Magnesium alloys are light-weight and cost-effective materials for structures, transportation parts, power-generation units, and temporary medical implants; however, their applications under chlorides are hampered by film deterioration, pitting, and hydrogen-evolution corrosion. This study focuses on the dry atmospheric CO$_2$ plasma treatment of high-purity magnesium, which results in the creation of carbonaceous and oxygen-rich Mg film comprising MgO, MgCO$_3$, and carbonates. Commercial Mg samples with about 99.98\% purity were ground with SiC paper and exposed to atmospheric plasma created from 100\% CO$_2$. The amount of the plasma exposure was set to one, three, and five swings. Pure Mg sample 600 grit proved to be highly hydrophilic, having a contact angle less than 5$^\circ$; whereas one-, three-, and five-swing CO$_2$ plasma treatment of Mg resulted in a contact angle of 136.7 $\pm$ 14.3$^\circ$, 138.1 $\pm$ 5.6$^\circ$, and 151.3 $\pm$ 9.6$^\circ$, correspondingly. Electron microscopy revealed the existence of converted layer approximately 1--1.5 $\mu$m thick, being composed of the compact and porous carbon-containing outer layers. XRD analysis of Mg films showed peaks at angles close to 32.0$^\circ$, 43.0$^\circ$, 46.5$^\circ$, and 52.0$^\circ$ for MgCO$_3$, as well as 37.2$^\circ$ and 43.0$^\circ$ for MgO. Using XPS, carbonate carbon peak at 292 eV, carbon-carbon bonding at 285 eV, and O--C/O--Mg binding in O 1s region were also revealed. The effect of CO$_2$ plasma on electrochemical behavior of Mg was studied in 3.5 wt.\% NaCl solution. Both cathodic and anodic currents for hydrophobic plasma-treated Mg significantly diminished. Hydrophilic plasma-treated Mg possessed higher impedance values than that of untreated one after being immersed into NaCl. Evolution of hydrogen gas for untreated Mg sample of 120 grit was approximately 15 mL cm$^{-2}$, whereas that of hydrophobic plasma-treated one was estimated to be about 2.9 mL cm$^{-2}$. MD simulation showed that MgCO$_3$ is hydrophilic with a contact angle of approximately 20$^\circ$, whereas amorphous carbon formed a contact angle of 138.3 $\pm$ 4.0$^\circ$.

magnesium corrosion,atmospheric plasma,carbon dioxide,magnesium carbonate,magnesium oxide,hydrophobic surface,electrochemical impedance,hydrogen evolution,molecular dynamics