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TUTORIAL ON POLEXPERTTM AND THE CYCLIC POTENTIODYNAMIC POLARIZATION TECHNIQUE David C. Silverman |
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Effects of Experimental and Environmental Variables ASTM Standards G-5 (ASTM Standard G5, "Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements", Annual book of ASTM Standards, 03.02 (Philadelphia, PA: ASTM) and G-61 (ASTM Standard G61, "Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys", Annual book of ASTM Standards, 03.02 (Philadelphia, PA: ASTM) provide guidance on how to set up an experiment and standard curves against which to check equipment and algorithms. These standards should be consulted by anyone unfamiliar with this technology to ensure that their experimental apparatus is operating properly. The assumption is made in this tutorial that the experimental apparatus and algorithm are operating properly and the cell geometry is appropriate for the intended study. The focus here is on three areas that can detrimentally affect the experimental results even if the physical equipment is correct; excessive uncompensated fluid resistance, inappropriate scan rate, and inappropriate point of reversal of the polarization scan.Measurement of the corrosion potential is the starting point for many electrochemical techniques, including the cyclic potentiodynamic polarization scan. Understanding its meaning and measurement are critical to proper scan generation. Electrochemical reactions characteristic of the interaction between a surface and a solution govern the corrosion process of a metal immersed in a solution. For example, when iron corrodes in acid, one half of the reaction process is iron forming iron ions via the reaction Fe → Fe+2 + 2e. The other half of the reaction is hydrogen ions combining with the electrons generated by the dissolution of iron to form hydrogen gas (2H+ + 2e → H2). Since electrons are exchanged during such a corrosion process, the reactions involved create an electrochemical potential. This potential is called the "corrosion potential" (sometimes called the "open circuit potential"). Corrosion is a process and not just a property of the material. The corrosion potential, therefore, is uniquely characteristic of the physics and chemistry at the solution-surface interface not just of the alloy itself. This potential is a measure of all of the electrochemical processes occurring at the surface and the environmental influence on those processes. The corrosion potential is normally measured as the voltage difference between an electrode made of the corroding material and an appropriate reference electrode, for example a saturated calomel electrode, a silver-silver chloride electrode, or a mercury-mercurous sulfate electrode. An excellent classic discussion on reference electrodes is D. J. G. Ives and G. J. Janz, Reference Electrodes, Academic Press, New York, 1961. Further information can be found in the tutorial on reference electrodes elsewhere on this site. The corrosion potential is measured using an electrometer with a high input impedance to limit current flow. An electrode made from the metal or alloy is placed in solution. One lead of the electrometer is connected to that electrode. The other lead is connected to the reference electrode. The measurement is the voltage difference between the two. Since the corrosion potential is created by the electrochemical process, its value reflects all of the processes causing corrosion. For the cyclic potentiodynamic polarization scan to be able to predict longer term propensity for such phenomena as localized corrosion or high levels of general corrosion the scan must be a reasonable copy of that which would be generated on an electrode of the same alloy generated after extended exposure to the actual environment. The surface structure of the test electrode might then be assumed to be similar to that of the actual alloy after extended exposure. For the features of the cyclic potentiodynamic polarization scan to have any chance of reflecting those that would occur at steady state (e.g. longer term exposure), the scan should be generated only after the corrosion potential reaches steady state. Potentials such as the pitting potential and repassivation potential have little meaning in and of themselves. Only their values relative to the corrosion potential have meaning for prediction. If the open circuit potential is not at steady state when the polarization scan is started, the differences between the pitting potential and corrosion potential or repassivation potential and corrosion potential could have little predictive value. If the polarization scan is generated from a surface state similar to that for the alloy in the field, then the features of the scan used for interpretation should also reflect those for the alloy in the field. POLEXPERT was created to provide guidance in relating the cyclic potentiodynamic polarization scan to the propensity of pitting, crevice corrosion, and general corrosion occurring after longer term exposure to an environment. The potentiodynamic polarization scans used in developing POLEXPERT, therefore, were generated after at least 18 to 24 hours of immersion to establish close to steady state conditions to try to ensure similarity to the surface condition after long-term exposure. The predictive power of POLEXPERT would be most applicable when cyclic potentiodynamic polarization scans to be interfaced with it have been generated only after the corrosion potential has reached steady state or the electrode has been immersed in the solution for between about 18 and 24 hours whichever is sooner. The typical arrangement in an