Hot-dip galvanizing, hot-dip galvanizing and electrogalvanizing (cold galvanizing)

1. what is hot plating?

1. Definition and basic characteristics of hot plating

Hot dip plating (hot dip), referred to as hot dip plating, is a method of plating metal materials immersed in other liquid metals or alloys with lower melting points.

The basic feature of hot-dip galvanizing of steel materials is that there is an alloy layer (alloy layer) between the steel substrate and the zinc coating, and when the surface of the steel piece is raised from the molten metal zinc, a layer of molten metal zinc is attached to the surface of the alloy layer, and the metal zinc coating is formed after cooling and solidification.

2. Hot-dip substrate and coating material

Iron and steel material is the main base material of hot dip plating, so the melting point of the metal as the coating material must be much lower than the melting point of steel.

It is generally believed that steels suitable for hot-dip galvanizing are rimming steels with low silicon content, and the metal manuals in the United States indicate that the silicon content wSi is required to be less than 0.05%.

In the domestic literature pointed out that Q235 steel class B steel is the most suitable for hot-dip galvanizing, this kind of steel hot-dip galvanized to get close, continuous, uniform intermetallic compound organization.

The popularity of continuous steel casting technology has brought new problems to hot galvanizing. The new steel has a silicon content of more than 0.05 percent. Therefore, if it is not a low-carbon steel, or a workpiece with a low or high silicon content, it is difficult to work on hot-dip galvanizing, but hot-dip galvanizing can be achieved by other processes.

Plating material

Commonly used coating metals are zinc (melting point of 419.5 ℃), aluminum (melting point of 658.7 ℃), tin (melting point of 231.9 ℃) and lead (melting point of 327.4 ℃). Compared with other metals, zinc is a relatively cheap and easy to plate a metal, belongs to the low value anti-corrosion coating, is widely used to protect steel parts, especially to prevent atmospheric corrosion, and for decoration.

3. Plating technology

It mainly includes slot plating (or hanging plating), barrel plating (suitable for small parts), automatic plating and continuous plating (suitable for wire and strip).

At present, according to the classification of electroplating solution in China, it can be divided into four categories: cyanide zinc plating; zincate zinc plating; chloride zinc plating; sulfate zinc plating.

2. what is hot dip galvanizing?

1. Definition of hot galvanizing

Hot dip galvanizing, also known as hot dip galvanizing (galvanizing), is a method for steel components to be immersed in molten zinc to obtain a metal coating.

2. Formation process of hot-dip galvanizing layer

It refers to the process of forming an iron-zinc alloy between the iron matrix and the outermost pure zinc layer. The iron-zinc alloy layer is formed on the surface of the steel part during hot-dip plating, which makes the iron and the pure zinc layer well combined. The process can be simply described:

When the iron workpiece is immersed in molten zinc, a solid solution of zinc and alpha iron (body core) is first formed at the interface. This is the matrix metal iron in the solid state dissolved zinc atoms to form a crystal, between the two metal atoms is fusion, the attraction between the atoms is relatively small.

Therefore, when the zinc in the solid solution to reach saturation, zinc and iron two kinds of element atoms interdiffusion, diffusion into (or called infiltration) iron matrix zinc atoms in the matrix lattice migration, and gradually form an alloy with iron.

The iron that diffuses into the molten zinc forms an intermetallic compound FeZn13 with zinc and sinks into the bottom of the hot-dip galvanizing pot, which is zinc slag.

When the steel part is removed from the zinc immersion liquid, a pure zinc layer is formed on the surface, which is hexagonal crystal and its iron content is not more than 0.003%.

3. The whole process of hot-dip galvanizing:

Taking the "dry" hot-dip galvanizing as an example, the steel pipe is coated with zinc chloride and ammonium plating solvent, after being preheated to a temperature of 150~200 ℃ on the drying furnace, it enters the zinc liquid (bath) with a temperature of 440~450 ℃.

First of all, the iron and steel parts absorb a lot of heat in an instant, so that the zinc liquid on the surface of the iron and steel parts is suddenly cooled, so that it solidifies on the surface of the iron and steel parts in an instant, forming a layer of solid shell. Due to the continuous large supply of heat, this layer of shell is quickly heated and melted.

When the surface temperature of the steel and iron parts reaches equilibrium with the temperature of the molten zinc, the molten zinc begins to interact with the steel. After a period of time, the iron-zinc alloy layer is produced on the surface of the steel; the outer side of the iron-zinc alloy layer is connected with the pure zinc layer, and the pure zinc layer is crystallized after cooling, and the steel substrate is connected inside the zinc layer.