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Hot-dip galvanizing (or simply hot-dip galvanizing) is a method of plating plated fasteners by dipping them into liquid metal zinc or its gold with a low melting point. Hot-dip galvanized fasteners mostly use solvent method.

Before the purified steel is dipped into the plating pan, a layer of flux is formed on the surface to prevent oxidation and remove surface residues. During dipping, the flux layer is volatilized by heat, so that the fresh steel surface is in direct contact with the molten metal, and the infiltration, dissolution, chemical reaction and diffusion occur, and the γ phase, δ1 phase and ζ phase alloy layers are formed on the surface of the steel parts in order to attach the layer of molten metal zinc, and the η phase zinc coating is formed after cooling. The zinc content of each phase alloy increased gradually from 77% to 99.97%. In modern hot-dip galvanizing production, a small amount of aluminum is usually added to the molten zinc solution to improve the adhesion of the galvanizing layer. At this time, the main phase in the alloy layer is Fe2AI5, while the number of δ1 phase and 7 phase is very small. There is no brittle y phase.

Hot dip galvanizing layer is a cheap and good corrosion resistance coating. Because of the electrochemical properties of zinc, it has a sacrificial protective effect on the steel matrix. The corrosion product (basic zinc carbonate) of the zinc layer in the atmosphere is relatively dense, well attached and insoluble in water, and has a certain protective effect on the zinc layer. Therefore, hot-dip galvanizing is widely used to protect steel and its parts from atmospheric corrosion, especially for the mass production of standard parts and fasteners.
 

It is generally thought that in hot dip galvanizing, the formation process of galvanizing layer is as follows: molten zinc penetrates the matrix iron in iron and reacts with zinc in pig iron - zinc alloy - in iron - zinc alloy layer. On the formation of pure zinc layer, the basic principle of the formation of galvanized layer is high-temperature metallurgical reaction. The metal, fasteners for hot dip galvanizing is usually used plating aid method (also known as solvent method), plating aid method hot plating _ process is divided into wet method and dry method two. The coating obtained by wet process has poor adhesion and is rarely used at present. The dry process allows a wide selection of plating AIDS and good coating quality. Therefore, the hot dip galvanizing of structural parts, fasteners and hardware is generally selected by dry method.

Hot-dip galvanized fasteners have the following characteristics:

(1) Can obtain thick coating, good corrosion resistance.

(2) The coating has good adhesion, and can be properly formed, welded and painted after plating.

Due to the small size of the fasteners, irregular shape, and some need to be threaded, there are also some obvious defects in hot dip galvanizing.
 

(1) It is difficult to screw the thread after plating. After hot dip galvanizing, the residual zinc sticks to the thread and is not easily removed, and the thickness of the zinc layer is not uniform, which affects the fit of the threaded parts. It is specified in GB/T13912 -- 1992 "Technical requirements for hot dip galvanized layer of Metal Covering Steel products" and GB/T2314-1997 "General Technical Requirements for Electric power Fixtures"; The external thread of the fastener should be processed or rolled according to the GB196 standard before the hot dip plating, while the internal thread can be processed before or after the hot dip plating. However, in practical applications, customers often require galvanized layers for both internal and external threads, so people adopt a variety of measures to solve the hot dip galvanized problem of threaded fittings. Such as thread parts after plating back attack; Reserve a large matching gap; Centrifuge and other methods. Backtapping easily destroys part of the thread coating, and even exposes the steel matrix, resulting in corrosion of fasteners. Deliberately increase the nut aperture or reserve the fit gap beyond the range, it is easy to reduce the fit strength, which is not allowed for high strength fit. At present, mechanical centrifugal dumping of residual zinc is common in China. In addition, people consider from the hot-dip galvanizing process itself, adjust the hot-plating temperature and dipping time to improve the quality of the coating, and it does not fundamentally solve the problem.
 

(2) The high operating temperature of hot-dip galvanizing will reduce the mechanical strength of high-strength fasteners. The strength of some threads of 8.8 bolts after hot dip galvanized is lower than the standard requirements; The strength of the bolts above 9.8 can not meet the requirements after hot dip galvanizing.
(3) Poor working environment and serious pollution. The hot dip galvanizing process of fasteners is carried out at high temperature, and strong irritating hydrogen chloride = gas will be precipitated when solvent drying and zinc dipping of the workpiece to be plated into the pool. Zinc pool is in high temperature for a long time, zinc vapor is produced on the surface of zinc pool, and the atmosphere of the whole working environment is bad.

At present, many private enterprises still use coal-fired reflection heating furnaces for hot-dip galvanizing production, which emits a large amount of smoke containing So2, CO2, CO and dust into the atmosphere, causing serious atmospheric pollution. Although there are many defects in hot dip galvanized fasteners, the hot dip galvanized coating is thick, the bond strength is good, and the long-term corrosion effect is good. In the power, communications, transportation sectors - always respected. For example, JB/T8177-1999 "General Technical Conditions for hot dip galvanized layer of metal accessories" put forward specific requirements for hot dip zinc at the end of the terminal, and DL/ T764-2002 "Cold heading hot dip galvanized bolts and nuts for fastening transmission line towers and power fixtures" made specific provisions for the hot dip galvanized layer at the thread position.

With the great development of electric power and transportation in China, the development of fastener hot dipping zinc will be promoted; Therefore, it is very important to develop automatic centrifugal dumping equipment, improve fastener hot-dip galvanizing and improve the quality of hot-dip galvanizing fastener coating.