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Theory and Applications of the Measurement ofTheory and Applications of the Measurement of Corrosion Rate with Polarization Resistance

Dr. Pete PetersonIvium Technologieswww.ivium.com

Leaders in Corrosion Control Technology

Agenda1. Introduction

2. Electrochemical Fundamentals

3. Electrochemical Corrosion Theory

4. The Polarization Resistance Experiment


5. Practical Comments on the Polarization Resistance Technique

Goals of This Presentation

• Explain some electrochemical fundamentals and electrochemical corrosion measurements in practical terms.

• In doing so, I hope to alleviate some of the fear of electrochemistry, which has the reputation of being a difficult science. It’s a very powerful tool for the corrosion scientist and it’s relatively easy to put it to work in most corrosion labs.


Why Are Electrochemical Techniques Used for Corrosion Measurement?

• Corrosion is an electrochemical, or “redox”*, process involving “electron transfer”.Fe Fe2+ + 2e–

*redox = reduction‐oxidation

• A broad range of electrochemical techniques have been developed specifically for corrosion measurement.

• Electrochemical techniques are rapid.

• Electrochemical techniques can measure very low corrosion rates.

• Can be used in the lab or in the field.


• In 2011, electrochemical techniques are well‐accepted by most of the corrosion community. For example, see “Measuring Water Well Corrosion Rate In Situ Using LPR Technique” in MATERIALS PERFORMANCE (February 2011).

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Electrochemical Techniques are Rapid!

• Corrosion is an inherently slow process. A typical corrosion rate is 10 milli‐inches per year (mpy) or 0 254 millimeters per year (mmpy) or 254 microns per yearper year (mpy) or 0.254 millimeters per year (mmpy) or 254 microns per year.

• The most realistic corrosion tests are weight loss measurements after exposure. However, they are very slow (weeks, months, or years).

• Why are they fast? Because the potentiostat (the electrochemical instrument) “polarizes” the sample to accelerate the corrosion process and make the measurement in minutes or hours.


Electrochemical Techniques Are Sensitive!

• Electrochemical techniques can be used to measure very low corrosion rates.

• A sample with a low corrosion rate will exhibit a low current during the electrochemical experiment. Corrosion scientists often use “rate” and “current” interchangeably.

• Electrochemical instruments (the potentiostat) can be designed to measure low currents very accurately. A 1 cm2 iron sample with a corrosion rate of 1 mpy (milli‐inch per year) will exhibit a current in the range of 2‐5 μA, which

b d t l b d t ti t t


can be measured very accurately by a modern potentiostat.

• 4 milli‐inches corresponds roughly to the diameter of a black human hair.

Electrochemistry and the Corrosion Scientist

• Corrosion measurement is THE best example of applied electrochemistry.

• Corrosion scientists have developed a number of electrochemical techniques for corrosion studies.

• DC Techniques such as Polarization Resistance, Tafel Plots, and Cyclic Polarization are used strictly for corrosion measurement.

• Electrochemical Impedance Spectroscopy (AC Impedance) is a powerful and popular tool for corrosion scientists.

• Electrochemical Noise is used exclusively for corrosion studies.


• There are a number of societies and publications to serve the corrosion community, e.g., NACE International, The Electrochemical Society, International Society of Electrochemistry, the International Corrosion Congress, LatinCorr, and the European Federation of Corrosion (EUROCORR).

Before we continue, a brief word about the fundamentals of electrochemistry.


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Electrochemistry Is One of the Fundamental Sciences and Is Practiced in Many Laboratories Worldwide

• Major Applications of Electrochemistry– Corrosion Measurement– Corrosion Measurement– Battery Development– Physical Electrochemistry, aka, “Electrochemical Research”

• If you’re studying an electron‐transfer reaction, you must use a potentiostat.

S l h d l h i l i h d h


• Several thousand electrochemical instruments are purchased each year. Tens of thousands electrochemical instruments are in daily use around the world.

A Review of Electrochemistry

Electrochemistry – The study of chemical reactions accompanied by the exchange of electrons. Electron‐transfer is always a factor in electrochemistry Chemistry is sometimes a factor inelectrochemistry. Chemistry is sometimes a factor in electrochemistry.

O = Oxidized Species Red = Reduced SpeciesOxidation: Loss of electrons Fe Fe2+ + 2 e‐

Reduction: Gain of electrons 2H+ + 2e‐ H2“Oxidation” to the electrochemist is “corrosion” to the corrosion scientist.To help you remember which is which…LEO says GER!)


