Page 590 - Corrosion Engineering Principles and Practice

552 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 553

other amphoteric metals such as zinc and magnesium may actually
corrode more rapidly if subjected to excessive cathodic protection
than if not protected at all. Steel is not subject to this effect. Lead may
be to some extent, but usually is not a problem unless the CP current
source is interrupted for an extended period. In this case, the
alkalinity developed at the lead surface will have an opportunity to
corrode the lead.

13.5.2 Anodes
Sacrificial Anodes
Materials suitable for use as galvanic anodes include aluminum,
magnesium, and zinc. According to Faraday’s law, the amount of
electrical energy that can be obtained from a galvanic anode depends
on the electrochemical equivalent of the metal used and the efficiency
of the working anode. A useful unit to characterize the capacity of an
anode is the ampere-hour (Ah) that corresponds to 1 A flowing for 1
hour, or any combination that will be equal to this, such as 0.5 A
flowing for 2 hours, 2 A flowing for 0.5 hour, and so forth.
The efficiency of a galvanic anode is the ratio of an anode weight
sacrificed for CP purposes divided by the total theoretical ampere-
hours or capacity of the material actually consumed. Galvanic anodes
are subject to self-corrosion which uses some of their capacity. This is
why the efficiency is less than 100 percent.
For example, pure zinc has a theoretical maximum capacity of
820 Ah/Kg. This means that if a zinc anode were to discharge 1 A
continuously, 1 kg would be consumed in 820 hours. If this kilogram
was discharging 1/10 A, it would be totally consumed in 8200 hours
or 48 weeks. Actually, zinc anodes operate, typically, at about 95
percent efficiency. This means that the energy content available for
useful current output would be 820 × 0.95, or 779 Ah/kg.
Another useful way of expressing the anode consumption is in
terms of kg per ampere-year. At 779 Ah/kg useful output for zinc, the
conversion would be
1 −
8760 (h year ) = 11.2 (kg A year ) (13.1)
1 −
1 −
e
−1
779 (Ah kg )
This equation indicates that 11.2 kg of zinc would be consumed
from an anode discharging 1 A for 1 year.
Zinc anodes are made of high-purity zinc (99.99 percent purity or
better) for soil use. For marine use, small amounts of aluminum and
cadmium may be added to ensure maximum efficiency. If zinc of
lesser purity is used, efficiency may suffer and the anodes may tend
to become passive and cease to discharge useful amounts of protective
current. Zinc anodes’ working potential with respect to a copper
sulfate electrode is in the order of −1.10 V.