cells, returning to the source of the DC current, B. Local action current will
ceasetoflow when all the metal surfaceisatthe same potential as aresult of
the cathode areas being polarized by an external current to the open-current
potential of the anodes. As long as this external current is maintained, the
metal cannot corrode.
The corrosion rate will remain at zero if the metal is polarized slightly
beyondthe open circuitpotential of the anode.However,this excess current
has no value and may be injurious to amphoteric metalso rc oatings.
Therefore, in actual practice, the impressed current is maintained close to the
theoretical minimum.
If the current should fall below that requiredf or completep rotection,
somep rotection will still be afforded.
2.4M ethodo fApplication
Asource of direct currentand an auxiliaryelectrode are required to provide
cathodic protection. The auxiliaryelectrode (anode) is usuallycomposed of
graphite or iron and is located some distance away from the structure being
protected. The anode is connectedtothe positive terminal of the DC source,
and the structure being protected is connectedt ot he negative terminal.
This permits current to flow from the anode,t hrough the electrolyte, to the
structure. Applied voltages are not critical as long as they are sufficient to
supply an adequate current densityt oa ll parts of the protected structure.
The resistivity of the soil will determine the required applied voltage; when
the endo falong pipelinei st ob ep rotected, the voltage will have to be
increased.
Current is usually supplied by ar ectifier supplying low-voltage direct
current of several amperes.
2.4.1I mpressed Current Systems
Forimpressed currentsystems,the source of electricityisexternal. Arectifier
converts high voltageA Cc urrent to lowv oltage DC current. This direct
currentisimpressed betweenburiedanodesand thestructure to be protected.
It is preferred to use inert anodes that willl ast for the longest possible
times. Typical materials used for these anodes are graphite, silicon, titanium,
and niobium plated with platinum.
For agiven appliedvoltage, the current is limited by electrolyte resistivity
and by the anodic and cathodic polarization. With the impressed current
system, it is possible to impose whatever potential is necessary to obtain the
current density required by means of the rectifier.
Electric current flowsinthe soil from the buried anode to the underground
structure to be protected. Therefore, the anode must be connected to the
positive pole of the rectifier and the structure to the negative pole. All cables
Corrosion of Linings and Coatings
46
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54
ceasetoflow when all the metal surfaceisatthe same potential as aresult of
the cathode areas being polarized by an external current to the open-current
potential of the anodes. As long as this external current is maintained, the
metal cannot corrode.
The corrosion rate will remain at zero if the metal is polarized slightly
beyondthe open circuitpotential of the anode.However,this excess current
has no value and may be injurious to amphoteric metalso rc oatings.
Therefore, in actual practice, the impressed current is maintained close to the
theoretical minimum.
If the current should fall below that requiredf or completep rotection,
somep rotection will still be afforded.
2.4M ethodo fApplication
Asource of direct currentand an auxiliaryelectrode are required to provide
cathodic protection. The auxiliaryelectrode (anode) is usuallycomposed of
graphite or iron and is located some distance away from the structure being
protected. The anode is connectedtothe positive terminal of the DC source,
and the structure being protected is connectedt ot he negative terminal.
This permits current to flow from the anode,t hrough the electrolyte, to the
structure. Applied voltages are not critical as long as they are sufficient to
supply an adequate current densityt oa ll parts of the protected structure.
The resistivity of the soil will determine the required applied voltage; when
the endo falong pipelinei st ob ep rotected, the voltage will have to be
increased.
Current is usually supplied by ar ectifier supplying low-voltage direct
current of several amperes.
2.4.1I mpressed Current Systems
Forimpressed currentsystems,the source of electricityisexternal. Arectifier
converts high voltageA Cc urrent to lowv oltage DC current. This direct
currentisimpressed betweenburiedanodesand thestructure to be protected.
It is preferred to use inert anodes that willl ast for the longest possible
times. Typical materials used for these anodes are graphite, silicon, titanium,
and niobium plated with platinum.
For agiven appliedvoltage, the current is limited by electrolyte resistivity
and by the anodic and cathodic polarization. With the impressed current
system, it is possible to impose whatever potential is necessary to obtain the
current density required by means of the rectifier.
Electric current flowsinthe soil from the buried anode to the underground
structure to be protected. Therefore, the anode must be connected to the
positive pole of the rectifier and the structure to the negative pole. All cables
Corrosion of Linings and Coatings
46
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54