in tentative calculations. Field test may show
that this figure should be modified.
c. Bare pipelines can usually be protected by
11 to 22 mA/m
(1 to 2 mA/ft
). This is
seldom justifiable economically for extensive
or long lines, however, and the necessary
protection is usually afforded by the
application of cathodic protection to localized
areas called "hot spots."
d. Bare steel tanks are treated the same as
bare pipelines, inside steel surfaces in con-
tact with fresh water at zero or low velocities
require from 22 to 65 mA/m
(2 to 6 mA/ft
depending on the nature of the water. The
low value is used for water which is scale
forming. That is, the water will form a calcar-
eous coating on the surface of the metal.
e. Protecting steel surfaces in contact with
water in motion presents another problem.
Water in motion produces a scouring effect
which prevents the formation of the above-
mentioned coating and even the formation of
a hydrogen film. Therefore, surfaces ex.
posed to water in motion require a higher
current density. The amount required is hard
to predict. In this case, an experimental
determination of the current requirement
should be made.
6.4. Examples for designing a system.- Several
factors enter the determination as to how many
sacrificial anodes may be required for a given
structure and corrosion problem and the manner
of distributing them with respect to the location
where corrosion is occurring. The anode
requirements for a small installation will normally
involve the steps taken in the two following
examples. For cathodic protection of larger
structures involving use of six or more anodes or
an impressed current rectifier) system, additional
steps must be taken to assure
proper functioning of the system, i.e., proper
distribution of the anodes, prevention of damage
to other buried metal work, design of an economic
system, and proper operation and maintenance.
Problem 1: Determine the galvanic anode
requirements for a cathodic protection
system of 45.7 m (150 ft) of 0.1-m- (4-in.-)
coated pipe buried in the ground.
A. Knowledge of the condition of pipe
protective coating (
as basis for assumptions).
B. Soil resistivity in ohm-centimeters
(do not use sacrificial anodes in soil
whose resistivity exceeds about 3,000
C. Assume a current demand (see
D. Protective current required is equal
to area of bare metal to be protected
times the required current.
E. Number of anodes required must
Data and assumptions for the problem
A. Pipe surface 5 percent bare.
B. Soil resistivity determined as 1,000
C. Assume 11 mA/m
A. Protective current required is the
total area of bare steel in square me-
ters (square feet) times the required
current per square meter (square foot).
Amperes = length of pipe (m) x pipe
percent bare metal
ma / square meter
For the example of 45.7 m of pipe in
9 (FIST 4 -5)