from the rectifier to the anode and to the structure must be electrically
insulated. If not, those from the rectifier to the anodew ill acta sa na node
and deteriorate rapidly,w hereas, those from the rectifier to the structure
may pick up someo ft he electricc urrent that would then be lost for
protection.
2.4.1.1 Current Requirements
The specific metal and environmentw ill determine the current density
required for complete protection. The applied current densitym ust always
exceedthe current density equivalent to the measured corrosionrate under
the same conditions. Therefore, as the corrosion rate increases, the impressed
current density must be increased to provide protection.
There are several factors that affect the current requirements.These include:
1. The nature of the electrolyte
2. The soil resistivity
3. The degree of aeration
The more acidic the electrolyte, the greater the potential for corrosion and
the greater the current requirement.
Soils that exhibit ah igh resistance requirealower cathode current to
provide protection. However,i na reas of violent agitation or higha eration,
an increase in current will be required. The required current to provide
cathodic protection can vary from 0.5 to 20 mA/ft.
2 of bare surface.
Field testing may be required to determine the necessary current density
to provide cathodic protection in as pecific area. However,t hese testing
techniques will provide onlya na pproximation.A fter completion of the
installation, it will be necessary to conduct ap otential surveya nd make
necessary adjustments to provide the desired degree of protection.
For cathodically controlledc orrosion rates, thec orrosion potential
approaches the open-circuit anodic potential, and the required current
densityi so nly slightly greater than the equivalent corrosion current. The
required currentc an be considerablyg reater than the corrosionc urrentf or
mixed control, and for anodically controlled corrosionreactions, the required
current is even greater.
When aprotective currentcauses precipitation of an inorganic scaleonthe
cathode surface, such as in hard water or seawater,the total currentrequired
is gradually reduced. Thisisthe resultofaninsulating coating being formed.
However, the current densitya tt he exposed metal areas does not change;
only the total current density per apparent unit area is less.
2.4.2 Sacrificial Anodes
It is possible, by selection of an anode constructedo fametal more active
in the galvanic series than the metal to be protected, to eliminate the need
Cathodic Protection
47
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54
insulated. If not, those from the rectifier to the anodew ill acta sa na node
and deteriorate rapidly,w hereas, those from the rectifier to the structure
may pick up someo ft he electricc urrent that would then be lost for
protection.
2.4.1.1 Current Requirements
The specific metal and environmentw ill determine the current density
required for complete protection. The applied current densitym ust always
exceedthe current density equivalent to the measured corrosionrate under
the same conditions. Therefore, as the corrosion rate increases, the impressed
current density must be increased to provide protection.
There are several factors that affect the current requirements.These include:
1. The nature of the electrolyte
2. The soil resistivity
3. The degree of aeration
The more acidic the electrolyte, the greater the potential for corrosion and
the greater the current requirement.
Soils that exhibit ah igh resistance requirealower cathode current to
provide protection. However,i na reas of violent agitation or higha eration,
an increase in current will be required. The required current to provide
cathodic protection can vary from 0.5 to 20 mA/ft.
2 of bare surface.
Field testing may be required to determine the necessary current density
to provide cathodic protection in as pecific area. However,t hese testing
techniques will provide onlya na pproximation.A fter completion of the
installation, it will be necessary to conduct ap otential surveya nd make
necessary adjustments to provide the desired degree of protection.
For cathodically controlledc orrosion rates, thec orrosion potential
approaches the open-circuit anodic potential, and the required current
densityi so nly slightly greater than the equivalent corrosion current. The
required currentc an be considerablyg reater than the corrosionc urrentf or
mixed control, and for anodically controlled corrosionreactions, the required
current is even greater.
When aprotective currentcauses precipitation of an inorganic scaleonthe
cathode surface, such as in hard water or seawater,the total currentrequired
is gradually reduced. Thisisthe resultofaninsulating coating being formed.
However, the current densitya tt he exposed metal areas does not change;
only the total current density per apparent unit area is less.
2.4.2 Sacrificial Anodes
It is possible, by selection of an anode constructedo fametal more active
in the galvanic series than the metal to be protected, to eliminate the need
Cathodic Protection
47
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54