DICMAPI Seminars 3 rd -4 th December 2014, Naples Electrochemical Impedance Spectroscopy and Electrochemical Noise Analysis Michele Curioni Corrosion and Protection Centre University of Manchester UK

DICMAPI Seminars 3 rd -4 th December 2014, Naples Overview (2 days) Background: DC techniques – Linear polarization / potentiodynamic polarization Electrochemical Impedance – Concept – Equivalent circuits – Measurement Electrochemical Noise – Generation of noise – Measurement and analysis

DICMAPI Seminars 3 rd -4 th December 2014, Naples Corrosion Rate Measurement Similarity to Service Conditions Good Signal Time Resolution Avoid Polarization and/or Faradic Currents Perturb the Corroding Surface to Measure Response Minimize Measurement Time Amplitude of the AC/DC Signal Average Signals / Integration Time Mathematical/Statistical Elaboration

DICMAPI Seminars 3 rd -4 th December 2014, Naples Overview Anodic and Cathodic Reactions DC techniques EIS Basic Equivalent Circuit elements

DICMAPI Seminars 3 rd -4 th December 2014, Naples Electrochemical Reaction Cu 0 Cu 2+ 2 e - Cu 0 Cu 2+ 2 e - Anodic Reaction Cathodic Reaction Cu 0 Cu e - Cu 0 Cu e -

DICMAPI Seminars 3 rd -4 th December 2014, Naples Electrochemical Reaction Cu 0 Cu 2+ 2 e - Cu 0 Cu 2+ 2 e - 8 e - 4 Cu 2+ Cu 0 Cu e - Electrochemical Series Standard potential for the reaction Exchange Current

DICMAPI Seminars 3 rd -4 th December 2014, Naples Electrochemical Setup (three electrode cell and potentiostat) Electrolyte Working Electrode (sample) Counter Electrode (Platinum?) REF I I II Control Potential Amplifier Applied Current Measured Potential

DICMAPI Seminars 3 rd -4 th December 2014, Naples One electrode at equilibrium potential Cu 0 Cu 2+ 2 e - Cu 0 Cu 2+ 2 e - 8 e - 4 Cu 2+ Time V 0.34 Time I 0 All the electrons produced by the anodic reaction are consumed by the cathodic reaction

DICMAPI Seminars 3 rd -4 th December 2014, Naples Polarizing one electrode Cu 0 Cu 2+ 2 e - 6 e - 3 Cu 2+ Time V 0.34 Time I 0 Cu 0 Cu 2+ 2 e - Cu 0 Cu 2+ 2 e - Cathodic reaction goes slower Anodic reaction goes faster WE NEED TO REMOVE SOME ELECTRONS!

DICMAPI Seminars 3 rd -4 th December 2014, Naples Time V 0.34 Time I Anodic polarization The potentiostat brings the POTENTIAL UP To do so, current ENTERS the working electrode FROM THE CABLE and EXITS FROM THE METAL-SOLUTION INTERFACE OXIDATION goes FASTER

DICMAPI Seminars 3 rd -4 th December 2014, Naples Time V 0.34 Time I Cathodic polarization The potentiostat brings the POTENTIAL DOWN To do so, current EXITS the working electrode FROM THE CABLE and ENTERS FROM THE METAL-SOLUTION INTERFACE REDUCTION goes FASTER

DICMAPI Seminars 3 rd -4 th December 2014, Naples Anodic + Cathodic Polarization V OCP I V I Test 1Test 2 Time V I

DICMAPI Seminars 3 rd -4 th December 2014, Naples Anodic + Cathodic Polarization V-Vcorr I Anodic Branch Cathodic Branch log I V-Vcorr Anodic Branch Cathodic Branch Linear representation Logarithmic representation

DICMAPI Seminars 3 rd -4 th December 2014, Naples What happens during corrosion log I V e- O 2(g) 2H 2 O (l) 4OH - Cathodic Reaction Oxygen Evolution Oxygen Reduction Me Me n+ Anodic Reaction Metal Oxidation Metal Reduction

DICMAPI Seminars 3 rd -4 th December 2014, Naples What happens during corrosion Me Me n+ log I V Anodic Reaction O 2(g) 2H 2 O (l) 4OH - e- Cathodic Reaction Oxygen Reduction Metal Oxidation Corrosion Potential Corrosion Current We can physically measure ONLY THE RED LINE!!!

