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What is Galvanization?

Published by Rapid Metals on

Galvanization (also galvanisation or frequently referred to as galvanising/galvanizing) is a popular method of protecting metals that are more prone to oxidisation or rusting.  

The process works by applying a protective layer of zinc to the metal, most commonly used with steel or iron. There are several methods of galvanization available but the most frequently used process is known as hot-dip galvanizing, in which steel or iron metal is fully submerged into a molten zinc bath.  

Many products you see every day will have gone through the galvanization process, from street-lights and signs to galvanized sheet metal for roofs and galvanized steel for construction. It is frequently used for metals that are likely to be exposed to the elements for an extended length of time and has been in use since as early as the 17th century but was only officially patented in 1836.

What does Galvanization Mean?

The term galvanization or galvanizing specifically means to coat metal, usually iron or steel, with molten zinc to provide a protective layer. 

Galvanized metal has a number of beneficial qualities including; 

  • Much higher resistance to corrosion
  • Additional protection for the base metal should the zinc coating become scratched 
  • An insoluble zinc coating that protects steel and iron from rainwater 
  • More cost-effective than using stainless steel or aluminium 

Zinc metal works as a sacrificial anode; should the protective coating become scratched or damaged, the steel is still protected as the zinc will corrode first. This is a process known as galvanic corrosion and only works with certain metals used in combination. The atomic structure of zinc means it usually lends itself to being an anode when used alongside a large number of different metals and this helps to reduce the corrosion time of the base metal (also known as a cathode).

How to Galvanize Steel or Iron

Galvanizing metal can be done in several different ways;

Hot-Dip Galvanization

The most commonly used process for metal galvanizing. Hot-dip galvanization is the process of dipping the required base metal into a specialist bath of molten zinc to create a thick protective coating. 

To start the process, the base metal needs to be thoroughly cleaned. This is usually carried out using a caustic solution that will get rid of any residual grease, oil, dirt and paint. The solution is then rinsed from the metal before the base metal is pickled in an acidic solution. The acidic solution is intended to remove mill scale, the flakey surface that appears on hot rolled steel before the acidic solution is then washed off.  

The surface of the metal, now fully cleaned, is more prone to oxidation and is usually covered using a zinc ammonium chloride flux to protect against exposure to the air and help the molten zinc adhere more strongly to the base metal.  

Once the flux is fully dry, the base metal is then dipped into the bath of molten zinc. The base metal is held in the bath until the temperature of the dipped metal equalises with the temperature of the zinc bath at approximately 449 °C (840 °F). This creates a specific chemical bond between the two metals known as a metallurgical bond.  

Once removed from the bath, the base metal goes through a quenching process (is immediately submerged in cooled water known as quench tank) to reduce the temperature of the finished metal and prevent reactions between the slightly molten coating and the surrounding air.  

Hot-dip galvanized metals are easily recognisable for their surface pattern known as ‘spangle’. This is a crystalline looking pattern that covers the surface of a hot-dip galvanized metal and can be adjusted in its appearance by changing the number of added particles used for heterogeneous nucleation and/or by adjusting the time in the cooling process.

Pre-Galvanizing

Pre-galvanization of metal products usually takes place at the steel mill and is used to provide a uniform protective coating of zinc, similar to hot-dip galvanizing. This is an automated process for pre-formed materials such as steel tubes or steel sheet and is also used for wire.  

The metal goes through a thorough cleaning process to remove any surface imperfections and is then fed into the molten zinc bath for coating. Unlike the hot-dip galvanizing process where the metal is dipped, equalised and quenched – pre-galvanized metals go through an additional process of wiping or have steam blown down the centre of the bore (for steel tubes) to ensure a uniform coating. The coating is usually between 7-42 μm in thickness depending on the grade and most products tend to have a coating measuring around 20 μm in thickness.

Pre-galvanization is used for materials that require a thinner, uniform finish but this also comes at a cost as pre-galvanization doesn’t offer the sacrificial protection that a hot-dip galvanized product does. Additionally, pre-galvanized products are often sent for re-sale and on cutting for purchase, gain uncoated and unprotected edges.

