Analysis of surface treatment process of galvanized steel grating before painting

Analysis of surface treatment process of galvanized steel grating before painting

Hot-dip galvanizing (hot-dip galvanizing for short) on the surface of steel grating is the most common and effective surface protection technology for controlling environmental corrosion of steel parts. In general atmospheric environment, the hot-dip galvanizing coating obtained by this technology can protect steel parts from rusting for several years or more than 10 years. For parts without special anti-corrosion requirements, there is no need for secondary anti-corrosion treatment (spraying or painting). However, in order to save the operating costs of equipment and facilities, reduce maintenance, and further extend the service life of steel grating in harsh environments, it is often necessary to perform secondary protection on hot-dip galvanized steel grating, that is, apply summer organic coating on the hot-dip galvanized surface to form a double-layer anti-corrosion system.
Usually, steel gratings are generally passivated online immediately after hot-dip galvanizing. During the passivation process, an oxidation reaction occurs on the surface of the hot-dip galvanizing coating and the interface of the passivation solution, forming a dense and firmly adhered passivation film on the surface of the hot-dip galvanizing layer, which plays a role in enhancing the corrosion resistance of the zinc layer. However, for steel gratings that need to be coated with a summer primer to form a double-layer anti-corrosion system for protection, the dense, smooth, and passive metal passivation film is difficult to bond tightly with the subsequent summer primer, resulting in premature bubbling and shedding of the organic coating during service, affecting its protective effect.
In order to further improve the durability of steel gratings treated with hot-dip galvanizing, it is generally possible to coat a suitable organic coating on its surface to form a composite protective system for protection. Given that the surface of the hot-dip galvanized layer of the steel grating is flat, smooth, and bell-shaped, the bonding strength between it and the subsequent coating system is insufficient, which can easily lead to bubbling, shedding, and premature failure of the coating. By selecting a suitable primer or a suitable pretreatment process, the bonding strength between the zinc coating/primer coating can be improved, and the long-term protective effect of the composite protective system can be exerted.
The key technology that affects the protective effect of the hot-dip galvanized steel grating surface protective coating system is also the surface treatment before coating. Sandblasting is one of the most commonly used and reliable surface treatment methods for steel grating coating, but because the hot-dip galvanized surface is relatively soft, excessive sandblasting pressure and sand particle size can cause the loss of the galvanized layer of the steel grating. By controlling the spray pressure and sand particle size, moderate sandblasting on the surface of hot-dip galvanized steel grating is an effective surface treatment method, which has a satisfactory effect on the display of the primer, and the bonding strength between it and the hot-dip galvanized layer is greater than 5MPa.
Using a cyclic hydrogen primer containing zinc phosphate, the adhesion between the zinc coating/organic primer is basically greater than 5MPa without sandblasting. For the surface of hot-dip galvanized steel grating, when it is not convenient to use sandblasting surface treatment, when further organic coating is considered later, a phosphate-containing primer can be selected, because phosphate in the primer helps to improve the adhesion of the paint film and enhance the anti-corrosion effect.
Before the primer is applied in the coating construction, the hot-dip galvanized layer of the steel grating is passivated or not passivated. The pretreatment has no significant effect on improving the adhesion, and alcohol wiping has no obvious improvement effect on the bonding strength between the zinc coating/primer.