I'm writing a novel set in the mid-nineteenth century. One of my characters needs to cause an iron padlock to rust rapidly, ideally within a matter of hours.

The lock doesn't need to rust away completely - the goal is just to make it noisey and annoying so that it can't be opened in silence.

Is this a realistic goal? If so, what methods could someone use to cause rapid rusting? Bonus points for suggesting period-appropriate materials.

Update: Based on the answer and comments below, it sounds a period appropriate approach would be to mix fine copper filings with saltwater and apply it to the lock.

I've read that bleach is an oxidizing agent which was available in the 19th century. Would adding bleach to this mixture increase the rate of rusting without creating an overly dangerous mixture?

Update: I conducted an experiment in my garage. Iodine worked pretty well. Experiment Video

A few months later the iodine had eaten through the lid of the jar I tested it in:

Iodine Ate Through My Jar

  • $\begingroup$ I've removed the oxidizing agent thing because I'm afraid they would dissolve the rust... $\endgroup$
    – DHMO
    Sep 18 '16 at 16:31
  • $\begingroup$ @user34388 and others - I am not a chemist. What are your thoughts on the risks of trying this experiment? I find the oxidizing agent compelling and it sounds like that might be necessary to reach the goal of rust within hours. $\endgroup$
    – T.D. Smith
    Sep 18 '16 at 16:34
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    $\begingroup$ This experiment is not harmful... as long as you don't use oxidizing agents that are too dangerous such as (should I be listing them?) acidified potassium dichromate or acidified potassium permanganate. $\endgroup$
    – DHMO
    Sep 18 '16 at 16:35
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    $\begingroup$ @user34388 At worst I'd be using household bleach, however I'm intrigued by your suggestion with beans, that had not occurred to me... $\endgroup$
    – T.D. Smith
    Sep 18 '16 at 16:39
  • $\begingroup$ What do you mean by "rust"? Having the whole surface thoroughly cover with a layer of rust is not a problem. Run a piece of (unseasoned) iron through the dishwasher and you'll see it. Having the padlock rust through and through like in the fable of the rope made of ash is a different matter, as the rust on the surface will shield the metal from the stuff you through at it from outside. $\endgroup$
    – rumtscho
    Sep 18 '16 at 19:25

Here are factors that would speed up rusting:

  • Presence of water: make sure that the iron is wet.
  • Presence of oxygen: make sure the iron have access to air (the dissolved oxygen in water also works).
  • Presence of metal below iron in the metal reactivity series (see the picture below): usually copper is used. Tie copper to iron. Make sure that they are in contact. The larger the surface area of contact the better.
  • Temperature: make the iron as warm as possible.
  • Presence of salt: explained here. In short, salt increases the conductivity of water which makes the iron rusts faster, since rusting involves electron transfer.
  • Acidic environment: since rusting involves hydrogen ions ($\ce{H+}$), an acidic environment would increase the concentration of hydrogen ions, making the iron rust faster. However, this would dissolve the rust.
  • Presence of hydrogen peroxide: hydrogen peroxide ($\ce{H2O2}$) is an oxidizing agent which would not dissolve the rust: $$\ce{3H2O2 + 2Fe -> 3H2O + Fe2O3}$$

Metal reactivity series:

(source: bbc.co.uk)

Equation of rusting:

  • $\ce{Fe -> Fe^3+ + 3e-}$
  • $\ce{O2 + 4H+ + 4e- -> 2H2O}$
  • $\ce{2Fe^3+ + 3H2O -> Fe2O3 + 6H+}$

More equations can be found here.

