Cathodic Corrosion

What Does Cathodic Corrosion Mean?

Cathodic corrosion is an electrochemical process where two metals are placed in an electrolyte and one of the metals corrodes. Corrosion continues as long as the electric contact is maintained, since the electrons continue to flow to the cathode.

For cathodic corrosion to occur, three conditions must be fulfilled: there must be two dissimilar metals, an electrolyte of salt dissolved in water and a metal must maintain the electric connection.

The type of alloy used does not matter, although the most common metals used are steel and aluminum.

Corrosionpedia Explains Cathodic Corrosion

When freely-corroding steel is used, the following reaction is common:

2Fe = 2Fe₂+ + 4e-

There are several electrons released that travel to the less active site. The reaction here is:

O₂ + 4e- + 2H₂0 = 4OH-

The two equations can then be recombined to form the following full equation:

2Fe + O₂ + 2H₂O = 2Fe (OH)₂

Cathodic protection is protecting the anodic metal by reducing the activities. This is achieved by supplying electrons to the anode from an electric charge. A galvanic anode, that is more active than steel, sacrifices itself to protect the structure from corrosion, hence the name "sacrificial systems."

When aluminum anodes are used, four ions of aluminum and 12 electrons are ready for the reactions:

4Al = 4AL₃+ + 12 e-

At the same time, oxygen gas, in the form of its ions, combines with water to form hydroxyl ions:

3O₂ + 12e- + 6H₂0 = 12OH-

If you want to protect the structure, control the amount of electrons and ensure they reach on the steel faster than the oxygen gas. This way, no corrosion will take place. There are two methods that can be used to prevent cathodic corrosion:

1. Couple the structure with a more active metal. For example, when the structure is made from steel, you can couple it with magnesium or zinc. The active metal makes it possible for a galvanic cell work to produce more electrons, thus acting as an anode. The active metal wears out, hence the name "sacrificial anode."
2. Use a direct current between the inert anode and the structure. This will ensure that the structure does not produce electrons.