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Avancées récentes sur l'analyse des données d'impédance globale et développement de l'impédance électrochimique locale : application aux revêtements utilisés pour la protection contre la corrosion de l'alliage d'aluminium 2024

Abstract : The aim of this work is to obtain a better understanding of the degradation mechanisms, as a function of exposition time in an aggressive environment, of commercial coatings (epoxy-polyaminoamide waterborne paint) used in aeronautical industry for corrosion protection of 2024 aluminium alloy by global and local impedance techniques. The coatings formulated with either strontium chromate (SrCrO4) or Cr(VI)-free pigments were compared. The behavior of dry coatings (in contact with Hg) was close to that of an ideal capacitor and could be accurately modelled with the power-law model corresponding to a CPE (Constant Phase Element). Upon immersion in NaCl solution, the behavior of the wet coatings became progressively less ideal, i.e. farther from a capacitive behavior. The impedance data was analyzed with the Young model that take into account the exponential variation of the coating resistivity along its thickness. This analysis confirmed that penetration of water and ions occurs on different time scales. The former process is faster and affects permittivity more strongly than resistivity; the latter is slower and affects almost exclusively resistivity. The models allows to explain not only the CPE or pseudo-CPE behaviors observed on impedance diagrams but also to determine the coatings water uptake during the test which is in good agreement with gravimetric measurements. Then, the self-healing properties of artificially damaged coatings were studied by local electrochemical impedance spectroscopy (LEIS). The local diagrams and the mappings (2D or 3D) allowed self-healing processes for the chromated system or corrosion developments for the unchromated system to be observed. The present work proposed a methodology to develop and to characterize coatings containing environmentally friendly inhibitors, and particularly the self-healing process.
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https://tel.archives-ouvertes.fr/tel-01700997
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Anh Son Nguyen. Avancées récentes sur l'analyse des données d'impédance globale et développement de l'impédance électrochimique locale : application aux revêtements utilisés pour la protection contre la corrosion de l'alliage d'aluminium 2024. Matériaux. Université Paul Sabatier - Toulouse III, 2016. Français. ⟨NNT : 2016TOU30326⟩. ⟨tel-01700997⟩

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