Modern anticorrosion technology of fasteners
Metal is affected by the environment, and the destructive damage caused by chemical or electrochemical reaction is called corrosion. Almost all metal products will have some forms of corrosion in a certain environment. Screw is a metal product, so the problem of metal corrosion can’t be avoided. The corrosion phenomenon also has obvious differences when it is used in different environment and time.
Taiwan, for example, has a narrow terrain, surrounded by the sea, and has an island climate. It has extremely high salinity and humidity in the atmosphere. There are also many polluting industries, such as steel, petrochemicals and thermal power generation. In addition, in recent years, the number of cars has increased dramatically, and a large number of exhaust gases have caused an increase in the proportion of corrosive sulfide and nitride gases and particles in the air The main reason of metal corrosion. For example, the corrosion severity of screws, bolts, structural steel, pipelines and other metals of Formosa Plastics Liuqing Mailiao plant, which is located at the seaside of Yunlin, Taiwan, may have exceeded the original estimate before the plant was built, forming a problem that engineers must pay attention to and give priority to.
Metal corrosion will cause huge equipment maintenance costs and serious economic losses, such as increased design, manufacturing, maintenance and repair costs due to corrosion, and even harm and disaster.
Screws have a wide range of applications and a large number of demands. Their corrosion resistance determines their durability and has a significant impact on the safety and reliability of equipment. Therefore, it is very necessary and meaningful to study the corrosion protection methods of screws and other fasteners and develop and apply modern corrosion protection technology.
Corrosion protection method
There are four main methods for metal corrosion protection, namely, control the material itself, the interface between the material and the environment, and the design of the improved metal structure. If the screw is made of completely corrosion-resistant alloy, it is not practical from the economic aspect unless special needs are needed; or the environmental factors that cause corrosion on the screw surface are completely isolated, which is actually the case It’s also very difficult.
The modified metal structure design can improve the influence of special conditions in some cases, but most screw designs cannot be fully modified and their protective effect is not permanent. Therefore, this method can not solve the problem fundamentally. Only effective protective measures are taken on the interface, that is, surface corrosion treatment is the most widely used method at present.
Surface corrosion treatment refers to the application of various methods to apply protective layer on the metal surface. Its function is to isolate the metal from the corrosive environment to inhibit the corrosion process, or reduce the contact between corrosive medium and metal surface, so as to prevent or reduce the corrosion.
The protective layer shall be able to meet the following requirements:
- Corrosion resistance, wear resistance, high hardness;
- The structure is compact, complete and the pore is small;
- It is firm and has good adhesion with base metal;
- It is evenly distributed and has a certain thickness.
The protective layer is usually divided into two categories: metal coating and non-metallic coating. Metal coating refers to the formation of protective layer on the surface of metal with strong corrosion resistance, which is also called coating. There are many methods and types of metal coating, among which the most common method is electroplating, followed by molten metal dip (hot dip) and chemical surface treatment.
Nonmetallic coating refers to the use of organic polymer materials such as paint, and inorganic materials such as ceramics, to form a protective layer on the surface of metal equipment or parts. The protective layer can completely isolate the base metal from the environmental medium to prevent the corrosion caused by contacting the corrosion medium.
Electroplating refers to the coating on the surface of the substrate by electrolysis with the material or object as the cathode in the solution containing the metal ions to be plated. In the process of electrolysis, chemical reactions occur on the interface between the electrode and electrolyte, oxidation reaction occurs in the anode release electrons and reduction reaction occurs in the cathode absorption electrons.
Electroplating can cover a uniform and good binding coating for materials or articles, so as to change its surface characteristics and appearance, and achieve the purpose of material protection or decoration. Direct current (low voltage and high current) must be used in electroplating. Zinc, cadmium, copper, nickel and other metals are commonly used for corrosion-resistant coatings or zinc, nickel, nickel tin, nickel copper, copper and zinc alloys. According to the electrode potential difference when the coating and the base metal form the couple, the anode coating and the cathode coating can be divided into two types.
In corrosion medium, the potential of anode coating is more negative than that of base metal. For example, zinc plating or cadmium on steel. When the coating is damaged, the coating will be dissolved as anode, which has the electrochemical protection effect on steel, that is, the “sacrificial” protection effect. The thicker the coating, the greater the protection effect.
