Corrosion basics: DC electrochemical characterization of a corrosion system

Introduction to Corrosion

Corrosion is an unwanted spontaneous electrochemical process leading to structural degradation of a material. This process involves electrochemical reactions taking place at the interface between the material and the electrolyte. The electrochemical characterization of a corrosion system (material + electrolyte) consists of determining certain parameters such as:

• Corrosion potential
• Corrosion current
• Tafel parameters
• Pitting and repassivation potentials (for passivable materials)

During the corrosion of a metal, the metal oxidizes according to a positive current $I_\text{ox}$ which follows an exponential law with respect to the potential $E$ (Fig. 1a).
The negative reduction current, $I_\text{red}$, of the oxidant responsible for the corrosion of the metal, oxygen or proton for example, also follows an exponential law, in the absence of limitation by the transfer of oxidant. (Fig. 1b).

• The two currents $I_\text{ox}$ and $I_\text{red}$ are not measurable, only their sum $I=I_\text{ox}+ I_\text{red}$ is measurable (Fig. 1c).
• The total current $I$ is zero at the corrosion potential $E_\text{corr}=E_\text{I=0}$ (Fig. 1d).
• At the corrosion potential, the current $I_\text{ox}$ and the absolute value of $I_\text{red}$ are equal to the corrosion current $I_\text{corr}$ (Fig. 1e).

The value of $I_\text{corr}$ cannot be determined simply by using the $I\;vs.\;E$ curve (Fig. 1f).

Figure 1: Current vs potential curves for a metal dissolved in an oxidant, partial and total currents.

Two types of methods are used to carry out this characterization: DC (Direct Current) and AC (Alternating Current) methods.

DC Methods for Corrosion Analysis

1. Tafel Plot

The Tafel plot is one of the most commonly used DC methods for the electrochemical characterization of a corrosion system. It directly provides the value of the corrosion current of the material and consequently, its corrosion rate.

Stern/Wagner-Traud Method

This method utilizes a semi-logarithmic representation, where the decimal logarithm of the absolute value of the current, $\log|I|$, is plotted against the potential $E$. In this representation:

• The graphs of the two currents $I_\text{ox}$ and $I_\text{red}$ are straight lines.
• The semi-logarithmic plot of the total current shows two asymptotes.
• The abscissa of the intersection point of the two straight lines is equal to the corrosion potential $E_\text{corr}$.
• The ordinate of the intersection point is a measure of $\log I_\text{corr}$.

Figure 2: Semi-logarithmic plot of the current vs potential curves.

2. Stern-Geary Method (Linear Polarization Resistance)

The Stern-Geary (or Wagner-Traud) Method, also known as Linear Polarization Resistance (LPR), is a widely used technique for measuring corrosion rates. It is less destructive than the Tafel method and allows for quick, repeated measurements of the polarization resistance of the material.
The method is based on the observation that the potential-current curve is approximately linear for a small range of potentials near the corrosion potential ($E_\text{corr}$), typically ±10 mV.

Key Concepts:

Polarization Resistance ($R_\text p$): This is defined as the slope of the potential-current density curve at the corrosion potential.

The expression of the polarization resistance can be defined as:

$$R_\text P = \frac{1}{dI/dE} \tag{2}$$

Which gives, at $E= E_\text {corr}$, using Eq. 1:

$$R_{\text p,E_\text {corr}}=\frac{\beta_\text a \beta_\text c}{I_\text {corr}(\beta_\text a + \beta_\text c)\text{ln}10} \tag{3}$$

and knowing the values $R_{\text p,E_\text {corr}}$, $\beta_\text a$ and $\beta_\text c$, we can figure out the value of $I_\text {corr}$ with the following relationship:

$$I_\text{corr}=\frac{\beta_\text a \beta_\text c}{R_{\text p,E_\text {corr}}(\beta_\text a + \beta_\text c)\mathrm{ln}10} \tag{4}$$

The value of $R_{\text p,E_\text {corr}}$ can be simply determined by displaying the graph giving $E_\text{WE}$ vs. $I$ around the corrosion potential and calculating the slope of the curve.

The Stern-Geary Method provides a practical approach for rapid corrosion rate measurements, making it valuable for both research and industrial applications. Its non-destructive nature allows for repeated measurements on the same sample, enabling the study of corrosion kinetics over time.

Comparison of Tafel and Stern-Geary Analysis

Both Tafel and Stern-Geary methods are valuable tools in corrosion science, each with its own strengths and limitations. The choice between the two often depends on the specific requirements of the study, the nature of the corrosion system, and the need for repeated measurements on the same sample.

Both methods are applicable for Tafelian systems where the corrosion process is controlled by the electron transfer reaction (limiting reaction).

Limitations and Considerations

For real corrosion systems, many processes on the material can impact the kinetics of reactions involved in corrosion, such as:

• Adsorption
• Diffusion (mass transport)
• High resistance of an electrolyte (generating an ohmic drop during measurement)

For systems with high electrolyte resistance, an Ohmic drop compensation is needed for reliable measurement. This compensation can be done automatically using ZIR protocol available in EC-Lab^® software.

Corrosion Rate Calculation

The corrosion current ($I_\text{corr}$) obtained from these methods is used to calculate the corrosion rate ($\text{CR}$) of a uniformly corroding sample:

$$\text{CR}=\frac{I_\text{corr}\text{K EW}}{dA}$$

with

$\text{CR}$ in mmpy (millimeter per year) or mpy (milliinch or mil per year).

$I_\text{corr}$ the corrosion current in A.

$\text{K}$ a constant that defines the units of the corrosion rate.
It is equal to 3272 to have $\text{CR}$ in mmpy and 1.288 x 105 to have $\text{CR}$ in mpy.

$\text{EW}$, the equivalent weight in g/equivalent.
It is defined as the molar mass of the oxidized metal divided by the number of electrons involved in the dissolution reaction.

$d$ is the density of the metallic sample in g/cm3.

$A$ is the sample exposed area in cm2.

Both Tafel and Stern-Geary methods aim to determine $I_\text{corr}$, which is crucial for calculating the corrosion rate and understanding the corrosion behavior of materials in specific environments.

For further information please see Application note number 10: Tafel plot LPR – Corrosion