Aluminized Steel (Steel Service Centers)

Chapter 1: What is Aluminized Steel?

Aluminized steel is steel that has been coated with a layer of aluminum or aluminum-silicon to enhance its properties, particularly its resistance to corrosion and rust. The hot dipping process, which involves immersing the steel in molten aluminum, improves the steel's surface characteristics, making it more durable and performance-effective, comparable to stainless steel.

The manufacturing process of aluminized steel combines the structural strength of steel with the surface properties of aluminum to create a material with outstanding durability and appearance. Steel offers strength, hardness, and good mechanical properties at a low cost, while aluminum provides a corrosion-resistant, attractive oxidized coating. This fusion enhances the steel's durability and expands its applicability, resulting in a metal that benefits from both the robust characteristics of steel and the protective qualities of aluminum.

Aluminized steel is supplied by steel service centers that process from steel mills. Aluminizing, a process similar to galvanizing, is one of the surface treatments performed on ordinary steel to meet the needs of downstream manufacturing sectors such as construction, transportation, automotive, aerospace, shipbuilding industries, appliances, HVAC systems, bakeware, and fireplaces.

Aluminized steel leverages the strengths of both metals. Steel, with a density and weight 250% greater than aluminum, is significantly stronger. However, uncoated steel is prone to rust and corrosion when exposed to the atmosphere. In contrast, aluminum's natural coating protects it from rust and corrosion, making it an ideal complement to steel in aluminized steel products.

Chapter 2: What are the characteristics of aluminum coatings?

Aluminum is one of the most commonly used metals today. They are particularly known for their high strength-to-weight ratio, making them suitable for automobiles and aircraft parts. Aside from its lightness, aluminum has many desirable surface characteristics. Its ability to protect the material from corrosion tops the list. Other properties such as conductivity, toughness, and reflectivity are also described below.

Corrosion Resistance

Corrosion resistance is a key attribute of aluminized steel. Aluminum inherently possesses high resistance to corrosion, effectively shielding the metal from two primary mechanisms of corrosion: direct chemical attack and electrochemical action.

Direct chemical attack, also known as pure chemical corrosion, occurs when a highly reactive agent comes into direct contact with the bare surface of a metal. This spontaneous reaction produces liquid and gaseous byproducts that escape or disperse from the corrosion site, while solid byproducts, such as rust or metal oxides, remain. Over time, the accumulation of these metal oxides on the surface can help to protect the underlying metal from further corrosion.

Aluminum resists corrosion effectively due to the formation of a thin, protective layer of aluminum oxide on its surface. This oxide layer, created by a direct chemical reaction, prevents further degradation of the material. It forms almost instantly upon exposure to the atmosphere and can regenerate if damaged, ensuring nearly permanent corrosion resistance.

Another protective mechanism of aluminum is its electrochemical behavior. Electrochemical corrosion occurs when an electrolyte solution connects the metal with a corrosive agent, such as acids or cations from less active metals.

Typically, the aluminum oxide layer provides sufficient corrosion resistance. However, in situations where this layer cannot regenerate, such as in highly acidic environments, the underlying aluminum alloy still offers protection against electrochemical corrosion. During such reactions, the aluminum coating, being highly active, corrodes instead of the steel. Thus, aluminum acts as a sacrificial anode, providing cathodic protection. It is an effective anodic material, comparable to zinc.

Electrical Conductivity

Aluminum has about 61% of the electrical conductivity of copper. Despite this lower conductivity, aluminum is often chosen over copper for specific applications because of its lower density and cost-effectiveness.

Thermal Conductivity

Aluminum conducts heat at twice the rate of brass and four times more efficiently than steel. This high thermal conductivity makes aluminum a popular choice for heat sinks in electronics and electrical components.

Low-temperature Toughness

Unlike steel, aluminum maintains its toughness even at low temperatures, where other metals might become brittle and fail. The mechanical properties of aluminum remain relatively stable across a wide range of temperatures.

Resilience and Impact Strength

Aluminum's inherent toughness gives it high resilience and impact strength. It can absorb sudden forces or shocks and elastically flex under dynamic loads.

