Galvalume Steel Coil: Guide to Durability and Performance

Galvalume steel coil, developed by Bethlehem Steel in 1972, combines the corrosion resistance of aluminum with the sacrificial protection of zinc, providing a lifespan about two to four times longer than traditional galvanized steel. Its durability makes it effective for roofing, siding, and various construction applications.

This versatile material is crucial in construction, manufacturing, architectural projects, automotive parts, agricultural equipment, and HVAC systems, offering enhanced protection against weathering and corrosion. 

Composition and Properties of Galvalume Steel Coil

The steel substrate is coated with a protective alloy composed primarily of 55% aluminum, 43.4% zinc, and 1.6% silicon. This coating alloy is critical in providing Galvalume steel coils with their corrosion-resistant properties.

Table of Coating Composition:

The microstructure of the coating also plays an essential role in how Galvalume steel coil performs, especially when it comes to protection. The aluminum provides a barrier against corrosion, while zinc acts actively to protect the steel where it is exposed, such as at cut edges. Silicon enhances the durability of the coating, enabling the product to withstand harsh environments.

Properties conferred by this alloy include high corrosion resistance, which outperforms galvanized steel on a per-thickness basis. Additionally, it offers excellent heat reflectivity and can withstand high temperatures without discoloration. 

Coating Microstructure(Corrosion resistance principle)

The 55% Al-Zn coating on Galvalume steel has a microstructure comprising two main phases: an aluminum-rich dendritic phase and a zinc-rich interdendritic region. The dendritic phase forms first during solidification, followed by the interdendritic areas as zinc concentration increases. The specific origin of these phases relates to the aluminum-zinc phase diagram. 

Additionally, the microstructure contains small silicon needles and an intermetallic layer at the interface with the steel. The unique microstructure, featuring a network of zinc-rich regions, is crucial for the coating’s corrosion resistance, which is enhanced by a labyrinth of these regions created during cooling after the hot-dip process.

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Galvalume Life Expectancy

Galvalume steel coils are resistant to corrosion and rusting, which means they can last for several decades even in harsh environments. According to research, galvalume has proven its outstanding performance in North America, which is prone to extreme conditions such as winter and acid rain.

The outdoor exposure tests on actual building installations prove that Galvalume steel sheets and coils are more resilient and have better corrosion resistance than zinc-coated sheets with the same coating thickness under the same harsh conditions. The steel’s service life expectancy can reach an average of up to 35 years without regular maintenance.

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The test also revealed that the service life of Galvalume could reach 40 to 60 years in both rural and industrial environments. The study consists of atmospheric tests in which researchers used flat coupon samples over a span of 17 years in Canada and over 36 years in the United States.

However, the actual service life can vary significantly based on several factors:

Environmental Conditions:
– Exposure to harsh weather conditions, such as high humidity, salt spray in coastal areas, industrial pollutants, and extreme temperatures, can affect the longevity of Galvalume steel.

Coating Thickness:
– The thickness of the Al-Zn coating directly influences the durability of the steel coil. A thicker coating typically leads to a longer lifespan.

Quality of Installation:
– Proper installation and maintenance are crucial for maximizing the life of Galvalume steel structures. Incorrect installation can lead to premature failure.

Manufacturing Process

Surface Preparation:

Before the application of the coating, the surface of the steel coil must be thoroughly cleaned to remove any oil, dirt, or scale. This is typically done using an acid bath (pickling) followed by a rinse and then a chemical treatment to enhance coating adhesion.

Galvalume Coating:

The clean steel coil is then passed through a bath of molten Galvalume. This process is known as hot-dip galvanizing. The steel strip passes through the molten bath and exits the other side, where excess coating is removed by air knives or mechanical wiping.

Cooling and Solidification:

After the excess coating is removed, the coated steel strip is cooled, allowing the Galvalume coating to solidify and bond to the steel surface.

Surface Treatment:

The coated steel may undergo additional surface treatments, such as oiling or passivation, to enhance corrosion resistance or improve paint adhesion. These treatments can also help to prevent any potential storage stains or fingerprints during handling and transportation.

Recoiling:

Once the inspection and any additional treatments are complete, the Galvalume steel coil is recoiled. This involves winding the steel strip onto a large reel or mandrel to create a coil of the desired size and weight for shipping and further processing.

Standards and Specifications

A. Overview of International Standards for Galvalume Steel Coil

Galvalume steel coil is subject to various international standards which specify the properties and quality requirements for coated steel products. ASTM A792 is a prominent standard set by the American Society for Testing and Materials, which outlines the specifications for 55% Al-Zn alloy-coated steel sheets. The EN 10346 standard, established by the European Committee for Standardization, also details requirements for continuously hot-dip coated steel flat products.

