1. Engineering Unit – Chemistry Section
Corrosion
Valentim M. B. Nunes
Engineering Unit – Chemistry Section
March 2018

2. Corrosion
The metallic corrosion is an electrochemical phenomena. It can be defined as the deterioration of a metal trough an electrochemical process, with the consequent loss of properties and functionality.
It is estimated that in advanced countries, the economical impact associated to corrosion is of the order of 3% of the Gross Domestic Product (GDP).

3. Corrosion of Iron
A familiar example of corrosion, is the formation of rust over the iron, in the presence of water and oxygen.
Anode: Fe(s)  Fe2+(aq) + 2 e-
Cathode: O2(g) + 4 H+(aq) + 4 e-  2 H2O(l)
Global: 2 Fe(s) + O2(g) + 4 H+(aq)  2 Fe2+(aq) + 2 H2O(l)
4 Fe2+(aq) + O2(g) + (4+2x)H2O(l)  2 Fe2O3•xH2O(s) + 8 H+(aq)
rust

4. Corrosion of aluminum
Other metals also suffer corrosion. Aluminum has a lower standard reduction potential lower that iron, then it has a higher tendency to suffer oxidation. But, the oxide formed (alumina: Al2O3) is non porous (contrarily to rust) and protects aluminum from corrosion.

5. Preventive measures to avoid corrosion
Avoid the contact with noble metals (higher Eº)
Remove O2 (air) and water.
Reduce the humidity, containing corrosive substances CO2, SO2, SO3, etc.
Utilize cathodic inhibitors.
Coat with organic materials.
Use metallic coatings.
Passivate the surface by anodization.
Vitrify or paint the surface.

6. Metallic Coatings
Coating of iron with zinc, that is more easily oxidized: Galvanization
Coating with tin. While there are tin on the surface corrosion of the iron does not occur. However a sting or risk accelerates corrosion!

7. Cathodic Protection
Cathodic protection is a process in which a metal to be protected is turned the cathode of a cell. The protection of submarine tanks, underground tubes, etc., is usually done this way.
Anode sacrificed
Mg  Mg e-
O2(g) + 4H+ + 4 e-  2 H2O(l)

8. Examples Protection of reinforced concrete in bridges.
Protection of metallic structures submersed or under the ground
Protection of navy's.

9. Effect of pH – Pourbaix diagrams
Corrosion also depends on the pH of the medium.
Applied potential < -0.7 V (contact with Zn or Mg – iron stays immunized.
Higher potential – iron suffers corrosion depending on the pH of the medium.
Pourbaix diagram of iron

10. Effect of pH – Pourbaix diagram for aluminum