controlled. Such control is more difficult in the field, and the technique
mustb eu sed more judicially there. Av ariety of methods and equipment
are used for such studies that are generally classified under the following
types:
PotentiostaticP otentialh eld constant
GalvanostaticC urrent heldconstant
Potentiodynamic( a) Potential changed continuously at aspecified rate
(b) Potential changedi nsteps and held constantat
eachstep
Galvanodynamic (a) Current changed continuouslyataspecified rate
(b) Current changed in steps and heldconstant at
eachstep (galvanos taircase)
Thep otentials or currentsa ppliedw itht hese techniques maym ake
irreversible changes to the metal-fluid interfaceb ecause theym ay
substantiallyp olarize the metal away from its natural state in the fluid
environment, generally preventing reuse of the test electrodes after one scan.
In common with all electrochemical methods,t hey can only be made in
sufficiently conductive media where the wetted electrode area is known.
Although corrosion rates can be estimated with this technique, it tends to be
less precise than linear polarization resistance (LPR)b ecause of the high
polarization potentials required.
11.1.9E lectrical Impedance Spectroscopy
Electrochemicali mpedance spectroscopy (ACi mpedance)i sanoww ellestablishedl aboratoryt echnique used to determinet he electrical impedance
of the metal–electrolyte interface at various AC excitationf requencies.
Impedancem easurementsc ombine the effectso fD Cr esistance with
capacitancea nd inductance.I no rder to make impedancem easurements, it
is necessary to have acorrosion cell of knowngeometry, areference electrode,
andi nstrumentation capableo fm easuring andr ecordingt he electrical
response of thetestcorrosion cell over awiderateofACexcitationfrequencies.
Again, with thee volutiono fm orer uggedc omputers,s omei nvestigation
of this method is nowb eing made in thefi eld. AC impedancei sc apable of
characterizing thec orrosion interfacem orec omprehensively andw ithg ood
qualitye quipment specifications of achievingm easurementsi nl ower
conductivity solution or high-resistivity coatings.A Ci mpedance measurementscan be used to predictcorrosion ratesand characterize systemsunder
study, andt heya re commonly used forp erformance studieso fc hemical
inhibitors andprotectivecoatingstoevaluatethe resistance of alloys to specific
environments,etc.
Corrosion of Linings and Coatings
538
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547
mustb eu sed more judicially there. Av ariety of methods and equipment
are used for such studies that are generally classified under the following
types:
PotentiostaticP otentialh eld constant
GalvanostaticC urrent heldconstant
Potentiodynamic( a) Potential changed continuously at aspecified rate
(b) Potential changedi nsteps and held constantat
eachstep
Galvanodynamic (a) Current changed continuouslyataspecified rate
(b) Current changed in steps and heldconstant at
eachstep (galvanos taircase)
Thep otentials or currentsa ppliedw itht hese techniques maym ake
irreversible changes to the metal-fluid interfaceb ecause theym ay
substantiallyp olarize the metal away from its natural state in the fluid
environment, generally preventing reuse of the test electrodes after one scan.
In common with all electrochemical methods,t hey can only be made in
sufficiently conductive media where the wetted electrode area is known.
Although corrosion rates can be estimated with this technique, it tends to be
less precise than linear polarization resistance (LPR)b ecause of the high
polarization potentials required.
11.1.9E lectrical Impedance Spectroscopy
Electrochemicali mpedance spectroscopy (ACi mpedance)i sanoww ellestablishedl aboratoryt echnique used to determinet he electrical impedance
of the metal–electrolyte interface at various AC excitationf requencies.
Impedancem easurementsc ombine the effectso fD Cr esistance with
capacitancea nd inductance.I no rder to make impedancem easurements, it
is necessary to have acorrosion cell of knowngeometry, areference electrode,
andi nstrumentation capableo fm easuring andr ecordingt he electrical
response of thetestcorrosion cell over awiderateofACexcitationfrequencies.
Again, with thee volutiono fm orer uggedc omputers,s omei nvestigation
of this method is nowb eing made in thefi eld. AC impedancei sc apable of
characterizing thec orrosion interfacem orec omprehensively andw ithg ood
qualitye quipment specifications of achievingm easurementsi nl ower
conductivity solution or high-resistivity coatings.A Ci mpedance measurementscan be used to predictcorrosion ratesand characterize systemsunder
study, andt heya re commonly used forp erformance studieso fc hemical
inhibitors andprotectivecoatingstoevaluatethe resistance of alloys to specific
environments,etc.
Corrosion of Linings and Coatings
538
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547