2 If a major food supply is combined with oxygen restriction, the corrosion action when
located against an underground bare metal structure can be intensified. An example is a piece of wood lying
against a bare metal pipe, bacteriological corrosion can begin.
(b) Combination Sources of Corrosion Cells.
1 A corrosion problem at a given point on an underground structure is not necessarily
confined to just one of the previously discussed types of corrosion cells.
2 Two or more sources of corrosion cell current may be acting at a given location, such as
dissimilar metals and differential aeration.
8.
Factors Affecting Rate of Corrosion.
NOTE: The following factors that can affect the rate of corrosion once a corrosion cell is established.
a. There has to be a d-c potential (voltage) between the anode and cathode of the corrosion cell in order
for corrosion current to flow.
b. The higher the potential, the higher the current flow and the greater the amount of corrosion.
9.
Corrosion Controls.
a. Corrosion control procedures can be used and can result in a high degree of freedom from corrosion.
b. The use of preventive corrosion control measures put into effect when an underground metallic
structure is built is the best way to go, If this is not done, and corrosion does occur, after-the-fact corrosion control
measures can be applied to alleviate further corrosion.
(1) Coatings.
(a) It has been learned that whenever direct current flows from an underground metallic structure
into a surrounding electrolyte (earth or water), the metal will be consumed (corroded) at the points of current
discharge.
(b) Following simple logic, if an insulating barrier (a coating) were to be placed between the
metal and the surrounding electrolyte, corrosion current could not flow and there would be no loss of metal.
QM5200
16-6
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