UFC 3-570-06
JANUARY 31 2003
Figure 2-7. Concentration Cell Caused by Concrete and Soil Electrolytes
Cathodic Areas on Pipe
Anodic Area
Anodic Area
Current Flow in the Earth
2-2.2.6
concentration cells, except that the "backfill impurities" are materials that do not
normally occur in the soil, but are foreign materials mixed into the electrolyte during or
between the excavation and the backfill process. This can be any material that forms
anodic or cathodic areas on the structure. It can also be an isolating material that forms
different conditions in the electrolyte, or a metallic material which actually becomes an
anode or cathode when in contact with the structure (galvanic corrosion).
2.2.2.7
surface of a metal, causing a different environment that in some cases may be
extremely corrosive to the metal. Most bacteria that have been implicated in corrosion
grow best at temperatures of 15 C to 45 C (60 F to 115 F). These bacteria are
generally classed by their oxygen requirements, which vary widely with species, and
may be aerobic or anaerobic. Their metabolism products influence the electrochemical
reaction by forming materials or films (slime) that act as a diffusion barrier, or change
ion concentrations and pH. Some bacteria are capable of being directly involved in the
oxidation or reduction of metal ions and can shift the chemical equilibrium that
influences the corrosion rate. Aerobic bacteria form oxygen and chemical concentration
cells, and in the presence of bacteria capable of oxidizing ferrous ions, further
accelerate corrosion. Many produce mineral or organic acids that may also breakdown
structure coatings. The breakdown products are then sometimes usable as food,
leading to accelerated corrosion.
2-2.3
Galvanic Corrosion. This type of corrosion is caused by an
electrochemical corrosion cell developed by a potential difference in the metal that
makes one part of the cell an anode, and the other part of the cell the cathode.
2-10
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