What is Hard Anodising?
Hard anodising is very similar to sulphuric anodising, but produces a thicker oxide layer and therefore increased corrosion resistance and increased wear resistance compared to standard sulphuric anodising. A thickness of 20µm to 70µm can usually be achieved with this process, with thickness up to 100µm possible on some aluminium alloys. Hard anodizing is sometimes referred to as hardcoat or Type 3 anodizing in North America.
By tight control of process parameters and taking into account part dimensions and geometry, we can achieve precise control over thickness whilst maintaining uniformity of finish. Metal Finishings Ltd ensures the performance of the hard anodised coatings we produce by carrying out Taber wear testing, neutral salt spray testing and verification of coating mass. As specialist hard anodisers we are able to achieve high coating thicknesses and wear resistance, even on notoriously difficult alloys such as the 2000 series (high copper) eg. HE15, L168.
Hard anodising can be coloured by dyeing the anodic film. Hard anodising colours (except clear and black) are much less appealing than those of sulphuric anodising because the additional thickness gives a more dull, grey finish. Our standard colours are:
- Clear/natural/silver coloured (no dye)
- Black anodising
- Red anodising
- Blue anodising
- Purple anodising
- ISO 10074 (replaces Def Stan 03-26, NES1005/2-3, Def 151 Type 3 and BS 5599)
- MIL-PRF-8625 Type III (formerly MIL-A-8625 Type III)
- Def Stan 21-5 2-3
- ASTM B580 Type A
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.
Hard anodised aluminium shows high corrosion resistance in aggressive environments. 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.
Hard anodising is used for improved wear resistance. The best wear resistance is achieved at the higher thickness and by leaving the coating unsealed. Its wear resistance can be further improved by coating with PTFE.
Anodic films are naturally porous, this allows them to be dyed. Hard anodising is common in both the sealed and unsealed states.
We offer three sealing methods: cold sealing (nickel sealing), hot water sealing (boiling water sealing) and dichromate 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.
Unsealed coatings are generally preferred when the hard anodising is for wear resistance. A suitable lubricant could be used to regain some of the corrosion resistance.
Except components that are dichromate sealed, all of our hard anodising is RoHS compliant.