1. Corrosion Behavior of Buried Pipeline in Presence of AC Stray Current in Controlled Environment
Elmira Ghanbari, M. Iannuzzi, M. Rincon Ortiz & R.S.Lillard.
Department of Chemical and Biomolecular Engineering.
National Center for Education and Research on Corrosion and Materials Performance.
The University of Akron, Akron, OH.
3/18/2015
Acknowledgments
The authors wish to thank Dan Dunmire and Richard Hayes (DoD CPO). Project funded by U.S Department of Defense grant No. W9132T

2. Background:
Stray current from nearby power sources can cause a structure to corrode.
Induced current by electromagnetic field.
Due to low resistivity of the electrolyte (soil):
the current flows through the structure.
The current gets discharged.
Positive metal ions leaves the metal.
Corrosion happens.

3. Motivations for study
What is the underlying mechanism of AC induced corrosion?
How does time averaged positive current
correlate to the corrosion current density?
How does AC influence the Faradic resistance?
How does corrosion relate to Root Mean Square of current (RMS)? or Peak values of AC current?

4. Experimental design: + - Carbon steel API grade X65 Steel.
DC & AC
New solution
+ -
Carbon steel API grade X65 Steel.
0.1 M sodium chloride solution.
Solution replenishment and stirring at 60 rpm.
Characterizing iron ion content using Ultraviolet Visible
Spectroscopy (UVS).
Characterizing the pH.
Testing with and without AC (RMS=600mV, 60 Hz).
Weight loss analysis as per ASTM G1.
Electrochemical tests:
Potentiostatic tests.
Linear polarization (LP).
Electrochemical Impedance Spectroscopy .
Old
solution
Corrosion rate:
Measured current.
Mass loss.
Faraday’s law

5. Results: Potentiostatic tests with AC.

6. AC corrosion measurements (60 Hz):
Effects of AC:
Increase in corrosion rate (≈10 times at OCP ).
Shift in corrosion potential to lower values.
Results from mass loss analysis in no AC condition, agree well with the data from polarization curve.
Fig1.Effect of AC (600 mV, 60 Hz) on corrosion rate of carbon steel at different DC biases. (Data obtained by postmortem mass loss (dW)).
Fig2.Effect of AC (600 mV, 60 Hz) on current density of carbon steel at different DC biases.

7. AC corrosion measurements (60Hz):
Decrease in corrosion rate ratios with increasing DC potentials for all AC RMS levels.
More pronounced effects of AC near OCP.
Increase in the corrosion rate ratios with increasing the AC RMS potential values.
OCP
Fig3.Effect of different AC (60 Hz) amplitudes on the ratio of corrosion rate with AC to the corrosion rate without AC at different DC potentials.

8. Measured current vs. Faradaic current (60 Hz):
The average of AC current, at 60 Hz, is a good indicator of the corrosion rate.
Mean value of positive current
Mean value of full current
Mean value of negative current
Fig5.Schematic of sinusoidal AC current density from potentiostatic test.
Fig4.Comparison of current density with AC (600mV and 60 Hz) from weight loss and three different cycles of potentiostatic test.

9. Equivalent Circuit: Rs: Solution resistance. Ra: Anodic resistance.
Rc: Cathodic resistance.
Cdl: Double layer capacitance.
Corrosion
Charge of Cdl
Discharge of Cdl
Anodic reaction on the surface of working electrode results in positive current.
Cathodic reaction on the surface of working electrode results in negative current.

10. Development of the model:
Kirchhoff’s voltage law.
(1)
(2)
(3)
Considering mass transport of oxidants (cathodic reaction under mix control).
(4)
Nonlinear differential equation:
(5)

11. b) Total current = Ia+Ic+Icd.
Simulation results
Dominance of capacitive current in high frequencies
At 60 Hz the total current is dominated by the capacitive current.
Anodic Faradic current is an order of magnitude lower than the total current.
Cathodic Faradic current is insignificant.
a) Faradic current.
b) Total current = Ia+Ic+Icd.
Fig6. Current response from simulation. Inputs : f=60Hz, Cdl=0.0005F, Rs=8, r=0.15,RMS=600 mV, OCP.

12. Potentiostatic test with AC at 60Hz:
At f=60 Hz, there is hydrogen evolution only on the surface of the counter electrode.
Observations are in agreement with the simulation results.
F=60 Hz, RMS=600 mV, OCP

13. Effect of AC at different DC potentials (60 Hz)
Simulation results:
Effect of AC at different DC potentials (60 Hz)
a) Anodic Faradaic current.
b) Cathodic Faradaic current .
c) Total current = Ia+Ic+Icd.
Fig7. Current response from simulation. Inputs : f=60Hz, Cdl=0.0005F, Rs=8, r=0.15,RMS=600 mV.

14. AC corrosion measurements at different frequencies:
Distortion from full sinusoidal waveform at lower frequencies (0.01 and 0.1 Hz).
Superposition of positive and negative rectified sinusoidal waveform at 0.01 Hz.
Additional features at both positive and negative currents at f=0.1Hz due to the increase in capacitive current.
Overall similarity between simulation and experimental results.
a) f=0.01Hz.
c) f=60Hz .
b) f=0.1Hz.
Fig9.Simulationl results: Total Current passing through the cell. Inputs :Cdl=0.0005F, Rs=8, r=0.5,
RMS=600 mV, OCP. .
Fig8.Experimental results: Total Current passing through the cell with 600 mV RMS at OCP.

15. Potentiostatic test with AC at 0.01Hz:
AC makes cycling between anodic and cathodic reaction:
Hydrogen evolution during:
Positive current on the counter electrode.
Negative current on the working electrode.
I(A)
E(V)

16. Measured current vs. Faradaic current:
Increasing the frequency, decreases the corrosion rate.
At higher frequencies, the time-averaged current from full cycle agrees with weight loss.
At f=0.01 Hz, the time-averaged current from positive cycle agrees with weight loss, since Icd is about zero.
Fig10.current density with AC (RMS=600mV and DC=-500mV) from weight loss and three different cycles of potentiostatic test at different frequencies with Rs=2.

17. Conclusions: There is a more pronounced effect of AC near OCP.
Increasing the ratio of DC to AC potential decreases corrosion rate.
A model was developed to describe the Faradaic and capacitive current in presence of time varying potential.
This model can be used for the development of cathodic protection criteria for AC induced corrosion.
By increasing the frequency the capacitive current increases and cathodic current can be neglected.
The time average current of full cycle at 60 Hz, is a good indicator of the corrosion rate.
At lower frequencies (0.01Hz), the average of positive AC current agrees with the corrosion rate.