Galvalume (Al-Zn) Coil Explained: AZ Coatings, Corrosion Performance & Roll Forming Differences

The coating is applied via a continuous hot-dip process similar to galvanizing.

Metallurgy, Performance, Forming Behaviour & Selection for Roll Forming

1. What Is Galvalume (Al-Zn Coated Steel)?

Galvalume (also called Aluzinc or Al-Zn coated steel) is carbon steel coated with an aluminum-zinc alloy typically composed of:

55% Aluminum
43.4% Zinc
1.6% Silicon

The coating is applied via a continuous hot-dip process similar to galvanizing. The silicon controls the growth of the intermetallic layer and improves coating adhesion to the steel substrate.

Galvalume combines:

Aluminum’s barrier corrosion protection
Zinc’s sacrificial protection
Improved heat reflectivity
Enhanced paint adhesion

It is commonly designated as AZ followed by coating mass.

2. AZ Coating Designations Explained

The “AZ” number refers to total coating mass in grams per square meter (both sides combined).

Designation	Total Coating Mass (g/m²)	Typical Application
AZ50	~150 g/m²	Interior / light exposure
AZ55	~165 g/m²	Standard roofing (USA common)
AZ100	~300 g/m²	Industrial exposure
AZ150	~450 g/m²	Coastal / aggressive zones

Important clarification:

AZ55 is not equivalent to Z275 galvanized.
They are different corrosion systems with different performance characteristics.

3. How Galvalume Protects Steel

Galvalume works through two mechanisms:

3.1 Barrier Protection (Aluminum)

Aluminum forms a stable oxide layer that slows oxygen and moisture penetration. This provides long-term atmospheric resistance.

3.2 Sacrificial Protection (Zinc)

Zinc protects exposed steel areas (such as cut edges) through galvanic action. However, AZ contains less zinc than pure galvanized coating, so cut-edge behavior differs.

4. AZ vs Galvanized (Z Coating) – Real Engineering Differences

Property	Galvanized (Z)	Galvalume (AZ)
Corrosion life (atmospheric)	Good	2–4x longer
Cut-edge protection	Strong	Moderate
Heat reflectivity	Moderate	High
UV resistance	Good	Excellent
Marine performance	Moderate	Good (higher AZ needed)
Acidic resistance	Better	Weaker
Appearance	Bright spangle	Matte silver

Key Engineering Difference

Galvanized relies primarily on zinc sacrificial protection.
Galvalume relies primarily on aluminum barrier protection.

That difference explains most real-world performance variations.

5. Cut-Edge Behaviour in Roll Forming

This is critical for roofing manufacturers.

In galvanized steel:

Zinc migrates aggressively to exposed edges.
Cut edges tend to remain protected longer in wet conditions.

In Galvalume:

Aluminum dominates corrosion protection.
Zinc content is lower.
In severe coastal conditions, red rust may appear at edges sooner than heavy galvanized coatings.

For high-salt environments:

AZ150 is strongly preferred over AZ55.
Edge sealing may be required for warranty compliance.

6. Steel Grades Used with AZ Coatings

Common substrate grades:

G250 (mild structural)
G300
G350
G450
G550 (high tensile roofing)

Higher tensile grades:

Increase springback in roll forming
Increase micro-cracking risk in tight radii
Require better roll tooling polish and alignment

AZ coatings on G550 require careful forming radius control.

7. Forming Characteristics in Roll Forming Machines

Galvalume behaves slightly differently than galvanized.

7.1 Surface Hardness

The aluminum-rich surface can be slightly harder and more abrasive to tooling.

Recommendation:

Use polished roll tooling
Avoid damaged chrome plating
Maintain clean strip surface

7.2 Micro-Cracking Risk

On tight bends, especially in:

Standing seam locks
Small rib profiles
High tensile grades

Micro-cracks may form in the coating.

Prevention:

Maintain correct bend radius
Avoid excessive forming pressure
Ensure proper pass design
Use incremental forming rather than aggressive early bending

8. Thickness & Base Metal Considerations

Common AZ base metal thicknesses:

0.30 mm
0.35 mm
0.40 mm
0.45 mm
0.50 mm
0.60 mm
0.75 mm

Important:

Total thickness = Base metal thickness + coating thickness.

Engineering decisions must be based on base metal thickness, not total thickness.

9. Paint Systems Over AZ

Most roofing applications use:

AZ substrate
→ Pretreatment
→ Primer
→ Topcoat (Polyester, SMP, PVDF)

Advantages of AZ for painted systems:

Improved underfilm corrosion resistance
Better paint adhesion
Reduced blistering compared to galvanized

This is why AZ is dominant in prepainted roofing markets.

10. Environmental Performance

Rural Environments

AZ55 often exceeds 20 years
Minimal corrosion progression

Industrial Zones

Aluminum barrier layer slows pollution-driven corrosion

Coastal Environments

AZ150 recommended
Fastener selection becomes critical
Cut edges may require additional protection

High Heat / Desert Climates

High reflectivity reduces roof temperature
Improved paint longevity
Lower cooling loads for buildings

11. Common Buyer Mistakes

Assuming AZ55 equals heavy galvanized
Choosing low coating mass for coastal jobs
Ignoring tensile grade during profile design
Not verifying coating mass on mill certificate
Using AZ in strongly acidic agricultural environments
Poor storage causing white rust formation

12. Storage & Handling Best Practices

To avoid white rust:

Store coils dry
Avoid condensation between wraps
Elevate from ground
Use breathable covers
Do not trap moisture under plastic

Moisture entrapment causes early cosmetic corrosion before forming.

13. International Standards

Common standards governing Al-Zn coated steel:

ASTM A792 (USA)
EN 10346 (Europe)
JIS G3321 (Japan)

Always verify:

Coating mass tolerance
Mechanical properties
Surface finish class
Base metal thickness tolerance

Summary: When to Choose AZ

Choose Galvalume (AZ) when:

Long atmospheric corrosion life is required
Roofing performance is priority
Heat reflectivity is beneficial
Painted systems are used
Industrial pollution exposure exists

Use caution when:

Exposure is highly acidic
Constant salt spray exposure exists without heavy coating
Tight radius forming is required on high tensile grades

AZ is not “better” in every condition — it is engineered for specific environmental advantages.

Frequently Asked Questions (6)

1. Is Galvalume better than galvanized?

In most atmospheric roofing applications, yes — AZ coatings typically last 2–4 times longer. However, galvanized may perform better at heavily exposed cut edges in wet environments.

2. What does AZ55 mean?

AZ55 refers to approximately 165 g/m² total aluminum-zinc coating mass applied to both sides combined.

3. Can Galvalume be used in coastal areas?

Yes, but higher coating mass such as AZ150 is recommended for long-term durability near salt exposure.

4. Does Galvalume crack during roll forming?

It can micro-crack on tight bends, especially on high tensile grades like G550. Proper pass design and radius control reduce risk.

5. Is Galvalume more expensive than galvanized?

Typically slightly higher upfront cost, but lower lifecycle cost due to longer corrosion performance.

6. How can I verify AZ coating quality?

Review the mill test certificate for coating mass, confirm compliance with ASTM A792 or EN 10346, and perform coating weight testing if required.