Abstracts séléctionnés 2018

Progress in Corrosion Science at Atomic and Nanometric Scales. Maurice, V.; Marcus, P. Progress in Materials Science 2018, 95, 132–171. https://doi.org/10.1016/j.pmatsci.2018.03.001.
Contemporary aspects of corrosion science are reviewed to show how insightful a surface science approach is to understand the mechanisms of corrosion initiation at the atomic and nanometric scales. The review covers experimental approaches using advanced surface analytical techniques applied to single-crystal surfaces of metal and alloys exposed to corrosive aqueous environments in well-controlled conditions and analysed in situ under electrochemical control and/or ex situ by scanning tunneling microscopy/spectroscopy, atomic force microscopy and X-ray diffraction. Complementary theoretical approaches based on atomistic modeling are also covered. The discussed aspects include the metal-water interfacial structure and the surface reconstruction induced by hydroxide adsorption and formation of 2D (hyd)oxide precursors, the structure alterations accompanying anodic dissolution processes of metals without or with 2D protective layers and selective dissolution (i.e. dealloying) of alloys, the atomic structure, orientation and surface hydroxylation of ultrathin passive films, the role of step edges at the exposed surface of oxide grains on the dissolution of passive films and the effect of grain boundaries in polycrystalline passive films acting as preferential sites of passivity breakdown, the differences in local electronic properties measured at passive films grain boundaries, and the structure of adlayers of organic inhibitor molécules.

ToF-SIMS and ERDA Study of Hydrogen and Deuterium in Nickel-Base Alloys Oxidized in Water. Wu, X.; Seyeux, A.; Vickridge, I.; Voyshnis, S.; Marcus, P. Corrosion Science 2018, 140, 151–158. https://doi.org/10.1016/j.corsci.2018.06.006.
The depth profiling of hydrogen and deuterium in polycrystalline and monocrystalline Nickel-base alloys (Ni-16Cr-8Fe, wt%) was performed by Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Elastic Recoil Detection Analysis (ERDA) after oxidation in simulated primary medium conditions containing 9 at.% D2O. The use of deuterium isotopic tracer in ToF-SIMS has demonstrated the presence of deuterium in the oxide layer for oxidation times of 300 min and 100 h. The quantities of deuterium were determined by ERDA using a 2 MeV 4He ion beam. Finally, the penetration coefficient α was calculated, and ranged from 0.01 to 0.04.

Nanoscale Intergranular Corrosion and Relation with Grain Boundary Character as Studied In Situ on Copper. Bettayeb, M.; Maurice, V.; Klein, L. H.; Lapeire, L.; Verbeken, K.; Marcus, P. Journal of the Electrochemical Society 2018, 165 (11), C835–C841. https://doi.org/10.1149/2.1341811jes.
The initiation of intergranular corrosion at various types of grain boundaries (GBs) was studied at the nanometer scale on microcrystalline copper in 1 mM HCl aqueous solution. In situ Electrochemical Scanning Tunneling Microscopy (ECSTM) and Electron Back-Scatter Diffraction analysis of the same local microstructural region were combined using an innovative methodology including micro marking performed with the STM tip. The results demonstrate that electrochemically-induced intergranular dissolution, at the surface termination of GBs, is dependent on the grain boundary character. It is found that random high angle boundaries as well as Σ9 coincidence site lattice (CSL) boundaries are susceptible to nanoscale initiation of intergranular corrosion while for Σ3 CSL boundaries the behavior is dependent on the deviation angle of the GB plane from the exact orientation. For the Σ3 twins, a transition from resistance to susceptibility occurs between 1° and 1.7° of deviation as a result of the increase of the density of steps (i.e. misorientation dislocations) in the coincidence boundary plane. The work 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.

Role of Segregated Iron at Grain Boundaries on Mg Corrosion. Mercier, D.; Swiatowska, J.; Zanna, S.; Seyeux, A.; Marcus, P. Journal of the Electrochemical Society 2018, 165 (2), C42–C49. https://doi.org/10.1149/2.0621802jes.
To elucidate the role of noble metal impurities on corrosion of Mg, 3D time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging in combination with X-ray photoelectron spectroscopy and optical microscopy measurements were carried out on Mg samples (99.9%) before and after Mg polarization at EOCP+0.5 V in 0.1 M NaCl. A significant segregation of Fe (and of Mn and Al) metallic impurities at grain boundaries (GBs) was observed on the Mg surface by 3D ToF-SIMS. A 3-step mechanism of Mg corrosion was proposed, including a catalytic effect of Fe segregated at the GBs on the hydrogen evolution reaction (HER). In the 1st stage, the initiation of Mg corrosion is accompanied by the HER occurring over Fe impurities segregated at GBs leading to formation of small circular defects, and propagation with occurrence of dark filiform-like pattern corrosion enriched in Cl. In the 2nd stage, segregated Fe metallic particles are released by Mg matrix undermining, and are dissolved into Fe2+ ions and in the 3rd stage a redeposition of Fe in the clear areas of Mg surfaces, corresponding to areas with low concentration of Cl, takes place.