Once the metal is cleaned, treated, and painted, the strip is rewound into a coil size prescribed by the customer. From there, the coil is removed from the line and packaged for shipment or additional processing.
After the primer is applied and cured, then the metal strip enters the finish coat station where a topcoat is applied. Topcoats provide color, corrosion resistance, durability, flexibility and any other required physical properties. Like primers, the topcoat is cured using thermal cure ovens.
Oven
Coil coating ovens can range from 130 feet to 160 feet and will cure the coatings in 13 to 20 seconds.
During this stage, the strip enters the prime coat station whereby a primer is applied to the clean and treated metal. After the primer is applied, the metal strip travels through a thermal oven for curing. Primers are used to aid in paint adhesion, improve corrosion performance and enhance aesthetic and functional attributes of the topcoat.
S Wrap Coater
The S wrap coater design allows for primers and paints to be applied to the top and back side of the metal strip simultaneously in one continuous pass.
The cleaning and pretreating section of the coil coating process focuses on preparing the metal for painting. During the cleaning stage, dirt, debris, and oils are removed from the metal strip. From there, the metal enters the pretreatment section and/or a chemical coater whereby chemicals are applied to facilitate paint adhesion and enhance corrosion resistance.
Dried-In-Place
In this stage a chemical that provides enhanced corrosion performance is applied. This treatment can be chrome free if required.
The accumulator is a structure that adjusts up and down to store material, which makes continuous operation of the coil coating process possible. This accumulation will continue to feed the coil coating processes while the entry end has stopped for the stitching process. As much as 750 feet of metal can be collected.
Hot-dip galvanizing is essentially a process that coats steel with a protective layer of zinc alloy to protect it from corrosion. Not only is it a relatively simple and cost-effective process, but the resulting benefits will last for decades.
The Process
Preparation
First and foremost, the steel itself must be thoroughly inspected to ensure that drainage and venting requirements are met. It wouldn’t do to have a flawed product at the outset.
Cleaning the Steel Prior to Hot-Dip Galvanizing
Before the galvanizing process can begin, the steel must be cleaned as thoroughly as possible. This isn’t just for appearances. The zinc coating simply can’t bond with a steel surface if foreign materials are present. There are several steps to cleaning:
The steel is submerged into a caustic tank at 180° F to remove any paint residue. It is then rinsed in a freshwater tank at ambient temperature to remove the caustic material.
The steel is pickled by being dipped into a tank of heated sulfuric acid at 140°-145° F to remove any mill scale. Afterward, it is again rinsed in a freshwater tank.
Lastly, the steel is washed in a flux tank containing a mixture of zinc chloride and ammonium chloride at 140°-170° F to provide protection against oxidation before galvanizing.
After these steps have been completed, the steel is ready to receive its new protective coating.
The Hot-Dip Galvanizing Kettle
The freshly cleaned steel is submerged into a kettle of molten zinc at 830°-850° F. The zinc then bonds with the steel in a diffusion reaction, creating a brand-new layer of metallurgically bonded zinc alloy. Before the newly coated steel can be removed from the tank, the surface of the molten zinc must be swept to remove any residue to ensure that the steel is exiting through pure zinc.
Cooling Down
As soon as it’s removed from the galvanizing kettle, the steel is quenched in aqueous sodium dichromate at ambient temperature to help cool it down before the final cleaning process. This also helps the galvanized steel retain its shiny surface for a longer period of time.
Final Cleaning
Once galvanized, the coated surface of the steel is cleaned of any dirt, drip marks or excess zinc to ensure the final product is ready for testing.
Testing Hot-Dip Galvanizing Results
The steel is then moved outside and placed on poles for millage tests. These tests verify that the process was successful and that the zinc coating is the proper thickness. Different applications for the steel can call for different coating thicknesses.
Other Key Considerations for Hot-Dip Galvanizing
Protection Against Rust via Hot-Dip Galvanizing
The main purpose of hot-dip galvanizing steel is to protect it against rust, the natural predator of steel. One of the main benefits of the zinc alloy coating, other than generally being very resistant to abrasion, is that zinc and rust are inherent enemies. Rust simply can’t eat through zinc, so it follows that it can’t eat the steel beneath the zinc. either.
