The corrosion of spring can be divided into chemical corrosion and electrochemical corrosion according to the type of reaction.They are the result of changes in the metal atoms on the surface of the spring or the loss of electrons to an ionic state.
If the metal on the surface of the spring simply reacts with the surrounding medium, and the spring causes corrosion, it is called chemical corrosion.For example, the spring is oxidized in the particularly dry atmosphere to form an oxidation film, and the spring is chemically changed with the liquid or impurity in the non-electrolyte liquid, which is a 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 an acid or a salt solution, which is an electrolyte. Due to defects or impurities on the surface of the spring, different potential differences are formed, so that the spring is constantly subjected to electrolytic corrosion.For example, the spring is in the humid atmosphere, because the water vapor in the atmosphere condenses into water film or water bead on the spring surface, and the corrosive gases in the atmosphere (such as sulfur dioxide and hydrogen sulfide in industrial waste gas or salt mist in the ocean atmosphere, etc.) dissolve in water film or water bead to form electrolyte.In addition, impurities or defects of the spring metal can form electrodes with different potential differences, and the spring also produces electrolytic corrosion.These are electrochemical corrosion.
Chemical corrosion of springs is small and slow, while electrochemical corrosion is major and common.But generally speaking, chemical corrosion and electrochemical corrosion exist simultaneously.
The spring is often corroded by the surrounding medium in the process of manufacture, storage and use.As the spring works under the influence of the spring force, the spring force will change and lose its function after being corroded.Therefore, preventing the corrosion of the spring can guarantee the stability of the spring and extend its service life.
The anti-corrosion method of spring generally adopts protective layer, which can be divided into metal protective layer, chemical protective layer, non-metal protective layer and temporary protective layer according to the property of protective layer. The first three methods are emphatically introduced here.
Stainless steel spring and copper wire spring have certain antisepsis ability themselves, do not undertake anticorrosive treatment commonly so.
1. Metal protective layer of spring
There are many types of metal protective coatings. In the case of springs, the metal protective coatings are usually obtained by electroplating.Electroplated protective coating not only protects against corrosion, but also improves the appearance of the spring.Some electroplated metals can also improve spring performance, such as improving surface hardness, increasing wear resistance, improving thermal stability and preventing radiation corrosion.However, if the corrosion of the spring is only for the general electric galvanizing layer and plating cadmium layer.
Zinc in dry air is more stable, almost do not change, do not change color.A white film of zinc oxide or carbon zinc carbonate is formed in moist air.This dense film will prevent further corrosion.Therefore, the galvanizing layer is used for the spring corrosion protection under general atmospheric conditions.Zinc coating should not be used for all springs that are in contact with solutions such as sulfuric acid, hydrochloric acid and caustic soda, as well as springs that work in humid air such as sulfur trioxide.
After passivation, the passivation can improve the protective performance of the coating and increase the appearance of the surface.
In maritime or high temperature of the atmosphere, as well as spring contact with the sea, in the 70 ° C hot water used in the spring, cadmium is more stability, strong corrosion resistance.Compared with zinc coating, cadmium coating is brighter and more beautiful, soft and more plasticity than zinc coating.However, cadmium is rare, expensive, and toxic to the environment.As a result, use is limited.Therefore, most of the springs used in the aviation, navigation and electronics industries only use cadmium plating as a protective layer.
In order to improve the corrosion resistance of cadmium coating, passivation can be carried out after plating.
The thickness of zinc and cadmium coating determines the level of protection.Thickness should generally be selected according to the working environment at the time of use. Hardness of the galvanizing layer is recommended in the range of 6-24 schedule m.The thickness of the cadmium plating layer is recommended within the range of 6~12 mm.
The spring galvanization and cadmium plating are carried out in cyanated electrolyte.In the electroplating process, in addition to zinc or cadmium plating, some of the reduced hydrogen permeates into the plating layer and crystal lattice of the matrix metal, causing internal stress and making the plating layer and spring on the spring brittle, also known as hydrogen embrittlement.Due to the high strength of the spring material and the large deformation of the spring forming process, it is particularly sensitive to hydrogen embrittlement. If it does not remove hydrogen in time, it will often cause the spring to break.In order to eliminate some defects in electroplating process, improve the physical and chemical properties of the spring, extend the service life of the spring, and improve the corrosion resistance of the coating, it is necessary to carry out post-plating treatment, i.e. dehydrogenation treatment.Dehydrogenation is performed immediately or within a few hours after plating.After electroplating of spring in 200 ~ 215 ° C temperature, heat 1 ~ 2 h (or more than 2 h, if the time is too long chromium prone to brittle), can achieve the goal of except hydrogen.
