Handbook of Electrical Installation Practice - BeKnowledge

Fig. 8.1 Galvanic corrosion.
back to the power source through the cathode. The anode is made positive with
respect to the cathode by the externally applied e.m.f.
This reversal of polarity of the external terminals of an electrolyte cell, as compared
with a galvanic cell, is a source of confusion; this confusion can be avoided if
it is remembered that in all types of electrochemical cells, current always enters the
electrolyte from the anode and leaves the electrolyte at the cathode.
PRINCIPLES OF CATHODIC PROTECTION
Cathodic Protection 183
In all corrosion cells, corrosion normally occurs only at anodes or anodic sites.There
are, however, exceptions; these include copper and aluminium, which are amphoteric.
If an entire structure buried in the soil, or immersed in water, is made the
cathode of an electrochemical cell so that the entire surface receives current from
the electrolyte, the structure will not corrode. Under these conditions, the structure
is said to be cathodically protected.
The cell, of which the protected structure is the cathode, may be either galvanic,
with current entering the electrolyte from the galvanic anodes more electronegative
than the protected structure, or electrolytic, with the anode electrically connected
to the positive terminal of an external power source. In any cathodic