Everything You Need to Know About Galvanized Steel

Introduction

Galvanized steel is all around us, and it’s a necessary part of the environment. Steel is immersed in molten zinc during hot-dip galvanizing to obtain a metallurgically bonded coating. As well as corrosion protection, the process comes with several other benefits that can often be overlooked, forgotten, or misunderstood.

Why should steel be galvanized?

To prevent premature rust and corrosion on steel, Galvanization is used to extend its life. As a result of its special properties, galvanized steel offers lower maintenance and repair costs for advocates of using it, such as when constructing or repairing steel structures.

Painting and plastic coating are alternative methods, but both have their drawbacks. Once the steel is damaged, the coating fades away, which makes it unreliable.

What is galvanized steel used for?

Galvanized steel has been used in various environments for over 150 years, proving its performance on countless occasions.

Galvanized steel can be found in many different industries, the energy industry, agriculture, water and waste management, and sports and leisure. Sculptures, bridges, gantries, signal gantries, gates, and even buildings can benefit from this material. Steel should be galvanized whenever corrosion is a risk.

Galvanizing process stages

• The first step is cleaning the steel in a greasing removal solution. 
• After the cleaning process, a vat of diluted hot sulfuric acid is lowered into a vat of cleaned steel.
• The next step is steel fusing into a solution of zinc-ammonium chloride.
• Next, hot zinc is poured into a vat to galvanize the steel.
• A final inspection is performed to ensure that the steel is consistent and well coated.

What is the life expectancy of galvanized steel?

Usually, in rural environments, the life expectancy of galvanized steel is around 50 years, while in urban ones, it’s 20-25 years. And in an industrial environment, galvanized steel can last up to 72-73 years.

Does Galvanized Steel freeze?

Galvanized steel can resist freezing and cracking and is particularly suited to withstand cold and freezing temperatures.

Detailing for hot-dip galvanizing

There are three main stages to hot-dip galvanizing: surface preparation, galvanizing, and inspection. Standards for hot-dip galvanizing iron and steel are based on ASTM International standards.

A steel component is first placed in a basket or fixed to a fixture. Pre-degreasing may be used before degreasing/cleaning if the part has gone through previous lubricating steps, for example, bending, rolling, or drilling.

Step 1: Preparing the surface

Hot-dip galvanizing begins with surface preparation. If the steel surface is not properly cleaned, the zinc will not react with the steel surface, failing the coating.

There are two or three steps in the surface preparation process if you are using a wet or dry hot-dip galvanizing technique. During wet hot-dip galvanizing, components are degreased and pickled before being transferred to a flux bath. In dry hot-dip galvanizing, the components are transferred to an additional flux bath after degreasing and pickling.

Steel components are either submerged in a bath of caustic soda or mild acid or a biological cleaning solution during the degreasing process. A variety of organic contaminants, including grease, oil, dirt, and paint markings, are removed during this process. This depends on the number of contaminants.

Degreasing should take no more than five or ten minutes. However, The cleaning solution can also be slightly agitated during submersion to increase efficiency.

Adding sandblasting to the process will remove additional contaminants, like epoxy and welding slag, that cannot be cleaned during this process. A water rinse follows the degreasing step.

The second step of surface preparation includes pickling, which is submerging the material in an acid solution containing 60-70° C hot dilute sulphuric acid or ambient hydrochloric acid to remove mill scale and corrosion from the surface. Generally, an explanation of 10-20 Vol.% acid is sufficient to remove excess scale and rust from steel components depending on the oxide scale present.

To ensure that the components and pickling kettle are protected against corrosion, inhibitors are also needed in the bath as there is a good chance of pitting at the surface of the component and accelerated destruction.

Performing the pickling process incorrectly can cause a significant reduction in the protective coating quality. The scales and rust from the pickling bath enter the galvanizing bath, causing the galvanizing process to be less efficient and waste more zinc.

Following pickling, the steel component is immediately placed in a galvanizing kettle if wet hot-dip galvanizing is used. A zinc ammonium chloride solution is immersed in a pool of pickled components in the dry galvanizing method.

Before galvanizing, a thin protective layer is applied to the steel surface to prevent the recurrence of oxides. This process removes all the oxides from the steel surface. The component is then dried at 120°C after fluxing.

Step 2: Galvanizing

A galvanizing kettle is used to immerse the cleaned component in a molten zinc bath after completing the surface preparation. Zinc is usually melted at about 450 degrees Celsius, about 30 degrees higher than its melting point. Due to the high temperature of the zinc molten, it is more fluid and adheres better to steel. 

Submerged in molten zinc, multiple intermetallic alloys are formed at the surface of the material when the zinc reacts with the iron. Normally, pure zinc is the outermost layer.

After the process is complete, the zinc is removed from sthe galvanized products by draining, vibrating, and centrifuging any small pieces, such as nuts and bolts, from the bath.

Water is then used to quench the components. It has been demonstrated that if the zinc coating is not cooled fast enough, the Kirkendall effect (the motion created by the difference in diffusion rates between zinc atoms and iron atoms) occurs, resulting in pitting and scaling of the zinc coating.

Step 3: Final Inspection

If any remaining zinc remains on the surface of the galvanized products after cooling to room temperature, it can be removed by using proper methods, such as filing. However, excessive filing will damage the coating. There is also the issue of eliminating excess zinc that remains on the surface of the product. 

If the zinc coating is damaged or uncoated, it should be repaired with the manufacturer’s permission.

Transporting galvanized goods in moist environments requires packing the components separately and ensuring that air can pass through gaps between them at all times. 

References:

1. https://galvanizeit.org/inspection-course/galvanizing-process/galvanizing
2. https://huntergal.com.au/galvanizing/what-is-galvanizing/
3. https://millenniumgalvanizing.com/our-process
4. https://fercon.ee/en/sandblasting

If you read something you liked on our website, or we helped you with an issue, consider buying us a coffee!