Fasteners are the most common components used in mechanical equipment for fastening connections. They are all used under specific conditions. The long-term interaction between fasteners and the environment always causes changes in their state and performance, namely corrosion. It is one of the main forms of fastener failure.
Even a slight corrosion of fasteners can affect the disasability and re-installation of the threads, while severe corrosion can damage the strength of the connection between components and even lead to the sudden failure of the workpiece, causing catastrophic accidents. Therefore, the anti-corrosion of fasteners has always been a topic of great concern.
Common Anti-corrosion Techniques for Fasteners
Common anti-corrosion techniques for fasteners usually involve forming a coating or protective layer on the surface of the workpiece through certain methods to prevent the influence of the external environment on the fastener itself, achieving the effect of corrosion resistance.
The main anti-corrosion techniques for fasteners include the following four types: film layer treatment technology, metal coating technology, coating technology, and changing the internal structure of the metal (such as stainless steel).
1. Film layer treatment technology
Film layer treatment technology mainly refers to the process of generating a stable chemical (electrochemical) conversion film on the metal surface using chemical or electrochemical methods. For example, in urban rail vehicles, the film layer treatment of fasteners commonly used is blackening/blueing treatment and phosphating treatment.
1.1 Blackening and Blueing
In a concentrated alkaline solution containing oxidants, after processing at 140°C for a certain period of time, a chemical oxidation film is formed on the surface of the steel component (mainly composed of FeO).
Blackening/blueing treatment characteristics:
1) Film layer thickness 0.5 – 1.5 μm.
2) The neutral salt spray test (NSS) generally lasts only 2 – 5 hours, at which point the oxide film layer has broken, and even a large amount of rust may appear, as shown in Figure 1.
3) Low hydrogen embrittlement sensitivity, suitable for use as high-strength bolts.
4) The torque-preload consistency of the fastener is poor.
5) Bright color, better decorative effect.
6) Low cost.
1.2 Phosphating treatment
Steel components are immersed in a solution containing manganese, phosphoric acid, phosphate salts, and other reagents, causing the metal surface to form a layer of insoluble phosphate conversion film. This process is called phosphating treatment. The characteristics of phosphating treatment.
1) The film layer is firmly bonded to the base material (1 – 50 μm thick).
2) NSS can last up to 10 – 20 hours, or even 72 hours.
3) Mechanical strength is poor, brittle.
4) As a fastener, its torque-preload consistency is very good.
5) The color is light gray or dark color, poor decorative effect.
6) Low hydrogen embrittlement sensitivity, suitable for use as high-strength bolts.
7) Low cost.
2. Metal coating technology
Metal coating technology mainly involves using coating technology on the surface of metal materials to form a thin metal layer, giving the metal material decorative or protective properties as a surface treatment process. In urban rail vehicles, the metal coating technology for fasteners mainly includes galvanizing, and other special metal coatings (chromium plating, nickel plating, cadmium plating, silver plating, etc.).
2.1 Galvanizing
Zinc can dissolve with iron, and its standard electrode potential is -0.76 V. For the steel base material, the zinc coating is an anodic coating, which can better protect the steel base material, so the galvanizing technology is widely used in fasteners. Common galvanizing methods include three types: hot-dip galvanizing, electro-galvanizing, and mechanical galvanizing.
2.1.1 Hot-dip galvanizing
Hot-dip galvanizing refers to immersing the steel component in molten liquid zinc, causing a series of physical and chemical reactions on the surface of the component, thereby forming a metal galvanized layer. The hot-dip galvanizing layer is very thick (up to 30 – 60 μm), and its corrosion resistance is very good, widely used in outdoor steel components (such as TV towers, highway guardrails, etc.) for long-term use.
For fasteners, hot-dip galvanizing is generally applicable to M6 and above bolts, but it cannot be used for high-strength fasteners, mainly because the operation temperature of the hot-dip galvanizing process is very high (400°C – 500°C), which is prone to cause tempering softening of high-strength fasteners.
2.1.2 Electro-galvanizing
Electro-galvanizing uses electrolysis to form a uniform, dense, and well-bonded galvanized layer on the surface of the steel component. The zinc layer thickness is thin (5 – 30 μm), and its corrosion resistance is the worst among galvanizing anti-corrosion treatments, but its process is simple, cost is low, and it has less impact on the thread engagement problem, and is widely used in fasteners.
Due to the high hydrogen embrittlement sensitivity of electro-galvanizing and the difficulty in completely removing hydrogen (the surface of the electro-galvanizing layer will flake off or fall off at temperatures above 100°C), electro-galvanizing cannot be used for high-strength fasteners.
2.1.3 Mechanical Galvanizing
Mechanical galvanizing refers to the surface treatment process in which steel components form a galvanizing layer by being impacted with an impact medium on their surface under the action of chemical substances such as zinc powder, dispersants, and promoters. The thickness of the mechanical galvanizing layer is generally 5 to 50 μm.
The surface of the galvanizing layer is dense and uniform, with good decorative effects and excellent corrosion resistance; and it has no disadvantages such as high-temperature tempering or hydrogen embrittlement like hot-dip galvanizing or electro-galvanizing, and is a particularly suitable surface treatment process for fastener corrosion prevention.
2.2, Other Metal Galvanizing Layers
2.2.1 Chromium Plating
Chromium as a metal galvanizing layer has strong adhesion, good wear resistance, excellent decorative effect, and high heat resistance (can be used normally below 500°C). Therefore, chromium plating as a metal galvanizing layer for fasteners is very ideal.
