Hot-dip galvanization

Process of coating iron or steel with molten zinc

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Galvanised hand rail Crystalline surface of a hot-dip galvanized handrail, known as "spangle" Protective effect: completely rusted letter box mounted to a hot-dip galvanized wall

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 450 °C (842 °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").[1]

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), according to the American Galvanizers Association. The use of galvanized steel at temperatures above this will result in peeling of the zinc at the inter-metallic layer[citation needed]. Electrogalvanized sheet steel is often used in automotive manufacturing to enhance the corrosion performance of exterior body panels; this is, however, a completely different process which tends to achieve lower coating thicknesses of zinc.

Like other corrosion protection systems, galvanizing protects steel by acting as a barrier between steel and the atmosphere. However, zinc is a more electropositive (active) metal in comparison to steel. This is a unique characteristic for galvanizing, which means that when a galvanized coating is damaged and steel is exposed to the atmosphere, zinc can continue to protect steel through galvanic corrosion (often within an annulus of 5 mm, above which electron transfer rate decreases).

Process

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The process of hot-dip galvanizing results in a metallurgical bond between zinc and steel, with a series of distinct iron-zinc alloys. The resulting coated steel can be used in much the same way as uncoated.

A typical hot-dip galvanizing line operates as follows:[2]

• Steel is cleaned using a caustic solution. This removes oil/grease, dirt, and paint.
• The caustic cleaning solution is rinsed off.
• The steel is pickled in an acidic solution to remove mill scale.
• The pickling solution is rinsed off.
• A flux, often zinc ammonium chloride is applied to the steel to inhibit oxidation of the cleaned surface upon exposure to air. The flux is allowed to dry on the steel and aids in the process of the liquid zinc wetting and adhering to the steel.
• The steel is dipped into the molten zinc bath and held there until the temperature of the steel equilibrates with that of the bath.
• The steel is cooled in a quench tank to reduce its temperature and inhibit undesirable reactions of the newly formed coating with the atmosphere.

Lead is often added to the molten zinc bath to improve the fluidity of the bath (thus limiting excess zinc on the dipped product by improved drainage properties), help prevent floating dross, make dross recycling easier and protect the kettle from uneven heat distribution from the burners. Environmental regulations in the United States disapprove of lead in the kettle bath. Lead is either added to primary Z1 grade zinc or already contained in used secondary zinc. A third, declining method is to use low Z5 grade zinc.[3]

Steel strip can be hot-dip galvanized in a continuous line. Hot-dip galvanized steel strip (also sometimes loosely referred to as galvanized iron) is extensively used for applications requiring the strength of steel combined with the resistance to corrosion of zinc, such as roofing and walling, safety barriers, handrails, consumer appliances and automotive body parts. One common use is in metal pails. Galvanised steel is also used in most heating and cooling duct systems in buildings

Individual metal articles, such as steel girders or wrought iron gates, can be hot-dip galvanized by a process called batch galvanizing. Other modern techniques have largely replaced hot-dip for these sorts of roles. This includes electrogalvanizing, which deposits the layer of zinc from an aqueous electrolyte by electroplating, forming a thinner and much stronger bond.

History

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In , French chemist Paul Jacques Malouin described a method of coating iron by dipping it in molten zinc in a presentation to the French Royal Academy.

In , Luigi Galvani (Italy), for whom galvanizing was named, discovered the electrochemical process that takes place between metals during an experiment with frog legs.

In , Alessandro Volta furthered the research on galvanizing when he discovered the electro-potential between two metals, creating a corrosion cell.

In , French chemist Stanislas Sorel obtained a patent for a method of coating iron with zinc, after first cleaning it with 9% sulfuric acid (H2SO4) and fluxing it with ammonium chloride (NH4Cl).

Specification

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A hot-dip galvanized coating is relatively easier and cheaper to specify than an organic paint coating of equivalent corrosion protection performance. The British, European and International standard for hot-dip galvanizing is BS EN ISO , which specifies a minimum coating thickness to be applied to steel in relation to the steels section thickness e.g. a steel fabrication with a section size thicker than 6 mm shall have a minimum galvanized coating thickness of 85 μm.

Further performance and design information for galvanizing can be found in BS EN ISO -1 and BS EN ISO -2. The durability performance of a galvanized coating depends solely on the corrosion rate of the environment in which it is placed. Corrosion rates for different environments can be found in BS EN ISO -1, where typical corrosion rates are given, along with a description of the environment in which the steel would be used.

See also

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References

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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.

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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