Due to factors such as production process and finished product packaging of hot-dip galvanized steel sheet, some manufacturers may have the following surface appearance defects:
(1) defects of the original plate.
The surface defects of the original galvanized sheet, such as creases, pits, wavy edges, etc., continue to affect the galvanized sheet and become its surface defects. Therefore, before production, producers need to test the original board.
(2) black spot.
There are many reasons for black spots, including:
A. the purity of zinc coating is not enough. When metal impurities form primary batteries on the surface of zinc, it is easy to produce black spots on the surface of zinc layer when it is favorable for hydrogen evolution.
B. the content of copper, iron, arsenic and other impurities in the zinc coating is high. After passivation, these impurities are black.
C. the treatment of dirt on the surface of the original plate is insufficient. In the process of production, the dirt will form residue on the surface, which will make some parts unable to be plated with zinc, resulting in the defect of zinc coating and easy falling off, thus causing black spots.
D. missing plating occurs during galvanizing. The missing part is easy to form black spots in post-treatment.
E. there is residue or small black ash (carbon or carbon compound) in the galvanizing solution. It adheres to the surface of zinc layer or resides in the zinc layer, and is firmly bound with the zinc layer. Black spots can be seen on the finished galvanized sheet.
F. abrade black spots. It is caused by rough operation during transportation and stacking.
Zinc coil, aluminum zinc plate, Tianfeng steel
(3) white rust.
Zinc is oxidized to produce white powder at a certain temperature when the surface of galvanized steel sheet is damped or immersed in rainwater. The anti-corrosion ability of the surface with white rust will be greatly reduced, which will affect its service life.
(4) the zinc coating is uneven.
Thezinc layer on the surface of galvanized sheet is uneven, uneven in thickness, and even small particles appear on the surface. There is excess zinc liquid on the surface of the strip steel coming out of the galvanizing pot. The excess zinc liquid 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 gas knife, the distance between the gas knife and the strip steel, and the speed of the strip steel will directly affect the thickness of the zinc layer. If the air supply of the air knife is not stable or the pressure of the air knife is insufficient, the zinc coating will accumulate, that is, the zinc scar.
(5) there are residues or dross (metal or non-metallic inclusions) in the slag containing galvanizing solution.
Sometimes these inclusions are mixed in the zinc coating, making small round spots appear on its surface, even making the original plate not stained with zinc, which is easy to cause iron spots.
(6) scratches and abrasions.
When the galvanized coil passes through the tension leveler, there are foreign matters in the leveler that cause surface scratches, scratches, etc.
(7) chromic acid dirt.
In order to improve the anti-corrosion ability of galvanized steel sheet, a very thin passivation layer can be formed on the surface of the strip, which can be treated by chromic acid, that is, chrome treatment. However, if the chromate treatment is poor and there is residual liquid, the surface of galvanized sheet will appear light yellow band or stain, or even black spot.