Mailbag: How Not to Clean a Rusty Trunk

June 14th, 2011

Larry the Crystal Guy e-mailed me the following discussion from a science forum.  It’s ancient, but interesting.  The exchange begins with an inquiry from a citizen with a rust problem.  I’ve reproduced the authors’ typos and larger morsels of stupidity unedited…

I have recently acquired a large trunk for sotring things in, however the metal on the edges and corners has gone very rusty, through being sat in a cupboard for a long time.  Anyone have any ideas, or know any good ways for how I can get rid of it?  Cheers!
After one board member suggested using vinegar and a second—predictably—told the original poster to search Google, in stepped user lightarrow, a “Hero Member” of the forum:
The best chemical I know to dissolve rust is hydrofluoric acid HF. Here in italy is sold in dilute acqueous solution (~10%) to use with textiles. On the other hand, it’s better not to use more concentrated solutions or there is the risk to attack the metal too.  If you want to use it with a piece of metal, you should find a way to make the solution in a gel or sirup form, otherwise the solution slips away and/or dries too quickly. Once, I dissolved a large amount of sugar to that solution for this purpose (but there will surely be better ways than this one); of course the reactivity is lower.
Yes, please, don’t use anything more concentrated because it might damage the metal.  And just ignore the mild burning sensation—that just means it’s working.  Ugh.  The next day and several posts later, daveshorts (another “Hero Member”) weighed in with safety concerns:
Don’t go anywhere near HF, working in a materials lab for 5 years, this was the only chemical people were actually scared of – a suggested damage limitation measure invovles an axe! If you get it on your skin it will get rapidly into your flesh and then proceed to break down your bones…
Affronted, lightarrow defended his advice:
I wrote: “dilute acqueous solution (~10%)”. It’s sold in most supermarkets!  I have tried oxalic acid, phosphoric acid and acetic acid (the active chemical in vinegar) and only HF works, unless the rust is thin or in a “good” composition/physical structure. Infact, rust is not a specific compound, it can vary from Fe(OH)3 to FeOOH to Fe2O3..ecc. The first form, Fe(OH)3, is the easiest to remove chemically. But try to dissolve Fe2O3 with vinegar or oxalic acid.   (I have tried).
I’ve been to a supermarket in Italy, but I don’t recall seeing 10% HF.  Maybe I missed it behind the gnocchi?
Seriously, kiddies…don’t try this at home.  Hydrofluoric acid can do bad, bad things to you (warning: gross).  I would not characterize 10% HF as “dilute”, either.

9 Responses to “Mailbag: How Not to Clean a Rusty Trunk”

  1. excimer Says:

    They sell dilute HF solutions (I don’t remember the concentration) at supermarkets in the US. Which makes my head asplode a little.

  2. Neil Says:

    I don’t know if you guys saw it, but there was a link doing the rounds on Twitter a while ago about someone who made his own electron microscope. Anyway, I was browsing that blog and discovered to my horror that this fella had bought some 50% HF to etch a ZrO2 ball!

    His attitude in the comments when people warn him about HF dangers is…worrying.

  3. azmanam Says:

    Thanks for the gross warning. Yikes. HF burns over 30% of his body and he recovered?

    I found HF was a good reagent for a tandem tris-silyl ether deprodection/spiroketal cyclization. Working on small scale with only a few drops of the con’c reagent, I still double or triple gloved, and kept a homemade calcium gluconate paste solution on the bench next to me (homemade b/c we didn’t have any commercial gel in our lab). Reagent worked really well – but scared the crap out of me.

  4. qvxb Says:

    Whink Rust Stain Remover contains 1.5-3.5 weight percent HF. The MSDS is here:

  5. ZAL Says:

    Well, I’m from Italy and I most definitely have never seen 10% HF in supermarkets. On the other hand, I have never actually looked for it…

  6. Hap Says:

    Derek Lowe had this a while ago (or his alter ego, or someone else that he posted with)

    Milkshake also commented about a coworker who decided to impress his boss with the effects of HF.

    I think we’ll just assume that lightarrow had some weapons-grade stupidity that he needed to get rid of on the open market. Doesn’t he know that the prices on things like that have come down lately?,1152/

  7. Friday chemical safety round-up | The Safety Zone Says:

    [...] Houshold chemistry by ChemBark: How not to clean a rusty trunk [...]

  8. wolfie Says:

    you’ll never get rid of me

  9. Chemical Products Says:

    This bid made me cringe “then proceed to break down your bones”. The stupidity of some people is amazing. Just throw away the trunk.

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