Analysis of Quality Problems and Causes of Galvanized Barrel Raw Materials

Considering the production process of galvanized barrel hot-dip galvanized steel sheet and finished product packaging and other factors, hot-dip galvanized steel sheet may have some of the following surface appearance defects:

(1) Original plate defect

The surface defects of galvanized original plate, such as creases, pits, wavy edges, etc., continue to affect the galvanized sheet and become the surface defects of its galvanized barrel. Therefore, before production, producers must inspect the original plate.

(2) Dark spots

A. The purity of the galvanized layer is not enough, and the metal impurities form the primary battery on the zinc surface, which is beneficial to hydrogen evolution, and it is easy to produce black spots on the surface of the zinc layer.

B. When the content of copper, iron, arsenic and other impurities in the galvanized layer is high, after passivation treatment, these impurities appear black.

C. The surface of the original plate is not adequately treated with contaminants, forming residues, so that some parts cannot be galvanized, resulting in problems in the galvanized layer and easy to fall off, resulting in dark spots.

D. Leak plating occurs during galvanizing, and dark spots are easily formed during post-processing.

E. The galvanizing solution of the galvanizing barrel is mixed with residue or small black ash (carbon or carbon compound), which adheres to the surface of the zinc layer or lives in the zinc layer, and is firmly combined with the zinc layer. Dark spots can be seen on the finished galvanized sheet.

F. Abrasive black spots. Caused by rough work during transportation and stacking.

(3) White rust

The surface of galvanized steel plate is damp or rain immersed, and at a certain temperature, zinc is oxidized to produce white powder. The anti-corrosion ability of the surface that produces white rust is greatly reduced, which affects its use.

(4) Uneven galvanizing layer

The zinc layer on the surface of the galvanized sheet is uneven, uneven in thickness, and even small particles on the surface. There is excess zinc liquid on the surface of the strip steel that comes out of the galvanizing pot, and these excess zinc liquid are blown off with an air knife (spraying high-pressure gas) to control the thickness of the zinc layer. The gas pressure of the air knife, the distance between the air knife and the strip steel, and the speed of the strip steel will directly affect the thickness of the zinc layer. If the air knife supply is unstable or the air knife pressure is insufficient, the galvanized layer will accumulate, that is, zinc scars.

(5) Slag inclusion

There are residues or dross (metal or non-metal inclusions) in the galvanizing liquid, and sometimes these inclusions are mixed in the galvanizing layer of the galvanizing drum. Small round spots appear on the surface of the galvanizing drum, and even the original plate cannot be stained with zinc, which can easily cause iron spots.

(6) Scratches and abrasions, etc

When the galvanized coil passes through the tension straightening machine, there are foreign objects in the straightening machine that cause surface scratches, abrasions, etc.

(7) Chromic acid fouling

In order to improve the anti-corrosion ability of galvanized steel sheet and form a very thin passivation layer on the surface of the strip, the galvanized drum can be treated with chromic acid, that is, chromium treatment. However, if the chromic acid treatment is not good and there is residual liquid, the surface of the galvanized sheet will appear light yellow band or stain, or even black spot.