What is the rust? Rust defined as a red substance tends to the brownish, formed on the surface of iron and steel as a result of the impact of corrosion. The iron rust happens because of the oxidation of the atoms of the surface of the iron as a result of touching these atoms with the oxygen of the air in the presence of the moisture, several forms of rust are distinguishable both visually and by spectroscopy, and form under different conditions. Moreover, rust consists of hydrated iron(III) oxides Fe2O3·nH2O and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3) with the presence of water molecules. Onset of rusting can also be detected in laboratory with the use of ferroxyl indicator solution. The solution detects both Fe2+ ions and hydroxyl ions. Formation of Fe2+ ions and hydroxyl ions are indicated by blue and pink patches respectively.
There are several types of the rust. Firstly, Pitting and cavity corrosion which occurs on the surface of unprotected steels used in infrastructure which lead to a huge decrement of its toughness and thickness. Secondly, contact corrosion is one of these types, it happens when there is a contact between a non-rusting metal and a rusting one, which leads to an iron oxide deposits at the moment of this contact and it might be continue further from this moment. Thirdly, crevice corrosion, this kind of rust happens on the limited areas and spaces, its rust intensity depends on some factors such as humidity, temperature, and geographical elements (nearer the sea, windy sites, particularly rainy etc.).
There are some chemical reactions expresse how the corrosion or the rust happen. Rust is formed when the oxygen of the moisture air reacts with iron in addition to water:
iron + water + oxygen ? hydrated iron(III) oxide
This is an example of an electrochemical reaction and corrosion. Two distinct electrochemical reactions occur, there is anodic dissolution or oxidation of iron going into aqueous (water) solution:
2Fe ? 2Fe2+ + 4e-
Cathodic reduction of oxygen that is dissolved into water also occurs:
O2 + 2H2O + 4e- ? 4OH-
The iron ion and the hydroxide ion react to form iron hydroxide:
2Fe2+ + 4OH- ? 2Fe(OH)2
The iron oxide reacts with oxygen to yield red rust, Fe2O3.H2O.
The following redox reaction also occurs in the presence of water and is crucial to the formation of rust:
4 Fe2+ + O2 ? 4 Fe3+ + 2 O2?
In addition, the following multistep acid–base reactions affect the course of rust formation:
Fe2+ + 2 ?H2O ? Fe(OH)2 + 2 ?H+Fe3+ + 3 ?H2O ? Fe(OH)3 + 3 ?H+as do the following dehydration equilibria:
Fe(OH)2 ? FeO + ?H2OFe(OH)3 ? FeO(OH) + ?H2O2 FeO(OH) ? Fe2O3 + ?H2O