Galvanising is a fundamental process in precision mechanical engineering, used to protect metal from corrosion.
This technique consists of coating a piece of metal with a layer of zinc, creating a physical barrier that prevents oxygen, water and any other potential corrosive agent from reaching the underlying metal.
The various methods of galvanising include mechanical and electrolytic galvanising.
The Role of Galvanising in Precision Mechanics
The use of galvanising is a common practice in Precision Mechanics, but also in all industries that require high durability, such as automotive, electronics and marine engineering.
In addition to corrosion protection, it provides improved aesthetics to metal parts, which can acquire a polished and professional appearance.
It is precisely these characteristics that make it a highly versatile solution that can be applied in a variety of contexts.
Below, we go into detail on the processes and steps involved in mechanical and electrolytic galvanising.
Mechanical Galvanising: What It Is and How It Works
Mechanical zinc plating, also known as sherardising, is a coating process in which zinc particles are applied to small metal parts, such as screws, nails and washers, through a mechanical method.
The process is performed inside a rotating cylinder, where the part, zinc (in powder or granules) and a refining agent such as glass or ceramic are placed. Sometimes, white sand is used as an adjuvant.
First, the metal part is thoroughly cleaned to remove any residual grease, oil or rust. Next, it is immersed in the cylinder, which in turn is heated to between 320 and 400 degrees. The combination of the heat and the rotation of the mixer encourages the zinc to adhere to the metal. Refining agents ensure that the coating created is uniform and adheres firmly to the surface.
The thickness of this coating can vary from 5 to 30 microns, depending on the specific result desired. In addition, after galvanising, parts can be phosphated to further improve their strength and appearance.
Finally, it should be emphasised that mechanical galvanising, as it does not require electricity or aggressive chemical solutions, is an ecologically more sustainable process than other types of processing, representing a more environmentally friendly solution.
The Stages of the Mechanical Galvanising Process
The process of mechanical galvanising, as it turns out, is basically a physical method. Although it is rather complex, it can be divided into three key steps: preparation, coating and finishing.
The preparation of the metal is a crucial step.
First, the metal part is carefully cleaned to remove any contaminants, including oil, grease and rust. This is done by sandblasting or acid washing. It is almost superfluous to emphasise the importance of this step: a cleaned part ensures good contact between the zinc and the metal, allowing the coating to adhere properly.
The coating phase takes place inside a rotating cylinder.
The component, zinc (in powder or granule form) and the refining agents are placed in the cylinder, a kind of cement mixer. When the container is heated and set into rotation, the zinc melts and adheres to the surface of the part, creating a coating.
In the finishing phase, the coated part is removed and possibly subjected to further treatments. These may include phosphating and other processes that you can learn about via our Precision Mechanics Glossary.
Electrolytic Galvanising: What It Is and How It Works
Electrolytic galvanising, also known as electroplating, is another common method of applying a protective zinc coating to metal parts.
Unlike its mechanical counterpart, it uses the principle of electroplating to create the protective layer. It does this by using electric current to reduce the zinc ions present in a solution and deposit them on the part.
This treatment also ensures the creation of an extremely uniform and smooth coating, even on metal parts with complex shapes or intricate details. However, compared to the mechanical process, it tends to produce a less thick layer, which is why it may not be the best choice for parts destined for very aggressive environments or intensive use.
The Stages of the Electrolytic Galvanising Process
Although more complex than the previous one, the electrolytic galvanising process can be divided into three main phases: cleaning, electroplating and finishing.
In the cleaning phase, the part is prepared for coating through a series of washes and chemical treatments. As is the case with any surface treatment, it is necessary to remove all residues of other substances before action is taken: in their presence, even in minute quantities, the coating may not adhere properly, resulting in a decrease in performance.
Subsequently, during the electrodeposition phase, the part is immersed in an aqueous solution containing zinc salts. The part acts as the cathode, while a piece of pure zinc acts as the anode. With the application of an electric current, the zinc ions dispersed in the solution are reduced and deposited on the piece of metal, forming a protective coating.
Finally, in the finishing stage, the coated product is removed from the solution to undergo further treatments to improve its corrosion resistance or aesthetic appearance. These may include passivation, sealing or painting.
Differences between Mechanical and Electrolytic Galvanising
Both mechanical and electrolytic galvanising are effective techniques for protecting metal parts from corrosion, but they have significant differences that may make them more or less suitable for specific applications:
- Thickness of coating: mechanical galvanising tends to produce a thicker coating than electrolytic galvanising. This makes it a better choice for parts intended for extreme conditions or intensive use.
- Uniformity of the coating: electro-galvanising, using electric current, can create an extremely uniform and smooth coating, perfect for metal parts with complex shapes or intricate details.
- Environmental sustainability: as the mechanical process does not require an electric current or aggressive chemical solutions, it is generally considered more environmentally friendly than the electroplating method.
- Aesthetics: both treatments can improve the aesthetic appearance of the part, but electrolytic galvanising can offer a more polished and professional appearance.
- Cost: the cost of the two methods can vary depending on factors such as size, quantity of zinc and time required for the process. In general, electrolytic galvanising can be more expensive due to the electricity and chemicals used.