1.How to improve the corrosion resistance of galvanized layer?
Increasing the Thickness and Uniformity of the Zinc Coating:
The zinc coating is the core of base layer corrosion protection. In highly corrosive environments, galvanized coils with a zinc coating thickness of ≥120g/m² (on both sides) should be selected. Improved galvanizing processes (such as air knife control in continuous hot-dip galvanizing) should ensure uniformity of the zinc coating to avoid the "short plate effect" caused by thinner areas.
Using zinc alloy coatings (such as zinc-aluminum-magnesium):
Compared to pure zinc coatings, zinc-aluminum-magnesium coatings can improve salt spray resistance by 2-3 times by forming a denser corrosion product film. When combined with a coating, this can significantly extend the life of the base layer protection.
Controlling Spangle Formation:
Using small or no spangle galvanized coils can reduce uneven coating thickness caused by spangle protrusions (which can lead to thinner coatings at protrusions), thereby reducing the risk of localized corrosion.
2.How to strengthen surface preparation?
Choose an efficient passivation process:
Traditional chromate passivation (containing hexavalent chromium) is effective but environmentally unfriendly. Chromium-free passivation (such as silane and titanium-zirconium systems) can be used. By forming a dense nanoscale film, it enhances the adhesion between the zinc coating and the coating (cross-hatch adhesion ≥ 0) and prevents electrochemical corrosion at the zinc-coating interface.
Add phosphating treatment (for demanding applications):
For pre-painted coils used outdoors for extended periods, a phosphating step can be added after passivation to form a porous zinc phosphate crystal layer, further enhancing the anchoring effect of the primer and reducing the risk of coating blistering (no blistering after 1000 hours of wet heat testing).
3.How to improve the adhesion and anti-corrosion synergy of primer?
Choose a high-corrosion primer:
Epoxy primer (5-8μm thickness) is preferred. The epoxy groups in its molecules react with the hydroxyl groups on the zinc coating to form a chemical bond. The epoxy's chemical resistance also prevents corrosive media from penetrating the zinc layer. For high-temperature environments (such as rooftops), a high-temperature-resistant polyester primer (temperature resistance ≥150°C) can be used instead.
Control the primer's curing degree:
By adjusting the baking temperature (180-220°C) and time (60-90 seconds), ensure that the primer's crosslinking degree is ≥90% (determined by gel content method) to avoid a decrease in water resistance due to insufficient curing.
4.How to improve weather resistance and anti-aging performance in a targeted manner?
For environments with high UV and severe corrosion: Choose a PVDF topcoat. Its fluorine content is ≥70%, its molecular structure is stable, it resists UV degradation, and it offers salt spray resistance of over 5,000 hours.
For moderately corrosive environments: Choose an SMP topcoat. It is lower in cost than PVDF and offers better weather resistance than standard polyester, suitable for a 10-15 year service life.
For low-cost applications: Standard polyester topcoats require the addition of antioxidants and UV absorbers to slow chalking.
5.How to optimize the production process?
Surface Cleaning Before Coating
A three-stage cleaning process of "alkaline cleaning + electrolytic cleaning + rinsing" is used to thoroughly remove oil, dirt, and dust from the galvanized coil surface, preventing contaminants from causing poor coating adhesion or localized corrosion.
Curing Process Parameter Optimization
A hot air circulation oven is used to ensure uniform temperature within the oven, preventing localized overheating that could cause coating degradation.
The baking time is controlled to match the coating thickness to ensure complete coating cure.