This article gives industry insights into aluminized steel.
Read further to answer questions like:
- What is aluminized steel?
- Characteristics of aluminum coatings
- Advantages of aluminized steel
- And much more…
Chapter 1: What is Aluminized Steel?
Aluminized steels are steels that have been hot-dip coated with pure aluminum or aluminum-silicon alloys. This hot-dip coating process is termed hot-dip aluminizing (HAD). Through hot-dip aluminizing, ordinary steels are given more superior surface properties that are comparable to that of more expensive materials such as stainless steel.
The structural properties of steel and surface properties of aluminum are combined to produce a material with a unique set of properties. Steel has good strength, hardness, and other useful mechanical properties at a low cost, while aluminum has excellent surface properties and corrosion resistance. These combined properties broaden the capability of the material.
Aluminized steels are usually supplied by steel service centers. Steel service centers perform further processing to steel produced from metal foundries and steel mills. Aluminizing is one of the surface treatment processes done to ordinary steel to fit the needs of downstream manufacturing sectors such as construction, transportation, automotive, aerospace, and shipbuilding industries.
Chapter 2: 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 is the most important property of aluminized steel. Aluminum has an inherently high corrosion resistance compared to most metals. It protects the metal from the two mechanisms of corrosion: direct chemical attack and electrochemical action.
Direct chemical attack, or pure chemical corrosion, is corrosion resulting from the direct exposure of a highly reactive agent to the bare surface of the metal. The corrosion process is spontaneous to the area in contact. Liquid and gaseous products of the reaction eventually escape or disperse from the corrosion site. However, solid products remain. These solid products are typically rust or metal oxides. The gradual buildup of metal oxides to the outer layer of the metal can hinder further corrosion.
Aluminum does not readily corrode because it tends to form a thin protective layer of aluminum oxide on its surface. This aluminum oxide layer is a product of a direct chemical attack that does not progress to the degradation of the material. This imperious and adherent layer forms instantaneously upon exposure to the atmosphere. Moreover, when the layer is broken, it can regenerate by forming new aluminum oxide layers on the freshly exposed surface. This gives the material an almost permanent corrosion resistance.
Another protective feature of aluminum is electrochemical. Electrochemical corrosion involves an electrolyte solution to link the metal to a corrosive agent. This corrosive agent can be acids or cations of less active metals.
In most conditions, the outer aluminum oxide layer is enough to provide corrosion resistance. But for cases where the protective layer is unable to regenerate such as in the presence of a strongly acidic solution, the aluminum alloy underneath can still protect against electrochemical corrosion. During an electrochemical reaction, instead of consuming the steel, the highly active aluminum coating is corroded instead. Thus, aluminum provides cathode protection by acting as a sacrificial anode. Aluminum is one of the best anodic materials comparable to zinc.
The electrical conductivity of aluminum is around 61% that of copper. It is preferred over copper for certain applications due to its lower density and cheaper cost.
Aluminum conducts heat twice that of brass and four times that of steel. This means aluminum is extensively used in heat sink applications in electronics and electrical components.
In contrast with steel, aluminum retains its toughness at low temperatures. Low temperatures typically make metals fail under brittle fracture. The mechanical properties of aluminum are almost constant across all temperatures.
Resilience and Impact Strength:
Because of its natural toughness, aluminum has high resilience and impact strength. Aluminum parts can absorb sudden forces or shocks and can elastically flex from dynamic loads.
Aluminum has some of the highest reflectivity of any metal in the 200 to 400 nm range, much better than gold and silver. Aluminum coatings, rather than silver, are commonly applied to glass to make mirrors. Depending on its finish, aluminum can reflect about 90% of light across the wavelengths of the visible spectrum.
Unlike steel, aluminum is not ferromagnetic but paramagnetic. This means it does not acquire a magnetic charge when subjected to strong magnetic fields. This makes aluminum coatings suitable for electronic and electrical enclosures and parts that emit high electromagnetic fields. Moreover, along with its electrical conductivity, aluminum can be used to create electromagnetic field shields.
