5. Cathodic Protection

5.3 Cathodic Protection Systems [6/8]

Cathodic Protection Installation

A typical cathodic protection installation is shown in the figure below. An anode bed comprising of five anodes is connected to the positive terminal of a transformer-rectifier and the negative is connected to a pipe. Electrons flow from the anode through the soil to the pipe to be protected. A very simplified diagram is shown below.

Characteristics of an Impressed Current System

Following are the typical anodes used in an impressed current anode system.

Graphite Anodes: They are made by fusing by coke and carbon particles at a very high temperature. Their operation limit is 0.3 amp/ft2 in fresh water,  1.0 amp/ft2 in salt water and 1.0 amp/ft2 in soil.

 
High Silicon, cast iron Typical composition is 

 
Silicon 14.35% min
Carbon 0.85% max
Manganese 0.65 % min
Iron  - remainder -
 

The received current density is 1-5 amp/ft^2. It is good for soil and fresh water and sea water.

Silicon -  Iron tubulars

 
Scrap iron The consumption rate is 9 kg/amp/year. There are no restrictions on maximum voltage.

 
Platinized Platinum The rate of consumption is 0.006 g/amp/year. It is very useful for protecting offshore structures.

 
Lead silver anodes Their composition is

 
Lead, 1-2 % Bismuth
Lead, 2 % Silver
Lead,  10% Silver, 6% Antimony
They are operated with current densities in the range of 15-50 amp/ft2. The consumption rate is less than 0.2 lb/amp/year.