The corrosion of springs can be divided into chemical corrosion and electrochemical reaction according to the types reflected. They are all the result of the transformation of metal material molecules on the surface of the spring or the gain and loss of electronic devices into positive ions.
If the metal material on the surface of the spring only produces chemical changes with the surrounding materials, and the corrosion caused by the spring is called chemical corrosion. For example, the spring is oxidized and converted into an air oxide film in particularly dry air, and the spring reacts with the liquid or the residue in the liquid in a non electrolyte liquid, which belongs to chemical corrosion.
If the spring touches the acid and alkali of the solution, the etching caused by the action of the micro battery is called electrochemical reaction. For example, the spring is in contact with acid-base or acid salt water solution, which is all electrolyte solution. Because of the defects or residues on the surface of the spring, the electric level with different potential difference will be generated, and the spring will continue to be eroded by electrolysis; Another example is that the spring is in wet and cold air, because the water vapor in the air turns into shrinkage water or water droplets on the surface layer of the spring, and the corrosive vapor in the air (such as sulfur dioxide and hydrogen chloride in organic exhaust gas or corrosion resistance in deep sea air) melts into the shrinkage water or water droplets to generate electrolyte solution. In addition, the residues or defects of the spring metal materials can also produce electric levels with different potential differences, and the spring also causes electrolytic etching. This is an electrochemical reaction.
The chemical corrosion of spring is small and slow, while the electrochemical reaction is critical and extensive. But generally speaking, chemical corrosion and electrochemical reaction exist at the same time.
In the whole process of production, storage and application, springs are often eroded by surrounding materials. Because the spring is fully exerted by its elasticity during operation, the elasticity will change and lose its function after the spring is etched. Therefore, avoiding the corrosion of the spring can ensure the smooth operation of the spring and increase its service life.
Generally, the protective layer is selected as the anti-corrosion method of the spring. According to the characteristics of the protective layer, it can be divided into metal material protective layer, organic chemical protective layer, non-metal material protective layer and transient protective layer. Here, the first three methods are mainly introduced in detail.
The stainless steel plate spring and the copper core wire spring have certain anti-corrosion work ability, so generally no anti-corrosion solution is carried out.
Metal material protective layer of spring
There are many types of metal material protective layer. As far as the spring is concerned, the metal material protective layer is generally obtained by electro gold plating. The protective layer of electroplating process can not only protect against corrosion, but also improve the appearance of the spring. Some plating process metal materials can also improve the working characteristics of the spring, such as improving the surface strength, improving the wear resistance, improving the heat resistance, and avoiding radiation corrosion. However, if the simplicity is to better etch the spring, the phosphating treatment layer and the plating process cadmium layer should be generally used.
Zinc is relatively stable in dry air, hardly changes and is not easy to fade. It will be converted into a layer of active zinc oxide or carbon type zinc carbonate milky white plastic film in wet and cold air. This layer of high-density plastic film can prevent re etching. Therefore, the hot-dip galvanized layer is used as the corrosion protection layer of the spring under the general air standard. Zinc coating is not suitable for springs that contact with aqueous solutions such as hydrochloric acid, sulfuric acid and anhydrous aluminum chloride, and work in humid and cold air with sulfur trioxide and other atmospheres.
Generally, the hot-dip galvanized coating is passivated by stainless steel. The passivation treatment can improve the maintenance characteristics of the coating and the appearance of the surface layer.
In the marine climate or high-temperature air, and the springs that touch the sea surface, the springs that are applied in 70 ° C boiling water have relatively stable cadmium and strong corrosion resistance. Cadmium coating is brighter and more beautiful than zinc coating. It is soft and has better ductility than zinc coating. The coating has less alkali brittleness. Z is suitable for spring as protective layer. However, cadmium is scarce and expensive, and the toxic side effects of cadmium salt are large, which causes serious air pollution. And thus is limited in application. Therefore, most of the springs commonly used in Airlines, ocean voyage and electronic industry only use cadmium coating as the protective layer.
In order to improve the corrosion resistance of cadmium coating, stainless steel passivation can be carried out after plating.
The thickness of zinc and cadmium coating determines the maintenance capacity. The thickness shall be selected according to the office environment during application. The strength of hot-dip galvanized layer is strongly recommended to be 6 ~ 24 μ M category; The thickness of cadmium plating layer is strongly recommended to be 6 ~ 12 μ M.
Hot dip galvanizing and cadmium plating of springs are carried out in the electrolyte of lithium cyanide battery. In the whole process of electroplating, in addition to zinc or cadmium, some recovered hydrogen permeates into the lattice constants of the coating and base metal materials, resulting in thermal stress, which makes the coating on the spring and the spring brittle, also known as alkali embrittlement. Because of the high compressive strength of the raw material of the spring and the large deformation of the spring during forming, it is particularly sensitive to alkali embrittlement. If the hydrogen is not removed immediately, the spring will usually be broken. In order to better eliminate some shortcomings caused by the whole process of electroplating, improve the physical characteristics of the spring, increase the service life of the spring, and improve the corrosion resistance of the coating, it is necessary to carry out the post-treatment process, that is, hydrogen removal. The hydrogen removal solution is carried out immediately or within a few hours after the electroplating process. After the plating process, the spring is heated for 1 ~ 2H at a temperature of 200 ~ 215 ° C (or above 2h, if the time is too long, it is very easy to cause chromium embrittlement), so as to achieve the goal of hydrogen removal.
Hydrogen removal is generally carried out in the drying oven. The actual effect of hydrogen removal is related to temperature, time and residence time after electroplating process. Generally speaking, high temperature, long heating time, and short interruption time after plating can achieve the actual effect of hydrogen removal. Therefore, the hydrogen removal temperature of the spring can be selected higher.
In addition to the above hot-dip galvanizing and cadmium plating, the protective layer of metal materials also includes copper plating, stainless steel, nickel plating, tin plating, gold plating, hot-dip galvanized aluminum alloy, etc. the spring designer can select the coating according to the working place of the spring.
