The effect of hydrogen on the early stages of oxidation of a magnesium alloy

Abstract

The early stage oxidation of germanium-doped magnesium alloys was investigated via atom probe tomography. A catalytic reaction cell attached to a local electrode atom probe was used for controlled exposure of electropolished specimens to oxygen gas. Hydrogen was detected in certain samples, thought to be introduced during electropolishing. A preferential interaction/reaction between oxygen and hydrogen was found in samples containing hydrogen-rich regions prior to oxidation. These results suggest that hydrogen plays an important role as a catalyst to oxidation, where the increase in the H evolution rate experienced when oxidising Mg alloys, may be directly related to the presence of H within the material prior to oxidation. Results revealed the ability of this unique experimental setup to provide new information about surface/sub-surface hydrogen and its effects on magnesium oxidation. Main Text The natural abundance of magnesium (Mg) and desirable strength to weight ratios make lightweight Mg alloys appealing candidates for application within the transport industry. However, their wider application as structural components is hindered by poor corrosion resistance [1, 2] and a susceptibility to hydrogen embrittlement [3-5]. Hydrogen (H) is absorbed into engineering alloys on contact with gaseous molecular hydrogen (H2), or by a parasitic partial reaction that accompanies the hydrogen evolution reaction (HER) during metal corrosion [6]. Atomic H typically either permeates or diffuses through metal lattice interstices and may become trapped at defects such as vacancies, grain boundaries, or dislocations [6, 7]. It may also form hydrides with suitable metals [8]. The diffusible H concentration (CH, Diff) represents the resident, mobile H atoms within alloys that can readily diffuse towards regions of tri-axial stress resulting in H-assisted cracking or H embrittlement

Nanoscale Perspectives of Metal Degradation via In Situ Atom Probe Tomography

“…Recent developments in APM offer promise for investigating the complex composition and structure changes accompanying gas/surface interactions. The in situ reactor, for APT analysis first described by others [6,[36][37][38][39] and more Fig. 9 Variations of the composition in Co n+ , Co 2 O n+ , CoO n+ , O n+ and O 2 n+ species detected (at% on the left scale and the O:Co ratio on the right scale) as a function of the static voltages (V stat ) and pulse frequencies applied broadly for APM analysis reported here and elsewhere [5], provides a new pathway for understanding atomic-scale processes involved in gas/surface reactions in a wide range of material systems, from single crystals of pure metals like Rh, to more complex alloys such as zirconium alloys.…”

“…The ability to experimentally control the ambient reaction conditions relevant to an atomic to nanoscale understanding of oxidation or catalysis on a metal or metal alloy surface is enabled by a reactor connected to an atom probe instrument. The design, fabrication, and demonstration of a gasphase reactor attached to an atom probe has been previously reported by Bagot and Visart de Bocarmé et al, and was demonstrated to explore reactive gas interactions on bimetallic metal systems: H 2 /Pd [36], O 2 /Mg alloy [36,37], NO/ PtRh [38], O 2 /PtRh [6] and O 2 /AuAg [39]. They reported local chemical segregation within the bulk and on material surfaces such as Ag-and Rh-enriched surface layers induced by O 2 exposure of AuAg and PtRh respectively, or Rh depletion caused by NO adsorption on PtRh.…”

“…We hypothesize that this acceleration probably reflects the fact that the oxygen atoms penetrate the Co needle isostatically from every direction. Note that here we are considering this penetration with only one dimension and thus is only a first approximation [37]. The next steps of this work will consist of improving the reconstruction scaling based on correlative TEM imaging of the needle shape taken before/after oxidation.…”

“…Beyond catalytic materials, the in situ APT approach is also valuable for studying early stages of alloy oxidation relevant to various applications, from nuclear energy to structural materials [37,54]. Traditionally, alloy oxidation behavior is studied with longer-term experiments (on the order of several to tens to even hundreds of days) in which samples are exposed to the oxidative or corrosive environment of interest.…”

Rapid assessment of structural and compositional changes during early stages of zirconium alloy oxidation

“…The recently developed reaction chambers attached to the vacuum systems of APT instruments have been used to study composition changes of oxide layers. These chambers have demonstrated their usefulness in analysing near-surface compositional changes during oxidation of catalyst materials such as Pt-Rh, Pt-Ru, Pt-Ru-Rh, Pd-Rh, and Rh 5,6,37 and Mg alloys 38 .…”