Sulphuric Acid Anodising

Main Attributes of Sulphuric Acid Anodising

Although the process is controlled, anodising needs to be regarded as an “artisan” process and not a high precision process. Sulphuric acid anodising leaves a clear (“natural”) coloured finish, which can be subsequently dyed. This process occurs naturally at a much slower rate for aluminium exposed to oxygen in the air. Clear coatings (natural colour) are normally 5 to 15 microns and dyed coatings are normally 10 to 25 microns.

A major benefit of anodising is that because the oxide film is electrically resistant, it is distributed across the component very evenly. This happens because when the film starts to grow, that area becomes more resistive until all other areas on the component have achieved the same thickness thus equalising the relative resistivity and ensuring even growth over the whole component.

Sulphuric acid anodising is generally used where a decorative and corrosion resistant finish is required.

As anodising does not involve the coating of the aluminium with a different metal (or other man made coating), the process will not hide any surface imperfections or blemishes in the aluminium substrate. In fact, the vigorous nature of the process will generally degrade the surface finish of the aluminium – this should be taken into account when considering the quality of the surface material to be treated if a highly decorative finish is required.

The electrolytic process in anodising requires good electrical contact point at all times during the treatment. This means that a very small set of jig marks will always appear after treatment. It is important that we are made aware of any specific jigging requirements where certain surfaces must be free from jig marks for decorative purposes or for critical bores and sealing surfaces.

Anodising is a cost effective process to deliver the requirements for components used in general industrial OEM equipment. Where a consistent high quality aesthetic finish is required then care needs to be taken to partner with a company capable of delivering the finish. Processing costs then start to rise.

Sulphuric acid anodising is also used to apply anodic films on Titanium. These films are relatively thin but prevent natural oxidation, provide an extra "Key" for subsequent coating and can also be used to colour the surface of the component.

Corrosion Resistance

Anodised aluminium shows high corrosion resistance in aggressive environments. Anodising will be vulnerable to alkalis and some acids - we will be able to advise you on this and are happy to provide compatibility testing. All sealing methods provide good corrosion resistance but we've found that the best corrosion resistance is achieved with dichromate sealing.

Sealing

Anodic films are naturally porous which allows them to be dyed. Most sulphuric anodising is sealed and this is our default process. We offer five sealing methods: cold sealing (nickel fluoride sealing), hot water sealing (boiling water sealing), dichromate sealing, nickel acetate sealing and Surtec 650 sealing.

For the majority of applications, the most economic and suitable sealing method is either cold sealing or hot water sealing. Dichromate sealing is heavily used in the aerospace and defence industries, but has a distinctive yellow/green colouration, which makes it unsuitable for parts where coating is cosmetic.

Good sealing is crucial to corrosion resistance and is necessary for light-fastness on dyed coatings. Surtec 650 sealing is only suitable for non-dyed parts and behaves somewhat differently to the conventional seals.

A minority of sulphuric anodising is left unsealed. This is usually when it is going to be painted or bonded. Leaving the anodising unsealed provides a good key for painting.

Resistance to Scratching and Abrasion

Aluminium oxide has an improved hardness compared to bare aluminium which makes it resistant to scratching and abrasion. Due to the thinner film nature of Sulphuric Acid Anodising, with constant rubbing or abrasion the film will wear away to reveal the bare substrate metal. Where wear resistance is a serious concern, hard anodising, which creates a much thicker, harder coating should usually be preferred.

Notes for Designers - Jigging & 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.

Further Reading & Resources

Surface Engineering Association - www.sea.org.uk

Institute of Material Finishing - www.materialsfinishing.org