Similar to a DC test, the Corrosion Rate or Penetration Rate can be found if you can define the Corrosion Current Density (i_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).

From the Stern and Geary relationship, the Corrosion Current (I_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

In order to determine the Corrosion Rate, we need to find the Corrosion Current Density (i_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

Once the i_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_corra/nF

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

Penetration Rate = i_corra/nFρ

Alternating Current (AC) Tests