Cathodic Protection
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not be pore-free because the protective current flows preferentially to the
exposed metal areas that require protection. Such coatings are useful in distributing the protective current, in reducing total current requirements, and
in extending the life of the anode. For example, in a coated pipeline, the current distribution is greatly improved over that of a bare pipeline, the number
of anodes and the total current required are less, and one anode can protect
a much longer section of pipeline. Because the Earth is a good electrical conductor and the resistivity of the soil is localized only within the region of
the pipeline or electrodes, the limiting length of pipe protected per anode is
imposed by the metallic resistance of the pipe and not the resistance of the
soil.
One magnesium anode is capable of protecting approximately 100 ft (30 m)
of bare pipeline, whereas it can provide protection for approximately 5 mi (8
km) of coated pipeline.
In a hot-water tank coated with glass or an organic coating, the life of the
magnesium anode is extended and more uniform protection is supplied to
the tank. Without the coating, the tendency is for excess current to flow to the
side and insufficient current flows to the top and bottom. Because of these
factors, cathodic protection is usually provided with coated surfaces.
9.4 Economics
The cost of cathodic protection is more than recovered by reduced maintenance costs, by reduced installation costs, or both. For buried pipelines,
the guarantee that there will be no corrosion on the soil side of the pipe
has made it economically feasible to transport oil and high-pressure natural
gas across the North American continent. It has also permitted the use of
thinner-walled pipe. Wall thicknesses need only be sufficient to withstand
the internal pressures. No extra allowance has to be added for external corrosion. This saving alone has sometimes more than paid for the installation
of the cathodic equipment.
Similarly, other cathodic protection systems have more than paid for their
installation costs, reduced maintenance costs by longer operating periods
between routine inspections and maintenance periods.
307
not be pore-free because the protective current flows preferentially to the
exposed metal areas that require protection. Such coatings are useful in distributing the protective current, in reducing total current requirements, and
in extending the life of the anode. For example, in a coated pipeline, the current distribution is greatly improved over that of a bare pipeline, the number
of anodes and the total current required are less, and one anode can protect
a much longer section of pipeline. Because the Earth is a good electrical conductor and the resistivity of the soil is localized only within the region of
the pipeline or electrodes, the limiting length of pipe protected per anode is
imposed by the metallic resistance of the pipe and not the resistance of the
soil.
One magnesium anode is capable of protecting approximately 100 ft (30 m)
of bare pipeline, whereas it can provide protection for approximately 5 mi (8
km) of coated pipeline.
In a hot-water tank coated with glass or an organic coating, the life of the
magnesium anode is extended and more uniform protection is supplied to
the tank. Without the coating, the tendency is for excess current to flow to the
side and insufficient current flows to the top and bottom. Because of these
factors, cathodic protection is usually provided with coated surfaces.
9.4 Economics
The cost of cathodic protection is more than recovered by reduced maintenance costs, by reduced installation costs, or both. For buried pipelines,
the guarantee that there will be no corrosion on the soil side of the pipe
has made it economically feasible to transport oil and high-pressure natural
gas across the North American continent. It has also permitted the use of
thinner-walled pipe. Wall thicknesses need only be sufficient to withstand
the internal pressures. No extra allowance has to be added for external corrosion. This saving alone has sometimes more than paid for the installation
of the cathodic equipment.
Similarly, other cathodic protection systems have more than paid for their
installation costs, reduced maintenance costs by longer operating periods
between routine inspections and maintenance periods.