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  1. #1
    BobL is offline Member: Blue and white apron brigade
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    Default Solubility of mild steel in various rust removal acids.

    Two commonly used acids used as rust removers are citric acid and vinegar (acetic acid). I've use vinegar before for short periods (no more than one day) but have always been nervous about forgetting that I have something in the vinegar bath and that I would come back a month after to find my workpieces had corroded away.

    To test this I set up containers containing vinegar. citric acid and oxalic acid,. I use oxalic because I had some readily available and although not that common is supposedly an excellent rust converter.

    I placed pieces of similarly shaped pieces of mild steel of know weights into the containers of acid and every half day or so pulled the pieces out and dried them and measured their weights. The steel had been sanded back to raw steel ie no rust as all I wanted to determine was the dissolution rate of the metal

    The Acids were nominally 4% in strength but more appropriately I measure the pH (Acidity) of the solutions and they were as follows.
    Acetic 2.9
    Oxalic 1.0
    Citric 1.9
    Which are all consistent with their know acidic strengths. For those that are unaware, the lower the pH translates to higher acidic strength, so the strongest acid of this trio is the Oxalic acid.

    I could go on and on about procedure but will cut to the chase and the following are the % of the workpieces dissolved after 115 hours (~5 days) of exposure.
    Acetic 0.48%
    Oxalic 0.12%
    Citric 0.25%

    Extending that to one month translates to losses of
    Acetic 2.6%
    Oxalic 0.7%
    Citric 1.4%

    Interestingly the most acidic (oxalic) has the lowest losses and the most losses are by the weakest acid (vinegar). This is because these acids interact with metals more via a chelating reactions than by the usual acid/metal reactions. The vinegar and citric produced a clean dull grey surface but the oxalic covered the steel with a thin layer of pale yellow powder (ferrioxalate?) which I was able to mostly brush/wipe off the surface with a paper towel

    This is a very simple experiment because it does not take the surface area to volume ratio of the pieces into consideration. Also the dissolution rate could be affected by the thickness of any rust when used during rust removal .

    Whether these losses are OK depends on your application, If it's a chunky piece of steel it probably doesn't matter but if it's something more delicate with a lot of surface area then you may need to be a bit careful.

  2. #2
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    What results have you got for hydrachloric.

  3. #3
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by pipeclay View Post
    What results have you got for hydrachloric.
    I didn't test hydrochloric as it is a non-chelating acid and a 4% strength would have a pH of zero, so these would mean much faster dissolution of the steel.

    If the rust is variable in thickness and contains sand/dirt or has underlaying paint it can dissolve any exposed/raw steel before the rest of the thicker layers of rust are removed.

    My aim was to test acids that are "throw the pieces in a tub of acid and if you happen to forget about them for a couple of days it won't be too destructive" class.

    I have used HCl for a thick chunky piece of very rusty steel and found like scale removal about 30 minutes was more than enough time to remove the rust but it did significantly etch the rest of the steel. I would not recommend it for rust removal from thin sheet or threads.

    BTW the above tests are for mild steel only, things can vary depending on the type of steel as Grahame found out with his old bit of CI in molasses.

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    Did you measure the pH at the end of the test to determine how much of the acid was used.

  5. #5
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by alfclp View Post
    Did you measure the pH at the end of the test to determine how much of the acid was used.
    Good question.
    I only thought about measuring the pH after I finished the test and because there were <0.5% changes in the weight loss I assumed very little of the acid was consumed.
    However I was able to go back and measure the pH of the original stock solutions and the only acid that changed pH within the tolerance of my pH meter was the acetic acid and that started out at 2.4.
    The other two remain unchanged.

    I should add the ~weights of steel used was ~70g in about 600 mL of acid.

  6. #6
    BobL is offline Member: Blue and white apron brigade
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    Just for fun I measured HCl as I thought it would only take minutes to see a measurable effect.

    So same as all the others, 4% solution (pH of ~0.1) and after 360 minutes only 0.21% of the steel has dissolved, pH has not changed.
    This translates to an ~4% dissolution over the same time period (115 hours) as for the acids above
    I have to admit to being surprised and assumed more would dissolve more rapidly.
    No doubt if the concentration was increased we'd see a bit more action.

    I have now decided to move onto part two of the experiment and will let the pieces of steel rust and then compare the effectiveness of removal over time.

    This what the steel pieces look like after being in the acids and dried and will now let them rust for a week or two.