electrochemical cell is for the sensing point of an external reference electrode to be brought close to the working electrode by means of a Luggin-Haber capillary. A rule-of-thumb is that the sensing point can be no closer to the working electrode surface than about two outer diameters of the capillary. (An "after-the-fact" way to identify if the sensing point was too close to the surface during the experiment is to examine if a pit-like structure appears on the electrode surface opposite the sensing point after the experiment.) The potentiostat can compensate for the voltage drop between the counter electrode and the sensing point. The potentiostat is blind to the voltage drop (resistance) between the sensing point and the working electrode. This resistance includes the solution resistance, any passive film resistance on the working electrode surface, and the electrical resistance of the electrode and leads. This resistance is the uncompensated resistance. The uncompensated voltage drop (uncompensated resistance times current between working and counter electrode) becomes greater as the current between the working and counter electrodes increases. In routine corrosion testing, environments can vary. Very often the effects of this resistance are not considered while generating the scan or analyzing the results. Sometimes the required test environments can either be non-aqueous or can be aqueous but with low ionic content. Under these circumstances, understanding what errors the uncompensated resistance can introduce is important because they can affect the interpretation. This figure The effect that the uncompensated resistance can have on the effective potential (as opposed to the potential believed to be applied by the potentiostat) and the effective scan rate (as opposed to the scan rate believed to be applied by the potentiostat) has been analyzed mathematically and reported in the literature (F. Mansfeld, Corrosion, 38, 10(1982): p. 556 and K. Schwabe, W. Oelssner, and H. D. Suschke, Prot. Metals, 15(1979): p. 126). In summary, if the uncompensated voltage drop becomes significant, the applied potential can be much greater than the voltage that is actually affecting the corrosion processes. In addition, the applied scan rate can be much greater than the effective scan rate. More importantly, the differences will be a function of the magnitude of the current passed between the working and counter electrodes, becoming greater as the current increases. The example described in D. C. Silverman, "Rotating Cylinder Electrode For Velocity Sensitivity Testing", Corrosion, Vol. 40, No. 5, p. 220 (1984).1  (466k)
shows how this phenomenon might manifest itself. In this case, the solution conductivity
was 100 µmho/cm. Mass loss of the electrode measured as a function of fluid velocity
demonstrated that the corrosion rate of steel in this environment was equal to the mass
transfer rate of iron ions from the surface. Under these circumstances, the polarization
scans would have been expected to have an anodic current density that increased with flow
rate while maintaining a constant passivation potential (similar to steel in
flowing concentrated sulfuric acid).
But, instead of the anodic current increasing with velocity and the passivation potential remaining constant, the mass transfer limiting current remained constant while the "measured" passivation potential increased with velocity. This result was an artifact of uncompensated resistance. The passivation potential should be independent of fluid velocity. The voltage drop became so large the 100 volt compliance of the potentiostat used for that study was most likely reached. The applied potential was much greater than the effective potential at the electrode interface. The observed passivation potential was much greater than the actual passivation potential. This point was confirmed by the independence of the characteristic potentials found on the return scan when the current densities became much lower and the compliance of the potentiostat was not exceeded. Had the effect of uncompensated resistance not been realized, the erroneous interpretation might have been that the corrosion rate was not sensitive to fluid velocity and that the passivation potential was a function of velocity. The following discussion assumes that proper account has been taken of any uncompensated resistance. The rate at which the potential is ramped, the polarization scan rate, is an experimental parameter over which the user has control. If not chosen properly, the scan rate can alter the polarization scan. The result can be a polarization scan that does not reflect the corrosion process. The problem is best understood by picturing the surface as a simple resistor in series with a parallel combination of a resistor and capacitor. The capacitor could represent the double layer capacitance and the resistor in parallel with it could represent the polarization resistance (inversely proportional to the corrosion rate). The series resistor would be the solution resistance. The goal is for the polarization scan rate to be slow enough so that the capacitors remain fully charged and the current/voltage relationship reflects only the interfacial corrosion process at every potential of the polarization scan. If not, some of the current generated would reflect charging of the surface capacitance in addition to the corrosion process. The measured current would then be greater than the current actually generated by the corrosion reactions. The scan would not represent the corrosion process alone. The outcome could be an incorrect polarization scan and an erroneous prediction from it. The question becomes, what is that proper scan