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Theory and Applications of the Measurement of Corrosion Rate with Polarization Resistance Dr. Pete Peterson Gamry Inst

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    1. Theory and Applications of the Measurement of Corrosion Rate with Polarization Resistance Dr. Pete Peterson Gamry Instruments www.gamry.com

    2. Agenda Introduction Some Electrochemical Fundamentals Some Electrochemical Corrosion Theory The Polarization Resistance Experiment What Can Go Wrong in Polarization Resistance? Some Examples of Polarization Resistance Applications

    3. Assumptions You have a serious interest in corrosion measurement. You would like to know more about electrochemical corrosion measurement. You are skeptical. Goal Explain electrochemical corrosion measurement in simple terms and demystify the jargon that electrochemists have carefully developed over the years to create the impression that these techniques are very, very difficult. In my opinion, electrochemistry is very poorly taught in US universities, so it would not be unusual if YOU had a negative opinion of electrochemistry.In my opinion, electrochemistry is very poorly taught in US universities, so it would not be unusual if YOU had a negative opinion of electrochemistry.

    4. Two Different Views of Electrochemical Corrosion Measurements Electrochemical corrosion measurements are easy because most of the techniques are standardized and are centered about Eoc. Electrochemical corrosion measurements are difficult because the Working Electrode is actively reacting!

    5. Why Are Electrochemical Techniques Used for Corrosion Measurement? Corrosion is an electrochemical (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 fast. Electrochemical techniques are sensitive. Can be used in the lab or in the field. In 2008, electrochemical techniques are well-accepted by the corrosion community. Wellmost of the corrosion community!

    6. Electrochemical Techniques are Fast! 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 year. The best corrosion tests are weight loss measurements after exposure. However, they are very slow (weeks, months, or years). Why are they fast? Because electrochemical instruments polarize the sample to accelerate the corrosion process and make the measurement in minutes or hours.

    7. Electrochemical Techniques Are Sensitive! Electrochemical techniques can 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 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 A, which can be measured very accurately by a modern potentiostat. 4 milli-inches corresponds roughly to the diameter of a human hair.

    8. 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). Short Courses in Electrochemical Corrosion Measurement are taught each summer at Pennsylvania State University and North Dakota State University.

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

    10. 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 in electrochemistry. O = Oxidized Species Red = Reduced Species Oxidation: Loss of electrons Fe Fe2+ + 2 e- Reduction: Gain of electrons 2H+ + 2e- H2 Ox + ne- Red The Oxidized species accepts an electron to form the Reduced species. Remember that LEO says GER!

    11. Redox Reactions (Half-Reactions) in Corrosion Oxidation: The Metal Being Tested Fe Fe2+ + 2e Reduction: Usually Some Solution Species Hydrogen ion: 2H+ + 2e H2 Water: 2H2O + 2e H2 + 2OH Oxygen: O2 + 2H2O + 2e 4OH (neutral/alkaline) O2 + 4H+ + 4e H2 + 2OH (acid) Oxidation is always accompanied by Reduction in both the Real World and the electrochemical experiment.

    12. What is Potential? Potential or Voltage (E, sometimes V): Unit: Volt The Potential is the driving force for the redox reaction. The potential is related to the thermodynamics of the system: ?G = -n F ?E (negative ?G 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 happen at +1 volt. Remember: There is no correlation between the thermodynamics of the chemical system and the kinetics (rate) of the reaction.

    13. What is Current? Current (i): Unit: Ampere Electron flow is the result of a redox reaction. Current measures the rate of the reaction (electrons per second). Zero current is nothing, i.e., if the current is zero, no redox reactions are occurring (thats not quite true in corrosion!). Anodic (oxidation) and cathodic (reduction) currents have different polarity (signs). Current may be expressed as current or current density. Dont worry about which way the current flows.

    14. A Simple Description of Potential and Current as a Flowing Water Circuit

    15. 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). Thats 12-13 orders of magnitude! Modern potentiostats are capable of auto-ranging the current over 7-11 decades of current, which makes your life relatively easy.

    16. Electrochemical Techniques Electrochemistry is simple in principalthere are only three variables: potential (E), current (i), and time. Active Experiments: Apply an excitation, measure a response. Make something happen! Potentiostatic: Control E, Measure i, Plot E vs. i or i vs. t Galvanostatic: 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. Active experiments give faster results, but risk changing the sample. Polarization Resistance is an active experiment.

    17. 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 experiments. The (three-electrode) Potentiostat is the electronic instrument that controls the potential between the Working Electrode (your sample!) and the Reference Electrode while 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 will allow a redox reaction to occur and the current will be indicative of the rate (kinetics) of the reaction.

    18. 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.

    19. The Potentiostat The Potentiostat is the electronic instrument that controls potential or current, and measures current or potential, respectively. A Potentiostat is complicated! Build it yourself at your peril! 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.