Reflectivity

Aluminum is highly reflective, especially in the 200-400 nm range, surpassing gold and silver. Therefore, aluminum coatings are often used on glass to create mirrors. With the right finish, aluminum can reflect approximately 90% of visible light.

Non-magnetic

Aluminum is paramagnetic, not ferromagnetic like steel. It doesn’t become magnetized under strong magnetic fields, making it ideal for electronic and electrical enclosures and components that emit high electromagnetic fields. Its electrical conductivity also allows it to serve as an electromagnetic field shield.

Non-sparking

Aluminum, whether pure or alloyed, does not produce sparks, making it suitable for manufacturing tools used in flammable or explosive settings.

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Chapter 3: What are the advantages of Aluminized Steel?

Aluminized steel offers several advantages over other materials made for the same purpose. Stainless steel and galvanized steel are two extensively used materials, particularly for purposes needing corrosion protection. Aluminized steel is now becoming a better alternative because of its unique qualities.

Dual-feature Corrosion Protection

As discussed earlier, aluminized steels are protected against direct chemical attack and electrochemical action. Direct chemical attack, commonly known as dry corrosion, occurs without the need for an electrolyte. Electrochemical action, referred to as wet corrosion, involves an electrolyte solution that facilitates the corrosion process. Both types of corrosion are prevalent in many industrial environments.

Coupling with Other Metals

Stainless steel is a widely used material known for its superior resistance to corrosion, largely due to its chromium content, which forms a protective oxide layer. However, when stainless steel is paired with other metals like carbon steel, it can lead to galvanic corrosion. In this electrochemical reaction, the carbon steel, acting as the anode, corrodes more rapidly compared to the stainless steel.

In contrast, aluminized steel does not encounter issues related to galvanic corrosion. The aluminum layer on this steel is more anodic than the steel itself, meaning it corrodes preferentially, thereby protecting the underlying metal. Other metals such as magnesium, beryllium, and zinc are even more anodic than aluminum.

With aluminized steel, galvanic corrosion is not a concern because the aluminum coating, being more anodic than the steel, undergoes corrosion in preference to the adjacent metal components. Additionally, metals like magnesium, beryllium, and zinc, which are even more anodic than aluminum, are used in various applications for their high corrosion resistance.

Lower Cost

Aluminized steels are much cheaper than stainless steels. This is because, in aluminized steel, the main structural component of the finished part is still plain carbon steel. The aluminum alloy coating is only 30 to 270 grams per square meter or 0.10 to 0.90 ounces per square foot of surface area. Most of the cost of aluminized steel goes into the operating expenses of the hot dipping process.

The coating of steel with alloys to offer extra protection is very common and includes several different metals. Anodizing, electroplating, and powder coatings have been used for years to protect the surface of steel. Although hot dipping for aluminized steel is costly, its costs are comparable to other steel coating methods. In the galvanizing process, carbon steel is coated with zinc, which provides the same protection as aluminum. Anodizing involves the application of various types of non-ferrous metals, such as copper, lead, and tin.

Excellent High-Temperature Performance

Aluminized steels are effective for use in environments up to 1292°F (700°C). The maximum temperature they can handle depends on the specific aluminum coating used and the type of carbon steel base. At these elevated temperatures, the base material retains its mechanical properties. Aluminized steel is well-suited for applications such as heat exchanger tubes, automotive mufflers, exhaust pipes, and structural components in furnaces, water heaters, and burners.

The performance of stainless steel can vary depending on its grade. Certain stainless steel alloys, such as 316Ti, are alloyed with stabilizing elements like titanium and niobium, making them suitable for high-temperature applications. Without these stabilizers, stainless steel may be susceptible to oxidation at elevated temperatures. When exposed to temperatures above 932°F (500°C), carbide precipitation can occur, leading to intergranular corrosion. As a result, standard stainless steel grades are generally limited to lower-temperature environments.

Galvanized steels are capable of withstanding temperatures up to around 482°F (250°C). Beyond this temperature, the free zinc in the coating begins to react with the steel, forming an iron-zinc alloy. While this iron-zinc layer offers some protection, prolonged exposure to high temperatures can lead to cracking and peeling of the coating.

