Cathodic Protection

Cathodic protection:

Metals undergo corrosion due to the formation of galvanic cells. It is also anode which suffers corrosion. Therefore the principle involved in this method is to force the metal to be protected to behave as cathode, there by corrosion does not occur. This is done by converting anodic side into cathodic side. This phenomenon is known as cathodic protection. Cathodic protection can be achieved by following methods:

1. Sacrificial anode method.
2. Impressed current method

1. Sacrificial anode method:

In this method, the metal to be protected is converted into cathode by connecting it to a more active metal. The more reactive metals like Zn, Mg, Al and their alloys can be used as anodes, which when connected to the metal object undergoes preferential corrosion (sacrifices itself) protecting the metal structure. The sacrificial anode continuously corrodes and gives the electrons to the protected metal and forces it to act as cathode, which remains unaffected.  Exhausted sacrifial anodes are replaced by new ones as and when required.

Application:

a)    This method is used for protecting storage tanks with Mg blocks.

b)    Mg bars are fixed to the sides of ocean going ships to act as sacrificial anodes.

c)    This method is used for protecting buried iron pipe lines with Mg blocks.

(2) Impressed current method:

Another method providing cathodic protection is by applying a direct current larger than the corrosion current. The protected metal is made cathodic by connecting to the negative terminal of the d.c power supply. The positive terminal of the d.c power supply is connected to an inert electrode like graphite. The metal structure being cathodic does not undergo corrosion.  Anode being inert remains unaffected. 

Advantages:

(1)  One installation can protect large area of metal.

(2)  Low installation cost.

Applications:

Impressed current or impressed voltage method is used to protect

i) Buried oil pipe line.

ii) Marine pipes.

iii) Buried water storage tanks.