Electrochemical Impedance Spectroscopy Manual

7/28/2019 Electrochemical Impedance Spectroscopy Manual

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ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY .

Introduction.

Electrochemical impedance spectroscopy is a recent tool in corrosion laboratoriesthat is slowly making its way into the service environment as units are decreased in size and become portable. Impedance Spectroscopy is also called AC Impedance or just ImpedanceSpectroscopy. The usefulness of impedance spectroscopy lies in the ability to distinguishthe individual contributions of components under investigation. For example, if the

behavior of a coating on a metal when in salt water is required, by the appropriate use of impedance spectroscopy, a value of resistance and capacitance for the coating can bedetermined through modeling of the electrochemical data. The modeling procedure useselectrical circuits built from components such as resistors and capacitors to represent theelectrochemical behavior of the coating and the metal substrate. Changes in the values for the individual components indicate their behavior and performance. Impedancespectroscopy is a non-destructive technique and so can provide time dependent informationabout the ongoing processes. Below is a listing of the advantages and disadvantages of thetechnique.

Advantages.

1. Useful on high resistance materials such as paints and coatings.

2. Time dependent data is available

3. Non- destructive.

4. Quantitative data available.

5. Use service environments.

Disadvantages.

1. Expensive.

2. Complex data analysis for quantification.

Impedance Spectroscopy Technique.

In impedance spectroscopy, a small sinusoidal voltage is placed on the sample over

a wide frequency range, from 105

to 10-3

Hz. It is therefore and alternating current (AC)technique. The controlling computer system measures the magnitude of the current induced

by the potential and in addition the phase angle between the potential and current maxima.A modified Ohms law is applied:


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For DC conditions:-V=iR

For AC conditions:-V=iZ

where Z is the IMPEDANCE of the system.From this modified Ohms Law for AC conditions the impedance can be calculated

by setting the input potential and measuring the induced current. When the phase angle, , between the voltage applied and the current induced is zero, then a pure resistance is

present. When a phase angle of 90o

is measured between the voltage and current at thesame frequency, a pure capacitance is present. Angles between these values can mean acombination of a capacitor and resistor are present. It is therefore important to plot theimpedance, usually the log of the impedance magnitude, as a function of frequency and inaddition the phase angle as a function of frequency. These are known as Bode plots.

log Z

Frequency

Rs

Rt+Rs

PhaseAngle

0

90

An example of a Bode plot is shown above for a simple electrical circuit called aRandles circuit. It consists of a resistor and a capacitor in parallel. This circuit simulates thetype of data found for a simple corroding interface, such as steel in seawater. When thecircuit has a maxima in the phase angle as shown in this case, it is caused by a combinationof a resistance and a capacitor in parallel and is called an RC circuit. It also leads to models

of electrochemical interfaces. One simple model is called the double layer model. In thisthe interface for a metal in electrolyte is envisioned as a layer of ions adjacent to theelectrode surface with a further layer of solvated ions further away from the surface. Thetwo layers of ions leads to a capacitor as they store charge, called the double layer capacitance, Cdl. It also leads to a resistor as charge leaks across the capacitor, called thecharge transfer resistance, Rs. Therefore the models of electrochemical interfaces arerepresented as a capacitor and resistor in parallel. In series with this is a resistor representing the solution resistance, Rs. A typical interface is shown below along with its


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electrical analogs.By using the electrical circuit analog approach, the data can be modeled to provide

quantitative values for Rs, Cdl and most importantly Rt. Rt is proportional to corrosionresistance of the electrode. The higher this value the more resistant to corrosion. Typical

values for Rt are 104

.cm2

for steel in seawater and 106

for titanium. Metals which will

tend to pit are in the 10 5 region.

A more complex analysis is available for paints on steel and other coated samples.In this case the circuit involves two parallel capacitors and resistors in series along with asolution resistance. In this case the parameter of importance is the pore resistance or Rp in

the figure below. This value extracted from models is often around 1010

, but if it decreases

below 106

it implies that corrosion is occurring below the paint film and the paint system isgenerally non-protective. This is commonly used in paint and coatings development todetermine the most effective protection scheme.

