Analysis of Corrosion Problems in Centrifuge Applications

Separation technology is an extremely widely used technology, applicable to medicine, chemical industry, metallurgy, textile, environmental protection, food, military and other industries, it is to promote the use of centrifugal force, liquid - solid, liquid - liquid or liquid —— Liquid-solid and other suspensions or emulsions According to the characteristics of physical properties, different separation requirements to achieve different separation purposes of a device, 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 broad.

In recent years, China’s national economy has continued to develop, new technologies and new technologies have emerged endlessly, and the application of centrifuges has become more widespread. However, in this process, most manufacturers are more concerned with the separation performance and strength of centrifuges. Requirements, the impact of corrosion factors are often not enough, resulting in some negative effects, this article on this issue for some analysis and explanation for everyone to discuss.

As we all know, the harm of corrosion is extremely large. There are many examples in the world to illustrate that the centrifuge is a high-speed rotating device. Its safety requirements are as important as the pressure vessels. Most of the centrifuge manufacturers are designing, selecting, and In the application, more consideration was given to the effect of uniform corrosion on the strength parts, and the structural design, processing technology, and other adaptations to the corrosive environment were neglected, resulting in some serious consequences. Not only does the part have a tendency to corrode, it pollutes the material being separated, and even more serious causes the machine to die.

First, the centrifuge environment

Centrifuges are divided into different types of equipment by function and structure, but they all have common features:

1, drum for high-speed rotating parts;

2, the main part of the drum to form a separate space;

3, drum was shaped structure;

4. There are some other connectors or accessories in the drum.

These characteristics of the drum illustrate that the core part of the centrifuge is a stress piece. Its irregular geometry leads to the multi-zone stress distribution of parts and the cooperation of multiple parts, which in turn makes it possible to form electrocouple pairs. This is something worth noting.

Second, the possible corrosion phenomenon

Metal corrosion forms are mainly divided into two types: uniform corrosion and local corrosion. The former is easy to find because of apparent phenomena, and it is clearly stated in most manual data. It will not be described in detail here, and only the latter will be discussed.

Localized corrosion occurs only locally and is a very serious and harmful damage, such as pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion, wear and corrosion, etc. In centrifuges, localized corrosion is widespread. Emphasis is placed on analyzing the mechanism of its formation, taking measures and overcoming them.

1, pitting corrosion is a highly corrosive phenomenon, mainly present in easily passivated metals, such as stainless steel, due to possible defects in the local surface, there are active ions in the solution can damage the passive film (such as halogen Ion), the passivation film is locally destroyed, thus forming a galvanic couple, causing pitting corrosion. After the pitting is formed, due to the effect of 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 static pitting phenomenon.

2. Intergranular corrosion can cause parts to lose strength and elongation, causing brittle fracture of the part. It is a kind of harmful damage that develops from the surface inward along the grain boundaries and has no signs of corrosion on the outer surface.

3, wear and corrosion that is part of the surface at the same time subject to 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, external tensile stress, corrosive infiltration environment, and local defects.

The above four kinds of corrosion are extremely serious to the centrifuge and may cause serious consequences.

Third, measures

1. Different materials and separation requirements require different models. After determining the model and main parameters, according to the corrosion resistance of different materials in different environments, comprehensive factors such as physical and chemical properties, cost-effectiveness, and other factors, the strength of the part material is determined 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, must fully understand the application environment, targeted materials, such as stainless steel is not suitable for concentrated sulfuric acid carbon steel is suitable.

b. Stainless steel is not stainless. It is only stainless steel in air and other adaptive environments.

c. In the same environment, choose the same material or materials with similar potentials as much as possible to avoid causing the galvanic couple to the environment.

d, comprehensive cost-effective, with particular emphasis on the impact of processing technology.

2, structural design

An excellent design can prolong the service life of the equipment and ensure the safety of the equipment. For the structural design, the design flaws that are most likely to accelerate corrosion are stress concentration tendency and crevice environment. Sometimes, although the corrosion rate is small in the metal soaking solution, the gap is formed. Due to changes in the chemical and electrochemical state of the internal solution (PH and potential drop), severe crevice corrosion will occur, so design should pay attention to:

a. Try to avoid gaps;

b. In the same environment, try to use the same material;

c. Ensure the homogeneity of the parts or increase the compressive stress;

d, to avoid the formation of dead ends, to ensure fluid flow.

3, surface protection measures

A basic idea for controlling corrosion is to isolate the corrosive environment. In the design of centrifuges, surface treatment methods such as flanges, galvanizing, chrome plating, electroless plating and the like are often used. These methods are effective in many environments, but an important phenomenon must be noted for rotating parts: Materials and coatings are two kinds of materials, their linear expansion coefficients are different, will produce different deformation when the rotating part is elastically deformed, which will cause a large number of micro cracks; if the aforesaid phenomenon exists, it will accelerate the formation of corrosion. Therefore, the method of surface protection for rotating parts should be used with caution.

4, corrosion inhibitor

Centrifuge itself involves many disciplines, to use a good centrifuge, in addition to some of the above measures, it should also pay attention to study its application environment. The application of corrosion inhibitors is also widely used in practical applications. For certain harsh environments, the use of appropriate corrosion inhibitors will enable the parts to reach a suitable level. As long as the process permits, the use of corrosion inhibitors should be an effective way for the application of centrifuges.

5, the correct process

The correct design and reasonable material selection are only prerequisites for solving the problem of corrosion of the centrifuge; the correct application of the processing technology also affects the anti-corrosion capability of the centrifuge to some extent. Processing methods usually leave many defects on the parts or equipment, such as misalignment, local stress, etc. Misalignment may cause gaps. Unreasonable positioning method of clamping may cause eccentricity of parts. Unreasonable pass may cause surface roughness, and assembly does not Appropriate, welding, unreasonable heat treatment, unequal tightening of fastening bolts, etc. may cause local stress concentration. Therefore, finding the right processing method is also an effective way to prevent corrosion.

The problem of corrosion in centrifuges is a complex and complex problem, due to the large differences between the centrifuge's working environment and the general data book's static data. Therefore, when referring to relevant data, in addition to the above analysis, the influence of many factors such as dissolved oxygen in the solution, temperature, high-velocity fluid, and flow state in the separation zone should also be taken into consideration, and appropriate measures should be taken to ensure the safe and reliable operation of the centrifuge.