1.How do the chemical compositions and protection mechanisms of galvanized coils and zinc-aluminum-magnesium coils compare?
Galvanized Coil:
Composition: Primarily pure zinc (Zn content > 99%).
Protection Mechanism: Sacrificial anodic protection. Zinc is more chemically active than iron. When the coating is damaged, the zinc corrodes before the iron, thereby protecting the underlying steel. This is a form of "self-sacrificial" protection.
Zinc-Aluminum-Magnesium Coil:
Composition: Zinc (Zn) is the base, with added aluminum (Al, approximately 5-11%) and magnesium (Mg, approximately 1-3%), as well as trace amounts of silicon and other elements.
Protection Mechanism: Triple protection mechanism:
1. Sacrificial anodic protection: Similar to galvanizing, zinc and magnesium provide sacrificial protection.
2. Dense Barrier Protection: Aluminum forms a very dense aluminum oxide film on the surface, effectively blocking the intrusion of corrosive media.
3. Self-Healing Protection: This is the most critical difference. The presence of magnesium allows corrosion products (such as magnesium hydroxide and basic zinc chloride) to stably coat cuts and scratches, forming a dense protective film that significantly slows the spread of corrosion. This is the so-called "self-healing" function.
2.How does the overall corrosion resistance of ZnAlMg and galvanized coils compare?
At the same coating weight (e.g. g/㎡), the corrosion resistance life of zinc-aluminum-magnesium is usually 2 to 10 times or even higher than that of ordinary galvanizing. The advantages are particularly prominent in harsh conditions such as industrial atmosphere and marine environment.
3.What is the difference in cut and scratch protection between galvanized coil and zinc-aluminum-magnesium coil?
Galvanized coils: Their greatest weaknesses are cuts and processing damage. Once the steel sheet is cut, the bare steel substrate is directly exposed, and the zinc layer can only provide limited sacrificial protection to the immediately adjacent areas. Red rust at the cut can quickly spread across the entire sheet surface.
Zinc-aluminum-magnesium coils: Thanks to their "self-healing" properties, corrosion products actively coat the cut surface, forming a protective layer that effectively prevents the lateral spread of red rust. Even after more than a decade, the extent of corrosion at the cut remains very limited.
4.How does the scratch resistance of zinc-aluminum-magnesium coil compare to that of galvanized coil?
ZnAlMg coatings are generally harder and more wear-resistant than pure zinc coatings, and are less susceptible to scratching during processing, transportation and installation, thereby better maintaining their protective properties.
5.What are the situations for choosing galvanized coil?
Budget-constrained and cost-sensitive.
For use in dry indoor environments, with minimal corrosion risk.
The product is not susceptible to rusting on cutouts (e.g., on completely encased or painted structural parts).
For short-term projects with minimal material performance requirements.