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Chemistry of Corrosion



The Chemistry of Corrosion


As in all chemical reactions, corrosion reactions occur through an exchange of electrons. In electrochemical reactions, the electrons are produced by a chemical reaction in one area, the anode, travel through a metallic path and are consumed through a different chemical reaction in another area, the cathode. In some cases, such as the common dry cell battery, electrochemical reactions can be used to supply useful amounts of electrical current.



In order for electrochemical reactions to occur, four components must be present and active. These components are the anode, cathode, electron path, and electrolyte.



In an electrochemical cell, the anode is the site where electrons are produced through the chemical activity of the metal. The anode is the area where metal loss occurs, the reaction is called oxidation. The metal loses electrons and migrates from the metal surface through the environment. The electrons remain in the metal but are free to move about in response to voltage gradients.



The cathode in an electrochemical cell is the site where electrons are consumed. For each electron that is produced at an anodic site, an electron must be consumed at a cathodic site. No metal loss occurs at sites that are totally cathodic.



Electrolytes are solutions that can conduct electrical currents through the movement of charged chemical constituents called ions. Positive and negative ions are present in equal amounts. Positive ions tend to migrate away from anodic areas and toward cathodic areas. Negative ions tend to migrate away from cathodic areas and towards anodic areas.




Anodic Reactions.

Metal loss at anodic sites in an electrochemical cell occurs when the metal atoms give up one or more electrons and move into the electrolyte as positively charged ions.


Typical Reactions.

The generic chemical formula for this metal loss at anodic sites is:


M ---> M+ + e-



M = uncharged metal atom at the metal surface

M+ = positively charged metal ion in the electrolyte

e- = electron that remains in the metal



This type of chemical reaction is called oxidation even though it does not directly involve oxygen but only results in an increase in positive charge on the atom undergoing oxidation.

More than one electron can be lost in the reaction as in the case for iron where the most common anodic reaction is:


Fe ---> Fe+ + 2e-



Fe = metallic iron


Fe+ = ferrous ion that carries a double negative charge



Cathodic Reactions.


The electrons that are produced at anodic sites are consumed at cathodic sites. The type of chemical reactions that consume electrons are called reduction and have the generic chemical formula:


R+ + e- --> Ro


R+ = a positive ion in solution

e- = an electron in the metal

Ro = the reduced chemical


During Corrosion of Steel in contact with water the Cathodic Reactions usually is:



O2 + 2H2O + 4e-    -----> 4OH-


The final reaction in the chemistry of corrosion is the formation of Iron oxides:


Fe2+  +  2 OH-    ------>    Fe(OH)2


Fe(OH)2 will then react to give Fe2O3 and Fe3O4 (rust)



corrosion chemistry

Source : "Corrosion Control" NAVFAC MO-307 September 1992;