« A new corrosion protection approach for aeronautical applications combining benzoxazine resins applied on sulfo-tartaric anodized aluminum » par M. Alexis RENAUD.

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Le 27 février 2018
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Campus Polytech - Auditoire 03

Soutenance publique de thèse de Doctorat

Promoteur : Marjorie OLIVIER


Résumé :

Aluminum alloys used in aircraft applications offer improved mechanical properties compared to
pure aluminum, but are strongly sensitive to corrosion. In the aircraft industry, the most classical
corrosion protection system consists in an electrochemically grown porous oxide layer coated with
an epoxy primer filled with corrosion inhibition pigments. Both layers contain hexavalent chromium
species: porous oxide layers are usually obtained from chromic acid anodizing (CAA) and classical
corrosion inhibition pigments are chromate salts. Hexavalent chromium provides an efficient and
durable corrosion protection in case of defect/damage. However, major health and environmental
issues are raised by such compounds. Alternative systems with a lower toxicity and comparable
performances should then be developed.
In this thesis work, alternative corrosion protection systems have been elaborated combining
porous oxide layers obtained from sulfo-tartaric anodizing (TSA) with benzoxazine resins. TSA is
one of the most promising alternatives to the toxic CAA. Benzoxazine resins display high thermal,
mechanical and chemical resistances, which can overtake classical epoxy resins.
Two laboratory-synthesized benzoxazines have been used: the Phenol-para-PhenyleneDiAmine
(P-pPDA) and the 4-EthylPhenol-para-PhenyleneDiAmine (4EP-pPDA). In a first stage, the
potential of such resins for corrosion protection applications has been evaluated by Electrochemical
Impedance Spectroscopy (EIS) performed on clear P-pPDA coatings applied on bare aluminum
alloys. As the high initial barrier properties appeared to progressively degrade when immersed in
a saline electrolyte, the curing process of the resin has been optimized in order to achieve a durable
protection. Cerium-based additives were also added to the formulation, improving the barrier
properties of the benzoxazine and their durability thanks to the good compatibility between the
matrix and the additives.
In a second stage, the two benzoxazines have been combined with porous oxide layers obtained
by TSA. The P-pPDA resin has been coated on top of the anodic layer, and the excellent interface
between the two layers offered high and durable barrier properties to the system. In the case of the
4EP-pPDA benzoxazine, the very low viscosity of the resin at high temperature permitted the full
impregnation of the porosity of the oxide during the curing step, leading to the sealing of the anodic
film. Obtained hybrid layers exhibited high and durable barrier properties, which again appeared to
be strongly impacted by the curing process duration and temperature.
Keywords: Aluminum; Corrosion; Benzoxazine resins; Organic coatings; Anodizing; Cerium;
Electrochemical Impedance Spectroscopy

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