AC-induced corrosion - Towards a nonlinear impedance model
P. LinhardtG. Ball
Vienna University of TechnologyInstitute for Testing and Research in Materials Technology (TVFA)
Karlsplatz 13, 1040 Vienna, Austria
ECHEM Center of Competence for Applied ElectrochemistryViktor-Kaplan-Straße 2
2700 Wiener Neustadt, Austria
IntroductionAC induced corrosion is a problem typically affecting pipelines influenced by railways, power-transmission lines etc.
AC voltages: up to > 100 V are reported AC-currents: up to 1000 A/m² are reported at technical frequencies (50, 60 & 16 2/3 Hz)
AC influence 16 2/3 Hz
Laboratory experiments in soil boxes [1] pSPICE modelingmodel Tafel-behavior by analog behavior modeling (ABM) [2]
set up & analyze electrical equivalent circuits by transient analysis [3]
ProblemAssess electrode processes by electrochemical measurements during AC-corrosion experiments in soil boxes
ApproachAPPLY sinusodial AC to specimen galvanostatically – MEASURE the distorted potential response – ANALYZE based on a nonlinear impedance model
Conclusions
• pSPICE appears as a powerful tool to set up nonlinear impedance models and to carry out large signal transient analysis.
• The analysis of nonlinear distortions is a promising tool for non invasive assessment of electrochemical processes during AC corrosion experiments.
• The distortion residual can be extracted regardless of IR-drop, making it an attractive source of information about the electrode processes.
• Futher work will address the development of nonlinear impedance modeling including diffusion phenomena andsystematically explore the correlation between the distortion residual with the corresponding electrode processes.
+
-~ =
IAC+DCIAC
IDC
CE WE
QRE
soil box
Emeasured
References[1] P. Linhardt, G. Ball, AC induced corrosion: Results from laboratory investigations and from a failure analysis, CORROSION 2006, paper 06160.[2] pSPICE Reference Guide, Cadence Design Systems Inc., USA, 2005.[3] pSPICE Student Version Release 9.1, Orcad Inc. 2000.
EXAMPLE20 bar gas transmission linebitumenous coatingcathodic protection
Case history: [1]
~ =
IAC+DC
Emeasured
electrolyte metal
EIRdrop Eelectrode
nonlinearimpedance
Rel
-5 0 5 10
-50
-40
-30
-20
-10
0
10
20
30
40
50
IAC
(mA
/cm
²)
Eelectrode (V)
0 10 20 30 40 50 60 70 80 90 100
-5
0
5
10
15
-50-40-30-20-10
01020304050
-50-40-30-20-10
01020304050
t (ms)
Eel
ectr
ode
(V)
Em
easu
red
(V)
IAC
(mA
) 16 2/3 Hz
0 10 20 30 40 50 60 70 80 90 100
-5
0
5
10
-60-50-40-30-20-10
0102030405060
-50-40-30-20-10
01020304050
t (ms)
Eel
ectr
ode
(V)
Em
easu
red
(V)
IAC
(mA
) 16 2/3 Hz
-5 0 5 10
-50
-40
-30
-20
-10
0
10
20
30
40
50
IAC
(mA
)
Eelectrode (V)
0 10 20 30 40 50 60 70 80 90 100
-5
0
5
10
-60-50-40-30-20-10
0102030405060
-50-40-30-20-10
01020304050
t (ms)
Eel
ectr
ode
(V)
Em
easu
red
(V)
IAC
(mA
) 16 2/3 Hz
-5 0 5 10
-50
-40
-30
-20
-10
0
10
20
30
40
50
IAC
(mA
)
Eelectrode (V)
-5 0 5 10 1510-5
10-4
10-3
10-2
10-1
|I| (A
)
Eelectrode (V) -5 0 5 10 1510-5
10-4
10-3
10-2
10-1
|I| (A
)
Eelectrode (V)
working electrode (WE)steel specimen, 1cm²
Pt-wirequasi reference electrode (QRE)
required to avoid AC-distortionsdrawback: missing DC potenial
AC-Galvanostat±70 Vpeak, ±100 mApeak, 0-100 µADCcathodic
Example of data, measured during specific phases of long term testing (>30 days)Numeric IR-drop correction, based on Rel estimated by impedance spectroscopy (EIS)
simulation results from transient analysis
Parameters empirically chosen
DC behavior as Tafel-plots
model Astrong cathodic activity
wide anodic passive range
model Bcathodically and anodically active
includes electrode surface coverage effects
model Amodel B
model A model B
characteristic features of the Lissajous-figure are better mimiced by model B
• Fourier analysis allows to separate the linear part from thepotential signal, yielding the distortion residual.
• The distortion residual reflects the nonlinear partof the electrode impedance.
• The distortion residual is not influenced by the IR-drop.
Excursion: the distortion residual
soil box
Counter Electrode (CE)stainless steel
sample holder