A Long History of Hot-Dip Galvanizing
Galvanizing metal with zinc is a process that we’ve been using for over 150 years. The first known use was in 17th-century Indian armor, and the process was industrialized to protect the iron in British naval ships. Since then, it’s caught on in almost every application of steel and iron throughout the developed world.
Aesthetics
The crystallites of the zinc coating, or the grain, are a visible feature of galvanized steel. This feature is called “spangle” and can be varied and adjusted to taste by altering the number of particles added in heterogeneous nucleation, as well as the cooling rate during the hot-dip galvanizing process. If desired, the spangle can be made to appear uniform, and the grain will be too small to be seen with the naked eye.
Common Uses for Hot-Dip Galvanizing
Hot-dip galvanizing of steel is most often used for construction purposes. The process provides protection for building frameworks, walkways, staircases and more in major cities all around the world. Most of us have almost assuredly come in contact with hot-dip galvanized steel without even realizing it.
Galvanized piping is also used heavily in irrigation and other outdoor water storage and water transportation solutions due to its superior ability to withstand the corrosive forces of the elements and resistance to rust.
The Cost-Effectiveness of Hot-Dip Galvanizing
Hot-dip galvanizing is popular amongst farmers, engineers, architects and more for its protective qualities, but it also provides other more economic benefits.
Because of its longevity and toughness, hot-dip galvanized steel requires less maintenance and can span decades before needing to be repaired or replaced. The resistance to abrasion also makes for smoother surfaces that are more easily cleaned and painted.
Hot-dip galvanized steel is well suited for use in a variety of environments and fabrications, and sometimes is placed in contact with different metals including, among others, stainless steel, aluminum, copper and weathering steel.
Metals near each other in the galvanic series have little effect on each other. Generally, as the separation between metals in the series increases, the corroding effect on the metal higher in the series increases as well.
Relative surface areas of contacting dissimilar metals is also relevant in determining which metal exhibits accelerated corrosion. It is undesirable to have a large cathode surface in contact with a relatively small anode surface.
Galvanic corrosion occurs when two different metals are in contact in a corrosive environment: one of the metals experiences an accelerated corrosion rate. The contacting metals form a bimetallic couple because of their different affinities (or attraction) for electrons. These different affinities create an electrical potential between the two metals, allowing current to flow.
The metal higher in the galvanic series of metals, the anode, provides protection for the metal lower in the series, the cathode.
As can be seen from the galvanic series, zinc protects the lower-order steel.
With respect to contacting surface areas of the two metals, although the corrosion current that flows between the cathode and anode is independent of area, the rate of penetration at the anode does depends on current density. Thus, a large anode area in contact with a relatively small cathode area is generally not problematic. Regardless, environmental conditions remain large determinants of corrosion rates.
Design considerations courtesy of American Galvanizers Association. The AGA also has the publication, The Design of Products to be Hot-Dip Galvanized After Fabrication, available for download.
Most ferrous materials are suitable for hot-dip galvanizing. Cast iron, malleable iron, cast steels, hot-rolled steel, and cold-rolled steels all can be protected from corrosion with zinc applied in the hot dip galvanizing process. Weathering steel and certain stainless steels(300 series) also can be galvanized.
It is important to understand that the chemical composition of the material being galvanized can affect coating characteristics. However, corrosion protection typically is not affected by these coating characteristic differences.
Fabrications with varying steel chemistries can form the galvanized coatings iron-zinc intermetallic layers at different rates, resulting in a non-uniform coating appearance. Again, corrosion protection is not affected by non-uniform appearances.
Variations in color and texture do not affect the corrosion protection provided by the galvanized coating. It is important to understand that upon normal weathering in the environment, the entire coating commonly evolves to a uniform appearance.
While the overwhelming determinant of coating thickness is steel chemistry, varying surface conditions and/or different fabrication methods also can affect coating appearance. This is because different parameters for pickling (immersion time, solution concentrations, temperatures) and galvanizing (bath temperature, immersion time) are required for:
Excessively rusted surfaces
Machined surfaces
Malleable iron
Hot-rolled steel
Cold-rolled steel
Castings, especially with sand inclusions
Pitted surfaces
Steel with relatively high levels of carbon, phosphorus, manganese, and/or silicon
Many coatings such as paint and lacquer cannot be removed from the steel with the chemical cleaning process used in the galvanizing facility. As perfectly cleaned steel is required for the metallurgical reaction to occur in the galvanizing kettle, these contaminants need to be removed mechanically from the surface prior to sending the fabrication to the galvanizer.
