Getting Started with Electrochemical Corrosion Measurement
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1.
Review of the Electrochemical Basis of Corrosion 2.
Quantitative Corrosion Theory 4.
Calculation of Corrosion Rate from Corrosion Current 6.
Current and Voltage Conventions 7.
References on Corrosion Theory and Electrochemical
Corrosion Tests
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Review of the Electrochemical Basis of CorrosionMost
metal corrosion occurs via electrochemical reactions at the interface between
the metal and an electrolyte solution. A thin film of moisture on a metal surface forms the electrolyte
for atmospheric corrosion. Wet concrete
is the electrolyte for reinforcing rod corrosion in bridges. Although most corrosion takes place in
water, corrosion in non-aqueous systems is not unknown. Corrosion normally occurs at a rate determined by an
equilibrium between opposing electrochemical reactions. The first is the anodic reaction, in which a
metal is oxidized, releasing electrons into the metal. The other is the cathodic reaction, in which
a solution species (often O2 or H+) is reduced, removing electrons
from the metal. When these two
reactions are in equilibrium, the flow of electrons from each reaction is
balanced, and no net electron flow (electrical current) occurs. The two reactions can take place on one
metal or on two dissimilar metals (or metal sites) that are electrically
connected. Figure 1-1 diagrams this process. The vertical axis is potential and the horizontal axis is the logarithm of absolute current. The theoretical current for the anodic and cathodic reactions are shown as straight lines. The curved line is the total current -- the sum of the anodic and cathodic currents. This is the current that you measure when you sweep the potential of the metal with your potentiostat. The sharp point in the curve is actually the point where the current changes signs as the reaction changes from anodic to cathodic, or vice versa. The sharp point is due to the use of a logarithmic axis. The use of a log axis is necessary because of the wide range of current values that must be displayed during a corrosion experiment. Because of the phenomenon of passivity, it is not uncommon for the current to change by six orders of magnitude during a corrosion experiment. Figure 1-1
The
potential of the metal is the means by which the anodic and cathodic reactions
are kept in balance. Refer to Figure
1-1. Notice that the current from each
half reaction depends on the electrochemical potential of the metal. Suppose the anodic reaction releases too
many electrons into the metal. Excess
electrons shift the potential of the metal more negative, which slows the
anodic reaction and speeds up the cathodic reaction. This counteracts the initial perturbation of the system.
The
equilibrium potential assumed by the metal in the absence of electrical
connections to the metal is called the Open Circuit Potential, Eoc. In most electrochemical corrosion
experiments, the first step is the measurement of Eoc.
The
value of either the anodic or cathodic current at Eoc is called the Corrosion
Current, Icorr. If we could measure
Icorr, we could use it to calculate the corrosion rate of the metal. Unfortunately, Icorr cannot be measured
directly. However, it can be estimated
using electrochemical techniques. In
any real system, Icorr and Corrosion Rate are a function of many system
variables including type of metal, solution composition, temperature, solution
movement, metal history, and many others. The
above description of the corrosion process does not say anything about the
state of the metal surface. In
practice, many metals form an oxide layer on their surface as they corrode. If the oxide layer inhibits further
corrosion, the metal is said to passivate.
In some cases, local areas of the passive film break down allowing
significant metal corrosion to occur in a small area. This phenomena is called pitting corrosion or simply pitting. Because
corrosion occurs via electrochemical reactions, electrochemical techniques are
ideal for the study of the corrosion processes. In electrochemical studies, a metal sample with a surface area of
a few square centimeters is used to model the metal in a corroding system. The metal sample is immersed in a solution
typical of the metal's environment in the system being studied. Additional electrodes are immersed in the
solution, and all the electrodes are connected to a device called a potentiostat. A potentiostat allows you
to change the potential of the metal sample in a controlled manner and measure
the current the flows as a function of potential. Both
controlled potential (potentiostatic) and controlled current (galvanostatic)
polarization is useful. When the
polarization is done potentiostatically, current is measured, and when it is
done galvanostatically, potential is measured.
