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# Alternating Current (AC) Tests

_{corr}).

As presented in the Schematic Nyquist Plot in the Figure below, the value that will be obtained during EIS (

*Electrochemical Impedance Spectroscopy*) will either be the R

_{ct}(

*Charge Transfer Resistance*) or the R

_{p}(

*Polarization Resistance*).

_{corr}) in the next equation can be calculated if the Anodic (βa) and Cathodic (βc) Tafel Constants have been found previously, and the Polarization Resistance (R

_{p}) is found during the current experiment. In the simplest case, the value for the Charge Transfer Resistance (R

_{ct}) is the same as for the Polarization Resistance (R

_{p}). Even in more complex situations, using the value for Charge Transfer Resistance (R

_{ct}) may provide an estimate of the Corrosion Rate.

I_{corr} = βaβc/2.3(βa + βc)R_{p}

_{corr}). Most electrochemical software will do this automatically if you enter the Area of the sample. The i

_{corr}is simply the Corrosion Current (I

_{corr}) per Unit Area (A), as follows:

i_{corr} = I_{corr}/A

_{corr}is known, it is a simple matter to find the corrosion rate (CR). In the following equation we already know the Atomic Mass (a), the Valence Change (n), and Faraday's Constant (F).

CR = i_{corr}a/nF

A Penetration Rate (such as mmpy or mpy) can be found by dividing by ρ (density).

**Penetration Rate = i _{corr}a/nFρ**

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Page last updated: 11/1/22