Hot dipped galvanized steel is a form of galvanizing for building bridges, skyscrapers, and structures in the harshest environments. The process passes through painstaking stages to ensure that the steel receives corrosion protection that could last for over 70 years.
Find out in this article how hot-dipped galvanized is processed and its long-lasting benefits to structures.
What Is Hot-Dipped Galvanized?
Hot-dip galvanization is a form of galvanization. It is the process of coating iron and steel with zinc, which alloys with the surface of the base metal when immersing the metal in a bath of molten zinc at a temperature of around 449 °C (840 °F). When exposed to the atmosphere, the pure zinc (Zn) reacts with oxygen (O2) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO2) to form zinc carbonate (ZnCO3), a usually dull grey, fairly strong material that protects the steel underneath from further corrosion in many circumstances. Galvanized steel is widely used in applications where corrosion resistance is needed without the cost of stainless steel, and is considered superior in terms of cost and life-cycle. It can be identified by the crystallization patterning on the surface (often called a “spangle”).
Galvanized steel can be welded; however, one must exercise caution around the resulting toxic zinc fumes. Galvanized fumes are released when the galvanized metal reaches a certain temperature. This temperature varies by the galvanization process used. In long-term, continuous exposure, the recommended maximum temperature for hot-dip galvanized steel is 200 °C (392 °F)
Hot dipped galvanized is a galvanizing method by coating the steel and iron with zinc. The steel enters a hot bath of molten zinc at around 842 degrees Fahrenheit. During this process, the zinc reacts with oxygen after exposure to the atmosphere.
It leads to the formation of zinc carbonate, which offers rust and corrosion protection to the steel. Hot-dip galvanization offers a superior life cycle and is cost-efficient due to its anti-corrosion properties.
The critical feature of HDG products is longevity and durability. The first maintenance time of hot-dipped galvanized steel is proportional to the zinc coating thickness. It passes through different processing state that yields zero maintenance in the years to come.
What is Galvanizing?
Galvanizing, or galvanization, is a manufacturing process where a coating of zinc is applied to steel or iron to offer protection and prevent rusting. There are several galvanizing processes available, but the most commonly offered and used method is called hot-dip galvanizing.
Galvanized steel is among the most popular steel types because of its extended durability, having the strength and formability of steel plus the corrosion protection of the zinc-iron coating. The zinc protects the base metal by acting as a barrier to corrosive elements, and the sacrificial nature of the coating results in a long-lasting and high-quality steel product.
This versatility makes it applicable to a variety of projects and industries, including agriculture, solar, automotive, construction, and so on. Below, we aim to provide a comprehensive description of how galvanized steel is processed, different galvanization methods, its benefits, and how it is used in these various industries.
Hot Dipped Galvanized Vs. Galvanized
The difference between galvanized and hot-dipped galvanized steel lies in the texture and finishing.
Galvanized materials have sharp and smooth finishing, while hot-dipped galvanized have a rough texture. Galvanization passes through different stages to form a layer of zinc carbonate on the metal surface to protect it from corrosion.
Hot-dip galvanization undergoes a process to form a pure layer of zinc on the surface, which passes through three stages: preparation, galvanizing, and inspection.
For your easy understanding, galvanized materials have a sharp and smooth texture, while hot dip galvanized have a rough surface. Hot dipped galvanized materials are less expensive than galvanized materials if you consider their performance and longevity.
Hot-dip galvanized materials are mistaken for more costly than it is. People perceive hot-dipped galvanizing as expensive because it underwent a high-performance coating process.
If you think about its longevity of fifty to seventy years, hot-dipped galvanized is a more inexpensive form of galvanization.
Hot Dipped Galvanized Process
Hot-dip galvanizing undergoes meticulous stages to develop a metallurgical bond between the steel and zinc. Before the steel goes to the galvanizing kettle, it passes through a four-step cleaning process.
The steel passes through the caustic tank at 180 degrees Fahrenheit. Immersing the steel in an acidic solution removes the steel’s paint, dirt, grease, oil, and residues. The steel enters the fresh water tank to rinse the acidic cleaning solution.
It goes to a pickling tank of hot acidic solution at 140 to 145 degrees Fahrenheit to remove the mill scale and then rinsed off. The steel immerses in a flux zinc solution consisting of zinc ammonium chloride at 140 to 170 degrees Fahrenheit.
Flux provides oxidation on the clean surface once exposed to the air before the steel undergoes galvanization. The flux dries on the surface to adhere and helps in the liquid zinc wetting process.
Once the surface is debris-free, the steel enters the galvanizing kettle, containing molten zinc at 830 to 850 degrees Fahrenheit. The steel remains in the kettle until it reaches the steel and molten zinc bath temperatures.
A diffusion-reaction occurs when the steel is in direct contact with the molten zinc. The reaction leads to the formation of a metallurgically bonded zinc alloy layer. In some instances, the molten zinc bath is added with lead to encourage bath fluidity.
Its primary purpose is to limit excess zinc on the steel by improving the drainage properties, easy dross recycling, preventing floating trash, and protecting the galvanizing kettle from irregular heat distribution from the galvanizing burners.
Removal of residues from the steel ensures that the metal has a pure zinc coating. Dipping the steel in a chromate quench tank is the final step in the hot-dip galvanization process.
Its purpose is to reduce the steel temperature, prevent a reaction from air exposure, and retains the brightness of the zinc coating during storage.
A quality control team inspects the HDG steel for consistency and fabrication of its component before sending it to a galvanizing machine.
The quality assurance team will check the product for possible factory defects when the galvanizing process is over. They also measure the thickness of the zinc coating.
How Long Does Hot-Dip Galvanizing Last?
The process of hot-dip galvanization is essential in protecting the steel from rusting and corrosion in the harshest environments. It does not require maximum maintenance to last more than fifty years.
However, its corrosion resistance depends on the type of surroundings and elements that may impact its longevity. According to a study, bare steel’s decomposition rate is 1/30 in the same environment.
Five factors contribute to the performance of hot-dip galvanized steel: rainfall, humidity, temperature, air salinity, and sulfur dioxide air concentration.
Dr. Gregory Zhang of Teck Metals Limited developed the accurate global performance data known as the Zinc Coating Life Predictor. Using the TFM chart, the first maintenance for the zinc coating shows that 95 percent of the zinc coating is intact.
It recommends initial maintenance of the material to prolong the longevity of the structure or application.
The governing specification for hot-dipped galvanized material, ASTM A123, says steel products with over ¼ inch thickness should have 3.9 mils of zinc coating. Steel more than ¼ inch thick can last for 72 to 73 years to first maintenance, even if it has exposure to the harshest environments.
Hot-dip galvanizing is a complete immersion process after submerging the steel into the acidic solution, pickling solution, flux, and molten zinc. It ensures the exterior and interior sides have a full zinc coating.
This process ensures that no corrosion could occur even in the steel structures’ hollow, tubular, and fastener threads.
Corrosion and rusting happen in the interior and concave parts, painted but without corrosion protection. The interior of hot-dipped galvanized material receives ample protection as condensation and humidity have higher rates.
Is Hot-Dip Galvanized Rustproof?
Yes, hot-dip galvanized steel is rustproof depending on its exposure to specific environments. Its rust and corrosion resistance depends on the thickness of the zinc coating and the harshness of the environmental conditions.
Generally, bare steel products in the same environment have a corrosion rate of 1/30.