DIY Home Aluminum Anodizing


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  • Hard Anodizing / Hard Coat Anodize

    On your travels around the internet you might come across the term "Hard Anodizing" or Hard Coat Anodizing. I for one was confused. Surely the whole point of anodising is to make the surface of the aluminium hard? So I presumed for a long time that "hard anodising" was just another term to discribe the process of anodising aluminium.

    Recently I learnt that this is not the case. There are infact various types of anodising.

    • Type I - this is a specialist anodising using chromic or phosphoric acid instead of sulphuric acid,.
    • Type II - this is the anodising that we are familiar with, using sulfuric acid and coloured dyes.
    • Type III - this is hard coat anodization, or Hard Anodising.

    Hard Anodising or Type III anodising uses sulphuric acid again, but a weaker solution. The thickness of the anodising coat or 2 to 4 times that of normal type II anodising. Type 3 is much harder to die because the pores in the anodised layer are much smaller than in Type II so the die finds it more difficult to penetrate. It often has a dark grey or nearly black colour.

    Hard anodising is performed with a weaker solution of sulfuric acid, and a lower temperature acid bath. Normally for Type II anodising we have our acid bath around 15-20 degrees. The temperature often rises during the anodising process. For type III hard anodizing, the temperature must be only 2-4C. The acid bath must be constantly mixed, otherwise a warm layer of acid will build up around the parts. The electric current will be much higher, perhaps 20 amps per sq foot. This requires bath mixing machines, cooling equipment and I dunno what else.

    I guess the principle is that with Type II anodising, once the layer gets to a certain thickness, the reaction is dissolving the anodised layer as fast as it is being made, by keeping the bath very cold, a thicker layer can form.

    I have not tried hard coat anodising, and, to be honest, I don't think I will, as there is not much point for my applications and it sounds like a lot of effort.


    Contact me on tomh at tomh@tomhow.me.uk

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