Hard anodising is primarily a functional surface finish applied
where aluminium components require increased wear resistance
and surface hardness. A 25-50µm thick hard anodised
film will typically have a Vickers rating of around 1100.
Surface coatings are achievable up to 75µm thick for
some alloys, however optimum hardness is usually achieved
with a film thickness of approximately 40µm. Hard anodising
also provides a high degree of electrical insulation.
The hard anodising process uses sulphuric acid (at a higher
concentration than for normal anodising) but it is carried
out at a solution temperature close to 0oC with
a significantly higher voltage being applied. We have specialist
chilling equipment and a modern pulse rectifier to achieve
these conditions. The resulting anodic film has more uniform
crystalline structure which gives the increased hardness.
applications for hard anodised components are:
- box and braking equipment for the motor sport sector.
- Hydraulic systems, valves, pumps and pulleys.
- Deep sea components which are subject to harsh salt
- Moving parts in machinery for packaging systems
and general engineering applications.
The colour and appearance of hard anodised
components varies significantly depending on the alloy used.
For example, hard anodised 6000 series alloys will have a
dark grey colour in a natural finish, whereas 2000 series
alloys will give a light sandy colour finish. These differing
natural coating colours will clearly influence the final colour
of any components which require dyeing after hard anodising
and it is usually appropriate for us to provide a colour sample
for approval before production batch sizes are processed.
There are a few considerations to be aware of when considering
hard anodising. If these apply then maybe semi-hard
would make a suitable alternative.
anodising may impair the surface finish, so surfaces
to be treated should have a finish superior to that
required on the processed item.
suitable for alloys containing more than 4% of copper
or more than 7% silicon.
of items to be hard anodised need to be radiused.
matching is more difficult with a hard anodised
component. Colours will tend to be darker and will
vary widely between alloys such as 2000 and 6000
holes will receive a much lower coating thickness
which decreases with depth.
The table below outlines the maximum size criteria for hard
We predominantly work to 2 specific standards as detailed
below although we are happy to work to any specification provided.
STAN 03-26 (formerly DEF 151 Type 3)
Anodised parts can be treated by immersion into a highly concentrated PTFE dispersion. This process will produce an abrasion resistant coating with a high degree of lubricity.
The following sealing options are available for hard anodised
- Cold Seal
- Hot Water Seal
- Nickel Acetate Seal – for increased
Dichromate – produces a dark green finish and
suitable for marine applications
- Unsealed – to give optimum adhesion
for further treatments (eg paint)
and Component Preparation
All components must be held rigidly on jigs
to maintain the electrical contact during the anodising process.
This is even more important for hard anodising as higher voltages
are applied to the parts. It is usually preferable for jigs
to be attached to features such as threaded holes to give
the best contact. In some cases it may be appropriate for
additional holes to be made purely for jigging purposes. This
can also prevent the need for jig marks appearing on decorative
or critical surfaces. It is always recommended that jigging
requirements be shown on drawings or discussed with us in
advance of processing. It is important that any non-aluminium
parts (such as helicoils or inserts) are removed before anodising
to avoid melting of the components.
can carry out basic masking of components to prevent individual
surfaces or holes from being anodised. This is usually performed
to maintain areas of electrical conductivity for earthing
Colour options are available with hard anodising for an additional
charge. The effectiveness of dyeing will be affected by the
choice of alloy as noted above and generally we recommend
only a limited selection of dye colours for hard anodised
parts. We have considerable experience in the use of black
and blue dyes with hard coatings. It should be noted that
no guarantee can be made in respect of long term colour matching
over larger production batches, however we are usually able
to keep consistency of colour with a pre-agreed tolerance
of shading based on initial colour samples.