it can effectivelyb eu sed to eliminate corrosion fatigue,i ntergranular
corrosion, stress-corrosion cracking,d ezincification of brass,o rp itting of
stainless steels in seawaterors teel in soil.
In 1824,S ir HumphryD avid reported that by coupling iron or zinc to
copper,the copper couldbeprotected againstcorrosion.The BritishAdmiralty
hadblocksofironattachedtothe hullsofcopper-sheathed vesselstoprovide
cathodic protection.Unfortunately,cathodicallyprotected copper is subjectto
foulingb ym arinel ife, whichr educed thes peed of vesselsu nder sail and
forced theadmiraltytodiscontinue thepractice. However, thecorrosion rate
of thec opperhad been appreciablyr educed.Unprotected copper suppliesa
differentnumberofcopperionstopoisonfouling organisms.
In 1829,E dmundD avyw as successful in protecting thei ronp ortionso f
buoysbyusing zinc blocks,and in 1840,RobertMalleteproducedazinc alloy
that wasp articularlys uiteda sasacrificial anode. Thefittingofzincslabs to
thehulls of vesselsbecamestandardpracticeaswoodenhulls were replaced.
This provided localizedprotection, specificallyagainst thegalvanicactionof
ab ronzep ropeller.O verall protection of seagoing vessels wasn ot
investigated until1950whenthe Canadian Navy determined that theproper
useofanti-fouling paints in conjunctionwithcorrosion resistantpaintsmade
cathodic protection of shipsf easablea nd couldr educem aintenance costs.
About 1910–1912t he firsta pplication of cathodic protection by means of
an impressed currentw as undertaken in England and the United States.
Since that time, the general use of cathodic protection has been widespread.
There are thousands of miles of buried pipeand cables that are protected in
this manner.
This form of protection is also used for water tanks, submarines, canal
gates, marine piling, condensers, and chemical equipment.
2.3T heory
Cathodic protection is achieved by applying electrochemical principles to
metallic components buried in soil or immersed in water.Itisaccomplished
by flowing acathodic current through ametal–electrolyte interface favoring
the reduction reaction over the anodic metal dissolution. This enables the
entire structure to work as ac athode.
The flux of electrons can be provided by one of two methods. By use of
ar ectifier,adirect current may be impressed on an inert anode and the
components.T hese components receivea ne xcess of electronsa nd
are, thereby,cathodically protected.
Thea lternative method is to couple the components with am ore
active metal, sucha sz inc or magnesium, to create ag alvanic cell. Under
these conditions, the active metal operates as an anode and is destroyed while
protecting the components that are cathodic.Suchananode is referred to as
a sacrificial anode.
Corrosion of Linings and Coatings
44
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54
corrosion, stress-corrosion cracking,d ezincification of brass,o rp itting of
stainless steels in seawaterors teel in soil.
In 1824,S ir HumphryD avid reported that by coupling iron or zinc to
copper,the copper couldbeprotected againstcorrosion.The BritishAdmiralty
hadblocksofironattachedtothe hullsofcopper-sheathed vesselstoprovide
cathodic protection.Unfortunately,cathodicallyprotected copper is subjectto
foulingb ym arinel ife, whichr educed thes peed of vesselsu nder sail and
forced theadmiraltytodiscontinue thepractice. However, thecorrosion rate
of thec opperhad been appreciablyr educed.Unprotected copper suppliesa
differentnumberofcopperionstopoisonfouling organisms.
In 1829,E dmundD avyw as successful in protecting thei ronp ortionso f
buoysbyusing zinc blocks,and in 1840,RobertMalleteproducedazinc alloy
that wasp articularlys uiteda sasacrificial anode. Thefittingofzincslabs to
thehulls of vesselsbecamestandardpracticeaswoodenhulls were replaced.
This provided localizedprotection, specificallyagainst thegalvanicactionof
ab ronzep ropeller.O verall protection of seagoing vessels wasn ot
investigated until1950whenthe Canadian Navy determined that theproper
useofanti-fouling paints in conjunctionwithcorrosion resistantpaintsmade
cathodic protection of shipsf easablea nd couldr educem aintenance costs.
About 1910–1912t he firsta pplication of cathodic protection by means of
an impressed currentw as undertaken in England and the United States.
Since that time, the general use of cathodic protection has been widespread.
There are thousands of miles of buried pipeand cables that are protected in
this manner.
This form of protection is also used for water tanks, submarines, canal
gates, marine piling, condensers, and chemical equipment.
2.3T heory
Cathodic protection is achieved by applying electrochemical principles to
metallic components buried in soil or immersed in water.Itisaccomplished
by flowing acathodic current through ametal–electrolyte interface favoring
the reduction reaction over the anodic metal dissolution. This enables the
entire structure to work as ac athode.
The flux of electrons can be provided by one of two methods. By use of
ar ectifier,adirect current may be impressed on an inert anode and the
components.T hese components receivea ne xcess of electronsa nd
are, thereby,cathodically protected.
Thea lternative method is to couple the components with am ore
active metal, sucha sz inc or magnesium, to create ag alvanic cell. Under
these conditions, the active metal operates as an anode and is destroyed while
protecting the components that are cathodic.Suchananode is referred to as
a sacrificial anode.
Corrosion of Linings and Coatings
44
CAT8247—CHAPTER 2—6/10/2006—12:05—SRIDHAR—XML MODEL B–pp. 43–54