From Estimated Exposed Surface Area
Estimating current requirements from expected exposed surface is always subject to
error. There are many factors, which affect the results.
• Total surface area in contact with soil or other electrolyte.
• Dielectric properties of any protective coating.
• Factors which may damage a protective coating during installation.
• Expected protective coating life under service conditions.
• Expected percentage coverage by protective coating.
• Past experience with coating applicators and construction contractors.
• Current density required for cathodic protection of the metal(s) in the
In the end, the expected current requirement depends on calculating the area of
exposed metal in contact with the electrolyte and multiplying it by the “best estimate”
of current density for the conditions present.
There is an alternate approach for coated electrically isolated structures (pipes, under-
ground storage tanks, etc.) where there is data available on existing cathodic protection
The approach requires reliable local data on:
• Expected leakage conductance (Siemens/unit area) in 1000 ohm cm. soil for a
class of coating (epoxy, polyethylene tape, etc.) and type of service
(transmission pipeline, gas distribution, fuel tank).
• Soil resistivity in the service area.
• Structure to soil potential shift required to produce polarization needed to meet
cathodic protection criteria. This is the immediate change in potential of an
isolated structure measured to a point at “remote earth” when cathodic
protection is applied.
The value is not a criteria for protection. However, under a given set of operating and exposure conditions, a potential shift will provide a good estimate of current needed to meet accepted criteria.
The approach is best understood by using an example.
A gas utility is planning to install 3049 meters (10,000 feet) of 5.1 cm (2 inch) coated
steel distribution mains in a new development. The average soil resistivity in the area
is 5,000 ohm cm. The corrosion engineer wishes to estimate the approximate current
required to cathodically protect the pipes.
Experience in the utility has developed the following data on cathodic protection
Average leakage conductance G for distribution type service is 2.14 × 10−3S/m2in
1000 ohm cm soil.
Average potential shift measured to “remote earth” to achieve protection is −0.250
Total surface area of the proposed pipe.
As=πd L = (5.1 × 3.1416/100) × 3049 = 488 sq. meters
Estimated leakage conductance of new pipe in 1000 ohm cm soil.
g = G × A = 2.14 × 10−3×488 = 1.04 Siemens
Since resistance = 1/conductance
Resistance to remote earth = 1/1.04 = 0.96 ohm
Estimated resistance to remote earth in 5000 ohm cm soil. (Resistance is directly pro-
portional to resistivity).
0.96 × 5 = 4.8 ohms
Estimated current to shift pipe potential to remote earth −0.250 volt. From Ohm’s
Law (I = E/R)
0.250/4.8 = 0.052 A.
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