There are many types of metal protective layers. In terms of springs, electroplating 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 the spring. , Such as improving surface hardness, increasing wear resistance, improving thermal stability, preventing ray corrosion, etc. However, if it is purely for the corrosion of the spring, the electrogalvanized layer and the electroplated cadmium layer should generally 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 as the anti-corrosion protection layer of the spring 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., is not suitable for zinc coating. Plating. Zinc spring
The general galvanized layer is passivated after plating, and passivation 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 and more beautiful than zinc coatings, softer, and more plastic than zinc Good, the coating has low hydrogen brittleness and is most suitable for springs as a protective layer. However, 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 them are only used in aviation and navigation And the springs used in the electronics industry 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.
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 thickness of the zinc coating is recommended to be selected within the range of 6~24/μm; the thickness of the cadmium coating is recommended to be 6~12μm Select within the range.
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 plating layer and the crystal lattice of the base metal. Stress makes the coating and spring on the spring brittle, which is 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, It often causes the spring to break. In order to eliminate some defects generated during 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, is necessary. The treatment is after electroplating. Immediately or within a few hours. The electroplated spring is heated at a temperature of 200 to 215 ℃ for 1 to 2 hours (or more than 2 hours) to 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 electroplating. Generally speaking, high temperature, long heating time, short pause time after plating, and its hydrogen removal effect is good. The spring hydrogen removal temperature can be selected higher.
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.