A Simple 3-step Galvanizing Process Protects Steel for ...

Galvanized steel is prized all over the world for its durability and corrosion-resistance. But for as strong as the finished material is, the process is surprisingly simple and straightforward. Galvanizing steel consists of just three steps:

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1. Surface Preparation.
2. Galvanizing.
3. Inspection.

Surface Preparation

Like most things, the quality of a finished galvanized product is directly influenced by the amount of effort put into the preparation. Poor surface prep can cause the galvanization to fail, but galvanization has built-in quality control of sorts. The zinc used in galvanizing won't react with an unclean steel surface. This makes it easy to see poorly coated areas as soon as the piece is pulled out of the galvanization tank. Unclean areas remain uncoated, allowing technicians to correct the problem right away.

Surface preparation consists of three steps:

1. Degreasing/Caustic Cleaning. An acidic 'bath' removes organic contaminants from the steel surface. Dirt, paint, grease, and oil can be removed this way. Materials that cannot be removed via the bath include: epoxies, vinyls, asphalt, and slag. These contaminants are removed by degreasing, grit-blasting, sand-blasting, or other mechanical means.
2. Picking. A dilute solution of acid is used to remove mill scale and rust from the steel. Instead of or in place of the acid, an abrasive cleaner or air blasted sand could be used.
3. Fluxing. Fluxing is a zinc ammonium chloride solution that removes remaining rust particles and adds a protective layer to the steel, helping to prevent any further oxides from forming on the surface before it can be galvanized.

Galvanizing

Once the steel is ready for galvanizing, it is immersed in molten zinc. The chemistry of the bath must meet certain standards, requiring at least 98% pure zinc and a temperature of 815-850 F. During the bath, the zinc reacts and bonds with iron in the steel to form an extremely hard alloy layer that strengthens and protects the steel. Excess zinc is removed from the galvanized piece by draining, vibrating and/or centrifuge but the metallic reaction continues as long as the steel remains near bath temperature. Galvanized pieces are cooled by immersing them in a passive solution, in water, or by being left in open air.

Inspection

The last step, the inspection process, is also the quickest. Coating thickness and surface condition are both closely scrutinized for integrity. Several tests are used to determine coating thickness, uniformity, adherence, and appearance. Results are compared to long-established and accepted standards set by the ASTM, the International Standards Organization (ISO), the Canadian Standards Association (CSA), and the American Association of State Highway and Transportation Officials (AASHTO).

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Galvanizing steel is the process of submerging steel in a bath of molten zinc which creates a protective layer that prevents future corrosion. Rust is potentially dangerous and can cause metal parts to become stiff and weaken the integrity of the metal and thus the structure.

Repairing deteriorated steel bridges after decades of use is time-consuming, expensive, and difficult to keep up with.

Hot-dip galvanization for steel not only protects metals from corrosion but also improves maintenance-free longevity. Thus, in the long run, businesses are saving money by keeping their steel galvanized and protected. In addition, the galvanization process is environmentally-friendly and resistant to conditions.

Galvanizing Steel: Three Main Steps

Step 1: Surface Preparation

The first and most crucial step is preparing the surface of the material for the coating process. Zinc will not react to unclean metal, so a successful surface prep is a must. Preparing the steel begins by removing all the dirt, grease, and other contaminants from the metal surface. This method is completed by degreasing the metal in a hot alkali solution or other acidic baths.

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Pickling

Then the steel material is pickled through a diluted acidic solution for the removal of mill scales and any rust buildup. Depending on the company, this process is either replaced with or accompanied by abrasive cleanings, such as air blasting sand or metallic shot.

Fluxing

The final process for surface prep is fluxing. This procedure eliminates oxides while simultaneously creating a protective coating for the prevention of the formation of any additional oxides.

Step 2: Galvanizing Steel

After the surface of the metal has been prepped, it's submerged in a large bath of 98% pure molten zinc at 830° F. The zinc reacts to the clean metal and generates a protective layer.

When the steel has reached the same temperature as the zinc bath, it is then withdrawn, drained, and cooled off before moving to the final step. Depending on the size and thickness of the material, this process could take up to 10 minutes or less.

Post-Treatment

It's possible that a post-treatment coating will be applied to the metal. This process is called quenching. The steel is dipped in mostly water and some chemicals to create a protective layer. Frequently, finishing steps will include grinding off zinc drips for a smooth finish.

Step 3: Inspection

During the inspection phase, the coating thickness, weight, and appearance is meticulously analyzed and compared against the ASTM Standards. There are two different procedures that can be performed to measure the coating's level of thickness: magnetic gauges or optical microscopy. Magnetic thickness gauge is the most simple and non-destructive thus making it the most popular method.

Analyzing Coating Weight

The second element that is analyzed is the material's coating weight. The coating weight refers to the volume of zinc coating on the metal surface. Like the coating thickness inspection process, coating weight has two different methods for measuring the weight. Both the weigh-galvanize-weigh and the weigh-strip-weigh procedure should only be applied to single specimen inspection. These methods have their pros and cons so it's important to choose the procedure that works best for you.

Final Galvanizing Steel Step

The last component that is inspected is the steel's appearance and coating finish. All sections of the material are inspected to ensure that it meets the proper standards. As mentioned before, zinc will not react to unclean metal. Thus, the steel's appearance and finish provides useful information such as revealing any errors that occurred during the galvanization process.

U.S. Bridge is a Certified Steel Fabricator

U.S. Bridge has been serving communities for more than 80 years, building steel bridges that withstand time. We're a steel fabricator by the AISC for Certified Bridge Fabrication ' Advanced (Major) and continue to be a leader in the American steel industry.

Still unsure if galvanizing steel is the right step for your bridge construction? Check our other blogs on the galvanization process and construction services for more information. Contact us today and let's talk about how we can make your bridge project a long-lasting success.

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