Anodic Protection for corrosion prevention ...

1. Anodic Protection to Prevent Corrosion
**Definition:
Anodic protection (AP) is a technique to control the corrosion of a
metal surface by making it the anode of an electrochemical cell and
controlling the electrode potential in a zone where the metal is
passive.
This method of protection tested and demonstrated by Edeleanu in
1954
AP is used to protect metals that exhibit passivation in environments
whereby the current density in the freely corroding state is
significantly higher than the current density in the passive state over a
wide range of potentials.
Anodic protection is used for carbon steel storage tanks containing
extreme pH environments including concentrated sulfuric acid and 50
percent caustic Soda where cathodic Protection is not suitable due to
very high current requirements.
Name: Bapi Mondal
Department: Applied Chemistry and Chemical
Engineering.
Course Name: Corrosion Engineering.

2. Process :
1.Simply anodic protection is based on the formation of protective
film on metals by externally applied anodic currents.
2. It appears that the application of anodic current to a structure
should tend to increase the dissolution rate of a metal and decrease the
rate of hydrogen evolution.
3. This usually does occur except for metal with active-passive
transition such as Ni,Fe,Cr,Ti and their alloys.
4. If carefully controlled the anodic currents are applied to these
materials they are passivated and the rate of metal dissolution is
decreased.
*Corrosion Prevention By Anodic Protecction:
To anodically protect a structure a device called a potentiostat is
required.
Potentiostat: A potentiostat is an electronic device that maintain a
metal at a constant potential with respect to a reference electrode.

3. Fig: Anodic Protection of a steel storage tank containing Sulfuric acid.
1. The potentiostat has three terminals one connected to the tank,
another to an auxiliary cathode (a platinum clad electrode )and the
third to a reference electrode.
2. In operation, the potentiostat maintain a constant potential between
the tank and the reference electrode.
3. The optimum potential for protection is determined by
electrochemical measurements.
* Polarization curves of anodic protection

4. Fig:Polarization curves depicting principles of anodic protection
If an active-passive alloy such as stainless steel is maintained in the
passive region through an applied potential (or current) from a
potentiostat, its initial corrosion rate (icorr) can be shifted to a low
value at ipass as shown in this Figure.
Applied anodic current density = oxidation current density –
reduction current density.
**Anodic protection parameters include:
a) Protection range – range of potentials in which the metal/alloy
exhibits stable passivity.
b) Critical anodic current density.
c) Flade potential.

5. *Advantages:
1. Anodic protection unlike cathodic protection is ideally suited for
protection of active-passive alloys in aggressive environments such as
high acidity and corrosive chemicals.
2. Hence anodic protection is the most preferred choice for protection
of chemical process equipment.