General Echem Reaction: Ox + ne‐ RedThis equation is commonly used in the literature when discussing electrochemistry. The

Oxidized species accepts an electron to form the Reduced species.

Half‐Reactions in Corrosion

Oxidation: The Metal Being TestedFe Fe2+ + 2e–Fe Fe + 2e

Reduction: Usually Some Solution SpeciesHydrogen ion: 2H+ + 2e– H2

Water: 2H2O + 2e– H2 + 2OH–

Oxygen: O2 + 2H2O + 2e– 4OH– (neutral/alkaline)O2+ 4H+ + 4e– H2 + 2OH– (acid)


O2 4H 4e H2 2OH (acid)

• Oxidation is always accompanied by Reduction.

What is Potential?

Potential or Voltage (E, sometimes V):

– Unit: Volt (milliVolt, microVolt)

– The Potential is the driving force for the redox reaction.

– The potential is related to the thermodynamics of the system:ΔG = ‐n F ΔE (negative ΔG (Gibbs Free Energy) is spontaneous)

– Potential is always measured versus a Reference Electrode.

– A positive voltage is oxidative and a negative voltage is reductive.

0 Volts is not nothing! Redox reactions happen at 0 volts that do not


– 0 Volts is not nothing! Redox reactions happen at 0 volts that do not happen at +1 volt…or ‐1 volt.

Remember: There is no correlation between the thermodynamics of the chemical system and the kinetics (rate) of the reaction.

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What is Current?Current (i or I):

– Unit: Ampere (mA, µA, nA, pA, fA)

Current measures the rate of the reaction (electrons per second)– Current measures the rate of the reaction (electrons per second).

– Electron flow is the result of a redox reaction.

– Zero current is nothing, i.e., if the current is zero, no redox reactions are occurring (that’s not quite true in corrosion!).

– Anodic (oxidation) and cathodic (reduction) currents have different polarity (signs).


– Current may be expressed as current or current density (current/area).

Potential and Current Can Be Related to a Flowing Water Circuitg

Water Pressure = Electron Pressure = Potential

Water Flow Rate = Electron Flow = Current


Range of Potential and Current that is Encountered in Corrosion Experiments

• 95% of electrochemical corrosion experiments take place within ± 2 volts vs. SCE.

• Current can vary from hundreds of milliamps to femtoamps (10‐15 amps). That’s 12‐13 orders of magnitude! Modern potentiostats are capable of auto‐ranging the current over 6‐12 decades of current, which makes your life relatively easy.


• The Current Ranges in a modern potentiostat are almost transparent to the user, but they are very important!

Electrochemical Experiments

• In principal, electrochemistry is simple…there are only three i bl t ti l (E) t (i) d tivariables: potential (E), current (i), and time.

• We can use our electrochemical instrument (Potentiostat/Galvanostat) to control Potential and measure Current (Pstat), or control Current and measure Potential (Gstat).


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Electrochemical Techniques

“Active Experiments”: Apply an excitation, measure a response. Make something happen!

• Potentiostatic: Apply & Control E, Measure i, Plot E vs. i or i vs. t• Galvanostatic: Apply & Control i, Measure E, Plot i vs. E or E vs. t

“Passive Experiments”: Observe the experiment electrochemically.• Potentiometric: Measure E at i=0 (for example, a pH Meter) • Zero Resistance Ammeter (ZRA): Measure i between two connected electrodes.

Examples: galvanic corrosion and electrochemical noise.

A i i i f l b i k h i h l


• Active experiments give faster results, but risk changing the sample.

• Polarization Resistance is an active experiment.

The Nernst Equation? Not so much.

• The Nernst Equation describes the concentration‐potential relationship of an electrochemical system at equilibrium.

• The Nernst Equation is not a particularly useful tool for the electrochemical measurement of corrosion rates.


• So…fuhgeddabowtit!

Controlled Potential (Potentiostatic) Experiments are Most Commonly Used in the Corrosion Lab

• In most electrochemical corrosion experiments, the potential is controlled. Because of the relationship between the potential and the thermodynamics of the system, controlled potential experiments are more informative than controlled current experimentsexperiments.

• The (three‐electrode) Potentiostat is the electronic instrument that controls the potential between the Working Electrode (your sample!) and the Reference Electrodewhile it measures the current between the Working Electrode and the Counter Electrode.

