Analysis of a film-forming amine response in impedance spectra

In this study, a detailed analysis of the dielectric response of a film-forming amine (1,3 N-oleyl propanediamine (OLDA)) in impedance spectra is presented, addressing both "bulk" OLDA and adsorbed OLDA layer on a carbon steel surface. The dielectric response of the "bulk" OLDA in the absence of electrolyte over an extensive temperature range (-150 degrees C to 50 degrees C) was explored by broadband dielectric spectroscopy (BDS). Subsequently, the response of the OLDA adsorbed layer on the steel surface was analysed by electrochemical impedance spectroscopy (EIS), for various immersion times (2 min to100 min) and temperatures (25 degrees C to 75 degrees C) in an aqueous NaCl solution. Impedance spectra (BDS and EIS), interpreted through the complex conductivity formalism, showed a resistive behaviour in the mid-frequency range and a capacitive behaviour at higher frequencies. The high frequency capacitive response in EIS spectra was shown to contain contributions from the OLDA layer and from the electrochemical double layer, as well. The temperature dependency of the OLDA conductivity was found to be similar to the temperature dependency of the electrolyte resistance, suggesting that the electrolyte (water and ions) crossed the OLDA layer to reach the steel surface. The presence of gauche defects in the OLDA alkyl chain, revealed by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) analyses, may be a contributing factor to the disorder in the adsorbed layer, facilitating electrolyte infiltration.