Where the electricity leaves the metal
Fundamentals of Corrosion
Corrosion is caused by the flow of electric current from areas of a metal surface through a conducting solution (or environment). Eating away of metal due to this electric current occurs at areas which are called anodes, where the electricity leaves the metal, entering the environment exposed to it.
Fe+2 ------------> Fe+3 + 1 e-
O2 (g) + 2 H2O + 4e- ------> 4 OH-
For corrosion to occur, an anode, a cathode and electrolyte are essential as shown in the corrosion triangle. If by any means linkage within this triangle is broken, corrosion can be controlled and prevented.
They aid in modifying the environment around our system so as to prevent or slow corrosion down. Inhibitors retard corrosion by increasing polarization of anode and cathode, and by increasing the electric resistance due to formation of a deposit on the surface of metal. Examples of corrosion control using inhibitors include, modifying structures to provide desired drainage, using inhibitors in power plants or engine cooling systems.
3.)Cathodic protection:
Impressed current:
In this method, we supply direct electric current from any other external source (sometimes even using a rectifier to convert AC to DC). An inert anode material is used in the process to complete the circuit.
Cathodic protection of steel:
In some protective coatings there is present a high zinc particle (or any other active metal) loading. These particles are in electric contact with each other as well as with the underlying metal. Thus a sort of cathodic protection is created by the protective film.
Due to formation of a stable oxide layer on the surface, further corrosion is prevented. Thus at first stage, metal which forms stable oxide layer must be selected, or/and oxidation resistance of some metals (generally alloys) has been improved by adding alloying elements like chromium, silicon, aluminium etc which on reaction with oxygen tend to form a stable oxide film which act as protective coating for metal when exposed to their service environments.
Away from dry and moist environments:
[3] http://www.materials.manchester.ac.uk/research/groups/corrosionandprotection/areas/index.html#aqueous
[4] http://www.corrosionist.com/corrosion_control_methods.htm
[9] Principles and prevention of corrosion, Denny A. Jones, 1996 second edition.
[10] Corrosion: Understanding the basics, J. R. Davis, 2000