Page 500 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

Electron flow
18.4 Cathodic potential sensors 495
Corrosion
Cations (+)
+ Ion flow –
Anions (–)
Electrolyte
No corrosion
Voltage difference
FIGURE 18.20
The metal corrosion process.
The dissimilar metals can be different elements (steel, aluminum, etc.) or simply impurities/non- homogeneous metals on a single metal sheet. Once the above conditions are satisfied, the combina- tion forms an electrochemical cell allowing the following chemical reactions to occur:
2Fe.2Fe11 1 4e2
This process produces four free electrons ready to combine with water to form hydroxyl ions in
the following reaction:
O 1 4e2 1 2H O.4OH2 22
Once the above reaction takes place, the main attraction (i.e., the really expensive heartbreak) takes place—namely, iron combining with oxygen and water to form ferrous hydroxide:
2Fe11 1 4OH2.2FeðOHÞ2
This process follows the second law of thermodynamics’ theoretical trend toward maximum entropy, tracing the steady flow of highly organized metals toward their disorganized oxidized metal combination. The two electric potentials of the dissimilar metals form an electrical path from the anode (area of highest activity) to the cathode (area of lowest activity). The anode, therefore, is corroded away—and if that anode happens to be your steel support structure, it will eventually fail and collapse.
An unprotected metal surface of a submerged steel structure becomes an anode (active area) once the conditions for an electrochemical cell are satisfied, thus allowing the electrochemical pro- cess described above to corrode the metal.
Cathode
Anode