The potential of cathode coating is corrected compared with base metal. For example, aluminum, copper and nickel are plated on steel. When the coating is damaged, the corrosion of exposed parts of the base metal will be accelerated. Therefore, the coating should pay attention to its tightness and no damage. It can only protect the coating when it is intact. Moreover, the protective ability depends on its porosity, low porosity and great protective effect. The corrosion resistance effect is to use the plating The layer isolates the external environment, so that the corrosion medium can not penetrate into the base metal.
Hot dip coating is a method of immersing the plated part into molten metal solution to form coating on its surface. The melting point of the hot dip coating metal is much lower than that of the base metal, such as zinc, aluminum, lead and its alloy.
The coating can be obtained by hot dip method, and few holes are available, which has better corrosion resistance compared with other plating layers. In the same corrosion environment, the service life of the hot dip coating is longer and has strong binding force.
But it is difficult to control the thickness of the coating. Generally, it is easier to obtain the coating thickness of 50g/m2 by hot dip coating method, while the thickness of the coating below 50g/m2 can be obtained economically by electroplating.
Successful hot dip coating must combine the underlying metal with the coating metal to form alloy, not only the coating metal covering the base metal. In the crystal structure, the hot dip coating has at least two layers, the inner layer is alloy diffusion layer and the outer layer is the coating. The outer layer is basically the same as the coating metal, but the inner diffusion layer is usually hard and brittle, and it may be easy to peel or crack after bending. Therefore, the deformation procedure that the processing part can bear depends on the metal, coating and other factors used. Therefore, hot dip coating is usually carried out after deformation processing.
Zinc is cheaper than other electroplating metals, and its corrosion resistance is far better than steel in most cases. Therefore, zinc coating can protect the steel exposed in discontinuous parts, holes, cracks and cutting edges.
Therefore, the general screws or screws prone to atmospheric corrosion are galvanized, but the screws are not regular in shape or smooth in surface as the plate or rod. Therefore, the thickness of galvanized layer is difficult to keep uniform in each part, and the head is the thickest, the front end of the screw hill is second, and the thread Valley is the thinnest.
In order to meet the large production of screws, reduce costs and increase production, electroplating is usually used to galvanize the barrel. This method is to pass the current from the electrode of the barrel holder into the lead in the barrel, and the wire contacts the screw and conducts electricity. Meanwhile, the drum turns the mixing screw to make the screw and the electrode contact evenly. However, the current is intermittent, so it is difficult to form a uniform current density, so the thickness of zinc plating is easy to be different.
The two kinds of electroplate can be used for galvanizing. Zinc sulfate is the main component of the acid electroplating solution, which is cheap and efficient, and the plating speed is fast and easy to manage. It is also less likely to cause hydrogen embrittlement than the alkaline electroplating solution, but the crystallization is slightly coarse, the luster is not good, and the ability of plating is poor.
The alkaline plating solution is mainly composed of zinc cyanide, although the price is high, but the ability of plating is good. If the management is right, the luster coating can be obtained. If chromate treatment is further implemented, the brighter coating can be obtained and the corrosion resistance is good.
Hot dip galvanizing is also used for the surface corrosion treatment of screws, bolts and nuts. The screws with hot dip galvanizing have better corrosion resistance than those of galvanized screws, but the cost is higher, and the bolts with high strength are not suitable for hot dip galvanizing, because the coating surface is easy to form hard and brittle, and fracture may occur in high temperature or high load conditions.
The durability of zinc coating in corrosive environment is linear with its thickness, that is, the thicker the coating has longer life, but it is poor in sulfide containing industrial area. At the same time, a large number of corrosive products will be produced in marine and tropical climate, and must be prevented by chromate treatment.
In the United States, researchers have used thin galvanized test pieces with film thickness of 0.03mm to test. The results show that the average life of zinc coating in rural, coastal and industrial areas is about 3:2:1, indicating that in polluted air, the zinc coating is most likely to corrode, because the waste gas discharged by factories in industrial areas contains many sulfur oxides (SOx) and nitrogen oxide (NOx) which will destroy the zinc coating; in coastal areas, the zinc coating is destroyed by the presence of sulfur oxides (SOx) and nitrogen oxides (NOx); in coastal areas, the average life of zinc coating is about 3:2:1, indicating that in There are a lot of salt crystallization and liquid particles (chlorides) in the atmosphere, which will cause metal corrosion, and the relative humidity of the atmosphere will also significantly affect the corrosion rate.
