How Does Steel Corrode in Water?

To understand cathodic protection one must first understand the corrosion mechanism. 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

The two dissimilar metals may be totally different alloys, such as steel and aluminum, but are more usually microscopic or macroscopic metallurgical differences on the surface of a single piece of steel.


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).

by: Richard Baxter, Jim Britton

Source: http://www.cathodicprotection101.com/