What is Galvanization?
Galvanization or galvanizing is the process of applying a protective layer of zinc to steel or iron to prevent rust. The most common method is hot-dip galvanizing, in which the parts are immersed in a bath of molten hot zinc.
Galvanization is one of the most widely used methods of protecting metal from corrosion. A thinner zinc layer is applied to a thicker base metal to shield it from the environment.
The next time you’re in your car, check out the street signs and lampposts you pass by. Many of them are dumb, silver in color. This “silver” is actually the zinc coating.
Proponents of galvanized steel, which can be used to build or repair steel structures, for example, benefit from lower maintenance and repair costs due to its special properties.
Without protection, steel will rust over time due to atmospheric conditions. The degree of rust depends on the environment in which the product is located. Rust is an iron oxide (usually a red oxide) that is created by the reduction and oxidation reaction of iron and oxygen in the presence of water or humidity.
There are protection methods such as painting or plastic coating, but these methods have disadvantages. If damaged, parts of the steel rust, and the protective layer falls off, making these types of protection not permanent and unreliable.
One of the better ways to protect steel materials from corrosion is hot-dip galvanizing. In hot-dip galvanizing, all surfaces of the material are dipped in molten zinc and completely coated with it. The corrosion of zinc is very slow, which gives it a longer lifespan while protecting the base metal.
How Does Galvanization Protect the Base Metal Underneath?
- The zinc coating prevents corrosive substances such as acid (e.g. acid rain) from reaching the metal underneath.
- Galvanization protects against rust. If the coating is scratched, zinc is still more easily “sacrificed” its anode than the base metal, which protects it from rust.
- The zinc protects the metal by corroding faster than the base metal. Sometimes a special ingredient called “chromate” is added to the galvanized metal, which makes it corrode even faster. It might sound like this isn’t what you want, but keep in mind that the zinc will corrode from the metal, which will protect the metal underneath from rust.
Galvanizing a metal only gives it anti-corrosion properties. Without the protective zinc coating, the metal would remain exposed to the elements and potentially oxidize and corrode much faster. Galvanized steel is an inexpensive alternative to materials like austenitic stainless steel or aluminum to prevent corrosion.
How Does Galvanization Work?
Galvanizing can protect metal in a number of ways. First, a protective coating is created that protects the metal from the environment.
The zinc layer prevents water and moisture, as well as other elements in the air, from attacking the steel underneath. Should the zinc coating be scratched deep enough, the metal would be exposed and susceptible to corrosion.
Galvanizing can also protect metal through a process known as “galvanic corrosion”. Galvanic corrosion occurs when two metals of different electrochemical compositions are brought into contact with one another with an existing electrolyte such as saltwater.
Depending on the atomic structure of the two metals, one metal is the anode and the other is the cathode.
The anode corrodes faster than itself and the cathode corrodes slower than itself. Zinc is used in electroplating because it has an affinity for the anode when it comes into contact with many different types of metals.
Since the zinc coating in contact with the base metal is usually the anode, it slows down the corrosion of the base metal or cathode.
Different Methods of Galvanizing
There are several different processes for galvanizing metal:
1. Hot-Dip Galvanizing
Hot-dip galvanizing involves dipping iron or steel in a bath of molten zinc to create a corrosion-resistant, multi-layer coating of zinc-iron alloy and zinc metal. As the steel is dipped into the zinc, a metallurgical reaction takes place between the iron in the steel and the molten zinc.
This reaction is a diffusion process so the coating forms perpendicular to all surfaces and creates a uniform thickness throughout the part.
The hot-dip galvanizing process has been used since 1742 and offers long-lasting, maintenance-free corrosion protection at a reasonable price for decades. Although hot-dip galvanizing has been used to protect steel for generations, the galvanizing process is constantly evolving with new technologies and creative chemistries.
The three main steps of the hot-dip galvanizing process are surface preparation, galvanizing, and post-treatment, each of which is discussed in detail. The process is inherently simple, which is a decisive advantage over other corrosion protection methods.
