Effects of scan rate on the potentiodynamic polarization curve obtained to determine the Tafel slopes and corrosion current density

doi:10.1016/j.corsci.2008.12.005

Corrosion Science

Volume 51, Issue 3, March 2009, Pages 581-587

https://doi.org/10.1016/j.corsci.2008.12.005 Get rights and content

Abstract

Effects of charging current on the potentiodynamic polarization curve that is obtained to determine the Tafel slopes and corrosion current density are reported in this paper. The potentiodynamic polarization curves are obtained at different scan rates for Ti6Al4V in naturally aerated 3.5% NaCl solution. The results show that the potential where the external current density equals to zero does not equal to the open circuit potential. The extent of the distortion of the polarization curve can be reflected the difference between the two potentials. Some significant errors are introduced into the values of the corrosion current density and Tafel slopes due to this distortion. In addition, severe distortion of the polarization curve can lead to misunderstanding of the electrode process. A new method is adopted to eliminate this distortion, and the potential-dependent of charging current density can also be obtained.

Introduction

The modern theory of aqueous metallic corrosion is now based firmly on electrode kinetics. For the corrosion system consisted of a cathodic reaction and an anodic reaction, the application of the Butler–Volmer equation [1], [2] and the mixed potential theory [3] results in the basic kinetic equation: $i = i_{corr} \{\exp [\frac{2.303 (E - E_{corr})}{b_{a}}] - \exp [- \frac{2.303 (E - E_{corr})}{b_{c}}]\}$ where E is the potential applied to polarize the corrosion system; i is the external current density; E_corr and i_corr are the corrosion potential and corrosion current density, respectively; b_c and b_a are the cathodic and anodic Tafel slope, respectively. When E is far away from E_corr, Eq. (1) gives the famous Tafel law [4]: $E = a \pm b \log | i |$ where a is a constant, b equals to b_c or b_a. Eq. (2) indicates that the logarithm of the external current density varies linearly with the potential at high overpotential. The corrosion current density can be determined by extrapolating the straight line of $E \sim \log | i |$ back to the corrosion potential.

The application of Eq. (2) is based on the assumption that the external current density comes predominately from the corrosion reaction. However, in practice, the charging process of the interfacial capacitance usually makes a contribution to the external current density. In a potentiodynamic scan experiment, a charging current always flows [5] because the continuously changing electrode potential leads a continuous change to the charge density stored at the electrode/solution interface. The charging current density is proportional to the product of scan rate and interfacial capacitance, which is similar to the electrical behavior of a capacitor. As the interfacial capacitance usually varies with the electrode potential [6], the charging current density is hard to be separated directly from the external current density. Thus, the potentiodynamic polarization curve is easily distorted around the corrosion potential where the faradaic current density is small. If this distortion is quite apparent in a wide potential range and not corrected, erroneous values of b_a, b_c, and i_corr will be obtained.

It can be found that the extent of the distortion of the potentiodynamic polarization curve depends on the scan rate. For most of the corrosion systems, the disturbance of the charging current is negligible by adopting a low scan rate, and thus, the distortion of the potentiodynamic polarization curve can be neglected. But for the corrosion system with an extremely low corrosion rate, the scan rate must be controlled in a very low level to make the disturbance of the charging current negligible. Doing so will extend greatly the polarization time, and it is easy for the polarization potential to cause some irreversible changes to the interfacial structure, especially in the anodic scan region. In this case, other errors will be introduced into the measurement results. Therefore, for these corrosion systems, it is very necessary to adopt a new method to obtain the polarization curve without the disturbance of the charging current.

Titanium and its alloys show very good corrosion resistance in various media because of its ability to form a passive film in the presence of air or water [7], [8], [9]. Potentiodynamic polarization technique has been widely used to study these corrosion systems [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. But the adopted scan rates in the literatures vary from 10 to 0.1 mV/s, and there are few instances of research work about the effect of scan rate on the potentiodynamic polarization curve. In this paper, the corrosion behavior of Ti6Al4V in naturally aerated 3.5% NaCl solution will be studied to illustrate how the potentiodynamic polarization curve is distorted by the charging current, and a new method is adopted to eliminate the disturbance of the charging current.

Section snippets

Some basic analysis

The external current density in a potentiodynamic scan experiment is the sum of the charging current density i_c and the faradaic current density i_f. If i = 0, then $i_{f} = - i_{c} \neq 0$ .

As is well known, the faradaic current density equals to zero at the open circuit potential E_op, which is also the corrosion potential. Thus, the zero-current-density potential of the potentiodynamic polarization curve, E₀ does not equal to E_op.

If the applied potential increases linearly with time, i.e., scanned in a positive

Experimental

The electrodes of Ti6Al4V were prepared by epoxy cold resin mounting, leaving areas of 1 cm² for exposure to the electrolyte. The surfaces exposed to the electrolyte were prepared by sequential grinding with silicon carbide paper up to #2000 finishing, degreased in an ultrasonic bath containing ethanol for about 60 s, and then rinsed with distilled water. A three-electrode cell arrangement was used for the electrochemical measurements, with a saturated calomel electrode (SCE) as reference

Variation of the open circuit potential with time

Fig. 1 shows the variation of open circuit potential (E_op) with immersion time of Ti6Al4V (TC4) alloys in naturally aerated 3.5% NaCl solution at the room temperature. At the initial stage, E_op increases rapidly from the approximate value of −0.53 V (SCE) to about −0.4 V (SCE), and then, the variation of E_op with time is very slow. This indicates that the corrosion resistance of TC4 increases with time and eventually reaches a relatively stable value.

Electrochemical impedance spectroscopy

The EIS spectra obtained at the open circuit

Conclusions

In a potentiodynamic scan experiment, the potential where the external current density equals to zero, E₀ does not equal to the open circuit potential E_op due to the disturbance of the charging current. For the positive and negative scan, the disturbance of the charging current makes E₀ negative and positive to E_op, respectively. The difference between the two potentials can reflect the extent of the distortion of the potentiodynamic polarization curve. The increase of the scan rate makes the

Acknowledgement

This work was supported in part by Harbin Special Foundation of Fellow Creation for Science and Technology of China (Grant No. 2006RFQXG032).

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Effects of scan rate on the potentiodynamic polarization curve obtained to determine the Tafel slopes and corrosion current density

Abstract

Introduction

Section snippets

Some basic analysis

Experimental

Variation of the open circuit potential with time

Electrochemical impedance spectroscopy

Conclusions

Acknowledgement

J. Electroanal. Chem.

Electrochim. Acta

Biomaterials

Mater. Chem. Phys.

Surf. Coat. Technol.

Scripta Mater.

Electrochim. Acta

Biomaterials

Surf. Coat. Technol.

Corros. Sci.

Corros. Sci.

J. Electroanal. Chem.

Corros. Sci.

J. Electroanal. Chem.

Biomaterials