Platinum-clad or 2% silver–lead electrodes that use impressed current
have been employed for the protection of structures in seawater. The latter
anodesa re estimated to last ten years, whereas, sacrificial magnesium
anodesr equirer eplacement every two years. On occasion, aluminum
electrodes have been used in freshwaters.
Because the effectiver esistivity of soil surrounding an anode is limited
to the immediate area of the electrode, this local resistance is generally
reduced by usingb ackfill. For impressed current systems, the anode is
surrounded with athick bed of coke mixed with three to fourparts of gypsum
to one part of sodium chloride.T he consumptiono ft he anode is reduced
somewhat because the coke backfill is ac onductor and carries part of the
current. If the anodei si mmersed in ar iver bed, lake, or ocean, backfill is
not required.
Auxiliary anodes need notb ec onsumed to fulfillt heir purpose.
Conversely,sacrificialanodesare consumed no less to supplyanequivalent
current than is required by Faraday’sl aw.
For magnesium anodes, backfill has the advantage of reducing the
resistance of insulating corrosion-product films as well as increasing the
conductivity of the immediate area. Atypical backfill consists of amixture of
approximately 20% bentonite (for retention of moisture), 75% gypsum, and
5% sodiumsulfate.
2.5U se with Coatings
Insulating coatings are advantageous to use with either impressed current or
sacrificial anodeswhensupplying cathodic protection. These coatings need
not be pore free because the protective current flowsp referentially to the
exposed metal areas that requirethe 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 ac oated pipeline,
the current distribution is greatly improved over that of abare pipeline, the
number of anodes and total current requiredi sl ess, and one anode can
protect am uch longer section of pipeline. Because the earth is ag ood
electrical conductor and the resistivity of the soil is localized onlywithin the
region of the pipelineorelectrodes, the limitinglength of pipeprotected per
anode is imposed by the metallic resistance of the pipe and not the resistance
of the soil.
One magnesium anodei sc apable of protecting approximately 100 ft.
(30m)o fabare pipeline,w hereas,i tc an providep rotection for
approximately 5miles (8 km) of ac oated pipeline.
In ahot-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
Cathodic Protection
51
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54
have been employed for the protection of structures in seawater. The latter
anodesa re estimated to last ten years, whereas, sacrificial magnesium
anodesr equirer eplacement every two years. On occasion, aluminum
electrodes have been used in freshwaters.
Because the effectiver esistivity of soil surrounding an anode is limited
to the immediate area of the electrode, this local resistance is generally
reduced by usingb ackfill. For impressed current systems, the anode is
surrounded with athick bed of coke mixed with three to fourparts of gypsum
to one part of sodium chloride.T he consumptiono ft he anode is reduced
somewhat because the coke backfill is ac onductor and carries part of the
current. If the anodei si mmersed in ar iver bed, lake, or ocean, backfill is
not required.
Auxiliary anodes need notb ec onsumed to fulfillt heir purpose.
Conversely,sacrificialanodesare consumed no less to supplyanequivalent
current than is required by Faraday’sl aw.
For magnesium anodes, backfill has the advantage of reducing the
resistance of insulating corrosion-product films as well as increasing the
conductivity of the immediate area. Atypical backfill consists of amixture of
approximately 20% bentonite (for retention of moisture), 75% gypsum, and
5% sodiumsulfate.
2.5U se with Coatings
Insulating coatings are advantageous to use with either impressed current or
sacrificial anodeswhensupplying cathodic protection. These coatings need
not be pore free because the protective current flowsp referentially to the
exposed metal areas that requirethe 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 ac oated pipeline,
the current distribution is greatly improved over that of abare pipeline, the
number of anodes and total current requiredi sl ess, and one anode can
protect am uch longer section of pipeline. Because the earth is ag ood
electrical conductor and the resistivity of the soil is localized onlywithin the
region of the pipelineorelectrodes, the limitinglength of pipeprotected per
anode is imposed by the metallic resistance of the pipe and not the resistance
of the soil.
One magnesium anodei sc apable of protecting approximately 100 ft.
(30m)o fabare pipeline,w hereas,i tc an providep rotection for
approximately 5miles (8 km) of ac oated pipeline.
In ahot-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
Cathodic Protection
51
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54