This discussion will concentrate on controlled potential methods, which
are much more common than galvanostatic methods. With the exception of Open Circuit Potential vs. Time,
Electrochemical Noise, Galvanic Corrosion, and a few others, potentiostatic
mode is used to perturb the equilibrium corrosion process. When the potential of a metal sample in
solution is forced away from Eoc, it is referred to as polarizing the
sample. The response (current) of the
metal sample is measured as it is polarized.
The response is used to develop a model of the sample's corrosion
behavior. Suppose
we use the potentiostat to force the potential to an anodic region (towards
positive potentials from Eoc). In
Figure 1-1, we are moving towards the top of the graph. This will increase the rate of the anodic
reaction (corrosion) and decrease the rate of the cathodic reaction. Since the anodic and cathodic reactions are
no longer balanced, a net current will flow from the electronic circuit into
the metal sample. The sign of this
current is positive by convention. Click here for a discussion of
electrochemical sign conventions. If we take
the potential far enough from Eoc, the current from the cathodic reaction will
be negligible, and the measured current will be a measure of the anodic
reaction alone. In Figure 1-1, notice
that the curves for the cell current and the anodic current lie on top of each
other at very positive potentials.
Conversely, at strongly negative potentials, cathodic current dominates
the cell current. In
certain cases as we vary the potential, we will first passivate the metal, then
cause pitting corrosion to occur. With
the astute use of a potentiostat, an experiment in which the current is
measured versus potential or time may allow us to determine Icorr at Ecorr, the
tendency for passivation to occur, or the potential range over which pitting
will occur. Because
of the range of corrosion phenomena that can be studied with electrochemistry,
the ability to measure very low corrosion rates, and the speed with which these
measurements can be conducted, an electrochemical corrosion measurement system
has become a standard item in the modern corrosion laboratory. Quantitative Corrosion TheoryIn the
previous section we pointed out that Icorr cannot be measured directly. In many cases, you can estimate it from
current versus voltage data. You can
measure a log current versus potential curve over a range of about one half
volt. The voltage scan is centered on
Eoc. You then fit the measured data to
a theoretical model of the corrosion process. The
model we will use for the corrosion process assumes that the rates of both the
anodic and cathodic processes are controlled by the kinetics of the electron
transfer reaction at the metal surface.
This is generally the case for corrosion reactions. An electrochemical reaction under kinetic
control obeys Equation 1-1, the Tafel equation.
I = I0 exp(
2.303(E-Eo)/b) Equation
1-1 In this
equation, The
Tafel equation describes the behavior of one isolated reaction. In a corrosion system, we have two opposing
reactions – anodic and cathodic. The
Tafel equations for both the anodic and cathodic reactions in a corrosion
system can be combined to generate the Butler-Volmer equation (Equation 1-2). where What
does Equation 1-2 predict about the current versus voltage curve? At
Ecorr, each exponential term equals
one. The cell current is therefore
zero, as you would expect. Near
Ecorr,
both exponential terms contribute to the overall current. Finally, as the potential is driven far from
Ecorr by the potentiostat, one exponential term predominates and the other term
can be ignored. When this occurs, a
plot of log current versus potential becomes a straight line. A log I
versus E plot is called a Tafel Plot.
The Tafel Plot in Figure 1-1 was generated directly from the Butler-Volmer equation. Notice the
linear sections of the cell current curve. In
practice, many corrosion systems are kinetically controlled and thus obey
Equation 1.2. A log current versus
potential curve that is linear on both sides of Ecorr is indicative of kinetic
control for the system being studied.
However, there can be complications, such as: 1. Concentration
polarization, where the rate of a
reaction is controlled by the rate at which reactants arrive at the metal
surface. Often cathodic reactions show
concentration polarization at higher currents, when diffusion of oxygen or
hydrogen ion is not fast enough to sustain the kinetically controlled
rate. 2. Oxide
formation, which may or may not lead to passivation, can alter the surface of
the sample being tested. The original
surface and the altered surface may have different values for the constants in
Equation 1-2. 3. Other
effects that alter the surface, such as preferential dissolution of one alloy
component, can also cause problems. 4. A mixed
control process where more than one cathodic, or anodic, reaction occurs
simultaneously may complicate the model.
