The steel components are first placed in baskets or fixed to specially designed fixtures. If the component has gone through previous procedures that require lubrication, such as rolling, bending, or drilling, a pre-degreasing process might be applied before the material goes through the degreasing/cleaning stage.

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The hot-dip galvanizing technique consists of three main stages: surface preparation, galvanizing, and inspection. The specifications for the process are determined based on ASTM International standard specifications for hot-dip galvanizing of iron and steel products.

Step 1: Surface preparation

Surface preparation is the first and essential stage of hot-dip galvanizing. Without proper surface cleaning, the necessary reactions between the steel surface and zinc will not take place resulting in the discontinuity of the coating and failure of the galvanized product.

The surface preparation process involves two or three main steps depending on whether wet or dry hot-dip galvanizing technique is employed. Wet hot-dip galvanizing involves degreasing and pickling, while dry hot-dip galvanizing includes an additional fluxing step in which the component is transferred to a flux bath after degreasing and pickling.

During the degreasing process, the steel component is submerged in a bath of either hot alkali solution, such as caustic soda at 70-90 oC, mild acid, or biological cleaning solution. This process removes dirt, paint markings, grease, oil and other organic contaminants from the surface of the material. The immersion duration depends in the degree of contamination. However, a typical degreasing duration of 5-10 mins is usually enough.  Furthermore, a slight agitation of the cleaning solution during submersion can increase the efficiency of the process.

Additional contaminants, such as epoxies and welding slag, which cannot be cleaned during this step, should be removed by a mechanical process, e.g. sand blasting. The degreasing step is followed by rinsing the component in water.

The second step of surface preparation involves pickling, in which the material is submerged in a bath of hot (60-70 oC) diluted sulphuric acid solution or ambient hydrochloric acid solution to remove mill scale and rust. The acid concentration is chosen based on the degree of oxide scale present at the surface of steel component; however, a solution of 10-20 Vol.% is normally enough to remove the excess scale and rust. Since there is a good chance of pitting at the component surface as well as an accelerated destruction of the pickling kettle, inhibitors are also required in the bath in order to ensure that the component and the pickling kettle are protected against corrosion.

A mechanical cleaning process, such as abrasive cleaning, or air blasting of sand or grit, can also be used along with, or as an alternative to chemical pickling. This step is also followed by rinsing in water.

It is worth mentioning that an improper pickling step can significantly reduce the quality of the protective coating. In addition, the excess amount of scale and rust, which are not removed during the pickling process, enter the galvanizing bath and increase the amount of zinc ash/dross, hence increasing the consumption of zinc and decreasing the efficiency of the galvanizing process.

If a wet hot-dip galvanizing technique is used, the steel component is transferred to the galvanizing kettle straight after pickling. In the case of dry galvanizing method, the pickled component is submerged in a bath of zinc ammonium chloride solution. This process removes all the residual oxides from the steel surface and allows for a thin protective layer to adhere to the surface in order to prevent the reoccurrence of oxides prior to galvanizing. The fluxing is followed by drying the component at ~ 120 oC.

Step 2: Galvanizing

Once the surface preparation is complete, the clean component is transferred to a galvanizing kettle and immersed into a molten zinc bath. The temperature of the molten zinc bath is usually kept at ~ 450 oC, which is about 30 oC higher than the melting point of zinc. Such high temperature of the molten zinc allows for higher fluidity and better adherence to the steel surface. During the submersion of the component, the molten zinc reacts with iron at the material surface and forms several layers of zinc-iron intermetallic alloys. The outmost layer of the coating normally consists of pure zinc.

After the completion of the galvanization process, the galvanized products are withdrawn from the bath and the excess zinc is removed via draining, and vibration, as well as centrifuge for small pieces, such as nuts and bolts.

The components are then quenched in water. If the cooling is not done fast enough, the mutual diffusion of zinc and iron atoms results in the Kirkendall effect (the motion of the boundary layer due to the difference in the diffusion rate of zinc and iron atoms), which can cause pitting and scaling of the zinc coating.