The Institute of material protection of Beijing University of science and technology has also conducted the atmospheric exposure corrosion test of zinc coating in different regions of the mainland. The test period is one year. The results show that the highest corrosion rate is in the area with the highest SO2 and chloride (sea salt particles) content and the highest relative humidity in the atmosphere. The relative humidity of the atmosphere is considered to be the main factor of corrosion, and the pollutants in the atmosphere, especially SO2 and chloride, are the main factors to promote corrosion.
During the process of production, storage and transportation, the galvanized layer of galvanized screw may be white, dark and blackened, which is mainly caused by corrosion caused by the contact between galvanized layer and corrosive medium and water, oxygen and carbon dioxide in the atmosphere.
To prevent this phenomenon, the materials with better surface condition can be selected before screw processing. In the process of galvanizing, the defects on screw surface are easy to infiltrate corrosion media such as acid, alkali and salt, which is difficult to clean and lead to corrosion.
Stress concentration should be reduced as much as possible during screw processing, because the stress concentration area is easy to produce corrosion during pickling and derusting. The thickness of oxide film should be minimized during heat treatment of screws. Especially, the oil and rust on the surface must be removed thoroughly before plating, and the time of pickling and rust removal should be properly controlled to avoid affecting the electroplating quality.
During electroplating, attention should be paid to the selection of galvanized solution with good deep plating capacity, strengthening the maintenance and management of bath solution, and properly controlling the current density and additive dosage. After plating, it is necessary to thoroughly clean to eliminate the harm of electroplating solution and organic matter to the coating. In addition, the exposure time of screws in the atmosphere should be shortened as far as possible and sealed as soon as possible.
Packaging materials should be selected reasonably to avoid the use of plastic products incompatible with zinc. The best packaging is metal materials such as galvanized iron sheet. Appropriate desiccant can be put into the packing box to reduce the humidity in the box. The relative humidity of the warehouse shall be reduced and harmful gas erosion shall be prevented.
Currently, there is a metal surface treatment called longzinc (dacrust) which is suitable for screws and nuts. The treatment method is to dip degreased and derusting screws and nuts into the Dacron rust rust inhibitor tank, or form a protective film of zinc and chromium compounds on the metal surface by spray plating, and then dry it. After cooling, it is treated again. The film thickness is about 5-8 μ m, which is silvery gray. According to the data provided by the manufacturer, the coating obtained by this method has good rust prevention and corrosion resistance characteristic.
Adding aluminum into zinc can improve the corrosion resistance of the coating. When the content of aluminum is 30%, the corrosion resistance of zinc aluminum alloy is the best, so the chemical stability and corrosion resistance of the coating are better than that of zinc plating alone.
Chemical surface treatment
Metal alloy can form protective surface coating, i.e. protective film or chemical conversion film under controlled conditions in proper chemical solution. Because of the chemical treatment, the surface metal is transformed into a combination of compound and underlying metal, so it produces a surface layer with resistance, which is a part of the underlying metal in physical and chemical properties.
The protective film is mainly used for:
- Improve the stability of oxide film on metal surface, and prevent metal or coating metal from further corrosion in the atmosphere;
- Reduce the corrosion of metal to the lowest degree;
- Improve the viscosity and shrinkage of paint and coating.
The common chemical surface treatment includes phosphate treatment, chromate treatment and oxidation treatment.
Phosphate treatment is to put metal into a solution containing phosphate for chemical treatment, which makes the metal surface form a phosphate film which is difficult to dissolve in water and has good adhesion, that is, phosphate film. It is mainly used as the base layer of paint and the adsorption layer of lubricant during cold metal processing.
Because phosphate film is composed of a series of zinc, manganese and iron phosphate salts and phosphate crystals of different sizes, the film is porous, with large surface area and good adsorption, and can absorb paint or a large number of lubricants. Therefore, it is often used as the bottom layer of paint, or cold stamping to reduce friction and cold drawing of steel pipe and steel wire.
Phosphate film is not soluble in water, and is stable in the atmosphere, and can be used as a protective layer with low rust protection requirements. If the phosphate film is sealed, soaked or painted, the corrosion resistance of phosphate film can be further improved.
The protective system composed of phosphate film and nitro paint or platform resin paint has a very good corrosion resistance to steel. When adjusting the viscosity of paint, the paint can freely penetrate through the film hole. The drying temperature of the film should be between 150 ℃ and 180 ℃, and it is better to dry in the oven.
