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Click here for "Rust's A Must", the epic
poem to corrosion!
Using Gamry Electrochemical
Instrumentation to Measure and Control Corrosion
Corrosion is the chemical or
electrochemical reaction between a material, usually a metal, and its
environment that produces a deterioration of the metal and its properties (see
ASTM G 15). A chemist would classify corrosion as an oxidation, an
electron-transfer reaction involving the loss of electrons.
Since corrosion is an
electrochemical process, it follows that electrochemical techniques and
electrochemical instrumentation can be used to study the corrosion
process. Indeed, a number of electrochemical techniques have been
developed over the years especially for the measurement of corrosion
processes. Electrochemical techniques are very well accepted by the
corrosion community and it is the rare corrosion laboratory that does not make
routine use of these tools.
The reasons for the popularity
of electrochemical techniques for corrosion measurement are based in
practicality:
They are fast. Corrosion, even rapid corrosion, is a slow
process. Real-time weight loss measurements need days and sometimes weeks
to make a reliable measurement of corrosion rate. Electrochemical
instrumentation can make a corrosion rate measurement in minutes or hours.
They are sensitive. Modern, well-designed electrochemical
instrumentation can measure extremely low corrosion rates.
They are accurate. Electrochemical techniques have been
exhaustively tested before finding general acceptance. Organizations such
as ASTM and NACE have sponsored a number of symposia and published peer-reviewed
papers describing the use and results of these techniques.
They are versatile. Electrochemical techniques can be used to study a
wide range of corrosion-related phenomena. The rate of uniform corrosion
can be measured. The tendency of a metal to exhibit localized (pitting or
crevice) corrosion can be measured. The passivation behavior of a
corroding system can be studied. Galvanic corrosion can be quantitated.
Sensitization effects can be studied. Electrochemistry can be used in the
laboratory or in the field. Measurements can be made on the lab bench or
in a pipeline or in an autoclave or in a slow strain rate machine.
Electrochemical
Techniques Used in Corrosion Measurement
Since electrochemistry was
recognized many years ago as the basis for corrosion, a number of
electrochemical techniques have been developed specifically for corrosion
measurement. These are generally referred to as "DC
Techniques". Among these techniques are Polarization Resistance, Tafel
Plots, Potentiodynamic Plots, Cyclic Polarization, and Electrochemical
Potentiokinetic Reactivation. Even though some of these techniques are
quite a verbal mouthful, they are all very similar and also very
straightforward to employ in a corrosion laboratory.
Gamry recently introduced the EFM140 Software to perform Electrochemical
Frequency Modulation (EFM). EFM is an interesting electrochemical
approach to the measurement of corrosion rate that has several potential
advantages. The corrosion scientists who use EFM will render the decision
on its usefulness.
Corrosion scientists also make
extensive use of Electrochemical Impedance Spectroscopy
(EIS). Indeed,
corrosion science was the first major scientific discipline to embrace EIS in
the early 80s and is largely responsible for the popularity of this powerful
technique.
To learn more about
electrochemical corrosion measurement, go to the literature. The place to
start is the Gamry website. You can get a quick overview of DC
Corrosion Techniques by reviewing our Application Note titled Getting
Started with Electrochemical Corrosion Measurement. There are a
number of references at the end of this Application Note that you are encouraged
to purchase. We particularly recommend the first two references ("DC
Electrochemical Test Methods" and "Principles and Prevention of
Corrosion").
Electrochemical Impedance
Spectroscopy can be a bit intimidating both theoretically and
instrumentally. We assure you, however, that it is worth your effort to
become comfortable with EIS, because it provides information that cannot be
obtained with DC techniques. EIS can study systems in which multiple
electrochemical reactions are occurring and also gives some insight into
the capacitive nature of your electrochemical cell. For an excellent and
extensive discussion, see our EIS
Primer.
A newer technique with
extraordinary potential is Electrochemical Noise. Researchers have
applied electrochemical noise to the measurement of uniform corrosion rate,
pitting corrosion, and coatings evaluation. The unique feature of
electrochemical noise techniques is that the sample is not perturbed at
all. Since the sample is maintained in an entirely natural state, there is
the possibility of obtaining results of very high accuracy. There are two
excellent references for electrochemical noise:
(1) Electrochemical
Impedance and Noise, Robert Cottis and Stephen Turgoose, National Association of
Corrosion Engineers, 1440 South Creek Drive, Houston, TX 77084-4906. Phone: 281-228-6200.
