Homemade rust

We can make pure rust, useful for painting and weathering our models, without the need of scraping some old iron piece!

I will explain here two methods, simple and cheap, I used to create a very fine orange rust powder, useful in painting and in weathering. You can use it as a pigment, fixing it with alcohol or turpentine, or mix it with an oil paint (like Burnt Sienna) in order to enrich its hues (thanks to Augusto for the tip!).

Warning: the methods described below do not involve dangerous procedures or harmful substances. Nonetheless, you ought to follow simple safety rules, like working in a well ventilated area, not swallowing the substances, avoiding contact with eyes (and with skin, for sensible persons), regularly monitor functioning electrical equipment and not using them in a way hazardous for people and things.

Obviously, if you notice any inaccuracy or simply you want to give me some advice, feel free to leave a comment!

Chemical corrosion

This is the simplest method, but also the slowest. You have to put into a jar some iron pieces (clips, nails, bolts, steel wool and so on, just not stainless steel) and submerge them in hydrogen peroxide; the reaction will be faster if you add salt and vinegar as a catalyst. Just after few minutes you will see some oxygen bubbles coming out from the iron pieces, and after a few days the situation will be like this:

Note that the clips on the left, that once were polished, now are corroded and rusty. You have to collect the orange "slime" on the right, made of ferric hydroxide —our rust. If you have used the catalyst, it will be mixed with salt and other substances, but you can put it into clean water and wait: the rust will settle to the bottom, while the other substances will remain dissolved in the water, ready to be removed. After that, let it dry and then "mill" what remains. This is what you will get:

Just for curiosity, the reaction happens between iron (Fe) and hydrogen peroxide (H₂O₂), obtaining, through a complex series of chemical reaction in acid environment, ferric hydroxide (Fe(OH)₃), water and gaseous oxygen:

$\text{Fe} + \text{H}_2\text{O}_2 \rightarrow\cdots\rightarrow \text{Fe(OH)}_3 + \text{H}_2\text{O} + \text{O}_2$

Electrolysis

This is the fastest (it takes hours instead of days) but also the most complex method. Take a jar and fill it with saturated salted water (drop salt into the water until you see it depositing on the bottom). Fix two electrical wires to the jar with scotch tape and link them to some iron pieces (clips, nails but not iron wool), so that these pieces —but not the wires— are submerged into the solution without touching each other (see the picture below). Now take an universal DC power supply, set it to at least 12 V and link it to the two wires. As soon as you turn it on, some hydrogen bubbles will begin sparkling from the cathode (- sign), while a yellow-green halo will emanate from the anode (+ sign):

After a couple of minutes the yellow-green halo (ferrous chloride) expands, while on the surface an orange foam begins to form. Meanwhile a white and dark green gel starts to cover the anode, and slowly settles on the bottom. Check, from time to time, whether the power supply overheats, and in case unplug it until it cools down.

After about three hours (but the time may vary) the situation is like in the photo below. The bubbles on the cathode have almost disappeared, while an amount of dark green substance has settled on the bottom. The rest of the solution is yellow-orange, without any trace of green. You can now unplug the adapter and remove the electrodes.

Note that while the cathode (blue wire) remained intact, the anode has worn out until its tip became as sharp as a needle.

After that, stir the solution and let it settle, then remove the excess salty water. Now you can choose between two ways:

Let it dry at a not-so-high temperature, under the sun or with a heater. The color will change from dark green to light orange (ferric hydroxide).
Put the solution in an old pot and boil it, until the water dries, then leave it on the fire until it becomes reddish-brown (ferric oxide).

In the first case you will get a light orange color, like in the previous method, while in the second one you will obtain a darker hue, like in the photo below.
In both cases you will need to collect the precipitate, "wash out" the salt like I did in the first method and then "mill" with a mortar or between two metal pieces, in order to obtain a fine powder.

The sodium chloride (table salt, NaCl) in aqueous solution dissociates itself into its ions:

$\text{NaCl}\rightarrow \text{Na}^+ + \text{Cl}^-$

The sodium will bind with the water, forming sodium hydroxide and gaseous hydrogen, while the chlorine will bind with iron, creating ferrous chloride:

$2\text{Na}^+ + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2\\ \text{Fe} + 2\text{Cl} \rightarrow \text{FeCl}_2$

The ferrous chloride reacts with sodium hydroxide, forming sodium chloride, that thus returns into the solution, and ferrous hydroxide, that is insoluble in water and thus precipitates:

$2\text{NaOH} + \text{FeCl}_2\rightarrow \text{Fe(OH)}_2 + 2\text{NaCl}$