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TUTORIAL ON POLEXPERTTM AND THE CYCLIC POTENTIODYNAMIC POLARIZATION TECHNIQUE David C. Silverman |
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Features Useful in Interpreting Scan Certain characteristics were identified as being important very early in the development of this technology. Two characteristic potentials identified as important for determining the propensity for localized corrosion were the protection potential (now often called the repassivation potential) and pitting potential and their relationship to the steady state corrosion potential (M. Pourbiax, L. Klimzack-Mathieu, and Cl. Vanleugenhaghe, Corros. Sci., 3(1963): p. 329 and M. Pourbaix, Corrosion, 26,10(1970): p. 431.). Over the years, investigators have re-examined this technique to determine the relevancy of these and other parameters to the interpretation of polarization scans.POLEXPERT uses the parameters shown in the table below to make its prediction from the polarization scan. While these parameters may not be the only ones that can be used, they are the features that provided consistency between observation and prediction. POLEXPERT requires that these features be used together.
The following four figures show typical polarization scans that might be observed in practice. They represent how polarization scans might appear for the different types of corrosion phenomena shown in the titles. Since scans with these characteristics are fairly common, they are used as part of the consistency check within POLEXPERT. The figures are drawn assuming an arbitrary minimum recorded current (e.g. 10-2 to 10-1 mamp/cm2) that could lie above the actually measured minimum current (e.g. 10-3 mamp/cm2) sometimes observed in an experiment. Features that are used by POLEXPERT are shown on the four figures above. These figures should be consulted when reading the discussion that follows. 1. Pitting and Repassivation Potentials Two potentials that are often thought to characterize an alloy in terms of localized corrosion are the "repassivation potential" (or "protection potential" as it is sometimes called) and the "pitting potential" and their values relative to the corrosion potential. The differences between these potentials and the corrosion potential are used as the two features, not the values of the potentials themselves. A common interpretation is that pitting would occur if the trajectories of the forward and reverse scans appear as in the scan titled "Polarization Scan for Localized Corrosion" and the corrosion potential is equal to or anodic (noble) with respect to the pitting potential. In this case, as the voltage is increased from the corrosion potential, the forward trajectory would show an extremely large increase in current even at potentials close to the corrosion potential. Preformed pits, e.g. crevices, might be expected to grow if the corrosion potential lies between the pitting and repassivation potentials. In this case, as the voltage is increased from the corrosion potential, the trajectory might show only a slight increase in current at most until a certain potential is reached. Beyond that latter potential, the forward trajectory would show a large increase in current for small increments in voltage. The alloy would be expected to resist localized corrosion if the corrosion potential is cathodic (active) with respect to the repassivation potential and the polarization scan would appear as in the scan titled "Polarization Scan for Passive Behavior".Different methods have been used to choose the repassivation potential. One way is to choose it as the potential at which the anodic forward and reverse scans cross each other. Alternatively, it can be chosen as that potential at which the current density reaches its lowest readable value on the reverse portion of the polarization scan. POLEXPERT was created by following the second procedure. The reason for this approach is that for some polarization scans, the forward and reverse portions of the polarization scan may not cross each other. The choice must be consistent for all scans in any screening study. The "pitting potential" is that potential at which the forward or ascending portion of the scan shows a rapid rise in current, (d(lnI)/dE approaches infinity) followed by reverse portion of the scan remaining at higher currents than the forward portion at the same potential as in the scan labeled "Polarization Scan for Localized Corrosion" Often, the electrode surface exhibits small pits after the experiment. Controversy surrounds the meanings of these potentials. The values measured are not intrinsic properties of the alloy being examined. In a perfect world they would be properties of the alloy – environment interaction. But, in reality, their values are influenced by a number of experimental variables, some of which are discussed in Effects of Experimental and Environmental Variables. The pitting potential as determined by the potentiodynamic polarization scan has been shown to be related qualitatively to the resistance of a material to a loss of passivity by pit initiation (B. E. Wilde, Corrosion, 28, 8(1972): p. 283). If a portion of the specimen becomes part of a crevice as, for example, between the electrode and the holder, the pitting potential may reflect the breakdown of passivity in that crevice, not pitting of the bare surface. The pitting potential