Step 3: Inspection

Once the galvanized products are cooled down to room temperature, any residual zinc remained on the surface of the product is removed by proper methods, such as filing. It is noteworthy that excess filing can cause damage to the coating. Another issue is removing the traces of excess zinc that remain on the product surface. Any discontinuity of the zinc coating should be mended with the permission of the producer and based on ASTM International standard practice for the repair of damaged and uncoated areas.

The galvanized products should be transported inside moisture-free containers and the components must be packed separately so that air can flow in between the gaps at all times.

Ohio Galvanizing
467 West Fairground St.
Marion, OH
(740) 387-

 

Hot Dip Galvanizing 101

Important Things About Galvanizing You Should Understand...Made Simple

• The Hot Dip Galvanizing Process
• The Thickness and Appearance of Our Coating
• The Expected Outdoor Service Life of Galvanized Coating
• Designing Products for Galvanizing
• Typical types of Products that are Galvanized

THE HOT DIP GALVANIZING PROCESS
At Ohio Galvanizing, the galvanizing process consists of three basic steps: 

• Surface Preparation
• Galvanizing
• Inspection

Surface Preparation
This is the critical initial step. When a coating fails before the end of its expected service life, it is most often due to incorrect or inadequate surface preparation.

In the galvanizing process, zinc simply will not adhere with a steel surface that is not perfectly clean. At Ohio Galvanizing, we cannot take responsibility for properly galvanizing your product unless we correctly clean it first.

The company is the world’s best Hot Dip Galvanizing Machine For Metal Product supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

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Surface preparation for galvanizing typically consists of these three steps:

1. Caustic Cleaning. Your product is dipped in a hot alkali solution to remove organic contaminants such as dirt, paint markings, grease, and oil from the metal surface. It is then dipped in a rinsing tank. Note: epoxies, vinyl, asphalt, or welding slag must be removed before galvanizing by grit blasting, sand blasting, or other mechanical means.
   
   
2. Pickling. Your product is then dipped in a dilute solution of hydrochloric acid.  Called pickling, this step removes scale and rust from the steel surface. It is then dipped in a second rinsing tank.
   
   
3. Fluxing. This is the final surface preparation step in the galvanizing process. Fluxing removes oxides and prevents further oxides from forming on the sur­face of the metal prior to galvanizing and promotes bonding of the zinc to the steel or iron surface. At Ohio Galvanizing your product is dipped or pre-fluxed in an aqueous solution of zinc ammonium chloride. The material is then air dried.

Galvanizing
In this step, your product is completely immersed in a bath consisting of a min­imum of 98% pure molten zinc. The bath chemistry is specified by the American Society for Testing and Materials (ASTM) in Specification B6.

Your material is immersed in the bath long enough to reach bath temperature, or about 850°F (454°C), then withdrawn slowly from the galvanizing bath whereby the excess zinc is removed by draining.

The chemical reactions that result in the formation and structure of the galvanized coating continue after the articles are withdrawn from the bath as long as the product is near the bath temperature. Your product is then cooled in a quench tank or ambi­ent air immediately after withdrawal from the bath.

Inspection
The two properties of the hot dip galvanized coating that are closely scrutinized after galvanizing are coating thickness and coating appearance. A variety of simple physical and laboratory tests may be performed to determine thickness, uniformity, adherence, and appearance.

Products are galvanized according to long-established, well-accepted, and approved standards of the ASTM. These standards cover everything from the minimum required coating thicknesses for various categories of galvanized items to the composition of the zinc metal used in the process.

Testing methods and interpretation of results are covered in the publication, The Inspection of Products Hot Dip Galvanized after Fabrication, published by the American Galvanizing Association (AGA). Because we are a member, this publication is available from Ohio Galvanizing.

According to numerous national and international studies, hot dip galvanizing produces no significant changes in the mechanical properties of the structur­al steels or welds commonly used throughout the world. The galvanized product's underlying steel is chemically and metallurgically equivalent to the uncoated steel.

THE THICKNESS AND APPEARANCE OF OUR COATING
ASTM specifications establish minimum standards for the thickness of galvanized coatings on various categories of items. These minimum standards are routinely exceeded by galvanizers due to the nature of the galvanizing process.

Some factors influencing the thickness and appearance of the galvanized coating Ohio Galvanizing can control-steel surface condition, bath temperature, bath immersion time, bath withdrawal rate, and steel cooling rate. Some factors are inherent in the product, such as the chemical composition of the steel and any cold working of the steel prior to galvanizing.

