MIL-HDBK-1004/10
Section 5:
PRECAUTIONS FOR CATHODIC PROTECTION SYSTEM DESIGN
Introduction. Cathodic protection systems depend upon the
5.1
application of electrical currents to counterbalance the natural tendency of
metals to revert to their low energy combined states. In addition to the
hazards involved with the operation of any electrical device, cathodic
protection systems may, through the flow of current in undesirable paths,
accelerate the corrosion of other structures in the vicinity of the structure
being protected. Potentials exceeding those given in para. 3.2 can result in
system damage. Potentials less than those required for complete protection
usually result in some protection on most areas of the structure and do not
result in attack at rates greater than for completely unprotected structures
of similar materials and conditions.
Excessive Currents and Voltages. Flow of current in undesirable
5.2
paths is proportional to the total amount of current flowing in the cathodic
protection system, all other factors being equal. Thus, protection of well
coated structures that require small amounts of current for protection will
cause fewer problems than protection of poorly coated or bare structures that
require substantially more current. Assuming that damaging potential levels
such as those resulting in coating damage or hydrogen embrittlement are not
reached, excessive currents will not cause deterioration of the structure
being protected but can cause severe localized attack of adjacent structures.
Interference. The flow of current in undesirable paths resulting
5.2.1
in accelerated attack of adjacent structures is called "interference." The
adjacent structures are commonly called "foreign" structures. In Figures 19
and 20, the mechanisms of two typical situations resulting in interference are
illustrated.
In Figure 19, the undesired current flows through two buried
pipelines in the vicinity of the protected system. Corrosion is accelerated
at locations where positive ions are forced to enter the environment from the
metal surfaces. Although corrosion is not usually accelerated on the
protected structure, cathodic interference can disturb the desired current
distribution on the structure being protected and can reduce the amount of
protection being received in some areas to a level below that required for
complete protection. Corrosion occurs where the current leaves the structure
and enters the electrolyte. In some models of electricity, the flow of
positive ions in the electrolyte is used to establish the flow of imaginary
positive charges in the metallic path used to describe conventional "current
flow." However, conventional current flow is widely used in the cathodic
protection industry and the flow of ions (positive) through the electrolyte
should be described for each situation in order to avoid confusion. This
description of electron and ion flow will be used throughout to explain what
is actually happening.
In Figure 20, the potential gradient surrounding an impressed
current anode results in an electron flow along a foreign structure. This
flow is described as positive ion (conventional) current flow from the anode,
through the electrolyte, to the pipe (cathode) and continuing on along the
pipe (imaginary) to the point of discharge (anode area) where accelerated
corrosion occurs.
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