Impedance analysis of a film-forming amine for the corrosion protection of carbon steel
Résumé
During shutdown periods of pressurized water reactors (PWR), corrosion phenomena can affect some components of the secondary circuit. The use of environmentally friendly corrosion inhibitors such as film-forming amines (FFA) is an interesting approach to attenuate or to stop the corrosion phenomena that can affect carbon steel, which constitutes the major part of the circuit.The aim of the study is to investigate the behaviour of a FFA for the protection of carbon steels under PWR representative conditions. A carbon steel rod of 1 cm2 cross-sectional area was used as rotating disk working electrode. The test solution was 10-3 M Na2SO4 prepared with deionized water. The pH was adjusted with ammonia. A heating bath was used to control the electrolyte temperature. Electrochemical impedance data were analysed to extract physical parameters of the fatty amines (FA) thin film formed on a carbon steel surface. The analysis was based on the use of the power-law model corresponding to a constant phase element (CPE) behaviour [1-3]. The electrochemical results were supplemented by surface observations and drop angle measurements. The film thickness was estimated from X-ray photoelectron spectroscopy (XPS).EIS data analysis allowed the permittivity value and the FA film thickness to be determined. An in-depth resistivity profile was calculated from the impedance results which highlighted a high resistivity at the metal/FA film interface. The extracted film thickness was about 20 nm. XPS analysis confirmed the presence of a very thin and homogeneous organic layer and the presence of iron oxide/hydroxide layer underneath the FA film. The total layer thickness (oxide and FFA) was lower than 10 nm. The difference in thickness between EIS measurements and XPS analysis can be explained by the ripening effect when the film was removed from the electrolyte and kept in dry conditions, damaging the weakly adsorbed layers of FFA.This work brought some insights and methodologies for the in situ analysis of organic thin films on conducting surfaces particularly for the determination of parameters, namely the thickness and the permittivity, that are usually difficult to access.