Fax:
281-228-6300. ISBN: 1-57590-093-9.
(2) Electrochemical Noise Measurement for Corrosion Applications, STP
1277, Edited by Kearns, Scully, Roberge, Reichert, and Dawson, ASTM, 100 Barr
Harbor Drive, West Conshohocken, PA 19428. Phone: 610-832-9500.
Fax: 610-832-9555,
ISBN 0-8031-2032-X.
Corrosion
Short Course
We also recommend the Short
Course on "Corrosion: Fundamentals and Experimental Methods" that's held at
Pennsylvania State University every summer. The faculty for the Short
Course is from both Penn State and Ohio State. The Short Course is the
best way to jump-start your use of electrochemical corrosion techniques. Need
more information?
EIS Short
Course
EIS is a powerful
technique for corrosion measurement and now there's a Short Course on this
subject. "Electrochemical
Impedance Spectroscopy: Theory, Applications, and Laboratory Instruction"
is held at the
University of Texas Health Sciences Center in Houston with faculty from
UTHSC, Ohio
State, Rockwell Science Center, and Battery Design Corp.
Click here for more details.
An
Excellent Review of Methods for Corrosion Rate Determination
Drs. John Scully
and Rob Kelly of the University of Virginia have published a very complete
review of the various methods, electrochemical and others, for the
determination of corrosion rate. The article, entitled
“Methods for Determining Aqueous Corrosion Reaction Rates” is
contained in ASM Handbook,
Volume 13A, Corrosion:
Fundamentals, Testing, and Protection. The review
discusses uniform corrosion, pitting, electrochemical noise,
galvanic corrosion, paints, and nonelectrochemical methods.
The article is
available for sale as a PDF download on the ASM Web site at
www.asminternational.org. The cost is $25 ($20 for ASM
Members).
The Handbook Volume can also be purchased from ASM International for
$264 ($212 for ASM Members) or by calling ASM Member Services at
(800) 336-5152 (US and Canada), Extension 0.
High Temperature
Corrosion Measurements
The environmental conditions in nuclear
reactors or industrial boilers is, to say the least, harsh!
Temperatures typically exceed 300° C! But the corrosion
rates are usually low, so making the corrosion rate measurement
electrochemically saves lots of time. Workers at Penn State
have published a paper on high-temperature corrosion using a
flow-through electrode assembly ("Experimental System for
Electrochemical Studies of Aqueous Corrosion at Temperatures above 300°C",
Balashov et al, J. Electrochem. Soc., 156,
C209 (2009). The corrosion rate was determined by fitting the
Butler-Volmer equation to a Tafel Plot and by Electrochemical Impedance
Spectroscopy.
Corrosion
Measurements of Reinforcing Bar in Concrete
Corrosion of rebar in
concrete continues to be a serious safety and financial issue for
bridges, port facilities, and other structures. It's a worldwide
problem, not confined to coastal areas with high chloride-containing
humidity or cold climates where road salt is needed to keep the roads
safe. Your Gamry Corrosion Measurement System is a powerful tool
to understand and control rebar corrosion. Dr. Andres
Torres-Acosta is working in this area at the Instituto Mexicano del
Transporte and has kindly provided some details on making
electrochemical measurements on rebar in concrete. More to come.
Instrumentation
for Electrochemical Corrosion Measurement from Gamry
Gamry has the world's widest range of electrochemical instrumentation for
corrosion measurement. Gamry also boasts two other features that may be
particularly important for corrosion measurement:
Grounded Samples: Gamry Potentiostats are designed to be electrically isolated from
ground, which allows them to be used to perform electrochemical experiments on
grounded samples. If your sample is in an autoclave, a strain rate tester,
if it's a bridge deck or a pipeline, then it's grounded and your Gamry Potentiostat will work just fine. Many non-Gamry potentiostats require
that the sample be floating (isolated from ground) and will not work with a
grounded sample. Note: If your sample, or Working Electrode, is grounded,
the Reference and Counter Electrodes must NOT be grounded.
Portability: Interface your Gamry Potentiostat to a laptop computer and
you can easily take it down the hall or across the country.
Gamry
products for the Corrosion Scientist are described below. For more detail
on any product, click on the hyperlink.
