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 used**. Galvanic anode materials are subject to self-corrosion
which uses some of its energy. This is why the efficiency is less than 100%.

A prospective sacrificial anode must possess a large number of
**electrons per unit mass** and should deliver these electric charges efficiently. Thus the electrical
output of an anode is given by current capacity which is expressed in
**Ah kg**^{-1}** or kg A**^{-1}**
y**^{-1}. The value of the current
capacity is determined by the electrochemical equivalent, the density and the
efficiency of the anodic material. The electrochemical equivalent, which is
dependent on the atomic weight and valence, is a characteristic of the anode
material.

For example**, ****pure **
**zinc** has a theoretical maximum capacity
of **820 Ah per kilogram** (373 Ah per pound).
This means that if a zinc anode were to discharge one ampere continuously, one kilogram
would be consumed in **820 hours**. If this
kilogram was discharging one tenth of an ampere, it would be totally consumed in
**8200 hours or 48 weeks**. Actually, zinc
anodes operate, typically, at about **95 % efficiency**.
This means that the energy content available for useful current output would be
**820 x 0.95, or 779 Ah per kg**.

Another way of
expressing this is in terms of **kg per ampere-year**.
At 779 Ah per kg useful output for zinc, the conversion would be:

The actual anode efficiency is determined by a number of factors including nature of the environment, operating current density and metallurgical microstructure. It is apparent that if the cathode reaction rate on the anode is low then the efficiency will be high, so that there is minimum self corrosion. Similarly large operating currents will yield high anode efficiency. It should be added that the type of corrosion attack experienced by the anode also significantly affects the magnitude of the anode efficiency. For instance, severe pitting and intergranular attack may result in a chunk of the anode to become detached without complete consumption of the electric charge in that piece.