Steel Coating Types Explained — Zinc, Galvalume & Prepainted

Steel Coating Types (Zinc, Galvalume, Prepainted)

Complete Guide for Roofing & Roll Forming Applications

Introduction

Steel used in roll forming and roofing applications is rarely bare.

It is protected by coatings designed to prevent corrosion, improve durability, and extend service life.

The three most common coating types are:

• Zinc (Galvanized Steel)

• Galvalume (Aluminum-Zinc Alloy Coating)

• Prepainted Steel (Color-Coated Coil)

Each coating behaves differently in:

• Coastal environments

• Industrial zones

• Agricultural settings

• High humidity regions

Understanding coating types is critical for:

• Roofing manufacturers

• Coil buyers

• Roll forming machine operators

• Structural designers

• Warranty planning

This guide explains the differences clearly and technically.

1️⃣ Zinc Coating (Galvanized Steel)

What Is Galvanized Steel?

Galvanized steel is:

Carbon steel coated with a layer of zinc.

Zinc protects steel through:

1. Barrier protection

2. Sacrificial protection (cathodic action)

Even if scratched, zinc corrodes before steel does.

Zinc Coating Designations

Common designations:

Coating	Meaning
Z100	100 g/m² zinc
Z275	275 g/m² zinc
G60	0.60 oz/ft²
G90	0.90 oz/ft²

Higher number = thicker zinc coating.

Where Galvanized Is Used

• Agricultural buildings

• Structural framing

• Interior applications

• Cost-sensitive roofing

Less ideal for marine environments.

Advantages

• Cost effective

• Good corrosion resistance

• Easy to form

• Widely available

Limitations

• Less corrosion resistance than Galvalume

• White rust in high humidity

• Surface dulls over time

2️⃣ Galvalume (Aluzinc Coating)

What Is Galvalume?

Galvalume is:

Steel coated with approximately 55% aluminum, 43.4% zinc, and 1.6% silicon.

Common designation:

AZ150 (150 g/m² aluminum-zinc coating)

How Galvalume Protects Steel

Aluminum provides:

• Barrier corrosion protection

Zinc provides:

• Sacrificial protection

Combination creates superior corrosion resistance.

Advantages

• Excellent corrosion resistance

• Better performance in coastal zones

• Longer service life than galvanized

• Strong heat reflectivity

Limitations

• Slightly higher cost

• Not ideal in animal agriculture (ammonia exposure)

• Cut edges require protection

Common Uses

• Commercial roofing

• Industrial buildings

• Warehouses

• Export roofing systems

Galvalume is dominant in modern roofing markets.

3️⃣ Prepainted Steel (Color-Coated Coil)

What Is Prepainted Steel?

Prepainted steel is:

Galvanized or Galvalume steel coated with primer and topcoat paint before forming.

Common base coatings:

• Zinc (GI)

• Galvalume (AZ)

Paint systems include:

• Polyester

• SMP (Silicone Modified Polyester)

• PVDF (High-performance architectural)

Coating Structure

Typical layers:

• Steel Base
• → Metallic Coating (Zinc or AZ)
• → Chemical Treatment
• → Primer
• → Topcoat Paint

This layered system increases corrosion protection.

Advantages

• Aesthetic finish

• Color variety

• UV resistance

• Improved corrosion resistance

• Longer warranties

Limitations

• Higher cost

• Surface scratching during forming

• Requires polished tooling

4️⃣ Galvanized vs Galvalume Comparison

Property	Galvanized	Galvalume
Corrosion Resistance	Moderate	High
Coastal Performance	Moderate	Better
Cost	Lower	Slightly higher
Heat Reflectivity	Lower	Higher
Lifespan	Good	Longer

Galvalume generally outperforms zinc in roofing.

5️⃣ Coating Weight & Performance

Coating thickness directly impacts corrosion resistance.

Example:

Z100 < Z275
AZ100 < AZ150

Thicker coating = longer protection.

Always specify coating weight in contract.

6️⃣ Coating & Roll Forming

Coating affects forming behavior.

Galvanized:

• Softer surface

• Easier to form

Galvalume:

• Slightly harder surface

• May increase roll wear

Prepainted:

• Requires smooth rolls

• Lower forming speed recommended

Surface protection is critical during forming.

7️⃣ Coating & Machine Wear

Metallic coatings increase friction.

Prepainted coil increases:

• Surface marking risk

• Tooling wear

Chrome-plated rolls recommended for painted production.

8️⃣ Coating & Cut Edge Corrosion

Galvanized:

Zinc sacrifices at cut edges.

Galvalume:

Aluminum does not sacrificially protect cut edges as effectively.

Design consideration for trimming operations.

9️⃣ Coating & Warranty

Typical warranties:

• Galvanized: 10–20 years (environment dependent)

• Galvalume: 20–30+ years

• Prepainted PVDF: 30+ years

Warranty depends on:

• Environment

• Installation

• Coating weight

• Maintenance

🔟 Coating Selection by Environment

Coastal Areas

Galvalume or prepainted with AZ base recommended.

Industrial Pollution

Galvalume preferred.

Agricultural Buildings

Heavy galvanized may perform better than Galvalume in ammonia-rich environments.

Architectural Projects

Prepainted PVDF recommended.

1️⃣1️⃣ Coating & Steel Grade

Coating type does not change steel grade.

Example:

0.5mm G550 AZ150
0.5mm G550 Z275

Same yield strength.
Different corrosion performance.

1️⃣2️⃣ Common Coating Mistakes

• ❌ Assuming Galvalume and galvanized are identical
• ❌ Not specifying coating weight
• ❌ Using light coating in marine zones
• ❌ Ignoring cut edge protection
• ❌ Not confirming base steel grade

Coating selection must match environment.

1️⃣3️⃣ Real-World Example

Two warehouses:

Warehouse A:
0.5mm G550 Z100

Warehouse B:
0.5mm G550 AZ150

In coastal environment:

Warehouse A shows corrosion in 8–10 years.
Warehouse B lasts significantly longer.

Coating choice affects lifecycle cost.

1️⃣4️⃣ Engineering Summary

Coating type determines:

• Corrosion resistance

• Surface durability

• Warranty life

• Machine wear

• Environmental performance

For roofing and roll forming:

Galvalume is typically superior for corrosion resistance.
Prepainted systems add aesthetic and UV durability.

FAQ Section

What is the difference between galvanized and Galvalume?

Galvanized is zinc-coated; Galvalume is aluminum-zinc alloy coated.

Is Galvalume better than galvanized?

Generally yes for roofing, especially in coastal areas.

What does AZ150 mean?

150 g/m² aluminum-zinc coating weight.

What is Z275?

275 g/m² zinc coating weight.

Does coating affect forming?

Yes. Prepainted and Galvalume require smoother tooling.

Can coating affect warranty?

Yes. Coating type and weight determine corrosion warranty.

Internal Linking Strategy

Link this page to:

• Understanding Steel Grade

• Gauge vs Millimeter Thickness

• What Is Material Yield Strength

• Roofing Profiles

• How Profile Geometry Affects Coil Width

• Roll Forming Machine Engineering Guide