• Why three electrodes? A three‐electrode Potentiostat allows potential control and current measurement at the Working Electrode with no “interference” from other electrochemical events in the cell.


• To perform an electrochemical measurement, we change the potential in some systematic way and measure the current response. The applied potential may “force” a redox reaction to occur and the current will be indicative of the rate (kinetics) of the reaction.

An Electrochemical Corrosion Measurement System

• A Corrosion Measurement System consists of a potentiostat, software, a computer, and a cell.

• Analog (non‐computerized) systems are not recommended for corrosion experiments.

• The corroding sample is placed in a Electrochemical Cell and connected to the Potentiostat.


• Prices range from $5000 to $40,000. In general, prices have come WAY down since 1980!

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The Potentiostat

• The Potentiostat is the instrument that controls potential or current, and measures current or potential, respectively.

• A Potentiostat is a sophisticated electronic device.

• The performance (accuracy, precision, stability, and bandwidth) of a potentiostat are affected by the electrical characteristics of the Working Electrode, which are unknown to the manufacturer. That can cause problems.


p

• Working Electrode: Corrosion sample being studied. The area must be known.

• Reference Electrode: Saturated Calomel (SCE) or Silver‐Silver Chloride (Ag/AgCl). Others are available (Cu/CuSO4). A Corrosion Engineer will sometimes use a “pseudo‐reference electrode”.

The 3‐Electrode Electrochemical Cell

• Counter Electrode: Should be conductive and inert (Graphite or Platinum).

• Note: Notice that we do not use the terms “anode” or “cathode”. Why?

• Solution may be stirred and deaerated to remove O2. The Working Electrode may rotate!


• Temperature control is encouraged. Rates of chemical reactions are verytemperature dependent.

• There are many different types of cells, e.g., autoclaves, rotating cylinder, measurements in the field, and others.

A Typical Electrochemical Cell(Courtesy ASTM G5)


Controlling the Temperature during a Corrosion Rate Measurement is Important!

• Rule of Thumb: Increasing the temperature 10° C doubles the rate of a chemical reaction.

• Mild Steel in 1 M HCl + 2x10‐2 M BTA1

– Rp@ 40° C = 250 Ω‐cm2

– Rp@ 50° C = 110 Ω‐cm2

– Rp@ 60° C = 49.6 Ω‐cm2

1 BTA = Benzotriazole Rp = Polarization Resistance


BTA = Benzotriazole Rp = Polarization Resistance

A. Popova, “Temperature Effect on Mild Steel Corrosion in Acid Media in Presence of Azoles”, Corrosion Science, 49, 2144 (2007).

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I. The Open Circuit Potential

• The Open Circuit Potential, EOC, is the potential difference between the metal Working Electrode and the Reference Electrode when immersed in the electrolyte.

• EOC is where corrosion occurs in service.

• EOC is the starting point for virtually all electrochemical corrosion experiments.

• A stable EOC is taken to indicate that the corroding system has reach a “steady state” and the experiment may begin. This may require minutes to days. No measurable current is flowing at Eoc.


• Important (And maybe a little confusing)! If we apply a potential equal to Eocwith our potentiostat, the current will be zero.

.

II. The Open Circuit Potential

• An applied voltage that is positive of EOC will accelerate an oxidation (corrosion) reaction. An applied voltage that is negative of EOC will accelerate a reduction

i hi k lif h l k h ’ ireaction. This makes life somewhat easy – you always know what’s going on at the Working Electrode.

• The terms “EOC” and “Ecorr” (corrosion potential) are often used interchangeably.

• The value of the EOC is not particularly useful as a predictive tool.

• E is amixed potentialwhose value is determined by the potentials of the two or


• EOC is a mixed potentialwhose value is determined by the potentials of the two or more half‐reactions of the electrochemical system, not by the potentiostat!

Controlled Potential (Potentiostatic) Experiments

• The Potentiostat controls the potential between theWorking Electrode and• The Potentiostat controls the potential between the Working Electrode and the Reference Electrodewhile it measures the current between the Working Electrode and the Counter Electrode.

• To run an experiment, change the potential in some systematic way and measure the current response. The applied potential will allow a redox reaction to occur.


