## 2 Materials and Methods  
Magnesium disks (15 mm diameter \(\times\) 2 mm height) were cut from Matthey commercial rods of high purity. Mg: 99.775 wt.% Mg, 0.0185 wt.% Al, 0.030 wt.% Fe, 0.010 wt.% Zncontents were measured by X-ray fluorescence (XRF) elemental analysis in a spectrometer ZSX Primus II (Rigaku, Tokyo, Japan) equipped with an X-ray tube with Rh anode, 4.0 kW power, with front Be window, using EZ scan combined with RPF-SQX software. Before tests, the disks were cleaned in ethanol and dried in air.  
For carbonatation, the Mg bare disks were immersed in carbonated water (Romaqua Group, with a concentration of 7.7 mmol/L Ca\({}^{2+}\), 31.0 mmol/L HCO\({}_{3}\)\({}^{-}\), 4.7 mmol/L Mg\({}^{2+}\), 3.6 mmol/L Na\({}^{+}\) and 56.8 mmol/L CO\({}_{2}\)) at standard laboratory conditions of temperature and pressure. Immersion time varied from a few minutes to one week.  
The carbonate coating composition was determined by elemental analysis using XRF. The crystal structure was studied by X-ray diffraction (XRD) in a diffractometer Ultima IV, operated at 30 mA and 40 kV, in thin film geometry, using Cu K\({}_{\alpha}\) radiation (\(\lambda\) = 1.5405 A). The crystallite size was calculated using the Scherrer method. Phase analysis was also carried out by using infrared and Raman spectroscopy. Raman spectra were recorded in a LabRam HR spectrometer (Horiba Jobin Yvon, Palaiseau, France) using a 325 nm excitation laser and a NUV 40x objective. Fourier transformed-infra-red (FT-IR) spectra were recorded using a 4100 ExoScan Spectrometer Agilent diffuse external reflectance system at sample surfaces, between 600-4000 cm\({}^{-1}\), with a resolution of 4 cm\({}^{-1}\) and 15 s measurement time.  
Morphology and surface topography were analyzed by scanning electron microscopy (SEM) in a field emission gun microscope Quanta 3D (FEI, Eindhoven, The Netherlands) at operating voltages between 5 and 20 kV equipped with a dispersive energy X-ray) spectrometer (EDS) and by atomic force microscopy (AFM) using an XE100 (Park Systems, Suwon, Korea) equipment with flexure-guided, cross -talk eliminated scanners in \"true non-contact\" working mode to minimize interaction between tip and sample. All measurements were made with sharp tips (NCLR from NanosensorsTM) of less than 8 nm apex radius, 225 \(\upmu\)m mean length, 38 \(\upmu\)m mean width, -4 \(\upmu\)m thickness, 48 N/m spring constant and -190 kHz resonance frequency. The topographical 2D and 3D AFM images were processed with the XEI program (v1.8.0, Park Systems) for tilt correction and subsequent data analysis, including the evaluation of the roughness parameters. The root-mean-squared roughness (Rq) represents the standard deviation of the height value in the image, the average roughness (Ra) is the average deviation from the mean height, the area between the roughness profile and its mean value per unit length, while the peak-to-valley parameter (Rpv) of the image or line, is the height difference between minimum and maximum. AFM images are presented in \"enhanced color\"TM view mode to highlight morphological details. Representative line scans were selected to show in detail the surface profiles. Optical images were obtained with a digital camera with optical zoom.  
The coatings' wettability was investigated by contact angle measurements using a contact angle meter drop shape analysis system, model DSA1 (FM40 Easy Drop, DIPROBOT DR-3, Riegler and Kirstein, Berlin, Germany). The static contact angle of deionized water on coated surfaces was obtained by analyzing the captured images, and the water contact angles were automatically calculated by the software of the instrument analyzer at room temperature. An average value of the contact angle between the sample surfaces and a minimum of five deionized water drops placed in various regions of the film surface was measured using the sessile drop method.