Polarization resistance measurement

Linear polarization re.slstance probe.s. LPR probes are more recent in origin, and are steadily gaining in use. These probes work on a principle outlined in an ASTM guide on making polarization resistance measurements, providing instantaneous corrosion rate measurements (G59, Standard Practice for Conducting Potentiodynamic Polarization Resistance Measurements ). 

ASTM G59, Standard Prac tice for Conducting Potentiodynamic Polarization Resistance Measurements, provides instructions for the graphical plotting of data (from tests conducted using the above-noted ASTM Standard G103) as the hnear potential versus current density, from which the polarization resistance can be found. 

To obtain the corrosion current from Rp, values for the anodic and cathodic slopes must be known or estimated. ASTM G59 provides an experimental procedure for measuring Rp. A discussion or the factors which may lead to errors in the values for Rp, and cases where Rp technique cannot be used, are covered by Mansfeld in Polarization Resistance Measurements—Today s Status, Electrochemical Techniques for Corrosion Engineers (NACE International, 1992). 

Impedance spectroscopy This technique is essentially the extension of polarization resistance measurements into low-conductivity environments, including those listed above. The technique can also be used to monitor atmospheric corrosion, corrosion under thin films of condensed liquid and the breakdown of protective paint coatings. Additionally, the method provides mechanistic data concerning the corrosion processes, which are taking place. 

Measurement of mass lost is the conventional method for determining the corrosion rate. The mass loss of an Fe specimen immersed in a corrosion test potential is determined by weighing, (b) Convert the mass loss rate 2.34 x 102 g tf rrT2 into icon using the atomic weight 55.847. (c) What is the difference between the results of the mass loss measurement and the polarization resistance measurement (Numata)... 

ASTM standards D-2776 (9) and G-59 (10) describe standard procedures for conducting polarization resistance measurements. Potentiodynamic (11), potential step, and current-step methods (12,13) have all been described to determine the linear E-i behavior of an electrode near Eco . The current step method has been cited to be faster than potentiodynamic methods and less susceptible to errors associated with drift in Econ. This issue will be discussed below. Regardless of the method used, independent determination of pa and pc is still required. 

E. Complications with Polarization Resistance Measurement by the Linear Polarization Method... 

Standard practice for conducting potentiodynamic polarization resistance measurements. ASTM Standard G-59, ASTM Annual/Book of Standards, ASTM. 

Figure 9 Calculated error in polarization resistance measured, ReU, compared to true polarization resistance, Rp. on a disk electrode with the reference electrode at the center, edge, and infinity as a function of conductivity, K, disk radius, r0, and Rcn. (From Ref. 18.)...

ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY AND POLARIZATION RESISTANCE MEASUREMENTS... 

Experimental studies usually yield good agreement between the rates of corrosion obtained from polarization resistance measurements and those derived from weight-loss data, particularly if we recall that the Tafel slopes for the anodic and the cathodic processes may not be known very accurately. It cannot be overemphasized, however, that both methods yield the average rate of corrosion of the sample, which may not be the most critical aspect when localized corrosion occurs. In particular it should be noted that at the open-circuit corrosion potential, the total anodic and cathodic currents must be equal, while the local current densities on the surface can be quite different. This could be a serious problem when most of the surface acts as the cathode and small spots (e.g., pits or crevices) act as the anodic regions. The rate of anodic dissolution inside a pit can, under these circumstances, be hundreds or even thousands of times faster than the average corrosion rate obtained from micro polarization or weight-loss measurements. 

B = 26 mV. The expected error in the calculated value of icorr (Eq 6.28) when using B = 26 mV as an approximation (as compared with extreme values of 13 and 52 mV) should be less than a factor of two. Therefore, the following approximation provides a reasonably good estimate of icorr from polarization-resistance measurements ... 

Standard Practice for Conducting Potentiodynamic Polarization Resistance Measurements, G 59-91, Annual Book ofASTMStandards, Vol 03.02, ASTM, 1995... 

ASTM G59-97. (2009) Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements, American Society for Testing Materials. 

Recommendation on polarization resistance measurements . Materials and Structures, in press,... 

C. Andrade, ). A. Gonzales, Quantitative measurements of corrosion rate of reinforcing steels embedded in concrete using polarization resistance measurement . Materials and Corrosion, 1978, 29, 515-519. 

Just as the Butler-Vohner equation was shown to be linear near E " with a slope inversely proportional to j o (low-field approximation. Sect. 1.3.8), the net current in a mixed potential system, represented by Eq. (41), is Knear near corr. with a slope inversely proportional to icorr-Since polarization resistance measurements are made near the corrosion potential, the net currents are often low. Therefore, errors associated with ohmic potential drop are usually small. However, there are cases for which ohmic potential drop creates significant error in the Rp measurement. The measured R is the sum of the trae polarization resistance. 

ASTM G59 descrihes a method for conducting potentiodynamic polarization resistance measurements [14]. It descrihes a setup for type 430 stainless steel in sulfuric acid, as for ASTM G5 described earher, and the same apparatus is specified. According to this standard, the potential should he scanned from 30 mV negative of the corrosion potential to 30 mV positive of the corrosion potential at a rate of 0.167 mV s . ... 

---