Chemistry - How could I cause metal to rust rapidly?

Solution 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:

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

Solution 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.