The cyclic potentiodynamic polarization technique provides a reasonable, rapid method for qualitatively predicting the propensity of an alloy to suffer localized corrosion in the form of pitting and crevice corrosion. Better and more accurate techniques are available for estimating general corrosion rates, This paper outlines the features that have been found useful for interpreting the polarization scan along with an example of how such an interpretation might be made. The paper follows with a discussion of some of the effects that uncompensated solution resistance, inappropriate scan rate, and improper point of scan reversal can have on the polarization scan features and how these effects might influence the interpretation.
Electrochemical technology tends to be used for laboratory or in-plant corrosion prediction when one desires a rapid indication of the risk of corrosion. One electrochemical technique that has gained widespread acceptance as a general tool for assessing the possibility of an alloy suffering localized corrosion in the form of pitting or crevice corrosion is the cyclic potentiodynamic polarization technique. This technique is used to generate the polarization scan. This technique has been especially useful to assess localized corrosion for passivating types of alloys such as 316ss, nickel-based alloys containing chromium, and refractory type of alloys such as titanium and zirconium, all in an array of environments.
The cyclic potentiodynamic polarization technique for corrosion studies was introduced in the 1960?s and refined especially during the 1970?s into a fairly simple technique for routine use. A brief description of the technique is as follows. The voltage applied to an electrode made from the alloy under study is ramped at a continuous, often slow, rate relative to a reference electrode using a potentiostat. The voltage is first increased in the anodic or noble direction (forward scan). The voltage scan direction is reversed at some chosen current or voltage and progresses in the cathodic or active direction (backward or reverse scan). The scan is terminated at another chosen voltage, usually either the corrosion potential or some potential active with respect to the corrosion potential. The potential at which the scan is started is usually the corrosion potential. The sample is often immersed in the environment long enough for the corrosion potential to reach steady state. The corrosion behavior is predicted from the structure of the polarization scan. An ASTM standard exists for this procedure? though the procedure as outlined there is not the only or even necessarily the best way to generate the polarization scan in all situations. Several vendors offer software with their potentiostats that enable the scans to be generated automatically under computer control.
Though the generation of the polarization scan is simple, its interpretation can be difficult. This interpretation is derived from the relationship between the current and voltage and differences in that relationship between the forward and the reverse portions of the scan. Certain characteristics were identified as being important very early in the development of this technology. Characteristic potentials identified as important for de