Good Heat Reflectivity

As previously noted, aluminum exhibits high reflectivity in the visible spectrum, and this extends to infrared radiation as well. Infrared radiation, which is a common type of thermal energy found in furnaces and burners, can be effectively reflected by aluminum. Aluminized steel can reflect up to 80% of incoming infrared radiation, depending on the quality of the coating's surface.

Chapter 4: What are the types of Aluminized Steel?

Hot-dipped aluminized steel is categorized into two primary grades based on the composition of the aluminum bath. The first grade, known as Type 1, consists of an aluminum-silicon alloy. The second grade, Type 2, is made from pure aluminum. Each type of aluminized steel exhibits distinct properties tailored for different applications.

• Type 1:

  Type 1 aluminized steel is hot dipped to form a thin layer of aluminum with 5% to 11% silicon added to create better adhesion of the aluminum layer. Silicon helps in the formation of a brittle intermetallic layer between the thin outer aluminum coating and various types of steel base metals.

  Silicon slows the growth and controls the formation of the intermetallic layer, which improves the heat resistance and workability of aluminized steel. Although the addition of silicon is beneficial in enhancing the bonding of the aluminum layer and the base steel, it does slightly deteriorate its corrosion resistance and electrical conductivity and creates black spots on the finished aluminized steel. Type 1 aluminized steel is typically used in industrial equipment and commercial products such as mufflers, furnaces, ovens, ranges, heaters, water heaters, fireplaces, and baking pans.

  

  
  
• Type 2:

  The molten aluminum bath of type 2 aluminized steel is composed of commercially pure aluminum. It is designed for use in conditions where the primary requirement is resistance to atmospheric corrosion. Type 2 aluminized steels are used as structural materials for enclosures, sewage piping, corrugated roofing, siding, grain bins, drying ovens, and air conditioner condenser housings.

  

  
  

Chapter 5: what is base metals?

Aside from carbon steel, several types of steel are used in the aluminizing process. The positive properties of aluminum are ideal for the protection of all steels, including commercial grade, forming, deep draw, structural, and high-strength low alloy steels, all of which are susceptible to the same negative effects as carbon steel. Applying aluminum alloy increases the longevity of the steels and the number of applications for which they can be used.

Commercial Steel (CS)

Commercial steel, also known as mild steel or commercial quality steel, is widely used for general-purpose applications. It is characterized by its ductility, softness, and malleability, allowing it to be bent in any direction without cracking. This steel typically has a carbon content of approximately 0.10%.

Forming Steel (FS)

Forming steel is distinguished by its lower carbon content compared to commercial steel, making it more ductile and malleable. It contains between 0.02% and 0.10% carbon.

Deep Drawing Steel (DDS)

Deep Drawing Steel is used in processes where the metal is radially drawn from a forming die. These steels have a carbon content of about 0.06%.

Extra Deep Drawing Steel (EDDS)

Extra Deep Drawing Steel is similar to Deep Drawing Steel but features an even lower carbon content, around 0.02%, which enhances its ductility.

Structural Steel (SS)

Structural Steel is an industrial-grade steel with a carbon content ranging from 0.20% to 0.25%, making it harder than commercial steel. Its high tensile strength makes it ideal for construction and building applications. This category includes various types such as carbon steel, high-strength carbon steel, weathering steel, and fire-resistant steel.

High-Strength Low-Alloy Steel (HSLAS)

High-Strength Low-Alloy Steel is defined by its mechanical properties rather than specific chemical compositions. It typically has yield strengths ranging from 250 to 590 MPa.

Ferritic Stainless Steel (FSS)

Ferritic Stainless Steel, an alternative to austenitic stainless steel, is enhanced by the aluminizing process. It is a low-carbon, high-chromium stainless steel with minimal or no nickel content. Known for its ductility, corrosion resistance, and magnetic properties, Ferritic Stainless Steel belongs to the 400 series of stainless steels, categorized by their composition and applications.

Chapter 6: What is manufacturing process?

Hot dipping is the primary method used to produce aluminized steel due to its cost-effectiveness and relatively simple equipment requirements. While hot dipping is the most common process, several alternative aluminum coating methods are also available. These methods are summarized and briefly explained below.

