Electrochemical-Impedance-Spectroscopy-Intro电化学阻抗谱.doc

Electrochemical Impedance Spectroscopy

Electrochemical Impedance Spectroscopy (EIS) is an electrochemical technique with applications in corrosion, battery development, fuel cell development, paint characterization, sensor development, and physical electrochemistry.  The reason for this popularity is the high information content of EIS.  EIS provides a more thorough understanding of an electrochemical system than any other electrochemical technique. 

Why is EIS so powerful? 
Because the EIS experiment involves the application of a sinusoidal electrochemical perturbation (potential or current) to the sample that covers a wide range of frequencies. 
This multi-frequency excitation allows
(1) the measurement of several electrochemical reactions that take place at very different rates and
(2) the measurement of the capacitance of the electrode.

AC Impedance
A typical experiment sweeps from 10 kHz to 0.01 Hz with a 10 mV perturbation around the rest potential. The usual result is an Nyquist impedance plot of half a semi-circle: the high frequency part giving the solution resistance and the width of the semi-circle giving the corrosion rate in the same manner as LPR. The analysis of this data is performed by circle fitting in the analysis software. One useful benefit of AC is the ability to measure the solution resistance at high frequency. This allows any instrument that incorporates AC to perform automatic IR compensation during DC tests.

At each frequency a sine wave is generated and fed into the potentiostat. This wave is then imposed on the cell and its potential and current flow measured. The measured values of current and voltage are compared for amplitude and phase and an impedance calculated. This is repeated for the rest of the frequencies and a plot generated. The standard starting point with AC impedance is the basic Randles circuit below.

An alternate name for AC impedance is Electrochemical Impedance Spectroscopy (EIS).

Concrete
Steel encased in intact concrete with no chloride carbonation stray currents or cracks should experience a pH of 12.5 and passivate remaining good for centuries. In practice problems do occur and finding the problem deep under the surface is no easy task. Some of the problems faced include; very high resistivity difficult connection to the reinforcing bar and the geometry of the test. Often the auxiliary and reference sit on the surface of the concrete with multiple strands of rebar some distance beneath. Knowing the extent of polarization is often a matter of experience. The simplest electrochemical test is potential mapping this gives an idea of corrosion activity but does not give corrosion rate data. More sophisticated tests use the Field Machine to determine corrosion rate from LPR IR compensated LPR and AC Impedance. Variants of the Field Machine have been supplied incorporating a guard ring to focus the measurement on a better defined area of rebar.

A test has been devised to measure chloride uptake in the laboratory. It consists of three bars mounted one above the other two in concrete and connected by a ZRA. A bath of 3% NaCl is mounted on the top and when the chloride permeates through the top bar becomes and anode with respect to the bottom two as Cathode.

This test block can also be used to measure IR compensated LPR and AC Impedance by applying a test frequency from 10000 Hz to 1 mHz mechanistic information of the corrosion processes can be obtained by studying the time constants revealed..

Linear Polarization Noise for Corrosion Monitoring in Multiple Phase Environments.

Linear Polarization Resistance Noise gives two results: the average monitored corrosion rate and the corrosivity of the conductive fluid. It works in situations where other more established techniques have technical difficulties and especially in every location where Current & Voltage Noise is used to calculate corrosion rate.

In many cases the Corrosivity of the Conductive Fluid is the most valuable result. For instance in a pipeline where there is a possibility of separation and localized pooling the corrosivity of the conductive fluid can be of greater value when calculating the injection rate for inhibitors.

Multiple Phase Systems are typically any which interrupt the conductivity between the test electrodes used to obtain the measurement. Examples are electrodes situated in a splash zone or multiple phase flow involving mixtures of saline water gas and oil. Traditional techniques such as Current and Voltage Noise Linear Polarization Resistance and Electrical Resistance all have limitations when operating in these environments. These are briefly noted below.
Linear Polarization Resistance is generally upset in multiple phase environments leading to erratic spiky results if the conductivity between the electrodes changes significantly throughout a test. The situation is worse in three electrode systems where a feedback loop through the cell is used to control the Cell Potential.

