of current from an anodic section of the steel plate
to a cathodic section of the steel plate to a
cathodic section of the same plate. The same is
sacrificial anode of magnesium has been added.
Since the magnesium anode is the least noble of
the three metals in the ground (electrolyte), it
becomes the sacrificial metal and deteriorates
from the corrosive action in lieu of the iron plate
and steel pipe.
9. Evaluation and solution of a field corrosion
9.1. Initial examination.- When the existence
of significant corrosion of a structure is
revealed, a method of controlling it must be
selected, and the procedure outlined in
paragraphs under this heading should be
lollowed by O&M personnel. A preliminary
investigation should first be made to ascertain
the extent and severity of the damage.
Enough of the corroding surface should be
exposed to permit adequate examination at
various locations. Special note should be
taken of the type and condition of any
protective coatings on the surface. The
character of the corrosion is of considerable
importance; that is, whether corrosion is of the
pitting type which may rapidly perforate a
pipeline or general surface corrosion which
may consume considerable metal before
failure or structural weakening would occur.
With the results of the inspection in hand,
consideration can be given to application of
protective coatings, cathodic protection, or
other remedies as means of controlling the
9.2. Sources of information.- Protective coat-
ings are discussed in this bulletin especially in
conjunction with cathodic protection, but the
Paint Manual provides a more comprehensive
treatment of coatings in general. The two
types of cathodic protection installations, the
sacrificial anode and impressed current or
rectifier systems, must be investigated to
enable selection of the best method which
depends on the complexity of the structure
and on whether the corrosion is galvanic or
stray current. Design of systems for long,
large, or complex structures and for
prevention of stray current corrosion requires
specialized knowledge, experience, and
techniques. Likewise, economic considerations
enter the picture with the more costly
installations. Thus, specialist advice or a field
survey may be necessary.
9.3. Protection methods for various situations. -
The summary below lists a number of common
field exposures where corrosion problems may be
encountered. One or more protection methods
may be appropriate when further corrosion must
be prevented. These methods are numbered
below and listed as possible solutions.
1. Application of protective coating.
2. Install cathodic protection by sacrificial
3. install cathodic protection by impressed
Metal surfaces exposed
to the atmosphere.
Metal surfaces inter-
mittently exposed to
1 and 2 or 3.
water and atmosphere.
Gates, piping, or other
metal work in water,
1 and 2 or 3.
including interior of
Interior of water pipes.
1 and 2.
Exterior of pipe tanks,
1 (touch up defects
or conduit buried in
in good existing
Guy rods and anchors
*The above listing is not an attempt to list the possible solutions in
order of their preference, and the normal design practice is to
provide the initial corrosion protection by selection of material and
through painting. However, if damaging corrosive conditions still
exist, additional corrosion prevention can usually be obtained by the
addition of a sacrificial anode or an impressed current system.
17 (FIST 4- 5)