Galvanized iron sheet has good corrosion resistance because the zinc layer can be used as a sacrificial anode to protect the underlying iron sheet from corrosion. However, in the environment of acid, alkali and salt medium, because the zinc layer is very active, it will darken and corrode. In order to improve the life of the galvanized iron sheet, it is generally used to apply paint to solve it.
Galvanized iron sheet components are often phosphating or passivated before painting, and usually just wipe off the grease and paint with ordinary paint (red lead paint, alkyd enamel paint), etc. It is found that the paint film will peel off after a long time, which is a common disadvantage of painting on galvanized iron sheets or zinc-based alloy sheets. Often, many paint users do not understand the reason for the peeling of the paint film. They only think that the quality of the paint is poor and there is a problem with the construction, but they do not know the reason for the improper selection of the coating design and the matching of the paint variety.
Grease coatings or alkyd coatings contain drying oil and many double bonds, which will be oxidized rapidly and dry to form a film under the catalysis of cobalt and jade soap. However, after their film formation, the oxidation does not stop, and continues slowly. As a result of oxidation, many by-products aldehyde, carboxylic acid and formic acid are produced. Among organic acids, formic acid is quite strong, and its ionization constant is 1.77×10- 4 (25°C), it can react with zinc substrates to form zinc formate. Zinc formate has certain water solubility and cannot form an insoluble corrosion product shielding layer. As a result of continuous corrosion, new loose zinc formate is formed between the zinc surface and the paint film, which is about 10 times larger than the original zinc volume. times, thereby reducing the adhesion, forming a large peeling off of the paint film. White powder can be seen behind the peeling paint film, which is mostly zinc formate after analysis.
Coating design of galvanized iron sheet or zinc-based alloy sheet:
1) Zinc-rich primer or calcium lead acid primer.
2 ) Apply X06-1 phosphating primer.
3 ) Epoxy asphalt paint or chlorosulfonated polyethylene paint.
The above-mentioned paint can be coated once after the surface of the zinc layer is degreased, and then the top coat can be applied. Because they do not contain grease double bonds, they will not oxidize to produce zinc formate and cause the paint film to peel off, so they have good adhesion to the zinc surface.
