Fasteners are a class of mechanical parts that are used to fasten connections and are widely used. Fasteners are generally made of metal and, depending on the material, have different corrosion conditions in various environments.
Corrosion is one of the main forms of fastener failure. Corrosion can affect the disassembly and repeated installation of threads, or damage the strength of the connection between parts, or even lead to the sudden failure of the workpiece, causing a catastrophic accident, so the corrosion of fasteners has always been a topic of great concern.
Commonly used anti-corrosion techniques for fasteners
Fasteners commonly used anti-corrosion technology fasteners are generally through a certain method in the workpiece surface to form a cover or corrosion-resistant layer to hinder the impact of the external environment on the fastener itself, to achieve the effect of corrosion resistance. Fasteners anti-corrosion technology mainly have the following four kinds: film treatment technology, metal plating technology, coating technology and change the internal structure of the metal (such as stainless steel).
1. Membrane layer treatment technology
Film treatment technology mainly refers to the use of chemical or electrochemical methods in the metal surface to generate a layer of stable chemical (electrochemical) transformation film treatment process. Such as in urban rail vehicles, the fasteners of the film treatment is more often used blackening / blue treatment and phosphate treatment.
1.1 Technical characteristics of the blackening/blue treatment.
The process of forming a chemical oxide film on the surface of steel parts (the main component is Fe, O,) after a certain period of treatment at around 140C in a concentrated alkaline solution containing an oxidising agent.This technique has several features：
★ Film thickness 0.5-1.5 μm.
★ The neutral salt spray test (NSS) is generally only 2 ~ 5 Hrs, when the oxide film layer has broken down, and even a large number of rust stains will appear.
★ Low sensitivity to hydrogen embrittlement, can be used for high strength bolts.
★ As a fastener its torque a preload consistency is poor.
★ Brighter colour, better decorative effect.
★ Low cost.
1.2 Phosphating treatment
Steel parts immersed in a solution containing manganese, phosphoric acid, phosphate and other reagents, so that the metal surface to generate a layer of insoluble phosphate conversion film process, called phosphate treatment.This technique has several features：
★ The film layer is firmly bonded to the substrate (1 ~ 50 μm thick).
★ NSS can reach 10-20 Hrs, even 72 Hrs.
★ Poor mechanical strength, brittle.
★ Good consistency of torque and preload as fasteners.
★ The colour is light grey and other dark colours, the decorative effect is poor.
★ Low sensitivity to hydrogen embrittlement, can be used as high strength bolts.
★ Low cost.
2. Metal plating technology
Metal plating technology is mainly used on the surface of metal materials to form a thin metal layer, in order to give the metal material decorative or protective surface treatment process. In urban rail vehicles, the metal plating technology for fasteners is mainly galvanised, as well as other special metal plating (chromium, nickel, cadmium, silver, etc.).
2.1 Galvanized
Zinc is miscible with iron and has a standard electrode potential of -0.76 V. For steel substrates, zinc plating is an anodic coating that better protects the steel substrate, so zinc plating technology is commonly used in fasteners. There are three common methods of zinc plating: hot dip zinc plating, electronics zinc plating and mechanical zinc plating.
2.1.1 Hot-dip zinc plating
Hot dip zinc plating is the process of dipping steel parts into molten liquid zinc, which causes a series of physical and chemical reactions on the surface of the workpiece, resulting in a zinc plating metal layer. The thickness of the coating is very thick (up to 30 ~ 60 μm) and the corrosion resistance is so good that it is widely used for long-term outdoor use of steel parts (e.g. TV towers, highway barriers, etc.). For fasteners, hot-dip zinc plating is generally suitable for bolts of M6 and above, but it cannot be used for high-strength fasteners, mainly because of the high operating temperature of the hot-dip zinc plating process (400C~500C), which is prone to tempering and softening of high-strength fasteners.
2.1.2 Electroplating with zinc
Electro-galvanising is the use of electrolysis to form a uniform, dense, well-bonded zinc layer on the surface of steel parts. Electro-galvanisation has a thin zinc layer (5-30μm) and the worst corrosion resistance of any galvanised anti-corrosion treatment, but it is a simple, low-cost process that has less impact on the screwing problem and is widely used in fasteners. Zinc plating is not used for high strength fasteners because of its high susceptibility to hydrogen embrittlement and the difficulty of complete de-hydrogenation (the surface of zinc plating peels or flakes at 100C and above).
2.1.3 Mechanical zinc plating
Mechanical zinc plating is a surface treatment process in which the surface of a steel part is impacted by an impact medium in the presence of zinc powder, dispersant and accelerator chemicals to form a zinc plating layer. With a thickness of 5 to 50 μm, mechanical zinc plating provides a dense and uniform surface, good decorative effect and excellent corrosion resistance.
2.2 Other metal plating
2.2.1 Chrome plating
Chromium as a metal coating has strong adhesion, good wear resistance, excellent decorative effect, and high heat resistance (below 500C can be used normally), so chromium plating as a metal coating for fasteners is very ideal.
The main disadvantages of chromium plating are as follows.
★ The process is complex and must be plated with nickel or copper before chromium plating.
★ Expensive.
★ Chrome plating is hard, brittle and easy to fall off.
