Anodized aluminium is used in external architectural applications where appearance and long life are important. Such applications include window frames and facade coverings.
There are several different types of anodising, which include natural anodising, colour anodising and hard anodising.
The anodizing process
Anodization of aluminium is a method which has been used for several decades. It is an electrochemical process that forms an anodic oxide layer on the aluminium surface. With anodization process, the aluminium surface is converted into an anodic oxide finish that is attractive, durable and corrosion-resistant. This aluminium oxide film, that seals the aluminium from further oxidation when it is exposed to air.
The anodizing process includes the following steps:
- Cleaning: With this pre-treatment process, mill oils, grease, aluminum oxides, and dirt are “chemically” removed from the wrought aluminum.
- Etching: With this pre-treatment, the aluminum surface is prepared for anodizing by removing a thin layer of aluminum.
- Anodizing: With the anodized process, the surface of the aluminum is converted to anodic oxide barrier layer (also called the anodic film) with pores. The thickness and density of the anodic film depends on the density of current applied, time, temperature, and concentration of the electrolyte during anodizing.
- Coloring: The coloration process works by injecting a color pigments into the bottom of empty pores of the anodic oxide layer. The strength of the colour will depend on how thick the coating layer produced during the anodising process is, and thus how much dye it can absorb. So light pastel colours can often be obtained with a minimal anodising thickness. Howver to get a fully saturated dark black (jet-black) colour may require a greater anodised oxide thickness.
- Sealing: To prevent the colour from leaching out and fading over time and to avoid any problems, the pores of the anodic film are permanently sealed off.
Thickness of anodising
There are different layers of thickness of anodising applied to aluminium to achieve different benefits. The thicker the anodic layer, the better corrosion resistance and the better the finish.
Natural (silver) anodised aluminium
Natural anodising or silver anodising is an uncoloured anodic oxide coating which is transparent and shows the appearance of the underlying metal. No colours or sulphate process has been used. Natural anodising is the least expensive anodic finish.
Brushed anodised aluminium
Colour options for anodised aluminium
Typical colour range for anodized aluminium includes silver, gold, bronze, copper, grey, black, red, blue and green. Bronze is the most common architectural anodized colour – bronze coluring ranges from champagne, light, medium, dark, to black.
Below are some of the more comon anodised finish colours.
Special colours like stainless steel look, chrome-look or titanium-look can be obtained by a combination of mechanical pre-treatment and colour anodising.
Frequently Asked Questions
Does anodised aluminum scratch?
However, aluminum isn’t a very hard metal and can be scratched fairly, the anodizing process creates an extremely hard surface that will be far more resistant to scratches and wear compared to untreated aluminum. In order to improve the scratch resistance of aluminum, The thicker oxide layer is created on the aluminium surface.