DICMAPI Seminars 3 rd -4 th December 2014, Naples Why do we need to do that??? Because it is impossible to measure directly the corrosion current. Me Me n+ O 2(g) 4OH - e- The corrosion current is INSIDE the metal!

DICMAPI Seminars 3 rd -4 th December 2014, Naples Equations The current that we physically measure The anodic area The anodic reaction exchange current density The potential at which we are keeping the electrode The equilibrium potential for the anodic reaction The Tafel Coefficient for the Anodic reaction

DICMAPI Seminars 3 rd -4 th December 2014, Naples Equations Anodic ReactionCathodic Reaction What we want to know What we can measure V a1 =V c1, V a2 =V c2, V a2 <<V c2

DICMAPI Seminars 3 rd -4 th December 2014, Naples CuCu 2+ Cu Cu 2+ One electrode with two reactions V a1 =V c1 V a2 =V c2 e-e- O2O2 2 H 2 O OH - O2O2 + 2 H 2 O + 4e - 4OH - O2O2 + 2 H 2 O + 4e - 4OH -

DICMAPI Seminars 3 rd -4 th December 2014, Naples 2Cu2Cu 2+ Cu 2+ Cu Cu 2+ One electrode with two reactions V a1 =V c1 V a2 =V c2 4e - O2O2 2 H 2 O 4OH - O2O2 + 2 H 2 O + 4e - 4OH - O2O2 + 2 H 2 O + 4e - 4OH -

DICMAPI Seminars 3 rd -4 th December 2014, Naples 2Cu2Cu 2+ Cu 2+ Cu Cu 2+ One electrode with two reactions V a1 =V c1 V a2 =V c2 4e - O2O2 2 H 2 O 4OH - O2O2 + 2 H 2 O + 4e - 4OH - O2O2 + 2 H 2 O + 4e - 4OH - If two reactions take place on one electrode and they have different equilibrium potentials, one will be the anodic reaction and one will be the cathodic reaction. We can simplify the equation! The Corrosion Potential will be somewhere between the potential of the anodic reaction and the potential of the cathodic reaction

DICMAPI Seminars 3 rd -4 th December 2014, Naples Simulated Polarization Curve I corr A Ba (V/deca de) bc (V/deca de)

DICMAPI Seminars 3 rd -4 th December 2014, Naples Polarization Resistance

DICMAPI Seminars 3 rd -4 th December 2014, Naples Polarization Resistance If we know the polarization resistance and Tafel slopes we can calculate the corrosion current

DICMAPI Seminars 3 rd -4 th December 2014, Naples Corrosion Current DISTRUCTIVENON DISTRUCTIVE

DICMAPI Seminars 3 rd -4 th December 2014, Naples 1)we cannot separate the electrolyte resistance 2)we don’t get any information on the electrode capacitance and 3)it is quite sensitive to non stationarity/linearity of systems (data can be quite noisy or the line can be not straight) 4) is unsuitable for coated electrodes. 5)Activation control 1)the experiment is destructive due to the large polarization applied 2)If the anodic and cathodic reaction are not under pureactivation control the extrapolation to Icorr of the potentiodynamic curves might not give the correct result 3)the curves might not be so clean as the simulated ones due to oveerlapping processes, thus the extrapolation becomes a bit arbitrary 4)it is unsuitable for coated electrodes in most cases.

DICMAPI Seminars 3 rd -4 th December 2014, Naples Summary DC techniques The purpose of most electrochemical techniques is to obtain a value of corrosion current. If you have a value of polarization resistance and have the Tafel coefficients you can calculate the corrosion current. Potentiodynamic polarization is destructive due to large current applied Linear polarization is less destructive but might suffer from non stationarity

DICMAPI Seminars 3 rd -4 th December 2014, Naples Evaluation of Corrosion Rate Similarity to Service Conditions Good Signal Time Resolution Avoid Polarization and/or Faradic Currents Perturb the Corroding Surface to Measure Response Minimize Measurement Time Amplitude of the AC/DC Signal Average Signals / Integration Time Mathematical/Statistical Elaboration

