(Invited) Organic Coatings for Corrosion Protection: Highlighting the Polymer Molecular Mobility Contribution to the EIS Response
Authors: Pebere Nadine , Causse Nicolas , Dantras Eric , Jero Deni , Roggero Aurélien 
Published: December 2024

Organic coatings are the single most widely applied method for corrosion protection of metallic materials. The coatings have generally low permeability to water and oxygen [1,2] and high electrical resistivity (specifically ionic resistance) to impede electrochemical reactions [3,4]. Protective performance of organic coatings are commonly assessed by electrochemical impedance spectroscopy (EIS) [5]. Monitoring of impedance diagrams over immersion time in an electrolytic solution generally allows corrosion appearance in the impedance spectra to be detected, hence predicting the coating’s lifetime. Moreover, most failure modes leading to corrosion damage at the macroscopic scale do not occur without water uptake first taking place [6,7]. EIS can be used to monitor the water uptake [8-11] but rarely at elevated temperatures [12].

In the field of polymer physics, molecular mobility is of prime importance as it governs the physical behavior of these macromolecular material. Broadband dielectric spectroscopy (BDS) is a widespread technique in the polymer physics community as it yields the dipole relaxation modes associated with molecular mobility in polymers [13].

EIS studies of organic coatings hardly ever approach the barrier properties from a molecular mobility point of view. In recent papers [14-18], our group has studied industrial organic coatings with the objective of identifying the contribution of the molecular mobility on their electrochemical impedance responses. The coatings were first analyzed in the dry state by means of BDS, and then in the wet state by EIS. By performing EIS measurements at various constant temperatures and by applying the dielectric permittivity formalism to the impedance data, the signature of the molecular mobility (α-mode, dielectric manifestation of the glass transition) was shown in their impedance response.

Through our recent studies, the presentation will highlight that temperature-controlled EIS (including ambient temperature) brings insights in the molecular mobility of protective organic coatings. Over long immersion times, such studies identified the influence of plasticization (linked to water uptake) or ageing of the polymer matrix (leaching of plasticizers, for example), which has a direct impact on the coatings performance against corrosion. The dielectric manifestation of the glass transition was evidenced from EIS measurements through the use of the dielectric permittivity formalism.

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