SACRIFICIAL ANODE SYSTEM DESIGN
Theory of Operation. The basic principle of cathodic protection
using sacrificial anodes is the electrochemical cell (refer to para. 2.2). As
in the case of impressed current cathodic protection, high energy (potential)
electrons are forced to flow from the anode to the structure to be protected.
The structure-to-electrolyte potentials required for protection are identical
to those for impressed current cathodic protection systems. These potential
requirements are given in para. 3.2. The high potential electrons are
generated through the corrosion of an active metal such as magnesium or zinc
(refer to para. 3.2). In this type of system, the anode material is consumed,
or sacrificed in the process, and the anodes must be periodically replaced in
order to obtain continued protection. In order to minimize periodic anode
replacement, sufficient anode material is normally provided so that the anode
replacement interval is a desired number of years. Common practice for buried
systems is to design the system for a 10- to 15-year anode life. For
submerged systems, or for buried systems where anode replacement is difficult,
longer (20- to 30-year) anode life is often used as a design criteria.
Advantages of Sacrificial Anode Cathodic Protection Systems. The
primary advantage of sacrificial anode cathodic protection systems over
impressed current cathodic protection systems is their simplicity and
reliability. There are fewer critical components such as rectifiers in
sacrificial anode systems. The critical cable from the anode to the impressed
current anodes which is prone to failure is not a factor in sacrificial anode
cathodic protection systems. The anode-to-structure cable in sacrificial
anode systems is at a negative (protected) potential. Sacrificial anode
cathodic protection systems are also in some cases less costly to install and
maintain than impressed current cathodic protection systems. This is
particularly true for systems with small current requirements (0.5 A or less
per 100 lineal feet of structure). There are no power costs or costs
associated with furnishing power at a remote site associated with sacrificial
anode cathodic protection systems.
Another major advantage of sacrificial anode cathodic protection
experienced when this type of system is used. Sacrificial anode cathodic
protection systems are commonly of the distributed anode type. This is
usually necessary because of the limited driving potential of the anode
Disadvantages of Sacrificial Anode Cathodic Protection Systems.
The primary disadvantages of sacrificial anode cathodic protection systems are
associated with the limited driving potential between the structure and the
anode materials used. This limits the current output of the anodes and
restricts the area of structure which can be protected using a single anode.
Anode consumption is also inherent in sacrificial anode systems and allowances
for periodic anode replacement must be made.
Sacrificial Anode Cathodic Protection System Design Procedures.
The basic principles for the design of sacrificial anode cathodic protection
systems are described in para. 4.2. First, the total amount of current is
determined, then the output per anode is determined. Then the number of
anodes required and the life of the anodes is determined. If desired, the
system parameters (anode size or type) are adjusted to give desired system
performance, primarily to achieve desired anode life.