Special attention should be paid to prevent white rust on zinc coating when storing galvanized steel sheet. White rust not only damages the appearance, but also reduces the zinc coating, resulting in the deterioration of product quality. Therefore, white
rust is a warning signal indicating that the storage environment is poor and should be treated immediately.
1. Causes of white rust formation
The surface of galvanized steel sheet will be oxidized in contact with air, and form a very thin oxide film, which can be used as a protective film to prevent further oxidation. However, if it is stored for a long
time under the condition of high humidity in the air, the carbon dioxide (C02) in the air will combine with zinc to form white powder white rust.
Generally speaking, white rust is produced rapidly after being soaked by dew or rain. In particular,
it occurs when the moisture is exposed.
2. What is condensation
Condensation refers to the condensation of supersaturated water vapor into liquid water due to the change of temperature. Common examples in daily life: if the container containing
ice water is taken out from the refrigerator, there are water droplets outside the container. In addition, when the temperature is high during the day and it becomes cold at night, dew will appear on trees and flowers. In the same way, dew will be produced
on the surface of steel plate.
3. How to prevent white rust:
(1) The surface treatment is carried out in the production of galvanized steel sheet.
a. Chromic acid treatment
Dip galvanized steel sheet into chromic acid solution
to form low activity film on the surface of zinc layer. Even if this kind of chromate film is slightly bruised, the chromate near the wound will dissolve in water and gather to the wound site to form a new membrane again. This kind of self-protection
can effectively prevent rust.
Chromic acid treatment is mainly used for the purpose of not painting; moreover, the surface coated with chromic acid is slightly yellow and should not be confused with rust.
The method of coating
oil on galvanized steel sheet to prevent direct contact between air and zinc. For condensation and water immersion, it has a strong anti rust effect. However, the oil on the steel plate surface must be wiped off by the user before painting.
If galvanized steel is to be used for painting, phosphate coating may be applied. Phosphate will adhere firmly to the galvanized surface, and the surface will be loose and porous, which can provide a good foothold for the coating and obtain
strong adhesion effect. On the user's side, paint can be applied as long as simple degreasing is applied.