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Analysis Of Quality Problems And Causes Of Hot Dip Galvanized Steel Plate

Considering the factors such as the production process and finished product packaging of hot dip galvanized steel plate, there may be some surface defects as follows:


(1) defects of the original plate

Surface defects of galvanized original sheet, such as creases, pits, wavy edges, etc. continue to affect galvanized sheet and become its surface defects.

Therefore, before production, the producer must inspect the original board.


(2) black spots

A. galvanized layer purity is not enough, metal impurities on the zinc surface to form a galvanic cell, beneficial to hydrogen evolution, easy to make the zinc layer surface black spots.

B. Copper, iron, arsenic and other impurities in the galvanized layer are high, after passivation treatment, these impurities appear black.

C. the surface of the original plate corrosion treatment is not sufficient, the formation of residue, so that some parts of the zinc plating, leading to zinc layer problems and easy to fall off, resulting in black spots.

D. Missing plating occurs during galvanizing, which is easy to form black spots during post-treatment.

E. The zinc solution is mixed with residue or small black ash (carbon or carbon compounds), which adheres to the surface of the zinc layer or resides in the zinc layer and is firmly combined with the zinc layer.

Black spots can be seen on the finished galvanized sheet.

F. Abrasive black spots.

By the transportation, stacking process caused by rough operation.


(3) white rust

The surface of galvanized steel plate is wet or soaked by rain. Under a certain temperature, zinc is oxidized and white powder is produced.

The antiseptic ability of the surface with white rust is greatly reduced, which affects its use.


(4) uneven galvanized layer

The zinc layer on the surface of galvanized sheet is uneven, uneven in thickness, and even small particles appear on the surface.

Excess liquid zinc on the surface of the strip coming out of the galvanizing pan is blown off with an air knife (injection of high pressure gas) to control the thickness of the zinc layer.

The gas pressure of air knife, the distance between air knife and strip steel, and the speed of strip steel will directly affect the thickness of zinc layer.

If the gas supply of air knife is unstable or the pressure of air knife is insufficient, it will cause the accumulation of zinc layer, that is, zinc scar.


(5) slag inclusion

There is residue or scum (metal or non-metal inclusion) in the zinc plating solution, sometimes these slag inclusion in the zinc layer, the Shanghai model ship model, mechanical model, industrial model, the surface of the small round spots, and even the original plate is not stained with zinc, it is easy to cause iron exposure.


(6) scratches and abrasions, etc

When the galvanized coil passes through the tension straightener, the surface scratches and abrasions are caused by foreign bodies in the straightener.


(7) chromic acid fouling

In order to improve the anti-corrosion ability of galvanized steel sheet, a very thin passivation layer is formed on the surface of strip steel, which can be completed by chromic acid treatment, that is, chroming treatment.

But if chromic acid treatment is poor, there is residual liquid, galvanized sheet surface will appear light yellow belt or stain, or even black spot.

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