TM 5-811-7
foot of bare area. The amount of current required
Table 2-2. Typical current density requirements for cathodic
for complete cathodic protection can be determined
protection of uncoated steel
three ways:
Environment
Current density (mA/sq ft)
rardb
--An actual test on existing structures using
AFM 88-9a
Ger
a temporary cathodic protection setup.
Neutral soil
0.4
to 1.5
0.4 to 1.5
--A theoretical calculation based on coating
Well aerated neutral soil
2
to 3
2 to 3
efficiency.
Wetsoil
1
to 6
2.5 to 6
Highly acidic soil
3
to 15
5 to 15
--An estimate of current requirements using
Soil supporting active
tables based on field experience.
sulfate-reducing bacteria
6
to 42
Up to 42
Heated soil
3
to 25
5 to 25
(1) The second and third methods above can
Stationary freshwater
1
to 6
5
be used on both existing and new structures.
Moving freshwater containing
Appendix B contains a detailed review of current
dissolved oxygen
5
to 15
5
to 15
requirement testing.
Seawater
3
to 10
5
to 25
(2) Current requirements can be calculated
based on coating efficiency and current density
a
Data are from Air Force Manual AFM 88-9, Corrosion Control (U.S.
(current per square foot) desired. The efficiency of
Air Force, August 1962), chap 4, p 203.
the coating as supplied will have a direct effect on
the total current requirement, as equation 2-1
b
Data are from J.S. Gerrard, "Practical Applications of Cathodic
shows:
Protection," Corrosion, Vol 2 (L.L. Shreir, Ed.), Newnes-
Butterworths, London, 1976, p 11:65. Used with permission.
I = (A)(I0)(1.0-CE),
j. Coating resistance. A coating's resistance
(eq 2-1)
decreases greatly with age and directly affects
structure-to-electrolyte resistance for design calcu-
lations. The coating manufacturers supply coating
where I is total protective current, A is total struc-
ture surface area in square feet, I0 is required cur-
resistance values.
k. Protective current required. By knowing the
rent density, and CE is coating efficiency. Equation
physical dimensions of the structure to be pro-
2-1 may be used when a current requirement test is
tected, the surface area can be calculated. The pro-
not possible, as on new structures, or as a check of
duct of the surface area multiplied by current den-
the current requirement test on existing structures.
sity obtained previously in I above gives the total
Coating efficiency is directly affected by the type of
current required.
coating used and by quality control during coating
l. The need for cathodic protection. For exist-
application. The importance of coating efficiency is
ing structures, the current requirement survey (I
evident in the fact that a bare structure may require
above) will verify the need for a cathodic protec-
100,000 times as much current as would the same
tion system. For new systems, standard practice is
structure if it were well coated.
to assume a current density of at least 2 milliam-
(3) Current requirements also can be esti-
peres per square foot of bare area will be needed to
mated from table 2-2. The table gives an estimate
protect the structure. (However, local corrosion
of current, in milliamperes per square foot, required
history may demand a different current density.) In
for complete cathodic protection. That value,
addition, cathodic protection is mandatory for
multiplied by the surface area of the structure to be
underground gas distribution lines (Department of
protected (in square feet) gives the total estimated
Transportation regulations--Title 49, Code of
current required. Caution should be used when
Federal Regulations, Oct 1979) and for water stor-
estimating, however, as under- or overprotection
age tanks with a 250,000-gallon capacity or
may result.
greater. Cathodic protection also is required for
underground piping systems located within 10 feet
2-2
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