Dear Dr. Askar Soltani ,

I suggest you to have a look at the interesting, following technical note:

-Basics of Electrochemical Impedance Spectroscopy By Gamry Instruments

Available at: https://www.gamry.com/application-notes/EIS/basics-of-electrochemical-impedance-spectroscopy/

and at the following paper:

-Electrochemical Impedance Spectroscopy for the Measurement of the Corrosion Rate of Magnesium Alloys: Brief Review and Challenges

Sebastián Feliu, Jr.

Metals, 10(6), 775; https://doi.org/10.3390/met10060775 (2020)

Available at: https://www.mdpi.com/2075-4701/10/6/775/htm

Best regards, Pierluigi Traverso.

Dear Askar,

The double layer is a thin film layer and its thickness actually depend on the electrolyte properties such as concentration, ionic species and experimental conditions such as voltage, frequency, scan rate( CV). This is useful in the study of adsorption–desorption behaviour of the inhibitors studied using capacitance-potential and the constant phase element exponent-potential profiles obtained from dynamic electrochemical impedance spectroscopy measurements. The double layer capacitance values can show a rather consistent increasing trend with some theoretical parameters namely, the fraction of transferred electrons, the highest occupied molecular orbital energy level, and the permanent dipole moments of the inhibitor molecules. In corrosion studies, this is advised that the value of capacitance double layer or constant phase element should be in the range of 1E-6 to 1E-4 Fcm-2. In fitting EIS data (in corrosion of metal alloy studies) by means of electrical equivalent circuit (EEC), double layer capacitance (Cdl) value or constant phase element (Q) value in the circuit should be higher than 1E-4 Fcm-2.

Ashish

Dear Ashish,

Thanks fir your conprehensive answer. As you said double layer capacitance properties depend on some factors like the ionic power of the electrolyte and inhibitors properties and soe other factors including temperature and etc.

Based on the capacitance formula if we could decreae dielectric coefficient between two capacitor surfaces we would able to decrease the capacitance value and therefore decrease the Q amount and related current passed the capacitor. So, if we substitute the water molecules in diuble layer by inhibitor molecules which have lower dielectric coefficient we would able to decrease the current passed the capacitir and therefore decrease the corrosion current. In corrosion studies double layer capacitance is normaly in the prder of 20-60 micro F/cm2. By adding inhibitors into the solution we decrease the capacitor’s value and get less corrosion rates. So, please discuss about the ciruit model when two capacitors including doule layer capacitance and inhibitor’s capacitance are formed on the surface of the metal. Do you believe that inhibitor molecukes substitute by water molecules ir they form another capacitance in parallel with double layer capacitance and what woukd be differences and effects in these two different ideas ?

Dear Dr Ashish Thakur

thanks for your conceptual answer. I want to discuss on electrical concepts of Cdl, when inhibitors are injected into the solution. As you know, in AC circuits, Cdl is periodically charged and discharged because there is a phase difference of 90 degree between sinusoidal applied voltage and current. So, when double layer capacitance is periodically charged and discharged, how we can say that for corrosion mitigation we need to decrease dielectric coefficient in double layer capacitor ?