What is Anodising?

Sulphuric anodising is an electrolytic treatment for aluminium that produces a coating of aluminium oxide. This coating exhibits excellent corrosion resistance and can be dyed in a variety of colours. The coating is available up to around 25 microns in thickness (beyond this thickness, hard anodising is required). Clear (natural colour) coatings are typically 5 to 15 microns and dyed (coloured) coatings are typically 10 to 25 microns. Sulphuric acid anodising is sometimes referred to as Type 2 anodizing in North America.

Colour Anodising

Anodising can be coloured by dyeing the anodic film. Our standard colours are:

  • Clear/natural/silver coloured (no dye) anodising
  • Black anodising
  • Red anodising
  • Blue anodising
  • Purple anodising

By varying the dyeing process, we are able to achieve a wide range of shades of the above colours (for example, we can do an electric blue shade and dark blue shade). We can also produce a matt or satin finish if required. Please enquire for more details, we are happy to supply samples. We can use RAL numbers and other colour standards to home in on the colour that you require, but we prefer to work from an agreed anodised aluminium sample when in production. We can introduce new colours if required.

Specifications Offered

We can anodize to the following specifications:

  • ISO 7599 (replaces BS 1615 and BS EN 12373-1)
  • AMS 03_25 / Def Stan 03-25 (replaces Def 151 Type 1, NWS 1005/2-1 and NES 1005/2-1)
  • MIL-PRF-8625 Type 2 (formerly 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

We can work to many other specifications, but the above are our most popular. Please contact us with your requirements.

Information for Designers

Thickness and Growth

Anodising is an electrolytic conversion coating, it transforms aluminium on the surface of components into aluminium oxide. Since aluminium converts rather than deposits, some of the thickness of the coating is ingress into the material and some of the thickness is growth out of the material. A good rule of thumb is that 50% of the anodic film is below the original surface and 50% is above. So a part that is anodised with a coating thickness of 10µm per surface would only have about 5µm of growth per surface.

You can use our anodising growth calculator to estimate the size of a feature after anodising. We recommend that you let us check your calculations.

Standards such as ISO 7599 make use of AA designations to specify coating thickness: AA5, AA10, AA15, AA20, AA25. Based on these it establishes minimum thicknesses. Most customers find it easier to simply specify a thickness range.

Corrosion Resistance

Anodised aluminium shows high corrosion resistance in aggressive environments. Our dichromate sealed anodising exceeds 500 hours of neutral salt spray testing to the requirements of MIL-PRF-8625. Anodising will be vulnerable to alkalis and some acids, our technical specialists will be able to advise you on this and we are happy to provide compatibility testing. We have found best corrosion resistance with dichromate sealing, but all sealing methods provide good corrosion resistance.


Anodic films are naturally porous, this 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 most applications, the most economic and most 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, this makes it resistant to scratching and abrasion. Where wear resistance is a serious concern, hard anodising should usually be preferred. For situations where low friction is desirable, we can coat the anodising with PTFE.

Alloy Selection

Please see our article on the best alloys to use for anodising and do not hesitate to contact us for more information.

RoHS and REACH Compliance

Except components that are dichromate sealed, all of our sulphuric anodising is RoHS compliant. All of our anodising is REACH compliant.


Metal Finishings Ltd are anodisers to the aerospace industry.