The corrosion of springs can be divided into chemical corrosion and electrochemical corrosion according to the type of reaction. They are all the result of the change of metal atoms on the surface of the spring or the gain or loss of electrons into an ionic state.

If the metal on the surface of the spring simply reacts chemically with the surrounding medium, and the spring causes corrosion, it is called chemical corrosion. For example, the spring is oxidized in a particularly dry atmosphere to form an oxide film, and the spring chemically changes with the liquid or impurities in the liquid in a non-electrolyte liquid, which belongs to chemical corrosion.

If the spring is in contact with the electrolyte solution, the corrosion caused by the action of the micro battery is called electrochemical corrosion. For example, the spring is in contact with acid or salt solutions. These solutions are electrolytes. Due to defects or impurities on the surface of the spring, electrodes with different potential differences are formed, so that the spring is continuously electrolytically corroded; for example, the spring is in a humid atmosphere. The water vapor in the atmosphere condenses into a water film or water droplets on the surface of the spring, and the corrosive gases in the atmosphere (such as sulfur dioxide and hydrogen sulfide in industrial waste gas or salt spray in the marine atmosphere, etc.) dissolve in the water film or water. An electrolyte is formed in the beads. In addition, the impurities or defects of the spring metal can also form electrodes with different potential differences, and the spring also produces electrolytic corrosion. These are all electrochemical corrosion.

The chemical corrosion of the spring is small and slow, while the electrochemical corrosion is the main and common. But generally speaking, chemical corrosion and electrochemical corrosion exist at the same time.

Springs are often corroded by surrounding media during manufacturing, storage, and use. Since the spring works by elastic force, the elastic force will change after the spring is corroded and lose its function. Therefore, preventing the corrosion of the spring can ensure the stable operation of the spring and prolong its service life.

The anti-corrosion method of the spring generally adopts the protective layer. According to the nature of the protective layer, it can be divided into: metal protective layer, chemical protective layer, non-metallic protective layer and temporary protective layer, etc. The first three methods are introduced here.

Stainless steel springs and copper wire springs themselves have a certain anti-corrosion ability, so generally do not carry out anti-corrosion treatment.

1. Metal protective layer of spring

There are many types of metal protective layers. As far as springs are concerned, gold plating is generally used to obtain metal protective layers. The electroplated protective layer not only protects from corrosion, but also improves the appearance of the spring. Some electroplated metals can also improve the working performance of the spring, such as increasing surface hardness, increasing wear resistance, improving thermal stability, preventing radiation corrosion, etc. However, if it is purely for the corrosion of the spring, the electroplated zinc layer and the electroplated cadmium layer should generally be used.

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 will form in humid air. This dense film prevents further corrosion. Therefore, the galvanized layer is used as a corrosion protection layer for springs under normal atmospheric conditions. All springs that are in contact with solutions such as sulfuric acid, hydrochloric acid, and caustic soda, as well as those that work in humid air such as sulfur trioxide, should not be coated with zinc.

Generally, after the galvanized layer is passivated, passivation can improve the protection performance of the coating and increase the appearance of the surface.

Cadmium is relatively stable and has strong corrosion resistance for springs used in oceanic or high temperature atmospheres, as well as springs in contact with seawater, and springs used in hot water at 70°C. The cadmium coating is brighter and more beautiful than the zinc coating, the quality is softer, and the plasticity is better than that of the zinc coating. However, cadmium is scarce, expensive, and highly toxic, causing serious environmental pollution. Therefore, it is restricted in use. Therefore, most springs used only in the aviation, marine and electronic industries use cadmium plating as a protective layer.

In order to improve the corrosion resistance of cadmium coating, passivation treatment can be carried out 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 hardness of the galvanized layer is recommended to be selected in the range of 6~24μm; the thickness of the cadmium layer is recommended to be selected in the range of 6~12μm.

The zinc and cadmium plating of the spring is carried out in a cyanide electrolyte. During the electroplating process, in addition to zinc or cadmium plating, a part of the reduced hydrogen penetrates 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 the spring material and the large deformation when the spring is formed, it is particularly sensitive to hydrogen embrittlement. If the hydrogen is not removed in time, the spring will often break. In order to eliminate some defects generated 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, that is, hydrogen removal treatment, must be carried out. The hydrogen removal treatment is carried out immediately or within a few hours after electroplating. The purpose of removing hydrogen can be achieved by heating the electroplated spring at a temperature of 200~215°C for 1~2h (or more than 2h, if the time is too long, chromium embrittlement may easily occur).

The removal of hydrogen is generally carried out in an oven. The hydrogen removal effect is related to temperature, time, residence time after electroplating, etc. Generally speaking, the higher the temperature, the longer the heating time and the shorter the pause time after plating, the better the hydrogen removal effect is. Therefore, the selection of the dehydrogenation temperature for the spring can be higher.

