Impact of Oxygen on Corrosion
Electrochemical corrosion is the most important classification of corrosion.
Four conditions must exist before electrochemical corrosion can proceed:
(1) there must be something that corrodes , the metal anode, where the oxidation reaction take places
(2) there must be a cathode, where the reduction reaction take places
(3) there must be continuous conductive liquid path (electrolyte, usually condensate and salt or other contaminations), for example: water, seawater, condensing water, humidity..
(4) there must be a conductor to carry the flow of electrons from the anode to the cathode.
The oxygen in the air dissolves in the water and causes rust to form. There are always two distinct chemical reactions in a corrosion process, the basic chemistry of corrosion is:
1) Anodic Dissolution of Metal (Iron) that goes into solution (water)
Fe -----> Fe2+ + 2e-
2) Cathodic Reduction of Oxygen dissolved into water
O2 + 2H2O + 4e- ---->
4OH-
The final reaction is: Fe2+ + 2OH- -----> Fe(OH)2