When phosphate film is used for cold deformation processing and reducing friction surface wear, the application effect of the coating can be improved by immersion with oil or soap lubricating material. The most convenient and effective method is to impregnate in potassium soap solution. The most widely used phosphate film is iron phosphate film, zinc phosphate film and manganese phosphate film, in which zinc phosphate film and manganese phosphate film are not affected by the oxidation of oxygen in the atmosphere, and the corrosion resistance is better.
Taking the Taiwan screw raw material manufacturer, Sinosteel as an example, the disc yuan produced by the company is processed into wire rod and then transported to the screw factory. If the material is carbon steel, the process includes hydrochloric acid pickling and rust removal and phosphate treatment to form a lubricating phosphate film, which is conducive to drawing line and easy to cold forging (squeeze to make forging coarse). If the material is alloy steel, spheroidizing treatment is required after pickling, and soft steel is formed by annealing to make it easy to be machined. Then pickling, phosphate treatment or lime soap coating are used for pumping. If the material is stainless steel, the lubricating film shall be formed by oxalic acid coating to facilitate the pumping. After the screw factory has finished the inspection of wire diameter, true roundness, surface scar or corrosion, the wire rod is directly sent to the molding machine for material collection and cold forging, and then it is made into screw by the hobbing mechanism.
Chromate treatment is a process of immersing metal or metal coating into chromate or chromate solution containing some additives. By chemical or electrochemical method, chromate film composed of trivalent chromium and hexavalent chromium is formed on the metal surface. It is often used as the post-treatment of zinc coating (galvanizing, hot dip galvanizing, zinc alloy) and cadmium coating to improve the corrosion resistance of the coating, and also can be used as the surface corrosion protection of other metals such as aluminum, copper, tin, magnesium, nickel and its alloy.
Chromate film has close structure, good adhesion and good chemical stability. It has no holes after drying, which can be used as a barrier to prevent water from reaching the bottom metal, and has reliable isolation and protection on metal surface in corrosive environment.
The main components of chromate film are compounds containing trivalent chromium and hexavalent chromium, as well as chromate of base metal or coating metal. The insoluble trivalent chromium compounds form the skeleton of the membrane, which makes the film have a certain thickness and high stability, so it has good strength.
The soluble hexavalent chromium compounds dispersed in the inner part of the film have a good filling effect. When the film is scratched, it can make the film passivate again, that is, it has the inhibition effect to prevent the corrosion of the underlying metal. In salt spray test and thermothermal test, the corrosion resistance of the zinc coating with chromate coating is more than ten times higher than that of the non film coating.
Different base metals are treated with different chromate solutions, and the composition of the films and films is different. The Cr 6 + / SO42 ratio directly affects the color and thickness of the film in chromate solution. When the concentration of sulfate is constant, the content of trivalent chromium is higher, so the amount of chromate film is more.
Aluminate film cannot be formed in chromate solution without acid or alkali addition, so to obtain thicker film, the solution must be kept at the best pH value. The aluminate film cleaned with water cannot be dried at a temperature above 50 ℃, because when the temperature is higher than this temperature, hexavalent chromium compounds in the film will change from solubility to insoluble, reduce the self-healing ability, and even crack the film with the temperature rise.
The iron and steel parts are immersed in alkaline solutions containing oxidants, such as potassium nitrate or sodium hydroxide solution of sodium nitrate, and keep their boiling point of about 140 ℃, or the steel parts are immersed in molten salt of sodium nitrate and potassium nitrate, and then quenched in water. A layer of black oxide, i.e. iron oxide film, can be obtained on its surface. The composition is the mixture of ferrous oxide and iron oxide, which is to improve the black black A simple and economical method for corrosion resistance of color metals.
The thickness of chemical oxide film of steel is very thin, generally 0.5-1.6 μ m, the corrosion resistance is limited, the thickness of the film is 2 μ m, the appearance is dim and light-free, black or black gray, and the film has good adsorption. If the anti rust oil or paint is immersed, its anti salt spray corrosion performance can be increased several times to dozens of times.
Steel parts need to be degreased and acid washed before oxidation treatment. Parts with less oil on the surface and no corrosion products can be directly put into concentrated alkaline solution for chemical oxidation. The primary oxidation treatment is completed in one tank solution, while the second oxidation treatment is completed in two different bath solutions with different composition. The film produced by primary oxidation treatment is thin and has poor corrosion resistance. The film produced by two oxidation treatment is thick and corrosion resistance is better. The parts after oxidation treatment must be thoroughly cleaned, then boiled in soap water, dried and oiled according to the steps.
