Aluminized steel

Contents

Overview

Aluminized steel is steel that has been hot-dip coated both sides with aluminum-silicon alloy. This process assures a tight metallurgical bond between the steel sheet and its aluminum coating, producing a material with a unique combination of properties possessed neither by steel nor by aluminum alone. Aluminized steel shows a better behavior against corrosion[1] and keeps the properties of the base material steel for temperature lower than 800 °C(1,472 F). For example, it is commonly used for heat exchangers in residential furnaces, mufflers, ovens, ranges, water heaters, fireplaces, bar-b-que burners, and baking pans.

Characteristics are defined by the exact metals used, as well as the process used.

Types

Hot-dip coated with an aluminum/silicon alloy containing 5% to 11% silicon to promote better adherence. It is intended principally for heat resisting applications and also for uses where corrosion resistance and heat are involved. Possible end uses are mufflers, furnaces, ovens, ranges, heaters, water heaters, fireplaces, and baking pans. [1] Type 1 is coated with a thin layer of aluminum and silicon. Type 1 is more commonly found in industrial products.

Hot-dip coated with commercially pure aluminum. It is intended principally for applications requiring atmospheric corrosion resistance. Type 2 may ultimately be manufactured into corrugated roofing and siding, grain bins, air conditioner housings and drying ovens. [2] Type 2 is coated with a thin layer of pure aluminum.

Properties

The basic structure of aluminized steel is a thin aluminum oxide layer outside, then an intermetallic layer that is a mix of aluminum, silicon, and steel, and finally a steel core.[2] Aluminized Steel is highly resistant to corrosion and shows high levels of heat reflectivity.

Crystal Structure

Aluminum has a Face Centered Cubic crystal structure.

Both Type 1 and Type 2 show excellent high reflectivity characteristics. At temperatures up to 842°C(900°F), aluminized steel reflects up to 80% of heat projected onto it.[3] Aluminized steel has the ability to maintain its strength at temperatures up to 677°C(1,250°F). Although stainless steel is the stronger of the two, aluminized steel has a greater electrostatic surface and can therefore reflect heat better.

Aluminized Steel is highly resistant to corrosion because of the thin layers of aluminum and silicon, which keep the underlying steel from oxidizing. This thin layer also keeps pit corrosion from occurring when exposed to salts that affect most other metals. Now as resistant to corrosion as it may be, if the aluminum layer breaks exposing the steel, the steel may oxidize and corrosion may occur.

Consumption

In North America nearly 70,000 tons of aluminized steel are consumed annually.[4] Some of the common products made from aluminized steel include water heaters, ranges, furnaces, space heaters and grills. [3]

Processing

Aluminized steel can be made using a variety of processes, cladding,hot dipping, galvanic coating, metallizing, and calorizing, but the most effective process is hot dipping. The process of hot dipping starts by cleaning the steel, then placing the steel in a bath of Al-11%Si at a temperature of 988K and shaken, then pulled out and air dried.[5] The aluminum diffuses into the steel, creating an intermetallic layer above the steel base layer, but below the outside aluminum coating. The aluminum coating is oxidized to help protect the inner steel from corrosion and further aluminum diffusion.[6] The silicon is added to the aluminum bath to create a thinner layer of aluminum on the steel. The hot dipping process is cheaper and more effective to produce aluminized steel than any other process.[7]

History and Development

Aluminized steel has a good history and was developed for providing more structural durability and a high yield strength in high corrosive environments. Aluminized steel maintains a high alloy’s steel strength at a fraction of the cost Aluminized steel is cheaper to produce than high alloy steels and thus a preferred material for manufacturing automobile and motorcycle exhaust gas systems.[8]

Pictures and Graphs

Structure of aluminized steel under a) light microscope and b) Scanning Electron Microscope. 1) oxidative layer 2) Aluminized layer 3) substrate

References

  1. ^ "Aluminized Steel Offers Attractive Physical Characteristics For Use In Industrial Duct Construction". Sheet Metal and Air Conditioning Contractors’ National Association. Retrieved 26 Feb 2011.
  2. ^ Kee-Hyun, Kim. Van-Daele, Benny. Van-Tendeloo, Gusfaaf. and Jong-Kyu, Yoon. (2006). “Observations of Intermetallic Compound Formation of Hot Dip Aluminized Steel”. Materials Science Forum, 519-21(2), 1871-75.
  3. ^ http://www.atlassteel.com/Products/aluminizedSteel.aspx
  4. ^ http://www.blocksteel.com/facts.htm
  5. ^ Rajendran, R. Venkataswamy, S. Jaikrishna, U. Gowrishankar, N. and Rajadurai, A. (2006). “Effect of Process Parameters in Hot Dip Aluminizing of Medium Carbon Steel”.
  6. ^ Deqing, Wang. and Ziyuan, Shi. (2003) “Formation of Al2O3 Layer on Steel”. Journal of Materials Science Letters, 22(14), 1003-1006.
  7. ^ Wang, Chaur. Jeng. Badaruddin, Mohd.. (2010) "The dependence of high temperature resistance of aluminized steel exposed to water-vapour oxidation". Surface and Coatings Technology, 205(5), 1200-1205.
  8. ^ Wang, Chaur. Jeng. Badaruddin, Mohd.. (2010) "The dependence of high temperature resistance of aluminized steel exposed to water-vapour oxidation". Surface and Coatings Technology, 205(5), 1200-1205.

Patent owner AK Steel USA .

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