In bimetallic corrosion, the more reactive metal corrodes faster in case where a pair of metals with electrical and physical contact is dipped in a corrosive electrolyte. The less corrosion resistant or the “active” member of the couple experiences accelerated corrosion while the more corrosion resistant or the “noble” member of the couple experiences reduced corrosion due to the “cathodic protection” effect. The most severe attack occurs at the joint between the two dissimilar metals. Further away from the bi metallic joint, the degree of accelerated attack is reduced.
It is a technique to control the corrosion of a metal surface by making it work as a cathode of an electrochemical cell. This is achieved by placing in contact with the metal to be protected another more easily corroded metal to act as the anode of the electrochemical cell.
Cathodic protection systems are most commonly used to protect steel, water or fuel pipelines and storage tanks, steel pier piles, ships, offshore oil platforms and onshore oil well casings.
Galvanizing of iron is an another example.
Protection of a metal by rendering its surface inactive by chemical methods is called passivation.
In the context of corrosion, passivation is the spontaneous formation of a hard non reactive surface film that inhibits further corrosion. This layer is usually an oxide or nitride that is a few molecules thick.
Stainless Steels can be passivated using a solution of nitric acid, to remove foreign particles from the surface and promote the growth of a protective oxide layer. Nickel can be used for handling elemental fluorine, due to a passivation layer of nickel fluoride.