Paint is an organic coating, mainly used to protect metal parts, equipment and structures to resist atmospheric corrosion, and is used as a barrier between the metal and the environment. It has a long history to prevent metal corrosion with paint, and it is also the most widely used and common corrosion protection technology, which is convenient to construct and low cost.
The effective life of paint coating depends on the durability of paint itself and the adhesion of coating to the metal to be protected, and the better adhesion and protection function also need to be applied reasonably to the paint.
According to the test, the most important factor affecting the life of paint is the proper treatment of metal surface. The treated metal surface must be free of dust, oil, grease, rust, oxide and any material that can cause initial corrosion. This factor is more important than the quality of paint itself. The paint with poor quality is used on the metal with good surface treatment, and the effect of better paint is better than that of the good paint on the metal with poor surface treatment. All dirt, oil stains and grease on the metal surface can be removed by solvent or alkaline solution before washing. Rust and oxide can be removed by pickling or sandblasting.
After the metal surface is clean, the first coat shall be applied to the dry surface immediately to achieve good adsorption. It is better to apply a layer of phosphate and then the first coat. Usually, the paint must be applied several times, and the number and type of coating determine the rust prevention effect.
In the process of painting, the primer is very important. In order to form basic adhesion on metal, generally, two coats of antirust primer and one finish coat shall be applied. Some researchers believe that steel spray should be painted at least four coats in corrosive environment and the thickness should not be less than 0.13MM. Many companies in Taiwan use three coats of primer, one or two finishing coats.
The common antirust primer includes red lead paint and zinc rich paint. Red lead paint is made of red lead (lead oxide) and various paint bases. Zinc rich paint is made up of a large amount of zinc powder and a small amount of film forming material (binder). After the paint film is dry, zinc powder accounts for the majority (about 90%). Lead oxide in red lead paint has a inhibition effect on metal corrosion, so it can prevent rust. Zinc rich coating relies on the cathodic protection of zinc on metals, so it has good anti rust performance.
The common top coats to prevent metal corrosion include epoxy resin paint, silicon resin paint, vinyl chloride resin paint, melamine resin paint, alkyd resin paint, among which epoxy coal tar asphalt paint belongs to epoxy resin paint is one of the most widely used high-efficiency anti rust coatings in the world, and its adhesion, firmness, water resistance and chemical corrosion resistance are excellent.
Ceramic protective film
Ceramics are the general names of metal oxides, carbides, borides, silicides and nitrides. They have the characteristics of high melting point, high hardness and brittle properties. At present, they are mostly used oxides and carbides, and a small amount of borides and silicides are used.
The ceramic particles in molten state are sprayed on the metal surface to form a coating, i.e. ceramic protective film, by spraying. Ceramic protective film is applied to metal surface to increase its resistance to high temperature oxidation and heat resistance. It has good corrosion resistance for all corrosion environments except alkali and hydrofluoric acid, and has the advantages of wear-resistant, easy to clean, beautiful and good insulation.
The components after spraying ceramics can be used at high temperature. The ceramic coating can prevent the intergranular corrosion between metal and alloy, as the barrier of corrosion gas isolation, reduce the probability of hot spot occurrence, and replace low alloy metal for important parts and resist the erosion of pollutants. If the dip coating method is used, it is necessary to dry in the oven after drying to form a protective layer with good adhesion.
Metal corrosion protection technology involves many disciplines, widely used and rich in content. For the corrosion phenomenon of fasteners such as screws, which protective measures should be adopted can not be generalized. If the cost of protection far exceeds the cost of screw purchase, it is more economical to choose to replace the new screw directly.
It is necessary to pay attention to the proper selection of materials and surface treatment methods, such as stainless steel, alloy steel, carbon steel or galvanized screw, etc., which should be comprehensively considered in advance. Otherwise, it is more difficult to take remedial measures such as protection after the event, and the effect is not good. Sometimes, even if the environment will cause corrosion isolation, corrosion can be effectively controlled. For example, the unit equipment of nuclear power plant is built in closed plant. Some people also use a special plastic cap (such as radolid) for screws, which can be covered with screws or nuts exposed to the atmosphere in various sizes to achieve the corrosion resistance effect.
In addition, it is necessary to avoid the connection of different metal materials with screws, otherwise, the screws are easy to cause corrosion due to the potential difference between the two materials. Therefore, the most economical and effective measures must be determined according to the reasons, environmental conditions, protection requirements and effects of screw corrosion, cost and economic benefits, and finally achieve the purpose of protection.
Source: Network Arrangement – China Fasteners Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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