An example of mixed control is the simultaneous reduction of oxygen and
hydrogen ion. 5. Finally,
potential drop as a result of cell current flowing through the resistance of
your cell solution causes errors in the kinetic model. This last effect, if it is not too severe,
may be correctable via IR compensation in the potentiostat.
For a good discussion on IR Compensation, see Gamry's
Application Note. In most
cases, complications like those listed above will cause non-linearities in the
Tafel plot. The results derived from a
Tafel Plot that does not have a well-defined linear region should be used with
caution. Classic
Tafel analysis is performed by extrapolating the linear portions of a log
current versus potential plot back to their intersection. See Figure 1-2 (this is Figure 1-1 reprinted
with annotations that demonstrate the analysis). The value of either the anodic or the cathodic current at the
intersection is Icorr. Unfortunately,
many real world corrosion systems do not provide a sufficient linear region to
permit accurate extrapolation. Most
modern corrosion test software, such as Gamry Instruments' DC105 DC Corrosion
Techniques software, performs a more sophisticated numerical fit to the Butler-Volmer equation. The measured
data is fit to Equation 1-2 by adjusting the values of Ecorr, Icorr, ba, and bc.
The curve fitting method has the advantage that it does not require a
fully developed linear portion of the curve. Figure 1-2
Polarization ResistanceEquation
1-2 can be further simplified by restricting the potential to be very near to
Ecorr. Close to Ecorr, the current
versus voltage curve approximates a straight line. The slope of this line has the units of resistance (ohms). The slope is, therefore, called the
Polarization Resistance, Rp. An Rp
value can be combined with an estimate of the Beta coefficients to yield an
estimate of the corrosion current. If we approximate the exponential terms in Equation 1-2 with
the first two terms of a power series expansion (ex = 1+x +x2/2...)
and simplify, we get one form of the Stern-Geary equation:
Icorr = (1/ Rp) (babc / (2.303 (ba + bc) ) Equation 1-3 In a Polarization Resistance experiment, you record a current versus voltage curve as the cell voltage is swept over a small range of potential that is very near to Eoc (generally ± 10 mV). A numerical fit of the curve yields a value for the Polarization Resistance, Rp. Polarization Resistance data does not provide any information about the values for the Beta coefficients. Therefore, to use Equation 1-3 you must provide Beta values. These can be obtained from a Tafel Plot or estimated from your experience with the system you are testing. Calculation of Corrosion Rate from Corrosion CurrentThe
numerical result obtained by fitting corrosion data to a model is generally the
corrosion current. We are interested in
corrosion rates in the more useful units of rate of penetration, such as
millimeters per year. How is corrosion
current used to generate a corrosion rate?
Assume an electrolytic dissolution reaction involving a chemical
species, S:
S -->
Sn+ + ne- You can
relate current flow to mass via Faraday's Law.
Q
= n F M
Equation
1-4 where Q is the charge in coulombs resulting
from the reaction of species S A more
useful form of Equation 1-4 requires the concept of equivalent weight. The equivalent weight
(EW) is the mass of
species S that will react with one Faraday of charge. For an atomic species, EW = AW/n (where AW is the atomic weight
of the species). Recalling
that M = W/AW and substituting into Equation 1-4 we get:
W = EW
Q / F
Equation
1-5 where
W is the mass of species S that has reacted. In cases
where the corrosion occurs uniformly across a metal surface, the corrosion rate
can be calculated in units of distance per year. Be careful - this
calculation is only valid for uniform corrosion, it dramatically underestimates
the problem when localized corrosion occurs!
