Introduction Of Commonly Used Anti-corrosion Technology For Fasteners

 Fasteners are the most common parts of mechanical equipment used for fastening connections. They are all used in specific environments, and the long-term interaction between fasteners and the environment will always cause changes in their state and performance. Change, that is, corrosion, is one of the main forms of fastener failure. Light corrosion of fasteners will affect the detachability and repeated installation of threads, and severe corrosion will damage the strength of the connection between components, and even lead to sudden failure of workpieces, resulting in catastrophic accidents. Therefore, the anti-corrosion of fasteners has always been of great concern to everyone. topic of.
Anti-corrosion technology commonly used for fasteners

Commonly used anti-corrosion technology for fasteners The anti-corrosion treatment of fasteners generally forms a covering layer or anti-corrosion layer on the surface of the workpiece by a certain method to prevent the influence 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 layer 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 process of forming a stable chemical (electrochemical) conversion film on the metal surface by chemical or electrochemical methods. For example, in urban rail vehicles, the film layer treatment of its fasteners is mostly black/blue treatment and phosphating treatment.
1.1, black and blue

In a concentrated alkaline solution containing an oxidant, after a certain period of treatment at about 140C, the process of forming a chemical oxide film on the surface of the steel part (mainly composed of Fe, O,).
Technical characteristics of blackening/bluening treatment:
1) The film thickness is 0.5-1.5 μm.
2) The neutral salt spray test (NSS) is generally only 2 ~ 5 Hrs. At this time, the oxide film layer has been broken, and even a large amount of rust will appear, as shown in Figure 1.

3) Low susceptibility to hydrogen embrittlement, can be used as high-strength bolts.
4) As a fastener, its torque-pretightening force consistency is poor.
5) The color is brighter and the decorative effect is better.
6) Low cost.
1.2. Phosphating treatment

The process of immersing steel parts in a solution containing manganese, phosphoric acid, phosphate and other reagents to form a layer of phosphate conversion film that is insoluble in water on the metal surface is called phosphating treatment. Technical characteristics of phosphating treatment.
1) The film layer is firmly bonded to the substrate (1-50 μm thick).
2) NSS can reach 10~20 Hrs, even 72 Hrs.
3) Poor mechanical strength and brittle quality.
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) The susceptibility to hydrogen embrittlement is low, so it can be used as high-strength bolts.
7) The cost is lower.
2. Metal coating technology

Metal coating technology is mainly a surface treatment process that uses coating technology to form a thin metal layer on the surface of metal materials to endow metal materials with decorative or protective properties. In urban rail vehicles, the metal coating technology of fasteners is mainly galvanized, and other special metal coatings (chrome plating, nickel plating, cadmium plating, silver plating, etc.).
2.1 Galvanized

Zinc and iron can dissolve each other, and its standard electrode potential is -0.76 V. For the steel substrate, the zinc coating is an anodic coating, which can better protect the steel substrate. Therefore, galvanizing technology is widely used in fasteners. There are three commonly used galvanizing methods: hot-dip galvanizing, electro-galvanizing and mechanical galvanizing.
2.1.1 Hot dip galvanizing
Hot-dip galvanizing means that steel parts are immersed in molten liquid zinc, so that a series of physical and chemical reactions occur on the surface of the workpiece, thereby forming a metal galvanized layer. The coating thickness of hot-dip galvanizing is very thick (up to 30-60 μm), and its corrosion resistance is very good. It is widely used in steel parts that are used outdoors for a long time (such as TV towers, highway guardrails, etc.). For fasteners, hot-dip galvanizing is generally suitable for bolts of M6 and above, but it cannot be used for high-strength fasteners, mainly because the operating temperature of the hot-dip galvanizing process is very high (400C~ 500C), it is easy to temper and soften high-strength fasteners.
2.1.2 Galvanizing
Electrogalvanizing uses electrolysis to form a uniform, dense, and well-bonded galvanized layer on the surface of steel parts. The thickness of the zinc layer of electro-galvanizing is relatively thin (5~30μm), and its corrosion resistance is the worst in galvanized anti-corrosion treatment. widely used in applications. Since electro-galvanizing has a high susceptibility to hydrogen embrittlement, and it is difficult to completely dehydrogenate (the surface of the electro-galvanized layer will peel or fall off above 100C), so electro-galvanized cannot be used for high-strength fasteners.
2.1.3 Mechanical galvanizing
Mechanical galvanizing refers to the surface treatment process of iron and steel parts using impact medium to impact the surface of steel parts under the action of chemical substances such as zinc powder, dispersant and accelerator to form a galvanized layer. The thickness of the mechanical galvanized layer is generally 5-50 μm, the surface of the coating is dense and uniform, the decorative effect is good, and the corrosion resistance is excellent; and the coating has no shortcomings of hot-dip galvanizing and electro-galvanizing such as high temperature tempering and hydrogen embrittlement. A surface treatment process especially suitable for corrosion protection of fasteners.
2.2. Other metal coatings

