Immediately after anodising the anodic film is still fresh, uncontaminated and hygroscopic. It is at this point that the properties of the anodic film may be modified by further chemical treatment. The most important of these are: sealing - a process which effectively closes the pores of the anodic film , colouring - where species are encouraged to fill the pores changing the film colour, and our ano-lube PTFE process, which gives a low co-efficient of friction surface.
COLOURING-DYEING
Anodic films readily absorb certain dyes. A large range of colours are available although most jobbing shops hold only a few. Ideally for the best appearance films should not he hard, as increased film density and smaller pore sizes associated with hard films reduce dye uptake. Additionally hard films are naturally darker in colour than softer sulphuric films often masking any attempt at colouring. Hard anodised films may be dyed but success is very alloy dependant.
There are several different anodising colouring systems in the market at the moment. Simple immersion in a bath of organic dye or in-organic dye being the most common. Other processes include integral colour anodising, where modified electrolytes give rise to very high quality coloured films, usually bronzes or black without the need for a dye vat after anodising and electrolytic dying where pigments based in cobalt, nickel or tin can be plated into the pores of the film giving rise to colours based on relative absorption / reflection of incident wavelengths or on interference colours.
EXAMPLES OF COLOURED ANODISING
Whilst the organic immersion dyes are the cheapest options there is some question about light fastness especially
when exposed to sunlight.
The in-organic systems which include the plated in metal complexes seem to have very good light fastness and also
thermal stability as the organic dyes tend to break down much over 150°C.
Colouring must usually be followed by sealing.
SEALING
Freshly formed anodic film formed in previously described processes are porous. In this condition the film is at its hardest but is also in a reactive condition (this is the stage at which dyeing is carried out). There are three alternatives at this juncture depending upon the objective.
A: For corrosion resistance and / or to prevent dyes leaching out
This applies to both hard and sulphuric films. Sealing in hot, dichromate solutions gives the optimum corrosion resistance leaving within the film a strong oxidising agent which helps preserve the substrate from attack. The resulting colour is yellow / green on sulphuric acid anodised film and similar to "Nato Green" on thick hard films.
Sealing in hot water avoids the colour changes caused by dichromate solution but with some loss of corrosion resistance.
Both of the above processes rely on hydration of the anodic film which increases the volume of the oxide thereby closing up the pores. The temperature of sealing is critical as different hydration states are produced at different temperatures with varying sealing quality. The seal bath must at 960° Centigrade minimum to get the best sealing quality.
Sealing in a 'cold' system favoured in some commercial applications using nickel-based solutions is said to be equivalent to 'hot water' sealing. This system uses a chemical reaction to ‘bung up’ the pores. Our in house tests prove this to be correct in most cases.
B: Maximum Hardness or Wear Resistance
Achieved by avoiding the hot sealing processes. Use cold sealing to prevent finger marking/staining etc.
C: Other Sealing Mediums May be Used.
For example using fatty acid solutions to increase electrical resistance. The anodic film is said to be at its hardest before sealing but we believe that the application of PTFE dispersions or oil etc. does give a degree of sealing with attendant gains to wear resistance.
