V

Structure being protected

Inert anode

FIGURE 11.22 Schematic arrangement illustrating cathodic protection system.

diagram where unreacted metal is the most stable product. Overall the following processes take place:

• The potential of the metal is polarized below the free corrosion potential.

• Aggressive ions such as chloride (Cl-) are repelled from the negatively charged metal surface (like charges repel).

• Oxygen is consumed at the metal surface.

• Hydroxyl ions are formed at the metal surface promoting the formation of a passive film (note for Al and Zn, alkali corrosion may occur at high pH values).

An alternative to the use of a direct current (dc) voltage to lower the potential of the metal to be protected is that of the application of a sacrificial anode. This method effectively creates a galvanic couple with the metal to be protected acting as the noble metal (cathode) while the sacrificial anode dissolves preferentially. Table 11.8 identifies various impressed current and sacrificial anodes used in CP systems. A full description of the principles and practice of cathodic protection can be found in references [52-54].

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