Corrosion Initiation Mechanisms at the Nanometric/Atomic Scale

Informations projet

CIMNAS

N° de convention de subvention: 741123

DOI

10.3030/741123

Projet clôturé

Date de signature de la CE 19 Mai 2017

Date de début 1 Septembre 2017

Date de fin 28 Février 2023

Financé au titre de

• EXCELLENT SCIENCE - European Research Council (ERC)

Coût total

€ 1 657 055,52

Contribution de l’UE

€ 1 657 055,52

Coordonné par

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 4 - CIMNAS (Corrosion Initiation Mechanisms at the Nanometric/Atomic Scale)

Période du rapport: 2022-03-01 au 2023-02-28

Résumé du contexte et des objectifs généraux du projet

The failure of metallic materials caused by corrosion strongly impacts our society with cost, safety, health, and performance issues. The mechanisms of corrosion propagation are fairly well understood, and various means of mitigation are known even if research is still necessary to improve this knowledge or to develop corrosion protection for the application of new materials. The vision of CIMNAS is that a major breakthrough for corrosion protection lies in a deep understanding and control of the initiation stage triggering corrosion. Corrosion initiation takes place at the atomic/molecular scale or at a scale of a few nanometres (the nanoscale) on metal and alloy surfaces, metallic, oxidised or coated, and interacting with the corroding environment.

The mission of CIMNAS was to challenge the difficulty of understanding corrosion initiation at the nanometric/atomic scale on such complex interfaces, ultimately aiming at designing more robust metallic surfaces via the understanding of corrosion initiation mechanisms.
The project was structured in three tasks to achieve three knowledge breakthroughs, each implementing a new vision of corrosion science and addressing a key issue for the understanding of corrosion initiation on metal and alloy surfaces: (i) Understanding the stability of surface oxide films, (ii) Understanding corrosion initiation versus passivation at the surface terminations of grain boundaries, (iii) Understanding the corrosion inhibition mechanisms of surfaces not uniformly passivated.

Resources included a team of highly experienced and recognised researchers headed by the PI, a unique apparatus recently installed at the PI’s lab, integrating surface spectroscopy, microscopy, and electrochemistry for in situ measurements in a closed system, novel experimental approaches, and a strong complementarity of experiments and modelling.

The ambition of the project was high. The scientific expertise of the PI and his team, the equipment available at the PI’s lab, the resources provided by the ERC were perfectly aligned with this ambition, so the action was fully implemented, and the objectives of the research program have been reached.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

The work on Task #1 emphasizes that the pre-passivation oxidation mechanism is at the origin of Fe-rich structural/chemical heterogeneities/defects generated at the nanoscale in the surface oxide and compromising the stability of the protective oxide film. Further work performed on Mo-bearing stainless steel and on new Cr-Fe-Co-Ni-Mo multi principal element alloys (MPEA) revealed that these Fe-rich weak points of protection are sealed by the enrichment in Mo, which enhances passivity in aggressive environments. Pretreatment by controlled surface pre-oxidation was also found instrumental to promote the initial Cr and Mo enrichments, thus paving the way for an innovative strategy for enhancing the resistance to localized corrosion of stainless alloys.

The work on Task #2 emphasizes the differences in corrosion initiation at the nanoscale and passivation behaviors between complex (random) and simpler (Coincidence Site Lattice) grain boundaries, as well as the key role of the deviation of the grain boundary plane from perfect geometry in the initiation of corrosion for most simple (twin) boundaries. Investigating the effects of organic inhibitors on the local corrosion properties of emerging grain boundaries showed that the inhibition efficiency against active dissolution depends on the GB character with the more reactive random boundaries being less efficiently protected. Electrochemical oxidation is poisoned by pre-adsorbed inhibitor layers, however with residual reactivity characterized by residual dissolution or passivation depending on the local barrier effect of the pre-formed inhibitor layer on oxide formation.

The work on Task #3 emphasizes the role of the surface oxide in preventing partial dissociation of the adsorbed molecules and promoting the formation of molecular layers more homogeneous at the nanoscale, as well as the destabilizing effects of water vapor reacting with pre-adsorbed organic multilayers. DFT modelling shows that the adsorbed inhibitor molecules can bond to both metallic copper atoms and to unsaturated oxygen atoms of copper oxide, and thus can protect partially oxidized copper surfaces. In aqueous solution, the results emphasize the enhanced inhibition obtained by electrochemical control of the interface buildup with removal of the native oxide.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

Progress achieved in Task #1 is beyond the state of the art. Cr enrichment in the surface oxide film was already known to be a key factor for passivity of stainless steels, but the origin of nanoscale heterogeneities of Cr-enrichment self-generated by the oxidation (pre-passivation) mechanisms was unknown, and unveiled by the ERC CIMNAS work. The effect of Mo on corrosion resistance in aggressive environments was known, but the detailed mechanisms were unknown, or only hypothesized. The role of Mo species, sealing the nanoscopic weak points in the passive film, was elucidated in the ERC CIMNAS work, achieving progress beyond current knowledge.

For Task #2, the dependence of intergranular corrosion on the crystallographic character of grain boundaries has been long debated. However, the nanoscale initiation of dissolution as well as the passivation of grain boundary terminations had never been interrogated, neither their local relationship with the grain boundary crystallographic character. The work performed emphasizes the precision needed in the design of the grain boundary network in applications where intergranular corrosion or its initiation must be controlled at the nanoscale. The effects on organic inhibitor on local dissolution and passivation at the surface emergence of grain boundaries were also unknown and the CIMNAS action brings new nanoscale insight on local inhibition mechanisms at emergent microstructure defects.

Progress achieved in Task #3 is beyond current knowledge brought by previous experimental studies. The role of the surface oxide on the adsorption of the 2-mercaptobenzothiazole organic inhibitor molecules as well as on the nanoscale morphology of adsorbed molecular layers was never interrogated before on well-controlled surfaces. Progress achieved on partially oxidized surfaces with DFT modelling is beyond current knowledge obtained by DFT modelling.

Results were disseminated via 41 scientific articles published in international journals. 69 oral presentations, including 29 invited ones, were given by the PI and permanent members of the team as well as by PhD students at international conferences among the leading ones in the field of corrosion.

The fundamental scientific achievements in Task #1 have opened the way to the design of an innovative thermo-chemical surface treatment that would enhance the durability of metallic materials, and an ERC-POC project entitled Surface Oxide Nanoengineering has been submitted.

The CIMNAS results have opened the way to a new field of research, namely corrosion science at the atomic or nanometric scale, using a surface science approach. Followers are already observed in different countries in Europe, USA and likely soon in China.
The beneficial societal impact of this new field is significant, due to the importance of enhanced durability of metallic materials in the context of zero net CO2 emission for mitigating the climate change.

Corrosion inhibition of copper and role of surface oxide at nanometric and molecular scales

Novel methodology for local analysis of corrosion properties at grain boundary terminations

Surface structure of austenitic stainless steel at the nanometric and atomic scales

Integrated Surface Analysis, Imaging and Electrochemistry under Gas and Liquid Environments