Electrode

+

+

+

Adsobed ionsSolvated

ions

e

e

e

e

e

electrons

Rs

Rt

Cdl

Simple Randles circuit for an electrochemical interface


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Rs

Rt

Cdl

Rp

Cc

Electrochemical model of a coated metal electrode, for example epoxy paint on a steel.Rp - pore resistance of coating, Cc - capacitance of coatingRt - charge transfer resistance of the coating substrate interface, Cdl capacitance of interface.

Impedance Testing

The experimental setup for impedance spectroscopy testing is shown below.A glass tube is clamped above the sample using half a spherical glass joint. Differentsurface areas can be exposed by using different diameter glass joints. An O ring separatesthe glass tube from the surface of the sample.

DATA ACQUISITION SYSTEMAND POTENTIOSTAT

CONNECTIONINTERFACE

TEST SPECIMENIN TUBE

REFERENCE ELECTRODE IN STAND


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Conductive Copper Tape

Clamp

O-Ring coated with RTV sealer

Saturated Calomel ReferenceElectrode

Platinum Counter Electrode

Counter Electrode Connector

Plastic Isolator Aluminum Test Specimen

Test Stand

Test Tube Filled with0.5 Normal NaCl

A stainless steel clamp holds the sample onto the joint, however an insulating pieces isrequired between the clamp and the underside of the sample. The sample is the workingelectrode and so electrical connection is made to it either directly or by placing a wireunderneath it. A reference electrode and a counter electrode are placed in the electrolyte.

The connections from these electrodes firstly go into a frequency response analyzer. Thishas the capability to impose on a voltage signal a small sinusoidal potential over a widefrequency range. It also can detect the phase angle and magnitude of the induced current inthe sample. The impedance is directly calculated in the frequency response analyzer. Thisdata is transmitted back through the potentiostat to the controlling computer and can bestored and plotted . The setup is simple and allows for multiple samples. Coated samples can

be tested in same manner. Solution conditions can be varied to include high resistancesolutions such as simulated concrete environments.


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The data below is for a steel placed in salt water. The experimental data as well as amodel fit using a Randles circuit is shown. For this simple case the model is very good atfitting and predicting the data.

Generally a 10mV potential is imposed on the sample and the current measured.


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However for very resistive samples such as thick paints and very low resistive solutions ahigher potential amplitude is used. Values up to 100 mV were reported in the literature. Thereason for increasing the potential is to produce a reasonable signal to noise ratio so that thedata is meaningful, especially for highly resistive coatings. It should be noted that the

potential cannot be too high otherwise the sample is polarized and anodic and cathodic

reactions are initiated. The idea of the small potential it to avoid major current flow and somaintain the technique as non destructive.Long term studies can be done by this technique to determine the behavior of

coatings for example. In addition, quantitative information is available which cannot bedetermined by salt spray (ASTM B117) or other techniques. For example if severalcoatings pass the salt spray test, the best one is still not known. However impedance testingcan determine the best coating. In addition the long term measurement capability alongwith the non destructive and quantitative nature of the test will sometimes indicatedimportant information, such as the coating with the highest resistance at the initiation of atest will degrade very quickly and not last as long as a coating which started at a lower value and did not decrease as much.

Using commercial software the data can be modeled and values extracted.

Modified Mansfeld ModelRsoln: 2.877 ohmsRmetal: 33.49 KohmsRoxide: 13.80 Kohms

Rsoln:

Rsoln + Roxide

Rsoln + Roxide + Rmetal


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The data above is for aluminum foil in seawater. The individual points are the data and thecontinuous line is the model fit for a two RC circuit model. Shown on the figure are theresistance values used by the model when fitted to the data. Different models can be appliedwhen different mechanisms are operating. For example a phase angle horizontal arrest at45o indicates that diffusion control is operating and a Warburg impedance componentwould be placed in the circuit to account for this.In addition to the horizontal arrest at 45the slope of the impedance magnitude as a function of frequency should be 0.5 when aWarburg impedance is operating.

Electrochemical Impedance Spectroscopy Manual

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