Hot-Dip Galvanizing: Purpose, Process, and Uses

The purpose of hot-dip galvanizing is to protect the material from corrosion by creating a layer of zinc that acts as a protective coating. The process of hot dipping is to submerge the metal into a bath of molten zinc, which metallurgically bonds itself to the material (usually steel). This then protects the material against external elements such as: humidity, moisture, sodium chloride, and acid. The hot-dipped galvanized steel can be used across a wide range of industries, for things such as: roofing, structural beams, automotive bodies, rebar in cement, metal fences, and electrical pylons.

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This article will discuss hot-dip galvanizing, how it works, its purpose, uses, advantages, and disadvantages.

What Is Hot-Dip Galvanizing?

The hot-dip process creates steel with a thin protective layer of zinc by dipping the steel into hot molten zinc. During the dipping, the zinc reacts with the steel to make a zinc-iron layer which can protect against corrosion. Hot-dip galvanizing is also referred to as galvanization and galvanizing. 

What Is the Purpose of Hot-Dip Galvanizing?

The purpose of hot-dip galvanizing is to form a protective iron-zinc coating on the surface of the steel to protect against corrosion. The coating should cover both internal and external sections of products which are hollow or shelled. The galvanization coating is a sacrificial coating that reacts with the environment to protect the steel beneath it. Hot-dip galvanization also leads to an impact and abrasion-resistant surface when compared to other protective treatments.

What Is the Process of Hot-Dip Galvanizing?

With any parts to be galvanized, it is easier for any welding to be completed before galvanization, as galvanized steel is harder to weld than pre-treated steel. For the galvanization process, the first step is to degrease and clean the surface of the steel of any residues or particles on the surface. This allows the coating to react fully and adhere to the steel. The best method of cleaning is to submerge the part into an alkaline or acid solution and then rinse with cold water. After cleaning, a coating of flux is applied by dipping the part into a 30% zinc ammonium solution at 65&#;80 °C. This removes any remaining oxide from the surface to allow the molten zinc to react with the steel. The part is then dipped in the molten zinc at approximately 450 °C for approximately 4&#;5 minutes depending on the thermal inertia of the part. After the hot dip, the now galvanized steel can be air-cooled or quenched if required.

What Is the Right Temperature for Hot-Dip Galvanizing?

The temperature at which steel should be galvanized is 450 °C. The zinc needs to be at this temperature to allow a metallurgical reaction to occur between the zinc and the steel. Any colder and the wetting of the steel would not be optimal and a significant increase in temperature would begin to soften the steel. Additionally, hotter temperatures could cause the part to warp if there are significant internal stresses already in the steel before hot dipping.  

Are Hot-Dip Galvanizing and Galvanizing Processes the Same?

No, hot-dip galvanizing and galvanizing processes are not the same. While both achieve the same result they both have different processes. The other processes for galvanizing are galvannealing and electro-galvanization. The galvannealing process is the same as the galvanization process, however after the hot dipping the part is heated up to over 1,050 °C and then air cooled. The annealing part of galvannealing encourages further reaction between the steel and zinc. In electro-galvanization, the steel is electroplated with zinc using zinc salt. This leaves behind a much thinner layer of zinc compared to regular galvanization.

What Is the Minimum Steel Thickness for Hot-Dip Galvanizing?

The thickness and silicon content of the steel being coated will dictate the minimum thickness of coating required. The ASTM A123 specification states that hot-dip galvanization should produce a coating between 1.4 to 3.9 mm thick for the best protection. The silicon content of the steel matters because an increase in silicon content makes the zinc more reactive and therefore a thicker coating will be required to maintain the longevity of the part. A thicker steel will also produce a thicker coating. As there is more iron available in a thicker steel part, more zinc will be required to react with the iron. This will produce a thicker protective layer. 

Does Hot-Dip Galvanizing Warp Steel?

Yes, hot-dip galvanization can result in warping. When the steel is hot dipped it heats the steel which can allow the internal stresses in the steel to warp the part. To reduce the chances of this happening the steel can be stress relieved/tempered to reduce the internal stresses of the material.