The metal protective layer, in addition to the above-mentioned zinc and cadmium plating, also has copper plating, chrome plating, nickel plating, tin plating, silver plating, galvanized titanium alloy, etc. The spring designer can choose the plating layer according to the working occasion of the spring.

2. The chemical protective layer of the spring

A dense protective film is formed on the surface of the spring by chemical reaction to prevent the spring from corroding. Usually, oxidation treatment and phosphating treatment are used.

Oxidation treatment and phosphating treatment have low cost and high production efficiency. Generally, spring manufacturers use oxidation treatment as anti-corrosion treatment.

(1) Oxidation treatment The oxidation treatment of steel is also known as bluing, blackening, and boiling black. After the oxidation treatment, a protective magnetic iron oxide is formed on the surface of the spring. This oxide film is generally blue or black, and sometimes dark brown. Its color depends on the surface state of the spring, the chemical composition of the spring material and the oxidation treatment process.

The methods of oxidation treatment include: salty oxidation method, alkali-free oxidation method and electrolytic oxidation method. The most common is the alkaline oxidation method.

The alkaline oxidation method is to put the spring into a temperature of about 140 ° C, soak it in a sodium hydroxide solution containing an oxidant for a certain period of time, and the oxidant and sodium hydroxide react with iron to generate sodium ferrite and sodium ferrite, and then react with each other, Generates magnetic iron oxide.

The thickness of the oxide film layer is about 0.6~2μm. Although the oxide film can improve the corrosion resistance of the spring, because the film is thin and has pores, its protection ability is poor, and it can only be used for springs that work in less corrosive media. The level of its anti-corrosion performance depends on the compactness of the oxide film and its thickness, which is also determined by factors such as the concentration of sodium hydroxide, the concentration of oxidant, and the temperature of the solution in the oxidation treatment.

In order to improve the corrosion resistance and lubricating ability of the oxide film, the treatment before and after the oxidation treatment should be strengthened. Before the oxidation treatment, the rust, scale, oil, heat-treated salt residue, surface contact layer, etc. on the surface of the spring must be thoroughly removed. After the oxidation treatment, the spring is usually filled with soap solution or dichromate, then washed with flowing warm water, blown dry or dried, and finally the water film is used to replace the anti-rust oil or the mechanical oil of a certain temperature. Oil immersion treatment.

Oxidation treatment erodes the surface grain boundaries of some hot-rolled spring materials, which will reduce the fatigue strength to a certain extent. Therefore, care should be taken when using oxidation treatment.

The traditional oxidation treatment requires heating. In recent years, some domestic manufacturing units also use room temperature blackening agent, which overcomes the defects of traditional blackening process and saves a lot of energy.

The blackening agent is blue-green concentrated liquefaction, free of impurities, no peculiar smell, non-flammable, non-explosive, non-corrosive, and safe to transport.

The blackening agent is diluted with water, and the commonly used dilution ratio is about 1:5 when the spring is blackened.

The operation process of the blackening agent is relatively simple, and its process route is: degreasing, rinsing, acid running, rinsing, blackening (2~5min at room temperature) rinsing, and water film replacement of anti-rust oil. It should be noted that the oil on the spring workpiece must be completely removed, and after cleaning, it can be placed in the normal temperature agent for oxidation treatment.

The oxidation treatment has low cost, simple process formula, high production efficiency, and the oxide film has a certain elasticity, which basically does not affect the characteristic curve of the spring. Decorative measures.

The quality inspection of the spring after oxidation treatment includes appearance inspection and corrosion resistance inspection.

(2) Phosphating treatment The method of placing the spring in a phosphate solution containing manganese, iron and zinc to form a water-insoluble phosphate film on the metal surface is called phosphating. The appearance of phosphating film is dark gray, gray or dark gray, without gloss. The thickness of the phosphating film is generally 5~20μm. The phosphating film is relatively stable under general atmospheric conditions, and its corrosion resistance is 6-24 times that of the blue-emitting film. The phosphating film has a microscopic pore structure and has a good adsorption capacity for paints and oils. Therefore, phosphating is often used in conjunction with coating methods such as painting. Dichromate filling, oil immersion or painting after phosphating can further improve its corrosion resistance. Phosphating film is relatively stable in animal oil, vegetable oil, mineral oil, and in some organic solutions (such as benzene, toluene).

Phosphating film can withstand high temperature of 400~500°C. Therefore, some springs that work at high temperatures, such as the springs of the projectile launching part, are often treated with phosphating.

The spring is preferably sandblasted before phosphating. After sandblasting, it should not be left for too long, and phosphating should be carried out immediately. If there is no sandblasting equipment, chemical degreasing and pickling methods can also be used to remove oil stains. The spring produces a lot of hydrogen during the phosphating process, so the spring after phosphating has the phenomenon of hydrogen embrittlement. For springs in key parts, dehydrogenation treatment should be carried out after phosphating treatment.