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Corrosion Initiation Mechanisms at the Nanometric/Atomic Scale

Periodic Reporting for period 2 - CIMNAS (Corrosion Initiation Mechanisms at the Nanometric/Atomic Scale)

Reporting period: 2019-03-01 to 2020-08-31

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 is 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 imitation mechanisms.

The project is 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 include 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.

It is envisioned that the model approach used and the achieved breakthroughs will open up a new horizon for research on corrosion initiation mechanisms at the nanoscale, and new opportunities for a knowledge-based design of novel corrosion protection technologies.
"Task #1: Understanding the stability of surface oxide films
The work has been conducted on model austenitic stainless steel single crystal surfaces on which reactivity towards oxygen was well controlled with sub-monolayer precision in order to better understand, at the nanometric and atomic scales, the mechanisms leading to the Cr enrichment in the surface oxide and their key role in providing passivity and stability to these alloys. The results obtained so far emphasize that the pre-passivation oxidation mechanism is at the origin of iron-rich structural/chemical heterogeneities/defects generated at the nanoscale in the surface oxide and thought to be weak points for the stability of the protective oxide film.

Task #2: Understanding the corrosion initiation and passivation at the surface terminations of grain boundaries
The work has been conducted on model copper microcrystalline samples using a novel methodology combining local microscopic in situ surface characterization of corrosion under electrochemical control (EC-STM) with local ex situ analysis of the microstructure (EBSD). The results obtained so far emphasize the differences in corrosion initiation at the nanoscale and passivation behaviors between complex (random) and more simple (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.

Task #3: Understanding of the corrosion inhibition of surfaces not uniformly passivated.
The work has been conducted on model copper single crystal surfaces in well controlled metallic or pre-oxidized state with deposition of 2-mercaptobenzothiazole organic inhibitor molecules from the vapor under ultra-low pressure. The results obtained so far emphasize 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 and thus possibly better protective. DFT modelling shows that the adsorbed molecule can bond to both metallic copper atoms and to unsaturated oxygen atoms of copper oxide on partially oxidized copper surfaces.
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"Progress achieved on understanding the stability of surface oxide films (Task #1) is beyond the state of the art. Cr enrichment in the surface oxide film was already well-established as being the key to passivity of stainless steels, but the origin of nanoscale heterogeneities of Cr-enrichment self-generated by the oxidation (pre-passivation) mechanisms was unknown. Further work should help establishing that such nanoscale heterogeneities/defects are weak points for passivity in aggressive environments and understanding how they are cured in Mo-bearing alloys or can be reinforced by acting on the pre-passivation mechanisms.

Progress achieved on understanding corrosion initiation and passivation at the surface terminations of grain boundaries (Task #2) is also beyond the state of the art. 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 has never been interrogated, neither its 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. Further work should bring new nanoscale insight on the inhibition mechanisms at the surface terminations of grain boundaries in the presence of corrosion inhibitors and how the relation with the grain boundary character is modified.

Progress achieved on the understanding of the corrosion inhibition of surfaces not uniformly passivated (Task #3) is beyond current knowledge brought by previous experimental studies, including on the most studied benzotriazole/copper system for which the adsorption of the inhibitor was only studied on copper surfaces well controlled in the metallic state. 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 on well-controlled surfaces. Progress achieved on partially oxidized surfaces with DFT modelling is beyond current knowledge obtained by DFT modelling of the benzotriazole/copper system. Further work should bring similar new insight on the interaction of copper surfaces with the 2-MBI (2-mercaptobenzimidazole) inhibitor.
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