What is hot dip galvanising?

Steel is one of the most popular construction materials due to its incredible strength and engineering capabilities. However, steel is also susceptible to rust and deterioration, especially when it is exposed to harsh conditions. To counter this, steel components are often hot dip galvanised. 

In this article, we explore the following:

• What is hot dip galvanised steel?
• The process of applying a hot dip galvanised coating.
• The pros and cons of hot dipped steel.

What is hot dip galvanising?

Hot dip galvanising is a thick, durable coating that is applied to steel components by immersing them in molten zinc/alloy. This outer zinc layer protects the inner core from moisture and corrosion, maximising longevity and durability. 

The process of hot dip galvanising

There are three main steps in the hot-dip galvanising process — surface preparation, hot dipping, and post-treatment. 

1. Surface preparation

The steel component is hung on a steel chain and immersed in a degreasing bath, such as an alkaline caustic solution. This removes any organic contaminants such as dirt, oil, and grease from the steel’s surface. When this is complete, the steel is rinsed with water. 

Next, the steel component is submerged in a dilute solution of hydrochloric or sulfuric acid, removing any oxides and mill scale. When this pickling process is complete, the component is rinsed with water again and dipped in a hot flux to clean the steel of any oxidation developed since the pickling and activate its surface. 
 
Finally, the component is dried with hot air to reduce splash and improve dip speeds. This multi-step process creates a clean surface that the zinc can react and bond with, achieving a thorough and consistent galvanised coating. 

2. Hot dipping

Next, the steel is fully submerged in a bath of highly pure molten zinc heated to 450℃. The liquid zinc coats the entire surface evenly, forming a consistent layer over every surface of the steel member, this includes the internal surfaces of tube members.

When the steel matches the bath’s temperature, the diffusion reaction of iron and zinc is complete and the product is withdrawn from the zinc kettle. The dip is typically complete in under ten minutes, depending upon the component’s size and thickness. 

3. Post-treatment

When the steel component is taken out of the zinc kettle, the zinc coating will gradually harden. In most cases, it undergoes a post-treatment process to improve the effectiveness of the coating. 

One of the most common treatments is quenching, where the steel is submerged in a quench tank containing water and a small portion of chemicals. This creates a passivation layer, protecting the galvanised steel during transportation and storage. 

The hot-dip galvanising process. Source. 

Hot dip galvanised vs pre-galvanised steel

Hot dip galvanising and pre-galvanising are two different processes, used for different purposes. 

Pre-galvanising, also called pre-gal, dura-gal or inline gal, is used on steel members that originated from coiled steel. Due to the speed at which the coil is run through the zinc bath, it produces a far thinner coating than hot dipping. It’s also worth noting that, as its name suggests, ‘pre-gal’ is completed BEFORE roll-forming and manufacturing are completed meaning that any bends, cuts, welds or punches become weak points in the member as the coating is compromised.

Hot dip galvanised steel creates a far thicker coating and produces a much more durable result with the process being completed AFTER fabrication ensuring no compromise of the protective coating.

Advantages of hot dip galvanised steel

Three types of protection

Hot-dipped steel has three crucial forms of protection. The first is barrier protection, providing full coverage of the steel surface and acting as a barrier from the surrounding environment. Secondly, cathodic protection prevents damage from coating imperfections like abrasions, cuts, or bends. 

Thirdly, hot-dipped steel has an insoluble zinc patina on its surface, slowing down the corrosion rate of the zinc and therefore delaying the corrosion of the inner steel.

Suitable in high corrosion zones

With its long-lasting protection against corrosion, hot-dipped steel is an excellent choice for buildings in high corrosion zones. For example, it is often used in coastal areas prone to sea spray. 

Long-lasting and low-maintenance

Hot-dip galvanised coatings are tough and durable, requiring very little maintenance. 
This reduces the need for repairs or replacements, which can be costly and time-consuming.

Scratch resistant

With some protective coatings, a slight scratch will expose the inner steel to corrosion and quickly lose its structural integrity. However, hot-dipped zinc coatings are incredibly abrasion-resistant, meaning they’re perfect for busy environments with vehicles or equipment. Even if the steel is hit, the coating will not be damaged. 

Disadvantages of hot dip galvanised steel

May react with acids

In some cases, zinc can react with acidic products, reducing the consistency of the coating. 
This is worth noting if you will be storing or using certain fertilisers around your new shed. Some chemical storage facilities may require an additional powder-coated layer over the hot dip galvanising for added protection. For more information about the products this applies to, speak with our team. 

More expensive for large clearspans

Due to the steel thicknesses and weights, hot dip galvanising may be more expensive for a large steel building. Unlike basic paint systems that are based on surface area, hot dip galvanising is a commodity tied to weight, which means the heavier the steel componentry the more costly it is to surface treat with hot dip galvanising. This is particularly true for large clear-span sheds requiring larger steel members.

Industrial finish

Due to the application method and the thickness of the finished coating, it is almost impossible to ensure an ‘architectural-level’ consistency. In addition, small variations in the silicon or phosphorus content in the steel can mottle the colouring of the final finish as it reacts with the molten zinc. For this reason, galvanising is considered to be an ‘industrial finish’ and may not be suited to projects with high architectural exposure.

Steel sheds Australia

At ABC Sheds, we have supplied structural steel sheds all over Australia for over 25 years. Engineered for the harsh Australian climate, our sheds are built with high-quality Australian structural steel with a hot-dipped galvanised coating. This combination comes with a 25-year guarantee and exceeds Australian building standards, ensuring 100% customer satisfaction.

At ABC Sheds, we tailor your new shed to meet your needs. With our robust structural steel construction, the options are almost endless. Ready to start your next project? Try our online Shed Builder Tool today and get a visual of how your dream shed will look. Alternatively, contact our team of experts to discuss your upcoming project.