MICROBIAL CORROSION.
An emphasis on corrosion so far is that four processes are needed for corrosion to occur. One method for these processes to occur involves biological processes, where organisms can produce electron flow, or modify the local environment to change from a non – corroding to a corrosive one. Some of these processes will be outlined in the following section.
Deposits.
When microbial deposits form on the surface of a metal, one case that exists is that they can be regarded as inert deposits on the surface, shielding the area below from electrolyte. A differential aeration cell can be considered to form, even for a very small colony. The area directly under the colony will become the anode and the metallic surface just outside the contact area will support the reduction of oxygen reaction and become the cathode. Metal dissolution will occur under the microbial deposit, and so would resemble pits, but the density of local dissolution areas should match closely the colony density.
Product Corrosion.
Microbial deposits can produce components that will change the local environment and thereby induce corrosion. Both inorganic and organic acids can be produced that will initiate corrosion when they are produced at the colony to metal interface.
The production of inorganic acids will also lead to hydrogen ion production which may contribute to hydrogen embrittlement of the colonized metal.
Sulfate Reducing Bacteria
In anaerobic conditions, some bacteria can reduce the sulfate ion to produce oxygen and the sulfide ion. The sulfide ion then combines with ferrous ions to form iron sulfide. The metal surface is dissolved. The oxygen produced reacts with hydrogen to form water molecules.
Hydrogen ions are produced, along with hydroxyl ions, by the breakdown of water, and the electrons to form atomic hydrogen from the hydrogen ion are produced by the oxidation of atomic iron. The reaction is therefore multi staged, depending upon anodic,cathodic, water dissociation and bacterial reactions. The source of electrons is the oxidation of the metal while the electron sink is reduction of hydrogen ions.
Metal Reactive Bacteria.
Some bacteria can directly reduce metal atoms to ions.
Testing Techniques.
Impedance spectroscopy is one test technique that is applicable to bio corrosion. Potentiodynamic scans have also been used to determine the effect of biofilms on both anodic and cathodic behavior.
References.
1. Microbiological Corrosion, Dr. Richard Lutey, NACE 80, Paper 39, pub NACE, Houston Texas, 1980.