Fasteners are the most common parts of mechanical equipment for fastening and connection, which are used in a specific environment, and the long-term interaction between fasteners and the environment will always change their state and performance, that is, corrosion, which is one of the main forms of fastener failure. The corrosion of fasteners will affect the detachability and repeated installation of threads, or damage the strength of the connection between components, and even lead to the sudden failure of workpieces, resulting in catastrophic accidents. Therefore, the anti-corrosion of fasteners has always been a topic of great concern.
Common anti-corrosion technology of fasteners
The common anti-corrosion technology of fasteners. The anti-corrosion treatment of fasteners generally forms a covering layer or anti-corrosion layer on the workpiece surface through certain methods to hinder the impact of the external environment on the fastener itself and achieve the effect of corrosion resistance. There are four main anti-corrosion technologies for fasteners: film treatment technology, metal coating technology, coating technology and changing the internal structure of metal (such as stainless steel).
1. Film treatment technology
Film treatment technology mainly refers to the treatment process of generating a stable chemical (electrochemical) conversion film on the metal surface by chemical or electrochemical methods. For example, there are many fasteners with black coating and blue coating in the vehicle.
1.1 blackening and bluing
The process of forming a layer of chemical oxide film (mainly composed of Fe, O,) on the surface of iron and steel parts after treatment at about 140C for a fixed time in a concentrated alkaline solution containing oxidant.
Technical features of blackening / bluing treatment:
1) Film thickness 0.5-1.5 μ m。
2) The neutral salt spray resistance test (NSS) is generally only 2 ~ 5 hrs. At this time, the oxidation film has been broken and even a large amount of rust will appear, as shown in Figure 1.
3) It has low sensitivity to hydrogen embrittlement and can be used as high-strength bolt.
4) As a fastener, its torque preload consistency is poor.
5) Bright color and good decorative effect.
6) Low cost.
1.2 phosphating treatment
Phosphating is a process in which iron and steel parts are immersed in a solution containing manganese, phosphoric acid, phosphate and other reagents to form a phosphate conversion film insoluble in water on the metal surface. Technical characteristics of phosphating treatment.
1) The film is firmly bonded to the substrate (1 ~ 50 μ M thick).
2) NSS can reach 10 ~ 20 hrs, even 72 hrs.
3) Poor mechanical strength and brittle.
4) As a fastener, its torque preload consistency is very good.
5) The color is light gray and other dark colors, and the decorative effect is poor.
6) It has low sensitivity to hydrogen embrittlement and can be used as high-strength bolts.
7) Low cost.
2. Metal coating technology
Metal coating technology is mainly a surface treatment process that uses coating technology to form a metal thin layer on the surface of metal materials, so as to endow metal materials with decoration or protection. In urban rail vehicles, the metal coating technology of fasteners is mainly zinc plating and other special metal coatings (chromium plating, nickel plating, cadmium plating, silver plating, etc.).
2.1 galvanizing
Zinc and iron can dissolve each other, and its standard electrode potential is -0.76 v. for steel substrate, zinc coating belongs to anodic coating, which can better protect steel substrate. Therefore, zinc plating technology is widely used in fasteners. There are three common galvanizing methods: hot dip galvanizing, electro galvanizing and mechanical galvanizing.
2.1.1 hot dip galvanizing
Hot dip galvanizing refers to that steel parts are immersed in molten liquid zinc, resulting in a series of physical and chemical reactions on the surface of the workpiece, so as to form a metal zinc coating. The coating thickness of hot-dip galvanizing is very thick (up to 30 ~ 60 μ m) , its corrosion resistance is very good, and it is widely used in outdoor steel parts used for a long time (such as TV Tower, highway protective fence, etc.). For fasteners, hot-dip galvanizing is generally applicable to bolts of M6 and above, but it can not be used for high-strength fasteners, mainly because the operation temperature of hot-dip galvanizing process is very high (400C ~ 500C), which is easy to temper and soften high-strength fasteners.
2.1.2 electro galvanizing
Electro galvanizing is the use of electrolysis to form a uniform, dense and well bonded zinc coating on the surface of iron and steel parts. The zinc layer thickness of electro galvanizing is relatively thin (5 ~ 30 mm) μ m) , the corrosion resistance is the worst in the galvanizing anti-corrosion treatment, but it is widely used in fasteners because of its simple process, low cost and little impact on thread screwing. Since zinc plating has high hydrogen embrittlement sensitivity and it is difficult to completely remove hydrogen (the surface of the electro galvanized layer will peel or fall off above 100C), electro galvanized cannot be used for high-strength fasteners.
2.1.3 mechanical galvanizing
Mechanical galvanizing refers to the surface treatment process of forming zinc coating by impacting the surface of steel parts with impact medium under the action of chemical substances such as zinc powder, dispersant and accelerator. The thickness of mechanical zinc coating is generally 5 ~ 50 μ m. The coating has compact and uniform surface, good decorative effect and excellent corrosion resistance; And the coating has no disadvantages of hot-dip galvanizing and electro galvanizing such as high-temperature tempering and hydrogen embrittlement. It is a surface treatment process especially suitable for anti-corrosion of fasteners.
2.2 other metal coatings
2.2.1 chromium plating
As a metal coating, chromium has the characteristics of strong adhesion, good wear resistance, excellent decorative effect and high heat resistance (normal use below 500C). Therefore, it is very ideal to use chromium coating as a metal coating of fasteners.