What Is Hot-Dipped Galvanized Steel Used For?

Hot-dip galvanized steel is used where steel is being used in a corrosive environment. This could be in salt-laden air or marine applications, but could also apply when steel is embedded in soil, concrete, or water. Galvanized steel is a versatile metal that is used in car bodies, bikes, fasteners, structural beams in buildings, rebar in concrete, metal fencing, and electrical pylons.

What Industries Utilize the Use of Hot Dipped Galvanized Steel?

Hot-dipped galvanized steel is used in just about every industry. Hot-dipped galvanized steel is used in the automotive, construction, telecommunications, oil & gas, and aerospace industries, for any application in which steel is in a corrosive environment. 

What Are the Properties of Hot-Dipped Galvanized Steel?

Table 1 below describes the properties and benefits of hot-dipped galvanized steel:

Part of an ongoing series on Galvanized Steel

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.

The steps in the galvanizing process are as follows:

• The steel is cleaned in a degreasing solution
• After being cleaned, the steel is pickled by being lowered into a vat of diluted hot sulfuric acid
• The steel is then fluxed in an aqueous solution (typically zinc-ammonium chloride)
• After the flux, the steel is galvanized through immersion in a vat of molten zinc
• Afterward, the steel is inspected for consistency and a complete coating

What are the advantages of galvanized steel?

Many different industries utilize galvanized steel primarily because it has such a wide array of benefits for industries to take advantage of, including having:

Low initial cost compared to most treated steels. In addition, galvanized steel is immediately ready to use when delivered. It does not require additional preparation of the surface, inspections, painting/coatings, etc. sparing companies more costs on their end.

Longer life. With galvanization, a piece of industrial steel is expected to last more than 50 years in average environments, and can last over 20 years with severe water exposure. There is no maintenance required. The increased durability of the steel&#;s finished product also increases the product&#;s reliability.

The sacrificial anode ensures that any damaged steel is protected by the surrounding zinc coating. It doesn&#;t matter if the steel section is completely exposed; the zinc will still corrode first. The coating will corrode preferentially to the steel, creating a sacrificial protection to the areas that are damaged.

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Rust resistance from the zinc coating. The iron elements in steel are incredibly prone to rusting, but the addition of zinc acts as a protective buffer between the steel and any moisture or oxygen. Galvanized steel is very protective, including sharp corners and recesses that couldn&#;t be protected with other coatings, making it resistant to damage.

Different methods of galvanizing

As stated above, there are several different processes for galvanizing steel.

Hot-Dip Galvanizing

The first and foremost method for galvanization is hot-dip galvanizing. The process is very similar to what the name suggests! In this method, steel or iron is dipped in a molten pool of zinc that maintains a temperature of around 860°F (460 °C). This molten bath begins a metallurgical bond between the zinc and the receiving metal. After the metal is pulled from the bath, it reacts to being exposed to the atmosphere, and the pure zinc mixes with oxygen to form zinc oxide. The zinc-oxide further reacts to carbon dioxide and forms zinc carbonate, which makes up the final protective coating on the material. The tell-tale sign of a hot-dipped galvanized material is the presence of a crystalline-like pattern on the surface, sometimes referred to as &#;spangle.&#;

The hot-dipped galvanizing method is an economical choice that can be quickly executed on both simple and complex shapes. 

The new coated material can be worked and machined in a similar fashion to uncoated materials. Galvanized steel can be used in high-temperature applications up to 392 °F, but use in temperatures exceeding that level will cause the zinc-carbonate layer to peel off. 

Galvannealing

Galvannealing is the outcome of combining the annealing and hot-dip galvanizing processes in order to produce a specialized coating on steel. The process of galvanization is performed via hot-dipping and instantaneous annealing, which produces a matte gray finish.

