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.

Answer 3

The squeaky performance of the padlock need not come completely from within. If the idea of copper filings is acceptable (they would certainly seem out of place), perhaps a little plaster of Paris could be made into a paste and shoved into the keyhole. After it sets, the operation of the lock would be stiff, perhaps noisy. The plaster could be retarded or not, so as to set hours later or within minutes.

For chemical complexity, you could add iron (not copper!) filings - let the added metal do the rusting. It could look like the padlock was just wearing out. Plaster need not stay white; if the iron filings don't color it enough, there's always carbon black. And NaCl if you want some rusting; and some oxalic acid, or cream of tartar for acidity - or NaHSO₄.

This could work even if the padlock is brass, although brass filings would look more innocent. But the plaster should be colored so as to look like dirt.

Perhaps even a little Portland cement, as a paste, maybe a little sand, maybe some iron filings for extra rusting. This would be more adhesive, but would take longer to set, unless you add a little CaCl₂ for acceleration.

Answer 4

Science At Play: Instant Rust | Connecticut Science Center describes a children's experiment (including a YouTube video ) for causing iron to rust almost instantly:

In a non-metallic container, add:

• eight parts hydrogen peroxide (standard 3% concentration).
• one part vinegar (standard 5% acetic acid).

While stirring, slowly add until no more will dissolve:

• table salt.

Then:

• Place an uncoated non-galvanized nail in the liquid.
• Watch it develop rust.
• Carefully remove and allow to air dry before touching the rust.

In our experiment, mixing hydrogen peroxide (H₂O₂) and distilled vinegar together creates a small amount of something called peracetic acid. Acid is corrosive and can cause things like metal to break down. Hydrogen peroxide is made of hydrogen and oxygen, but it’s the oxygen that’s key to creating rust on metal.

The molecules of iron on the surface of the nail exchange atoms with the oxygen in the solution and produce a new substance. You guessed it–rust! (or iron oxide as scientists would call it!)

This whole process is helped along by the salt we added to the solution. Its job in this whole process is to act as an electrolyte which lowers the electrical resistance in the solution, helping the oxygen and the nail to trade atoms more easily.

This is intended to be kid-safe (with appropriate precautions), but for your situation, a more strongly concentrated solution might be considered.