The spring is often corroded by the surrounding medium in the process of manufacture, storage and use.As the spring works under the influence of the spring force, the spring force will change and lose its function after being corroded.Therefore, preventing the corrosion of the spring can guarantee the stability of the spring and extend its service life.
The corrosion of spring can be divided into chemical corrosion and electrochemical corrosion according to the type of reaction.They are the result of changes in the metal atoms on the surface of the spring or the loss of electrons to an ionic state.
If the metal on the surface of the spring simply reacts with the surrounding medium, causing corrosion of the spring is called chemical corrosion.For example, the spring is oxidized in the particularly dry atmosphere to form an oxidation film, and the spring is chemically changed with the liquid or impurity in the non-electrolyte liquid, which is a chemical corrosion.
If the spring is in contact with the electrolyte solution, the corrosion caused by the action of the micro battery is called electrochemical corrosion.For example, the spring is in contact with an acid or a salt solution, which is an electrolyte. Due to defects or impurities on the surface of the spring, different potential differences are formed, so that the spring is constantly subjected to electrolytic corrosion.For example, the spring is in the humid atmosphere, because the water vapor in the atmosphere condenses into water film or water bead on the spring surface, and the corrosive gases in the atmosphere (such as sulfur dioxide and hydrogen sulfide in industrial waste gas or salt mist in the ocean atmosphere, etc.) dissolve in water film or water bead to form electrolyte.In addition, impurities or defects of the spring metal can also form electric levels with different potential differences, and the spring also produces electrolytic corrosion.These are electrochemical corrosion.
Chemical corrosion of springs is small and slow, while electrochemical corrosion is major and common.But generally speaking, chemical corrosion and electrochemical corrosion exist simultaneously.
Spring corrosion can be divided into medium corrosion, contact corrosion and stress corrosion according to its working environment or conditions.
Medium corrosion is the corrosion caused by the spring's contact with the corrosive medium, which generally causes corrosion damage to the whole spring.For example, springs on chemical equipment or Marine vessels are often corroded by the medium.
Contact corrosion is a kind of local corrosion caused by the long term contact between the spring and the metal with different electric potential.For example, corrosion of the end of a spring in contact with other metals.
Stress corrosion is a kind of corrosion under the joint action of stress and corrosive medium.It usually produces cracks in the section with large tensile stress and gradually expands, resulting in the spring break.The degree of stress corrosion is closely related to the type of spring material and surrounding medium.
In order to prevent the corrosion and damage of the spring, the surface treatment of the spring should be generally applied to cover the spring surface with a protective layer.
According to the nature of protective layer, the commonly used protective layer in engineering can be divided into:
1) chemical protective layer.By means of chemical reaction, a compact protective film is formed on the surface of the spring to prevent the corrosion of the spring. The chemical protective layer commonly used by the spring is oxidized (also known as blue or black) and phosphorized.
2) metal protective layer.There are many types of metal protective coatings.As far as springs are concerned, metal protective coatings are generally obtained by means of electroplating.Electroplating protection layer can not only protect the spring from corrosion, but also play a decorative role.Some electroplating metal protective layer can also improve the working performance of the spring, such as improving surface hardness, increasing wear resistance, improving thermal stability and preventing radiation corrosion.
The spring's metal protective layer is generally galvanized layer and cadmium plating layer.
3) non-metal protective layer.The protective coating is a coating of organic matter or minerals that is dipped or sprayed on the surface of the spring.
The materials that belong to this protective layer are paint, asphalt, paint, lubricating oil, plastics, paraffin, etc.
4) temporary protective layer.The protective layer mainly prevents the spring from corroding during processing or storage.
The commonly used temporary protective layer is dipped in wax, coated with antirust water, coated with antirust paper or peeling plastic.
Surface treatment of springs, regardless of the type of protective layer, should include two processes: pretreatment before surface treatment and surface treatment.