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In vitro analysis of insoluble salt formation mechanism associated with Mg corrosion—variations depending on the diffusion environment in model tissue

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Magnesium (Mg) alloys have attracted attention as biodegradable metals, but the details of their
corrosion behavior under biological environment have not been elucidated. Previous studies have
suggested that diffusion through blood flow may influence Mg corrosion. Therefore, to understand
the degradation behaviors of Mg, we analyzed insoluble salt precipitation associated with Mg
corrosion in model tissue with different diffusion rates. A pure Mg specimen was immersed into a
model tissue prepared with cell culture medium supplemented by a thickener at a different
concentration (0.2%–0.5%) to form the gel. Micro-focus x-ray computed tomography of the gel
was performed to observe gas cavity formation around the specimen. The insoluble salt layer
formed on the specimen surface were analyzed by scanning electron microscopy with
energy-dispersive x-ray spectroscopy, and Raman spectroscopy. As results, gas cavity formation
was observed for all specimens. At day 7, the gas cavity volume was the highest at 0.5% thickener
gel followed by 0.3% thickener gel. The insoluble salts were classified into three types based on
their morphology; plate-like, granular-like, and crater-like salts. The crater-like salts were observed
to cover 16.8 ± 3.9% of the specimen surface immersed in the 0.5% thickener gel, at the specimen
area contacted to the gas cavity. The crater-like salts were composed by Mg hydroxide and
carbonate from the deepest to the top layer. In plate-like or granular-like salts, Mg carbonate was
formed in the deepest layer, but phosphates and carbonates, mainly containing calcium not Mg,
were formed on the surface layer. In conclusion, the increase in the thickener concentration
increased the gas cavity volume contacting to the specimen surface, resulting in the increase in
precipitation of Mg hydroxide and carbonate, composing crater-like salts. Mg hydroxide and
carbonate precipitation suggests the local increase in OH− concentration, which may be attributed
to the decrease in diffusion rate.

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  • 11/01/2024
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