2.2.1 Chrome plating
Chromium as a metal coating has the characteristics of strong adhesion, good wear resistance, excellent decorative effect, and high heat resistance (it can be used normally below 500C), so chromium coating is used as a metal coating for fasteners. very ideal.
Chromium plating mainly has the following disadvantages:
1) The process is complicated, nickel or copper must be plated before chrome plating.
2) Expensive.
3) The chrome plating is hard, brittle and easy to fall off.
2.2.2 Nickel plating
As a metal coating, nickel has good electrical conductivity, high hardness, good decorative effect, and good heat resistance (it can be used normally below 600C), so it is ideal to use nickel plating for fasteners.
Nickel plating mainly has the following disadvantages:
1) The process is complicated, and copper must be plated before chrome plating.
2) The nickel coating is porous, and the corrosion of the substrate will be accelerated when the coating is thin.
3) Expensive.
2.2.3 Cadmium plating
As a metal coating, cadmium is an anodic coating, which has strong hydrochloric acid corrosion resistance, low hydrogen embrittlement, and good decorative effects. It is especially suitable for fasteners used in marine environments (such as fast firmware).
Cadmium plating mainly has the following disadvantages:
① The environmental pollution is high, and the gas and soluble cadmium salts produced when cadmium is melted are poisonous.
②The price is expensive.
2.2.4 Silver plating
As a metal coating, silver has excellent electrical conductivity, excellent reflective properties, good lubricity and excellent heat resistance (it can be used normally below 870C), so silver plating is widely used in the fields of electronics, high-frequency components, etc. (such as generator conductive bolts, vehicle battery lead-out terminals).
Silver plating mainly has the following disadvantages:
① The process is complicated, and copper must be plated before silver plating.
②The price is very expensive.
2.2.5 Galvanized nickel
Zinc-nickel composite coating is a new type of alloy metal coating developed on the surface treatment process of galvanizing, which has many advantages.
1) NSS up to 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 hardness of the coating is high, and the decorative effect is very good.
4) There is almost no hydrogen embrittlement, and it can be used for high-strength fasteners.
5) Good heat resistance (it can be used normally below 8009C).
The main disadvantage of the current zinc-nickel coating is the higher price (about 6 times that of zinc plating), but its excellent comprehensive performance has been more and more widely recognized by people.
3. Coating technology

Coating technology refers to the application of specific coatings on the surface of objects with certain equipment and methods to form a dense, continuous and uniform film on the surface, which is then dried and cured by natural or artificial methods to form protective or decorative properties. A surface treatment technology for functional coatings.
In fasteners, the most widely used coating technology is zinc-chromium coating technology, which is a kind of coating formed on the surface of steel parts by coating zinc-chromium coatings on steel parts and baking them in a fully closed circuit. Layer, also called dacromet treatment, which has the following excellent characteristics.
1) NSS can reach 500 ~ 1000 Hrs.
2) Good permeability.
3) No susceptibility to hydrogen embrittlement.
4) Environmental pollution is low.
5) As a fastener, its torque-preload consistency is very good.
6) The price is moderate (about twice that of galvanized).
Dacromet treatment mainly has the following disadvantages:
1) Poor wear resistance (hardness is only 1 H).
2) The color is single (only silver white and silver gray), and the decorative effect is poor.
3) Poor conductivity, not suitable for parts with conductive connections.
4. Change the organizational form of steel

4.1 Changes in composition (such as stainless steel)

Stainless steel is the abbreviation of stainless acid-resistant steel, which has excellent corrosion resistance and good decorative effect, and is widely used in various fields. 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 the steel will rise from the negative electrode potential to the positive electrode potential, making the steel matrix itself "inert";
2) Cr will form a dense Cr-rich passivation film on the steel surface, thereby further protecting the substrate.
3) Stainless steel is divided into: martensitic steel, ferritic steel, austenitic steel, austenitic-ferritic stainless steel, etc., among which austenitic stainless steel has the best corrosion resistance, such as A2 , A4 stainless steel.
Stainless steel mainly has the following deficiencies: ①The yield strength is very low (generally no more than 300 MPa), which is not suitable for the connection of major structural parts.
②It is prone to thread seizure. When the stainless steel bolts are tightened, it is easy to cause the thread surface to be damaged. At this time, it will spontaneously produce a layer of oxide layer, which will intensify the adhesion and locking of the bolts.
③ prone to intergranular corrosion. C and Cr in stainless steel will form compounds at a certain temperature, especially near the grain boundary, which will cause a "Cr-poor area" at the grain boundary, resulting in grain boundary corrosion.
④ Poor corrosion resistance to CI medium (except A4 stainless steel).
⑤ The price is higher (about 4 times that of Dacromet).
4.2 Changes in heat treatment state

Iron and steel materials are mainly multi-phase structures (secondary phases such as impurities, carbides, and intermetallic compounds usually exist in steel as cathodes, and Fe matrix as anodes). There is a potential difference between the phases in the multiphase structure, forming a corrosion micro-battery. The second phase may be anodic passivation phase or cathodic dissolution phase, both of which will affect the corrosion resistance of the matrix.
Such as stainless steel, it must be very careful when welding and heat treatment. After stainless steel is subjected to high-temperature solution treatment, it is heated between 400C and 850C, and a large amount of CrsC is formed. And Cr, C; Carbide will precipitate along the grain boundary, so that a Cr-poor region is formed near the grain boundary. The carbide acts as the cathode of the corrosion cell, and the Cr-poor region acts as the anode of the corrosion cell, which leads to grain boundary corrosion and its corrosion resistance will be greatly reduced.