Electro-galvanizing

Also called ‘zinc electroplating’, electro-galvanization is the process of using electricity to coat iron and steel with a protective zinc layer. Steel materials (acting as the cathode) are submerged into a solution containing zinc salts. Electricity is then applied to start an electrochemical reaction which coats the surface of the steel with zinc metal.  

This is a technique commonly used in automotive and home/commercial appliance industries as it produces a more aesthetically pleasing finish. However, the zinc coating is much thinner than both pre-galvanizing and hot-dip galvanization, with a maximum thickness of up to 9 μm. This makes products that have been electro-galvanized unsuitable for outside use where they risk being exposed to the elements. Products that are to be used outside will require an additional protective coating, such as paint.

Can You Paint Galvanized Steel?

Yes, it is possible to paint galvanized steel, either for aesthetic or safety reasons or to provide additional protection where the external elements are more aggressive (exposure to acid rain for example). However, not every paint is suitable for use on galvanized metals and some surfaces may require additional preparation for painting. When a metal has been newly galvanized, the surface is shiny and smooth, making it difficult for most paints to properly adhere. 

There are four different types of methods for preparing a galvanized metal surface for painting;

  • T-Wash 
  • Etch Primers 
  • Sweep Blasting 
  • Weathering 

The type of paint used is also important as not every paint will be suitable for use on galvanized metal surfaces and the paint system used will be dependent on the installation environment. Popular paint types for galvanized metal include;

  • High-build epoxy 
  • Vinyl/vinyl co-polymer 
  • Polyurethane and acrylic urethanes 
  • Acrylic epoxies 
  • Polysiloxanes 

Make sure to thoroughly clean the metal surface prior to painting to remove any residual grease, oils and dirt.

Can You Weld Galvanized Steel?

It is possible to weld galvanized steel, but welding must be done extremely carefully as the fumes created are extremely toxic. Welding should be done outside where possible or in a completely ventilated environment with the correct protective equipment including breathing apparatus. The ventilation for your space when welding should remove toxic air as well as providing a constant supply of fresh air.  

Additionally, welding galvanized steel can be difficult as the protective zinc coating makes it tricky to adhere to. To get a sturdy, durable TIG or MIG weld, you will need to remove some of the zinc coating via grinding. This can be time-consuming but creates a clean join. Deeper penetrating rods are available to go through the protective coating, however, this process creates untidy beads with a lot of ‘spatter’. 

The most important thing to remember when welding galvanized steel is protective procedures to prevent immediate and future health concerns, including injury and death.  

Don’t forget to replace any lost corrosion protection from removing the zinc coating for welding, this can be done by re-galvanizing the metal or using a protective anti-corrosive paint layer.

How Long Does Galvanized Steel Take to Rust?

The timeline of oxidation for galvanized metals is dependent on the application. For example, steel roofs typically last for decades but when located nearer to the coast or in a marine environment, the exposure to seawater drastically reduces the lifetime of the protection. This happens as seawater has high electrical conductivity and gradually converts the solid zinc to zinc chloride, which is a soluble compound that simply washes away.  

Similarly, galvanized metal car bodies that exist in colder environments tend to last less time than their counterparts in warmer climates as the exposure to road salt corrodes the protective layer much faster. In the UK, the average atmospheric zinc corrosion rates are measured at being less than 1µm per year, meaning that metals with a 20 μm thick coating are likely to provide 20 years of maintenance-free protection.  

You can extend the lifetime of the protective zinc coating using additional protective paint coatings or by adding another sacrificial anode which will corrode faster than the underlying base metal.

Galvanized Steel from Rapid Metals

We are proud to be one of the UK’s leading stockists of galvanized steel tube and galvanized steel sheet. No matter the project size, we accept all orders great and small with no minimum quantity required. Order the galvanized metal you need cut to size with next day delivery from Rapid Metals today. 

If you need something that we don’t have in stock or listed online simply let us know, our fantastic supplier network ensures we have access to a whole range of specifications. You can reach our friendly team online and in-store, on the phone and via email – simply visit our contact page to find the nearest branch to you.

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