In the manufacturing industry, hot dip galvanising and zinc plating are two of the most common methods of applying a protective zinc coating to steel or iron components.
The main difference between the two is that hot dip galvanising involves immersing the steel component in a vat of hot molten zinc, whereas zinc plating uses electrolysis (electric current) to coat the zinc instead.
In this article, we will take a more in-depth look at the differences between the two processes, and how they affect the metallurgical properties of the steel component.
When And Why Are Zinc Coatings Needed?
Steel is one of the most commonly used materials in manufacturing owing to its versatility, strength and competitive pricing. However, steel has always had its Achilles heel; corrosion. This is the reddish brown, rough and flaky stuff you see on old bridges and shipwrecks. So, steel needs some modification in order to be more durable, particularly when exposed to the elements.
Corrosion occurs when the steel chemically reacts with the water and air in the environment, and forms an iron oxide. The iron oxide is the rust you see on the outside. In this instance, the steel surface has been oxidised by the oxygen in the air and water. Corrosion only happens to metal that is exposed to the external environment, so protecting the outermost layer of the metal is often sufficient.
Corrosion resistance is vital in small components like nuts and bolts, as well as washers and machine screws, as these types of fasteners are used to join multiple objects together. As a result, any damage to these components will be detrimental to the overall fixture, which can be both dangerous and costly to repair if precautions are not taken.
The solution to this problem has been to coat steel components with another metal. The other metal acts essentially as a sacrificial coating, being oxidised instead of the underlying steel to provide a protective barrier.
Zinc coatings are also utilised as a method to prevent hydrogen embrittlement. This process is the deterioration of a metal’s structural properties whereby the presence of diffusible hydrogen results in the metal becoming brittle and cracking.
As the degree of embrittlement is directly affected by the microstructure of the metal and the amount of hydrogen absorbed, minimising contact between the two is the best preventative measure. This is where the application of a coating, such as Zinc Plating, helps to protect the component surface from atomic hydrogen.
Accu can supply a range of high tensile fasteners which have been de-embrittled, a heat treatment which minimises the risk of the coating failing to protect against cracking.
This process is important as if a component, for example a Cap Head Screw, does not go through the de-embrittlement procedure, it will be unsuitable for use. The component will fail at under half the specified tightening force.
While zinc is one of the most popular protective coatings, other common metals used for plating include gold, silver and nickel. Accu offers plating in all of these metals, including zinc. We also offer a custom manufacture service, so you can get custom made screws that are plated with these metals.
What Is Hot Dip Galvanising?
When people say ‘galvanising’, they are typically referring to hot dip galvanising. During this process, the iron or steel component is submerged into a bath of hot, liquid molten zinc. The temperature of the molten zinc is around 450 °C (842 °F).
Here, the molten zinc chemically bonds with the steel or iron, producing an external protective layer of zinc as it cools and reacts with oxygen in the atmosphere, forming Zinc Oxide (ZnO). This then reacts with carbon dioxide too, causing a protective layer of Zinc Carbonate (ZnCO3) which provides the dull greyish appearance.
Steps In Hot Dip Galvanising
1) Cleaning cycle to prepare the surface
To get a good quality galvanising treatment, the metal surface has to be very clean. The surface cleaning processes involve 3 primary steps. Firstly, degreasing occurs to remove organic residues, dirt and oil. This is followed by pickling with an acidic solution to remove mill scales and any rust. Finally the fluxing process ensures that no oxidation occurs to the base metal before it is dipped into the molten zinc bath.
2) Drying process
The component undergoes rinsing in between degreasing, pickling and fluxing to remove any remaining traces of oxidation on the surface. Following this, the component is dried before it is ready for immersion into the molten zinc bath. It is vital at this stage that all contaminants have been removed from the surface of the component as unclean steel will not react with the zinc.
The clean component finally goes into the bath of molten zinc. This step usually only lasts around five minutes for small parts, but will take longer for larger parts.
4) Post galvanising treatment
The galvanised part is cooled and may undergo additional processes, such as painting or powder coating.
Why Not Use Stainless Steel Components Instead?
Both stainless steel and galvanised components are great for corrosion resistance. However, galvanising can often be far more cost effective than using stainless steel.
It is also one of the most environmentally friendly ways to provide corrosion resistance, as any zinc which does not form a coating on the steel or iron components during the hot dip will remain in the galvanising bath, ready for reuse.
Another significant factor in opting to use Steel components with a Hot Dip Galvanised instead of Stainless Steel components is the tensile strength considerations for the screw, nut or bolt. If an application requires structural bolts for high stress fastenings and to secure heavy objects together, a manufacturer may opt to use high tensile steel fasteners.
Whilst Stainless Steel components can be produced to high tensile strength grades, the significantly higher cost per unit can often mean Hot Dip Galvanised Steel components are a more attractive option.
What Is Zinc Plating?
Zinc plating refers to zinc electroplating, a method that uses electric current to break down zinc into ions (tiny charged particles) and deposit on the metal component. The process is done by forming a simple electrolytic cell consisting of the zinc, an aqueous solution and the component to be plated.
How The Zinc Plating Process Works
Here, the zinc is called the anode (positive charge) and the component to be plated is called cathode (negative charge). In simple terms, when you pass a direct electric current through the cell, the zinc anode begins to break down to tiny zinc ions.
Since the zinc ions are positively charged, they are attracted to the positive terminal (steel component that needs plating), so they flow towards it. The aqueous solution facilitates the movement of the ions. When they reach the component, the zinc ions form a neat outer layer of zinc.
Common Differences Between Galvanised And Zinc Plated Components
Galvanised components that are not treated often have a muted grey colour that is quite rough. Zinc plated parts are often smoother and shinier and can also feature a coloured finish, such as blue or yellow.
Zinc plating produces a coating that is quite thin, roughly in the range of 0.005 mm to 0.025 mm. It becomes more expensive to get a thicker coating than this with zinc electroplating. With hot dip galvanising, you can get a thickness of over 0.1 mm.
As you can achieve a greater thickness with hot dip galvanising, it means that galvanised parts can go years and even decades without rusting. The rusting process is so slow in fact that as the zinc rusts, it produces zinc oxide, which adds another layer of protection.
With zinc plating, the smaller thicknesses mean that zinc plated parts are not as durable as galvanised ones. Hence, it is unsuited for outdoor application unless it has been treated afterwards.
As galvanised parts have a thicker zinc coating, this means it is stronger and more resilient, especially in outdoor applications. Since the corrosion rate is quite slow, the coating retains its strength over a longer period of time.
This is where zinc plating wins out, being a more readily available metal, it is more cost effective than hot dip galvanising, especially for smaller components. Zinc is also considerably cheaper than other metals such as gold.
Hot dip galvanising can be used for almost any steel or iron component, large or small. They can also be used in outdoor and indoor applications.
Zinc plating is mostly limited to small metal parts such as nuts, bolts and screws. They are also mostly suited for indoor applications. However, they do have better weldability than galvanised parts.
Hot Dip Galvanising Vs Zinc Plating - Which Is The Best Option?
In conclusion then, owing to the lower cost per component and lower corrosion resistance overall, a zinc plated finish is best for smaller parts that are used indoors, like nuts, screws and bolts.
On the other hand, hot dip galvanising produces a thicker zinc coating, hence providing parts with a finish that is stronger and more durable. This makes it the better option for outdoor applications where you need stronger corrosion resistance.