Analysis of corrosion issues in centrifuge applications

Thread Content

This post was last edited by Wang Genrong on 2017-9-2 at 16:51. Separation technology is a widely applied technique used in various industries such as pharmaceuticals, chemicals, metallurgy, textiles, environmental protection, food processing, and the military. It makes use of centrifugal force as a driving force to separate suspensions or emulsions consisting of liquid and solid, liquid and liquid, or liquid, liquid, and solid, depending on the physical properties of the materials and the desired separation goals. Thanks to its simple structure, high efficiency, and purely physical separation process, it ensures that the properties of the separated materials are well preserved. Therefore, centrifuges have a very wide range of market applications, especially imported products from brands such as Thermo. In recent years, China’s national economy has continued to develop, with new technologies and processes emerging in large numbers, and the application scope of centrifuges has become even broader. However, in this process, most manufacturers focus more on the separation performance and strength requirements of centrifuges, paying insufficient attention to the impact of corrosion factors, which has led to some negative consequences. This article provides an analysis of this issue for further discussion. It is well known that the hazards of corrosion are extremely severe; there are many examples around the world to illustrate this. A centrifuge is a device that rotates at high speeds, and its safety requirements are just as important as those of pressure vessels. Most centrifuge manufacturers, when designing, selecting, and using such devices, focus more on the impact of uniform corrosion on components with high strength, while neglecting the adaptability of the structural design and manufacturing processes to corrosive environments, which has led to serious consequences. Not only do the parts show signs of corrosion and is the contaminated material separated, but in more severe cases it can even lead to machine damage and loss of life. I. Centrifuge environment: Centrifuges are classified into different types based on their function and structure, but they all share common characteristics: 1. The drum is a component that rotates at high speeds; 2. The drum forms the core of the separation space; 3. The drum has a non-standard shape; 4. There are also other connecting components or accessories inside the drum. These characteristics of the drum indicate that the core component of the centrifuge is a stress-bearing part; its irregular geometric shape results in a multi-zone stress distribution within this component. Moreover, the combination of multiple parts creates the possibility of the formation of electric couples, and these are all factors that deserve attention. II. Possible corrosion phenomena Metal corrosion can be mainly classified into two categories: uniform corrosion and localized corrosion. Uniform corrosion is easy to detect based on its observable symptoms, and it is described in detail in most manual documents; therefore, no further explanation is given here. Only the latter category will be discussed in some detail. Local corrosion occurs only in specific areas and represents a very serious form of degradation with significant consequences; it includes phenomena such as pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion, and wear corrosion. In centrifuges, local corrosion is widespread, and it is necessary to analyze its formation mechanisms in order to take measures to overcome it. 1. Pitting is a severe form of corrosion that occurs primarily in metals prone to passivation, such as stainless steel. Due to potential defects on the surface, along with the presence of active ions in the solution that can destroy the passivation layer (such as halide ions), the passivation layer is locally damaged, thereby forming an electrochemical cell that leads to pitting. After pitting forms, the effect of centrifugal force accelerates the dynamic process of the electrochemical couple within the pit, thereby ensuring the continuation of pit corrosion until perforation occurs, which is different from the phenomenon of pitting under static conditions. 2. Intergranular corrosion causes parts to lose their strength and ductility, leading to brittle fracture. It is a harmful form of damage that progresses from the surface along the grain boundaries inward, with no signs of corrosion on the outer surface. 3. Wear and corrosion refers to the simultaneous damage to a part’s surface caused by wear and corrosion. 4. Stress corrosion is damage that occurs in a corrosive environment due to the action of certain tensile stresses. It has the following characteristics: residual tensile stress, external tensile stress, a corrosive penetrating environment, and local defects. The above four types of corrosion have a highly severe impact on centrifuges and can lead to serious consequences. III. Measures 1: Different materials and separation requirements necessitate the use of different machine models. After the model and main parameters are determined, the material for the strength components is selected on a fundamental level by taking into account the corrosion resistance of different materials in various environments, their physical and chemical properties, cost-effectiveness, and other factors. From the perspective of the material itself, it is safe for the target substances it is intended for; therefore, the following principles should be followed when selecting materials: a) There is no universal anti-corrosion material – it is necessary to fully understand the application environment and choose materials accordingly. For example, stainless steel is not suitable for concentrated sulfuric acid, while carbon steel is appropriate for such applications. b、Stainless steel is not rust-proof; it only remains rust-free in air and other suitable environments. c. In the same environment, try to use the same material or materials with similar equilibrium potentials to avoid creating a galvanic couple in that environment. d. Comprehensive cost-performance; special attention should be paid to the impact of processing feasibility. 2. Structural design: An excellent design can extend the service life of equipment and ensure its safety. Regarding structural design, the design flaws that most easily accelerate corrosion are stress concentration and the presence of gaps. Sometimes, even though the metal is immersed in a solution, the rate of corrosion is low; however, the formation of gaps leads to changes in the chemical and electrochemical conditions of the liquid inside those gaps (a decrease in pH and potential), which results in severe crevice corrosion. Therefore, in the design process it is important to: a) avoid the formation of gaps as much as possible; b) use the same material wherever possible within the same environment; c) ensure uniformity in the structure of the components or apply compressive stresses; d) prevent the formation of dead zones to ensure smooth flow of fluids. 3. A basic approach to controlling corrosion through surface protection measures is to isolate the environment that facilitates corrosion. In centrifuge design, surface treatment methods such as flanging, galvanizing, chromium plating, and chemical coating are often employed. These methods are effective in many environments, but for rotating components, an important phenomenon must be taken into account: the base material and the coating are two different materials with distinct coefficients of linear expansion. This results in different amounts of deformation when the rotating component undergoes elastic deformation, leading to the formation of numerous microcracks. If this phenomenon occurs, it will accelerate corrosion. Therefore, surface protection methods for rotating parts should be applied with caution. 4. Corrosion inhibitors and centrifuges involve multiple disciplines; to make good use of centrifuges, in addition to the measures mentioned above, it is also necessary to pay attention to studying their application environment. The use of corrosion inhibitors is also widely adopted in practical applications. For certain harsh environments, the use of appropriate corrosion inhibitors can render components suitable for use. As long as the manufacturing processes permit it, the use of corrosion inhibitors should be an effective approach for centrifuge applications. 5. A proper design of the manufacturing process and the appropriate selection of materials are merely prerequisite conditions for preventing corrosion in centrifuges; the correct application of the processing techniques also affects the centrifuge’s resistance to corrosion to a certain extent. Processing methods often leave many defects on parts or equipment, such as misalignment and local stress. Misalignment can lead to gaps, while improper clamping and positioning methods may cause the parts to be off-center. Inappropriate tool movement can result in rough surfaces, and inadequate assembly, improper welding, incorrect heat treatment, as well as uneven tightening of bolts can all cause local stress concentration. Therefore, finding the right processing method is also an effective way to prevent corrosion. The corrosion problem of centrifuges is a complex and multifaceted issue, as the operating environment of centrifuges differs significantly from the data obtained in static conditions in standard manuals. Therefore, when referring to relevant information, in addition to the analysis mentioned above, it is also necessary to take into account the effects of various factors such as dissolved oxygen in the solution, temperature, high-speed fluids, and changes in flow patterns in the separation zone; appropriate measures should be adopted to ensure the safe and reliable operation of the centrifuge.