Separation technology is an extremely widely used technology, which is suitable for various industries such as medicine, chemical industry, metallurgy, textile, environmental protection, food, military, etc. It is driven by centrifugal force and will be liquid-solid, liquid-liquid or liquid. ——Liquid-solid suspension or emulsion according to the physical characteristics, different separation requirements to achieve a different separation purpose of the equipment, due to its simple structure, high efficiency, pure physical separation process, can fully guarantee the separation of materials characteristic. Therefore, the market for centrifuges is extremely extensive.
In recent years, China's national economy has continued to develop, new technologies and new processes have emerged, and the scope of application of centrifuges is more extensive. However, in this process, most manufacturers are more concerned about the separation performance and strength of centrifuges. Requirements, the impact of corrosion factors are often not enough, which brings some negative effects. This article makes some analysis and explanation for this issue for everyone to explore.
As we all know, the danger of corrosion is extremely huge. There are many examples in the world that the centrifuge is a high-speed rotating equipment, and its safety requirements are as important as pressure vessels. Most centrifuge manufacturers are designing, selecting and In the application, more attention is paid to the influence of uniform corrosion on strength parts, neglecting the adaptability of structural design and processing technology to corrosive environment, resulting in some serious consequences. Not only the parts tend to corrode, but also the materials that are separated from the pollution; even more serious, even the machine is destroyed.
First, the centrifuge environment
Centrifuges are divided into different types of equipment according to their functions and structures, but all have common characteristics:
1. The drum is a high speed rotating part;
2. The separation space is formed by the drum as the main body;
3. The rotating drum has a special-shaped structure;
4. There are other couplings or accessories in the drum.
These characteristics of the drum indicate that the core part of the centrifuge is a stress member, and its profiled geometry results in multiple stress distribution of the part, the cooperation of multiple parts, and the possibility of forming a pair of galvanic couples. These are some of the things that are worth noting.
Second, possible corrosion phenomena
Metal corrosion forms are mainly divided into two categories: uniform corrosion and local corrosion. The former is easy to find because of the apparent phenomenon, and it is clearly expressed in most manual data. It will not be described in detail here, and only the latter will be discussed.
Localized corrosion only occurs locally, which is an extremely serious and harmful damage, such as pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion, wear corrosion, etc. In centrifuges, local corrosion is widespread and should be Focus on the analysis of its formation mechanism and take measures to overcome it.
1. Pitting corrosion is a highly corrosive phenomenon, mainly in easily passivated metals, such as stainless steel. Due to possible local defects in the surface, there are active ions in the solution that can destroy the passivation film (such as halogen Ion), the passivation film is partially destroyed, thereby forming a pair of galvanic couples, causing pitting corrosion. After the pitting corrosion is formed, due to the centrifugal force, the dynamic process of the galvanic couple in the hole will be accelerated, thereby ensuring the continuity of the hole corrosion until the perforation, which is different from the pitting phenomenon under static.
2. Intergranular corrosion will cause the parts to lose strength and extensibility, causing the parts to be brittle. It is a kind of harmful damage that develops from the surface along the grain boundary and has no signs of corrosion on the outer surface.
3. Wear and corrosion means that the surface of the part is subject to both wear and corrosion damage.
4. Stress corrosion is damage caused by a certain tensile stress in a corrosive environment. It has the following characteristics: residual tensile stress, applied tensile stress, corrosive infiltration environment, local defects.
The above four types of corrosion are extremely severely affected by the centrifuge and may cause serious consequences.
Third, measures
1. Different materials and separation requirements require different models. After the model and main parameters are determined, according to the corrosion resistance of different materials in different environments, comprehensive physical and chemical factors, cost performance and other factors are combined to determine the strength component materials from the basic surface. From the point of view of the material itself, the target material it targets is safe, therefore, the selection of materials should follow the following principles:
a. There is no absolute universal anti-corrosion material, and the application environment must be fully understood. The targeted material selection, such as stainless steel, is not suitable for concentrated sulfuric acid carbon steel.
b. Stainless steel is not stainless, but it is not rusted in air and other adaptive environments.
c. In the same environment, choose the same material or material with similar equilibrium potential as much as possible to avoid galvanic environment.
d, comprehensive cost performance, pay special attention to the impact of processing technology.
2, structural design
An excellent design can extend the service life of the equipment and ensure the safety of the equipment. For structural design, the design defects that are most likely to accelerate corrosion are stress concentration tendency and gap environment. Sometimes, although the corrosion rate is small in the metal soaking solution, the gap is formed. Since the chemical and electrochemical states of the internal solution change (PH and potential drop), it will cause severe crevice corrosion, so the design should pay attention to:
a, try to avoid the occurrence of gaps;
b. In the same environment, try to use the same material;
c. Ensure the uniformity of the parts or increase the compressive stress;
d. Avoid forming dead angles and ensure fluid flow.
3. Surface protection measures
One of the basic ideas for controlling corrosion is to isolate the corrosive environment. In the design of centrifuges, surface treatment methods such as flange, galvanizing, chrome plating, and electroless plating are often used. These methods are effective in many environments, but for rotating parts, an important phenomenon must be noted: matrix The material and the coating are two materials, and their linear expansion coefficients are different, which will produce different deformation amounts when the rotating member is elastically deformed, thereby causing a large number of microcracks; if the above phenomenon exists, the corrosion formation will be accelerated. Therefore, the method of surface protection for rotating parts should be used with caution.
4, corrosion inhibitor
The centrifuge itself involves multiple disciplines. To use the centrifuge, in addition to the above measures, it should also pay attention to the application environment. The application of corrosion inhibitors is also widely used in practical applications. For some harsh environments, the use of appropriate corrosion inhibitors can make the parts suitable. As long as the process permits, the use of corrosion inhibitors should be an effective way for centrifuge applications.
5, the correct process
The design is correct and the material selection is reasonable. It is only the precondition for solving the corrosion of the centrifuge; the correct application of the processing technology also affects the anti-corrosion ability of the centrifuge to a certain extent. The processing method usually leaves many defects on the parts or equipment, such as misalignment, local stress, etc. Misalignment may cause gaps. Unreasonable positioning method may lead to eccentricity of parts, unreasonable pass may result in rough surface, and assembly is not Appropriate, unreasonable welding, heat treatment, uneven force on the fastening bolts will cause local stress concentration. Therefore, finding the right processing method is also an effective way to prevent corrosion.
The corrosion problem of the centrifuge is a comprehensive and complex problem, because the working environment of the centrifuge is quite different from the data obtained in the static data of the general data sheet. Therefore, in addition to the above analysis, in addition to the above analysis, consideration should also be given to the effects of dissolved oxygen, temperature, high-speed fluid and fluid state changes in the separation zone, and appropriate measures to ensure safe and reliable operation of the centrifuge.