Removal of hydrogen usually takes place in the oven.The effect of hydrogen removal is related to temperature, time and residence time after electroplating.Generally speaking, high temperature, long heating time, short pause time after plating, its dehydrogenation effect.Therefore, the temperature selection of the spring can be higher.
The metal protective layer, in addition to the aforementioned galvanized and cadmium plating, there are copper plating, chromium plating, nickel plating, tin plating, silver plating, galvanized titanium alloy, etc. The spring designer can choose the coating according to the working situation of the spring.
2. Chemical protective layer of spring
A compact protective film is formed on the surface of the spring by chemical reaction to prevent the corrosion of the spring.Generally, oxidation treatment and phosphating treatment are used.
Oxidation treatment and phosphating treatment have low cost and high production efficiency. Generally, spring manufacturers adopt oxidation treatment as corrosion prevention treatment.
(1) the oxidation treatment of oxidized steel is also known as bluing, blackening and boiling black.After oxidation treatment, the surface of the spring produces protective magnetic iron oxide, which is generally blue or black or sometimes dark brown.The color depends on the surface state of the spring, the chemical composition of the spring material and the oxidation process.
The methods of oxidation treatment are: salty oxidation, alkali free oxidation and electrolytic oxidation.Use alkaline oxidation method is much.
Alkaline oxidation is put a spring in around 140 ° C temperature, sodium hydroxide solution containing oxidant soaking time, oxidant and sodium hydroxide with iron, ferrous generate acid sodium and sodium, iron reacts to each other, to generate the magnetic iron oxide.
The thickness of the oxide film is about 0.6-2 depth m.Although the oxide film can improve the corrosion resistance of the spring, it has poor protective ability due to its thin film and air hole, so it can only be used for the spring in the medium with weak corrosion.The anti-corrosion property depends on the density and thickness of the oxide film, which is determined by the concentration of sodium hydroxide, the concentration of oxidant and the temperature of the solution.
In order to improve the corrosion resistance and lubrication ability of the oxide film, the treatment before and after oxidation treatment should be strengthened.Prior to the oxidation treatment, the spring surface rust, oxide skin, oil pollution, heat treatment of salt slag, surface contact layer and other thoroughly cleared.After oxidation treatment, the spring is usually filled in soap solution or bichromate, and then washed, blown dry or dried with flowing warm water. Finally, the rust prevention oil or mechanical oil at a certain temperature is replaced by the water film for immersion treatment.
The surface grain boundary of some hot-rolled spring materials has been eroded by oxidation treatment, which can reduce fatigue strength to a certain extent.
Conventional oxidation treatments require heating.In recent years, some domestic manufacturers also use room temperature blackening agent, which overcomes the defect of traditional blackening process and saves a lot of energy.
The blackening agent is concentrated and liquefied blue-green, without impurity, peculiar smell, fire, explosion and corrosion, and safe transport.
The dilution of the blackening agent with water, and the dilution ratio of the spring darkening is about 1 times 5.
The operation process of blackening agent is relatively simple, and its process route is: oil removal, rinsing, acid row, rinsing, blackening (2-5min at room temperature) rinsing, water film replacement anti-rust oil.It should be noted that the oil on the spring workpiece must be completely cleaned before it can be put into room temperature agent for oxidation treatment.
Oxidation treatment has low cost, simple process formulation, high production efficiency, and a certain elasticity of the oxide film, which basically does not affect the spring's characteristic curve. Therefore, oxidation treatment is widely used as anti-corrosion and decoration measures for forming spiral spring, spring washer and sheet spring.
The quality inspection of the spring after oxidation treatment includes appearance inspection and corrosion resistance inspection.
(2) phosphorization: the process of putting springs into a phosphate solution containing manganese, iron and zinc to form a layer of water-insoluble phosphate film on the metal surface is called phosphorization.The appearance of phosphating film is dark gray, gray or black gray, not shiny.The thickness of the phosphating film is usually 5~20 meters.Phosphating film is stable under general atmospheric conditions, and its corrosion resistance is 6~24 times that of bluing film.Phosphating film has micropore structure and good adsorption capacity for paint and oil.After phosphating, dichromate filling, oil soaking or painting can further improve its corrosion resistance.Phosphating film is stable in animal oil, vegetable oil and mineral oil, and also stable in some organic solutions (such as benzene and toluene).
Phosphating film can withstand temperatures up to 400 ~ 500 ° C, as a result, some of the work under the high temperature of the spring, such as shell part of spring that common phosphating treatment.
