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Chemical reactions of Microbiologically Influenced Corrosion (MIC)

 

The microscopic organisms and the by-products they produce influence the electrochemical corrosion of metals are known as MIC (microbiologically influenced corrosion).  As soon as a metal is placed in water a biofilm begins to form on its surface.   

In general, a protective slime, or biofilm, will build up on surfaces to facilitate an organic decomposition, which can lead to favorable conditions for corrosion initiation if the biofilm coverage is sporadic.  The formation of biological growth on surfaces in contact with natural water is known as biofouling.[1]  The biofilms traps nutrients and provides an ideal microenvironment for the bacteria.  Colonies are developed, trapping ions and creating localized chemical and physical gradients at the metal surfaces.  As the biofilms increases in thickness, oxygen permeability to the interior decreases creating a good environment for anaerobic bacteria; the remaining aerobic bacteria will consume any oxygen that does penetrate the biofilm.  As discussed below, a small electrochemical cell is cultivated, which can lead to metal dissolving under the biofilms and pitting 

The area under the deposit, which has the lowest oxygen concentration, will become the anode in the reaction while the area outside the deposit is the cathode, Figure 1.  The electrochemical reaction becomes self-sustaining.  As the pit grows, iron dissolves according to the anodic reaction:

Fe → Fe2+ + 2e-

(Eq . 1)

 
Assuming a basic or neutral environment, the cathodic reaction is reduction of dissolved oxygen outside the pit to form OH- according to: 
 

O2 + 2H2O + 4e- → 4OH

(Eq. 2)

 
If chloride ions are present, the chloride ions (-1) migrate to the anode area to maintain a charge balance and react with the iron ions (+2) to form ferric chloride, a corrosive solution. 
 

Fe2+ + Cl-→ FeCl2

(Eq. 3)

 
 

References

(1) S. W. Borenstein, Microbiologically influenced corrosion handbook, 1994.

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