Galvannealed steel is a zinc-iron alloy product, where the base metal is coated by the hot-dip process, then heated to induce alloying between the molten zinc coating and the steel. The resulting finish is a dull matte surface. Galvannealed steel is conducive to welding and the surface is excellent for paint adhesion.

Pre-galvanizing

Also similar to the hot-dip galvanizing method, but performed at the very first stage of production. Pre-galvanizing is a process that involves rolling the sheet metal through a cleaning agent to quickly prime material for galvanizing. Then, the metal is passed through a pool of molten liquid zinc and is immediately recoiled. The primary advantage of this method is that coils of steel sheet can be rapidly galvanized on a large scale with a more unified coating than the traditional hot-dipped method.

Electrogalvanizing

The most unique of these outlined methods, electrogalvanizing does not involve dipping the material in a molten vat of zinc. Instead, an electric current is introduced to an electrolyte solution that is applied to the steel, which reduces positively charged zinc ions to zinc metal &#; which is then deposited on the positively charged steel. Like pre-galvanizing, this method is typically done at the first stage of production.

Galvanizing advanced high-strength steel

For more information about the galvanization process for advanced high-strength steel, as well as our partner National Galvanizing, please click here.

What industries utilize galvanized steel?

Galvanized metals are used everywhere!

The bodies of cars and many bicycles are made from galvanized metals. Some drinking water pipes are still made from galvanized steel. Cool rolled sheet metal is also frequently galvanized. Nuts, bolts, tools, and wires of all kinds are now galvanized because it is a cheap process, and helps boost the metal&#;s lifespan!

Galvanized steel, in particular, is often what is used in modern &#;steel frame&#; buildings. Galvanized steel is also used to create structures like balconies, verandahs, staircases, ladders, walkways, and more. Galvanized metal is the ideal choice if your project will live outside after it&#;s done. Fences, roofs, outdoor walkways, these are all great choices for galvanized metal!

• Wind & solar industries &#; Solar projects must have a continuous workflow once installed and any repairs/maintenance results in disruption to service (a.k.a. revenue loss). This means that hot-dipped galvanized steels are popular in solar projects for their corrosion protection. It is also popular for its &#;environmental&#; friendliness because it does not produce emissions and ensures decades without maintenance. For example, it is often utilized in the agriculture industry because the equipment is susceptible to being easily corroded, creating a demand for tougher, more resilient equipment. Hot-dip galvanized steel provides corrosion protection that can often last for decades, even when exposed to the harsh environment of farming.

• Automotive industry &#; Though only used on luxury models up until the s, the use of zinc-coated bodies for automobiles is now the norm in auto manufacturing. The &#;body-in-white&#; of a car makes up about 80% of the body, all using galvanized steel. The rust -resistance of galvanized steel is also a good marketing tool for the automotive industry because it can provide &#;anti-rust warranties&#; to customers.
• Construction industry &#; Whether for residential or commercial, the durability of galvanized steel has made it popular for over a century in the construction industry. It is also selected for construction because of its aesthetics; The &#;shine&#; that galvanized steel provides gives it a contemporary feel and is popular in modern architectural designs. Also, it isn&#;t just used for large structural pieces but things like fencing, gutters, rails, tubing, poles, and much more.
• Telecommunication industry &#; lines are not an easy maintenance job, they are tall and often difficult to reach. Hot-galvanized steel can be used on wiring and equipment boxes which decreases the risk of damage and need for maintenance at all.

Well, there you have it! With our partner Heidtman Steel, National Galvanizing  runs a 245,000-square-foot facility, featuring pickling, galvanizing, galvannealing, and slitting, all in one location, convenient to major markets. This combination of capabilities provides our customers with unparalleled responsiveness to meet the ever-changing demands of today&#;s market.

About National Material L.P. &#; With more than 3,000 employees from a multinational portfolio of companies, NMLP provides engineered metal products which include aluminum extrusion and stainless steel rolled product companies to automotive, aerospace, construction, defense, electrical, and industrial markets. Request a Quote Online or give us a call (U.S.) 847-806-

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