Several protective layers about hardware springs
Hardware springs are generally electroplated to obtain a metal protective layer. Because the electroplating 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. Nickel-plated spring
Zinc and cadmium plating of the spring is carried out in a cyanide electrolyte. In the electroplating process, in addition to zinc or cadmium plating, part of the reduced hydrogen penetrates into the plating layer and the crystal lattice of the base metal. This causes internal stress and makes the plating layer 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 hardware spring during forming, it is particularly sensitive to hydrogen embrittlement. If the hydrogen is not removed in time, it will often cause the spring to break, in order to eliminate some defects generated in the electroplating process To improve the physical and chemical properties of the spring, extend the service life of the spring, and increase the corrosion resistance of the coating, post-plating treatment must be carried out, that is, hydrogen removal treatment. The hydrogen removal treatment is carried out immediately or within a few hours after electroplating. Heat the electroplated spring at a temperature of 200-215°C for 1 to 2 hours (or more than 2 hours) to achieve the purpose of hydrogen removal.
The 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, if the temperature is high, the heating time is long, and the pause time after plating is short, the hydrogen removal effect is good. Therefore, select a higher temperature for removing hydrogen from the spring.
Metal protective layer, in addition to the above-mentioned zinc and cadmium plating, there are 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.
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. Galvanized spring
Generally, the galvanized layer is passivated after plating. 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 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.
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 coatings 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 selected within the range of 6-12μm.