For a
complex alloy that undergoes uniform dissolution, the equivalent weight is a
weighted average of the equivalent weights of the alloy components. Mole fraction, not mass fraction, is used as
the weighting factor. If the
dissolution is not uniform, you may have to measure the corrosion products to
calculate EW. Conversion
from a weight loss to a corrosion rate (CR) is straightforward. We need to know the density, d, and the
sample area, A. Charge is given by Q =
I T, where T is the time in seconds and I is a current. We can substitute in the value of Faraday's
constant. Modifying Equation 1-5:
CR =
Icorr K EW / d A
Equation
1-6 CR
The corrosion rate. Its units are given by the choice of K (see
Table 1-1 below) Table
1-1 shows the value of K used in Equation 1-6 for corrosion rates in the units
of your choice. Table 1-1
See ASTM
Standard G 102, Standard Practice for Calculation of Corrosion Rates and
Related Information from Electrochemical Measurements, for further
information. The standard can be
obtained from ASTM, 1916 Race St., Philadelphia, Pennsylvania 19013-1187 (USA). IR CompensationWhen you pass current between two electrodes in a conductive solution, there will always be regions of different potentials in the solution. Much of the overall change in potential occurs very close to the surface of the electrodes. Here the potential gradients are largely due to ionic concentration gradients set up near the metal surfaces. Also, there is always a potential difference (a potential drop) due to current flow through the resistance in the bulk of the solution. In an
electrochemical experiment, the potential that you wish to control or measure
is the potential of a metal specimen (called the Working Electrode) versus a
Reference Electrode. You are normally
not interested in the potential drops due to solution resistances. The
Gamry Instruments Series G Potentiostat, like all modern electrochemical
instruments, is a three-electrode potentiostat. It measures and controls the potential difference between a
non-current carrying Reference Electrode and one of the two current carrying
electrodes (the Working Electrode). The
potential drop near the other current carrying electrode (the Counter
Electrode) does not matter when a three-electrode potentiostat is used.
Careful placement of the Reference Electrode can compensate
for some of the IR drop resulting from the cell current, I, flowing through the
solution resistance, R. You can think
of the Reference Electrode as sampling the potential somewhere along the
solution resistance. The closer it is
to the Working Electrode, the closer you are to measuring a potential free from
IR errors. However, complete IR
compensation cannot be achieved in practice through placement of the reference
electrode, because of the finite physical size of the electrode. The portion of the cell resistance that
remains after placing the Reference Electrode is called the uncompensated
resistance, Ru. Gamry Potentiostats use current interrupt IR compensation to
dynamically correct uncompensated resistance errors. In the current interrupt technique, the cell current is
periodically turned off for a very short time.
With no current flowing through the solution resistance, its IR drop
disappears instantly. The potential
drop at the electrode surface remains constant on a rapid time scale. The difference in potential with the current
flowing and without is a measure of the uncompensated IR drop. The
potentiostat makes a current interrupt measurement immediately after each data
point is acquired. It actually takes
three potential readings: E1 before the current is turned off, and E2 and E3
while it is off. See Figure 1-3. Normally, the latter two are used to
extrapolate the potential difference, delta E, back to the exact moment when
the current was interrupted. The timing
of the interrupt depends on the cell current.
The interrupt time is 40 microseconds on the higher current ranges. On lower current ranges, the interrupt lasts
longer. Figure 1-3
In
controlled potential modes, the applied potential can be dynamically corrected
for the measured IR error in one of several ways. In the simplest of these, the IR error from the previous point is
applied as a correction to the applied potential. For example, if an IR free potential of 1 volt is desired, and
the measured IR error is 0.2 volts, the potentiostat will apply 1.2 volts. The correction is always one point behind,
as the IR error from one point is applied to correct the applied potential for
the next point. In addition to this normal
mode, the Gamry PC4 offers more complex feedback modes in which the two points
on the decay curve are averaged. By
default in the controlled potential modes, the potential error measured via
current interrupt is used to correct the applied potential. In the controlled current modes, no
correction is required. If IR
compensation is selected, the measured IR error is subtracted from the measured
potential. All reported potentials are
therefore free from IR error. For a
detailed theoretical discussion of Uncompensated Resistance, see Keith B.