Applying the Voltage in a Controlled Potential Experiment

Computerized potentiostats generate a potential waveform that is described by the “Scan Rate”:

Scan Rate (mV/sec) = Step Height/Step Time( / ) p g / p

The Scan Rate is an important experimental parameter in the electrochemical corrosion experiment and, for corrosion experiments, it’s always slow, e.g.,

0.166 mV/sec!

ial,

V


Time

Pot

enti ΔE, mV

Δt, sec

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We now return you to your normally scheduled presentation.


Electrochemical Corrosion Applications

• Quantitative Corrosion Rate Measurements (for uniform corrosion only)

• Qualitative Indication of Passivation Tendencies• Qualitative Indication of Passivation Tendencies

• Qualitative Pitting and Crevice Corrosion Studies

• Galvanic Corrosion

• Intergranular Corrosion (Sensitization)


• Stress Corrosion Cracking (the sample is under load during the test)

• Evaluation of Organic Coatings with Impedance Spectroscopy

Quantitative Corrosion Rate Measurements

• Polarization Resistance and Tafel Plots are experiments designed to measure the rate of uniform corrosion in units of penetration (mmpy or mpy).

• Designed to measure the corrosion current, ICORR, from which we can calculate the corrosion rate.

• Unlike weight loss, electrochemical techniques provide a “snapshot” of the corrosion rate.

• Should be performed after when the system has reached “steady state”, indicated by a stable EOC.

• Note for the Purists: “Linear Polarization Resistance (LPR)” is not a correct term


• Note for the Purists: Linear Polarization Resistance (LPR) is not a correct term, but we use it anyway. The correct term is “Polarization Resistance”. (See CORROSION, November 2005.)

Corrosion Current, Icorr

• At EOC, imeas = 0, but we know that corrosion is occurring.Fe Fe2+ + 2 e‐

2H+ + 2e‐ H2

Fe + 2H+ Fe2+ + H2

• We need to somehow measure the current due to the anodic or cathodic half‐reaction. This current is called the Corrosion Current, ICORR.

• The goal of the Polarization Resistance experiment is to determine the Corrosion Current.


• From the Corrosion Current, we can calculate corrosion rate.

• Don’t forget the electrode area! iCORR is usually current (A) and ICORR is usually current density (A/cm2).

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Electrochemical corrosion measurements are not empirical…they are based on good solid science developed over many years.based on good solid science developed over many years.

So we’ll touch very briefly on the theoretical aspects of electrochemical corrosion measurement.


Mixed Potential Theory

The principal of charge conservation requires that there must be a least one reduction and one oxidation in an electrochemical reaction.

• 2H+ + 2e‐ H2

Z Z 2+ 2• Zn Zn2+ + 2e‐

• The anodic current equals the cathodic current at icorr.

• Both reactions must occur on the same surface, so their potentials must shift to a common value, which is Ecorr (same as EOC).


• This is an example of a “mixed potential system” and Eoc is called a “mixed potential”.

“Principles and Prevention of Corrosion”, Jones

Butler‐Volmer Equation

The Butler‐Volmer Equation is a general electrochemical equation that describes the relationship between the potential and the current (kinetics) in a mixed potential system.

I = Ia + Ic = ICORR(e(2.3(E‐Eoc)/a) – e(‐2.3(E‐Eoc)/c))

Where: I = cell current (A)ICORR = corrosion current (A) E = applied potential (V)

Rate of anodic reaction Rate of cathodic reaction


app ed po e a ( )Eoc = open‐circuit potential (V)a = anodic Tafel constant (V/decade)c = cathodic Tafel constant (V/decade)

Graphical Representation of the Butler‐Volmer Relationship between Potential and Current in a Mixed Potential System

M + 2H+ M2+ + H2

An experiment like this is called a “Tafel Plot” and is relatively common in today’s corrosion laboratory.

Experimental data from the corrosion measurement system.


J. Scully & R. Kelly, ASM Handbook, Volume 13A, 2003.

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Stern‐Geary Equation and Polarization Resistance

• The purpose of the Polarization Resistance experiment is to determine the Polarization Resistance (Rp).

• The Stern‐Geary Equation describes the relationship between the Polarization Resistance (Rp) and the Corrosion Current (iCORR).

• RP = E/i = ßaßc/2.3 iCORR (ßa + ßc)

• IMPORTANT: The Stern‐Geary Equation is derived from the Butler‐Volmer Equation via a series expansion in which ΔE /βa,c is assumed to be less than 0.1.


• Therefore, in the Polarization Resistance technique, the applied potential should be within ±5‐10 mV of Eoc.

