Aluminum-zinc alloy coated coil — sold under names like Galvalume, Zincalume, or simply AZ coil — looks like a small variation on standard galvanizing from a spec sheet. In production, it’s a genuinely different metallurgical process, with tighter bath control and a different cooling behavior than zinc coating requires. Understanding that process explains why AZ coil performs differently in the field, and what actually separates a well-produced coil from a marginal one.
The Bath Composition
Standard aluminum-zinc coating baths run at approximately 55% aluminum, 43.4% zinc, and 1.6% silicon by weight — a composition developed specifically because it sits near a eutectic point in the aluminum-zinc phase diagram, giving the alloy a lower melting point and better fluidity than either metal alone would provide at coating-line speeds. The silicon addition is not incidental: without it, a brittle intermetallic layer (Fe2Al5) grows excessively at the steel-coating interface, and silicon suppresses this growth to a controlled, thin layer that preserves coating adhesion and formability.
Bath temperature is held tighter than a standard galvanizing line — typically in the range of 590–610°C, versus roughly 450–460°C for pure zinc. This higher operating temperature is one reason AZ coating lines require different equipment specifications than a standard GI line rather than simply running a different bath chemistry through the same setup.
Line Process, Step by Step
- Strip cleaning — alkaline cleaning and rinsing to remove rolling oils and surface contamination before the strip enters the furnace section
- Annealing — the strip passes through a controlled-atmosphere furnace (typically a reducing atmosphere of nitrogen-hydrogen mix) to anneal the steel and prepare a clean, oxide-free surface for coating adhesion
- Molten bath immersion — the strip enters the aluminum-zinc-silicon bath, coating both sides simultaneously as it passes through
- Air knife control — pressurized air jets strip excess molten coating from the strip surface as it exits the bath, controlling final coating thickness; this step requires more precise control on AZ lines than GI lines due to the alloy’s different viscosity and surface tension behavior
- Cooling — controlled cooling rate directly determines the spangle pattern and microstructure of the solidified coating; AZ coating solidifies through a different phase sequence than pure zinc, producing its characteristic flower-pattern (spangle) appearance as primary aluminum dendrites form first, followed by the zinc-rich eutectic filling between them
- Chemical treatment / passivation — a chromate or chrome-free passivation layer is applied to protect the coating surface prior to coiling and improve paint adhesion for coil destined for further color coating
- Recoiling and inspection — coating weight, surface finish, and dimensional checks before final coiling for shipment
Why the Intermetallic Layer Matters
At the interface between the steel substrate and the aluminum-zinc coating, a thin intermetallic alloy layer forms during the coating process — typically an Al-Fe-Si compound a few microns thick. This layer is unavoidable and, at the right thickness, is actually beneficial: it provides strong metallurgical bonding between coating and substrate. But if bath chemistry, temperature, or line speed isn’t properly controlled, this layer can grow too thick, becoming brittle and prone to cracking during forming or bending — leading to coating flaking at bend lines, a defect that shows up during fabrication rather than at the mill, often well after the coil has already been sold and shipped.
This is one of the least visible but most consequential production variables in AZ coil manufacturing, and it’s part of why coating quality can vary meaningfully between mills even when the nominal bath composition and coating weight look identical on paper.
Coating Weight and How It’s Measured
| Coating Class | Weight (both sides) | Typical Application |
|---|---|---|
| AZ50 | 50 g/m² | Light-duty indoor components |
| AZ70 – AZ100 | 70–100 g/m² | General construction, ducting |
| AZ120 – AZ150 | 120–150 g/m² | Roofing, cladding, extended-life outdoor use |
| AZ180+ | 180 g/m² and above | Heavy-duty industrial and coastal-adjacent applications |
Coating weight is verified through triple-spot testing per ASTM A792 — stripping coating from sample areas and measuring mass loss — or through non-destructive magnetic or XRF gauging on the production line for in-process control. Buyers sourcing at volume should request batch-specific test data rather than relying on a general product specification, since bath composition drift and line speed variation can shift actual coating weight from stated nominal values over a production run.
Corrosion Behavior: A Different Mechanism Than Pure Zinc
Pure zinc coating protects steel almost entirely through galvanic (sacrificial) action. Aluminum-zinc coating works differently: the aluminum-rich dendrite structure forms a stable, passive oxide barrier on flat surface areas — similar in principle to how stainless steel resists corrosion — while the zinc-rich interdendritic phase still provides galvanic protection, concentrated mainly at cut edges where the coating structure is interrupted.
The production-level consequence is that flat-surface corrosion resistance on AZ coil generally exceeds pure zinc coating at equivalent coating weight, while cut-edge protection is somewhat more limited than pure zinc provides, since less of the coating volume is galvanically active. This isn’t a flaw in the coating — it’s a direct result of the alloy’s solidification structure, and it’s why AZ coil specification decisions should account for how much field cutting and edge exposure a project will actually involve.
Standard Specifications
| Specification | Range |
|---|---|
| Base metal thickness | 0.13mm – 1.5mm |
| Coating weight | AZ50 – AZ200 |
| Width | 600mm – 1500mm |
| Steel grade | SGLCC, S250GD, S550GD (structural grades) |
| Spangle finish | Regular spangle (visible flower pattern) or minimized spangle |
| Standards | ASTM A792, JIS G3321, EN 10346 |
What Separates a Well-Run Production Line
- Tight bath temperature control — minimizing fluctuation keeps intermetallic layer growth consistent and predictable across a production run
- Air knife calibration matched to AZ viscosity — reused GI-line settings without adjustment for the alloy’s different flow characteristics is a common source of coating weight inconsistency
- Controlled cooling rate — affects both spangle appearance and coating microstructure, with faster, more uniform cooling generally producing a more consistent, less porous coating
- Batch-level coating weight testing, not just line-start calibration checks carried forward as representative of an entire run
Specifying AZ Coil With the Production Process in Mind
Aluminum-zinc coating isn’t a simple substitution for zinc in the same equipment — it’s a distinct metallurgical process with its own temperature requirements, intermetallic layer behavior, and solidification chemistry, and production quality depends on how tightly a mill controls each of those variables. Asking a supplier about bath temperature control, intermetallic layer management, and batch-specific coating weight verification reveals far more about actual product quality than a coating weight number on a spec sheet alone.
For technical data sheets, coating weight test reports, or a quotation matched to your project’s exposure and fabrication requirements, our export team can help specify the right AZ coil for your application.