Generally, the steels requiring anti-corrosion coating mainly include hot-dip galvanized steel and hot sprayed galvanized steel. Zinc is an active metal. The purpose of coating zinc coating is to provide cathodic protection and rust prevention function
for steel, which is the same as that of zinc rich primer.
The surface coated with zinc metal coating has the following differences compared with bare steel:
1. Whether hot-dip galvanized steel or hot sprayed galvanized steel, the surface is usually smooth, so it is impossible to spray clean like bare steel
to meet certain roughness requirements;
2. The active zinc surface will produce zinc salt after a period of exposure;
3. Active zinc is easy to saponify with fatty acids in some oily paints (such as alkyd, phenolic, etc.) or some active components
in other paints;
4. Since zinc metal coating has been provided for protection, the protective coating system after coating can be thinned accordingly.
In the market, the most commonly recommended primers for hot-dip galvanized steel and hot
sprayed galvanized steel surfaces mainly include the following two types:
1. Ethylene phosphating primer, also known as etching primer and washing primer, is a special paint containing phosphoric acid. The film (8-15 microns) formed by phosphating
primer is firmly attached to the metal surface and plays the role of phosphating and passivation.
Phosphating primer is rarely used at present because it contains zinc, chrome yellow and high lead content. It is usually only used when durability
requirements are not high. It is coated with alkyd, phenolic, epoxy ester and other low-grade paints.
2. Epoxy general purpose primer. Some manufacturers also call it epoxy multifunctional primer. It is a kind of primer specially designed for smooth
non-ferrous metal surface (including galvanized surface). The adhesion of the paint is strengthened to ensure its adhesion strength. At present, this kind of primer is the mainstream in the market.