ASTM A792 Galvalume (AZ) Steel: Coating Designations, Grades & Ordering Guide

ASTM A792 (AZ / Galvalume) — Coating Designations & Ordering

Engineering & Procurement Guide for Roll Forming Applications

ASTM A792 is the primary North American specification governing aluminum-zinc alloy coated steel sheet — commonly marketed as Galvalume.

It defines:

• Mechanical grade (yield strength levels)

• Coating weight (AZ designations)

• Surface finish

• Chemical composition

• Thickness tolerances

• Testing requirements

For roll forming manufacturers producing:

• Roofing panels

• Standing seam

• Wall cladding

• Structural deck

• Solar mounting systems

Understanding ASTM A792 is critical for:

• Corrosion performance

• Springback control

• Coating cracking risk

• Warranty alignment

• Supplier communication

Misunderstanding AZ coating designations is one of the most common causes of premature corrosion and specification disputes.

1️⃣ What Is ASTM A792?

ASTM A792 covers:

“Steel Sheet, 55% Aluminum-Zinc Alloy-Coated by the Hot-Dip Process”

The coating composition is typically:

• ~55% Aluminum

• ~43.4% Zinc

• ~1.6% Silicon

The silicon improves coating adhesion and intermetallic layer stability.

Unlike ASTM A653 (galvanized zinc coating), A792 uses an Al-Zn alloy coating for enhanced atmospheric corrosion resistance.

2️⃣ Mechanical Grade Designations Under A792

ASTM A792 provides several strength categories.

2.1 Structural Grades

Designation	Minimum Yield Strength
Grade 33	~230 MPa
Grade 40	~275 MPa
Grade 50	~345 MPa
Grade 80	~550 MPa

Grade 50 is widely used in:

• Structural panels

• Purlin applications

• Load-bearing roofing

Grade 80 is high tensile and significantly increases springback.

2.2 Commercial & Forming Grades

• CS (Commercial Steel)

• FS (Forming Steel)

• DDS (Deep Drawing Steel)

These prioritize ductility over structural capacity.

Roofing panels often use structural grade for improved stiffness.

3️⃣ AZ Coating Designations Explained

ASTM A792 expresses coating weight in ounces per square foot.

AZ Designation	Total Coating Weight (oz/ft²)	Approx. g/m²
AZ50	0.50 oz/ft²	~150 g/m²
AZ55	0.55 oz/ft²	~165 g/m²
AZ60	0.60 oz/ft²	~180 g/m²
AZ70	0.70 oz/ft²	~215 g/m²

Most common in North America: AZ55.

Important:

The number represents total coating weight both sides combined, not per side.

4️⃣ AZ vs Galvanized (G90) — Performance Difference

Although AZ55 and G90 appear similar in coating weight, their corrosion mechanisms differ.

Property	G90 (Zinc)	AZ55 (Al-Zn)
Cut edge protection	Strong	Moderate
Flat surface corrosion	Good	Excellent
Heat reflectivity	Moderate	High
Coastal life	Moderate	Better (with higher AZ)

AZ coating relies primarily on aluminum barrier protection, not zinc sacrificial action.

5️⃣ Coating Thickness & Forming Behavior

AZ coatings are slightly harder than pure zinc coatings.

In roll forming:

• Increased surface hardness

• Slightly higher friction

• Micro-crack potential in tight bends

• Reduced galling vs pure zinc

However:

Cut-edge corrosion performance depends heavily on coating weight.

5.1 Coating Microstructure

Al-Zn coatings form:

• Aluminum-rich dendritic structure

• Zinc-rich interdendritic phases

This mixed structure:

• Slows corrosion propagation

• Reduces uniform rust spread

• Changes cut-edge behavior compared to galvanized

6️⃣ Bend Radius & Coating Cracking

Because AZ coatings are alloy-based:

• Micro-cracking may occur in tight radii

• High tensile substrate increases risk

• Overbending in tooling must be controlled

However:

Even when micro-cracked, AZ coatings maintain barrier protection due to aluminum oxide formation.

7️⃣ Corrosion Performance by Environment

Rural

AZ55 often exceeds 20+ years.

Urban / Industrial

Performs better than G90.

Coastal

AZ150 (outside A792 standard scope but available globally) preferred.

Salt exposure requires careful specification.

8️⃣ Thickness & Tolerance

ASTM A792 defines:

• Base metal thickness tolerance

• Coating mass tolerance

• Width tolerance

• Flatness & camber limits

Thickness tolerance directly impacts:

• Oil canning

• Panel flatness

• Lock seam accuracy

Precision roll forming requires stable thickness control.

9️⃣ Surface Finish Options

A792 coil may be supplied as:

• Regular spangle

• Minimized spangle

• Skin passed

Skin-passed coil improves:

• Surface smoothness

• Oil canning control

• Paint adhesion (for PPGL)

🔟 Ordering Checklist — What to Ask Suppliers

When ordering ASTM A792 coil, specify:

1. Grade (e.g., Grade 50, 80)

2. Coating designation (AZ55, AZ60, etc.)

3. Base metal thickness

4. Surface finish (skin passed?)

5. Mill test certificate

6. Yield & tensile results

7. Coating weight verification

8. Edge condition (mill edge vs slit edge)

9. Country of origin

10. Revision year of ASTM standard

Failure to define grade clearly may result in commercial steel instead of structural grade.

1️⃣1️⃣ Common Ordering Mistakes

1. Confusing AZ55 with AZ150

2. Assuming AZ55 equals G90

3. Not specifying structural grade

4. Ignoring edge condition

5. Ordering commercial grade for structural application

6. Not verifying coating mass tolerance

1️⃣2️⃣ Machine Design Implications

Switching from:

• G90 galvanized → AZ55

• Grade 33 → Grade 50

Changes:

• Springback compensation

• Forming pressure

• Roll wear rate

• Drive torque requirement

High tensile A792 material behaves like G350+ in roll forming.

Structural AZ panels require heavier machine platforms than roofing AZ.

1️⃣3️⃣ Buyer Strategy (30%)

When AZ55 Is Ideal

• Standard roofing panels

• Commercial buildings

• Moderate corrosion climates

When Higher AZ Is Needed

• Coastal exposure

• Long-term warranty projects

• Solar installations

When Galvanized May Be Better

• Environments requiring stronger sacrificial cut-edge protection

• Continuously wet edge conditions

6 Frequently Asked Questions

1. What does AZ55 mean?

It means 0.55 ounces of aluminum-zinc coating per square foot total, applied to both sides combined.

2. Is ASTM A792 better than ASTM A653?

For atmospheric corrosion resistance, AZ coatings under A792 typically outperform galvanized coatings under A653.

3. Can AZ55 be used in coastal environments?

Yes, but higher coating weights may be recommended for aggressive salt exposure.

4. Does AZ coating crack during forming?

Micro-cracking can occur in tight bends, especially on high tensile grades.

5. Is AZ55 equal to G90?

No. They have similar coating weight magnitude but different corrosion mechanisms.

6. Should structural panels use Grade 50 under A792?

Yes. Grade 50 (≈345 MPa) is common for structural roofing and deck applications.

Final Engineering Summary

ASTM A792 governs aluminum-zinc coated steel (Galvalume).

Correct specification must define:

• Mechanical grade

• AZ coating designation

• Base metal thickness

• Edge condition

• Surface finish

AZ coatings provide superior atmospheric corrosion resistance compared to galvanized steel but behave differently at cut edges and tight bends.

Proper ordering prevents structural mis-specification, corrosion failure, and production instability.