Overall equation:

$$\ce{4Fe + 3O2 ->[H+,H2O] 2Fe2O3}$$

  • $\begingroup$ Maybe an acidic environment would favour rusting. If you see the electrochemical aspect of rusting, the H+ ions are on the left hand side of the cell equation for rusting $\endgroup$ Sep 18 '16 at 15:50
  • $\begingroup$ @AmritanshSinghal But wouldn't acid dissolve the rust? $\endgroup$
    – busukxuan
    Sep 18 '16 at 16:00
  • $\begingroup$ @busukxuan I think the question poster just needs the padlock to go away, so it doesn't matter. $\endgroup$
    – DHMO
    Sep 18 '16 at 16:02
  • $\begingroup$ @busukxuan the question only asks methods to speed up the process of rusting. SO,the OP is interested in speeding up the oxidation of iron and for that increasing [H+] would serve the purpose.I do not know if acid would dissolve it or not but that is asked. However, if you know what shall be the reaction of Fe2O3.xH2O with H+(i.e. dissolution of Fe3+ in acid), do mention in the comment. I would like to know that. $\endgroup$ Sep 18 '16 at 16:07
  • $\begingroup$ @AmritanshSinghal Not long ago I used HCl to remove rust. Basically it's just the hydroxide reacting with acid to form water, while there acid's conjugated base forms a salt with the iron. $\endgroup$
    – busukxuan
    Sep 18 '16 at 16:12

Here's a less-known effect: Iron or steel exposed to iodine vapors will rust within hours.

Also, in a draft-free room, iodine crystals placed well above an iron object will evaporate, creating a descending vapor-plume which rusts everything directly below. Or just use overkill: affix a sack of black iodine powder over the steel to be rusted. (Also, iodine has a distinct odor, so an S. Holmes type might sniff the rust and learn much, while normal humans would never think to try.)

Historical: currently iodine crystals are only sold for treating lifestock, but 170 years ago it was a common photographic chemical (critical in Daguerrotype process,) and also a common antiseptic. Perhaps your local Victorian druggist would have kilos of raw solid iodine to supply the daguerrotype craze, if not just for mixing up some iodine tincture for treating small cuts.

  • $\begingroup$ I set up an experiment to test this. I put approximately 15g of iodine crystals on a plastic mesh about 2" above a steel nail. After about 18 hours I'm seeing a little rust starting to form. It's about 55 defrees Fahrenheit, so the sublimation of the iodine may be slower than yoir response expected. $\endgroup$
    – T.D. Smith
    Oct 30 '16 at 15:13
  • $\begingroup$ When I saw this happen, it was above 70F and quite humid as well. It might be humidity that does it, rather than just heat. Also, even a slight oil-film (such as with store-bought nails) might interfere ...which probably removes this idea from the running. Your door hinge would already have to be grease-free and probably squeaky. $\endgroup$
    – wbeaty
    Oct 30 '16 at 23:57
  • $\begingroup$ In the book the temperature will be comparable to my test. I expect I'll have the characters use a degreasing solvent (lye?) before applying iodine. $\endgroup$
    – T.D. Smith
    Oct 31 '16 at 0:08
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    $\begingroup$ @Tyler IDEA! If you have iodine crystals, you can use ammonia to make contact explosive paste. Ammonium iodide. Smeared on a hinge or in a keyhole, after drying, it goes POW when disturbed even slightly. Just use a small stick to thoroughly grind up about a cc of black iodine crystals immersed in a saucer of ammonia cleaner. Keep adding more ammonia until the violet turns blue-gray. Let it settle out. The paste can be made on the spot as needed. It's dangerous to store it wet in a sealed container, but can be done briefly (just not for ~days.) Don't make more than 1cc at a time! $\endgroup$
    – wbeaty
    Oct 31 '16 at 0:09
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    $\begingroup$ @Tyler Loud crackling floor: make an entire cc of the black stuff, fling it at the ground hard, so it spatters. Wait to dry. Now anyone walking on the floor makes all kinds of crackling noises, even some quite loud pops. Sometimes it lasts for weeks! . . Also the ammonium iodide makes a small purple cloud of iodine, and if exploding from a keyhole, will stain the hand. Which perp was messing with a prepared lock? Check for purple hand! $\endgroup$
    – wbeaty
    Oct 31 '16 at 0:19

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