B. Instrument Capabilities
Instantaneous Corrosion Rate Measurements
Instantaneouscorrosionrate measurementsaremade withthelinear
polarizationresistance(LPR)technique. Thistechnique isareliable
electrochemicalprocedurebased onprinciplesoutlinedinASTMG59
StandardPracticeforConductingPotentiodynamicPolarizationResis-
tanceMeasurements.1Anexcellenttechnicalreviewofthistechniqueis
givenelsewherebyMansfeld.2,3
Thedeterminationof a corrosionratefromthe parameters measured
withcorrosionprobesdependsonFaraday’slaw. Themassloss,
current,potential,andchanges inthesevariablesareconverted intoa
corrosionrate. ASTMG102StandardPractice for Calculation of
CorrosionRatesandRelatedInformationfrom ElectrochemicalMea-
surementsprovides allnecessaryinformationforthe directcalculationof
thecorrosionrate.4
ThemethodusedbytheMS1000 for calculating the corrosionrateisas
follows. Alow amplitudedcvoltageisapplied across theelectrodes
andthenthe resultingcurrent,normalized tocurrentdensity, ismea-
sured. Thevoltageis heldconstantforthirty secondsbeforemeasuring
thecurrent sothat anycapacitanceinthe oxidelayerwillbe fully
charged. Thepolarityofthe applied voltage isthenchangedandthe
measurementcycleis repeated. Polarityisreversedto minimizepoten-
tialerrorsdue topossibleelectrodedissimilaritiesorchangesin the
opencircuitpotentialoftheelectrodesduringthemeasurement.
Thepolarizationresistance,Rp, ofthecorrodingelectrodes is calcu-
latedfromtheslope of thepotential-currentdensityplot. Rpisdefined
as:
Rp=E
i
2
