How could I cause metal to rust rapidly?

Question

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:

Answer 1

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:

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

Answer 2

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.