13. Designing a Model of a Dual Cathodic Protective System


Objective Protection of internal surface of a steel tank containing salt water by galvanic anode and impressed current system.
Basis of Design

Tank (dimensions: H = 1250 mm, W = 50 mm, L = 100 mm). The internal surface to be coated with epoxy. Two faces of tank to be transparent for visual inspection.

Salt contents in the range of 150-200 ppm of brackish water
A drainage system
Portability of the model
Silicon anodes to be used for impressed current system
Magnesium anodes to be used for Galvanic system

Silicon iron anodes TA-4 with 1 meter lead Kyanar cable

Magnesium anode 2.3 Kg, with 1 meter lead

Shunt 50 A- 50 mV
Water trough
Power supply
SDDT toggle switch
Plugs and sockets


Components Photographs of the different components of the apparatus are shown below.
The tank assembly is shown in the adjacent figure. The transparent sides are clearly shown.


The coated surface of the pipe by epoxy is shown in this figure.


A side-view of the tanks is shown here.


The rheostat which is used for the adjustment of the current.


The voltage source for the impressed current is shown in this figure.


This is an electrical switch to select the protection system (galvanic and impressed current).


The connection board is shown in this figure.


The two protection systems installed in the tank. The silicon anode and magnesium anodes are hung as shown.


Figure shows the connection system on the top of the tank.



The Cu-CuSO4 reference electrode.


. A schematic of the model is shown in here. current passes from the AC supply to transformer rectifier on to the impressed current anode through the electrolyte to the structure (steel tank). The protective potential is measured at the test point by connecting the +ve terminal of the voltmeter to a Cu-CuSO4 reference electrode and the negative to the tank.


The system is switched to galvanic anode system for measurement of protection potential by the change over switch.

Magnesium anode is electrically separated by maintaining a distance of 10 cms from the bottom of the tank. The protective potential is measured by the digital voltmeter as described above.

Some useful data

(a)    Sacrificial System

     Wetted area of the tank = 6300 cm2

     Structure voltage  ( Cu-CuSO4) = -567.4 mV

     Magnesium anode voltage = -1.008 V

     Structure potential after switching the sacrificial system = -1.2 V

(b)   Impressed Current System

     Structure voltage (Cu-CuSO4) = 566.9 mV

     Structure potential after switching the impressed current system = -1.3 V


The working model described above can be used to successfully demonstrate the principles of cathodic protection system to students and engineers interested in cathodic protection.