paints and the procedures for their application
and maintenance.
and maintenance.
5. Cathodic protection
5.1. General.- Cathodic protection is the use
of an impressed or galvanic current to reduce
or prevent corrosion of a metal in an
electrolyte by making the metal to be
protected by the cathode of a corrosion cell.
The source of the protective current is
immaterial, and it may be derived from zinc or
magnesium anodes or external sources of
power, i.e., a rectifier. Whenever corrosion
takes place at the surface of steel in contact
with an electrolyte, it can be controlled by
cathodic protection. It is not always the most
economical method since other more
corrosion-resistant materials may be applied.
However, after careful study of all the factors,
cathodic control of corrosion by itself or in
conjunction with protective coatings will often
prove to be the most efficient means of
protecting buried or submerged metals.
Cathodic protection is not considered a practi-
cal means for protecting the interior surfaces
of smaller diameter pipelines. In this bulletin,
methods of using cathodic protection by sacri-
ficial anodes for protection of the exterior of
buried pipeline installations will be described.
Other applications of cathodic protection will
be briefly covered, and some reference to
adaptability of the systems to other structures
will be made. It must be remembered that for
each structure, protection is a specific
problem and has to be handled as such in
cathodic protection installations.
of an impressed or galvanic current to reduce
or prevent corrosion of a metal in an
electrolyte by making the metal to be
protected by the cathode of a corrosion cell.
The source of the protective current is
immaterial, and it may be derived from zinc or
magnesium anodes or external sources of
power, i.e., a rectifier. Whenever corrosion
takes place at the surface of steel in contact
with an electrolyte, it can be controlled by
cathodic protection. It is not always the most
economical method since other more
corrosion-resistant materials may be applied.
However, after careful study of all the factors,
cathodic control of corrosion by itself or in
conjunction with protective coatings will often
prove to be the most efficient means of
protecting buried or submerged metals.
Cathodic protection is not considered a practi-
cal means for protecting the interior surfaces
of smaller diameter pipelines. In this bulletin,
methods of using cathodic protection by sacri-
ficial anodes for protection of the exterior of
buried pipeline installations will be described.
Other applications of cathodic protection will
be briefly covered, and some reference to
adaptability of the systems to other structures
will be made. It must be remembered that for
each structure, protection is a specific
problem and has to be handled as such in
cathodic protection installations.
6. Sacrificial anode systems
6.1. Theory.- Sacrificial anodes, metallically
connected to a corroding structure and suit-
ably immersed in the electrolyte (water or
moist soil), create a simple galvanic cell in
which the structure is the cathode or
protected surface. By this device, detrimental
corrosion is replaced by localized and
controlled corrosion of an expendable anode
which can readily be examined and replaced
as necessary.
connected to a corroding structure and suit-
ably immersed in the electrolyte (water or
moist soil), create a simple galvanic cell in
which the structure is the cathode or
protected surface. By this device, detrimental
corrosion is replaced by localized and
controlled corrosion of an expendable anode
which can readily be examined and replaced
as necessary.
the mill scale is in moist soil or water. Since
the pipe metal is anodic to the mill scale and
all elements of a corrosion cell are present,
the pipe metal is anodic to the mill scale and
all elements of a corrosion cell are present,
current flows and corrosion (formation of
ferrous ion, Fe + +) progress at the break in
the mill scale
ferrous ion, Fe + +) progress at the break in
the mill scale
sufficient period of time, a pit is likely to
develop, possibility resulting in eventual
perforation and failure of the pipe. However,
as shown in
develop, possibility resulting in eventual
perforation and failure of the pipe. However,
as shown in
magnesium anode has created a new
corrosion cell in which the corrosion
(formation of magnesium ion, Mg + +) is now
taking place at the anode. The iron of the
pipe (as well as the mill-scale coating) has
become the cathode of the new cell and is
said to be cathodically protected. This type of
cathodic protection is easily recognizable as
the sacrificial anode type since the cell
generates all of the current for protection,
there being no external sources involved.
corrosion cell in which the corrosion
(formation of magnesium ion, Mg + +) is now
taking place at the anode. The iron of the
pipe (as well as the mill-scale coating) has
become the cathode of the new cell and is
said to be cathodically protected. This type of
cathodic protection is easily recognizable as
the sacrificial anode type since the cell
generates all of the current for protection,
there being no external sources involved.
shows that magnesium heads the list as the
most anodic metal and is widely separated
from iron in the galvanic series. Magnesium
coupled to iron provides sufficient galvanic
potential to provide positive protection. An
important feature of a sacrificial anode
system is that it is inherently a safe system
because the normal potentials generated are
insufficient to damage coatings present on
the surface to be protected. Because of the
low potentials generated, sacrificial systems
can be used only in low-resistance soils, i.e.,
with a resistivity less than 3000 ohm-
centimeters.
most anodic metal and is widely separated
from iron in the galvanic series. Magnesium
coupled to iron provides sufficient galvanic
potential to provide positive protection. An
important feature of a sacrificial anode
system is that it is inherently a safe system
because the normal potentials generated are
insufficient to damage coatings present on
the surface to be protected. Because of the
low potentials generated, sacrificial systems
can be used only in low-resistance soils, i.e.,
with a resistivity less than 3000 ohm-
centimeters.
6.3. Assumptions of protective current
require-merits and bare metal areas.- To
obtain a starting point, certain general
assumptions have been found helpful.
require-merits and bare metal areas.- To
obtain a starting point, certain general
assumptions have been found helpful.
a. For bare metal in the ground, a current of
11 to 22 mA/m
11 to 22 mA/m
2
(1 to 2 mA/ft
2)
of bare metal
surface has been found adequate, except
under extreme or unusual conditions. This
value must then be modified to suit the par-
ticular conditions.
under extreme or unusual conditions. This
value must then be modified to suit the par-
ticular conditions.
b For coated pipe, the current required is
difficult to estimate without field tests. The
primary reason is the unknown condition of
the protective coat which can vary from nearly
0 to 98 percent coverage. For a fairly new
protective coat properly applied, assume 2
percent bare and 22 mA/m
difficult to estimate without field tests. The
primary reason is the unknown condition of
the protective coat which can vary from nearly
0 to 98 percent coverage. For a fairly new
protective coat properly applied, assume 2
percent bare and 22 mA/m
2
(2 mA/ft
2
) for use
7 (FIST 4- 5)