Page 604 - Corrosion Engineering Principles and Practice

566 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 567

or other activities which may make the site untenable within
the expected life of the installation.
• Presence of metallic structures, other than the one to be
protected, which might be affected by the planned system.
• Presence of CP systems on other structures, which may have
an effect on the structure to be protected.
• Presence of sources of stray DC such as DC railway systems.
• Presence of AC systems which can have an effect on the
readings taken and on the criteria for full protection of the
metal.
• Unusual environmental conditions such as acid-waste
discharge from local manufacturing operations.

The current needed to protect a given buried metallic structure
may vary over wide limits, depending on the nature of the environment
and whether or not it has a protective coating, and if coated, on the
quality and effectiveness of the coating as applied. For example, if it
is assumed that a steel structure to be protected is buried in corrosive
soil and has an exposed surface area of 90 m , the current required,
2
assuming reasonably uniform current distribution, could range from
about 3 A if the structure is bare, to as low as 30 mA or less if the
structure has a superior coating. This means that a bare structure may
require 100,000 times as much current as the same structure would if
it were well coated.
It cannot be assumed, however, that just because a structure is
coated, it will take a small amount of current to cathodically protect
it. A poor coating material or an excellent material poorly applied can
take much more current than the low value given above. For the same
2
90 m structure, a relatively poor coating could result in a current
requirement for CP in the order of 15 mA or more. A current of 15 mA
may not sound like much, but it is 500 times as great as the 30 mA
value given for a superior coating. The difference is considerable
when working with large structures (such as large diameter cross-
country pipelines).

13.7.1 Tests for a Coated System
When surveying a coated system, data on the electrical strength of
the coating and on current requirements for CP can be taken
concurrently. If the coating is in reasonably good condition, current
requirements are obviously much smaller than on bare structures.
This makes it possible to test large surfaces, distances, or structures
with one test setup and a modest power supply. This may be done by
actually applying current using a temporary test setup and adjusting
the current from the power source until suitable protective potentials
are obtained [15].