Fasteners are the most common parts of mechanical equipment used for fastening connections. They are used in specific environments, and the long-term interaction of fasteners with the environment will always cause its state and performance to occur. Change, that is, corrosion, which is one of the main forms of fastener failure." The corrosion of fasteners will affect the detachment and repeated installation of the thread in a light degree, and will damage the strength of the connection between the components and even lead to the failure of the workpiece. Sudden failure results in catastrophic accidents, so the corrosion resistance of fasteners has always been a topic of great concern to everyone.
Commonly used anti-corrosion technology for fasteners
The commonly used anti-corrosion technology for fasteners The anti-corrosion treatment of fasteners is generally to form a coating or anti-corrosion layer on the surface of the workpiece by a certain method to hinder 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 treatment technology, metal coating technology, coating technology and changing the internal structure of metal (such as stainless steel).
1. Film processing technology
The film treatment technology mainly refers to the treatment process of forming a stable chemical (electrochemical) conversion film on the metal surface by chemical or electrochemical methods. For example, in the urban rail vehicle, the film treatment of its fasteners is more black/blue treatment and phosphating treatment.
1.1. Blackening and bluing
The process of forming a chemical oxide film on the surface of steel parts in a concentrated alkaline solution containing an oxidant at about 140C for a certain period of time (the main components are Fe, O,).
Technical Features of Blackening/Blueing Treatment:
1) The thickness of the film layer is 0.5-1.5 μm.
2) Neutral salt spray test (NSS) is generally only 2 ~ 5 Hrs, at this time the oxide film layer has been broken, and even a lot of rust will appear.
3) Low sensitivity to hydrogen embrittlement, can be used as high-strength bolts.
4) As a fastener, its torque-preload consistency is poor.
5) Brighter color and better decorative effect.
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 insoluble phosphate conversion film on the metal surface is called phosphating. The 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 brittleness.
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 hydrogen embrittlement sensitivity is low and 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 give decorative or protective properties to metal materials. In urban rail vehicles, the metal coating technology of fasteners is mainly galvanized, and other special metal coatings (chrome, nickel, cadmium, silver, etc.).
2.1 Galvanized
Zinc and iron are mutually soluble, and the 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 refers to the immersion of iron and steel parts in molten liquid zinc, which causes a series of physical and chemical reactions on the surface of the workpiece to form a metal galvanized layer. The thickness of hot-dip galvanized coating is very thick (up to 30 ~ 60 μm), and its corrosion resistance is very good. It is widely used in long-term outdoor steel parts (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 hot-dip galvanizing process is very high (400C~ 500C), it is easy to temper and soften high-strength fasteners.
2.1.2 Electrogalvanizing
Electrogalvanizing is the use of 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 thin (5~30μm), and the corrosion resistance is the worst in the galvanized anti-corrosion treatment. Widely used in parts. Electrogalvanizing cannot be used for high-strength fasteners because electrogalvanizing has high sensitivity to hydrogen embrittlement, and it is difficult to completely remove hydrogen (the surface of electrogalvanized layer will peel or fall off at temperatures above 100C).
2.1.3 Mechanical galvanizing
Mechanical galvanizing refers to the surface treatment process of iron and steel parts under the action of chemical substances such as zinc powder, dispersants and accelerators, using impact medium to impact the surface of iron and steel parts 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 high temperature tempering, no hydrogen embrittlement, etc. A surface treatment process especially suitable for fastener corrosion protection.
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