In its pure or alloyed form, aluminum does not produce sparks. This property is required for making tools used in flammable or explosive environments.
Chapter 3: Advantages of Aluminized Steel
Aluminized steel offers several advantages over other materials that are made for the same purpose. Stainless steel and galvanized steel are two extensively used materials, particularly for purposes wherein corrosion protection is needed. Aluminized steel is now becoming a better alternative because of its unique set of qualities.
Dual-feature corrosion protection:
As mentioned in the previous chapter, aluminized steels are protected against direct chemical attack and electrochemical action. Direct chemical attack is often referred to as dry corrosion while electrochemical action is termed as wet corrosion. Both corrosion mechanisms are present in most industrial environments.
Can be coupled with other metals:
Stainless steel is a typical material that competes with aluminized steel in terms of corrosion resistance. This material offers better resistance since it is homogeneously alloyed with chromium, an excellent oxide-forming metal. However, when coupled with other metals such as steel, it tends to promote a type of electrochemical reaction called galvanic corrosion. Steel that is connected to stainless steel corrodes faster by acting as an anode.
Aluminized steel presents no problem when it comes to galvanic corrosion. The aluminum coating is more anodic than steel. Thus, it is the one consumed by corrosion instead of the adjacent metal parts. Magnesium, beryllium, and zinc are a few metals that are more anodic than aluminum.
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.
Aluminized steels’ costs are comparable to that of galvanized steel. Galvanized steel is carbon steel coated with zinc, which provides the same protection mechanism as aluminum. This is another common product of the hot-dip coating process.
Excellent high-temperature performance:
Aluminized steels are suitable for applications up to 700°C. This maximum temperature varies with the type of aluminum coating and the carbon steel base. At these temperatures, the mechanical properties of the base are maintained. Aluminized steel is an effective material for heat exchanger tubes, automotive mufflers, exhaust pipes, and structural components of furnaces, water heaters, and burners.
When compared to stainless steel, results can vary depending on the stainless steel grade. Some stainless steel alloys have stabilizing elements, namely titanium and niobium, an example of which is stainless steel 316Ti. This stainless steel variant is intended for high-temperature use. However, without stabilizers, stainless steel is prone to oxidation at high temperatures. At temperatures above 500°C, carbide precipitation occurs, which eventually results in intergranular corrosion. This limits the application of the basic stainless steel grades to lower temperatures.
Galvanized steels can only withstand temperatures of approximately 250°C. At higher temperatures, free zinc in the coating reacts with the steel to form an iron-zinc alloy. Though the iron-zinc layer can provide protection, continued exposure to heat can cause the layer to crack and peel off.
Good heat reflectivity:
As mentioned earlier, aluminum has a high reflectivity in the visible spectrum. The same is true for reflecting infrared radiation. Infrared radiation is the common form of thermal energy or heat in furnaces and burners. Aluminized steel can reflect up to 80% of the incoming radiation. This depends on the coating’s surface quality.
Chapter 4: Types of Aluminized Steel
There are two main types of hot-dipped aluminized steel grades that are classified according to their aluminum bath composition. The first type, commonly called Type 1, is composed of an aluminum-silicon alloy. The second type, Type 2, is made from pure aluminum. Aluminized steel produced from each type has unique characteristics that are intended for specific applications.
Type 1 aluminized steel is made from a bath containing 5 to 11% silicon and aluminum as balance. The silicon in the molten bath has an important role in the proper formation of the brittle intermetallic layer between the outer aluminum coating and the inner base metal. Silicon is added to control the formation of this intermetallic layer by slowing its growth. This improves the heat resistance and workability of the finished product. However, some of the caveats of adding silicon are the slight deterioration of corrosion resistance, electrical conductivity, and bright finish. Type 1 aluminized steel is typically used in industrial equipment such as furnaces, heat exchangers, and burners.
