Fe --> Fe+2 + 2e-
Analogous reactions occur in other metals. The electrons migrate through the metal to the cathode area where they react in any one of several ways.
Some typical cathodic reactions are as follows:
Negatively charged ions, such as hydroxyl ions produced at the cathode, migrate to the anode of the corrosion cell. Positively charged ions will move toward the cathode.
This movement of ions can cause additional reactions at the
Fe+2 + 2OH- ----> Fe(OH)2
The ferrous hydroxide produced has a very low solubility and is quickly precipitated as a white floe at the metal-water interface. The floe is then rapidly oxidized to ferric hydroxide:
4Fe(OH)2 + O2 + 2H2O ----> 4Fe(OH)3
Dehydrolysis of this product leads to the formation of the corrosion products normally seen on ferrous surfaces, red dust and hydrated ferric oxide:
2Fe(OH)3 ----> Fe203 + 3H2O Fe(OH)3 ----> FeOOH + H2O
As solid corrosion products are precipitated at the anode, they may cause the precipitation of other ions from the water. Thus, a corrosion film may show traces of hardness salts, or suspended matter like mud, sand, silt, clay or microbiological slime.
The structure of the entire surface film, including corrosion products and inclusions, is a major factor in determining the total amount of corrosion which will take place. If a porous film forms over the metal, corrosion can continue, because metal ions can penetrate it and reach the solution interface. If, however, a tight, adherent film is formed, ionic diffusion is prevented and the metal will no longer dissolve.
Most corrosion occurs at the beginning of a metal's service life. Initially, metal dissolution is not impeded by a film of corrosion products. In time, the film will retard, or halt the corrosion. The degree to which such a film can impede corrosion is a complex function of the corrosion reactions, the structure of the deposit and the water velocity.