UFC 3-570-06
JANUARY 31 2003
Figure 2-16. Stray Current Corrosion Cell Caused by a Cathodic Protection
System
Cathodically Protected Structure (Cathode)
CP Power Source
CP Anode
2-2.4.5
cause induced current in metallic structures. Some areas may be prone to these effects
due to mineral deposits or other physical or environmental characteristics. These
currents may also be caused by severe sun spot activities. The varying earth's
magnetic field intercepting the metallic pipeline generates a voltage on the structure;
where this current leaves the structure to enter into the earth, corrosion occurs.
2-3
RATE OF CORROSION. Since almost all corrosion is an electrochemical
reaction, anything that affects the speed of a chemical reaction or the amount of current
flow will affect the rate of corrosion. Ohms law is applicable to the electrical portion of
the corrosion cell. The rate of corrosion is directly proportional to the amount of current
that flows in the electrochemical corrosion cell. If the current can be measured, an
exact calculation of the metal loss can be made. This means that a measurement in
amps or milliamps can be mathematically calculated in kilograms (pounds) per amp
year. One amp year is one amp flowing for a period of one year. Different metals have
different consumption rates.
2-3.1
Electrical Effects on the Rate of Corrosion. Any factor that affects the
amount of current flowing in a circuit will affect the rate of the electrical portion of the
electrochemical reaction (corrosion). Following is a description and example of the
factors affecting the rate of the electrical portion of corrosion.
2-3.1.1
Potential Difference. The potential difference between the anode and the
cathode is electromotive force and can be measured as voltage. The greater this
difference, or voltage, the greater the potential of corrosion. The voltage is directly
proportional to the current, and therefore the corrosion, in an electrochemical cell. If the
voltage is doubled, and all other factors remain the same, the amount of corrosion
2-19
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