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  1. #1
    Join Date
    Oct 2004
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    Default Strength of phosphoric acid?

    I want to buy some phosphoric acid for rust removal.
    The local hardware has small bottles of 85% concentration.
    A chemical supplier has 81%, which they say will be OK for my needs.
    Is the 4% difference anything to worry about?

    Jordan
    Last edited by nadroj; 23rd Nov 2011 at 01:37 PM. Reason: spelling

  2. #2
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    Default

    Hi Jordan,
    Well the rust remover I have in the shed is 35%. So assuming more is better, you're a long way in front of that.

    Stuart

  3. #3
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    Default no noticable difference

    4% will not be noticed for what you want to do
    Gfreg

  4. #4
    Join Date
    Nov 2008
    Location
    Perth WA
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    Default Maybe an alternative.

    Hello Jordan,

    When I acquired my little No. O mill it was in a fairly appalling condition as a result of exposure to water based coolant and general mistreatment. The non painted surfaces were all discoloured with rust, some of it quite heavy. My brother in law, a farmer, had encountered Corro Dip at some country show and swore by it. As a result of immersion of those parts in Corro Dip, I do too.

    index.html

    BT

  5. #5
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    Southern Highlands NSW
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    Default

    Thanks for the advice folks.
    Bob - the CorroDip looks like a quality product, and likely has phosphoric acid in it at a guess. I have some steel that I want to clean up before painting, and will give the 81% stuff a go.

    Jordan

  6. #6
    Join Date
    Nov 2008
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    Perth WA
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    Default

    Whatever you use Jordan, don't spill the stuff on concrete and leave it. I have a depression the size of a cereal dowl in the shed floor where some Corro Dip bled out over a few days from a useless plastic milk bottle..

    BT

  7. #7
    Join Date
    Oct 2007
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    Sydney
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    Default

    Quote Originally Posted by Stustoys View Post
    Hi Jordan,
    Well the rust remover I have in the shed is 35%. So assuming more is better, you're a long way in front of that.

    Stuart
    Yes mine is pretty much the same. I was using it earlier (3M "rust dissolver") and so checked the concentration. It was 30%.

    Pete

  8. #8
    Join Date
    Nov 2007
    Location
    melbourne australia
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    3,228

    Default

    Jordan,
    I'm not a chemist, but with acid I would not assume stronger is better. If you are going to use it for rust conversion I would dilute it to 30-35% if that's what concentration the commercial rust converting products use.

    Lower concentrations will be safer for you and anything you happen to spill the stuff on.

    BTW what purpose is the 85% phosphoric product sold for?

    Good luck with the project.

  9. #9
    Join Date
    Nov 2005
    Location
    Darkest NSW
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    42

    Default

    I've found the 30-35% "Rust Dissolver" to be fine for most purposes, especially when applied with fine steel wool.

    Just keep it away from any aluminium parts; it will remove anodising colour as well !

  10. #10
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    Default

    What would you thin it down with?
    Water just sounds wrong.

    Stuart

  11. #11
    BobL is offline Member: Blue and white apron brigade
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    Water is the go for dilution, plus a couple of drops of wetting agent per litre - dishwashing liquid is fine.

  12. #12
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    Nov 2005
    Location
    Darkest NSW
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    Default

    The wetting agent is a good idea, as the stuff doesn't wet a surface very evenly without.

    I use this stuff on machinery surfaces all the time - stinks a bit, but very effective indeed.

  13. #13
    BobL is offline Member: Blue and white apron brigade
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    If it too concentrated it will not dry properly and sits on the surface absorbing water and forms a horrible gooey messy. To speed up the drying I bake the derusted objects in a BBQ oven on low for an hour and it goes a sort of grey-white colour which can be readily painted over.

  14. #14
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    Default

    Quote Originally Posted by jack620 View Post
    BTW what purpose is the 85% phosphoric product sold for?
    Sorry, I didn't think to read the label, just saw it was 85%.
    It was in a Home Hardware shop in Mittagong NSW.

    A few good additional pieces of advice - thanks again fellas.

    I bought 35kg for just under $80.

    Jordan
    Last edited by nadroj; 24th Nov 2011 at 08:10 PM. Reason: addit

  15. #15
    BobL is offline Member: Blue and white apron brigade
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    Default

    Quote Originally Posted by nadroj View Post
    I bought 35kg for just under $80.
    Wow - that is a lota rust!

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