There are many types of metal protective layers. As far as the spring is concerned, the metal protective layer is generally obtained by electroplating. 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 the spring. , Such as improving surface hardness, increasing wear resistance, improving thermal stability, preventing ray corrosion, etc. But if it is purely for spring corrosion, generally electrogalvanized layer and electroplated cadmium layer should be used.
Zinc is relatively stable in dry air, hardly changes, and is not easy to change color. In humid air, a white film of zinc oxide or carbon zinc carbonate is formed. This dense film can prevent further corrosion. Therefore, plating The zinc layer is used for the spring’s anti-corrosion protection layer under general atmospheric conditions. Any spring that is in contact with sulfuric acid, hydrochloric acid, caustic soda and other solutions, and works in humid air with an atmosphere of sulfur trioxide, etc., should not be coated with zinc.
The thickness of the zinc and cadmium coating determines the level of protection. The thickness should generally be selected according to the working environment during use. The hardness of the zinc coating is recommended to be selected in the range of 6~24μm; the thickness of the cadmium coating is recommended to be in the range of 6~12μm Select.
The zinc and cadmium plating of the spring is carried out in the cyanide electrolyte. In the electroplating process, in addition to the zinc or cadmium plating, a part of the reduced hydrogen penetrates into the crystal lattice of the plating layer and the base metal, causing Internal stress makes the coating and spring on the spring brittle, also called hydrogen embrittlement. Due to the high strength of the spring material and the large deformation of the spring during forming, it is particularly sensitive to hydrogen embrittlement. If you do not remove hydrogen in time , Often cause the spring to break. In order to eliminate some of the defects generated in the electroplating process, improve the physical and chemical properties of the spring, extend the service life of the spring, and improve the corrosion resistance of the coating, post-plating treatment, that is, hydrogen removal treatment, must be carried out. Hydrogen treatment is carried out immediately or within a few hours after electroplating. The electroplated spring is heated at a temperature of 200 to 215 °C for 1 to 2 hours (or more than 2 hours, if the time is too long, it is easy to cause chromium brittleness), Can achieve the purpose of hydrogen removal.
Hydrogen removal is generally carried out in an oven. The effect of hydrogen removal is related to temperature, time, and residence time after plating. Generally speaking, high temperature, long heating time, and short pause time after plating are good for hydrogen removal. The spring hydrogen removal temperature can be selected higher.
The passivation treatment is generally 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 coatings are brighter, beautiful, softer, and more plastic than zinc coatings. Zinc is good, and the coating has low hydrogen embrittlement, which is most suitable for springs as a protective layer. However, cadmium is rare, expensive, and cadmium salt is highly toxic, which is very harmful to the environment. Therefore, it is restricted in use. Therefore, most of them are only used in aviation, The springs used in the navigation and electronics industries use cadmium plating as a protective layer.
In order to improve the corrosion resistance of the cadmium coating, passivation treatment can be performed after plating.
Metal protective layer, in addition to the above-mentioned zinc plating and cadmium plating, there are also copper plating, chromium plating, nickel plating, tin plating, silver plating, zinc-titanium alloy plating, etc.The spring designer can choose the plating layer according to the spring working occasion.
Spring surface treatment technology
In recent years, the surface of the spring has been light-decorated (finished) treatment has also been recognized by the spring designer. The light-finished (finished) process is to put the spring directly into the inclined centrifugal or spiral vibration type, eddy current type and other series of light decoration machines. Add appropriate amount of abrasive, abrasive and water for smoothing. Generally, the smoothing is about 20~30min. The specific smoothing 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 a few minutes and then take it out. At this time, a 5μm anti-rust oil film is attached to the surface of the spring, which protects the spring from rust. This treatment method greatly supports the traditional complicated procedures, such as degreasing, cleaning, pickling, dehydrogenation, etc.
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, etc. The springs for finishing (finishing) treatment and conventional oxidation or plating treatments, The comparison of performance (life) and corrosion resistance between the two does not wait for further testing and practice to make a more reliable conclusion.
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 result of the electron gain and loss becoming an ion state.
If the metal on the spring surface only chemically reacts with the surrounding medium, and the spring causes corrosion, it is called chemical corrosion. For example, the spring is oxidized in a particularly dry atmosphere to form an oxide film, and the spring is in a non-electrolyte liquid with the liquid or impurities in the liquid. Chemical changes, etc., belong to chemical corrosion.
If the spring is in contact with the electrolyte solution, the corrosion caused by the action of the microbattery is called electrochemical corrosion. For example, the spring is in contact with an acid or salt solution, which is an electrolyte, and a potential is formed due to defects or impurities on the surface of the spring. Different electrodes cause the spring to be continuously corroded by electrolysis; another example is that the spring is in a humid atmosphere, because the water vapor in the atmosphere condenses into a water film or water droplets on the surface of the spring, plus corrosive gases in the atmosphere (such as industrial waste gas) The sulfur dioxide and hydrogen sulfide or salt spray in the ocean atmosphere, etc.) dissolve in the water film or water droplets to form an electrolyte. In addition, the impurities 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 anti-corrosion method of the spring generally uses 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.
The chemical corrosion of the spring is small and slow, and 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 acts on the elastic force when working, the elastic force will change and lose its function after the spring is corroded. Therefore, preventing the corrosion of the spring can ensure the spring The work is stable and its service life is extended.
Stainless steel springs and copper wire springs have certain anti-corrosion ability, so anti-corrosion treatment is generally not carried out.