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Corrosion reactions
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It has been demonstrated that potential differences within a metal, or between two metals, will cause chemical reactions at the anode and cathode. Anodic reactions are typified by the dissolution of iron:

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:


a. Hydrogen ion reduction     2H+ + 2e- ---- > H2 Important in acidic solutions.


b. Reduction of water            2H20 + 2e- ---- > H2+2OH- Occurs normally in natural waters.


c. Oxygen reduction                 02 + 4H+ +4e- ---- > 2H20 Occurs in aerated acidic solutions. 


d. Oxygen reduction of water O2 + 2H2O + 4e- --- > 40H- Important in natural, aerated waters.


e. Ferric ion reduction             Fe+3 + e- --- > Fe+2    Occurs under acidic, turbulent conditions (e.g. acid cleaning). 


f. Sulfate ion reduction         4H2 + S04-2 ---- > S-2 + 4H2O Occurs in the presence of sulfate reducing bacteria.


g. Metal ion reduction (plating) M+N + Ne- ----> MO  Involves more noble metals in solution. The most frequent cathodic reactions are a, b, c and d. 

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 anode.
Hydroxyl ions will combine with the ferrous cations produced by dissolution of the metal:

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.


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