has been shown to vary with the amount of localized corrosion induced by the applied potential e.g. the chemistry changes within the localized area. The difference between the repassivation potential and the pitting potential has been found at least in some environments to be a measure of the extent of crevice corrosion suffered by the sample (B. E. Wilde and E. Williams, Electrochimica Acta, 16(1971): p. 1971). More detailed analysis has revealed that the value of the pitting potential is a function of the scan rate used to generate the polarization scan. The value seems to become less noble (more active) as the scan rate is decreased (N. G. Thompson and B. C. Syrett, Corrosion, 48, 8(1992): p. 649). Extrapolation of these potentials with scan rate has suggested that in the limit of low (e.g. zero) scan rate and with virtual elimination of occluded sites for crevice corrosion to initiate on the electrode, the repassivation and pitting potentials approach each other. The repassivation potential becomes more noble and the pitting potential becomes more active. The conclusion was made that the repassivation potential as determined from a cyclic potentiodynamic polarization scan is more conservative than the actual repassivation potential but the pitting potential so measured is less conservative. These conclusions were corroborated in another study of nickel alloys in nuclear waste (N. Sridhar, D. Dunn, and G. Cragnolino, Mat. Res. Soc. Symp. Proc., 353(1995): p. 663). Though the values ascribed to these two potentials can be a bit arbitrary, the potentials remain practical aids to predicting localized corrosion. The values used for POLEXPERT are the differences between the pitting potential and the corrosion potential and between the repassivation potential and corrosion potential. A rule-of-thumb that has sometimes been employed reasonably successfully has been to require that the corrosion potential be some value (e.g. 200 mV) more active than the repassivation potential for crevice corrosion not to be expected to be a problem. This requirement helps to further ensure that the prediction is conservative because of the above discussion about variation of repassivation potential with scan rate. The same rule might be employed for the pitting potential though the pitting potential measured under a finite scan rate tends to become less conservative as the scan rate increases (P. E. Manning, "The Effect of Scan Rate on Pitting Potentials of High Performance Alloys in Acidic Chloride Solutions", Paper 73, CORROSION/80, Chicago, Il., March 3-7, 1980). To use a rule-of-thumb like that mentioned above when screening alloys and conditions, the voltage scan rate, apparatus, and procedure must be kept constant for all polarization scans. Generating the polarization scan after the corrosion potential has reached steady state is best though that condition may not always be fulfilled. Scan rates up to about 0.5 mV/s have often been found to be acceptable for obtaining reasonable predictions for self-passivating alloys in a reasonable length of time. The hysteresis refers to a feature of the polarization scan in which the forward and reverse portions of the scan do not overlay each other. Examples of the hysteresis are shown in the two figures, "Polarization Scan for Passive Behavior" and "Polarization Scan for Localized Corrosion". The hysteresis is created by the current density difference between the forward and reverse portions of the scan at the same potential. It is a result of the disruption of the steady state surface structure by the increase in potential. If reflects the ease or difficulty with which that initial structure is restored as the potential is decreased back toward the corrosion potential at constant scan rate. From a comparison standpoint, for a constant and appropriate experimental procedure, the larger the hysteresis, the greater is the disruption of surface passivity, the greater is the difficulty in restoring passivity, and usually the greater is the risk of localized corrosion. Approaching a potential from more active potentials at a scan rate greater than zero will create a surface structure that is different from that created when approaching the potential from more noble potentials. The "positive" hysteresis shown in the figure entitled "Polarization Scan for Passive Behavior" results from the polarization to more noble potentials making the surface more passive so that when the same potential is reached on the return portion of the scan, the current emanates from a surface more passive than during the forward scan. Hence the current density is lower. The "negative" hysteresis in the figure entitled "Polarization Scan for Localized Corrosion" results from the opposite effect. Polarization to more noble potentials causes a decrease in passivity sometimes by the initiation of localized corrosion so that when the same potential is reached on the return portion of the scan, current emanates from a surface that is less passive. Hence, the current density is greater. This phenomenon is usually a reflection of a propensity for localized corrosion either in the form of pitting or crevice corrosion. From a practical standpoint, a positive hysteresis usually signifies that the alloy will be more resistant to localized corrosion than does a negative hysteresis. POLEXPERT uses information about the appearance of the hysteresis in its