Protective layer of the spring 
There are many types of metal protective layers. For springs, electroplating is generally used to obtain a metal protective layer. The electroplated protective layer not only protects against corrosion but also improves the appearance of the spring. Some electroplated metals can also enhance the working performance of the spring, such as increasing surface hardness, improving wear resistance, enhancing thermal stability, and preventing radiation corrosion. However, if the sole purpose is to protect the spring from corrosion, it is generally recommended to use a zinc plating layer and a cadmium plating layer.
    Zinc is relatively stable in dry air, hardly changes, and is not easily discolored. In humid air, a white film of zinc oxide or basic zinc carbonate forms. This dense film can prevent further corrosion. Therefore, the galvanized layer is used as a corrosion protection layer for springs under general atmospheric conditions. Springs that come into contact with solutions such as sulfuric acid, hydrochloric acid, caustic soda, and those working in humid air with atmospheres like sulfur trioxide should not use zinc plating.
    Generally, the galvanized layer undergoes passivation treatment after plating, which can enhance the protective performance of the coating and improve surface aesthetics. 
    In marine or high-temperature atmospheres, as well as springs in contact with seawater, and springs used in 70℃ hot water, cadmium is relatively stable and has strong corrosion resistance. The cadmium plating layer is brighter and more aesthetically pleasing than the zinc plating layer, softer, and has better plasticity than zinc. The hydrogen embrittlement of the coating is low, making it most suitable as a protective layer for springs. However, cadmium is rare, expensive, and its salts are highly toxic, causing significant environmental pollution. Therefore, its use is restricted. Most springs that use cadmium plating as a protective layer are only found in aviation, navigation, and electronic industries. 
    To improve the corrosion resistance of the cadmium plating layer, passivation treatment can be performed after plating. 
The thickness of the zinc and cadmium plating layers determines the level of protection. The thickness should generally be chosen based on the working environment during use. It is recommended that the thickness of the galvanized layer be in the range of 6 to 24 μm, while for the cadmium layer, a thickness in the range of 6 to 12 μm is suggested.
The galvanizing and cadmium plating of springs are carried out in cyanide electrolytic solutions. During the electroplating process, in addition to plating zinc or cadmium, some reduced hydrogen also permeates into the lattice of the plating and the base metal, causing internal stress, making the plating and the spring brittle, which is also called hydrogen embrittlement. Due to the high strength of spring materials and the significant deformation during spring formation, they are particularly sensitive to hydrogen embrittlement. If hydrogen is not removed in time, it often leads to spring fracture. 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 enhance the corrosion resistance of the coating, post-plating treatment, namely hydrogen removal treatment, must be performed. The hydrogen removal treatment is carried out immediately after electroplating 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 performed in an oven. The effectiveness of hydrogen removal is related to temperature, time, and the duration of the electroplated spring's stay. Generally speaking, the higher the temperature, the longer the heating time, and the shorter the post-plating pause time, the better the hydrogen removal effect. Therefore, the temperature selection for hydrogen removal of springs can be higher. 
    In addition to the aforementioned zinc and cadmium plating, there are also copper plating, chrome plating, nickel plating, tin plating, silver plating, and zinc-titanium alloy plating. Spring designers can choose the coating based on the working conditions of the spring.