    20. Working Electrode: Corrosion sample being studied. Reference Electrode: Saturated Calomel (SCE) or Silver-Silver Chloride (Ag/AgCl). Others are available (Cu/CuSO4). 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. Temperature control is encouraged! Rates of chemical reactions are very temperature dependent! The 3-Electrode Electrochemical Cell

    21. An Example of an Electrochemical Cell References suggest 1cm^2 test area per 2.5um (0.0001 inch) coating thicknessReferences suggest 1cm^2 test area per 2.5um (0.0001 inch) coating thickness

    22. 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 O-cm2 Rp@ 50 C = 110 O-cm2 Rp@ 60 C = 49.6 O-cm2 1 BTA = Benzotriazole A. Popova, Temperature Effect on Mild Steel Corrosion in Acid Media in Presence of Azoles, Corrosion Science, 49, 2144 (2007).

    23. Controlling the Temperature in the Cell Reference Electrode Working Electrode Counter Electrode Water Jacket for temperature Control with a Water Bath

    24. 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 a mixed potential whose value is determined by the potentials of the two or more half-reactions of the electrochemical system, not by the potentiostat! EOC is where corrosion occurs in service. Its very, very, very important! 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. .

    25. II. The Open Circuit Potential No measurable current is flowing at Eoc. 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 reaction. This makes life somewhat easy. The terms EOC and Ecorr (corrosion potential) are often used interchangeably. The value of the EOC is not particularly useful as a predictive tool. The chemistry at Eoc is at steady state, but no equilibrium. Equilibrium means that nothing is changing and that is not the case.The chemistry at Eoc is at steady state, but no equilibrium. Equilibrium means that nothing is changing and that is not the case.

    26. Controlled Potential (Potentiostatic) Experiments The Potentiostat controls the potential between the Working Electrode and the Reference Electrode while 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.

    27. Applying the Voltage in a Controlled Potential Experiment

    28. We now return you to your normally scheduled presentation.

    29. Electrochemical Corrosion Applications Quantitative Corrosion Rate Measurements (for uniform corrosion only) 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 EIS

    30. 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 when the system has reached steady state, indicated by a stable EOC. Note: Linear Polarization Resistance (LPR) is not a correct term, but we use it anyway. The correct term is Polarization Resistance. (See CORROSION, November 2005.)

    31. 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. Dont forget the electrode area! iCORR is usually current (A) and ICORR is usually current density (A/cm2). Area is very important. At a given potential, more current flows at a big electrode than at a small electrode.Area is very important. At a given potential, more current flows at a big electrode than at a small electrode.

    32. Precision of Electrochemical Corrosion Rate Measurements Corrosion is a messy business! -Dr. Jerry Frankel 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 for corrosion measurements. (Increase Your Confidence in Corrosion Test Data, Steve Tait, MATERIALS PERFORMANCE, March 2001) Error as a Function of Sample Size

    33. Comments on the Precision of Electrochemical Corrosion Measurements Electrochemical instrumentation is very accurate (<0.5% error) High purity Fe polished to a mirror finish gives great results! When corrosion occurs, anodic and cathodic reactions are occurring simultaneously on the surface of the sample The surface of a metal sample is not homogeneous. In fact, it may be very heterogeneous. 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 dynamic and the result ispoor precision

    34. Corrosion is a messy business, butit always involves electrochemistry Sometimes electrochemical techniques work greatand sometimes they worknot so great. Electrochemical corrosion measurement techniques always provide some information on the corrosion process. To avoid misinterpretation, its important to be able to recognize non-ideal behavior. Important: Use as many electrochemical (or other) tools (LPR, Tafel Plots, EIS, etc.) as possible to characterize a corrosion process.

    35. Electrochemical Corrosion Measurements are based on good solid science developed over many years. So well touch very briefly on the theoretical aspects of electrochemical corrosion measurement.

    36. Mixed Potential Theory The principal of charge conservation requires that there must be a least one reduction and oneoxidation in an electrochemical reaction. 2H+ + 2e- H2 Zn Zn2+ + 2e- The anodic current equals the cathodic current at icorr. Both reactions must occur on the same surface, so their potentials must change to a common value, which is Ecorr (same as EOC). This is an example of a mixed potential system and Ecorr is called a mixed potential.

    37. 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) Eoc = corrosion potential (V) ?a = anodic Tafel constant (V/decade) ?c = cathodic Tafel constant (V/decade)

    38. Graphical Representation of the Butler-Volmer Relationship between Potential and Current in a Mixed Potential System

    39. Butler-Volmer Equation The Butler-Volmer Equation only holds if: The rate is charge-transfer controlled (aka, activation-controlled, electron-transfer controlled). Complicating factors are: Passivity Diffusion-controlled (concentration polarization) Adsorption Only one reduction and one oxidation reaction are occurring (first order) Alloys can be complicated If the Butler-Volmer equation doesnt hold, then the electrochemical response will not be classic and must be interpreted in terms of the chemistry of the system.

    40. 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 = ac/2.3 iCORR (a + c) The Stern-Geary Equation is derived from the Butler-Volmer Equation via a series expansion in which ?E /a,c is assumed to be less than 0.1. Therefore, in Polarization Resistance the applied potential should be within 5-10 mV of Eoc. The plot of potential vs. current is approximately linear in this region. Emphasize the Stearn-Geary equations importance to PolRes.Emphasize the Stearn-Geary equations importance to PolRes.