The molten aluminum bath of type 2 aluminized steel is composed of commercially pure aluminum. Type 2 aluminized steels are intended to be used in conditions where the primary requirement is atmospheric corrosion resistance. They are used in common structural materials such as enclosures, sewage piping, and corrugated roofing.
Chapter 5: Base Metals
The structural properties of aluminized steels depend on the type of base metal. Hot-dip aluminizing is not limited to plain carbon steel. It can be done to other alloys as well. Below are some base steel alloys used in hot-dip aluminizing.
Commercial Steel (CS):
Commercial steel is often referred to as mild steel. They are used for general purpose applications. Commercial steels have a carbon content of around 0.10%.
Forming Steel (FS):
Forming steel has a lower carbon content than commercial steel. This makes them more ductile and malleable. Forming steels contain 0.02 to 0.10% carbon.
Deep Drawing Steel (DDS):
These steels are used in steel fabrication processes where the stock is radially drawn from a forming die. They contain approximately 0.06% carbon.
Extra Deep Drawing Steel (EDDS):
EDDS is similar to DDS. However, these steels have a smaller carbon content of around 0.02% which makes them more ductile.
Structural Steel (SS):
These are industrial-grade steels. They are composed of 0.20 to 0.25% carbon, which makes them harder than commercial steel.
High-strength Low-alloy Steel (HSLAS):
These steels are made according to a set of mechanical properties and not by specific chemical composition. They have yield strengths of around 250 to 590 MPa.
Ferritic Stainless Steel (FSS):
These are stainless steels that are further enhanced by aluminizing. The grades of ferritic stainless steel include types 409 and 439.
Chapter 6: Manufacturing Process
Hot dipping is the main process employed for producing aluminized steel. It is a widely used method since its operation is the least expensive and requires the simplest equipment. Aside from hot dipping, other aluminum coating methods exist. These are summarized and explained briefly below.
Calorizing:Calorizing involves diffusing aluminum into the surface of the workpiece such as plain carbon steels and high alloy steels. This is achieved through either pack diffusion or slurry method. Pack diffusion involves packing aluminum powders to the workpiece and baking them at high temperatures to create the diffusion layer. Slurry method, on the other hand, involves spraying or dipping the workpiece into a mixture of aluminum powders and binders. The metal is then baked and dried.
Electroplating:Electroplating is a general process defined as the formation of a metal coating onto another metal or other solid material. This is accomplished by passing a direct current between the anode (aluminum) and the cathode (base metal or workpiece). The current is conducted by aluminum ions present in an electrolyte solution. Aluminum ions are then deposited on the surface of the workpiece, forming a layer of coating.
Metal spraying, also known as metalizing, is the process of producing aluminized steel by spraying the workpiece with molten aluminum. Aluminum wire or powder is melted using electric arcs, oxy-acetylene flame, or plasma. It is then atomized and sprayed onto the surface.
Aluminum coating through the cladding process is achieved by rolling, extruding, or drawing steel together with an aluminum sheet or film. The process is done at elevated temperatures to facilitate the bonding of the aluminum coating to the steel part.
The Hot Dipping Process
Hot dipping is a straightforward process that involves three main operations, namely surface preparation, immersion, and finishing. Additional steps may be added to improve the process such as flux coating and heat treatment. These series of operations are explained in detail below.
Surface preparation is an assortment of processes that aims to obtain a metal surface that has a high level of 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 is done typically 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 it cannot thoroughly remove oil and oxides.
This stage covers pickling, electrolytic cleaning, and solvent cleaning. Pickling is a surface treatment that involves immersing 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 cleaning 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.
This process serves two purposes. The first is to prevent the oxidation of the surface of the workpiece after cleaning. Oxidation of metals 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 wetting characteristics of the metal. This ensures 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 as well.
This is the main aluminizing process where 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 that of 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 of the 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 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 had 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 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 cooling 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.
- Aluminized steels are steels that have been 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 provides 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.