On 6th april 2018, Samuel Leleu has defended his thesis. Awarded by the University of Toulouse, his work was supervised by both CIRIMAT laboratory (CNRS/UT3 Paul Sabatier/INP), the Science de la Matière doctoral school, and IRT Saint Exupéry.
The high quality and the relevance of Samuel’s thesis contributed to get significant results as a part of SURFINNOV project.
THESIS SUBJECT
“Toward a new “green” and efficient corrosion protection method of magnesium alloys for the aeronautic industry.”
About his thesis
Magnesium alloys represent an alternative to aluminum alloys and composite materials – especially in the aeronautic sector with the aim of reducing the structural mass. The main goal of this thesis is to participate in the development of new protection methods that would be more efficient and environmentally-friendly. To this end, electrochemical techniques – such as electrochemical impedance spectroscopy (EIS) – were used together with atomic force microscopy, inductively coupled plasma atomic emission spectrometry, time-of-flight secondary ion mass spectrometry and standardized industrial tests. First, the corrosion resistance in Na2SO4 solution of three magnesium alloys containing rare earths (WE43, EV31 and ZE41) was studied and compared to those of two aluminum-rich magnesium alloys (AZ31 and AZ91) and pure magnesium. For all the alloys, it was shown that intermetallic particles act as local cathodes. This galvanic coupling was stronger in the case of rare earths magnesium alloys, especially for the EV31 alloy. Their corrosion was controlled at the same time by the dissolution of magnesium-rich matrix and by the progressive covering of the alloy surface by an oxides/hydroxides layer. This film was more protective in the case of the alloys; but it is not sufficient to counterbalance the detrimental effect of intermetallic particles. Ultimately, the addition of rare earths increases the corrosion rate of magnesium alloys in Na2SO4 solution compared to aluminum magnesium alloys or pure magnesium. Concerning the WE43 alloy, it was shown that the protective oxide film was thinner and more stable than the one formed on pure magnesium, especially in the presence of chloride ions. These results were explained by the incorporation of alloying elements, like yttrium, that should be responsible of the formation of a more compact film. Then, several protection methods were considered to obtain a corrosion-resistant system that would be compatible with key requirements of the aeronautic industry. The combination of the anodization treatment developed by Prodem Company, called CEP, and the water-based or high-solid coatings offered by Mapaero Company was assessed and compared to classical chromated coatings. It was concluded that current chemical conversion treatments did not provide enough protection for magnesium alloys. The CEP treatment porous morphology acts as an efficient primer base. The best performances were obtained for the CEP treatment in combination with high solid chromate-free coatings. Complementary analysis showed that, with an additional varnish layer, the developed protection system is compatible with aeronautic key requirements.
SURFINNOV Project
Developing corrosion protection solutions for light alloys by improving the tribological properties of titanium and improving surface treatments versus the temperatures required.
Jury
- Mme. Nadine PEBERE – CNRS/CIRIMAT – PhD Advisor
- Mme. Karine GROENEN-SERRANO – Toulouse University/LGC – Examiner
- M. François-Xavier PERRIN – Toulon University/Mapiem – Examiner
- M. Bertrand RIVES – STELIA/IRT Saint Exupéry – Examiner
- Mme. Delphine VEYS-RENAUX – Univerisity of Lorraine/IJL – Rapporter
- M. Vincent VIVIER – CNRS/ LISE – Rapporter
- M. Laurent BORTOLOTTO – Airbus Helicopters – Invited
- Mme. Pauline CÔTE – MAPAERO – Invited
- M. Mathieu NICOLAS – PRODEM – Invited
PUBLICATIONS
N. Pébère, S. Leleu, N. Causse and B. Rives: “Corrosion resistance of rare-earth Mg alloys in a Na2SO4 solution”. 2017
S. Leleu, N. Pébère and B. Rives: “Evaluation of corrosion performance of surface treatments on aeronautical magnesium alloys by the impedance technique”. EUROCORR 2016, Montpellier (France). 2016
S. Leleu, N. Pébère and B. Rives: “Investigation of the corrosion behaviour of magnesium alloys by electrochemical impedance spectroscopy (EIS)”. ISE (International Society of Electochemistry, The Hague (Netherlands). 2016
S. Leleu, N. Pébère and B. Rives: “Caractérisation de la résistance à la corrosion d’alliages de magnésium par spectroscopie d’impédance électrochimique”. JTE (Journée Toulousaine d’Electrochimie), Toulouse (France). 2016
S. Leleu, B. Rives and N. Pébère: “Contribution to the understanding of the corrosion inhibition mechanisms on magnesium alloys”. Airbus Structure Days, Madrid (Spain). 2015