Aluminized Steel: Overview and Applications

What is Aluminized Steel?

Aluminized steel is steel that has been hot-dip coated on both sides with an aluminum-silicon alloy. This process ensures a strong bond between the steel sheet and its aluminum coating, creating a material with a unique combination of properties. Aluminized steel resists corrosion and retains the properties of steel at temperatures below 800°C (1,470°F). It is commonly used in heat exchangers for residential furnaces, automotive mufflers, ovens, and other applications that require efficient heat transfer.

Types of Aluminized Steel

Type 1

Type 1 aluminized steel is coated with a thin layer of aluminum-silicon alloy, containing 5% to 11% silicon. This coating promotes adherence and is primarily used in heat-resistant applications where corrosion resistance is also needed. It can withstand temperatures up to 550°C (1,022°F) with minimal change to the base material. It is used in products like mufflers, furnaces, ovens, and baking pans. However, it can develop black spots due to the silicon content. Type 1 has also started replacing galvanized or galvalume steel in bakery trays since it does not contain harmful lead.

Type 2

Type 2 is coated with commercially pure aluminum and is used in applications requiring atmospheric corrosion resistance. It is often used in corrugated roofing and siding, grain bins, drying ovens, and air-conditioner condenser housings.

Properties of Aluminized Steel

The structure of aluminized steel consists of a thin aluminum oxide outer layer, an intermetallic layer of aluminum, silicon, and steel, and a steel core. Both Type 1 and Type 2 aluminized steels have high reflectivity, reflecting up to 80% of heat at temperatures up to 842°C (1,548°F). They maintain strength at temperatures up to 677°C (1,251°F). Aluminized steel is more reflective than stainless steel due to its greater electrostatic surface.

The aluminum and silicon layers make aluminized steel highly resistant to corrosion by preventing the steel from oxidizing. These layers also protect against pit corrosion, especially when exposed to salts that typically corrode other metals. However, if the aluminum layer is damaged and the steel is exposed, oxidation and corrosion can occur.

Consumption and Uses

In North America, nearly 700,000 tons of aluminized steel are used annually. It is commonly used in products such as water heaters, ranges, furnaces, space heaters, and grills. Aluminized steel offers structural durability and high yield strength in corrosive environments. It is preferred for manufacturing automobile and motorcycle exhaust systems due to its strength and lower cost compared to high-alloy steels.

Processing of Aluminized Steel

Aluminized steel is produced through various processes, including cladding, galvanic coating, metallizing, and calorizing. However, hot dipping is the most effective method. This process involves cleaning the steel and immersing it in a bath of aluminum with 11% silicon at 988K. The aluminum diffuses into the steel, creating an intermetallic layer beneath the aluminum coating. This coating is oxidized to protect the steel from corrosion. Silicon in the bath helps form a thinner aluminum layer, making hot dipping a cost-effective method for producing aluminized steel.