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TUTORIAL ON POLEXPERTTM AND THE CYCLIC POTENTIODYNAMIC POLARIZATION TECHNIQUE David C. Silverman |
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Overview-Cyclic Potentiodynamic Polarization Scan Technique The cyclic potentiodynamic polarization technique for corrosion studies was introduced in the 1960’s and refined especially during the 1970’s into the fairly simple, routine technique it is today. The technique is built on the concept that predictions of behavior of a material in an environment can be made by forcing the material from its steady state and observing how it responds as the force is removed at a constant rate and the material is relaxed back to its steady state condition. In this case, the material is an alloy and the environment promotes electrochemical corrosion. Applied potential is the force. This potential is applied in a controlled manner to an electrode made from the alloy under study. The potential is ramped at a continuous, often slow rate relative to a reference electrode using an instrument called a potentiostat. Traditionally, the potential is first increased at a constant rate in the anodic or noble direction (forward scan). The scan direction is reversed at some chosen maximum current or voltage and progresses at the same rate in the cathodic or active direction (backward or reverse portion of the scan). The scan is terminated at another chosen voltage, usually either the original corrosion potential or some voltage active with respect to that corrosion potential. The voltage at which the scan is started is usually the corrosion potential. For best results, the sample is immersed in the environment long enough for the corrosion potential to reach steady state since all behavior is related back to this potential. The corrosion behavior is predicted from the structure of the polarization scan. Note that the polarization scan is best plotted as voltage (relative to the suitable reference electrode) versus the logarithm of the current density. This format is followed so that the structure in the low current region where much of the corrosion information lies is magnified. An ASTM standard exists to verify equipment and software for generating the potentiodynamic polarization scan. (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). Several vendors offer software with their potentiostats that enable the scans to be generated automatically under computer control.2. Generation of the Polarization Scan The electronic device used to generate the cyclic potentiodynamic polarization scan is the potentiostat equipped with the abilities to ramp the applied potential in a controlled manner and then measure the resulting current. Discussions about the electronics within the potentiostat itself and the interaction between those electronics and the experiment can be found in suppliers’ information sheets. The goal here is to provide background so that the polarization scans generated are consistent with the needs of POLEXPERT. When using POLEXPERT, the polarization scan generated has been assumed to reflect only longer-term alloy behavior in the environment.Three electrodes are required for the measurement, (1) the corroding or working electrode made of the material of interest, (2) the counter electrode made from an inert material such as graphite or platinum, and (3) a stable reference electrode. Sometimes more than one counter electrode is positioned in the fluid. All counter electrodes are connected together. One lead from the potentiostat is attached to each electrode. This figure shows a simple schematic of the set-up. This figure shows an example of a cell with the external reference electrode connected by means of capillary tube filled with the test solution. Under control of a computer, the potential at the point sensed by the reference electrode in the solution is ramped relative to the reference electrode. The current is generated between the corroding electrode and the counter electrode. The tip of the reference electrode or capillary attached to it should be close to the corroding electrode but not too close. One rule-of-thumb is that this sensing point should be no closer that 2 outer capillary diameters from the corroding electrode. The reference sensing point could be too close if after generating a polarization scan a pit or small round discolored region is observed on the corroding electrode directly opposite the point sensed by the reference electrode. The scan pattern could give a false indication of localized corrosion. The tip should be moved farther away, the electrode resurfaced (if reusable), and the scan repeated before POLEXPERT is used to provide interpretation of the results. The objective is to vary the counter electrode voltage so that the voltage difference between the solution side of the corroding electrode and the reference electrode is the voltage set on the potentiostat. That assumption means that any voltage drop between the sensing point of the reference electrode and the corroding electrode is negligible. Unfortunately, this assumption may not be valid in all situations. Solution resistance can greatly affect the validity of this assumption. The reason is that the potentiostat actually maintains the voltage at the point in solution sensed by the reference electrode (or the tip of the capillary) not at the corroding electrode. Any voltage drop across the distance between these two cannot be measured directly by the potentiostat. In addition, if the solution resistance is too low, the voltage required at the counter electrode to generate the polarization scan can exceed the maximum output of the potentiostat. Further discussion of this point along with a graph showing the magnitude of the voltage drop versus current density and conductivity and an example how the effect might manifest itself can be found later in this tutorial in the section entitled Solution Resistance. The potentiostat manufacturers’ literature usually is a good source on how to implement reasonable equipment-based techniques such as current interrupt to try to overcome the effect of high solution resistance. Knowledge of the conductivity is important for this evaluation to be made. POLEXPERT assumes that the voltages (relative to the corrosion potential) that are provided as input are obtained in the presence of little or zero uncompensated resistance. The scan rate or rate of increase or decrease of voltage is one of the most important variables that the user selects. The section entitled Scan Rate provides details on how scan rate can affect the polarization scan and the need to balance the desire to complete the scan in a reasonable time against the need to ramp reasonably slowly. As discussed in that section, the uncompensated resistance plays a role in the choice of scan rate. A gross estimate of the magnitude of the "appropriate" scan rate as a function of the uncompensated resistance is provided. POLEXPERT was developed using scan rates no higher than 0.5 mV/s. The suggestion is that in the absence of information and in order to balance accuracy versus the amount of time required to generate the scan, the voltage scan rate for cyclic potentiodynamic polarization scans should be no greater than 0.5 mV/s. Generating the cyclic potentiodynamic polarization scan requires that the scan direction be reversed when an appropriate current or voltage is reached. The question is the appropriate value. The section entitled Point of Scan Reversal provides background information to aid in that decision and on the consequences of the applied voltage being pushed too far in the anodic or noble direction. Experience has suggested that the current at the point of scan reversal should be no greater than about 100 to 1000 microamp/cm2 (10-3 to 10-4 amp/cm2) in many instances to avoid the types of problems discussed in the section "Point of Scan Reversal". But, that rule is not hard and fast. If the scan suggests general corrosion, then larger current densities may be required to identify a passivation potential as might occur when there is an "oxidizable or reducible film." POLEXPERT was trained using polarization scans that were reversed at about 100 microamp/cm2 (10-4 amp/cm2) when scans suggested "localized corrosion" or "passive behavior". Scans suggesting "general corrosion" could be allowed to reach higher current densities before being reversed. If the current density never reaches 100 to 1000 microamp/cm2 (10-3 to 10-4 amp/cm2) then the scan must be reversed at a certain voltage. The example in the section on Point of Scan Reversal shows what can happen when the polarization scan direction is reversed at too high of a voltage. A hard and fast rule about an appropriate reversal voltage cannot be made. Careful thought should be given to what extraneous reactions might initiate at elevated potential and if those reactions could alter the alloy surface so that such potentials as the pitting potential and repassivation potential no longer reflect the alloy behavior in the natural environment. In summary, to be consistent with the scans used for training POLEXPERT:
Previous Page: Using POLEXPERT - a step-by-step procedure Next Page: Effects of Experimental and Environmental Variables |
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David C. Silverman, Ph.D. - Primary Consultant |
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