BLANC - Blanc

CAractérisation des Propriétés mécano-électrochimiques de SUrfaces traitées mécaniquement par Laser – CAPSUL

Submission summary

The precise influence of mechanical surface states on localized corrosion properties (pitting and crevice corrosion) is not clearly identified up to day. Interactions between mechanics and electrochemistry form the so-called mechano-chemistry growing thematic. Among many other ones, the main questions are : (1) which is the real influence of residual or applied stresses on the initiation of localized corrosion, (2) how does a global mechanical state affect the properties of protective surface films (thickness, resistance), (3) which is the real effect of a local mechanical state on electrochemical phenomena occurring around precipitates, or around grain boundaries. A number of recent works revealed improvements of the surface reactivity after mechanical surface treatment, which result seems to be contrary to the mechano-chemical analytical theories. However, these studies were limited by the different scales involved during mechanical analysis (a few µm) and corrosion initiation (in the nm range), and by the difficulty to isolate the mechanical contribution from the physico-chemical or metallurgical effects.Due to the low work-hardening levels involved and to the little modification of the surface roughness, Laser Shock Processing is expected to be a good candidate to analyse specifically the influence of stresses or microstructures on surface reactivity. For instance, recent works have demonstrated significant improvements of pitting corrosion behaviours of various stainless steels in chloride solutions, due to the generation of a compressive stress field by LSP.On the other hand, LSP has never been applied to the localized corrosion behaviour (intergranular or pitting) of aeronautical aluminium alloys, which is expected to be promoted by high tensile applied or residual stress fields In this context, the present proposal aims at : (1) applying , optimising and modelling the LSP process to generate compressive stress fields within two 7XXX aluminium alloys, (2) investigating the specific influence of LSP-induced stresses on electrochemical reactivity at a global and local scale (around precipitates, around grain boundaries), and proposing mechano-electrochemical criteria. This project will be supported by mechanical and electrochemical characterizations at a global and local scale, and by FEM modelling of LSP-induced stress fields, with a specific focus on active sites environment (precipitates-matrix interfaces). The 4 laboratories listed here below, and already interacting in the GER MeCAM non-official, and not granted, structure (www.u-bourgogne.fr/REACTIVITE/GERMECAM.htm/), will be the partners of CAPSUL : (1) the 'LALP' laboratory (UPR 1578 CNRS, 94414 Arcueil), for the LSP experiments and modelling, for global electrochemical experiments, and for AFM-STM surface analysis; (2) the 'ICB' lab (UMR 5613 CNRS-Univ.Dijon, 21078 Dijon), for mechanical and electrochemical local experiments (microXRD, electrochemical microcell technique and difference viewer imaging technique); (3 The 'LIM' lab (UMR 8006 ENSAM-CNRS, 75013 Paris) for conventional and pseudo grazing incidence x-ray diffraction; (4)The 'LGECO' lab (INSA, 67084 Strasbourg), for nano-indentation tests and morphological analysis. Gathering the interdisciplinary skills from these 4 labs, the CAPSUL project aims at understanding the real influence of 1st and 2nd order residual stresses on the surface reactivity of Al alloys, in the specific frame of pitting corrosion in chloride solution. A better understanding of mechanisms involved requires the development of the multi-disciplinary approach proposed in CAPSUL project. More widely, the project aims at predicting the influence of residual stresses induced by any process, irrespective of any other surface modifications, on the localized corrosion reactivity of passivable metal surfaces.The following concrete results may be expected from CAPSUL program :1) Evaluating and quantifying the influence of mechanical surface states (stresses and work-hardening amplitudes or gradients) on the localized corrosion properties of 7XXX Al alloys; 2) proposing mechano-chemical criteria for the two alloys considered, and identifying the cathodic (passivity) or anodic (dissolution around precipitates) origin of modifications induced. This also includes a characterization of oxide films (thickness + structure + resistance) grown on pre-stressed substrates, and inclusion-matrix interfaces; 3) Evaluating experimentally and providing a FEM model for the determination of global and local residual stress fields induced by LSP treatments; 4) Applying the mechano-electrochemical approach for solving an industrial issue : the mechanical and electrochemical behaviour of heat-affected zones (HAZ) in friction stir welds at the meso- and macroscale. Consequences on the pitting susceptibility and role of the microstructure and the surface treatment will be addressed.

Project coordination

Patrice PEYRE (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ILE-DE-FRANCE SECTEUR PARIS A)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ILE-DE-FRANCE SECTEUR PARIS A
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ILE-DE-FRANCE SECTEUR PARIS A

Help of the ANR 332,000 euros
Beginning and duration of the scientific project: - 36 Months

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