Similar to the production process of hot-dip galvanized steel sheets, the production process of electro-galvanized steel also adopts the continuous electroplating method, which is divided into two steps: pre-treatment (cleaning section) and post-plating treatment. The quality of electroplated galvanized sheets depends to a large extent on the quality of washing before electroplating. The main purpose of electro-galvanizing post-treatment is to improve the surface painting performance and corrosion resistance of galvanized sheets to extend the service life of galvanized sheets. Post-plating treatment mainly includes phosphating treatment and chromating treatment. In addition to defects in the original board, improper operation of any process will cause quality problems. Common surface defects include:
(1) Pinhole
Refers to the pores on the surface of the coating that are similar to those drilled by a needle tip. Although their density and distribution are different, when observed under a magnifying glass, they are generally similar in size and shape. Pinholes are usually defects caused by the adsorption of hydrogen bubbles during the electroplating process. During the electroplating reaction, a large number of bubbles are generated. If these bubbles are not eliminated in time, they will remain between the surface of the strip and the anode. The anode and the surface of the strip are covered by the gas layer, which affects the conductivity, causing the surface of the strip to be partially unplated.
(2) Pitting
Refers to the irregular recessed holes on the surface of the coating, which are characterized by different shapes, sizes, and depths. Pitting is generally a defect caused by defects in the substrate or adhesion of foreign matter during the plating process.
(3) Burrs (or roughness)
It refers to the foreign matter that is raised on the surface of the plating layer and has a stinging feeling. It is usually characterized by being more prominent in the upper part of the plating or in the high current density area. The thickness of the zinc layer in electroplated zinc is controlled by controlling the current density and strip speed. That is, increasing the current density and reducing 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 vesicles on the surface of the coating, which are characterized by varying sizes and density, and are separated from the substrate. Generally, it is more obvious in zinc alloy and aluminum alloy coatings.
(5) Peeling (or falling off)
Refers to crack-like or non-crack-like defects caused by peeling off of coating and substrate. Usually caused by poor pre-plating treatment.
(6) Spots
Refers to defects such as color spots and dark spots on the surface of the coating. It is caused by poor metal ion deposition in the coating during the electroplating process, adhesion of foreign matter, or incomplete cleaning of the passivation solution after passivation treatment.
(7) Yin and Yang side
It refers to the defect of uneven local brightness or uneven color on the surface of the coating. In most cases, it shows a certain regularity in similar products.
(8) Partially without coating
There are defects such as missing plating, iron spots or points of different sizes and shapes on the surface of the coating (except for process requirements).
In addition to the above-mentioned surface defects, the coating surface sometimes has damage, scratches, white rust, roller marks, pits, creases, black spots, unwashed salt traces, water stains, wipeable or Defects such as brown and brown passivation films and dendritic, spongy and striped coatings. All these defects will have a certain impact on the use of galvanized sheets.