Original board defect
The surface defects of galvanized sheet, such as creases, pits and wavy edges, continue to affect the galvanized sheet and become its surface defects. Therefore, before production, producers must inspect the original board.
the purity of zinc coating is not enough and metal impurities form galvanic cell on the surface of zinc, it is easy to produce black spots on the surface of zinc coating when it is beneficial to hydrogen evolution.
b. When the content of copper,
iron, arsenic and other impurities in the zinc coating is high, these impurities will appear black after passivation treatment.
c. The surface of the original plate is not fully treated, forming residue, so that some parts can not be plated with
zinc, resulting in defects of the zinc coating and easy to fall off, resulting in black spots.
d. It is easy to form black spots in post-treatment due to missed plating during galvanizing.
e. The residue or small black ash (carbon or carbon
compound) in the zinc plating solution adheres to the surface of the zinc layer or lives in the zic layer, and is firmly combined with the zinc layer. Black spots can be seen on the finished galvanized sheet.
f. Abrasion black spots. It is caused by rough operation during transportation and stacking.
The surface of galvanized steel sheet is affected by moisture or rain. Under a certain temperature, zinc is oxidized to produce
white powder. The anti-corrosion ability of white rust surface is greatly reduced, which affects its use.
Uneven zinc coating
The zinc coating on the surface of galvanized sheet is uneven, uneven in thickness, and even small particles appear
on the surface. There is excess liquid zinc on the surface of the strip steel coming out of the galvanizing pot. The excess liquid zinc is blown off with an air knife (spraying high-pressure gas), so as to control the thickness of the zinc layer. The
gas pressure of the air knife, the distance between the air knife and the strip, and the speed of the strip will directly affect the thickness of the zinc layer. If the gas supply of the air knife is unstable or the pressure of the air knife is insufficient,
the zinc coating will accumulate, that is, the zinc scar.
There are residues or scum (metallic or non-metallic inclusions) in the zinc plating solution. Sometimes these inclusions are mixed in the zinc coating. Shanghai model
ship model mechanical model industrial model makes its surface appear small round spots, and even makes the original plate not stained with zinc, which is easy to cause iron dew point.
Scratches and abrasions
When the galvanized coil passes through the tension leveler, there are foreign matters on the surface of the leveler, such as scratch and scratch.