DICMAPI Seminars 3 rd -4 th December 2014, Naples EIS EIS involves the application of a small AC signal of variable frequency to gain information on the corrosion process introducing minimal damage to the corroding surface (non destructive) We need to understand: How electrode surface reacts to alternated current perturbation Series and Parallel High frequency and low frequency behaviour Equivalent circuits How to measure impedance Sources of problems

DICMAPI Seminars 3 rd -4 th December 2014, Naples 30 Impedance Measurement Modulus and phase at freq 1 Modulus and phase at freq 2 Modulus and phase at freq 3

DICMAPI Seminars 3 rd -4 th December 2014, Naples Alternated Current Perturbations Low Frequency

DICMAPI Seminars 3 rd -4 th December 2014, Naples Alternated Current Perturbations Medium Frequency

DICMAPI Seminars 3 rd -4 th December 2014, Naples Alternated Current Perturbations High Frequency

DICMAPI Seminars 3 rd -4 th December 2014, Naples Double Layer Formation Positive Negative Electrode Concentration

DICMAPI Seminars 3 rd -4 th December 2014, Naples The electrode surface

DICMAPI Seminars 3 rd -4 th December 2014, Naples Practical Measurement Setup

DICMAPI Seminars 3 rd -4 th December 2014, Naples R1R1 R2R2 R3R3 RtRt = i V Resistors-series

DICMAPI Seminars 3 rd -4 th December 2014, Naples R1R1 R2R2 R3R3 i V i RtRt = Resistors-parallel

DICMAPI Seminars 3 rd -4 th December 2014, Naples Capacitor Series

DICMAPI Seminars 3 rd -4 th December 2014, Naples Capacitor Parallel C t =C 1 +C 2 +C 3 …

DICMAPI Seminars 3 rd -4 th December 2014, Naples Low FreqHigh Freq R=R1 R1 R=0

DICMAPI Seminars 3 rd -4 th December 2014, Naples Low FreqHigh Freq R=infinity (circuit open) R1 R=R1

DICMAPI Seminars 3 rd -4 th December 2014, Naples Low FreqHigh Freq Rs =15Ω R=R1 Rp=1000 Ω R=R1+R2 Cdl=40 μF

DICMAPI Seminars 3 rd -4 th December 2014, Naples AC currents Instantaneous response to time varying signals Response depending on the derivative of the time-varying signal

DICMAPI Seminars 3 rd -4 th December 2014, Naples Impedances

DICMAPI Seminars 3 rd -4 th December 2014, Naples Impedances Z1Z2Z3 Series Z1 Z2 Z3 Parallel

DICMAPI Seminars 3 rd -4 th December 2014, Naples Rc circuits impedance Z

DICMAPI Seminars 3 rd -4 th December 2014, Naples The electrode surface

DICMAPI Seminars 3 rd -4 th December 2014, Naples Pstat V I Ref WkgCounter

DICMAPI Seminars 3 rd -4 th December 2014, Naples 50 Impedance Measurement Modulus and phase at freq 1 Modulus and phase at freq 2 Modulus and phase at freq 3

DICMAPI Seminars 3 rd -4 th December 2014, Naples Log Z Log F Phase

DICMAPI Seminars 3 rd -4 th December 2014, Naples Rs increase Log Z Log F Rs increase Phase

DICMAPI Seminars 3 rd -4 th December 2014, Naples Log Z Log F Phase Rp increase No effect

DICMAPI Seminars 3 rd -4 th December 2014, Naples Log Z Log F Phase Cdl increase No effect Cdl increase

DICMAPI Seminars 3 rd -4 th December 2014, Naples

Summary EIS is based on measuring the current response resulting from the application of a sinusoidal AC signal of variable frequency EIS is less distructive than DC techniques due to the small signal applied The simplest circuit representing an electrode is the (R(CR)) Information on solution resistance, polarization resistance and double layer capacitance can be obtained. Suggested Reading Corrosion Testing Made Easy: Electrochemical Impedance and Noise, R.A. Cottis, S. Turgoose, and R. Newman, NACE, 1999