Sulphuric Acid Anodising

Your Guide to Sulphuric Acid Anodising



High quality precision machined components

Specialising in intricate, tight tolerance work

Extensive experience and high customer satisfaction

Penta Precision is able to offer sulphuric acid anodising as part of our one-stop machining service in the UK.

As well as our own machining expertise, we are able to access our network of approved quality suppliers for the finishing of your parts, saving you the time and effort and helping to streamline your supply chain.

Customers turn to us when they have had quality and delivery issues with their existing suppliers and need a reliable machining partner.

Why Choose Sulphuric Acid Anodising?


  • Excellent corrosion resistance
  • Electrically insulating
  • Decorative qualities
  • Available in a variety of colours
  • Scratch resistant
  • Cost effective
  • 5 to 25μm coating thickness
  • RoHS compliant option

Sulphuric Acid Anodising


Sulphuric Acid Anodising Process

As aluminium “oxidises” (the natural reaction between of air and aluminium) it produces a loose dry white powder, aluminium oxide. Sulphuric acid anodising is an electrolytic process using electricity (DC current) and a sulphuric acid electrolytic solution causing oxidation of the aluminium surface to form a thin “harder” film of aluminium oxide under controlled conditions.

The passing of the current liberates high volumes of Oxygen at the surface of the aluminium thus aiding controlled formation of the oxide layer. These processes do not produce a coating but are a conversion of the surface aluminium to aluminium oxide and are therefore metallurgically bonded.

The technique produces a tight but porous layer which, because it is porous, requires a sealing process to close the pores and prevent further natural attack on the aluminium.

Sealing is carried out by immersing the component in near boiling de-mineralised water, which if required can have small quantities of other chemicals, such as Nickel acetate or sodium dichromate to further enhance corrosion resistance.

The process is the most common protective treatment for aluminium. The anodised layer forms both within the surface substrate (penetration) as well as on top of the surface, growing the size of the component.

Before sealing, immersion in specially formulated dyes can be carried out, the open pores soak up the dye which will then be locked in by the sealing process, this can produce very attractive finishes but will depend on the thickness of the anodised film as the thinner the film, the less dye will be absorbed.



  • ISO 7599 (replaces BS 1615 and BS EN 12373-1)
  • Standard Thickness:
    • Def Stan 151/1 Standard - 5-7µm
    • Def Stan 03-25 Standard - 5-7µm
    • BS1615 Grade AA3 - 3µm
    • BS1615 Grade AA5 - 5µm
    • BS1615 Grade 10 - 10µm
    • BS1615 Grade 15 - 15µm
    • BS1615Grade 20 - 20µm
  • AMS 03_25 / Def Stan 03-25 (replaces Def 151 Type 1, NWS 1005/2-1 and NES 1005/2-1)
  • MIL-A-8625 Type 2 (MIL-A-8625 Type II)
  • MIL-STD-171 7.2.1 and 7.2.2
  • Def Stan 21-5 2-1
  • JP 213 Method 1
  • JP 217
  • TS112 F3 + D2
  • DS26.00 F3 + D2
  • P&G STD 41-013
  • ASTM B580 Types B, C, D, E and F
  • AMS 2471
  • “Type 2” anodising (North America)
Components, Applications & Industries
Here are some examples of the industries and components for which sulphuric acid anodising is common and appropriate. This is by no means an exhaustive list. Please do get in touch if you want to talk through your project.

Typical Components & Applications: Domestic cookware | Hand tools | Fascias for computers and televisions | Automotive trim | Base plates | Panels | Enclosures | Brackets | Heatsinks | Frames

Industry Examples: Electronics | Motorsport | Aerospace | Defence/military | Medical/healthcare | FMCG | General OEM manufacture & engineering

    Main Attributes:

    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.


    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.

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