1, the principle is different

Cold galvanized coating mainly relies on electrochemical principle for corrosion protection; Hot dip galvanizing, also known as hot dip galvanizing, is a method to obtain metal coating by immersing steel components in molten zinc.

2. Different corrosion resistance.

The corrosion resistance of hot galvanizing is higher than that of cold galvanizing (also called electro-galvanizing). Hot dip galvanizing will not rust for several years, while cold dip galvanizing will rust within three months.

3. Different thickness

The zinc layer of hot dip galvanizing itself is relatively thick, and the thickness exceeds 10um. Although the surface is bright, it is rough and there will be zinc flowers. However, the galvanized layer is very thin, only 3-5um thick. Although the surface is smooth, it will be dark and dirty, with good processability and insufficient corrosion resistance.

Extended data:

Mechanical derusting (sandblasting, shot blasting, etc.) shall be carried out before spraying cold galvanized. ) It should be selected to make the steel surface clean to ISO 85O 1— 1 Sa 2.5, which is equivalent to GB 8923Sa2.5. Literally, it is defined as: "After mechanical derusting, the steel surface is free of dirt such as oil, rust and scale, or only leaves slight traces;" 95% of the steel surface is impacted by steel shot (sand), showing metallic luster. "

For another index of surface treatment quality-roughness, different cold galvanized materials have different requirements for roughness. In the construction instructions of Zinga, the average roughness Ra = 12.5 is required, that is, the roughness Rz = 55 ~ 75 microns. In the construction instructions of "Strong Zinc" and "Robaru", the requirement for roughness is slightly lower, Rz=40~60 microns.

If the cold-dip galvanized film is formed separately, the surface roughness can be slightly controlled to ensure the coating thickness of cold-dip galvanized. However, when cold galvanizing is used as the bottom layer of heavy-duty anti-corrosion coating, and the total thickness of the coating is greater than 180 micron, the surface roughness Rz of steel should be greater than 60 micron to ensure the adhesion of the whole coating.

It is best to use sandblasting to remove the old paint film and rust spots, and the steel surface still has a certain roughness. If conditions are limited, manual grinding and pneumatic (electric) tools can also be used for derusting, and the derusting grade standard reaches ISO 8501-1ST 3, that is, very thorough manual and electric tools for derusting. Its literal definition is: "The surface should be free of visible grease and dirt, and there is almost no scale, rust, old coating and foreign matter that is not firmly attached.

The surface should have the luster of a metal substrate. "Different brands of cold galvanized materials have different requirements for surface treatment. For example, before painting in Tianjin, Belgium, there is a slight corrosion area of about 5% on the steel surface; The coating of Roval 60 ~ 80 microns (dry film thickness) has excellent adhesion to steel, and the pulling method can reach 6. 1MPa when the steel surface is polished to st 3 level according to the test of China Institute of Shipbuilding Technology.

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