Rust and corrosion are costly to property owners and taxpayers. Decaying buildings, roads, bridges, etc. are expensive to repair and, without adequate corrosion protection, maintenance often takes place or, in the worst case, the structure has to be rebuilt.
In the face of the pursuit of sustainable development, defining durable structures that require little maintenance over time offers both environmental and economic benefits.
This process is very similar to hot-dip galvanizing, but it is carried out in the steel mill, usually on materials that already have a certain shape. In pre-galvanizing, the sheet metal is rolled in a similar cleaning process as in hot-dip galvanizing. The metal is then passed through a pool of hot, liquid zinc and then drawn off.
One advantage of this process is that, compared to hot-dip galvanizing, large sheet steel coils can be quickly galvanized with a more even coating. The disadvantage is that after the start of production of the pre-galvanized metal, there are exposed, uncoated areas.
This means that when a long sheet metal coil is cut into smaller dimensions, the edges where the metal is cut remain free.
Unlike previous methods, electroplating does not use a bath of molten zinc. Instead, this process uses an electric current in an electrolyte solution to transfer zinc ions to the base metal. In the process, positively charged zinc ions are electrically reduced to zinc metal, which is then deposited on the positively charged material.
Grain finer can also be added to ensure a smooth zinc coating on the steel. Similar to pre-galvanizing, electro-galvanizing is typically applied continuously to a roll of sheet metal.
Some advantages of this process are an even coating and a precise layer thickness. However, the coating is typically thinner than the zinc coating obtained by hot-dip galvanizing, which can lead to reduced corrosion protection.
What is Hot Dip Galvanizing? In Detail
Hot-dip galvanizing (HDG) is the process of coating steel by immersing it in a bath of molten zinc. There are three basic steps involved in hot-dip galvanizing; Surface preparation, galvanizing, and inspection.
Hot-dip galvanizing is a form of galvanizing. It is the process of coating iron and steel with zinc that when the metal is immersed in a bath of molten zinc at a temperature of about 450°C (842°F), alloy with the surface of the base metal.
In the atmosphere, pure zinc (Zn) reacts with oxygen (O2) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO2) to form zinc carbonate (ZnCO3), a mostly dull gray, fairly strong material that makes up the steel underneath many Protects against further corrosion.
Galvanized steel is widely used in applications where corrosion resistance is required without the cost of stainless steel and is considered to be superior in terms of cost and lifespan. It can be identified by the crystallization pattern on the surface.
1. Surface Preparation
When the fabricated steel arrives at the galvanizing plant, it is hung on wire or placed on a shelving system that can be lifted and moved through the process with overhead cranes. The steel then goes through a series of three cleaning steps; Degreasing, pickling, and flux.
Degreasing removes dirt, oil, and organic residues, while the acidic pickling bath removes mill scale and iron oxide. The final step in surface preparation, the flux, removes any remaining oxides and covers the steel with a protective layer to prevent further oxide formation before galvanizing.
Proper surface preparation is crucial as zinc does not react with contaminated steel.
After surface preparation, the steel is immersed in the molten (830 F) bath containing at least 98% zinc. The steel is lowered into the kettle at an angle so that air can escape from tubular shapes or other pockets and the zinc can flow in, over, and through the entire piece.
When immersed in the kettle, the iron in the steel reacts metallurgically with the zinc to form a series of zinc-iron intermetallic layers and an outer layer of pure zinc.
The final step is a review of the coating. A very precise determination of the coating quality can be achieved through a visual inspection since zinc does not react with unclean steel, which would leave an uncoated area on the part. In addition, a magnetic thickness gauge can be used to check that the coating thickness meets the specification requirements.
Benefits of Hot-dip Galvanizing
Hot-dip galvanizing offers a number of advantages to the steel it protects. The metallurgically bound zinc-iron alloy layers not only form a barrier between the steel and the environment but also protect the steel catholically.
Due to the cathodic protection that zinc offers, the galvanized coating sacrifice itself to protect the underlying steel from corrosion.
The firmly adhering coating with a bond strength of around 3,600 psi is also extremely abrasion-resistant, as the intermetallic layers are harder than the base steel. But even if the coating is damaged, the sacrificial effect of zinc protects exposed steel up to ¼ of an inch away.
