Cathodic Protection

How Does Steel Corrode in Water?
For corrosion to occur, three conditions 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.

I don't know where I got this information but at least it is very useful to understand how cathodic protection work as explanation below.



















• If the above conditions exist, at the more active metal surface (in this case we will consider freely corroding steel which is non uniform), the following reaction takes place at the more active sites: (two iron ions plus four free electrons)
2Fe => 2Fe++ + 4e-

• The free electrons travel through the metal path to the less active sites where the following reaction takes place: (oxygen gas 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).


• How Does Cathodic Protection Stop Corrosion?
Cathodic protection prevents corrosion by converting all of the anodic (active) sites on the metal surface to cathodic (passive) sites by supplying electrical current (or free electrons) from an alternate source.

Usually this takes the form of galvanic anodes, which are more active than steel. This practice is also referred to as a sacrificial system, since the galvanic anodes sacrifice themselves to protect the structural steel or pipeline from corrosion.

In the case of aluminum anodes, the reaction at the aluminum surface is: (four aluminum ions plus twelve free electrons)

• 4Al => 4AL+++ + 12 e-

and at the steel surface, (oxygen gas converted to oxygen ions which combine with water to form hydroxyl ions)
• 3O2 + 12e- + 6H20 => 12OH-

As long as the current (free electrons) is arriving at the cathode (steel) faster than oxygen is arriving, no corrosion will occur.


Impressed Current Cathodic Protection Systems
Due to the high currents involved in many seawater systems it is not uncommon to use impressed current systems. Impressed current systems use anodes of a type that are not easily dissolved into metallic ions, but rather sustain an alternative reaction, oxidization of the dissolved chloride ions.

2Cl- => Cl2 + 2e-
Power is supplied by an external DC power unit..