Oxidation-Reduction Reactions:

The corrosion of a metal (which is, the chemical transformation which is recognized as destructive to the metal) is the oxidation step of the whole oxidation-reduction process.  Oxidation is the procedure of losing electrons; deduction is the procedure of gaining electrons.  The metal atoms release electrons (are oxidized) and become positive ions. The site at that this occurs is called as the anode.  Classical oxidation half-reactions involve the following.

 Zn → Zn⁺² + 2e^-                                                                         (2-1)

Al → Al ⁺³ +3e^-                                                                             (2-2)

Fe → Fe ⁺² +2e^-                                                                             (2-3)

The cations (positive ions) might then go into solution, or they might combine along with any available anions (negative ions) or water to form ionic compounds.  The correct fate of the cations is important to following processes, other than the main effect is which atoms leave the metallic state and the metal deteriorates.

An oxidation process cannot take place without a concurrent reduction (gain of electrons) process. A nature of the reduction step in corrosion sometimes varies along with the metal and the environment to that it is exposed. For most metals within an aqueous environment, the most important reduction half-reaction is the reduction of hydronium ions (a hydronium ion is simply a hydrogen ion attached to a water molecule).

H₃0⁺ + e^- → H +H₂O                                                                          (2-4)

Small concentration differences inside a solution in contact along with the metal might also affect the rate and nature of corrosion reactions. Thus, it is frequent impossible to predict the exact nature of corrosion reactions. It is commonly found, therefore, that for most metals exposed to an aqueous environment the half-reactions included in corrosion are the reduction reaction of Equation (2-4) and an oxidation half-reaction of the category display in Equations (2-1) by (2-3).

General corrosion is the procedure whereby the surface of a metal undergoes a slow, associatively uniform; removal of material. That occurs on the surface of a single metal rather than dissimilar metals. In common corrosion, a nearly infinite number of micro-cells are established on the metal surface. The Oxidation occurs at anodic areas and reduction at cathodic areas.  A micro-cell is uniformly distributed over the metallic surface, and as the reaction proceeds the cells might migrate, and disappear and re-form.  That is, some particular micro-region might be instead cathodic and anodic.  The outcome is a uniform attack on the metal surface.

Under a few conditions, associatively huge regions become anodic or cathodic.  Such regions have fewer tendencies to migrate and might remain operative for long periods of time. In that case, there will be severe attack of the metal at the anodic (oxidation) region. The output might be a visible pit within the metal surface.