Sulphuric Acid Anodising

Notes for Designers – Jigging and Fixturing Considerations

We advise you to develope an understanding of the anodising process at the design stage.

Fixturing where possible is best near two extremities of the component.

Air is vigorously pumped to agitate the chemical fluid through the anodising process. Deep recesses, blind bores and holes can lead to air pockets being formed with resulting poor or inconsistent anodise layers being formed.

Whenever an aesthetic, consistent high quality level of finish is required, we recommend you discuss the processing issues of individual components with us. We also advise that on manufacturing drawings you specify jigging and fixturing points and the orientation of the part when in the process.

When larger parts are being anodised, good electrical contact is essential to avoid “blow” (melting) and damage to the component. Consider blind tapped holes at either end of the component so a bolt can be bottomed out in the hole and used for jig mounting.

Anodising is an “artisan” rather than a precision process. Although competent processors run a defined controlled process, factors that affect the results in the anodising process include:

• Variations in alloying content of aluminium from batch to batch
• The geometric shape and features of the part being anodised
• Where an individual component is situated within a tank
• How the component is orientated within the processing tank
• What other components are processed at the same time within the processing tank

If guaranteed tight tolerances are a prerequisite in aspects of the application (i.e. less the 30 microns total tolerance band) then designers need to consider post anodising finishing machining techniques such as lapping, grinding and honing. The significant uplift in the costs involved means this is only usual in motorsport, aerospace, defence and other highly validated demanding applications.

Standard anodising produces a “satin sheen” to the appearance. Preparing with a deeper etch “matts” the finish down.

Achieving consistent colour matching is problematic in anodising. Aluminium Alloy grades have a broad enough alloy content manufacturing tolerance that results in colour “tint” variation in different batches of material. 

Recently there has been developments with mixing sulphuric acid and either Tataric acid or Boric acid as a method of replacing the less environmentally acceptable Chromic acid solution.

Material Grades for Chromic Acid Anodising

1000 Series

1000 anodises well but is rarely used due to its mechanical properties

2000 Series

2000 can be anodised when handled with care

We have particular expertise in the handling of 2000 series (copper rich) alloys such as 2014A and 2024 where particular care is needed to avoid burning or melting of the alloy during the anodising process.

Corrosion resistance on 2000 series alloys is comparatively poor compared to 6000 series alloys. However, we work with a partner who has carefully developed an anodising process that ensures good corrosion resistance on 2000 series alloys.

Approximately 35 - 40% growth, 65-60% penetration

5000 Series

Approximately 50% growth, 50% penetration

6000 Series

Great for anodising. Approximately 50% growth, 50% penetration

7000 Series

Approximately 50% growth, 50% penetration

Chromic Acid Anodising Preparations, Colours & Variants

Deep etch to “matt” down the “satin sheen” appearance.

Bead blast or “brush finish” to give a uniform texture”. This “dulls” the surface. A chemical “Brightening” process is used to bring the surface “sheen” back.

Masking exacting engineering requirements now requires that specific areas of components should not be coated, although masking for anodising is particularly challenging. 

Where ever possible, in preference to masking if areas of a component need to be free from anodising it is recommended that components are left 100 microns up on tolerance and a post anodising machining processes is added to ensure an accurate, “clean line” visually pleasing finish is created.

Chromic Acid Anodising Colours

• “Natural” anodise (brings out the colour of the aluminium)
• “Silver” anodise - using a longer etch the colour takes a more silver appearance 
• Black sulphuric anodise is a popular variant to ensure a consistent colour and tone
• Gold
• Yellow
• Aluminium Orange
• Aluminium Bronze
• Pink
• Aluminium Fiery Red
• Aluminium Bordeaux
• Aluminium Violet
• Blue
• Aluminium Green
• Aluminium Grey
• Deep Black
• Olive Drab (for military applications)

Specialist anodisers can offer other special variants.