(1) Raw material steel sheet defects
Surface defects of the raw material steel sheet, such as creases, pits, wavy edges, etc., continue to affect the Hot Dipped Galvanized Steel Sheet and become its surface defects. Therefore, before production, the manufacturer must inspect the raw material.
(2) Dark spots
a. The galvanized layer purity is not enough, and the metal impurities form a primary battery on the zinc surface. It is easy to have black spots on the surface when it is favorable for hydrogen evolution.
b. When impurities such as copper, iron, and arsenic in the galvanized layer are high, it will appear black after passivation treatment.
c. The surface of the raw material is not sufficiently treated to form residues, so that some parts cannot be plated with zinc, which causes the galvanized layer to be faulty and easy to fall off, resulting in black spots.
d. Leakage occurs during galvanizing, and black spots are easily formed during post-processing.
e. Residues or minor black ash (carbon or carbon compounds) are included in the galvanizing solution, which adheres to the zinc layer surface or resides in the zinc layer, then firmly combined with the zinc layer. As a result, black spots are visible on the finished galvanized sheet.
f. Abrasive black spots. It is caused by rough work during transportation and stacking.
(3) White rust
When the surface of galvanized steel sheet is wet or immersed by rain at a specific temperature, the anti-corrosion ability of the zinc surface is significantly reduced, which affects its use.
4) The galvanized layer is uneven
The galvanized sheet zinc layer is uneven, the thickness is uneven, and even small particles appear on the surface. The reason is due to the uneven air supply of the air knife during hot-dip galvanizing or the improper spacing between the air knife and the strip.
(5) Zinc scar (galvanized layer accumulation)
There is excess zinc liquid on the surface of the strip steel coming out of the galvanizing pot, and these excess zinc liquids are blown off with an air knife (high-pressure jet gas) to control the zinc layer thickness. The air knife gas pressure, the distance between the air knife and the strip, and the strip speed will directly affect the zinc layer thickness. If the air supply of the air knife is unstable or the air knife pressure is insufficient, the zinc coating will accumulate, that is, zinc scarring.