Understand Deeply About The Difference Between Hot And Cold Galvanizing

Appearance

Hot dip galvanizing is a common galvanizing process because of its thick zinc layer, about 10um or more, strong corrosion resistance. However, it is very thin, about 3-5um. The surface of hot dip galvanizing is rough and bright. In serious cases, there are zinc flowers, while electroplating is smooth and gray or dirty. The galvanized steel plate produced by electroplating method has good processing performance, but the coating is thinner and less corrosion resistance than that of hot dip galvanized steel sheet. Cold galvanizing is electro-galvanizing, the amount of zinc is very small, only galvanizing on the outer pipe wall (hot plating - both inside and outside), only 10-50g / M2, its corrosion resistance is much different from that of hot galvanizing.

Chemical principle of galvanizing

In chemical industry, there are many kinds of electro-galvanizing, which are suitable for small parts; hot galvanizing is generally used for power equipment and components, and it is suitable for large parts and equipment.

Electro-galvanizing, commonly known as galvanizing, uses the electro-chemical method to take zinc as the old pole. When the zinc atom loses its electrons, it becomes an ionic state and dissolves into the electrolyte. When the steel belt is used as the cathode, the zinc ions are reduced to zinc atoms on the steel strip and deposited on the surface of the steel strip to form a coating. In order to remove the iron oxide on the surface of the steel pipe, after pickling the steel pipe shall be cleaned by ammonium chloride or chlorination solution or mixed solution of ammonium chloride and zinc chloride, and then sent to the hot-dip tank.

Zinc content

There is a great difference in the amount of galvanizing between hot-dip galvanized sheet and electro galvanized sheet. The amount of hot-dip galvanizing can not be too less, generally the minimum is 50 ~ 60g / m2 on both sides, and the maximum is 600g / m2. The galvanized layer of the electro galvanized sheet can be very thin, with a minimum of 15g / m2. However, if the thickness of the separate layer is required, the production line speed is very slow, which is not suitable for the process characteristics of modern units. Generally, the maximum is about 100g / m2. Because of this, the production of galvanized sheet is limited.

Coating structure

There are fundamental differences between hot-dip galvanized sheet and electro galvanized sheet in the coating structure. There is a slightly brittle compound layer between the pure zinc coating and the steel belt matrix of the hot-dip galvanizing plate. When the pure zinc coating is combined, it mostly forms flower, and the coating is uniform without pores. However, the zinc atoms in the electroplated layer are only deposited on the surface of the steel belt, and they are attached to the surface of the steel strip by physical action. There are many orifices, which are easy to cause pitting corrosion due to corrosive medium. Therefore, the hot-dip galvanized plate is more corrosion-resistant than the electroplated galvanized plate.

Heat treatment process

The heat treatment process of hot-dip galvanizing and electro-galvanizing is totally different. Generally, the hot-dip galvanizing plate takes cold plate as raw material, annealing and hot-dip galvanizing are carried out continuously on the galvanizing line, and the steel strip is heated in a short time and then cooled, so the strength and plasticity are affected to a certain extent, and its stamping performance is worse than that of the cold-dip galvanizing plate after degreasing and annealing on the same professional production line. The galvanized sheet is made of cold-rolled sheet, which basically guarantees the same processing performance of cold-rolled sheet, but its complex process also increases the production cost. In a word, the production cost of hot-dip galvanized sheet is lower and its application is more extensive, which has become the main variety of galvanized sheet market.