To allow for optimum coating development when combining dissimilar steels in a single fabrication, consider these guidelines:
Galvanize dissimilar steels separately and assemble after galvanizing
Avoid the use of old steel with new steel, or combining castings with rolled steel in the same assembly
Avoid using excessively rusted, pitted, or forged steels in combination with new or machined surfaces; if this is unavoidable, a thorough abrasive blast-cleaning of the assembly (normally before any machining is done) will allow for a more uniform galvanized coating.
Stainless steels in the 300 series can be galvanized because they contain nickel, which is necessary to initiate reaction between the steel and the zinc. Stainless steels in the 400 series do not contain nickel and cannot be galvanized.
Weathering steel also can be galvanized, although it typically contains relatively high amounts of silicon and will develop thicker coatings that are usually matte gray in finish. The corrosion protection provided by galvanizing is unaffected by coating appearance.
Design considerations courtesy of American Galvanizers Association. The AGA also has the publication, The Design of Products to be Hot-Dip Galvanized After Fabrication, available for download.
Climbworks Design, Holmes Solutions and Steelworks Supply worked together to build this unique project. Charlie Deathridge of Climbworks Design said he and the owners knew from the beginning they wanted to utilize hot-dip galvanizing to limit the challenges and costs associated with maintaining such a structure.
To help make that vision a reality, the team turned to AZZ Galvanizing – Nashville, which hot-dip galvanized pipe, rolled pipe, beams, a spiral staircase and strut arms for the attraction.
This roller coaster recently won the American Galvanizers Association (AGA) Artistic award for its innovative use of hot-dip galvanized steel.
About
The Flying Ox is both beautiful in design and structure. The rollercoaster was designed to be a piece of art that would attract families to the park. The design was challenging because of its cable-to-rail zip line features. The artists combined hot-dip galvanizing with paint to create a finish that could endure corrosion for years to come.
Why the Designer Chose Hot-Dip Galvanizing
The designers chose hot-dip galvanizing because of its sustainability. The rollercoaster was made to withstand hot summers and cold winters, which erode materials. Hot-dip galvanized steel addresses the economic and environmental factors while maintaining an artistic finish.
Project Recap
The success of this rollercoaster project and its high visibility at the park has made a positive impact in the galvanized steel market. The corrosion protection and aesthetics achieved from combining hot-dip galvanizing with paint have enhanced this project so families can enjoy the Flying Ox for years to come.
AZZ focuses on creating a stronger, safer world, evidenced by its focus on protecting infrastructure from corrosion. This project was an innovative way to show the possibilities of galvanizing. Learn more by exploring AZZ’s hot-dip galvanizing capabilities
Usually, these trailers are painted, which often results in chipping and premature wear. In the case of one particular trailer to be used along the coast, however, which can be especially harsh on painted steel, a different and more effective method was chosen.
The trailer was manufactured using hot-dip galvanizing, providing a durable and long-lasting coating to give the trailer an extended life with added durability, even in a harsh coastal climate.
This innovative take on large livestock trailers recently won the American Galvanizers Association (AGA) Original Equipment Manufacturing award for its unique approach to ensure a more durable and longer lasting coating.
AZZ Galvanizing – Houston West took on this critical project, delivering a stunning end result that the owner of the trailer was extremely pleased with.
About
In order to ensure quality work, the manufacturer and AZZ galvanizing representative worked together to prep the design. The trailer required an enormous amount of drain and vent holes, which took a lot of fine attention to detail.
Why the Designer Chose Hot-Dip Galvanizing
The designers chose hot-dip galvanizing because of its sustainability and ability to meet the requirement of a 50-year life with a low maintenance cost.
The farms and road can face harsh natural elements which are often highly corrosive. Hot-dip galvanized steel addresses the economic and environmental factors that will make the trailer in great condition for years to come.
Project Recap
The livestock industry is constantly growing to meet new demands. Livestock trailers are an essential product, and this one in particular stands out among the rest.
The trailer’s unique hot-dip galvanized steel coating will protect it for many years past other trailers on the market.
AZZ focuses on creating a stronger, safer world, evidenced by their focus on protecting infrastructure from corrosion. This project was an innovative way to show the possibilities of galvanizing. Learn more about AZZ today by exploring our hot dip galvanizing capabilities.
