Corrosion Rate Measurement by Linear Polarization

As mentioned in Chapter 1.3, the slope, d /di, at the zero-current potential is a measure of the polarization resistance, Rp. The corrosion can be determined from the polarization resistance using the Stern-Geary equation, if the Tafel slopes are known  

Recall that both Tafel slopes are positive in this form of the equation. The Stern-Geary equation is the basis for the linear polarization method in which the polarization resistance is determined typically by scanning the potential from a value slightly below the corrosion potential to one slightly above the corrosion potential. It is an extremely easy technique that has been put to considerable use in corrosion monitoring. The polarization resistance can be determined by a simple two-point measurement at values above 

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 .  

H2SO4 in which the potential was scanned from —20 to -1-20 mV relative to the OCP. A relatively linear response is observed in this potential range. From Fig. 4, the polarization resistance can be found to be 80 cm. Using the Tafel slopes for this system determined from the potentiodynamic polarization curve given in Fig. 3 and the polarization resistance taken from Fig. 4, the corrosion rate is found to he 1.4 x 10 A cm , which is close to the value determined hy the Tafel extrapolation technique. 

It is possible to use the data from a standard potentiodynamic polarization curve to determine polarization resistance. So, even though the scan was performed over a wide potential range, the data near the zero urrent potential can be analyzed. The Tafel slopes are determined from the potentials further away from the zero urrent potential, and the corrosion rate is determined from 

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Corrosion in brownstock and post-oxygen washers has been investigated by Bennett and Magar [795] using electrochemical procedures. Their results showed that corrosion is affected by pH, chloride concentration, temperature, and aeration. These tests were complemented with instantaneous corrosion rate measurements by linear polarization resistance methods and by weight loss tests. 

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