3. Employed Experimental Techniques
3.2 A tomic force microscopy (AFM)
18.104.22.168 Electrochemical impedance spectroscopy (EIS)
The use of electrochemical impedance spectroscopy in evaluation of electrochemical and corrosion mechanism is due to its ability of probing electrochemical systems even at very low frequencies. This technique was developed after the elaboration of the potentiostat in the 1940s followed by the frequency response analyzer in the 1970s (13). The EIS has become a powerful tool in the study of metal electrode/electrolyte interface, coatings, porous electrodes, electrode /layer/ electrolyte interface (55). Additionally, electrochemical reaction mechanisms, kinetic, passive surfaces, materials of dielectric and transport properties of an electrode, protection properties of corrosion inhibitors, e.g: corrosion inhibition via phosphonic acids formed by absorption on metal surfaces (7, 88), metal coatings and thin film had been studied (89). It is a non-destructive method (advantages over the DC methods) since it uses very small excitation potential amplitudes, and has a minimal perturbation to the working system.
EIS as an electrochemical measuring method quantifies the protective behavior of coatings. EIS results can indicate the state of coating degradation, the efficacy of corrosion protection and can even identify protective mechanisms. EIS measurements on organic coatings apply alternating
voltage over a counter electrode and the coated metal substrate, and the response of the system is registered. Impedance (Z) is a measure of a circuit’s tendency to resist (impede) the flow of an alternating electrical current and is the AC equivalent of electrical resistance (R) in DC applications.
A sine wave voltage applied to an electrochemical cell gives a current response, which is shifted in time due to the influence of the system. This time shift is expressed as the phase angle.Since the impedance is frequency dependent, the measurements go over a wide range of frequencies.
By measuring the high frequency impedance (105- 103 Hz) and using equivalent circuit modeling the time of measurement can be kept to a minimum.
Data achieved by impedance spectroscopy are fitted to an equivalent circuit when enough parameters (elements in the equivalent circuit) are used. It is preferred to fit the data to the most probable equivalent circuits. The fitting parameters are as follows: electrolyte resistance (Rs: the solution resistance between the working electrode and the reference electrode ), constant phase elements (CPE: used instead of a capacitor to represent the non-homogeneity such as the roughness or the energetic inhomogeneity of the system), coating resistance (Rc: electrochemical (corrosion) behavior of the system), metal double layer capacitance (Cdl: capacitance between the working electrode and the electrolyte; controlled by many variables such as temperature, electrode potential, types of ions, impurity adsorption, ionic concentrations, oxide layers, electrode roughness, etc), charge transfer resistance (Rct). Two parameters can characterize the CPE. The diffusion on the metal surface is represented by the Warburg element (W) that would appear at low frequency of the Nyquist plot (90, 91).
The EIS data can be displayed by two common methods, which are Nyquist and Bode plots.
Nyquist plots are usually used for mechanistic analysis due to mechanistic implications (e.g.
planar diffusion vs. pore diffusion) of number of relaxations (88). In the Nyquist plots the real part (Z’) is plotted on the x axis while the imaginary part (Z”) on the y axis. The low frequencies resemble to slower processes arising on the metal interface while the higher frequencies resemble reactions that are kinetically controlled as well as to the charge transfer processes that is usually related to the coating interface.
By the application of sinusoidal AC potential centered at the open circuit potential (EOCP) of the sample that causes it to cycle between the anodic and cathodic polarization as a result of a small AC current. Amplitude signal ≤ 10mV is applied (81) to the sample with the use of a potentiostat; the resulting current is analyzed to extract the phase and the amplitude relationship between the current and the voltage signals. Usually the impedance is measured as a function of frequency at a number of decades from 100 kHz to 1 mHz as an example (92). The low frequency choice is usually compromise between the resolution and the acquisition time, a common compromise between the time needed for data collection and accuracy is the choice of 5- 10 points per decade and a low frequency limit of 10- 1mHz. In Bode plots, the frequency is an independent variable where an accurate compression can be applied between the experimental and the calculated impedance spectra. In Bode graphs with the same data the logarithm of the modulus of the impedance where log |Z| vs. log ω is the Bode module plot and the phase angle (ϴ) vs. log ω is the Bode phase plot (8θ) (Figure γ.8).
Figure 3.8: EIS data interpretations.
33 3.5 Adsorption on metal surface
Adsorption is known to be the adhesion of ions, atoms, molecules of dissolved solids, liquid or gas that are called adsorbate to create a film on a surface, which is the adsorbent (93). This happens when a self-assembled monolayer of molecules is formed on a solid surface (94). The adsorption process on a metal surface is usually visualized by plots that show the concentration dependent change of the surface coverage on a solid surface. These graphs known as adsorption isotherms have several types of isotherms such as Freundlich, Langmuir and the Stephen Brunauer, Paul Emmet and Edward Teller which is called the BET isotherm. There are several factors that can affect the process of adsorption such as (93):
Condition of experiments, eg, pressure temperature, pH, etc.
Nature of adsorbent and adsorbate.
Activation of adsorbent.