Parts manufacturers tend to think of electroplating as a surface protection treatment for metals such as steel and iron. However, the electrochemical process is much more than that and has a high aesthetic value as the Zinc Plating Colors can be produced in a variety of colours. The traditional appearance of zinc plating is grey-blue or silver.
However, different technologies can change this colour. Therefore, this article will introduce the zinc coating colour technology, the zinc colours that can be achieved and the possible applications.
Galvanising or electroplating is the electrochemical or chemical deposition of a thin layer of zinc on metal parts. It is a protective surface treatment that increases the durability, corrosion resistance and service life of parts.
Zinc plating takes place using the following steps:
Cleaning of the substrate (metal parts) can be mechanical, electrochemical or chemical. Physical and electrochemical cleaning remove contaminants such as dirt, grease or rust, while chemical cleaning using acids removes oxides from the substrate surface.
In hot-dip galvanising, the substrate is immersed in a molten zinc solution; in electro-zinc plating, the substrate is immersed in a zinc solution containing zinc salts (zinc sulphate or zinc chloride) and some additives. A direct current is then applied to deposit the zinc on the substrate.
Post-treatment processes include chromate conversion, passivation, etc. These processes can improve the performance and aesthetic value of galvanised parts.
Zinc plating and its post-treatment processes (such as chromate conversion) produce different colours. Below is an overview of the different types of zinc passivation colour technologies.
The galvanising process involves immersing the substrate (usually steel) in molten zinc at a temperature of approximately 450°C (842°F). The zinc then bonds with the substrate.
The post-treatment process involves immersing the galvanised parts in a hexavalent chromium solution. This forms a thin chromate conversion layer.
The zinc coating coloring technology is to deposit a trivalent chromium conversion coating on the zinc-plated surface.
An electrochemical process used to deposit zinc on a substrate. The substrate is immersed in a zinc solution and then a direct current is passed through it.
Zinc alloys on electrochemically or mechanically deposited substrates
Depending on the passivation technology, coating thickness and additives, the zinc coating can appear in different colours. Its colour ranges from silver to yellow, black, blue or green.
Chromates are able to change colour because they contain a transition metal (chromium) in a variety of oxidation states. Each oxidation state has a unique effect on the colour of the zinc coating. For example, hexavalent chromium compounds have a different colour to trivalent chromium compounds. The colour of chromates also depends on the thickness of the coating and the concentration of the electrolyte. Thicker chromate coatings are lighter in colour and thinner coatings are lighter in colour.
The thickness of the zinc coating affects the colour. Thicker coatings are lighter than thinner coatings. Also, the thicker the coating, the less uniform the thickness of the chromate layer and the less uniform the colour distribution.
Post-treatment processes in zinc plating include chromate conversion coating, passivation, or sealing, and they have the following impacts on zinc-plated parts:
The post-treatment process is an additional barrier between the galvanized component and the environment. It therefore acts as an additional protection against corrosion.
Chromate conversion coatings can improve the colour of galvanised parts. As a result, parts manufacturers can achieve yellow, black and rainbow colours.
Chromate conversion coatings can improve the adhesion of paint and powder coatings to galvanised surfaces.
Post-treatment processes such as passivation can improve the surface finish of parts by making the surface smoother and more uniform.
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