There are many types of metal protective layers for springs. As far as springs are concerned, gold plating is generally used to obtain the metal protective layer. The plating protective layer can not only protect against corrosion, but also improve the appearance of the spring. Some electroplated metals can also improve the spring Work performance, such as improving surface hardness, increasing wear resistance, improving thermal stability, preventing ray corrosion, etc. However, 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 as the anti-corrosion protective 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., should not be coated with zinc. Generally speaking. After the galvanized layer is passivated, 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, more beautiful, softer, and more plastic than zinc coatings. Zinc is good, and the coating has low hydrogen embrittlement. It 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. 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 Select within the range of 6~24μm; the thickness of the cadmium plating layer is recommended to be selected within the range of 6~12μm.
The zinc and cadmium plating of the spring is carried out in the cyanide electrolyte. During 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 The internal stress makes the coating and spring on the spring brittle, also called hydrogen embrittlement. Because the strength of the spring material is high, and the deformation of the spring is large when it is formed, it is particularly sensitive to hydrogen embrittlement, if it is not removed 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), The purpose of hydrogen removal can be achieved.
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.