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VASP: an innovative technique for corrosion monitoring (VASP Rp determination) Corrosion – Application Note 36

Latest updated: July 31, 2023


Variable Amplitude Sinusoidal micro-Polarization technique is an electrochemical protocol based on EIS measurements using low voltage amplitude perturbation. Like the Tafel plot technique, the VASP allows the determination of the corrosion rate and Tafel parameters of a corrosion system without sample surface destruction. In this note, the VASP technique is introduced. The corrosion parameters of a pure Nickel sample in acid media were determined using VASP technique. The obtained results were compared with those obtained with Tafel plot technique.


Variable Amplitude Sinusoidal micro-Polarization technique (VASP) is a new electrochemical method integrated in EC-Lab® and EC-Lab Express®. It is based on an electrochemical impedance measurement that allows the study of corrosion mechanisms by the determination of Tafel parameters $\beta_a$, $\beta_c$; and the corrosion current $I_{corr}$.

The VASP technique consists of determining the change of the measured polarization resistance Rp with the potential amplitude variation. Rp is determined by Electrochemical Impedance spectroscopy measurements at a fixed and low enough frequency fs. fs determination is defined in the next paragraph.

The VASP experiment is performed by applying a sinusoidal electrical potential EWE(t):

$$E_{WE}\left(t\right)=V_asin{(}2\pi\ f_st) \tag{1}$$

with variable amplitude values and by determining at the frequency fs the polarization resistance for each amplitude.

The sinusoidal potential $E_{WE}(t)$ is applied around open circuit potential (OCP) with amplitudes increasing from $V_{a\ min}$ to $V_{a\ max}$.

Sinusoidal potential Ewe(t) with amplitudes increasing from Va min to Va max.
Figure 1: Sinusoidal potential $E_{WE}(t)$ with amplitudes increasing from $V_{a\ min}$ to $V_{a\ max}$.

The impedance response of a non-linear electrochemical system depends on the potential amplitude except for low amplitudes where the impedance response is independent of the potential amplitude.

The frequency fs is previously determined by an EIS technique on a wide frequency range. The Fig. 2 shows the Bode diagram for the Nickel electrode in 0.1 mol·L-1 HCl media.

Bode impedance diagram for fs frequency determination.
Figure 2: Bode impedance diagram for fs frequency determination.

The frequency fs is determined on the Bode plot and corresponds to the low frequency where Im(Z)=0 (i.e φ=0).

The electrode impedance at the frequency fs is a real parameter:

$$Z\left(f_s\right)=R_p \tag{2}$$

For negligible Ohmic drop, $R_e\approx0$, the Rp parameter can be expressed by the following relation [1,2]:

$$\frac{1}{R_p}=I_{corr}\sum_{k=0}^{\infty}{\frac{b_a^{2k+1}+b_c^{2k+1}}{2^{2k}k!\left(k+1\right)!}V_a^{2k}} \tag{3}$$

Where ba and bc are the Tafel slopes:

$$b_a=\frac{ln{(}10)}{\beta_a}, b_c=\frac{ln{(}10)}{\beta_c} \tag{4}$$

The corrosion parameters are determined by numerically fitting the data with Eq. 3.

This note explains how to perform a VASP experiment. The resulting data are compared with those obtained with a Linear Polarization Resistance (LPR) test.

Experimental setup

Investigations were performed at room temperature using a SP-300 potentiostat with EIS capability.

  • Working electrode: Pure Nickel wire with immersed area S = 0.6 cm².
  • Counter electrode: Platinum wire
  • Saturated Calomel Electrode (SCE)
  • Solution: HCl (1 mol·L-1)

On the setting window of VASP (Fig. 3), the user can set the frequency value of the applied potential. The maximum and the minimum values of the potential amplitude and the number of steps N are also entered.

In this application note, the chosen settings are shown on Fig. 3. The experiment is performed at the frequency fs = 0.23 Hz using N = 20 sinus amplitude. The frequency value fs was determined by the impedance measurement graph shown above (Fig. 2).

Setting window of VASP experiment.
Figure 3: Setting window of VASP experiment.

Commonly, amplitudes of applied potential are chosen between Va min = 10 mV vs. OCV and Va max = 100 mV vs. OCV. For N measured points, the amplitude of the applied potential increases from Va min to Va max by a step of $\frac{V_{\text{a max}} – V_{\text{a min}}}{N-1}$. In our case, the potential increases by 5 mV from one point to the next one.


Figure 4 illustrates the VASP plot from which the corrosion current $I_{corr}$ and the Tafel parameters $β_a$ and $β_c$ can be estimated.

VASP plot.
Figure 4: VASP plot.

In the Analysis/Corrosion menu of (EC-Lab® & EC-Lab Express®) the user can select the “VASP Fit” analysis. This tool allows the user to calculate the corrosion current Icorr and Tafel parameters βa, βc.

Once the numerical fitting is launched, the simulated plot is added to the VASP plot as shown in Fig. 5.

VASP Fitting plot.
Figure 5: VASP Fitting plot.
VASP Fit window.
Figure 6: VASP Fit window.

The user has the choice between βa, βc parameters and ba, bc Tafel slopes. These parameters can be obtained by ticking the “use ba and bc” box in the VASP Fit window (Fig. 6).

The results of the VASP experiment were compared with those obtained by the linear polarization technique.

As mentioned in the application note related to the linear polarization resistance (LPR) technique [3], the “Tafel Fit” tool provides an estimation of the corrosion rate and of the Tafel parameters. Fig. 7 shows the Tafel curve and the fitting plot obtained on pure Nickel in HCl media. The estimated corrosion parameters are illustrated in Fig. 8.

Tafel plot.
Figure 7: Tafel plot.

In similar experiment conditions, both techniques should provide similar results.

A slight difference can be observed between the Fit parameter’s values obtained by LPR and VASP techniques. It may be due to differences in the surface state between the two samples.

Tafel Fit window.
Figure 8: Tafel Fit window.

The table below summarizes the data obtained from the two experiments. The corrosion parameters obtained from VASP technique are compared with those obtained by classical linear polarization technique. Considering the surface change, the values of the corrosion parameters obtained with both techniques are in decent agreement.

Table I: comparison between VASP and LPR parameters.

Corrosion parameters LPR VASP Deviation (%)
Icorr/µA 10.3 11.9 13
βa/mV 69.5 74.73 07
βc/mV 249.1 190.4 23


This application note presented the use of the VASP technique integrated to EC-Lab® and EC-Lab Express® software. The corrosion parameters obtained from this technique are compared with those obtained by linear polarization resistance (LPR) method. It showed that the corrosion parameters fitted by VASP technique are close to those obtained by LPR technique. The VASP method is often a quickly implementable technique, easier to perform on several electrochemical systems.

Due to the low potential amplitudes, the VASP technique has the advantage of only slightly modifying the surface properties of an electrode compared to the LPR technique that alters the electrode surface. The VASP technique is recommended for potential sensitive surface electrode.

Data files can be found in :
C:\Users\xxx\Documents\EC-Lab\Data\Samples\Corrosion\PEIS_VASP, VASP_VASP and LP_VASP


1) K. Darowicki, Corros. Sci., 37 (1995) 913.
2) J.-P. Diard, B. Le Gorrec, C. Montella, Corros. Sci., 40 (1998) 495.
3) Application note #10 “Corrosion current measurement for an iron electrode in acidic solution

Revised in 08/2019

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