The metal protective layer of metal spring is usually electroplated, because electroplating can improve the working performance of spring, such as improving the hardness of spring surface, enhancing the anti-wear ability, improving the thermal stability, preventing the ray corrosion, etc. Electroplating can also improve the appearance of the spring and beautify it.
Zinc is a relatively stable metal in dry air, almost no change, not easy to change color. In moist air, a white film of zinc oxide or carbon zinc carbonate will be formed. This tight film prevents the spring from external corrosion. Therefore, zinc coating is used for spring corrosion protection under general atmospheric conditions. If the hardware spring is to contact with sulfuric acid, hydrochloric acid, caustic soda and other solutions, and works in humid air such as sulfur trioxide, it is not suitable to use zinc coating.
Cadmium plated spring
In the marine or high temperature atmosphere, as well as the spring in contact with sea water, the spring used in 70 ℃ hot water has relatively stable cadmium and strong corrosion resistance. Compared with zinc coating, cadmium coating is bright and beautiful, soft, plasticity is better than zinc coating, hydrogen brittleness of coating is small, it is most suitable for spring as protective layer. However, cadmium is rare, expensive and toxic to the environment. Therefore, it is limited in use. Therefore, most springs used in aviation, navigation and electronic industry only use cadmium coating as protective layer. The thickness of zinc and cadmium coating determines the protection ability.
Zinc plating and cadmium plating of springs are carried out in cyanide electrolyte. In the process of electroplating, in addition to zinc or cadmium, some reduced hydrogen seeps into the lattice of the coating and the base metal, causing internal stress and making the coating and spring on the spring brittle, also known as hydrogen embrittlement. Due to the high strength of spring material and the great deformation of metal spring during forming, it is particularly sensitive to hydrogen embrittlement. If hydrogen is not removed in time, the spring will break. In order to eliminate some defects in the electroplating process, improve the physical and chemical properties of the spring, prolong the service life of the spring, and improve the corrosion resistance of the coating, post plating treatment must be carried out Dehydrogenation treatment. Dehydrogenation is carried out immediately after plating or within a few hours
In addition to zinc plating and cadmium plating, the metal protective layer of spring also includes copper plating, chromium plating, nickel plating, tin plating, silver plating, zinc plating titanium alloy, etc. the manufacturer can choose according to the conditions.