Galvanization: Definition, Methods, and Process

What is Galvanization/Galvanizing?

Galvanization or galvanizing (also spelled galvanization or galvanizing) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged in a bath of molten hot zinc.

How does galvanizing prevent rusting?

Galvanizing protects against rust in various ways:

  • It forms a barrier that prevents corrosive substances from reaching the steel or iron below.
  • The zinc serves as a sacrificial anode so that the exposed steel remains protected by the remaining zinc even if the coating is scratched.
  • The zinc protects its base metal by corroding from the iron.
  • The zinc surface reacts with the atmosphere and forms a compact, adherent patina that is insoluble in rainwater.

History of Galvanization

In 1780, an Italian, Luigi Galvani, discovered the electrical phenomenon of twitching a frog’s leg muscles when touched by two different metals, namely copper and iron. Galvani incorrectly concluded that the power source was in the frog’s leg. The term electroplating (or electroplating) appeared in the dictionary and was in part related to the work of Michael Faraday.

In 1824, Sir Humphrey Davy showed that when two different metals were electrically connected and immersed in water, the corrosion of one was accelerated while the other was given some protection.

In 1829 Henry Palmer received a patent for “indented or corrugated sheet metal” from the London Dock Company. Its discovery had a dramatic impact on industrial design and galvanizing.

In 1836, Sorel in France granted the first of numerous patents for a method of coating steel by immersing it in molten zinc after the first cleaning. He named the process “galvanizing”. In addition to Sorel’s 1836 patent, William Crawford was granted a British patent in 1837 for a similar process.

Although uncertain, it is believed that the first use of galvanized corrugated iron for the Navy was in Pembroke Docks, Wales, in 1844.

In the mid-19th century, British sheets of galvanized corrugated iron were considered exciting and glamorous. The concept of making iron sheets stiffer not only enabled large span roofs to be built without the need for cumbersome load-bearing walls, but they also looked new in a built environment dominated by stone and wood. The silver reflective material was a sign of progress and success in the industry.

Why Galvanize?

Galvanizing a metal simply 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 using materials like austenitic stainless steel or aluminum to prevent corrosion.

How Does It 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 it does itself and the cathode corrodes slower than it does itself. The reason zinc is used in galvanizing is because 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

As the name suggests, this method involves dipping the base metal into a molten pool of zinc. First, the base metal must be cleaned either mechanically, chemically, or both to ensure that a quality bond can be established between the base metal and the zinc coating.

After cleaning, the base metal is allowed to flow to remove any oxides remaining after the cleaning process. The base metal is then immersed in a bath of heated zinc and a metallurgical bond is formed.

The advantages of this method are that it is economical; it can be done quickly and to complex shapes. However, the final coating may be inconsistent compared to other galvanizing processes

2. Pre-galvanizing

This method is very similar to hot-dip galvanizing, but it is done in the steel mill, usually on materials that already have a certain shape. During pre-galvanizing, the sheet is rolled using a cleaning process similar to that used in the hot-dip galvanizing process. The metal is then passed through a pool of hot, liquid zinc and then withdrawn.

One advantage of this process is that large sheet steel coils can be quickly galvanized with a more even coating compared to hot-dip galvanizing. One disadvantage is that once production of the pre-galvanized metal begins, there are exposed, uncoated areas.

This means that when a long sheet metal coil is cut into smaller sizes, the edges where the metal will be cut will remain exposed.

3. Electrogalvanizing

Unlike previous methods, electroplating does not use a molten zinc bath. 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 refiners can also be added to ensure a smooth zinc coating on the steel. Similar to the pre-galvanizing process, 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 the hot-dip galvanizing process, which can lead to reduced corrosion protection.

What is Hot Dip Galvanizing? In Detail

Hot Dip Galvanizing is a unique process. When clean steel is immersed into molten zinc, a series of zinc-iron alloy layers are formed by a metallurgical reaction between the iron and zinc, providing a robust coating that is an integral part of the steel.

Due to the fact that hot-dip galvanizing has been used to protect iron/steel for so long, various terms have been used to describe the process including galvanizing, hot-dip galvanizing, and hot-dip galvanizing.

