Environmental Engineering Reference
In-Depth Information
Figure 4.13
Stray currents produced by an on-shore welding generator damaging a
ship hull.
underneath; particularly the municipal water supply lines and sewage lines are
affected. The hull of a ship becomes prone to stray current corrosion if a welding
operation is carried out on it with the help of a welding motor-generator located
on shore. This is because current leaking from the hull enters water that returns
to the source. The situation is shown in Fig. 4.13. The incidence of corrosion
can be avoided by shifting the dc generator to the ship.
4.5 ANODIC PROTECTION
Anodic protection is based on the principle of passivation of metals. It has been
discussed in Section 2.4 that active-passive metals such as iron, chromium,
nickel, titanium, and their alloys attain passivity on anodic polarization and in
this state their rate of corrosion decreases many fold, sometimes by the order of
10 5 -10 6 . In anodic protection, the potential of the system is maintained in the
passive region with the help of a potentiostat. The structure to be protected is
connected to one terminal of the potentiostat, the second terminal is connected
to an auxiliary cathode, and a constant potential is maintained with respect to a
reference electrode convected to the third terminal. The anodic protection ar-
rangement for a steel tank containing sulfuric acid is shown in Fig. 4.14.
The principle of anodic protection is further illustrated in Fig. 4.15. An active-
passive metal corroding in an acid medium has been depicted. For the attainment
of passivity the anodic current must be increased to i critical , which corresponds to
E pp . The current automatically adjusts itself as the potentiostat sets the potential
to this value. The magnitude of applied current has been shown as i app(1) . However,
the potential has to be raised further to E Prot to maintain the metal in the passive
region and the corresponding applied current is i app(2) . The large difference in
magnitude of these two applied current values may be noted. The current required
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