Cyclic Potentiodynamic Polarization Scans

The cyclic potentiodynamic polarization technique for corrosion studies is an electrochemical technique that was introduced in the 1960’s and refined especially during the 1970’s into the fairly simple, routine technique it is today. This technique is especially suited for screening alloys in terms of their risk of suffering localized corrosion in the form of pitting corrosion or crevice corrosion. While cyclic potentiodynamic polarization scans are fairly easy to generate, their interpretation can be far more complex. 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 corrosion rate at a constant voltage scan rate and by observing how the current responds as the voltage force is applied and removed also at a constant voltage scan rate. The material is the alloy under consideration and the environment is that which promotes corrosion. Applied potential is the force. This potential is applied in a controlled manner to an electrode made from the alloy under study. A more complete tutorial on the technique and a web based application for interpreting the results can be found at http://www.argentumsolutions.com/tutorials/polexpert_tutorialpg3.html.

The Technique

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 as measured after the corrosion reaction has reached steady state. The corrosion behavior is predicted from the structure of the polarization scan. 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). The scan rates and experimental conditions used in this standard are not necessarily applicable to individual situations.

Equipment Overview

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. 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. An overview of the entire set-up is shown in this figure:

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 procedure is usually under computer control. 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.

Example of Polarization Scan

The polarization scan is plotted as potential versus logarithm of current. The reason is that this approach enables expansion of the low current portion of the response that is often important for the interpretation of the diagram. Shown in this figure is an example of a polarization scan.

This polarization scan is one that suggests the possibility of crevice corrosion and pitting corrosion. The arrows denote the direction of applying the voltage, first increasing voltage, then decreasing voltage. A number of features are used for the interpretation. They include the "pitting potential", the "repassivation potential", "hysterisis" between forward and reverse portions of the scan, and the current density near the corrosion potential. Further information on interpreting the polarization scan like the one above using these and other parameters can be found at http://www.argentumsolutions.com/tutorials/polexpert_tutorialpg5.html or in a number of texts on corrosion.

Experimentally induced artifacts can sometimes interfere with the values of the needed parameters. Some of these sources of error are:

  • inappropriately large voltage scan rate
  • large uncompensated (solution) resistance
  • inappropriate point at which the volatage ramp is reversed
  • varying corrosion potential (steady state not achieved)
  • non-uniform current and potential distributions

Further information on the effects of these artifacts and how to prevent their influence can be found at http://www.argentumsolutions.com/tutorials/polexpert_tutorialpg4.html

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