Chromium plating has the following disadvantages:
1) The process is complex. Nickel or copper must be plated before chromium plating.
2) It's expensive.
3) The chromium coating is hard, brittle and easy to fall off.
2.2.2 nickel plating
As a metal coating, nickel has good conductivity, high hardness, good decorative effect and good heat resistance (it can be used normally below 600C). Therefore, nickel plating is also ideal for fasteners.
Nickel plating has the following disadvantages:
1) The process is complex, and copper must be plated before chromium plating.
2) Nickel coating is porous, and the substrate corrosion will be accelerated when the coating is thin.
3) It's expensive.
2.2.3 cadmium plating
As a metal coating, cadmium is an anodic coating. It has the characteristics of strong hydrochloric acid corrosion resistance, low hydrogen embrittlement and good decorative effect. It is especially suitable for fasteners used in marine environment (such as fasteners of HNA aircraft and oil drilling platform).
Cadmium plating mainly has the following disadvantages:
① The environmental pollution is high, and the gas and soluble cadmium salt produced by cadmium melting are toxic.
② It's expensive.
2.2.4 silver plating
As a metal coating, silver has excellent conductivity, excellent reflective performance, good lubricity and excellent heat resistance (it can be used normally below 870c). Therefore, silver plating is widely used in electronic and electrical engineering, high-frequency components and other fields (such as generator conductive bolts, vehicle battery lead-out terminals).
Silver plating has the following disadvantages:
① The process is complex, so copper must be plated before silver plating.
② The price is very expensive.
2.2.5 galvanized nickel
Zinc nickel composite coating is a new type of alloy metal coating optimized and developed in the surface treatment process of zinc plating, which has many advantages.
1) NSS can reach 500 - 1500hrs.
2) The electrode potential of the coating is between Fe and Zn, which is more suitable for the assembly of aluminum parts.
3) The coating has high hardness and good decorative effect.
4) It has almost no hydrogen embrittlement and can be used for high-strength fasteners.
5) Good heat resistance (it can be used normally below 8009c).
The main disadvantage of zinc nickel coating is its high price (about 6 times that of zinc plating), but its excellent comprehensive properties have been widely recognized by people.
3. Coating technology
Coating technology refers to a surface treatment technology in which a specific coating is coated on the object surface with certain equipment and methods to produce a dense, continuous and uniform film on the surface, and then dried and solidified by natural or artificial methods to form a protective or decorative coating.
Among fasteners, zinc chromium coating technology is the most widely used coating technology. It is a coating formed on the surface of steel parts by coating zinc chromium coating on steel parts and baking through full closed-circuit cycle, also known as Dacromet treatment. It has the following excellent characteristics.
1) NSS can reach 500 ~ 1000 hrs.
2) Good permeability.
3) No hydrogen embrittlement sensitivity.
4) Low environmental pollution.
5) As a fastener, its torque preload consistency is very good.
6) The price is moderate (about twice that of Galvanized).
Dacromet treatment has the following disadvantages:
1) Poor wear resistance (hardness only 1 h).
2) Single color (only silver white and silver gray), poor decorative effect.
3) Poor conductivity, not suitable for conductive connection parts.
4. Change the microstructure of steel
4.1 change of composition (e.g. stainless steel)
Stainless steel is the abbreviation of stainless and acid resistant steel. It has excellent corrosion resistance and good decorative effect. It is widely used in various cities. It is generally believed that the corrosion resistance mechanism of stainless steel is mainly as follows.
1) When the Cr content exceeds 13%, the electrode potential of steel will rise from negative electrode potential to positive electrode potential, making the steel matrix itself "inert";
2) CR will form a dense CR rich passive film on the surface of steel to further protect the substrate.
3) Stainless steel is divided into martensitic steel, ferritic steel, austenitic steel, austenitic ferritic stainless steel, etc. among them, austenitic stainless steel has the best corrosion resistance, such as A2 and A4 stainless steel.
Stainless steel has the following disadvantages: ① the yield strength is very low (generally not more than 300 MPa), which is not suitable for the connection of major structural parts.
② Thread seizure is easy to occur. When the stainless steel bolt is tightened, it is easy to cause thread surface damage. At this time, it will spontaneously produce a layer of oxide layer, which will aggravate the bolt adhesion and locking.
③ Prone to intergranular corrosion. At a certain temperature, C and Cr in stainless steel will form compounds, especially near the grain boundary, which will lead to "CR poor zone" at the grain boundary and grain boundary corrosion.
④ Poor CI medium corrosion resistance (except A4 stainless steel).
⑤ The price is higher (about 4 times that of Dacromet treatment).
4.2 change of heat treatment state
Iron and steel materials are mainly multiphase structure (impurities, carbides, intermetallic compounds and other second phases usually exist in iron and steel as cathode, and Fe matrix as anode). There is a potential difference between phases in the multiphase structure, forming a corrosive micro cell. The second phase may be anode passivation phase or cathode dissolution phase, both of which will affect the corrosion resistance of the matrix.
For example, stainless steel should be very careful in welding and heat treatment. After high-temperature solution treatment, stainless steel is heated between 400C and 850c to produce a large amount of CRSC. And Cr, C; Carbides will precipitate along the grain boundary, resulting in the formation of Cr poor zone near the grain boundary. Carbides are used as the cathode of the corrosion battery and Cr poor area is used as the anode of the corrosion battery, resulting in grain boundary corrosion, and its corrosion resistance will be greatly reduced.