The spring had better be sandblasted before phosphating.After sandblasting treatment can not wait too long, should be immediately phosphate.If there is no sandblasting equipment, chemical oil removal and pickling can also be used to remove oil.The phosphating spring produces a lot of hydrogen in the process of phosphating, so the phosphating spring has hydrogen embrittlement.For the key part of the spring, after phosphating treatment should be dehydrogenated.
3. Non-metal protective layer of spring
Non-metal protective coating is a coating of organic material, such as paint, asphalt, plastics, etc. on the surface of the spring to protect the spring from corrosion.
Non-metal protective layer, thick film layer, good chemical stability, good mechanical corrosion protection, but low hardness, easy to scratch damage, and aging of film layer.
The paint used for the spring coating is usually of the following types:
A.Asphalt paint has good water resistance, moisture resistance, corrosion resistance, especially excellent acid resistance and good alkali resistance.But adhesion, mechanical strength, poor decorative.
B.Phenolic paint phenolic paint can be divided into two types: primer and topcoat. Phenolic primer has strong adhesion, good rust resistance, but poor mechanical strength and gloss.Phenolic coating film is hard, gloss good, but poor weather resistance, easy to yellow.
C.Alkyd paint alkyd paint film is tough, strong adhesion, good mechanical properties, excellent gloss, good durability and a certain oil resistance, insulation performance.Its drawback is that the surface dry fast and long adhesive time, easy wrinkle, water resistance, alkali resistance.
D.Epoxy paint epoxy paint has strong adhesion, high hardness, good toughness, flexural resistance, impact resistance, hard but not brittle, excellent resistance to water, acid, alkali and many organic solvents, especially alkali resistance.The disadvantages are fast surface flour and high solvent selectivity.Water-soluble epoxy paint for electrophoresis coating.
In general, the paint layer can be used alone or as a phosphide stain.Sometimes springs are distinguished by applying different colors of paint in order to classify them according to their loads.The paint of which type and brand should be chosen according to the working environment.If necessary, it shall be indicated in the spring pattern.
There are many methods of construction of paint, most of which are used for springs in the following ways:
A.Dip coating is to dip the spring into the paint tank, then take out, let the surface of the excess paint natural dripping, after drying on the surface of the spring covered with a paint film.
The dip-coating method has high production efficiency and can be applied to mechanization and automation production.And the technology is simple, operation is convenient.But the paint volatilizes quickly, the paint that contains heavy quality pigment and two component paint material (amine solidify epoxy paint, polyurethane paint etc.) should not be used.Dip coating film is not smooth enough, easy to produce thin and thick, edge flow.
B.Spray painting is to use the spray gun to spray paint into mist particles on the spring surface uniform deposition of a layer of paint film.
Spraying method has high efficiency, convenient construction and strong adaptability, and the film thickness is relatively uniform, smooth and smooth.However, the effective utilization rate of spraying method is only about 70%~80%, and more solvents are needed than other methods.In addition, the diffusion of paint mist particles and volatile solvent, resulting in environmental pollution, affecting the health of workers.
C.Electrostatic spraying applies thermosetting powder evenly on the surface of the spring at the moment when it passes through the orifice.Then spray spring is 150 ~ 180 ° C temperature in the oven heat insulation (plastic), to form a smooth solid surface protective layer, general heat preservation time of 40 ~ 60 min.
This process has high efficiency, suitable for flow production, good product quality, uniform film thickness, adhesion and other advantages.At present, this method is widely used in spring manufacturing industry.
D.In order to improve the service life of the spring, the surface corrosion prevention treatment, such as the valve spring, is changed to replace the traditional oxidation or coating treatment with special rust prevention oil (grease) after the spring cleaning.
In recent years, the surface of the spring has been treated by the spring designers.The finishing (finishing) process is to put the spring directly into the tilting centrifugal type or spiral vibrating type, eddy current type and other series of finishing machines, and add appropriate grinding materials, grinding agent and water for finishing.General optical finishing is about 20~30min, specific optical finishing time depends on the spring shape and the amount of device.After finishing the finish, remove the spring and rinse it with tap water, then immerse in the SM series water film replacement anti-rust oil for several minutes before removing.At this time, a 5 rust-proof oil film is attached to the surface of the spring, which protects the spring from rust.This treatment method largely feeds traditional complicated processes, such as oil removal, cleaning, pickling, hydrogen removal and so on.At present, the spring adopting light decoration (integral) technology is: plunger spring for oil pump and fuel injector pressure adjusting spring;Support spring for refrigerator compressor, etc.The comparison of the performance (life) and corrosion resistance between the photofinishing (finishing) treatment and the conventional oxidation or coating treatment of the spring, without further test and practice, a more reliable conclusion can be drawn.