The chemical composition of the steel being galvanized strongly influences the thickness and appearance of the galvanized coating. For example, silicon, phosphorous, or combinations of these two elements can cause thick, brittle galvanized coatings. The carbon, sulfur, and manganese content of the steel also may have a minor effect on the galvanized coating thickness.

The combination of the elements mentioned above, known as "reactive steel" to the galvanizing industry, tend to accelerate the growth of zinc-iron alloy layers. This may result in a finished galvanized coating consisting entirely of zinc-iron alloy. Instead of a shiny appearance, the galvanized coating will have a dark gray matte finish that provides just as much corrosion protection as a galvanized coating having the common bright appearance.

THE EXPECTED OUTDOOR SERVICE
LIFE OF GALVANIZED COATING

The graph below is a plot of the thickness of the galvanized coating against the expected service life of the galvanized coating under outdoor exposure conditions. Most galvanized applications are 4 mils of thickness minimum per surface.

The expected service life is defined as the life until 5% of the surface is showing iron oxide (rust). At this stage, it is unlikely that the underlying steel or iron has been weakened or the integrity of the structures protected by the galvanized coating otherwise compromised through corrosion.

DESIGNING PRODUCTS FOR GALVANIZING
The best way to ensure the safe, effective, and economical galvanizing of steel products is for the designer, fabricator, and galvanizer to work together before the product is manufactured. Ohio Galvanizing can assist you with the practices that should be followed in designing products for effective and safe galvanizing. These practices are easily applied and in most cases are routine methods used to ensure maximum corrosion protection...and the most cost effective galvanizing process for you.

Most ferrous materials are suitable for galvanizing. These include cast iron, malleable iron, cast steels, and hot and cold rolled steels. 

Sizes, Shapes and Dimensions     
Iron and steel products to be galvanized after fabrication range from small pieces of hardware to large welded steel assemblies. Ohio Galvanizing utilizes kettles that are 30 ½ feet long by 5 feet wide by 7 ½ feet deep.

If your product(s) is too deep or too long to fit into the kettle, it is often possible for us to utilize double-dipping to galvanize products that exceed the dimensions of our kettle.  Should you have questions about a product's galvanizability, contact Ohio Galvanizing today for a no-cost consolation.  

Filling and Vent Holes
Galvanizing requires that cleaning solutions and molten zinc flow into, over, through, and out of fabricated steel products. Designs that promote the flow of zinc are optimal.

Filling and vent holes must be provided to prevent pickling or other cleaning bath fluids from becoming trapped in an article. It is best to avoid narrow gaps between plates, overlapping surfaces, and back-to-back angles and channels. When overlapping or contacting surfaces cannot be avoided, all edges should be completely sealed by welding but provided with a small hole or a short gap in the welding to relieve pressure build-up in overlapping areas that exceed 16 square inches.

For more in-depth information, AGA's publication "The Design of Products to be Hot Dip Galvanized," or their CD "Designing with Hot Dip Galvanized Steel," are available from Ohio Galvanizing. In addition to covering the essential considerations for proper design and providing specific technical specifications and details, illustrations demonstrate both proper and improper design practices. These practices are also described in ASTM A 143, A 384, and A 385.

TYPICAL TYPES OF PRODUCTS THAT ARE GALVANIZED

Amusement rides    
Beams
Bicycle racks
Boat trailers
Box rail
Bridge rail
Bridge substructure
Bridge superstructure
Cat walks
Columns
Conveyor systems
Cooling tower parts
Dock hardware
Dock levelers
Fence posts
Flag poles
Garbage cans
Gas turbine skids
Grating
Handrails Highway guardrail/posts
Ladders
Lattice towers
Metal sculptures
Overhead cranes
Overhead sign supports
Pedestrian bridges
Picnic table frames
Pipe
Plates
Platforms
Rebar
Signal light poles
Stair treads
Sound barriers
Structural steel
Transmission poles
Utility trailers
Water/waste treatment structures
Wheelbarrows
Wire mesh

Galvanizing 101

The important things you should know about galvanizing, made simple.

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