• The plot of potential vs. current is approximately linear in this region.

Butler‐Volmer Equation

The Butler‐Volmer Equation only holds if:

• The rate is charge‐transfer controlled (aka, activation‐controlled, electron‐transfer ll d) l fcontrolled). Complicating factors are:

– Passivity– Diffusion‐controlled (concentration polarization)– Adsorption

• Only one reduction and one oxidation reaction are occurring.– Alloys can be complicated

• If the Butler Volmer equation doesn’t hold then the electrochemical response will


• If the Butler‐Volmer equation doesn t hold, then the electrochemical response will not be “classic” and must be interpreted in terms of the chemistry of the system.

That concludes the theoretical portion of this presentation…


The Polarization Resistance Experiment


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Experimental Procedure for Polarization Resistance

1. Measure Eoc and allow to stabilize.

2. Apply initial E that is 10 mV negative of Eoc . Wait 60 seconds to allow the system to stabilize.stabilize.

3. Scan at a slow scan rate (~0.166 mV/s) to a final E that is 10 mV positive of Eoc .

4. Measure current, plot E (Y‐axis) versus I (X‐axis)

5. Measure slope, which has units of resistance (E/i = R).

6 Convert R to i using the Stern‐Geary Equation


6. Convert Rp to icorr using the Stern‐Geary Equation.

7. Convert icorr to Corrosion Rate.

Result of the Polarization Resistance Experiment(Courtesy ASTM G3)

Experimental Data

Slope of the Tangent to the


Slope of the Tangent to the Data at Zero Current

Leaders in Corrosion Control Technology Leaders in Corrosion Control Technology

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Experimental Setup for Polarization Resistance is Relatively Easy with Modern Software

• Define the potential region to scan– This is tricky…the scan region depends upon the value of Eoc.– In this case, the computer is your friend!

f h l h f ( ) (– Define the potential region with respect to Eoc, e.g., scan from (Eoc ‐10 mV) to (Eoc + 10 mV). Problem solved.

• Define the scan rate– The Scan Rate is always slow. ASTM recommends 0.167 mV/sec.– The Scan Rate = ΔE/Δt. Keep ΔE “small” (1‐2 mV). Keep Δt “long” (1‐10 sec).

• Enter sample area, density, equivalent weight, and Tafel Constants.


• The Potentiostat and Software take care of everything else, including Eoc stabilization, setting the appropriate Current Range, and calculating the results.



Calculation of ICORR from RP

Stern‐Geary Equation

R = E/i = ß ß /2 3 I (ß + ß )RP = E/i = ßaßc/2.3 ICORR (ßa + ßc)

Icorr = ßaßc/ RP(ßa + ßc)

RP = Slope of the Polarization Resistance Plot in ohms or ohms‐cm2

iCORR = Corrosion Current, Amperes or Amperes/cm2.


ßa, ßc = Tafel Constants from a Tafel Curve, volts/decade of current.

Note: The area of the electrode must be taken into account somewhere!

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I. Calculation of Corrosion Rate from ICORR via Faraday’s Law

Q = nFm/A = it Faraday’s Law

Q = coulombs (A‐s/eq), n = number of electrons, F = the Faraday (96,487 coulombs/equivalent), m = mass (g), A = atomic weight, i = current (A), t = time (s).

Since Equivalent Weight (EW) = A/n,m = it(EW)/F


Corrosion Rate (g/s) = m/t = i(EW)/F

II. Calculation of Corrosion Rate from ICORR via Faraday’s Law

From the engineering standpoint, it is convenient to express Corrosion Rate in units of penetration, mpy (milli‐inches per year), mmpy (mm per year), or microns per year.microns per year.

Divide both sides of the equation Corrosion Rate (g/s) = m/t = i(EW)/F

by electrode area and density,Corrosion Rate (cm/s) = i(EW)/FdA

i/A = current density (I). After conversion of cm to inches or mm, and seconds to years


years,

Corrosion Rate (mpy) = 0.13 Icorr (EW)/dCorrosion Rate (mmpy) = 0.00327 Icorr(EW)/d


Precision of Electrochemical Corrosion Rate Measurements

• Corrosion is a messy business!‐Dr. Jerry Frankel

Error as a Function of Sample Replicates

The Ohio State University

• Corrosion is a surface process involving a large number of variables that are difficult to understand and/or control.

• A Relative Standard Deviation of 15% is Excellent!

