In recent years, with the rapid development of automobile, precision instrument and household appliances industries, the rapid development of bearing industries such as automobile bearings, precision machine tool bearings, railway bearings and household appliance bearings in my country has been driven. Bearings are generally composed of rings (inner and outer rings), rolling elements (steel balls and rollers) and cages. Except for some special bearings that are composed of non-metallic rolling elements and cages, most bearings are made of metal materials. It is a common problem that metal materials rust in the atmospheric environment. Rusting of bearings will affect their accuracy level and service life, and may even lead to scrapping or accidents during use. Therefore, bearing rust prevention is a problem that both bearing manufacturers and users are very concerned about.
1. Bearing corrosion mechanism
Corrosion refers to the chemical or electrochemical reaction of metals due to contact with the surrounding substances. Rust is usually the corrosion or discoloration caused by the influence of moisture, oxygen and other corrosive gases in the atmosphere. In the bearing industry, rust refers to the electrochemical reaction caused by the formation of local micro-batteries on the surface of bearing parts in the atmosphere.
2. Bearing corrosion phenomenon
Metal corrosion can be divided into general corrosion and local corrosion according to its damage form .
Comprehensive corrosion means that when a metal surface is subjected to the action of a corrosive medium, the corrosion rate spreads along the metal surface faster than it spreads deep into the metal, and the corrosion is distributed over the entire metal surface.
Localized corrosion refers to corrosion that occurs in individual areas of metal, mainly in the form of pitting corrosion, pitting corrosion, pitting corrosion, intergranular corrosion, corrosion cracking, subsurface corrosion, etc. Bearings generally suffer from localized corrosion.
Common rust on bearings during the production process include: flake corrosion, honeycomb corrosion, yellow rust, black rust, black spots, and rust caused by bumps, scratches, and abrasions.
Common corrosion of bearings during storage and use are:
The metal surface of the rolling bearing is corroded by acid and alkali solutions directly due to the destruction of the oil film. The rust occurs in different colors such as yellow rust, red rust, black rust, etc. The contact surface of the rolling bearing is corroded or rusted due to friction and wear when the speed is extremely slow, when it is shaking or stationary.
3. Analysis of the causes of bearing corrosion
In nature, most metals exist stably in various compound states. At present, the metals used by people are obtained by decomposing metal compounds through smelting or other chemical methods. In this process, the metal absorbs energy, so its chemical energy is higher, it is more active, and it has a natural tendency to regenerate various compounds. The corrosion reaction of metals is a natural phenomenon, which is the thermodynamic basis of metal corrosion. If the molecules on the metal surface react evenly with external substances to form a dense oxide film, it will prevent the metal from continuing to corrode. However, many reaction products of bearings are relatively loose and have a tendency to react continuously, and cannot form an oxide film. In particular, the electrochemical reaction of bearing steel has a self-acceleration phenomenon, and the rust reaction develops very quickly.
Bearing corrosion is the result of a combination of factors:
Air is an environmental factor, and the moisture content in the air will change greatly with the seasons and weather changes. In the rainy season, the relative humidity of the air can reach more than 80%, especially at the processing site. Due to the relatively closed environment, the evaporation of the grinding fluid used in the grinding process further increases the humidity of the air room temperature. When the bearings and their parts are affected by temperature changes, it is very easy to form a condensed water film on their surface. During the drying process of the bearings after cleaning, the surface solvent evaporates and absorbs heat, which reduces the surface temperature of the bearings and also provides conditions for the generation of condensed water.
In factory workshops, there are dense crowds, high mobility, and high proportions of dust and harmful gases. These substances can dissolve in different forms in the condensed water film on the bearing surface, causing bearing corrosion.
Bearing factories are generally located in industrially intensive areas, and various industrial waste gases can significantly deteriorate the air quality. Statistics show that the amount of sulfur dioxide produced by thermal power plants or other coal-fired boilers is alarming. With the increase in urban vehicles, the increasing exhaust emissions from automobiles have become a focus of environmental protection. These sulfur-containing gases are highly soluble in water, and their solutions are highly corrosive. In addition, rising temperatures will greatly accelerate the rust reaction.
4. Chemical reaction mechanism of bearing corrosion
There are two main chemical reaction mechanisms of bearing corrosion, namely chemical corrosion and electrochemical corrosion .
Chemical corrosion is corrosion caused by chemical reaction between metal and medium, and is a corrosion process without the generation of electric current.
Electrochemical corrosion is corrosion caused by electrochemical reactions between metals and media, and is corrosion caused by electric current.
Electrochemical corrosion is much more dangerous than chemical corrosion. In most cases, electrochemical corrosion occurs in bearing parts. The core reaction of electrochemical corrosion is the electrochemical reaction on the metal surface, and electrochemical reaction requires three conditions, namely electrodes (which can be understood as points with different potentials), conductors connecting the electrodes, and electrolyte solutions.
The internal non-uniformity of bearing steel causes the potential of different parts to be different; internal stress is generated during the processing; and changes in surface material are also the reason for the potential difference in bearing materials.
Bearing metal parts are all good conductors, and the metal conductors connecting different electrode points have excellent conductivity.
Electrolyte solution is a liquid containing ions that forms a closed circuit with electrodes and wires.
The air contains a certain amount of moisture and various inorganic compound gases or dust. During the processing, bearing parts will also come into contact with different types of inorganic salts and other corrosive substances soluble in water. When a condensation film is formed on the surface of the bearing in a humid environment, different components in the air dissolve in water to form an electrolyte solution. Other types of soluble substances dissolve in water, which increases the concentration and corrosiveness of the electrolyte solution, causing electrochemical reactions to begin. The substances generated by the reaction will accelerate the electrochemical reaction and the accompanying chemical reaction, exacerbating the corrosion.
This is the inherent reason why bearings rust in the air.
