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Electrochemical Techniques for Corrosion Measurements

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Assignment Present a critical examination of the proposed mechanisms of CO 2 and H 2 S corrosion with particular emphasis on the cathodic (oxidation) reactions. Due Date 5 June 2009

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3 Corrosion Measurements Involve the use of a potentiostat for applying a potential (relative to a reference electrode) and measuring the current (flowing from the working electrode to the counter or auxiliary electrode) *ensure specimen potential (w.r.t. counter) constant … even though solution resistance might alter.

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The Manual Potentiostat Variable High Voltage Source V High Impedance Voltmeter Ω Auxiliary Electrode Working Electrode Reference Electrode Ammeter (current)

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Electrochemical Impedance Spectroscopy Resistance Ohms Law For a resistor, R, it follows Ohms Law at all current and voltage levels The resistance value is independent of frequency AC current and voltage signals through a resistor are in phase with each other

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Electrochemical Impedance Spectroscopy Impedance Impedance applies to AC voltage and current Like resistance impedance is a measure of the ability of a circuit to resist the flow of electrical current The excitation potential or AC voltage can be expressed as a function of time

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Electrochemical Impedance Spectroscopy The relationship between radial frequency ω (radians s -1 ) and frequency (f) hertz is: The response to the AC voltage is given by:

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Electrochemical Impedance Spectroscopy Similar to Ohms law The important point to remember is that when an AC voltage is applied to a pure capacitor the resulting AC current is shifted in phase by 90 o There is no phase shift for a pure resistor

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Electrochemical Impedance Spectroscopy Current phase shift due to impedance. Through a capacitor this phase shift is 90 o Applied Voltage Resulting Current

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Electrochemical Impedance Spectroscopy Randles circuit for a simple corroding system R s = the solution resistance R ct = the charger transfer (polarisation resistance) C dl = the double layer capacitance

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Electrochemical Impedance Spectroscopy Nyquist plot for the Randels circuit Resistance Capacitance Increasing Frequency Solution Resistance Charge transfer resistance = R total - R s

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EIS Nyquist Plots A Nyquist plot is made up of a series of vectors representing the total magnitude of the resistance and capacitance components Phase angle Non Resistive Component

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Electrochemical Impedance Spectroscopy Bode impedance plot Impedance Frequency Solution resistance R ct

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Electrochemical Impedance Spectroscopy Bode Phase plot Frequency Phase angle

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EIS (Summary) We start here at the high frequency

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EIS Diffusion or Mass Transfer controlled process Nyquist plot - Warburg Impedance Frequency

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EIS Diffusion or Mass Transfer controlled process Bode Impedance plot Impedance Frequency

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EIS Diffusion or Mass Transfer controlled process Bode Phase plot Phase Angle Frequency

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EIS – Mass Transfer Controlled Process (Summary) Nyquist Bode Impedance Bode Phase

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EIS Equivalent Circuit for a Mixed Kinetic and Charge Transfer Controlled Process

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EIS Bode Plots for the Mixed Controlled Reaction Impedance Phase

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EIS Equivalent Circuit for a Filmed Corroding Surface (E.g. Failed Coating)

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EIS for a Filmed Corroding Surface (E.g. Failed Coating) Nyquist Bode Impedance (Magnitude Bode Phase Angle

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Linear Polarization Method Valid for corrosion under activation control. Involves applying a small perturbation to the potential around E corr (i.e., ± E 10 mV). Slope of summed curve (measure E vs i for system) is difference between slopes of curves for the coupled reactions: S a - S c N.B. i for summed curve = i a + |i c | (i a =x)

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The curves are ~linear within ~20mV – S a and S c are constant. ForE around E corr, S a and S c are related to i corr (the required quantity): assuming the high-field approximation for the individual reactions Now: Polarization Resistance.... is measured. The Tafel coefficient b a and b c must be known. or slope

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26 Remember: during linear polarization measurements we plot E vs i (not log i) around the corrosion potential: = polarization resistance

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Linear Polarization This involves the application of low over-potentials and therefore the currents are relatively very small. This means that the charging current (capacitance current) can make a significant contribution to the noise or background current. Use slow scan rates and perform a cyclic scan to check whether you are measuring capacitance. The reverse scan should produce an iE curve that retraces over the forward recorded iE curve. The iE curve can be curved due to a difference in the anodic and cathodic Tafel slopes.

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Linear Polarization It is important to view the iE curve. If the iE curve is curved, the polarization resistance can be obtained by drawing a line that is tangential to the curve at E corr and at zero current. Some portable instruments use a potential-step method. In this case the current, at, for example, -10 mV and +10 mV is measured and R p is computed from these measurements. The advantage of this technique is that the current measurements are made at a constant voltage and therefore the charging current is zero. The disadvantage is that no iE curve is recorded and therefore an error can be introduced if there is curvature in the iE graph

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Linear Polarization The portable instruments that use the potential-step technique, usually apply a high frequency AC signal before the measurement to determine the solution resistance and subtract this value from the measured polarization resistance.

