Industrial products are inevitably exposed to the adhesion and coverage of gas, liquid, and solid pollutants on the surface of the workpiece during manufacturing, assembly, installation, and transportation. These contaminants adhere to the surface of the workpiece, and some physical and chemical changes will occur.

1. Adhesion pollution of workpiece

Why do contaminants stick to the surface of the workpiece? We can observe from the perspective of the molecular structure of a substance. The forces on the molecules on the surface of the substance and the molecules inside are different. The intermolecular forces on the internal molecules are balanced, while the intermolecular forces on the molecules on the surface are unbalanced.

The intermolecular forces are shown in the following figure. The B molecule is inside the substance, and the intermolecular force on the surrounding molecules is balanced; the force on the A molecule on the surface is unbalanced, and it has a tendency to pull inward due to the gravitational force of the internal molecules; the C molecule At the corner of the material surface, only the gravitational force of the molecules on the right and the molecules below, has a tendency to pull to the lower right corner. It can be seen from this that there are dangling bonds in the material molecules on the surface of the workpiece, and other foreign molecules must be adsorbed to maintain equilibrium. The tiny particles floating in the air, dust and water vapor, have some electrical properties in the friction with air molecules. When they are in contact with the surface of the workpiece, they are easily caught by the dangling bonds, which will cause the workpiece to be contaminated by adhesion.

2. Metal corrosion contamination

In nature, most metals exist in the state of metal compounds. According to the principle of thermodynamics, most of the metal compounds are in a low-energy state: the materials used as metal workpieces are refined, processed, and reprocessed through multiple processes, and the single metals are in a high-energy state and are in a high-energy state. The metal compound is spontaneously transformed into a low-energy metal compound, and the oxidation-reduction reaction occurs spontaneously, or it is called an electrochemical reaction or a corrosion reaction. This is the cause of corrosion contamination of metal workpieces.

Generally, there are three thin layers on the microscopic surface of metal workpieces, namely strained layer, oxide layer and adhesion layer. As shown in the following figure, corrosion pollution occurs in adhesion layer and oxide layer.

According to the action mechanism, pollution can be divided into chemical corrosion and electrochemical corrosion (also called "microcell corrosion"). People's measures to deal with corrosion pollution are still industrial cleaning, cleaning the workpiece surface, then applying anti-corrosion treatment, coating protective layer and so on.

In practical application, it is impossible to achieve absolute surface purification, and pollution can only be reduced to the minimum according to the technological requirements of industrial products.

3. Material and surface energy of workpiece

Due to the different materials of the manufactured workpieces, the molecular weights of the workpieces are different, and the number of dangling bonds is also different. The surface molecular energy of this workpiece is called surface energy. Different substances have different surface energies. The larger the surface area of matter, the greater the surface energy.

After machining, the surface of the workpiece is damaged and absent, and the number of dangling bonds increases, which will increase the adhesion to pollutants. Rough machining in manufacturing process, uneven surface of workpiece, large surface area and large surface energy. On the workpieces with exactly the same size, the machining process is fine, the surface roughness of the workpieces is small, the surface area is relatively small, and the surface energy is small.

In the cleaning process of equipment, it is necessary to understand the differences in surface energy for workpieces of different materials, such as metal, plastic, ceramic, silicon wafer, glass, etc. Solid surfaces are divided into two categories, and the energy of metal and general inorganic surfaces is above 100 mN/m, which is called high-energy surface. The energy on the surface of organic matter such as plastic is low, which is called low energy surface. Surface energies of metals, inorganic substances and organic substances are shown in the following table.