Corrosion can be defined as the destructive attack of a metal through interaction with its environment.
DRIVING FORCE FOR CORROSION.
In order to answer the question What is Corrosion? is necessary to understand how corrosion works and why metals are affected by corrosion.
Metals used in the construction, e.g. steel, stainless steel grades, copper alloys, are subject to corrosion, this is due to the high energy content of the elements in metallic form.
In nature, most metals are found in chemical combination with other elements; these metallic ores are refined by man and formed into metals and alloys.
As the energy content of the metals and alloys is higher than that of their ores, chemical re-combination of the metals to form ore like compounds, i.e. the corrosion, is a natural process.
FUNDAMENTAL MECHANISM OF ATTACK.
Corrosion of metals takes place through the action of the electrochemical cell. Although this single mechanism is responsible, the corrosion can take many forms.
As in all chemical reactions, corrosion reactions occur through an exchange of electrons.
In electrochemical reactions, the electrons are produced by a chemical reaction in one area, the anode area, travel through a metallic path and are consumed through a different chemical reaction in another area, the cathode.
The generic chemical reaction for this metal loss at anodic sites is:
M ---> M+ + e-
M = uncharged metal atom at the metal surface
M+ = positively charged metal ion in the electrolyte
e- = electron that remains in the metal
This type of chemical reaction is called oxidation even though it does not directly involve oxygen but only results in an increase in positive charge on the atom undergoing oxidation.
More than one electron can be lost in the reaction as in the case of iron where the most common anodic reaction is:
Fe ---> Fe2+ + 2e-
Fe = metallic iron
Fe2+= ferrous ion that carries a double positive charge
The electrons that are produced at anodic sites are consumed at cathodic sites. The type of chemical reactions that consume electrons are called reduction:
One of the most common cathodic reaction is the reduction of oxygen:
O2 + 4e- + 4H2O ---> 4OH-
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