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Aluminised Steel
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Aluminised steel (also called aluminum-silicon coated steel) is a steel product that combines a carbon steel base with an aluminum-based protective layer. It is widely used in automotive exhaust systems, heat-resistant structures, HVAC systems, and industrial thermal equipment.
In industry practice, two related but different concepts often appear: hot-dip aluminized coating (aluminized steel) and aluminizing diffusion layer (pack aluminizing / diffusion aluminizing). They are often confused, but their processes and structures are fundamentally different.
1. Aluminized Steel (Hot-Dip Aluminizing Coating)
This is the most common commercial product form.
Process Principle
Steel is immersed in molten aluminum or aluminum-silicon alloy, forming a metallurgical coating layer on the surface.
Structure Characteristics
- Outer layer: aluminum-rich coating
- Middle layer: Fe-Al intermetallic layer
- Base layer: steel substrate
Key Features
- Coating thickness is relatively uniform
- Good corrosion and oxidation resistance
- Excellent heat reflectivity
- Suitable for mass production
Main Purpose
To provide a protective surface barrier against heat and corrosion.
2. Aluminizing (Diffusion Aluminizing / Pack Aluminizing)
This is a different surface treatment method used mainly for high-temperature performance enhancement.
Process Principle
Aluminum atoms diffuse into the steel surface at high temperature, forming an alloyed diffusion layer, rather than a simple coating.
Structure Characteristics
- No clear coating boundary
- Aluminum is diffused into steel surface
- Forms Fe-Al alloy phase layer
Key Features
- Extremely strong bonding with substrate
- Excellent high-temperature oxidation resistance
- Very hard surface layer
- More suitable for extreme environments
Main Purpose
To create a surface alloy layer for long-term high-temperature resistance.
3. Key Differences Between Aluminized Coating and Aluminizing Diffusion
| Item | Hot-Dip Aluminized Steel | Diffusion Aluminizing |
|---|---|---|
| Process | Immersion in molten aluminum | High-temperature diffusion reaction |
| Structure | Coating + intermetallic layer | Alloyed diffusion layer |
| Bonding | Mechanical + metallurgical bonding | Fully metallurgical bonding |
| Surface Appearance | Metallic coating visible | More uniform alloy surface |
| Heat Resistance | High | Very high |
| Corrosion Resistance | Good | Excellent |
| Production Efficiency | High (mass production) | Lower (special process) |
| Application | Automotive, HVAC, general industry | Furnace parts, extreme heat systems |
4. Why Silicon Is Important in Aluminized Steel
Silicon plays a critical role in hot-dip aluminized steel:
- Controls reaction speed between aluminum and steel
- Prevents excessive brittle Fe-Al layer growth
- Improves coating uniformity
- Enhances high-temperature stability
- Improves coating adhesion and durability
Typical aluminum-silicon coating contains:
- Aluminum: 85–92%
- Silicon: 8–12%
5. Application Differences
Aluminized Steel (Coated Type)
- Automotive exhaust systems
- Heat shields
- HVAC ducts
- Household appliances
- Industrial sheet metal
Diffusion Aluminizing
- Furnace fixtures
- Kiln components
- High-temperature tooling
- Aerospace thermal parts
- Chemical reactor components
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