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Tafel Extrapolation

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Tafel Method We can only measure the net current across the specimen electrode – at the corrosion potential there is no net current (only local anode – cathode currents which constitute the corrosion current). We cannot measure corrosion rate directly, though we need i corr. Measure potential and current at some distance on either side of E corr – extrapolate E - log i curves (in same quadrant) back to E corr … Plot of the total current (i T = i o + i c ) versus potential showing the extrapolation of the Tafel regions to the corrosion potential, E corr, to yield the corrosion current, i corr.

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Passivation Under certain conditions of potential and pH, some metals form protective films, i.e., they passivate Pourbaix diagram for the iron/water/dissolved oxygen system showing the effect of potential in moving the system from a corrosive (active) region (point 1) to a passive region (point 2) We can exam the kinetics using a potentiodynamic scan and Evans diagram

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33 The polarization curve for the anodic reaction of a passivating metal drawn for potentials more noble than the equilibrium potential (E e ) a Oxidative dissolution of oxide (e.g., Cr 2 O 3 CrO 4 2- ) (E e ) M/MO is the equilibrium potential for oxide/hydroxide formation Tafel region (i crit is min. reaction rate required to initiate film growth by precipitation of M n+ ) The region attained by the metal in a given environment depends upon the cathodic reaction i.e., where the cathodic curve cuts the above anodic curve. Flade

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Tafel Extrapolation Technique Involves measurements at high over-potential in which logi is recorded. The best method of performing these measurements is by : 1) Using two identical electrodes and recording the anodic curve on one electrode and the cathodic curve on the other electrode. In each case starting at the open circuit potential E oc (or E corr ) 2) Performing the cathodic curve on one electrode starting the scan from E oc. Turning off the potentiostat and monitoring E oc until it returns to its original value. The anodic scan is then recorded again starting at the E oc.

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Tafel Extrapolation Technique Before commencing a Tafel measurement, it is generally best to allow your metal electrode to reach a steady state potential. This can be observed by performing a potential time measurement in which E corr is monitored with time. Scan Rates are normally in the range of 0.1 mV to 5.0 mV per second. The cathodic plot is scanned to an over-potential of about 400 mV. Anodic potentials can be scanned much higher depending on what information needs to be obtained.

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Cyclic Pitting Scans The technique is used to evaluate the susceptibility of metals to pitting corrosion in a particular environment. It is applicable to metals such as stainless steels, high nickel alloys and aluminium, which form a passive protective film. With this technique, the potential is scanned to voltages in the transpassive region. Exceeding the passive region is indicated by a sudden increase in current. At this stage the voltage scan is reversed, usually when the current reaches a certain current density (0.5 mA cm -2 )

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Cyclic Pitting Scans The extent of the hysteresis in the reverse scan is an indication of the susceptibility to pitting corrosion. Pitting corrosion is considered to stop at the potential where the iE curve from the reverse scan crosses the iE curve of the forward scan. The sudden increase in current can be due to three processes: 1) Onset of pitting corrosion 2) Trans-passive uniform corrosion 3) The oxygen evolution reaction In the case of trans-passive corrosion, the slope of the iE curve is not as steep compared to pitting corrosion and oxygen evolution. In the case of oxygen corrosion, the reverse iE curve normally will retrace over the forward iE curve.

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Harmonic Analysis Butler Volmer Equation When an electrode is polarized near the corrosion potential by a sinusoidal voltage of frequency ω and amplitude U 0, then the current density of the Faradaic process is given by:

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Harmonic Analysis The current densities of the Faradaic process will have a distorted sinusoidal form due to the non- linear nature of the cathodic and anodic partial processes in a polarization curve. The amplitudes of the harmonic components can be obtained by Fourier series expansion of the exponential terms

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Harmonic Analysis The simplified magnitude of the first three harmonic components are given by:

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Harmonic Analysis The technique has been verified by the work of Will Durnie, Curtin University. He compared corrosion rates from HA with those obtained using linear polarization measurements and Stern Geary equation. When the Tafel slopes obtained from HA were placed in the Stern Geary equation an excellent correlation was obtained.

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Harmonic Analysis Durnie, W. H.,Curtin University

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Electrochemical Noise (ECN) ECN measures the current/voltage response between two (largely) identical electrodes. The two electrodes are coupled together (short circuited together) through a zero resistance ammeter (ZRA). The random fluctuations of current is measure by the ZRA. At the same time the random fluctuations in voltage noise at the coupled electrodes is measured with respect to a reference electrode.

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Electrochemical Noise Measurement A Working Electrode 2 Working Electrode 1 Zero Resistance Ammeter V Reference Electrode

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Rotating dual cylinder electrode (RDCE) Standard electrochemical cell with reference, auxiliary and RDCE The RDCE is useful for performing ECN Measurements ECN uses identical electrodes. In the example shown the electrode areas are not the same since this RDCE was used to investigate preferential weld corrosion

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Potential Noise and Current Noise

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Voltage Noise PSD

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