To understand cathodic protection, you must first understand how corrosion is caused. For corrosion to occur, three things must be present:
1. Two dissimilar metals
2. An electrolyte (water with any type of salt or salts dissolved in it)
3. A metal (conducting) path between the dissimilar metals
The two dissimilar metals may be totally different alloys – such as steel and aluminum – but are more likely to be microscopic or macroscopic metallurgical differences on the surface of a single piece of steel. In this case we will consider freely-corroding steel, which is non-uniform.
If the above conditions exist, the following reaction takes place at the more active sites: (two iron ions plus four free electrons):
2Fe => 2Fe++ + 4e-1
The free electrons travel through the metal path to the less active sites, where the following reaction takes place: (oxygen gas is converted to oxygen ion – by combining with the four free electrons – which combines with water to form hydroxyl ions).
O2 + 4e- + 2H20 => 4 OH
Recombinations of these ions at the active surface produce the following reaction, which yields the iron-corrosion product ferrous hydroxide: (iron combining with oxygen and water to form ferrous hydroxide).
2Fe + O2 + 2H2O => 2Fe (OH)2
This reaction is more commonly described as ‘current flow through the water from the anode (more active site) to the cathode (less active site).
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