Basic principles and process flow of hot-dip galvanizing
Hot-dip galvanizing production, according to the process, is generally divided into "continuous hot-dip galvanizing" process and "batch hot-dip galvanizing" process; professional galvanized sheets, galvanized pipes, strip steel and other industries generally use "continuous hot-dip galvanizing" Process: The hot-dip galvanizing industry for iron towers, fittings, steel structures and other miscellaneous parts generally adopts the "batch hot-dip galvanizing" process. The hot-dip galvanizing production that the author is engaged in is all the latter.
Hot-dip galvanizing production, according to process layout, is generally divided into European-style, Japanese-style and other production layouts. "Batch hot forging zinc" enterprises use both production line process layouts.
(1) Basic principles of hot-dip galvanizing
Hot-dip galvanizing, also known as hot-dip galvanizing, is different from electro-galvanizing! Hot-dip galvanizing process, among various coating methods for protecting steel substrates, hot-dip galvanizing is a very excellent one. In liquid zinc, after the steel workpiece has undergone physical and chemical treatment, the steel workpiece is immersed at a temperature of 440°C to 465°C (that is, the temperature of the linear corrosion zone and the high parasitic corrosion zone, the temperature suitable for hot-dip galvanizing of different steel workpieces The range is different), or the process of treatment in molten zinc at higher temperatures. The steel matrix reacts with the zinc liquid to form a Zn-Fe gold layer and a pure zinc layer covering the entire surface of the steel workpiece. The galvanized surface has a certain degree of toughness, can withstand great friction and impact, and has good bonding with the matrix.
This plating method not only has the corrosion resistance characteristics of electroplated zinc, but also has a Zn-Fe alloy layer. It also has strong corrosion resistance that cannot be matched by electroplated zinc. Therefore, this plating method is particularly suitable for strong corrosive environments such as various strong acids and alkali mist.
Hot-dip galvanizing is different from electro-galvanizing. Electro-zinc plating is a zinc coating formed on the cathode during the electrolysis process. In fact, electroplating zinc is a deposition process that gradually precipitates zinc atoms. That is, when electroplating begins, tiny crystal nuclei are generated on the surface of the iron base. These single crystal nuclei increase as the electroplating time increases, and finally join together to form a galvanized layer. In contrast, hot-dip galvanizing is not a deposition process, but a dissolution process of zinc to iron. It is a galvanized layer formed by the dissolution of iron in molten zinc liquid. This process is actually a "zinc welding film hanging" process.
The hot-dip galvanizing layer is formed by zinc in a high-temperature liquid state in three steps.
Three steps of hot dip galvanizing:
The formation of hot-dip galvanizing layer is achieved through the following steps:
(1) The surface is dissolved by the zinc liquid; the iron-based surface is dissolved by the zinc liquid to form a Zn-Fe alloy phase.
(2) The zinc ions in the alloy layer further diffuse to the matrix to form a Zn-Fe mutually dissolved layer; iron forms a Zn-Fe alloy during the process of being dissolved by the zinc liquid and continues to diffuse.
(3) A zinc layer is wrapped on the surface of the Zn-Fe alloy layer, and the zinc layer cools and crystallizes to form a zinc coating.
Therefore, the galvanized protective layer obtained by directly immersing the decontaminated and rust-removed steel materials in the molten zinc liquid is very strong. At the same time, its diffusion and coverage capabilities are far better than electroplated zinc.
The so-called "diffusion ability" and "covering ability" refer to the uniformity and integrity of the galvanized layer on the surface of the plated workpiece. "Diffusion ability" is also called "throwing ability", which refers to the ability to evenly distribute the thickness of the galvanized layer; "coverage ability" is also called "depth ability", "mouth cover ability" or "landing ability". In terms of these two capabilities, hot-dip galvanizing is much stronger than electro-galvanizing. For example, the deep recesses on the surface of the workpiece, or the inner hole wall of the tubular workpiece, are often very shallow or even impossible to be plated with electro-galvanizing. Hot-dip galvanizing, no matter how complex the shape of the workpiece, can obtain a good galvanized layer that is consistent inside and outside.
Hot dip galvanizing performance characteristics
It has a thick, dense layer of pure zinc covering the surface of the steel, which can avoid the contact of the steel matrix with any corrosive solution and protect the steel matrix from corrosion. In the general atmosphere, a very thin and dense zinc oxide layer is formed on the surface of the zinc layer. It is difficult to dissolve in water, so it plays a certain protective role on the steel matrix. If zinc oxide forms insoluble zinc salts with other components in the atmosphere, the anti-corrosion effect will be more ideal.
After hot-dip galvanizing, steel has a Zn-Fe alloy layer that is densely bonded and exhibits unique corrosion resistance in marine salt spray atmosphere and industrial atmosphere. Due to the strong bonding, Zn-Fe is mutually soluble and has strong wear resistance. Because zinc has good ductility, its alloy layer adheres firmly to the steel matrix. Therefore, hot-dip galvanized workpieces can be cold punched, rolled, drawn, bent, etc. without damaging the galvanized layer.
Hot-dip galvanizing of steel workpieces is equivalent to an annealing treatment, which can effectively improve the mechanical properties of the steel matrix, eliminate the stress during forming and welding of steel workpieces, and is conducive to turning steel workpieces.
The surface of hot-dip galvanized steel workpieces is bright and beautiful. The pure zinc layer is the most plastic zinc layer in hot-dip galvanizing. Its properties are basically close to pure zinc and has ductility, so it is flexible.