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  1. #1
    Join Date
    Mar 2008
    Location
    Nth Qld
    Posts
    68

    Default Acid bath strength

    I wish to make an acid bath to clean an old cast iron vice of old paint and burn marks from where the oxy torch has been applied to loosen screws holding the jaws.

    I have some hydrochloric pool used for the swimming pool.

    what sort of strength should I make this bath? eg 1 litre of water, 200ml acid ?

    all suggestions apprecitaed,

    regards,
    Jill

  2. #2
    Join Date
    Dec 2005
    Location
    Canberra
    Posts
    490

    Default

    The amount you suggested sounds probably a little strong - try 100 ml to a litre since it doesn't attack you skin quite as quickly if you splash it on; however I've used it at every strength from neat to 'a good slurp in a bucket'.

    Note that the acid etched surface will rapidly pick up fresh rust (within seconds!) once out of the acid; so make sure you rinse it underwater and then spray on WD-40 or oil it as soon as it comes out.

    When I'm being fussy, its hydrochloric acid, then rinse well, leave to dry (air compressor comes in handy for blow-drying it!) and then hit the ever so slightly rusty part with phosphoric acid pre-paint stuff, then paint.

  3. #3
    Join Date
    Mar 2008
    Location
    Nth Qld
    Posts
    68

    Default

    Hi Master Splinter. Many thanks for your advice. You are not wrong about the fresh rust appearing right away, but I made a mistake in not doing the phosphric acid trick before applying the primer. I hadn't thought of using the air compressor to blow-dry it faster.

    The 10:1 acid mix was OK, but it did not lift the old paint or the carbon / grease on the old vice
    So I hit it with degreasol and rags, then with paint stripper, and then with a high pressure water jet, and it was looking much better. Then I put it in the acid bath, and wow, millions of little bubbles started rising pretty quickly. Left it in for 10 minutes, then washed it down, and the whole vice immediately started to get this yellowy stain all over it - some form of rust? It came off with a wire brush, so then got some metal primer on it pretty quick.

    Took a few hours, this exercise, especially masking the running surfaces and jaws, but it looks good

    kind regards,
    Jill

  4. #4
    Join Date
    Jan 2008
    Location
    Melbourne
    Posts
    54

    Default

    wouldn't caustic soda be the better option? (in retrospect) You would get all the grease and paint off, but without the rust - and less fumy. (Still dangerous stuff though)

  5. #5
    Join Date
    Dec 2005
    Location
    Canberra
    Posts
    490

    Default

    Caustic would be an excellent first treatment; it'll strip paint and grease (and everything else vaguely organic) but won't really attack the metal...I use it on the odd stainless steel 'whoops, that got left on the stove a wee bit too long' saucepan.

    Boil it in caustic if you are really keen, then rinse and drop it in the acid to get rid of the rust.

    And yes, the yellow stain is rust; the metal has been acid etched, so there is a much bigger (and cleaner) surface full of iron atoms just waiting to donate electrons to the first passing water molecule and turn themselves back into Fe2O3.

  6. #6
    Join Date
    Mar 2008
    Location
    Nth Qld
    Posts
    68

    Default

    it has beena great learning experience. Many thanks for your help and advice

    kind regards,
    jill

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