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. Let’s take a look at the galvanizing process from start to finish.
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.
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.
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.
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.