Video from Dept. of Chemical & Electrical Engineering, University of Illinois.

Bill describes how metals like aluminum and titanium are made resistant to corrosion by growing an oxide layer into the metals. These is the same process used on many Apple products. This video is based on a chapter in the book Eight Amazing Engineering Stories; learn more at http://www.engineerguy.com/elements.

Different types of anodising

There are a few different types of anodising. In Graph, we do ‘sulphuric acid anodising’ – the parts are put into a sulphuric acid bath and an electric current is passed through to make the aluminium oxide grow on the surface. Depending on the temperature of the bath and the voltage of the current, the size of the holes or pores can be changed. Hard anodising is carried out at a lower temperature and with a higher voltage than clear anodising. This makes the pores smaller and the cells are packed more tightly together, which gives a harder coating. Because the pores are smaller and the cells are closer together, this means that hard anodising changes the colour of the part. The colour depends on the alloy or type of material. Clear anodising uses higher temperatures and lower voltage; this results in a more porous, less hard-wearing coating that is suitable for different applications.

Because clear and hard anodised coatings have different properties, they are used for different applications. Hard anodising is more expensive to carry out than clear anodising (because the temperature must be kept lower and the voltage is higher, plus it takes longer to get a thicker coating).

The Anodising Process

Introduction
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. 

Anodising steps
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.

Care & Cleaning of Anodised Parts

Anodising is surprisingly durable and resilient to corrosion. However, like its aluminium base, it is susceptible to strong alkalines, such as Lye or Sodium Hydroxide. It also can be damaged by strong acids, such as nitric or sulfuric. Beware of strong alkaline cleaners, which often contain ammonia or lye, as well as masonry or cement cleaners, which can contain strong acids.
Surprisingly, anodising is quite resistant to organic solvents. While alcohol or acetone will quickly remove ink from a permanent marker, for example, they will not damage or fade coloured anodizing, and can be excellent for cleaning purposes.

OK:  Alcohol, Mild soap/detergent, Acetone, MEK.

Not suitable:  Ammonia, Alkaline Cleaners, Lye, Strong Acids.
  

 

Colour Anodising

If you have a colour sample available, please send this either with the job or email a photo. This will help us to get the colour you are after. NOTE: While every effort will be made to match the colour this cannot be guaranteed as there are many variables that impact on the final colour. White is not available as a colour, this is due to the fact that the particles that make up white dye are too big to fit into the pores generated by anodising.

Welding & Anodising

If the parts require welding, this should be carried out before anodising. The anodic coating has very high electrical resistance, effectively stopping welding. We recommend the use of 5356 or Al-Mg 5 alloy type welding rods for the best looking results after anodising. In addition, the use of TIG instead of MIG welding is favoured as MIG welding produces very high localised temperatures with rapid cooling leading to structural changes in the aluminium which hinder the anodising and prevent the uptake of dyes. TIG welding typically lessens or eliminates this problem by preheating the aluminium around the weld area and the slower rate of cooling. 

Machining your parts before anodising

Before you deliver your parts to Graph

Scratches & Marks after machining

Anodising does not hide marks present on the surface of a part. Scratches and machine lines must be removed or polished out before anodising.  In some instances, etching will reduce the impact of these marks but results in a matt finish.  Etching will blend in fine lines but will have virtually no impact on more pronounced marks that may be present.