Non Hot-Dipping Processes

Calorizing

Calorizing is a surface treatment technique designed to apply aluminum coatings to steel, enhancing its resistance to heat and corrosion. In this process, steel is exposed to a mixture of aluminum powders within a furnace. The high temperatures in the furnace cause the aluminum to diffuse onto the steel's surface. Calorizing can be carried out using two main methods: pack diffusion and the slurry method.

• Pack diffusion - Pack diffusion involves packing aluminum powders onto the surface of the steel workpiece and baking it at high temperatures to create a diffusion layer. The process of pack diffusion is used to coat ferrous metals with non-ferrous metals for protection against corrosion.
• Slurry method – The slurry method involves spraying or dipping the workpiece into a mixture of aluminum powders and binders, after which it is baked and dried. It is used to place a coating less than one millimeter thick on the base metal. The process of slurry coating includes aluminum powder, an activator, adhesive, and binder. The key to the process occurs during curing, when the coating transfers onto the steel substrate.

Electroplating

Electroplating is a process used to deposit a thin metal layer onto an electrically conductive surface using two electrodes or conductors. In this process, the anode electrode, which is positively charged, causes metal ions to move toward the negatively charged cathode electrode, while the negatively charged ions return to the anode.

The metal to be coated is placed at the cathode, and the metal coating is positioned at the anode. As ions travel from the anode to the cathode, they adhere to and cover the steel workpiece. Direct current is applied to the anode, causing its metal atoms to oxidize and dissolve. When these ions reach the cathode, they are reduced and form a coating on the metal piece. The current is regulated so that the rate of metal dissolution at the anode equals the rate of plating at the cathode.

Electroplating, also known as electrodeposition, enhances the strength and durability of steel by applying an aluminum coating. It is commonly used in various industries, including aerospace, automotive, medical and dental tools, and microwave products.

Metal Spray Coating

Metal spray coating, also known as metalizing and thermal spraying, melts aluminum wire into a molten material and injects it into a compressed air stream, where the molten aluminum is atomized into fine droplets and sprayed onto the surface of the steel workpiece. The fine droplets quickly cool, fuse, and solidify to form a protective metallic layer or film with a thickness of 0.47 to 0.59 inches (12 mm to 15 mm).

For the metal spray coating process, the aluminum feedstock typically used is 85/15 wire. This method is designed to apply an aluminum coating to steel surfaces without forming an intermetallic layer, unlike hot dipping. However, metal spray coating often results in a less smooth finish compared to other coating methods, with surfaces appearing mottled, rough, and uneven.

Cladding

Aluminum coating via the cladding process involves rolling, extruding, or drawing steel together with an aluminum sheet or film. This procedure is performed at elevated temperatures to ensure a strong bond between the aluminum coating and the steel component.

• Roll Bonding – The aluminum layer and steel are passed through a pair of rollers that apply enough pressure to bond the aluminum to the steel. The pressure is sufficient to deform the aluminum and steel layers and reduce their combined thicknesses. Heat may be used to increase the ductility of the metals.
• Explosive Welding – With explosive welding, the pressure to create the bond between the steel and aluminum is created by the detonation of a sheet of chemical explosive that spreads aluminum over the steel and removes impurities and oxides. It is an ideal process for giving steel a corrosion-resistant surface.
• Laser Cladding – Laser cladding, also known as laser metal deposition, is similar to the metal spray coating process. It involves feeding aluminum feedstock, wire, or powder into a melt pool created by a laser beam as it passes over the surface of the steel. Laser cladding spreads the aluminum coating over the surface of the steel using minimal heat to create a tight, secure bond.

Hot Dipping Process

Hot dipping is a simple process comprising three primary steps: surface preparation, immersion, and finishing. To enhance the process, additional procedures such as flux coating and heat treatment may be included. The following sections provide a detailed explanation of each of these operations.

