When restoring a tool of any sort, the first step (after disassembling it) usually involves dealing with surface oxidation, or rust.
What is rust?
Rust is a general term used to describe a family of iron oxides. Rust is the weakening of iron that results from oxidation of its atoms, essentially a form of electrochemical corrosion. Rust forms in the presence of air (oxygen) and water – if one of them is missing, rust doesn’t form.
How does rust form?
Now to the chemistry of rust. Assume a drop of water placed on a piece of iron.
When water containing dissolved oxygen comes into contact with iron, the iron (Fe) in the middle of the drop begins to oxidize, loosing electrons.
Fe(s) → Fe²+(aq) + 2e-
Near the outer surface of the drop, water and oxygen receive the electrons and form hydroxide ions:
O2(g) + 2H2O(l) + 4e- → 4OH- (aq)
The hydroxide (OH-) ions react with the iron (II) ions to produce iron (II) hydroxides, sometimes known as green rust.
Fe²+(aq) + 2OH-(aq) → Fe(OH)2(s)
The iron (II) hydroxide is then oxidized by air, and converted to hydrated iron (III) oxide. The iron (II) hydroxide reacts further with oxygen and water to form hydrated iron (III) oxide = rust.
4Fe(OH)2 (s) + O2 (g) + 2H2O (l) → Fe2O3 · 3H2O(s)
To illustrate how rust forms, I put together some experiments. In the first experiment I placed some absorbent cotton disks in the base of a glass jar, and sprayed in some water. Then I added a block plane blade with some old rust spots – the sealed jar acts like a high humidity environment. I left the jar alone or three weeks. Surface rust formed easiest in regions of the blade where there were abrasions, existing “stable” rust, or areas where moisture collects, such as the grooves in the back of the blade. These regions of rust, as shown in Fig.1, fan out in almost fractal-like patterns. This is typical of rust formed in high humidity environments.
In the second experiment I submerged another blade in a sealed jar of water. This jar was left for three weeks as well. The result was a jar containing a reasonable amount of rust in suspension – hydrated iron (III) oxide. The images in Fig.2 show the amount of rust on the blade once the blade was removed from the water. The lower-right image shows the colour of the water and suspended material after the blade was removed. The glass jar ended up with being quite stained by the process.
Once the rust residue was cleaned off the blade, the area covered by the rust was shown to have converted to black rust. The image shown in Fig.3 shows the boundary of black rust region with the portion o f the blade not submerged in the water – and a transitionary region of flash rust (caused by the moisture from wiping off the rust). Black rust, or black iron oxide, forms on submerged steel where the environment has a low oxygen concentration. It is also known as magnetite (Fe3O4), and provides the blade with a thin protective coating (similar to the process of gun bluing).
The portion of the blade not submerged in the water developed localized rust clusters due to the humidity in the jar (Fig.3-right). Fig 4. shows the blade after being submerged for two days in water – notice the rust forming on the blades surface. When disturbed, the rust settles to the bottom of the jar.