Corrosion process on metal with access of air in aqueous environment could be describe by deterioration of iron which can be written as:
2Fe + O2 + 2H2O à 2Fe(OH)2 [1]
The product, ferrous hydroxide, is commonly further oxidized to magnetite (Fe3O4) or a hydrated ferric oxide (FeOOH), that is, rust. It is convenient to consider separately the metallic and nonmetallic reactions in Eqn [1]:
2Fe à 2Fe2+ + 4e [2]
O2 + H2O + 4e à 4OH- [3]
To balance Eqns [2] and [3] in terms of electrical charge, it has been necessary to add four electrons to the right-hand side of Eqn [2] and to the left-hand side of Eqn [3]. However, simple addition and rationalization of Eqns [2] and [3] yield Eqn [1]. We conclude that corrosion is a chemical reaction [1] occurring by an electrochemical mechanism (Eqns [2] and [3]), that is, by a process involving electrical and chemical species. Figure 1 is a schematic representation of aqueous corrosion occurring at a metal surface. Equation [2], which involves consumption of the metal and release of electrons, is termed an anodic reaction. Equation [3], which represents consumption of electrons and dissolved species in the environment, is termed a cathodic reaction. Whenever spontaneous corrosion reactions occur, all the electrons released in the anodic reaction are consumed in the cathodic reaction; no excess or deficiency is found. Moreover, the metal normally takes up a more or less uniform electrode potential, often called the corrosion or mixed potential (Ecorr).
Figure 1 : Schematic illustration of the corrosion of steel in an aerated environment. Note that the electrons released in the anodic reaction are consumed quantitatively in the cathodic reaction, and that the anodic and cathodic products may react to produce Fe(OH)2.
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