In addition to the cathodic protection that hot-dip galvanizing offers, there are a few other properties of the coating that ensure long service life.
First, the reaction in the galvanizing kettle is a diffusion process; which means the coating grows perpendicular to the surface and ensures that all corners and edges are at least the same thickness as flat surfaces. In addition, full immersion in the zinc bath provides complete coverage of the steel, including the interior of hollow structures.
Finally, the zinc coating naturally forms an impermeable layer of corrosion products on the surface, the so-called zinc patina. The patina, the cathodic protection, the complete coverage, and all these other properties give hot-dip galvanized steel a long, maintenance-free service life.
How long does the galvanizing process take?
A typical immersion time is around four or five minutes, but it can be longer for heavy objects with high thermal inertia or if the zinc is needed to penetrate indoors.
Upon withdrawal from the galvanizing bath, a layer of molten zinc is removed from over the alloy layer. Often times this cools down to reveal the bright shiny appearance of galvanized products.
Treatment after the galvanizing process can include quenching in water or air cooling. Conditions in the galvanizing plant such as temperature, humidity, and air quality have no influence on the quality of the galvanized coating.
In contrast, these are crucial for good painting quality. No post-treatment of galvanized items is required and a paint or powder coating can be applied to improve aesthetics or to provide additional protection in extremely aggressive environments. Chemical conversion coatings and other barrier systems can be used to minimize the occurrence of wet storage stains.
How Can You Use Galvanized Metal?
Galvanized metals are used everywhere!
The bodies of cars and many bicycles are made from galvanized metals. Some drinking water pipes are still made from galvanized steel. Cool rolled sheet metal is also frequently galvanized.
Nuts, bolts, tools, and wires of all kinds are now galvanized because it is a cheap process, and definitely helps boost the metal’s lifespan!
Galvanized steel, in particular, is often what is used in modern “steel frame” buildings. Galvanized steel is also used to create structures like balconies, verandahs, staircases, ladders, walkways, and more.
Galvanized metal is a great choice if your project will live outside after it’s done. Fences, roofs, outdoor walkways, are all great choices for galvanized metal!
Benefits of Galvanizing Metal
- Lowest first cost. Galvanizing is lower in first cost than many other commonly specified protective coatings for steel. The application cost of labor-intensive coatings such as painting has risen far more than the cost of factory operations such as galvanizing.
- Less maintenance/Lowest long-term cost. Even in cases where the initial cost of galvanizing is higher than alternative coatings, galvanizing is almost invariably cheapest in the long term (because it lasts longer and needs less maintenance). And, maintenance causes problems and adds to costs when structures are located in remote areas, and when plant shutdown or disruption to production is involved.
- Long life. The life expectancy of galvanized coatings on typical structural members is far in excess of 50 years in most rural environments, and 20 to 25 years plus, even in severe urban and coastal exposure.
- Reliability. Galvanizing is carried out to Australian/New Zealand Standard 4680, and standard, minimum coating thicknesses are applied. Coating life and performance are reliable and predictable.
- Toughest coating. A galvanized coating has a unique metallurgical structure which gives outstanding resistance to mechanical damage in transport, erection and service.
- Automatic protection for damaged areas. Galvanized coatings corrode preferentially to steel, providing cathodic or sacrificial protection to small areas of steel exposed through damage. Unlike organic coatings, small damaged areas need no touch up.
- Complete protection. Every part of a galvanized article is protected, even recesses, sharp corners and inaccessible areas. No coating applied to a structure or fabrication after completion can provide the same protection.
- Ease of inspection. Galvanized coatings are assessed readily by eye, and simple non-destructive thickness testing methods can be used. The galvanizing process is such that if coatings appear sound and continuous, they are sound and continuous.
- Faster erection time. As galvanized steel members are received, they are ready for use. No time is lost on-site in surface preparation, painting and inspection. When assembly of the structure is complete, it is immediately ready for use, or for the next construction stage.
- A full protective coating can be applied in minutes; The galvanizing process is not dependent on weather conditions