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Analysis Of Quality Problems And Causes Of Galvanized Sheet

Similar to the production process of hot galvanized steel plate, the production process of electric galvanized steel plate is also the use of continuous electroplating method, divided into pretreatment (cleaning), plating after treatment of two steps.


The quality of galvanized sheet depends on the washing quality before electroplating to a great extent.


The main purpose of electrogalvanizing post-treatment is to improve the surface coating performance and corrosion resistance of galvanized sheet, so as to prolong the service life of galvanized sheet.


The post-plating treatment mainly includes phosphating treatment and chroming treatment.

In addition to the original plate defects, any process improper operation, will cause quality problems.


Common surface defects are:


(1) pinhole

Refers to the surface of the coating and needle chisel similar holes, its density and distribution is not the same, but under a magnifying glass observation, generally its size, shape similar.

Pinholes are usually defects caused by the adsorption of hydrogen bubbles in the electroplating process.

In the electroplating reaction, a large number of bubbles are produced. If these bubbles are not removed in time, they will be detained between the strip surface and the anode. The anode and the strip surface are covered by a gas layer and affect the conduction, resulting in local plating on the strip surface.


(2) pitting

Refers to irregular concave holes on the surface of the coating, which is characterized by different shapes, sizes and shades.

Pitting is usually caused by substrate defects or adhesion of foreign bodies in the plating process.


(3) Burr (or rough)

Refers to the plating surface raised with a prickly feeling of foreign bodies, usually characterized by the plating to the upper or high current density area is more significant.

The thickness of zinc layer is controlled by controlling the current density and strip speed, that is, increasing the current density and decreasing the strip speed can increase the thickness of zinc layer.

But this is a certain limit, when the strip speed is very low, the current density is high, the coating surface will become rough, but affect the quality of the coating.


(4) bubbling

Refers to the bulging vesicles on the surface of the coating, which are characterized by different size, density, and separation from the substrate.

In general, zinc alloy, aluminum alloy coating is more obvious.


(5) Peeling (or peeling)

Refers to the coating and substrate peeling cracked or non-cracked defects.

It is usually caused by poor treatment before plating.


(6) spots

Refers to color spots, dark spots and other defects on the surface of the coating.

It is caused by poor deposition of metal ions in the plating process, adhesion of foreign bodies or cleaning of passivation solution after passivation treatment.


(7) Yin and Yang

Refers to the coating surface local brightness or uneven color defects, in most cases in the same product shows a certain regularity.


(8) no local coating

The surface of the coating has defects of different sizes and shapes such as leakage, iron spots or spots (except as specified in the process).


In addition to the surface defects described above, the surface of the coating is sometimes damaged, scratched, white rust, roll marks, pits, creases, black spots, unwashed traces of salt, water traces, erasable or brown, brown passivation film and dendritic, spengy and striped coating defects.

All these defects will have a certain impact on the use of galvanized sheet.

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