JANUARY 31 2003
M0 -> M+n++ ne-
M0 represents a metal atom such as iron or copper in a metallic structure.
The arrow represents the direction in which the reaction is occurring. The symbol M+
represents a metal ion. Metal ions formed in the corrosion reaction leave the metal
structure and enter the environment (electrolyte). The symbol e- represents the
negatively charged electron released by the formation of the metal ion. The free
electron that is formed in the corrosion reaction remains within the metal structure. For
a specific anodic reaction such as occurs in the corrosion of copper the reaction would
Cu0 -> Cu++ + 2e-
This represents the reaction of one copper atom to form one copper ion with
a charge of +2 and two electrons. Note that there is no change in total charge (0 = +2 +
-2). All metals can react to form metal ions and electrons. It is characteristic of anodic
reactions that metal ions and electrons are produced.
Cathode Reaction. At the cathode there are many possible reactions. The
simplest common cathodic reaction is the reaction of hydrogen ions, which are present
in water solutions, with electrons to form hydrogen gas. In chemical shorthand this
reaction is written:
2 H+ +2e- -> H2
This represents the reaction of two hydrogen ions (2H+) with two electrons
(e-) to form two hydrogen atoms, which then combine to form one molecule of hydrogen
(H2) gas. As in the case of anodic reactions, there is no change in net charge in this
reaction (+2 + -2 = 0).
Another common reaction at the cathode is the reaction of electrons with
dissolved oxygen and the breakdown of water into hydroxyl ions. In chemical shorthand
this reaction is written:
O2 + 2H2O + 4e- -> 4OH-
This represents the reduction of dissolved oxygen (O2) in alkaline electrolytes
where oxygen and the breakdown of two water molecules (2H2O) results in the
formation of four hydroxyl ions (4OH-).
Other Cathodic Reactions. In other cathodic reactions, different ions may
react with electrons, but the important characteristic of every cathodic reaction is the re-
bonding (gaining) of electrons, which is the main characteristic of a reduction reaction.
Metal ion reduction and metal deposition may also occur. Note that there is no direct
involvement of the metal itself in the cathodic reaction, except that if metal ions are