What is chemical rust


If metals are exposed to the weather, as in Fig. 1, ugly changes in the metal occur. This electrochemical reaction is generally referred to as corrosion. When it comes to iron, one often speaks simply of rusting. A distinction is made between surface corrosion with almost uniform corrosion and pitting corrosion (pitting), which only occurs on a small area of ​​the metal.
 

The rusting of iron can be described in the following reaction equations:

Local anode: FeFe 2+ + 2e - Oxidation (electron donation) /*2

Elemental iron (atom) changes into the iron (II) ion. Thereby 2 electrons are given off.


Local cathode: O2+ 2 H.2O + 4 e-4 OH - Reduction (electron uptake)

1 molecule of elemental oxygen combines with 2 molecules of water to form 4 hydroxide ions (OH-) Ions. Thereby 4 electrons are absorbed.

However, since the first reaction only emits 2 electrons and the cathode reaction requires 4 electrons, the anode reaction has to run twice. We multiply the anode reaction by the factor 2 and write ´ /*2 ´


Local anode: 2 feet2 feet 2+ + 4 e - Oxidation (electron donation)

Elemental iron (atom) changes into the iron (II) ion. Thereby 2 electrons are given off.


Local cathode: O2+ 2 H.2O + 4 e-4 OH - Reduction (electron uptake)

Now 4 electrons are given off in the anode reaction and 4 electrons are taken up in the cathode reaction, which are required for the formation of 4 hydroxide ions.

Bump Fe 2+ now on hydroxide ions (OH - ) This creates poorly soluble iron (II) hydroxide, which is oxidized to red-brown iron (III) oxide hydroxide (rust) by oxygen.


Fe2++ 2OH -Fe (OH)2  

   4 Fe (OH)2 + O24 FeO (OH) + 2 H2O



Ferric oxide hydroxide
is called in the technical language as rust.