Corrosion can be defined in a more simplistic way as deterioration of materials under the influence of an environment.
Without exception, the corrosion of metals and alloys (majority of materials used in industry) in aqueous environments (the most often encountered environment) is an electrochemical reaction. A brief description of the electrochemical principles for the beginners of corrosion is made before attempting on the forms of corrosion.
THERMODYNAMICS:
At the outset it is necessary to pose the question WHY DO METALS CORRODE? And set criteria for the process.
Corrosion of metals and alloys involves oxidation from its metallic state and therefore must obey the thermodynamic criteria as any other processes.
Take for example, the corrosion of Mg in water, which can be represented by the following reaction.
Mg + H2O(l) + 1/2 O2 = Mg(OH)2 _G° = -142.6 K Cals
The corrosion of Mg is spontaneous as it leads to a -ve change in free energy.
On the contrary the reaction of Au with H2O and O2 results in a +ve change in free energy and so Au cannot be corroded.
ELECTROCHEMICAL CRITERIA:
It must be remembered that corrosion of metals and alloys by aqueous media must necessarily proceed through electrochemical process; hence it is imperative to set electrochemical criteria for corrosion of metals.
As indicated earlier, corrosion of metals involves oxidation of metals and alloys from their metallic state in ionic form.
It is represented as
M = Mn+ + ne (Oxidation)
One can notice, the oxidation of metal simultaneously leads to release of electrons. Should the reaction to continue there must be a sink for electrons.
This is accompanied by a reduction reaction. WHAT ARE THESE CATHODIC REACTIONS? One or more of the following reactions are possible.
They are
2H+ + 2e = H2 (Hydrogen Reduction reaction)
O2 + 4H+ + 4e = 2H2O (Reduction of dissolved O2 in acidic media)
O2 + 2H2O + 4e = 2OH- (Reduction of dissolved O2 in basic media)
Mn+ + ne = M (Metal Deposition)
Mn+ + e = M(n-1)+ (Metal ion reduction)
Interestingly oxidation (also called anodic) and reduction (also called cathodic) reaction occur on the same surface of the metal.
In uniform corrosion these anodic and cathodic sites keeps interchanging with respect to time.