Aluminium metal is widely used to make machine parts because it is cheap, light, and easy to work with. It is also a very soft metal, so can easily be scratched or damaged, and it can be corroded. Raw aluminium will naturally form a very thin oxide layer on the surface, but this is not hard enough or thick enough to protect the part.
To make parts made from aluminium last longer, it is necessary to ‘finish’ the parts in some way. One of the main ways this is done is by anodising. When aluminium is anodised, some of the aluminium at the surface is combined with oxygen to form aluminium oxide, which is much harder than aluminium. This provides a protective coating.
The anodising process involves a number of pre-treatment steps – these are very important – if the surface of the part to be anodised is not extremely clean, the anodic layer will not form properly.
The first pre-treatment step is soak – this is like a stronger version of dishwashing liquid, it has very strong de-greasing properties and removes oil and grease from the part.
The second pre-treatment step is etch – this is a strong caustic soda solution, also known as sodium hydroxide. This step removes the natural oxide layer on the aluminium, giving a fresh surface for the anodising to take place. Etching will also remove some of the aluminium itself, and the longer the part is left in the etch, the more aluminium will be removed. For this reason, it is very important that work instructions are followed carefully so that too much aluminium is not removed from parts during processing.
After the etch, the parts go through a desmut in a nitric acid solution. Etching can leave a ‘smut’ on the surface of the part, this is made up of components of the aluminium that were not dissolved by the etch. The desmut removes this to make sure the surface is clean and fresh.
Finally, after all of this preparation, the parts can be anodised.
This is an electrochemical reaction (electricity and chemical) – the parts are placed in an acid bath (we use sulphuric acid), and an electric current is passed through them. At this point, the jigging of the parts really comes into play – a good contact between the part and the jig is needed to allow the electricity to flow through the parts. This results in an aluminium oxide layer growing up from and down into the surface of the aluminium.
Because the anodic coating forms in this way, tolerances of parts can be affected. If a part with tight tolerances is anodised and needs to be reworked, once this anodic coating is stripped off it might be impossible to build up enough coating to get it back within tolerance.
The aluminium oxide forms in a honeycomb structure, with pores or holes that reach almost all the way through the coating.
If these pores are left open, the part is more likely to become corroded. Because of this, after anodising, these holes are generally ‘sealed’ – this improves the corrosion resistance of the coating by making the coating continuous – the holes are blocked up to provide more protection to the underlying part.
Before sealing, the parts can be put into a dye bath, where dye particles go into the holes – this changes the colour of the coating. If the coating is thicker, there is more room for the dye particles to build up (longer pores) and so the colour can be darker. Likewise, if the part is left in the dye bath for a longer time, more dye gets into the pores and the colour is darker.