decision making. There are cases when the hysteresis does not reflect an ease of destruction of passivity. For example, polarization scans for metals suffering so-called active (or rapid) corrosion, e. g. steel in 1N hydrochloric acid, would show a rapid rise in current under slight polarization. After polarizing such alloys to 100 µamp/cm2 or 1000 µamp/cm2 and reversing the potential, the return scan would not necessarily overlay the forward scan exactly. This behavior is shown in the figure entitled "Polarization Scan for General Corrosion". Such appearance does not reflect hysteresis for an active-passive alloy but is a result of the relatively large amount of charge passed across the interface. Since the measured current does not represent the steady state current at the applied potential there is a difference in current at the same potential depending on scan direction. One important note of caution is that the current density or potential at which the polarization scan is reversed can have a large effect on the values of the potentials shown in Table I and even on the type of hysteresis (B. E. Wilde, Corrosion, 28, 8(1972): p. 283 and K. Ravichandran, M. Sivakumar, T. S. N. Sankara Narayanan, and S. Rajeswari, Mater. Sci. Lett., 14, 5(1995): p 317). This point is discussed in the previous section on Point of Scan Reversal. 3. Active-Passive Transition or "Passivation Potential" This feature reflects the following characteristics for some alloy-environment combinations. The measured current increases rapidly as the potential is ramped in the anodic direction near the corrosion potential. That current goes through a maximum value. That current then decreases rapidly to a low value as the potential is ramped still further. Iron or some austenitic alloys may demonstrate this type of behavior in acidic environments, for example. The rapid decrease in current may suggest an alloy surface undergoing some type of passivation process or valency change (Fe(II) to Fe(III)) as the potential is increased. Sometimes, the current does not decrease as potential increase but just reaches a plateau as a function of potential. The figure entitled "Polarization Scan for Oxidizable/Reducible Surface" shows both cases. POLEXPERT uses information about the presence of this feature in its decision making.The presence of this feature could mean that the alloy has a finite corrosion rate at the corrosion potential. The question is if this corrosion rate is unacceptable. In principle, one could curve-fit this scan in the vicinity (± 200 to 300 mV) of the corrosion potential and extract Tafel slopes and a polarization resistance and from this information estimate a corrosion rate. There are drawbacks to this procedure. The mechanism might change over that potential range. The approach requires assuming a corrosion mechanism which has one irreversible anodic contribution and one irreversible cathodic contribution. Such may not be correct. A better approach is to estimate the corrosion rate using the alternative techniques of polarization resistance, electrochemical impedance spectroscopy, or coupon mass loss that are more sensitive in estimating general corrosion rates. For this case, the polarization scan would serve as an indicator that general corrosion is a possibility and that a more accurate measurement is required to estimate the rate. 4. Anodic-to-Cathodic Transition Potential The potential at which current changes from anodic to cathodic current during the reverse portion of the scan is assumed to be the potential of the anodic-to-cathodic transition. The difference between this potential and the corrosion potential is an additional feature useful for screening alloys in the same environment as long as the experimental procedure is unchanged among tests. In theory, that transition should occur at the corrosion potential. But, in reality, changes in the surface structure during polarization can cause that potential to be above or below (anodic or cathodic with respect to) the corrosion potential.If the polarization scan appears as in the two figures, "Polarization Scan for Passive Behavior" and "Polarization Scan for Localized Corrosion", this potential still exists but the current at the transition is lower than the lowest recorded value of the current density. Under these circumstances, this potential might be assumed to be the potential at which the cathodic current rises above the lowest recorded value. Inter-scan comparisons can still be made as long as the choice is consistent among them. The difference between this potential and the corrosion potential would provide an additional indication of persistence of passivity. For that reason POLEXPERT requires information about the difference between this potential and the corrosion potential. For alloys that can passivate either by a change in oxidation state (ferrous to ferric) or a change in the passive layer (greater enrichment of chromium oxide, for example), polarization to more noble potentials relative to the corrosion potential might place the surface in a more passive state than at the corrosion potential at least until the transpassive region is reached. If the transition is cathodic or active with respect to the corrosion potential, the suggestion would be that passivity persists as the scan returns through the corrosion potential. Following that reasoning, the passive film that would normally develop