B. Specifics of Coating Weight and Thickness

The coating weight of Galvalume steel coil is precisely defined in the standards to ensure durability and resistance to corrosion. For instance:

Testing and Certification Processes

To verify that the Galvalume steel coil meets the required standards, it undergoes rigorous testing, which includes:

• Adherence Tests: To assess the coating’s grip on the steel substrate.
• Corrosion Resistance Tests: Such as salt spray tests to measure the material’s resistance to oxidization.
• Tensile Tests: To determine the mechanical strength of the steel.

Applications of Galvalume Steel Coil

Building and Construction:

   – Roofing materials: Galvalume steel is often used for residential, commercial, and industrial roofing due to its durability and resistance to corrosion.

   – Wall cladding: As an exterior protective layer on buildings, it provides a combination of strength and aesthetics.

   – Structural sections: It is used for framing and support structures due to its strength and resistance to environmental degradation.

   – Gutters and downspouts: Its corrosion resistance makes it suitable for rainwater collection systems.

Agricultural Equipment:

   – Grain bins and silos: The protective coating on Galvalume steel is beneficial for storing grain and other agricultural products, as it resists corrosion from natural elements.

   – Barns and animal shelters: These structures benefit from the use of durable, long-lasting materials like Galvalume steel.

Comparison With Galvanized Steel

A. Material Characteristics

Galvalume steel coil and galvanized steel are both coated steel products that are known for their durability and protection against corrosion. Galvalume is coated with an alloy of zinc, aluminum, and silicon, while galvanized steel is coated purely with zinc. The aluminum in Galvalume offers better surface smoothness and heat reflection, leading to higher temperature resilience compared to galvanized steel.

B. Corrosion Protection Differences

The protective layer on Galvalume coils consists of:

• 55% Aluminum: Provides barrier protection against corrosion.
• 43.4% Zinc: Offers galvanic protection to the steel.
• 1.6% Silicon: Enhances the adhesion and durability of the coating.

In comparison, galvanized steel’s zinc-only coating primarily provides galvanic protection which is effective but can erode over time, leading to potential rust.

C. Applications Contrast

Galvalume is optimal for roofing, siding, and trim applications, where long-lasting protection is crucial, especially in harsh environments. In contrast, galvanized steel is commonly used in interior applications or environments with fewer corrosive elements, such as framing in construction.

D. Economic and Efficiency Considerations

While the initial cost of Galvalume may be higher than galvanized steel, its longevity and durability can lead to reduced maintenance and replacement costs over time. The reflective properties of Galvalume may also lead to energy cost savings.

E. Aesthetic and Design Benefits

Galvalume steel has a distinct, smooth, spangled appearance that is often seen as more aesthetic than the matte finish of galvanized steel. This can be an important design factor when exposed steel is part of the architectural consideration.

Product Specifications and Thickness

The thickness of the coil varies according to its application and customer needs. The common thickness range is listed below:

• 0.15 mm to 1.5 mm for standard applications
• 1.6 mm to 2.0 mm for structural uses

Galvalume steel coils are available in several grades, which cater to different environments and purposes, outlined as follows:

• Commercial Grade (CS Types A, B, or C): Suitable for moderate forming.
• Forming Steel (FS type): Designed for more severe forming applications.
• Structural Grades (33/37/40/50/80): Used in load-bearing applications.

Coating weights are another crucial aspect, commonly expressed as AZ50 to AZ150 (which signifies 0.50 to 1.50 ounces of coating per square foot, respectively). This denotes the durability and lifespan of the material.

Coil Sizes:

• Width: Ranges from 600mm (23.6 inches) up to 1500mm (59.1 inches)
• Inside Diameter: Standard at 508mm (20 inches) or 610mm (24 inches)
• Outside Diameter: Typically up to 2000mm (78.7 inches)

Other Names

Galvalume steel coil is known by several other names, depending on the manufacturer or the region. Some of the alternative names:

Aluzinc: This is a common name used for Galvalume steel coil in Europe and some parts of Asia. The name is derived from the combination of aluminum and zinc in the coating.

Zincalume: Zincalume is a registered trademark of BlueScope Steel, an Australian company. It is essentially the same product as Galvalume, with a coating composition of 55% aluminum, 43.5% zinc, and 1.5% silicon.

Al-Zn Coated Steel: This is a generic name for steel coils coated with an aluminum-zinc alloy, which includes Galvalume.

Aluminium-Zinc Alloy Coated Steel: This is another generic name for steel coils coated with an aluminum-zinc alloy, including Galvalume.

Galval:  Galval is another name used for Galvalume steel coil, particularly in some parts of Europe.

Galvalume Plus: Galvalume Plus is a variant of Galvalume steel coil that has an additional clear acrylic coating applied over the Galvalume coating for added protection and enhanced aesthetics.

Conclusion

Galvalume steel coil offers distinctive advantages which are instrumental for its widespread use in the construction and manufacturing industries. Its alloy coating, predominantly consisting of aluminum, zinc, and silicon, provides exceptional corrosion resistance, making it a superior choice for roofing and siding applications. Moreover, the energy efficiency it confers to buildings, due to its reflective properties, can be critical in reducing overall energy costs.