Polarization Resistance Technique

The Technique

The polarization resistance technique (sometimes referred to as "linear polarization") is an experimental electrochemical technique that enables the estimation of the polarization resistance and from that the corrosion rate. Briefly, the experiment requires polarizing an electrode between about -20mV and +20mV relative to the steady state corrosion potential. The slope of the resulting curve at the corrosion potential is the polarization resistance (in the absence of ohmic, e.g. solution, resistance). An example of a methodology for generating the potentiodynamic sweep in the vicinity of the corrosion potential is described very adequately in ASTM Standard G59 "Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements" available at http://www.astm.org. The following simulated plot shows how such a scan might appear:

The data are usually analyzed by assuming that the relationship between the current and voltage in the polarization curve is given by: where i is the measured current density, i_corr is the corrosion current density, V is the applied (assumed to be actual) voltage, V_corr is the corrosion potential, and b_a and b_c are the anodic and cathodic Tafel slopes. The basis for this equation lies in the mixed potential theory proposed by Wagner and Traud in 1938. A translation of this paper has recently appeared in print (F. Mansfeld, Corrosion, Vol. 62, p. 843, 2006) available from http://www.nace.org. The corrosion current can be related to the Tafel slopes by: The above equation can be derived directly without assuming linearity between voltage and current. Combining the above equations enables Tafel slopes and the polarization resistance to be extracted from a curve-fitting of the actual data. Obtaining a good estimate of the polarization resistance can sometimes be much easier than obtaining good estimates for the Tafel slopes. When that issue occurs one can still estimate the corrosion current by assuming the equation: and by assuming that B lies between about 10 and 30 mV (often about 15 to 25 mV). Further information can be found at http://www.argentumsolutions.com/tutorials/polres_tutorialpg2.html and in F. Mansfeld, "The Polarization Resistance Technique for Measuring Corrosion Currents", in Advances in Corrosion Engineering and Technology (M. G. Fontana and R. H. Staehle, ed.), 6, ch. 3, Plenum Press, 1976 and more recently in J. R. Scully, "Polarization Resistance Method for Determination of Instantaneous Corrosion Rates", Corrosion, Vol.56, p. 199 (2000) available at http://www.nace.org.

Equipment Overview

The electronic device used to generate the scans necessary for the polarization resistance technique 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. Note that in some on-line applications using three electrodes, the counter and reference electrode are the same material as the working electrode. In some on-line applications using two electrodes, the counter and reference electrode are the same electrode.

Assumptions

The above analysis has certain inherent assumptions that must be kept in mind.

1. The reaction rate (corrosion current) can be expressed as being proportional to the exponential of the voltage offset from the corrosion potential for one oxidation (anodic) and one reduction (cathodic) reaction.
2. Uncompensated resistance in the electrolyte and leads is either absent or is much smaller than the polarization resistance. The polarization resistance as measured by this technique is equal to the sum of all resistances of which the actual polarization resistance is one contributor.
3. Mass transfer cannot be the controlling or rate limiting step and both the anodic and cathodic reactions must be under activation control. Otherwise, the Tafel slope corresponding to the mass transfer controlled process is infinite and the above equations have to be modified accordingly.
4. The corrosion potential does not lie close to the reversible potentials for the oxidation and reduction reactions. Being 25 mV or so from the reversible potential is often sufficient.
5. To estimate the rate of uniform corrosion from the polarization resistance, each reacting site across the entire electrode surface is assumed to function simultaneously as a cathode and an anode. The anodic and cathodic sub-reactions do not occur on different sites.
6. No additional electrochemical reactions are occurring to interfere with the current density versus voltage curve.

Experimentally induced artifacts can sometimes cause significant errors. Such errors can occur even if the assumptions outlined above are fulfilled. Some of these sources of error are:

• inappropriately large voltage scan rate
• large uncompensated (solution) resistance
• non-linearity in vicinity of corrosion potential when using a limited number of points or assuming a linear regression
• errors in Tafel slopes
• varying corrosion potential (steady state not achieved)
• non-uniform current and potential distributions

Further information on these sources of error and their effect on the measurement and estimation of the corrosion rate can be found at http://www.argentumsolutions.com/tutorials/polres_tutorialpg3.html or in the publications J. R. Scully, "Polarization Resistance Method for Determination of Instantaneous Corrosion Rates", Corrosion, Vol.56, p. 199 (2000) (available at http://www.nace.org) and D. C. Silverman, "Practical Corrosion Prediction Using Electrochemical Techniques", ch. 68 in Uhlig's Corrosion Handbook, 2^nd edition (R. W. Revie, ed.), The Electrochemical Society, 2000.

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