Environmental Remediation and Energy Production Technologies
2 H+(aq) + 2 e- ďƒł H2(g)
(10.3)
In some situations, a metallic ion with higher reduction tendency, i.e., an ion of a more noble metal, present in the media, may be reduced to a lower oxidation state causing the oxidative dissolution of the metal in the structure with the solution contacts. This may be the case of complex piping and container systems, with accessories of different materials or with complex electrolytic solutions (eg.: industrial, thermal power stations, primary solar circuits, WWTP piping systems). 10.2.2. The corrosion cell and the fight against corrosion As stated above, corrosion in aqueous media, wet corrosion, is an electrochemical or oxidationreduction (redox) process. Each redox reaction is composed by two half reactions: the reduction semi-reaction and the oxidation semi-reaction. In the particular case of corrosion reactions the semi-reactions is always that of the oxidation of the construction metal (eq. 1). The reduction semi-reaction is that of the reduction of a species in the surrounding environment, for instance, O2 reduction, in neutral or alkaline media (eq. 2), hydrogen evolution in acidic media (eq.3), etc. One distinctive characteristic of redox reactions is the transference of electrons from one species (the oxidized species or reductant) to the other (the reduced species or oxidant). Another special particularly of redox reactions among the different chemical reactions is that the reagents (species that interact) do not need to be in direct contact for reaction to take place! It is only needed that there is a way for electrons to flow from the reductant to the oxidant. The place where each of the semi-reactions happens is called electrode, normally, located at the surface of conducting metallic bodies. The place where oxidation semi-reaction takes place is called anode and the place where reduction occurs is the cathode. Therefore, frequently, oxidation semi-reaction is said the anodic process and reduction semi-reaction is named the cathodic process. Corrosion as any electrochemical or redox process may be described with the help of an electrochemical cell, having an anode, a cathode, an electronic conductor through which electrons flow from anode to cathode, and an electrolytic conductor, or simply, electrolyte, that allow the migration of positive and negative ions to the electrode with opposite electrical charge. Figure 1 shows a representation of the 4 essential elements (anode, cathode, electronic conductor, and electrolyte) of a corrosion cell.
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