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  • Nitric Acid: The Chemical for Preparing and Desmutting Aluminium

    The Chemical formula for Nitric Acid

    Nitric Acid is HNO3.

    About Nitric Acid

    Nitric Acid is really horrible stuff. In my opinion it is much nastier than sulfuric acid. The bottles I get are a sort of light yellow wee colour. You unscrew the cap, and a visible fume of white vapour comes out the top. Nasty. The yellow colour is due to the oxides of nitrogen forming over time. Really strong nitric acid (over 86%) is known as fuming nitric acid. This can either be white fuming or red fuming depending on how much nitrogen dioxide is present.

    Nitric acid is used as a lab reagent, and more famously in the making of exposives like TNT. It is also popular in making fertiliser, but that is far to dull to go into.

    I'm always careful with this acid, and keep a lot of alkaline nearby for emergencies.

    Where to buy Nitric Acid

    Nitric acid is not easy to buy. I use an online shop in the uk to buy bottles of 70% solution. They send this stuff through the post? Madness, but I love it!

    Nitric acid and aluminium anodising: Desmutting

    The purpose of nitric acid in the aluminium anodising process is not immediately aparent. Nitric acid is used to prepare the surface of the aluminium prior to the anodising itself.

    The first stage in preparing the aluminium for anodising is to clean off the existing oxide layer in a bath of caustic soda. This is very exciting and smelly: lots of frothing and steaming etc. Proper chemistry. Not of this mucking about with pathetic bits of litmus paper.

    After the caustic soda bath, the aluminium is washed. At this point, depending on the alloy of aluminium used, the surface appears to be covered in smut. Aluminium is not normally used in the pure form. It is an alloy of many different metals. Although the caustic soda removes the aluminium and aluminium oxide, it does not remove the other metals, and these remain on the surface.

    Enter the Nitric acid. All these other metals will be dissolved by nitric acid.. but not the aluminium. Therefore, after the caustic soda stage, the part is soaked in a bath of made of half water and half 70% nitric acid. This cleans up the part lovely.

    Again, the part is washed in water, and it is ready for anodising.

    Demo of nitric acid to clean and desmut aluminium

    A bit of scrap aluminium tube which has seen long service.

    Now it has been cleaned up in the caustic soda bath, see the black smut on the surface

    After dipping in nitric acid: Much cleaner!

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