Page 596 - Corrosion Engineering Principles and Practice

558 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 559

FIGURE 13.27
Impressed current
cathodic protection
anode made of
high-silicon cast
iron. A new anode
is on the left and New anode
a depleted anode
is on the right.
(Courtesy of Dean
Rookes, West
Coast Corrosion
Prevention Ltd.)

Depleted anode

Nonconsumable Anodes. Platinized anodes utilize a few micrometers
thick surface coating of platinum on titanium, niobium, or tantalum
substrates. Restricting the use of platinum to a thin surface film has
important cost advantages. For extended life, the thickness of the
platinum surface layer can be increased. The corrosion resistance of
the substrate materials is important due to the presence of
discontinuities in the platinum surface coating that invariably will
occur in practice. It is important that the potential of unplatinized
areas on these anodes does not exceed the critical depassivation value
for a given substrate material.
Platinized titanium is often used in marine environments. To avoid
the dissolution of titanium at unplatinized locations on the surface, the
operating voltage of the anode is limited by the anodic breakdown
potential of titanium which is in the range of 9 to 9.5 V in the presence
of chlorides. Hence the maximum recommended operating voltage of
platinized titanium anodes is 8 V. The corresponding maximum current