2.2.2 Nickel plating
Nickel as a metal plating has good electrical conductivity, high hardness, good decorative effect, good heat resistance (below 600C can be used normally), so the fasteners using nickel plating treatment is also more ideal.
The main disadvantages of nickel plating are as follows.
★ The process is complicated, and copper must be plated before chromium plating.
★ Nickel plating has porous, thin plating will accelerate the corrosion of the substrate.
★ Expensive.
2.2.3 Cadmium plating
Cadmium as a metal plating belongs to the anodic plating, which has strong resistance to hydrochloric acid corrosion, low hydrogen embrittlement, good decorative effect, especially suitable for the application of marine environment fasteners (such as seafaring aircraft, oil rig fasteners).
The main disadvantages of cadmium plating are as follows.
★ High environmental pollution, cadmium melting gas and soluble cadmium salts are toxic.
★ Expensive.
2.2.4 Silver plating
Silver as a metal plating has excellent electrical conductivity, excellent reflective properties, good lubrication and excellent heat resistance (below 870C can be used normally), so silver plating treatment is widely used in electronics, high-frequency components and other fields (such as generator conductive bolts, vehicle battery lead terminals).
The main disadvantages of silver plating are as follows.
★ The process is complicated and copper must be plated before silver plating.
★ The price is very expensive.
2.2.5 Zinc-nickel plating
Zinc-nickel composite plating is a new type of alloy metal plating optimised on the surface treatment process of zinc plating, which has many advantages.
★ NSS up to 500 – 1500Hrs.
★ The electrode potential of the coating is between Fe and Zn, which makes it more suitable for the assembly of aluminium parts.
★ High hardness of the coating, good decorative effect.
★ Almost no hydrogen embrittlement, can be used for high strength fasteners.
★ Good heat resistance (below 8009C can be used normally).
The main disadvantage of zinc-nickel plating is its high price (about 6 times that of zinc plating), but its excellent overall performance has become more and more widely recognized.
3. Coating technology
Coating technology refers to the application of a specific coating to the surface of an object using certain equipment and methods to produce a dense, continuous and uniform film, which is then dried and cured by natural or artificial means to form a protective or decorative coating. It is then dried and cured by natural or artificial means to form a protective or decorative coating.
In fasteners, the coating technology is the most applied zinc-chromium coating technology, which is zinc-chromium coating on the steel parts, after the full closed circuit coating baking cycle so as to form a coating on the surface of the steel parts, also known as Dacromet treatment, which has the following excellent characteristics.
★ NSS up to 500 ~ 1000 Hrs.
★ Good permeability.
★ No hydrogen embrittlement sensitivity.
★ Low environmental pollution.
★ Very good torque – preload consistency as a fastener.
★ Moderate price (about twice as much as galvanized).
The main disadvantages of Dacromet treatment are as follows.
★ Poor wear resistance (only 1 H hardness).
★ Single colour (only silver white and silver grey), poor decorative effect.
★ Poor electrical conductivity, not suitable for parts with electrical connections.
4. Changing the organisation of steel
4.1 Change of composition (e.g. stainless steel)
Stainless steel is short for stainless steel, which has excellent corrosion resistance and good decorative effect, and has a wide range of applications in various collar cities. It is now generally believed that the corrosion resistance mechanism of stainless steel is mainly as follows.
★ Cr content of more than 13%, the electrode potential of steel will rise from the negative electrode potential to the positive electrode potential, so that the steel matrix itself is “inert”.
★ Cr will form a dense Cr-rich passivation film on the surface of the steel, which further protects the substrate.
★ Stainless steel is divided into: martensitic steel, ferritic steel, austenitic steel, austenitic-ferritic stainless steel, etc., especially austenitic stainless steel corrosion resistance is the best, such as A2, A4 stainless steel.
Stainless steel mainly has the following shortcomings:
★ Yield strength is very low (generally not more than 300 MPa), is not suitable for the connection of major structural components.
★ Easy to appear thread galling. Stainless steel bolts are easy to cause damage to the thread surface when tightened, when it will spontaneously produce a layer of oxidation, thus increasing the bolt adhesion locking.
★ Easy to produce intergranular corrosion. At a certain temperature in the stainless steel C and Cr will form a compound, especially near the grain boundary, which will cause the grain boundary “poor Cr area”, resulting in grain boundary corrosion.
★ Poor corrosion resistance to CI media (except A4 stainless steel).
★ Higher price (about four times the Dacromet treatment).
4.2 Heat treatment state change
Steel materials are mainly multi-phase organization (impurities, carbides, intermetallic compounds and other second phase usually exist as a cathode in the steel, while the Fe matrix as the anode). In a multi-phase organization, there is a potential difference between phases, forming a corrosion microcell. The second phase may be an anodically passivated phase or a cathodically dissolved phase, both of which have an effect on the corrosion resistance of the substrate.
For example, stainless steel, its welding and heat treatment should be very careful. Stainless steel by high temperature solution treatment, heated between 400C ~ 850C, a large number of CrsC. and Cr,C; carbide will precipitate along the grain boundary, so that the formation of poor Cr area near the grain boundary. Carbide as the cathode of the corrosion cell, poor Cr area as the anode of the corrosion cell, resulting in grain boundary corrosion, its corrosion resistance will be greatly reduced.
Post time: Apr-18-2025