11.1.14 CORRATERS ystems
CORRATER systems measure the instantaneous corrosion rate of am etal
in ac onductive fluid usingt he LPR measurement technique. As described
earlier,c orrosion is an electrochemical process where electrons are transferred between anodic and cathodic areas on the corrodingm etal resulting
in oxidation (corrosion) of the metal at the anode and reduction of cations
in the fluid at the cathode.
Sterns and Geary originally demonstrated that the application of as mall
polarizing potential difference ( D E )f rom the corrosion potential ( E corr )o f
ac orroding electrode resulted in am easured current density ( i meas )t hat is
related to the corrosioncurrent density ( i corr )b yE quation 11.1:
D E
i meas
Z
b a b c
ð 2 ; 303 i corr Þðb a C b c Þ
;
(11.1)
where
b a Z anodic Tafel slope
b c Z cathodic Tafel slope.
Because the Tafel coefficients are more or less constant for agiven metal–
fluid combination, i meas is proportional to i corr that is proportional to the
corrosion rate. Equation 11.1 and the entire LPR technique are only valid
when the polarizingpotential difference is very low (typicallyupto20mV).
In this region, the curves are linear,h ence the term LPR. Inspection of
Equation 11.1 shows that the resultisaresistance, the polarization resistance,
R p .Strictly speaking, there are bothanodicand cathodic R p values that can
differ, they are usuallya ssumed to be equal. The resistance to current flow
between anode and cathode on the LPR probe is the sum of both polarization
resistance values and the resistance of the solution betweent he electrodes
( R s )ass hown in Equation 11.2:
D E Z i meas ð 2 R p C R s Þ
(11.2)
From Equation 11.1 and Equation 11.2, obtaining results from the LPR
technique would seem to requireonlyinstantaneous readings of resistance.
In practice,h owever,t he determination of polarization resistancei s
complicated by ac apacitance effect at the metal–fluid interface (doublelayer capacitance). Figure 11.2 is an equivalent electrical circuit of the
corrosion cell formed by the measuring electrodes and the fluid,s howing
the importance of R s and double-layer capacitance effects. The effect of
the double-layer capacitance is to require the direct current flow to initially
charge up the capacitors, resulting in ad ecaying exponential current
flow curve vs. time, after application of the polarizing potential difference.
At ypical LPR current vs. time curve is shown in Figure 11.3.E achm etal–
fluid interface has its own characteristic capacitance that in turn,determines
the amount of time requiredt oo btain valid measurements of i corr and
Corrosion of Linings and Coatings
542
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547
CORRATER systems measure the instantaneous corrosion rate of am etal
in ac onductive fluid usingt he LPR measurement technique. As described
earlier,c orrosion is an electrochemical process where electrons are transferred between anodic and cathodic areas on the corrodingm etal resulting
in oxidation (corrosion) of the metal at the anode and reduction of cations
in the fluid at the cathode.
Sterns and Geary originally demonstrated that the application of as mall
polarizing potential difference ( D E )f rom the corrosion potential ( E corr )o f
ac orroding electrode resulted in am easured current density ( i meas )t hat is
related to the corrosioncurrent density ( i corr )b yE quation 11.1:
D E
i meas
Z
b a b c
ð 2 ; 303 i corr Þðb a C b c Þ
;
(11.1)
where
b a Z anodic Tafel slope
b c Z cathodic Tafel slope.
Because the Tafel coefficients are more or less constant for agiven metal–
fluid combination, i meas is proportional to i corr that is proportional to the
corrosion rate. Equation 11.1 and the entire LPR technique are only valid
when the polarizingpotential difference is very low (typicallyupto20mV).
In this region, the curves are linear,h ence the term LPR. Inspection of
Equation 11.1 shows that the resultisaresistance, the polarization resistance,
R p .Strictly speaking, there are bothanodicand cathodic R p values that can
differ, they are usuallya ssumed to be equal. The resistance to current flow
between anode and cathode on the LPR probe is the sum of both polarization
resistance values and the resistance of the solution betweent he electrodes
( R s )ass hown in Equation 11.2:
D E Z i meas ð 2 R p C R s Þ
(11.2)
From Equation 11.1 and Equation 11.2, obtaining results from the LPR
technique would seem to requireonlyinstantaneous readings of resistance.
In practice,h owever,t he determination of polarization resistancei s
complicated by ac apacitance effect at the metal–fluid interface (doublelayer capacitance). Figure 11.2 is an equivalent electrical circuit of the
corrosion cell formed by the measuring electrodes and the fluid,s howing
the importance of R s and double-layer capacitance effects. The effect of
the double-layer capacitance is to require the direct current flow to initially
charge up the capacitors, resulting in ad ecaying exponential current
flow curve vs. time, after application of the polarizing potential difference.
At ypical LPR current vs. time curve is shown in Figure 11.3.E achm etal–
fluid interface has its own characteristic capacitance that in turn,determines
the amount of time requiredt oo btain valid measurements of i corr and
Corrosion of Linings and Coatings
542
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547