Fig. 1. Sample information. (a) SEM sideview image of a typical magnesium pillar with the diameter of ~ 100 nm. (b) Bright-field TEM image of the pillar framed by white dashed rectangle. (c) High resolution TEM image and the corresponding EELS mapping (d) of the magnesium-oxide interface, indicated by the black dashed frame in (b).

Fig. 2. Dynamic oxidation process and mechanism of Mg from room temperature to 200 °C. (a) TEM images showing the damage process of native oxide scale including the voids formation, voids growth and oxide scale cracking, as well as the following process of MgO ridges’ growth from cracks at high temperatures. The inset selected area electron patterns indicate the initial magnesium pillar with a thin amorphous oxide surface layer is oxidized into MgO completely at 200 °C. (b) Sketches showing the thermal oxidation mechanism. i. Oxygen adsorbed on the native oxide surface at room temperature. ii. With increasing temperature, the outward Mg2+ ions diffusion channeled by the already existing vacancies is accelerated along with oxide scale thickening gradually. iii. The simultaneous inward vacancy diffusion leads to vacancy segregation forming voids, which, together with the tensile internal stress induced by the MgO scale thickening, cause crack formation in the oxide scale. Oxygen penetrates inward and reacts with fresh Mg at cracks. iv. Catastrophic oxidation occurs with fast MgO ridge growth forming a spongy morphology.

Fig. 3. Thermal oxidation inhibition effect of MgCO3 scale directly transformed from the native oxide scale on the surface of magnesium. (a) Bright field TEM image and the corresponding SAEDP of a typical Mg pillar fabricate by FIB. (b) The pillar in (a) after carbonation inside E-TEM under 4 Pa CO2 and electron beam irradiation. Its SAEDP illustrates that carbonated surface layer is composed of the newly formed MgCO3 and some remained MgO, which are marked by the yellow and blue dashed curve, respectively. The main body of the pillar after treatment is still Mg crystal. (c) The carbonated pillar with MgcO3 scale was heated from room temperature to 350 °C in ~4 Pa oxygen. At 250 °C, SAEDP was taken, and compared with the diffraction patterns in (b), it shows no obvious change. Chose one diffraction spot (marked by the red circle) from the {024} diffraction ring of MgCO3 to take the corresponding dark-field TEM image (framed by red), showing that MgCO3 mainly distributes on the surface. Approaching 350 °C, the TEM projection image of the pillar looks like shortened because the weld (connection of hotplate and magnesium lamella base) fracture at high temperatures. Actually, during the entire heating process in oxygen, the tested pillar kept intact and free from obvious oxidation.

Fig. 4. Comparison of size changes of the untreated and carbonated magnesium pillars during the oxidation at different temperatures. For each pillar, the size changes of two different parts were measured (the top right inset schematic diagram). The black and red curves represent the size changes of the Mg pillar with native oxide layer and the Mg pillar with MgCO3 scale, respectively. For the untreated pillar, its size changes dramatically along with temperature, and the changes themselves are temperature dependent, indicating the oxidation mechanism transforming from diffusion controlled to chemical reaction controlled. While, for the carbonated pillar, its size almost keeps a constant, suggesting no obvious oxidation. The inset schematics illustrate the vacancy diffusion induced initial oxidation of Mg with oxide layer (left, black) and the oxidation inhibition effect of MgCO3 layer with few vacancies (right, red), respectively.