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Thread: Percent Phosphoric Acid for Rust Removal

  1. #1
    Join Date
    Sep 2002
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    Default Percent Phosphoric Acid for Rust Removal

    Hello,
    I have a jug of 85 percent phosphoric acid.
    I also have a rusty 15x42.9 S-Beam, 4 ft in length.

    What percent solution would be best for de-rusting.
    This will be brushed on.

    Is there a simple way to figure how much water is needed
    to change the solution.
    Last edited by Nicholas; 07-26-2019 at 06:44 PM. Reason: changed 44 to 42.9

  2. #2
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    Quote Originally Posted by Nicholas View Post
    Hello,
    I have a jug of 85 percent phosphoric acid.
    I also have a rusty 15x44 S-Beam, 4 ft in length.

    What percent solution would be best for de-rusting.
    This will be brushed on.

    Is there a simple way to figure how much water is needed
    to change the solution.
    I don't remember the kind of acid, but it was in a plastic jug bought at a pool supply store. It leaches out of the plastic and caused flash rusting on everything in the garage. Don't store it in the workshop if it's the same stuff

  3. #3
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    Quote Originally Posted by RB211 View Post
    I don't remember the kind of acid, but it was in a plastic jug bought at a pool supply store. It leaches out of the plastic and caused flash rusting on everything in the garage. Don't store it in the workshop if it's the same stuff
    That would be muriatic acid (dilute hydrocloric acid). Yep, keep it out of the shop! I keep mine in the shed outside with the paint and other chemicals.

    Phosphoric is much more benign - safe to handle and no nasty fumes.

    Nicholas, I can't say what the best dilution would be - but the way to figure out the amount of water is simple:

    Let's say you have 10 ounces of your 85% phosphoric. That means that 8.5 oz. of the 10 is acid. If you want, say, 20% dilution you add your acid to enough water to make a total of 42.5 oz. - that's 8.5/0.2 - the water required is 42.5 - 8.5 or 34 oz.

    Always add acid to water, never water to acid. That's very important with strong acids like HCl, less so with phosphoric, but a rule to follow faithfully anyway.

    -js
    There are no stupid questions. But there are lots of stupid answers. This is the internet.

  4. #4
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    Sep 2002
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    Sorry about not replying sooner, I was out in the shop spending some quality time with my g&e shaper, turning 2 in square blocks into 1-3/4 inch square blocks.

    RB211,
    I used to have some muriatic acid, after using it up I decided to go with the less lethal phosphoric, for the reasons you stated. It did work great tho.

    Jim Stewart,
    I will try the 20 percent, since you described how to dilute it.

    Thanks Guys.

  5. #5
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    Quote Originally Posted by Nicholas View Post
    Sorry about not replying sooner, I was out in the shop spending some quality time with my g&e shaper, turning 2 in square blocks into 1-3/4 inch square blocks.

    RB211,
    I used to have some muriatic acid, after using it up I decided to go with the less lethal phosphoric, for the reasons you stated. It did work great tho.

    Jim Stewart,
    I will try the 20 percent, since you described how to dilute it.

    Thanks Guys.
    Mind, I don't know the best dilution - but I think I've heard the 20% suggested. I just use the dilution that's specified on the bottle.

    PAGING J TIERS, WHITE COURTESY PHONE PLEASE.
    (Jerry has used lots of phosphoric for derusting, I'm sure he has a good number.)

    -js
    There are no stupid questions. But there are lots of stupid answers. This is the internet.

  6. #6
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    If you are talking % and then diluting to a specific value first figure out if the original is specified by by volume or weight... I haven't looked at Phosphoric, but know (I have to work with it) there is a big difference with Sulfuric.

    Oh... I just use it straight from Home Depot or your local concrete store. Mine say 30%, whatever that really means. 85% no matter how specified should be handled with care.

    Muriatic? Great for instantly removing galvanizing from steel, but not great in general and as mentioned above, keep it outside of your shop unless you really like rusted tools.
    Last edited by lakeside53; 07-26-2019 at 11:15 PM.

  7. #7
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    I buy phosphoric at about 40% and dilute it 1 or 2 to 1, mainly to make it go farther.

    The problem with brushing it on is that the volume of acid in the vicinity of the rust will be very small, and it will get spent quickly. Some sort of recirculating system would make it go better.

    Ed
    Last edited by ed_h; 07-27-2019 at 01:09 AM.
    For just a little more, you can do it yourself!

  8. #8
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    Jan 2006
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    If you want to store muriatic in the shop, dilute it down to 10% or so. It is still strong enough to de-ganlvanize and de-rust stuff, but won't off gas enough to hurt surrounding surfaces. I've always have a jug of 10% on the shelf in the shop.

    Ed
    For just a little more, you can do it yourself!

  9. #9

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    I've had plenty good results with 45% so if you can get 85% you can dilute it down and it should last you a bit longer and/or give you more volume to immerse things.

    Edit - Phosphoric, that is. I've never had occasion to touch muriatic.

  10. #10
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    Remove the lid, apply.
    http://www.ospho.com/

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