Key Takeaways
Coated sheet is classified by coating mass per unit area: Z-classes for pure zinc (for example Z80–Z275) and AZ-classes for the 55%Al–43.5%Zn–1.5%Si coating (for example AZ50–AZ200).
Coating mass is measured as the three-point average on both sides, in g/m², per the product standard.
AZ-coated sheet offers better corrosion resistance and higher service temperature than pure zinc coatings, while Z coatings give better cut-edge cathodic protection.
Coated sheet is typically supplied in thin gauges (commonly up to about 4 mm). Coating availability above the common classes is per mill capability and agreement.
1. Z and AZ Coating Systems
Two coating families dominate the market. The pure zinc coating (Z) protects by barrier action plus sacrificial (cathodic) protection of cut edges; common classes include Z80, Z100, Z120, Z140, Z180 and Z275 (g/m² total, both sides). The aluminum–zinc coating (AZ, approximately 55% Al, 43.5% Zn, 1.5% Si) combines aluminum's corrosion resistance and heat reflectivity with zinc's cut-edge protection; common classes include AZ50, AZ70, AZ100, AZ150, AZ180 and AZ200. Coating mass is reported as the three-point average on both sides per the product standard.
Galvanized steel sheet
2. Z vs AZ: How to Choose
| Aspect | Z (pure zinc) | AZ (aluminum–zinc) |
|---|---|---|
| Coating composition | About 99% Zn | About 55% Al, 43.5% Zn, 1.5% Si |
| Cut-edge protection | Strong (sacrificial) | Good but lower than Z |
| Corrosion resistance (general) | Good | Better in most atmospheres; higher service temperature |
| Typical use | Roofing, ducting, general sheet | Roofing, cladding, agricultural buildings, heat-reflective panels |
3. Standards and Measurement
Z and AZ coated sheet is specified per EN 10346 or ASTM A653 (Z classes), ASTM A792 (AZ coated) and GB/T 2518 in China. Coating mass is measured by the gravimetric or XRF method as the three-point average over the sheet width, both sides combined, and reported in g/m². The ordered class defines the minimum acceptable coating mass; the COA reports the delivered value. Coating classes above the common range are produced per mill capability and order agreement, not by a general rule about mill size.
4. Practical Notes
Most AZ-coated product is thin sheet (commonly up to about 4 mm), because the coating is applied on continuous lines optimised for thin gauges; heavy plate is coated by other routes. For painted product (PPGI-type), the AZ substrate is a common base for high-durability panels. Confirm the coating class, substrate grade, thickness and surface treatment with the supplier.
Frequently Asked Questions
Q1. What does AZ150 mean?
AZ150 means an aluminum–zinc coating mass of about 150 g/m² total (three-point average, both sides) per the product standard.
Q2. Which coating is better: Z or AZ?
It depends on service: Z gives stronger cut-edge sacrificial protection; AZ gives better general corrosion resistance and heat behaviour.
Q3. Is AZ coated sheet an alloy steel?
No. The steel substrate is carbon steel; the AZ coating is an aluminum–zinc coating on the surface.