• A minimum of 3 replicates is recommended f i t (“I Y


for corrosion measurements. (“Increase Your Confidence in Corrosion Test Data”, Steve Tait, MATERIALS PERFORMANCE, March 2001)

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Comments on the Precision of Electrochemical Corrosion Measurements

• Electrochemical instrumentation is very accurate (<0.5% error).

• The surface of a metal sample is nothomogeneous. In fact, it may be veryheterogeneous.

• When corrosion occurs, anodic and cathodic reactions are occurring simultaneously on the surface of the sample

• The anodic and cathodic sites may “turn on” or “turn off” and move around as the local environment changes

• Electrolyte impurities can also affect the results

• The corrosion environment is very


• The corrosion environment is verydynamic and the result is…poor precision

• Polishing the surface improves precision, but…does it reflect reality?

Corrosion is a messy business, but…it always involves electrochemistry

Sometimes electrochemical techniques work great. Sometimes they work…not so great.great.

Electrochemical corrosion measurement techniques always provide some information on the corrosion process.

To avoid misinterpretation, it’s important to be able to recognize non‐ideal behavior.

Important: Use as many electrochemical (or other) tools (LPR, Tafel Plots, EIS, weight


p y ( ) ( , , , gloss, etc.) as possible to characterize a corrosion process.

First, the Good News…Polarization Resistance Works Very Nicely for the Measurement of Corrosion Rates

• Very fast (< 5 minutes). Scanning 20 mV at 0.1 mV/sec requires only 200 dseconds.

• In general, Polarization Resistance Plots exhibit good linearity, so data interpretation is easy.

• Potential remains close to EOC, so it is essentially a non‐destructive technique and can be used for continuous monitoring for inhibitor


evaluation.

• Described in ASTM G 59 (Volume 3.02).

And Now, the Bad News…What Can Go Wrong with Polarization Resistance?

• You need the Tafel Constant(s) for the Stern‐Geary Equation.

• EOC may be unstable.

• Use of a Scan Rate that is too fast (Step Height too high or Sampling Period too short).

• Complications from alloys containing several electroactive metals


p y g(reduction or oxidation of more than one electroactive species).

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The Problem of Tafel Constants

For best results in Polarization Resistance, the Tafel Constants are needed in the Stern‐Geary equation.

• Because of non‐linearities in the Tafel Plot, it can be difficult to experimentally determine the Tafel Constants with a reasonable level of confidence.

• a is not expected to equal c…you must measure both.

• a varies from 0.06 to 0.12 V/decade. c varies from 0.06 to infinity (diffusion control). (Jones)

• Because of the form of the Stern‐Geary equation, ICORR is not hyper‐sensitive to the


Tafel values. Using 0.1 V/decade as a and c results in a maximum error of a factor of 2. (Jones)

• For “corrosion monitoring”, Tafel Constants are not required. Simply observe changes in Rp.

To Determine the Tafel Constants, Run a Tafel Plot

1. Measure EOC and allow to stabilize.

2. Apply initial E that is 250 mV negative of EOC.

3. Scan at a slow scan rate (0.166 mV/s) to a final E that is 250 mV positive of EOC.

4. Measure current, plot E (Y‐axis) versus log I (X‐axis).


The Perfect Tafel Plot

In the ideal Tafel Plot, the slope of the linear region is the Tafel Constant.

The extrapolated anodic and cathodic Tafel slopes intersect at the Corrosion Current.

The Tafel Plot is not symmetrical, i.e., the A di T f l C t t i t


Anodic Tafel Constant is not equal to the Cathodic Tafel Constant.

Sounds good.

Now let’s look at a real Tafel Plot!


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Measurement of Tafel Constants

• Insufficient linear region is a major problem.

• The cathodic Tafel Plot is likely to be reasonably linear.The cathodic Tafel Plot is likely to be reasonably linear.

• The anodic Tafel Plot is not likely to be reasonably linear.

• Remember, if all else fails, use 0.1 for the Tafel Constants and your results will be pretty good!

• If you’re just looking for a change in the corrosion rate, Tafel Constants are


not required. Simply observe changes in Rp.

• Note: A Tafel Plot can be used to measure Icorr and corrosion rate, but nonlinearity is also a problem for this application.

Eoc is Unstable

• A stable Open‐Circuit Potential indicates that the system hasA stable Open Circuit Potential indicates that the system has reached steady state.