    41. That concludes the theoretical portion of this presentation

    42. The Polarization Resistance Experiment

    43. Experimental Procedure for Polarization Resistance Measure EOC and allow to stabilize. Apply initial E that is 10 mV negative of EOC. Scan at a slow scan rate (~0.125 mV/s) to a final E that is 10 mV positive of EOC. Measure current, plot E (Y-axis) versus I (X-axis) Measure slope, which has units of resistance (E/i = R). Convert Rp to icorr Convert icorr to Corrosion Rate.

    44. If electrochemical data looks good, it almost certainly IS good.If electrochemical data looks good, it almost certainly IS good.

    45. The next three slides will explain the Corrosion Rate calculation.The next three slides will explain the Corrosion Rate calculation.

    46. Setting Up of a Polarization Resistance Experiment with Computer-Controlled Instrumentation

    47. Calculation of ICORR from RP Stern-Geary Equation RP = ?E/?i = ac/2.3 iCORR (a + c) Where RP = Slope at the origin of the Polarization Resistance Plot in ohms or ohms-cm2 iCORR = Corrosion Current, Amperes a, c = Tafel Constants from a Tafel Curve, volts/decade of current. Note: The area of the electrode must be taken into account somewhere!

    48. I. Calculation of Corrosion Rate from ICORR Faradays Law Q = nFm/A = it Q = coulombs (A-s/eq), n = number of electrons, F = the Faraday (96,487 coulombs), 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

    49. II. Calculation of Corrosion Rate from ICORR 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. 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, Corrosion Rate (mpy) = 0.13 Icorr (EW)/d Corrosion Rate (mmpy) = 0.00327 Icorr(EW)/d

    51. First, the Good NewsPolarization Resistance Works Very Nicely for the Measurement of Corrosion Rates Very fast (< 5 minutes). Scanning 20 mV at 0.1 mV/sec requires 200 seconds. In general, 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.

    52. And Now, the Bad NewsWhat Can Go Wrong with Polarization Resistance? You need to know the Tafel Constant(s). 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 (reduction or oxidation of more than one electroactive species). Uncompensated solution resistance.

    53. The Trouble with Tafel Constants For best results in Polarization Resistance, the Tafel Constants are needed in the Stern-Geary equation. It can be very difficult to experimentally determine the Tafel Constants with a reasonable level of confidence. ?a is not expected to equal ?c. ?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.

    54. To Determine the Tafel Constants, Run a Tafel Plot Measure EOC and allow to stabilize. Apply initial E that is 250 mV negative of EOC. Scan at a slow scan rate (0.125 mV/s) to a final E that is 250 mV positive of EOC. Measure current, plot E (Y-axis) versus log I (X-axis).

    55. Tafel Curve Analysis

    57. Examine the Tafel Region

    58. Measurement of Tafel Constants Insufficient linear region is a major problem. 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 youre 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!

    59. If EOC is Unstable, Using Galvanostatic Control Can Help A stable EOC indicates that the system has reached steady state. Despite my drifting EOC, I still need a Corrosion Rate! An LPR measurement with a drifting 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. Some work is usually required to determine the appropriate value of the applied current. Do NOT allow the potential to exceed 10 mV! F. Huet et al, Polarization Resistance Measurements: Potentiostatically or Galvanostatically?, CORROSION, 65, 136(2009).

    60. 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 = S fi/(ai/ni) = S (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?

    61. 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 system Scan Rate = ?E ? ? ? f At these scan rates, an experiment may take a long time. But thats why you have a computer-controlled instrument.

    62. As With Any Experiment, There Can Be Measurement Problems If theres a measurement problem, its either: The instrument The cell/electrodes The chemistry The local electronic environment

    65. A Few Examples of Real-World Applications of Polarization Resistance

    68. Evaluation of Corrosion Inhibitors 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.

    70. Corrosion of Rebar in Concrete Corrosion of reinforcing rods in concrete continue to be a concern in coastal areas and in colder climates where road salts are used. LPR and EIS are used to monitor corrosion rate. Concrete is a reasonably good electrolyte! Photos courtesy of Dr. Andres Torres-Acosta, Instituto Mexicano del Transporte

    71. Corrosion of Rebar in Concrete Experiments are run: With electrolyte ponded on the top of the concrete sample, and In a controlled humidity chamber with no ponded electrolyte

    72. Corrosion of Rebar in Concrete Identical rebar used for working and counter electrode. Titanium rod used for reference electrode. Be careful! Rebar in concrete has a very high capacitance (200 microF/cm2). This is the first mention of capacitance. EIS can be used to measure the capacitance.This is the first mention of capacitance. EIS can be used to measure the capacitance.

    76. 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 while youre in Atlanta. 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, 2003. Addresses many electrochemical and other methods to measure the rate of corrosion. Electrochemical Techniques in Corrosion Engineering, 1986, NACE International 36 papers. Covers the basics of the various electrochemical techniques and a wide variety of papers on the 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.

    77. Theory and Applications of the Measurement of Corrosion Rate with Polarization Resistance Dr. Pete Peterson Gamry Instruments www.gamry.com