Surface treatment of spring

In recent years, the surface finishing (finishing) treatment of springs has also been recognized by spring designers. The finishing (finishing) process is to put the spring directly into the inclined centrifugal, spiral vibrating, vortex and other series of finishing machines, and add an appropriate amount of abrasive, abrasive and water for finishing. Normally, it takes about 20~30min. The specific time depends on the shape of the spring and the amount of the device.

After finishing, take out the spring and rinse it with tap water, then immerse it in the SM series water film replacement anti-rust oil for several minutes and then take it out. At this time, a layer of 5μm anti-rust oil film is attached to the surface of the spring, which protects the spring from corrosion. This treatment method greatly supports the traditional complicated procedures, such as degreasing, cleaning, pickling, and dehydrogenation.

At present, the springs adopting the finishing process include: plunger springs for oil pump nozzles and pressure regulating springs for fuel injectors; support springs for refrigerator compressors. The comparison of performance (life) and corrosion resistance between the light-finished (finished) and conventional oxidized or plated springs does not wait for further tests and practices to draw more reliable conclusions.
The corrosion of the spring can be divided into chemical corrosion and electrochemical corrosion according to the type of reaction. They are the result of the change of the metal atoms on the surface of the spring or the gains and losses of electrons into ionic states.

If the metal on the surface of the spring only reacts chemically with the surrounding medium, the corrosion caused by the spring is called chemical corrosion. For example, the spring oxidizes in a particularly dry atmosphere to form an oxide film, and the spring chemically changes with the liquid or the impurities in the liquid in the non-electrolyte liquid, etc., which belong to 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 acid or salt solutions. Such solutions are all electrolytes. Due to defects or impurities on the surface of the spring, electrodes with different potential differences are formed, so that the spring is constantly subject to electrolytic corrosion. Another example is that the spring is in a humid atmosphere.

The water vapor in the atmosphere condenses into a water film or water droplets on the surface of the spring, and the corrosive gases in the atmosphere (such as sulfur dioxide and hydrogen sulfide in industrial waste gas or salt spray in the ocean atmosphere) are dissolved in the water film or water Electrolytes are formed in the beads. In addition, the impurity or defects of the spring metal can also form electrodes with different potential differences, and the spring also produces electrolytic corrosion. These are all electrochemical corrosion.

The chemical corrosion of the spring is small and slow, while the electrochemical corrosion is the main and common. But generally speaking, chemical corrosion and electrochemical corrosion exist at the same time.

In the process of manufacturing, storage, and use, the spring often suffers from the corrosion of the surrounding medium. Since the spring works by elastic force, the elastic force will change and lose its function after the spring is corroded. Therefore, preventing the corrosion of the spring can ensure the stable operation of the spring and prolong its service life.

The anti-corrosion method of spring generally adopts a protective layer. According to the nature of the protective layer, it can be divided into: metal protective layer, chemical protective layer, non-metal protective layer and temporary protective layer, etc. The first three methods are introduced here.

Stainless steel springs and copper wire springs have certain anti-corrosion ability, so anti-corrosion treatment is generally not carried out.

1. The metal protective layer of the spring

There are many types of metal protective layers. In terms of springs, gold plating is generally used to obtain the metal protective layer. The electroplated protective layer can not only protect from corrosion, but also improve the appearance of the spring. Some electroplated metals can also improve the working performance of springs, such as increasing surface hardness, increasing wear resistance, improving thermal stability, and preventing radiation corrosion.

But if it is purely for the corrosion of the spring, the electro-galvanized layer and the electro-cadmium layer should generally be used.

Zinc is relatively stable in dry air, hardly changes, and is not easy to change color. A white film of zinc oxide or carbon zinc carbonate is formed in humid air. This dense film prevents further corrosion. Therefore, the galvanized layer is used as the anti-corrosion protection layer of the spring under normal atmospheric conditions. All springs that are in contact with solutions such as sulfuric acid, hydrochloric acid, caustic soda, and working in humid air such as sulfur trioxide, should not be coated with zinc.

Generally, passivation treatment is carried out after the galvanized layer, which can improve the protective performance of the coating and increase the appearance of the surface.

In oceanic or high-temperature atmospheres, springs in contact with seawater, and springs used in hot water at 70°C, cadmium is relatively stable and has strong corrosion resistance. Cadmium coating is brighter and more beautiful than zinc coating, softer, and has better plasticity than zinc. The coating has less hydrogen embrittlement and is most suitable for springs as a protective layer. But cadmium is scarce, expensive, and cadmium salt is highly toxic, which is very harmful to the environment. Therefore, it is restricted in use. Therefore, most of the springs used in aviation, navigation and electronics industries use cadmium plating as a protective layer.