• The experiment can only begin after the system has reached steady state.

h ’ h d l h


• Sorry…there’s nothing you can do to accelerate the stabilization of Eoc.

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If Eoc is Unstable, Using Galvanostatic Control Can Help*

• An unstable Eoc indicates that the system has not reached “steady state”. There is nothing you can do to accelerate the stabilization.

• Despite my drifting Eoc, I still need a Corrosion Rate! An LPR measurement with a d if i E ill fl h i l h S G l i hidrifting Eoc will cause a current to flow that violates the Stern‐Geary relationship.

• In “non‐stationary” systems, i.e., systems in which the Eoc drifts, galvanostatic control (control and scan the current while measuring the potential) can give more reliable results.

• Galvanostatic control is a little tricky. Some work is usually required to determine the appropriate value of the applied current. Keep the applied current low and do not allow the potential to exceed ±10 mV!


not allow the potential to exceed ±10 mV!

* F. Huet et al, “Polarization Resistance Measurements: Potentiostatically or Galvanostatically?”, CORROSION, 65, 136(2009).

Scan Rates in Polarization Resistance

• If the Scan Rate is too high, errors can result due to the capacitance of the Working Electrode. The corrosion rate is usually overestimated.

• The solution? Scan rates are always slow, e.g., 0.125 mV/sec.

• EIS can be used to measure the appropriate scan rate for any systemScan Rate = E f

• Since the voltage range is typically 10‐30 mV, the Polarization Resistance experiment is fast even at slow scan rates


experiment is fast even at slow scan rates.

Equivalent Weight of an Alloy

• The Equivalent Weight of an alloy is the reciprocal of the total number of equivalents of all alloying elements (Jones)

• EWalloy = 1/ Neq

• Neq = Σ fi/(ai/ni) = Σ (fin/a)f = mass fraction of element i, a = atomic weight, n = # of electrons

• Example: 304 SS is 19% Cr, 9.25% Ni, and 71.75% Fe. Equivalent weight =


25.12

• Question: do all elements in the alloy corrode at the same rate?

A E l f R l W ld A li ti f P l i tiAn Examples of a Real‐World Application of Polarization Resistance


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As With Any Experiment, There Can Be Measurement Problems

• In most cases, a measurement problem can be easily recognized from a visual examination of the data. Any sort of noise or flatlined data is a bad sign!

• If there’s a measurement problem, it’s either:– The instrument

– The cell/electrodes

– The chemistry


The chemistry

– The local electronic environment


Evaluation of Corrosion Inhibitors

• This is probably the single most popular application of electrochemical• This is probably the single most popular application of electrochemical corrosion measurements.

• Make Polarization Resistance measurements as a function of time. Dose the inhibitor and observe the change in corrosion rate.

• This measurement can be automated at a very reasonable cost.


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References for Electrochemical Corrosion Testing

• Principles and Prevention of Corrosion, Denny A. Jones, Prentice‐Hall, Upper Saddle River, NJ 07458 (1996). ISBN 0‐13‐359993‐0.An excellent textbook covering all types of corrosion and corrosion testing.

• DC Electrochemical Test Methods, N. Thompson and J. Payer, NACE International, 1998, ISBN 1‐877914‐63‐0.A very good tutorial that is highly recommended. Buy it at the NACE Bookstore while you’re in Houston.

• “Polarization Resistance Method for Determination of Instantaneous Corrosion Rates”, J. R. Scully, CORROSION, 56, 19‐218 (2000).An excellent review of Polarization Resistance.

• Methods for Determining Aqueous Corrosion Reaction Rates, J. Scully and R. Kelly, ASM Handbook, Volume 13A. See www.asminternational.org. Addresses many electrochemical and other methods to measure the rate of corrosion.

• Electrochemical Techniques in Corrosion Engineering, 1986, NACE International36 papers. Covers the basics of the various electrochemical techniques and a wide variety of papers on the application of these techniques Includes impedance spectroscopy


application of these techniques. Includes impedance spectroscopy. • Corrosion Testing and Evaluation, STP 1000, Edited by R. Baboian and S. W. Dean., ASTM, 1989, ISBN 0‐8031‐

1406‐0.30 papers covering electrochemical and non‐electrochemical corrosion testing.

Theory and Applications of the Measurement of Corrosion Rate with Polarization Resistance

Dr. Pete PetersonIvium Technologieswww.ivium.com


theory and applications of 1. introductionthe measurement ... · pdf file1 theory and...

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