    Heres what they look like after their acid baths.
    The order from L to R is HCl, citric, oxalic and acetic.
    The thin yellow layer on the oxalic is discussed above.
    Start.jpg

  7. #7
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    Hi Bob,

    Won't the surface left by the oxalic acid act as a rust preventative ?
    Best Regards:
    Baron J.

  8. #8
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by BaronJ View Post
    Hi Bob,

    Won't the surface left by the oxalic acid act as a rust preventative ?
    Sure but that's also part of the testing in that I would like to know how quickly the steel corrodes after being exposed to these acids

    If it really hampers the rust production then I will remove it.

  9. #9
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    I seem to remember reading how Chlorine ions can remain in steel exposed to them, even after neutralisation and flushing with clean water, and these Cl ions accelerate the rusting process again. Would it be better to use another strong acid like sulphuric or nitric acids to avoid the supposedly troublesome Cl ions embedding themselves in the crystal lattice of the steel or cast iron?
    Thanks again BobL for taking the time to run your experiments, write them up and then disseminate your findings to the rest of us. I among many others I am certain, appreciate your considerable efforts.

  10. #10
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Ropetangler View Post
    I seem to remember reading how Chlorine ions can remain in steel exposed to them, even after neutralisation and flushing with clean water, and these Cl ions accelerate the rusting process again. Would it be better to use another strong acid like sulphuric or nitric acids to avoid the supposedly troublesome Cl ions embedding themselves in the crystal lattice of the steel or cast iron?
    Yep - the steel piece that was in the HCl bath is the first one that has obviously started to rust.
    Even though I have some on hand I decided not to test nitric or sulphuric as they are becoming increasingly harder to get.

    Thanks again BobL for taking the time to run your experiments, write them up and then disseminate your findings to the rest of us. I among many others I am certain, appreciate your considerable efforts.
    Cheers

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    As a side note Inhibited Hydrochloric acid has been used in industry (my connection is boiler cleaning a long time ago). A variation of this is Clarke's solution which is a inhibited HCl solution according to ASTM, made by adding 500 ml HCl to 500 ml water, and dissolve in 5 g of antimony trioxide, and 5 g of stannous chloride. There is further information available on the web, however I'm not sure how useful this is for the present tropic.

    Apologies if you believe it is inappropriate.

  12. #12
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Driftabout View Post
    As a side note Inhibited Hydrochloric acid has been used in industry (my connection is boiler cleaning a long time ago). A variation of this is Clarke's solution which is a inhibited HCl solution according to ASTM, made by adding 500 ml HCl to 500 ml water, and dissolve in 5 g of antimony trioxide, and 5 g of stannous chloride. There is further information available on the web, however I'm not sure how useful this is for the present tropic.

    Apologies if you believe it is inappropriate.
    Not at all and thanks for the info.

  13. #13
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    I'm with BobL on this drift about, thank you for your info, and if you or anyone else can explain a bit more about these inhibited solutions (which I assume are much less likely to etch the iron than their non inhibited counterparts) and how it all works, I for one would like to hear all about it.

  14. #14
    BobL is offline Member: Blue and white apron brigade
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    A couple of photos showing corrosion 12 and 24 hours after removal from acids and hanging them outside in the garden.

    NB the pieces were blown cry using compressed air.

    This morning the retic went off at 5am but these pieces wold have been a good 6ft above the highest water spray nevetheless they would have experienced some water vapour.

    The HCl is behaving as I expected although I would have thought maybe even more rusting would have taken place.
    It's hard to see but the piece in the Acetic acid does have a few specs of rust on it - but I would have expected more than this. In fact I have pulled some pieces of steel out of vinegar baths and they have developed visible rust within minutes
    I wonder if it is because I blew dry the pieces after taking them out of their acid bath?
    This has

    12and24hours.jpg

  15. #15
    BobL is offline Member: Blue and white apron brigade
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    4 days since the last photo and 5 days since the test pieces were removed from their acids and blown dry using a compressor.
    In the mean time the test pieces have been exposed to water from the retic twice and at least one rain shower.

    The HCl test piece has developed a bit more rust but nowhere near the amount I thought it would
    The Oxalic test strip is still rust free.
    The Acetic acid piece still has the same amount of rust (a few spots) it had 4 days ago
    The Citric acid test has over taken the Acetic acid and has about twice as much rust.

    The only thing I have done different compared to what I have done in the past is blown dry the work with a compressor.

    So much for my idea regarding getting these blighters to rust so I can test the rust removal capabilities of each acid.
    I am going to start again with raw mild steel, accelerate the rusting, and take it from there.

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