as fused epoxy can be applied to pump components using fluidized bed or electrostatic coating. As long as the coating remains intact, it can provide excellent corrosion protection. As a polymer coating, it is limited to use under low flow rates and is typically employed in clean water applications, where it can also improve hydraulic performance by smoothing the pump surface. Working with experienced materials engineers helps ensure that end-users receive the most suitable solution. However, coatings suitable for pipes may not be suitable for pump applications, as in pump applications the flow velocity is much higher, narrow channels cause the flow to concentrate, and moving parts are difficult to protect. Therefore, many methods commonly used in pipelines (such as electrical corrosion protection) are not suitable for pumps. In such cases, the coating is applied to specific areas where traffic is expected to increase, or to locations where collision damage is likely to occur (such as 90-degree bends). Hard coatings are typically applied using spraying methods such as air plasma spraying (APS) or high-velocity oxygen fuel (HVOF). The specific spraying method to be chosen will depend on the desired coating thickness and the composition of the paint. The cemented carbide coating deposited using the high-speed oxygen flame method is highly wear-resistant. Tungsten carbide combined with a cobalt, nickel, or cobalt-chromium matrix is preferred. Thanks to improvements in powder and thermal spraying techniques, the material not only possesses good wear resistance and toughness but also excellent corrosion resistance. The thermal spraying method can be applied to most substrates, but it is a \"line-of-sight\" process, which makes coating components with complex shapes (such as impellers) very difficult. For complex components that are difficult to coat using the thermal spraying method, the CVD process can be employed; it enables the formation of very hard coatings, but this process must be carried out at temperatures above 850°C. Such high temperatures limit the choice of substrates, as structural changes and partial deformation may occur during the cooling stage. Not all coatings are the same; coating systems play a positive role in improving the performance and durability of pumps, which is why there are numerous coating services available. It is relatively easy for people to obtain raw materials and basic equipment, and to use them to coat various types of devices. However, taking HVOF coatings as an example, their quality primarily depends on spraying parameters such as material temperature, coating speed, deposition rate, and the quality of the equipment used. Coatings like these require some time to be applied properly, which will inevitably affect the overall cost of pump repair. However, increasing the deposition rate will increase the stress within the coating, and over time, this may lead to a degradation of the coating’s performance and premature failure. Pump manufacturers such as Sulzer and their coating suppliers have spent many years developing the coating processes and settings they use ； Extensive knowledge and rich experience of the manufacturing process are the only way to improve it. The final processing procedures for each coating are strictly protected, with proprietary information ensuring that every customer around the world receives the same quality of coating. To illustrate the importance of these procedures, especially in pump applications, consider the process of installing and removing the impeller. In many cases, it is necessary to heat the impeller in order to be able to install it on the drive shaft or remove it from the drive shaft. This contraction and fitting process may cause the coating to be damaged during routine maintenance operations. Sulzer ensures that its coating technology can withstand such thermal shocks and continue to provide long-lasting corrosion protection. Modern coating technologies for maintaining traditional equipment can be used as part of a renovation plan for such equipment, thereby extending the lifespan of pumps. As part of the renovation project, applying a new coating can significantly improve the performance and reliability of existing equipment. Finally, the key to a successful anti-corrosion solution is to have a thorough understanding of the application in question and to use all available information to determine the most appropriate measures. For those who wish to refurbish existing assets and equipment, they can take a series of improvement measures to extend their service life and enhance the performance of the pumps. If a new pump design is required, there is an opportunity to determine the most suitable substrate and optimal coating system to improve the pump’s durability. In the future, as coating technologies continue to advance, end-users will be able to choose customized coatings that can be applied during the manufacturing process. However, even when using the most advanced coatings, research and development of coating processes is required in order to utilize these processes for applying the paint to the complex inner surfaces of cast impellers and volutes. Therefore, this remains the \"holy grail\" of pump design; once this challenge is overcome, the reliability and service life of industrial pumps will be further improved. Ownership costs can be minimized by properly using the substrate and protective coating, as well as by designing better pumps to provide comprehensive and cost-effective pump solutions.