Oldham et al, Analytical Chemistry, 72, 3972 & 3981(2000). Current and Voltage ConventionsCurrent
polarities in electrochemical measurements can be inconsistent. A current value of -1.2 mA can mean
different things to workers in different areas of electrochemistry or in
different countries or even to different potentiostats. To an analytical electrochemist it
represents 1.2 mA of anodic current. To
a corrosion scientist it represents 1.2 mA of cathodic current. A Gamry Potentiostat in default mode follows
the corrosion convention for current in which positive currents are anodic and
negative currents are cathodic. For the
convenience of our users around the world, Gamry Potentiostats can provide the
current polarity in your preferred polarity with a simple software command. The
polarity of the potential can also be a source of confusion. In electrochemical corrosion measurement,
the equilibrium potential assumed by the metal in the absence of electrical connections
to the metal is called the Open Circuit Potential, Eoc. We have reserved the term Corrosion
Potential, Ecorr, for the potential in an electrochemical experiment at which
no current flows, as determined by a numerical fit of current versus potential
data. In an ideal case, the values for
Eoc and Ecorr will be identical. One
reason the two voltages may differ is that changes have occurred to the
electrode surface during the scan. With
most modern potentiostats, all potentials are specified or reported as the
potential of the working electrode with respect to either the reference
electrode or the open circuit potential.
The former is always labeled as "vs. Eref" and the later is
labeled as "vs. Eoc". The
equations used to convert from one form of potential to the other are:
E vs. Eoc = ( E vs.
Eref) - Eoc Regardless
off whether potentials are versus Eref or versus Eoc, one sign convention is
used. The more positive a potential,
the more anodic it is. More anodic
potentials accelerate oxidation at the Working Electrode. Conversely, a negative potential accelerates
reduction at the Working Electrode. References on Corrosion Theory and Electrochemical Corrosion TestsDC
Electrochemical Test Methods, N.G. Thompson and J.H. Payer, National
Association of Corrosion Engineers, 1440 South Creek Drive, Houston, TX
77084-4906. Phone: 281-228-6200. Fax: 281-228-6300. ISBN: 1-877914-63-0. Principles and Prevention of Corrosion, Denny A. Jones, Prentice-Hall, Upper Saddle River, NJ 07458 (1996). ISBN 0-13-359993-0. Polarization Resistance Method for Determination of Instantaneous Corrosion Rates, J. R. Scully, Corrosion, 56, 199 (2000) Several electrochemical techniques have been approved by the ASTM (American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428. Phone: 610-832-9500. Fax: 610-832-9555, www.astm.org). They may be found in Volume 3.02 of the ASTM Standards: G 5: Potentiostatic and Potentiodynamic Anodic Polarization Measurements G 59: Polarization Resistance Measurements G 61: Cyclic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, and Cobalt-Based Alloys G 100: Cyclic Galvanostaircase Polarization G 106: Verification of Algorithm and Equipment for Electrochemical Impedance Measurements G 108: Electrochemical Potentiokinetic Reactivation (EPR) for Detecting Sensitization Electrochemical Techniques in Corrosion Engineering, 1986, National Association of Corrosion Engineers, 1440 South Creek Drive, Houston, TX 77084-4906. Phone: 281-228-6200. Fax: 281-228-6300. Corrosion Testing and Evaluation, STP 1000, Edited by R. Baboian and S. W. Dean, 1991. American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428. Phone: 610-832-9500. Fax: 610-832-9555. ISBN 0-8031-1406-0. Electrochemical Corrosion Testing, STP 727, Edited by F. Mansfeld and U. Bertocci, 1979. American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428. Phone: 610-832-9500. Fax: 610-832-9555. Corrosion and Corrosion Control, 3rd. Ed., Herbert H. Uhlig, John Wiley and Sons, New York, 1985.
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