rate? Though no recognized method exists to estimate this scan rate because the capacitance and resistance would be functions of the applied voltage, the relationship between the modulus and the frequency in an electrochemical impedance spectrum could be used to create a conceptual method of estimating if the chosen scan rate is reasonable (F. Mansfeld and M. Kendig, Corrosion, 37, 9(1981): p. 545). The conceptual approach uses the lowest breakpoint frequency (where the inverse of the phase angle passes through a maximum) of the impedance spectrum as the starting point. The premise is that the scan rate (rate of change of voltage) can be related to a frequency at every applied potential. That frequency must be low enough so that the impedance magnitude becomes independent of frequency. There the polarization or charge transfer resistance is being measured with no interference from surface capacitance. The mathematical model is shown in this figure The break-point frequency, the point at which the inverse of the phase angle goes through a maximum and the impedance magnitude goes through its inflection point, can be calculated easily (F. Mansfeld, Corrosion, 37, 6(1981): p. 301) for the case of a surface that can be modeled as a parallel combination of a resistor (polarization resistance) and capacitor (double layer capacitance) in series with a resistor (uncompensated resistance). That frequency is about at the position of middle arrowhead in the figure
The estimates are very conservative and are meant for illustrative purposes only. But, even so, they suggest that the maximum permissible scan rates at which the capacitive contribution is eliminated for very passive type of alloy-environment interactions are fairly low. These types of systems are those most likely to be encountered when using polarization scans to estimate the risk of localized corrosion for passive alloys. In fact for systems whose polarization resistances lie in the range of 105 to 106 ohm-cm2, not uncommon for some iron- and nickel-based alloys, even scan rates as low as 0.5 mV/s may not enable complete charging of the surface capacitance. This observation does not mean that the cyclic polarization scan cannot be used for screening or that scan rates as high as 0.5 mV/s cannot be used. What it does mean is (1) that an appropriate balance must be struck between the need for slow scan rates and the need to get the information quickly and (2) scan rates should be kept the same when screening alloys or environments with comparable resistance to general corrosion to help to ensure consistent comparisons. POLEXPERT was developed from potentiodynamic polarization scans in which the scan rates were in the range of 0.25 to 0.5 mV/s. The current density or potential at which the polarization scan is reversed can play a significant role in the appearance of the polarization scan and the value of the repassivation potential or protection potential discussed later. The reason is that the value of the repassivation potential is dictated by the amount of prior damage to the surface created as the potential is scanned in the anodic direction from the corrosion potential. The farther the polarization scan is generated in the anodic direction, the greater tends to be the degree of upset of the surface region. The effect of point of reversal on repassivation potential is especially pronounced if the pitting potential is exceeded or some other electrochemical transformation is precipitated especially if it does not reflect behavior at the corrosion potential. The result can be an erroneous prediction of corrosion behavior. A "best point of scan reversal" recommendation cannot be made because the amount of upset of the surface required for a prediction is somewhat dictated by the information desired. Maintaining a constant reversal point can be most important if alloys are being screened in a constant environment or if a single alloy is being evaluated across a number of environments. The polarization scan shown in the figure The nickel based alloy tested has a significant amount of chromium. Surface characterization results from similar alloys suggests that such alloys rich in chromium tend to have an enrichment of the surface in chromium, sometimes to the exclusion of all other components. This characteristic could give the alloy some of the same behavior as chromium. In the figure Under operating conditions, the alloy potential would never be such as to cause the Cr(III) to Cr(VI) transformation. Most likely, the potential would never reach that secondary repassivation of the surface, a rise of over 600 mV from the corrosion potential. The figure This exercise demonstrates that the procedure for generating potentiodynamic polarization scans to assess the propensity for localized corrosion must be consistent with the expected surface chemistry in the application. Erroneous conclusions result when the procedure creates surface chemistry and reactions that do not reflect the actual processes for which information is desired. This technique cannot be used without thinking about the surface chemistry that might be involved. Previous Page: Overview-Cyclic Potentiodynamic Polarization Technique Next Page: Features Useful in Interpreting Scan 1 © NACE International publication and year shown in citation above. All rights reserved. Displayed with permission from NACE International, Houston, TX (http://www.nace.org). Published in Corrosion, in the month and year shown in the citation above. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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David C. Silverman, Ph.D. - Primary Consultant |
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