for an external DC current. Ag alvanic cell willb ee stablished with the
current direction, exactly as described, by usinga ni mpressed electric
current. Theses acrificial anodes are usually composed of magnesiumo r
magnesium-based alloys.Occasionally,zinc and aluminumhave been used.
Because these anodes aree ssentially sources of portablee lectrical energy,
they areparticularly useful in areas where electric power is not available or
wherei ti su neconomicalo ri mpracticalt oi nstall power lines for
this purpose.
Most sacrificiala nodes in use in the United States are of magnesium
construction. Approximately 10 million pounds of magnesium is annually
used fort hisp urpose. Theo pen-circuit potential difference between
magnesium and steel is about 1V .T hism eans that one anodec an protect
onlyalimited length of pipeline. However,t his low voltage can have an
advantage over higher impressed voltages in that the danger of overprotection to somep ortions of the structure is less, and, because the total
current per anode is limited, the danger of stray-current damage to adjoining
metal structures is reduced.
Magnesium anode rods have also been placed in steel hot-watertanks to
increase the life of thesetanks. The greatest degree of protection is afforded
in “hard” waters where the conductivity of the water is greater than in
“soft” waters.
2.4.2.1 Anode Requirements
To provide cathodic protection, acurrent density of afew milliamps (mA)is
required. Therefore, to determine the anodic requirements, it is necessary to
knowthe energy contentofthe anode and its efficiency.From this data, the
necessary calculations can be madetosize the anode,determine its expected
life, and determine the number of anodes required. As previously indicated,
the threem ost common metalsu sed as sacrificial anodesa re magnesium,
zinc, and aluminum. The energyc ontenta nd efficiency of thesem etals are
as follows:
Metal
Theoretical
Energy Content
(A h/lb)
Anodic
Efficiency
(%)
Practical Energy (PE)
Constant (A h/lb)
Magnesium
1000
50
500
Zinc
370
90
333
Aluminum
1345
60
810
The number of pounds of metal required to provide ac urrent of 1Afor
ay ear can be obtainedf romt he following equation:
lb metal= A yr Z
8760 h = yr
PE
:
Corrosion of Linings and Coatings
48
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54
current direction, exactly as described, by usinga ni mpressed electric
current. Theses acrificial anodes are usually composed of magnesiumo r
magnesium-based alloys.Occasionally,zinc and aluminumhave been used.
Because these anodes aree ssentially sources of portablee lectrical energy,
they areparticularly useful in areas where electric power is not available or
wherei ti su neconomicalo ri mpracticalt oi nstall power lines for
this purpose.
Most sacrificiala nodes in use in the United States are of magnesium
construction. Approximately 10 million pounds of magnesium is annually
used fort hisp urpose. Theo pen-circuit potential difference between
magnesium and steel is about 1V .T hism eans that one anodec an protect
onlyalimited length of pipeline. However,t his low voltage can have an
advantage over higher impressed voltages in that the danger of overprotection to somep ortions of the structure is less, and, because the total
current per anode is limited, the danger of stray-current damage to adjoining
metal structures is reduced.
Magnesium anode rods have also been placed in steel hot-watertanks to
increase the life of thesetanks. The greatest degree of protection is afforded
in “hard” waters where the conductivity of the water is greater than in
“soft” waters.
2.4.2.1 Anode Requirements
To provide cathodic protection, acurrent density of afew milliamps (mA)is
required. Therefore, to determine the anodic requirements, it is necessary to
knowthe energy contentofthe anode and its efficiency.From this data, the
necessary calculations can be madetosize the anode,determine its expected
life, and determine the number of anodes required. As previously indicated,
the threem ost common metalsu sed as sacrificial anodesa re magnesium,
zinc, and aluminum. The energyc ontenta nd efficiency of thesem etals are
as follows:
Metal
Theoretical
Energy Content
(A h/lb)
Anodic
Efficiency
(%)
Practical Energy (PE)
Constant (A h/lb)
Magnesium
1000
50
500
Zinc
370
90
333
Aluminum
1345
60
810
The number of pounds of metal required to provide ac urrent of 1Afor
ay ear can be obtainedf romt he following equation:
lb metal= A yr Z
8760 h = yr
PE
:
Corrosion of Linings and Coatings
48
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54