• Surface Preparation: Surface preparation is an assortment of processes that aims to obtain a metal surface with high cleanliness and purity. Contaminants such as oil, grease, dirt, and rust can inhibit the binding of the molten aluminum to the base metal.
  • Mechanical Cleaning: Mechanical cleaning is typically done by grinding, brushing, or blasting. This process is used to physically remove contaminants on the surface of the workpiece. Subsequent chemical cleaning processes are usually employed after this step since they cannot thoroughly remove oil and oxides.
  • Chemical Cleaning:This stage covers pickling, electrolytic cleaning, and solvent cleaning. Pickling is a surface treatment that immerses the workpiece in a mild acid bath to remove scales. Electrolytic cleaning removes impurities on the surface of the workpiece by passing a current through the workpiece. Depending on the direction of current flow, oxygen or hydrogen gas is released, which lifts and removes contaminants on the surface of the workpiece. Another common method is solvent cleaning, which targets organic compounds such as oils, greases, and paint. Compatible solvents dissolve the contaminants, creating a solution. This solution can be easily removed from the workpiece.

    

    
    

• Fluxing: This process serves two purposes. The first is to prevent the oxidation of the surface of the workpiece after cleaning. Metals oxidize is faster at elevated temperatures. Since hot dipping occurs at the melting point temperature of aluminum alloy, the oxidation of the workpiece is accelerated. The newly created oxides can interact with the intermetallic layer and can decrease the bonding strength of the coating.

  The second benefit of fluxing is to improve the wettingcharacteristics of the metal, ensuring that the whole surface is covered and free from voids or discontinuities. This further enhances the bonding strength of the intermetallic layer.

  Aside from directly applying the flux coating onto the workpiece, fluxing can also be done by covering the molten aluminum bath with a flux solution. This is known as wet fluxing. Through wet fluxing, a layer of molten flux is applied to the surface of the aluminum bath. This not only prevents the workpiece from oxidizing upon contact with the molten bath but also prevents the bath from oxidizing.

• Immersion: This is the main aluminizing process in which the workpiece is dipped into the molten aluminum bath. At first glance, this is a fairly easy process. However, certain difficulties arise, which can result in problems such as peeling of the aluminum coating, poor mechanical properties of the finished part, surface discontinuities, and workpiece deformation. To correct these, a few requirements must be followed.

  

  
  
  • The workpiece must have a relatively high melting temperature compared to the aluminum alloy. This is to prevent any alteration of the mechanical properties of the workpiece.
  • The base metal and the aluminum alloy must be soluble and able to form alloys. This is to ensure the aluminum coating will wet and cover the entire surface of the workpiece.
  • The immersion time must be right to create the intermetallic layer of iron-aluminum alloy. This layer is necessary to metallurgically bind the aluminum coating with the base metal. However, this layer is brittle, and excessive immersion may result in a thicker layer. This thick, brittle layer can cause the aluminum coating to peel off from the base metal upon introduction to forming operations.

  The first and second requirements are easily met when steel is used as the base metal. Steel has a much higher melting point than aluminum. Also, both metals readily form alloys. The third requirement is based on the best practices of the manufacturer. Some variables that affect immersion time are the composition of the aluminum alloy, the target thickness of the intermetallic layer, and the molten bath temperature.

• Finishing: Finishing includes cooling, chemical treating, and coating. A selected type of finishing is utilized; the selection depends on the product specifications. The finished product is typically cooled at room temperature with or without subsequent heat treatment processes. Chemical treating involves applying proprietary solutions to protect the highly reflective surface from stains and scaling. Coated aluminized steel has oils or lubricants applied to aid the formability of the finished part. Oils and lubricants prevent the aluminum coating from being damaged during processing.
• Heat Treatment: Heat treatment is usually performed after the initial cooling process. This process involves placing the newly produced aluminized steel into a furnace to heat the metal and cool it at a controlled rate. This is done to further diffuse the aluminum into the base metal. The results are better anti-corrosive properties and improved bonding.

Conclusion

• Aluminized steel is steel hot dip coated with pure aluminum or aluminum-silicon alloys. This hot-dip coating process is termed hot-dip aluminizing (HAD).
• Corrosion resistance is the most important property of aluminized steel. Aluminum has an inherently high corrosion resistance compared to most metals.
• Aluminized steel protects protection against the two mechanisms of corrosion: direct chemical attack and electrochemical action.
• Aside from being corrosion-resistant, aluminized steel is also known for its low cost, excellent high-temperature performance, and heat reflectivity.

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