Hot-dip galvanizing offers external and internal protection in hollow profiles, repairs itself if damaged, sacrifices itself to protect the base metal, is environmentally friendly, has good impact and abrasion resistance, and maintenance-free service life of 50 years or more.

Galvanized steel is widely used in applications where corrosion protection is required and can be identified by the crystal pattern on the surface (often referred to as “flake”).

Galvanizing Process

Hot-dip galvanizing involves coating iron or steel with a layer of zinc by immersing the metal in a bath of molten zinc at a temperature of about 450°C (842 ° F). During the process, a metallurgically bonded coating is formed that protects the steel from harsh environments, whether outside or inside.

Galvanization or galvanizing (also spelled galvanization or galvanizing) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged in a bath of molten hot zinc.
Galvanizing

Often used in applications where corrosion resistance is required without the cost of stainless steel, galvanized steel can be identified by the crystal pattern on the surface (often referred to as “tinsel”). Galvanizing is probably the most environmentally friendly way of preventing corrosion.

Hot-dip galvanizing is a very simple process that protects steel structures from corrosion in the long term.

Hot-dip galvanizing process steps:

Main steps within the galvanizing process include:

  • Cleaning cycle – decreasing and chemical clean
  • Fluxing – helps to wet the surface of the steel
  • Galvanizing – immersion of steel into a bath of molten zinc
  • Post-treatment (optional)

Step-1. Cleaning cycle

The galvanizing reaction only takes place on a chemically clean surface. As with most zinc coating processes, the secret to a good coating lies in the preparation of the surface.

It is important that it is free of grease, dirt, and scale before galvanizing. These types of contaminants are removed by a variety of methods and it is common to first degrease with an alkaline or acidic solution in which the component is immersed. The article is then rinsed in cold water to avoid contamination of the rest of the process.

The article is then immersed in hydrochloric acid at ambient temperature to remove rust and scale. Welding slag, paint and heavy grease are not removed by these cleaning steps and should be removed by the manufacturer before sending the job to the electroplater. After further rinsing, the components are usually subjected to a flow process.

Step 2. Fluxing procedure:

This is usually applied by immersion in a flux solution – usually around 30% zinc ammonium chloride at around 65-80 ° C. Alternatively, some galvanizing lines can operate using a flux blanket on top of the galvanizing bath. The flow process removes the last traces of oxide from the surface and the molten zinc can wet the steel.

Step 3. The Galvanizing Process

When the clean iron or steel component is immersed in the molten zinc (which is usually around 450° C), a series of zinc-iron alloy layers are formed through a metallurgical reaction between iron and zinc. The rate of reaction between the steel and zinc is usually parabolic over time, so the initial rate of reaction is very fast, and considerable movement can be seen in the zinc bath.

The main thickness of the coating is formed during this time. Then the reaction slows down and the coating thickness is not increased significantly, even if the object is in the bath for a long period of time.

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.

Post-treatment

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.

Application of Galvanizing

  • To galvanized sheet metal.
  • To galvanized house-hold items such as buckets, tubs, and other containers.
  • To galvanized machine parts, tools, ships, tanks, and wires.
  • Metal pipes and wires are the most popular galvanized items which find application in industrial use as well as in articles made for domestic use.

FAQs.

What Is Galvanization?

Galvanization or galvanizing (also spelled galvanisation or galvanising) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged in a bath of molten hot zinc.

What is the process of galvanization?

Galvanization or galvanizing (also spelled galvanisation or galvanising) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged in a bath of molten hot zinc.

What is Hot Dip Galvanizing?

Hotdip galvanizing (HDG) is the process of coating fabricated steel by immersing it in a bath of molten zinc. There are three fundamental steps in the hotdip galvanizing process; surface preparation, galvanizing, and inspection

Why zinc is used for galvanizing?

The reason that the galvanizing process uses zinc instead of other metals is that zinc oxidizes and experiences acid corrosion “sacrificially” to steel. That means that when zinc is in contact with steel, oxygen, and acids will attack the zinc rather than the steel beneath it.