Chromium plating processing mainly has the following disadvantages:
1) The process is complex; chromium plating must be preceded by nickel or copper plating.
2) It is expensive.
3) The chromium plating layer is hard and brittle, and is prone to peeling off.
2.2.2 Nickel Plating
Nickel as a metal galvanizing layer has good electrical conductivity, high hardness, excellent decorative effect, and good heat resistance (can be used normally below 600°C). Therefore, nickel plating processing for fasteners is also a relatively ideal choice.
Nickel plating processing mainly has the following disadvantages:
1) The process is complex; nickel plating must be preceded by copper plating.
2) The nickel plating layer has porosity, and when the thickness of the coating is thin, it will accelerate the corrosion of the substrate.
3) It is expensive.
2.2.3 Cadmium Plating
Cadmium as a metal galvanizing layer belongs to an anodic coating, and it has strong resistance to hydrochloric acid corrosion, low hydrogen embrittlement, and excellent decorative effect. It is particularly suitable for fasteners used in marine environments (such as aircraft for ocean navigation and oil drilling platforms). Cadmium plating processing mainly has the following disadvantages:
① It has a high environmental pollution level; the gas produced when cadmium melts and the soluble cadmium salts are toxic.
② It is expensive.
2.2.4 Silver Plating
Silver as a metal galvanizing layer has excellent electrical conductivity, excellent reflective properties, good lubricity, and excellent heat resistance (can be used normally below 870°C). Therefore, silver plating processing is widely used in electronic and electrical engineering, and high-frequency components (such as generator conductive bolts, vehicle battery terminals).
Silver plating processing mainly has the following disadvantages:
① The process is complex; silver plating must be preceded by copper plating.
② It is extremely expensive.
2.2.5 Zinc-Nickel Coating
Zinc-nickel composite coating is a new type of alloy metal galvanizing layer that has been developed based on the electro-galvanizing surface treatment process. It has many advantages.
1) NSS can reach 500 – 1500 Hrs.
2) The electrode potential of the coating is between Fe and Zn, making it more suitable for assembly of aluminum components.
3) The coating has high hardness and excellent decorative effect.
4) It has almost no hydrogen embrittlement and can be used for high-strength fasteners.
5) It has good heat resistance (can be used normally below 800°C).
The main disadvantages of zinc-nickel coating are: a relatively high price (about 6 times that of electro-galvanizing), but its excellent comprehensive performance has been increasingly recognized by people.
3. Coating Technology
Coating technology refers to the surface treatment technology of applying a specific coating with certain equipment and methods onto the surface of an object to form a dense, continuous, and uniform film, which is then dried and solidified through natural or artificial methods to form a protective or decorative coating.
In fasteners, the most widely used coating technology is zinc-chromium coating technology, which is to apply zinc-chromium coating onto steel components, and through a closed-loop coating and baking process, forms a coating on the surface of the steel component, also known as Dacromet treatment. It has the following excellent characteristics.
1) The service life of NSS can reach 500 to 1000 hours.
2) Good permeability.
3) No hydrogen embrittlement sensitivity.
4) Low environmental pollution.
5) As a fastener, the torque-preload consistency is very good.
6) The price is moderate (about twice that of galvanized).
The disadvantages of chromic acid anodizing treatment are as follows:
1) Poor wear resistance (hardness is only 1 H).
2) Monotonous color (only silver white and silver gray), poor decorative effect.
3) Poor conductivity, not suitable for parts requiring electrical connection.
4. Altering the microstructure of steel
4.1 Change in composition (such as stainless steel)
Stainless steel is the abbreviation of stainless and acid-resistant steel. It has excellent corrosion resistance and good decorative effect, and is widely used in various fields. It is generally believed that the corrosion mechanism of stainless steel is as follows.
1) When the Cr content exceeds 13%, the electrode potential of the steel will rise from a negative electrode potential to a positive electrode potential, making the steel matrix itself “inert”;
2) Cr forms a dense Cr-rich passivation film on the steel surface, further protecting the base.
3) Stainless steel is classified by microstructure: 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 shortcomings: ① The yield strength is very low (generally not exceeding 300 MPa), not suitable for the connection of major structural components.
② It is prone to thread seizing. When tightening stainless steel bolts, the surface of the thread is likely to be damaged, and at this time, a layer of oxide layer will spontaneously form, thereby intensifying the bolt adhesion lock-up.
③ It is prone to intergranular corrosion. At a certain temperature, C and Cr in stainless steel will form compounds, especially near the grain boundaries, which will cause “poor Cr areas” at the grain boundaries, leading to grain boundary corrosion.
④ Poor corrosion resistance to CI media (except for A4 stainless steel).
⑤ The price is higher (about 4 times that of chromic acid anodizing treatment).
4.2 Change in heat treatment state
Steel materials mainly have a multiphase structure (impurities, carbides, intermetallic compounds, etc. are usually present as cathodes in steel, while the Fe matrix is the anode). There is an electric potential difference between the phases in the multiphase structure, forming corrosion micro-cells.
The second phase may be a positive extreme passive phase or a cathode dissolution phase, both of which will affect the corrosion resistance performance of the base.
For example, stainless steel, it is necessary to be very careful during welding and heat treatment. After high-temperature solution treatment of stainless steel, it is heated between 400°C and 850°C, and a large amount of CrC. and Cr,C; carbides will precipitate along the grain boundaries, forming a poor Cr area near the grain boundaries.
The carbides act as the cathode of the corrosion cell, and the poor Cr area acts as the anode of the corrosion cell, thereby causing grain boundary corrosion, and the corrosion resistance will be greatly reduced.
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