Tafel extrapolation method

Corrosion Rate by CBD Somewhat similarly to the Tafel extrapolation method, the corrosion rate is found by intersecting the extrapolation of the linear poi tion of the second cathodic curve with the equihbrium stable corrosion potential. The intersection corrosion current is converted to a corrosion rate (mils penetration per year [mpy], 0.001 in/y) by use of a conversion factor (based upon Faraday s law, the electrochemical equivalent of the metal, its valence and gram atomic weight). For 13 alloys, this conversion factor ranges from 0.42 for nickel to 0.67 for Hastelloy B or C. For a qmck determination, 0.5 is used for most Fe, Cr, Ni, Mo, and Co alloy studies. Generally, the accuracy of the corrosion rate calculation is dependent upon the degree of linearity of the second cathodic curve when it is less than... 

The time required to determine Icorr by Tafel extrapolation is approximately 3 h, which corresponds to the approximate time required for experimental setup and generation of a cathodic polarization curve at a commonly employed, slow scan rate of 600 mV/h. In comparison, a comparable gravimetric evaluation (mass-loss measurement) on a corrosion-resistant metal or alloy could take months, or longer. A limitation of the Tafel extrapolation method is the rather large potential excursion away from Ecorr, which tends to modify the WE surface, such that if the measurement is to be repeated, the sample should be re-prepared following initial procedures and again allowed to stabilize in the electrolyte until a steady-state Ecorr is reached. Consequently, the Tafel extrapolation method is not amenable to studies requiring faster, or even continuous, measurements of Icorr. 

In the Tafel-extrapolation method for evaluation of Icorr, why is the cathodic polarization curve generally analyzed rather than the anodic polarization curve ... 

Table 6.9 Corrosion Current Values of Selected Coatings Determined in 0.5 M H3BO3 Solution at 25 °C Using the Tafel Extrapolation Method...

Figure 10.6 Example of a polarisation curve (solid line) and Tafel extrapolation method (dashed line).

Another approach to obtain 4on- and Econ is to use the Tafel extrapolation method. Here a sufficiently large polarization from Ecorr is used so that one or the other of the terms in Equation 15.6 is negligible. For a positive (or anodic) polarization when (E—Ecorr) > 0> the second term in Equation 15.6 is negligible (recall that Be is negative) and the equation may then be rearranged to give an expression relating the anodic polarization rjd to the anodic current (ij ... 

FIGURE 15.5 Polarization diagram illustrating the Tafel extrapolation method. The soUd curve represents the experimental data, whereas the dashed Knes are the extrapolations of the linear branches. The dotted curves illustrate the effect of concentration polarization (illustrated for oxygen diffusion), showing limiting currents for two cases, labeled and i ta(2). 

Where, W is weight loss (mg), A is area of the specimen (cm ), D is density of the specimen (gm/cm ), T is exposure time (hours) and unit pm/year is micro-metre/year. Indirect methods of corrosion rate measurement involve anodic/ cathodic reaction, consideration of current potential relationship or polarisation resistance values. Tafel extrapolation method is the most popular laboratory methods for measuring corrosion rate of a metal from electrochemical data in a corrosive medium. 

Nondestructive Electrochemical Methods—Many of the disadvantages discussed above for the Tafel extrapolation method can be eliminated by using the polarization resistance technique. This nondestructive method can be used on systems that are under either activation or diffusion... 

---