on the alloy in the environment would be considered to be very stable. If this anodic-to-cathodic potential is more noble than (anodic to) the corrosion potential, the suggestion is that any passivity that might be created by a surface oxidation product upon polarization in the anodic (noble) direction would be somewhat reduced or even possibly absent at the corrosion potential. The inference would be that any film present at the corrosion potential might not be very passivating. Corrosion could be measurable at the corrosion potential. Using the polarization scan to estimate a general or uniform corrosion rate is not making best use of this electrochemical technology. Making such an estimate from the polarization scan requires the assumption of a mechanism and the curve-fitting of the scan to equations describing that mechanism over a significant potential range, e.g. several hundred millivolts. The assumption is that the corrosion mechanism does not change over this potential range. That assumption may or may not be valid. Better technologies are available for estimating corrosion rates, e.g. electrochemical impedance spectroscopy, polarization resistance, and coupon immersion tests. Estimating actual corrosion rates from polarization scans are not discussed in this tutorial. But the polarization scan can be used to infer if such alternative measurements might be warranted. Following is the way it is used in POLEXPERT. This parameter labeled in POLEXPERT as the "passive current" is the order of magnitude value of the current density that would reflect its value at or near the corrosion potential. The value provides information on the risk of the corrosion rate being a contamination rate versus something far worse. It is meant to act as a way of flagging the possibility and qualitative magnitude of general corrosion. The exact value is not needed, just an order of magnitude estimate as readable from the polarization scan itself. The value is meant to reflect the degree of surface passivity near but not necessarily at the corrosion potential. POLEXPERT was designed with the idea that in most instances, the accuracy of the current density measurement from the potentiodynamic polarization scan decreases when the current density is less than about 10-8 to 10-7 amp/cm2 (0.01 to 0.1 microamp/cm2. The following discussion, therefore, assumes that a vertical line is drawn parallel to the potential axis in this current density range and this line becomes the lowest measured current density. 1. If the forward portion of the polarization scan remains at the lowest current density and the reverse portion of the polarization scan decreases to and remains at the lowest recorded current density for some potential range near the corrosion potential, then the passive current can be assumed to be that lowest recorded value. Very likely, the corrosion rate will be at most a contamination rate. The figures showing "Passive Behavior" and "Localized Corrosion" demonstrate this type of behavior. 2. If the reverse portion of the polarization scan decreases to and remains at a certain current density over some potential range at or near the corrosion potential but not necessarily at the lowest recorded range, then the passive current is assumed to be that value. This choice is made even if the current density of the forward scan is at the lowest recorded current as defined above. The degree of risk of something greater than a contamination rate could depend on the magnitude of this value. This choice could lead to a more conservative prediction about general corrosion. The figures labeled as "Passive Behavior" and "Localized Corrosion" can show this type of behavior. 3. If the forward portion of the polarization scan increases to and remains at a certain current density plateau over some potential range at or near the corrosion potential, then the passive current is assumed to that value at the plateau. The plateau can be reached with or without a passivation potential. That choice is used even if the reverse portion of the polarization scan reaches the lowest recorded current density. The degree of risk of something greater than a contamination rate could depend on the magnitude of this value. This choice could lead to a more conservative prediction about general corrosion. When a passivation potential is found as in the figure corresponding to "Oxidizable/Reducible Surface" the value assumed is that at the current peak corresponding to that potential. 4. If the polarization scan shows a rapid increase in current to a high value with no current plateau as in the case of the figure entitled "General Corrosion" , then the risk exists of a large corrosion rate leading to possible failure. What this case means is that there is no passive current at or near the corrosion potential and a value of 105 microamp/cm2 can be used. 6. Additional Examples in Literature Several examples of how these parameters are derived and the ability of POLEXPERT to use them are in E. M. Rosen and D. C. Silverman, "Corrosion Predictions from Polarization Scans Using an Artificial Neural Network Integrated with an Expert System", Corrosion, 48, 9(1992): p. 7341 (774k).
Previous Page: Effexts of Experimental and Environmental Variables Next Page: POLEXPERT (the intelligent prediction tool) 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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