Hard Anodising

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 0^oC 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.

Typical 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 water environments.
• Moving parts in machinery for packaging systems and general engineering applications.

Choice Of Alloys

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.

Restrictions

There are a few considerations to be aware of when considering hard anodising. If these apply then maybe semi-hard anodising would make a suitable alternative.

• Hard anodising may impair the surface finish, so surfaces to be treated should have a finish superior to that required on the processed item.
• Not suitable for alloys containing more than 4% of copper or more than 7% silicon.
• Edges of items to be hard anodised need to be radiused.
• Colour 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 series alloys.
• Blind holes will receive a much lower coating thickness which decreases with depth.

Maximum Size Criteria

The table below outlines the maximum size criteria for hard anodising:

Dimension	Clear	Black
Length	1700mm	1200mm
Width	800mm	600mm
Depth	800mm	800mm

Available Standards

We predominantly work to 2 specific standards as detailed below although we are happy to work to any specification provided.

• DEF STAN 03-26 (formerly DEF 151 Type 3)
• MIL-A-8625 Type 3

PTFE Coating

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.

Sealing

The following sealing options are available for hard anodised components:

• Cold Seal
• Hot Water Seal
• Nickel Acetate Seal – for increased corrosion protection
• Sodium Dichromate – produces a dark green finish and suitable for marine applications
• Unsealed – to give optimum adhesion for further treatments (eg paint)
  

Jigs 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.

We 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 purposes.

Colour Dyes

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.