Current & Voltage Noise.
Often thought of as the solution to all monitoring problems the technique simply employs a two electrode Potentiostat that polarizes the electrodes at 0mV with respect to each other. A third electrode is then used to monitor Potential fluctuations. Unfortunately during periods of low conductivity large Potential Perturbations do not correspond to significant current activity due to high solution resistance. This is a fundamental error in the technique which leads to corrosion rates that are lower than expected.

Electrical Resistance.
This gives a reading of the metal loss on the metals surface. Trends in the data can then be used to calculate the historic corrosion rate. The technique is limited however in that it only gives the average historic corrosion rate at the probe and does not attempt to give the corrosivity of the conductive elements in the multiple phase medium.

Linear Polarization Noise.
This combines the practical nature of Linear Polarization Resistance with the subtlety of Noise. The technique works with two electrode probes of a standard design and is resistant to any changes in the medium surrounding the electrodes as like Current Noise no feedback mechanism operates through the test fluid. Two results are obtained from the technique the Average Monitored Corrosion Rate and the Corrosivity of the Conductive Fluid. Data trends are presented in real time in mpy or mmpy.

As with Current and Voltage Noise the technique has further possibilities with regards to monitoring of localized corrosion as well. Knowledge of the system under test is paramount.

The technique is available as an option for operation with the portable Field Machine or desk top models such as our Gill AC. Models suitable for in-situ monitoring on site can also be supplied on request.

Linear Polarization Resistance
The LPR technique is the most frequently used being both quick and easy. A small sweep from typically -10 mV to +10 mV at 10 mV/min around the rest potential is performed. The resulting current/voltage plot usually exhibits a straight line the inverse slope of which is proportional to the corrosion rate. The Gill AC Gill 8 and 12 the Field Machine the Pocket Machine the LPR meter and the Bubble Test software all use this method.

The step method is used in hand held instruments for example the pocket machine. The current is measured at points A and B once the initial current surge has steadied. The voltage sweep results in a response shown above a best fit straight line gives the charge transfer resistance A variant of the LPR test is the pitting index. This is a measure of the asymmetry between anodic and cathodic current response a feature built into to the portable LPR meter where it is available as a switched option between corrosion rate and pitting index. The LPR method is ideal for plant monitoring offering an almost instantaneous indication of corrosion rate allowing for quick evaluation of remedial action and minimizing unscheduled downtime. As an example and guide to allow the new operator to obtain a feeling for the numbers involved the table below gives a qualitative classification of corrosion rates of carbon steel in a water cooling system.
Corrosion rate mm/year
mils/year
Classification
 <0.03
 1.2
 Excellent
 0.03 – 0.08
 1.2 – 3.2
 Very good
 0.08 – 0.13
 3.2 – 5.2
 Good
 0.13 – 0.2
 5.2 – 8
 Moderate
 0.2 – 0.25
 8 – 10
 Poor
 >0.25
 >10
 Very poor
To convert a corrosion current in mA/cm2 to a corrosion rate in mm/year multiply the current by 12.
The sources of error in LPR tests include uncertainty in the parameter B used in the Stern and Geary equation where icorr = B/Rp B = (ba.bc)/(2.3(ba+bc)) choice of a scan rate that is too fast neglect of the solution resistance and non linearity. In practice a value of 20 mV usually works well for B a scan rate of 0.2 mV/sec is often adequate the solution resistance can be compensated by positive feedback and the non-linearity error is only a small percentage of the result. Linear polarization resistance can be done either three or two electrodes. The two electrode method relies on both electrodes been similar so that when they are coupled and offset the test is still in the linear region. This matching of electrodes is not needed when using the three electrode method as the potentiostat measures the rest potential and offsets the test around that.