Potentiostats
Gamry Potentiostats are the best choice for corrosion for several reasons, but
one of the most important is the low noise of a Gamry Potentiostat. Gamry
Potentiostats are simply the best Potentiostat for corrosion, which inevitably
requires the measurement of very low currents. The lower limit of
sensitivity is determined by the inherent noise level in the Potentiostat and
Gamry's noise specification is the best in the industry. Amazingly (to
us), many potentiostat manufacturers don't even specify noise. Do yourself
a favor...don't buy a potentiostat for which noise is not specified.
Click here for
Gamry's specifications.
For corrosion, you can use either the Series
G 300
or the Series G 750
Potentiostat. If
you think you'll routinely encounter higher currents (higher currents mean
higher corrosion rates), go with the Series G 750.
With the Series G 300, you'll be able to measure lower currents, which is
often an important factor in a corrosion lab.
The Reference 600 Potentiostat is an
outstanding Potentiostat and the preferred corrosion instrument from the
standpoint of performance.
It covers a wider current range than the Series G, so it might good for you if
you expect to encounter very low corrosion rates. If you're charged with
making both corrosion measurements and paint testing, then the Reference 600
is the best choice! Corrosion scientists also like the Reference 600
because it's small and can be easily carried out of the lab to make measurements
in the field using a notebook computer.
The Reference 3000 Potentiostat has
a very low current measurement capability and also works well for corrosion.
However, a corrosion lab really does not need the high-current capability of the
Reference 3000. The Reference 3000 is a good choice for labs involved in corrosion
measurement, providing there are other applications that can benefit from the 3-amp limit
of the Reference 3000.
Multiple-Potentiostat
Systems
For the highest sample throughput, consider a multi-channel system for performing simultaneous
experiments. A simultaneous system is more expensive than a sequential
system, but the sample throughput is awesome! Click
here for information on the Gamry MultEchem Electrochemistry Systems.
Multiplexer
The ECM8 Multiplexer is used with a Gamry
Potentiostat to automatically acquire data from eight (8) cells in a sequential
fashion. It's perfect for monitoring the change in corrosion rate with
time.
Software
DC Corrosion: The DC105 Corrosion
Techniques Software provides 12 techniques in an easy-to-use
point-and-click format.
Electrochemical Impedance Spectroscopy: The EIS300
Electrochemical Impedance Spectroscopy Software is all that is needed to
run EIS with a Gamry Potentiostat.
Electrochemical Frequency Modulation: The EFM140
Electrochemical Frequency Modulation Software puts this new technique to
work for you.
Electrochemical Noise: The ESA400
Electrochemical Signal Analyzer is a very sophisticated package for data
acquisition and analysis. Dare we say that there is no competitive product
that even begins to approach the power and flexibility of the ESA400? Yes,
we do, indeed, dare say! The EN120
Electrochemical Noise Software is used to acquire noise data for several
samples with the ECM8 Multiplexer.
Critical Pitting Temperature:
The Critical Pitting Temperature (CPT) is determined by increasing the
temperature of the sample while monitoring specific electrochemical parameters
in the cell. The CPT110 Critical Pitting
Temperature Software makes the measurement automatically. The CPT110 is
used with a Gamry Potentiostat, the TDC2
Temperature Controller, and the Flexcell.
Gamry is the only potentiostat manufacturer that provides an automated system
for CPT measurement.
Electrochemical
Cells
Your sample, whose corrosion characteristics you wish to study, must be mounted
in an electrochemical cell and connected to the potentiostat. For
cylindrical samples, try the Multiport Corrosion Cell
Kit or the EuroCell. Flat samples can be easily, and inexpensively, studied with
the PTC1 and PortHoles Electrochemical Sample
Masks. You can use a Rotating
Disk Electrode or a Rotating Cylinder Electrode to study the effects of
solution velocity on corrosion rate.
If your sample has a complex geometry, presenting the
sample properly to the instrument can be difficult. You need to know the
area and you must be sure that the electrolyte is contacting only the
test sample. If you need some ideas, call Gamry and talk to
David Loveday
or Jacob Ketter or Burak Ulgut or Pete Peterson. Also, you should be aware that electrochemical cells are
not potentiostat-specific. That means that a non-Gamry cell will work on a
Gamry Potentiostat, and vice versa.
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