Possible quality problems and causes of galvanized steel coils

1. Hot-dip galvanized steel

Taking into account factors such as the production process of hot-dip galvanized steel and the packaging of finished products, hot-dip galvanized steel may have the following surface appearance defects:

(1) Defects of the original board

The surface defects of the original galvanized coil, such as creases, pits, wavy edges, etc., continue to affect the galvanized coil and become its surface defects. Therefore, before production, the manufacturer must inspect the original board.

(2) Dark spots

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

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

c. The surface of the original plate 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 small black ash (carbon or carbon compounds) are included in the galvanizing solution, which adhere to the surface of the zinc layer or reside in the zinc layer, and are firmly combined with the zinc layer. Black spots are visible on the finished galvanized coil.

f. Abrasive black spots. It is caused by rough work during transportation and stacking.

(3) White rust

When the surface of galvanized steel coil is wet or immersed by rain, at a certain temperature, zinc is oxidized to produce white powder. The anti-corrosion ability of the surface with white rust is greatly reduced, which affects its use.

(4) The galvanized layer is uneven

The zinc layer on the surface of the galvanized coil is uneven, the thickness is uneven, 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, and these excess zinc liquid are blown off with an air knife (jet 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, and the speed of the strip will directly affect the thickness of the zinc layer. 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.



(5) Slag inclusion

There are residues or scum (metallic or non-metallic inclusions) in the galvanizing solution, and sometimes these slag inclusions are included in the galvanized layer. On zinc, it is easy to cause dew point.

(6) Scratches and abrasions, etc.

When the galvanized coil passes through the tension leveler, there are foreign objects in the leveler that cause surface scratches and abrasions.

(7) Chromic acid dirt

In order to improve the anti-corrosion ability of galvanized steel coil, a very thin passivation layer is formed on the surface of the strip steel, which can be completed by chromic acid treatment, that is, chromating treatment. However, if the chromic acid treatment is not good and there is residual liquid, the surface of the galvanized coil will appear yellowish bands or stains, or even black spots.

2. Electro-galvanized coil

Similar to the production process of hot-dip galvanized steel, the production process of electro-galvanized steel also adopts continuous electroplating method, which is divided into two steps: pretreatment (cleaning section) and post-plating treatment. The quality of electro-galvanized coil depends to a large extent on the quality of washing before electroplating. The main purpose of post-treatment of electrogalvanizing is to improve the surface paint performance and corrosion resistance of galvanized coil, so as to prolong the service life of galvanized coil. The post-plating treatment mainly includes phosphating treatment and chroming treatment. In addition to the original board defects, improper operation of any process will cause quality problems. Common surface defects are:

(1) Pinhole

Refers to the fine holes on the surface of the coating that are similar to those chiseled by the needle tip. Although their density and distribution are different, they are generally similar in size and shape when observed under a magnifying glass. Pinholes are usually defects created by hydrogen bubble adsorption during electroplating. In the electroplating reaction, a large number of bubbles are generated. If these bubbles are not removed in time, they will stay between the surface of the strip and the anode. The anode and the surface of the strip are covered by a gas layer, which affects the electrical conductivity, causing the surface of the strip to be partially plated. .

(2) pitting

Refers to the irregular concave holes on the surface of the coating, which are characterized by different shapes, sizes and depths. Pockmarks are generally caused by defects in the substrate or the adhesion of foreign objects during the electroplating process.

(3) Burrs (or roughness)

Refers to the foreign body that is raised on the surface of the coating and has a thorny feeling, usually characterized by being more prominent in the upper surface of the electroplating or in the high current density area. The thickness of the zinc layer in electro-galvanizing is controlled by controlling the current density and the strip speed, that is, increasing the current density and decreasing the strip speed can increase the thickness of the zinc layer. But this has a certain limit. When the strip speed is very low and the current density is high, the surface of the coating will become rough, which will affect the quality of the coating.

(4) Bubbling

Refers to the raised small bubbles on the surface of the coating, which are characterized by different sizes and density, and are separated from the substrate. Generally, it is more obvious in zinc alloy and aluminum alloy coating.

(5) Peeling (or falling off)

Refers to the cracked or non-cracked defects of the coating and the substrate peeling off. Usually caused by poor pre-plating treatment.

(6) Spots

It refers to the defects such as color spots and dark spots on the surface of the coating. It is caused by poor deposition of metal ions in the plating layer, foreign matter adhesion or poor cleaning of the passivation solution after passivation treatment during the electroplating process.

(7) Yin and Yang side

It refers to the defects of uneven brightness or uneven color on the surface of the coating, and in most cases, it shows a certain regularity in similar products.

(8) No local coating

The surface of the coating has defects of different sizes and shapes, such as leakage plating, exposed iron spots or point defects (except for process regulations).

In addition to the above-mentioned surface defects, the coating surface sometimes has damage, scratches, white rust, roll marks, pits, creases, black spots, unwashed salt marks, water marks, wipeable or Brown, brown passivation film and defects such as dendritic, spongy and striped coatings. All these defects will have a certain impact on the use of galvanized coil.

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