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Abstract :
[en] In automotive industry, the body car is composed of different metallic substrates such as steel, galvanized steel and aluminium alloys of 6xxx series. The corrosion protection of all the substrates is often obtained by a cataphoretic coating. For economic reasons, steel is more and more used but this substrate is very reactive and sensitive to corrosion and particularly to filiform corrosion. This specific type of corrosion is produced between the coating and the substrate by formation of filaments. The aim of this work is to study by electrochemical impedance spectroscopy the susceptibility to filiform corrosion of steel.
Electrochemical impedance spectroscopy can be used to evaluate in short time the sensitivity to filiform corrosion of the different substrates through the estimation of the exposed metallic area. This is possible if the corrosion products are correctly dissolved before characterization. Their presence may indeed hinder the metal activity leading to an underestimation of the delaminated area.
Different steel grades with and without pretreatment were analysed by EIS on scratched samples.
In order to remove the corrosion products and to avoid subsequent corrosion during the measurement, many parameters were optimized such as the choice of electrolyte solution, acid and dissolution time, EIS electrolyte and immersion time before EIS measurements, exposure time to climatic chamber. The steel is a very complex substrate and thus an additional cathodic polarization step must be added (time and potential to define) to evacuate corrosion products and homogenize pH underneath the delaminated coating.
