Electrochemical Techniques for monitoring and understanding the actual electrochemical process taking place on the metal surface

Corrosion of metal occurs through electrochemical reactions at the metal-solutioni nterface. Understanding the electrochemical phenomenon of a corrosion process will help to monitor and mitigate corrosion reaction. By using electrochemical methods, it is possible to monitor as well as understand the actual electrochemical process taking place on the metal surface. Unlike weight loss or gravimetric methods, electrochemical methods are fast in determining and analyzing the electrochemical properties. The electrochemical methods used for this study are described in the following sub-sections.

Linear polarization method

In a corroding electrode, a region of linear dependence exists between current and the potential applied over a small range close to the free corrosion potential. Free corrosion potential is the potential measured when no current flows through the electrode. This linear currentpotential response is due to the exponential relation of anodic and cathodic currents of a corroding electrode to potential, derived from Butler-Volmer equation45. Over a small potential range (<20 mV), the difference between cathodic and anodic exponential curves is almost linear. This linear dependence was first noted by Stern and Geary in 1957 and they derived an equation known as Stern-Geary equation, relating the slope of the linear region to the Tafel slopes and corrosion current.

   Stern − Geary Equation ∶Rp = (ba . bc) /(2.303*(ba + bc)*icorr) = ∆E/∆l ------------(5)

where, Rp is the polarization resistance, icorr is the corrosion current and ba , bc are anodic and cathodic Tafel slopes respectively. Eqn 5 served as an important tool for a new experimental approach to study the electrochemistry of corrosion reactions. The Linear polarization resistance (LPR) technique is based on the above mentioned theoretical fact. It is a non destructive method used for calculating polarization resistance which in turn used for calculating corrosion rate.

LPR technique generates a plot of current verses potential over a small potential range. In this method the metal sample is polarized step-wise, starting below the corrosion potential (usually -20 mV) and ending above the corrosion potential (usually +20 mV). The plot of current versus potential is linear, the slope of which gives the polarization resistance (Rp)46. The polarization resistance is inversely proportional to the uniform corrosion rate and can be used in the Stern-Geary equation to determine the corrosion current and corrosion rate47. A plot of current verses potential using LPR technique is shown below in Figure 2.1. The grey line on the graph is the sample plot of experimental data obtained using LPR technique. The linear fit of experimental data was performed using “Gamry- Echem analyst”
software and shown as a black line. From the experimental fit line the corrosion rate and polarization resistance are calculated. The calculation of corrosion rate using LPR technique is based on few fundamental assumptions such as the corrosion rate is uniform, both anodic and cathodic reactions are under activation control (kinetic control), a negligible solution resistance and most importantly known values of Tafel slopes.

The advantage of LPR technique is that data acquisition can be done quicker compared to other electrochemical methods. This method is non destructive since the potential applied to the sample is very small. The main demerit of this technique is that it needs Tafel data to calculate corrosion rate, which must be obtained either from literature or from other experiments.