## 1 Introduction  
Mg has some very advantageous properties for use in biodegradable implants: it is an intrinsic component of living organisms, can be assimilated naturally without any toxic effects and stored in the bones, and can also be easily eliminated from the body [1]. In addition, its mechanical strength and modulus of elasticity are close to those of human bones; therefore, Mg implants can avoid the'stress-shielding' effect that leads to a reduction of bone density around the implant [1, 2]. In addition, Mg is readily available, inexpensive and completely recyclable [3]. Mg is prone to corrosion in physiological fluids, the definition of a biodegradable material. However, controlling the rate of degradation of Mg-based biomaterials for their application in implants is still a challenge. The healing period required for bone regeneration varies from a few weeks to a few months [1, 4], during which time the implant material must maintain sufficient mechanical strength. The rapid localized corrosion of Mg in physiological fluids makes pure Mg unusable for most applications, as many studies show, even from the 19 th century [4]. Mg alloys are currently being used in fixation screws and plates, as well as porous scaffolds for bone regeneration and as degradable stents for angioplasty, but not for load -bearing applications [1]. The most critical issue is the control of the degradation rate [1]. Remediation of the rapid degradation of Mg materials in the physiological environment has been tried through different surface modification approaches, such as micro-arc oxidation [5], anodization [6], phosphating [7], electro-deposition [8; 9] or biomimetic deposition of calcium phosphate [10; 11], fluoride chemical conversion [12; 13] and alkaline heat treatment [14]. Biomineralization of calcium carbonate is the natural process that many marine organisms, such as corals, use for the growth of their skeletons and shells [15]. The natural formation of the coral reefs is a slow biomedical precipitation of calcium carbonate from seawater. Additionally, the coating of artificial coral reefs with calcium carbonate promotes the attachment and growth of coral larvae and photosynthetic epibiota on these surfaces [16]. Calcium carbonate coatings on bioinert Ti have been reported to improve the osteointegration process [17]. Furthermore, magnesium and calcium carbonate coatings have been reported to enhance the corrosion resistance of Mg and Mg alloys. Carbonate formation induced by immersion in sodium hydrogen carbonate aqueous solution substantially improved the corrosion resistance of pure Mg [18]. Hybrid polymer coatings have also been developed [19; 20]. Coatings with mesquehonite (MgCO\({}_{3}\cdot\)3H\({}_{2}\)O) with acicular morphologies can be obtained on pure Mg by chemical conversion coating in sodium hydrogen carbonate solution, which is useful as pre-coating for polycaprolactone deposition [19]. Composite coating composed of polycaprolactone and amorphous CaCO\({}_{3}\) can remarkably improve the degradation behavior of AZ60 alloy [20]. The formation of a two-layer coating on Mg alloy AZ91D: an inner Al-hydrotalcite (Mg\({}_{6}\)Al\({}_{2}\)(OH)\({}_{16}\)CO\({}_{3}\cdot\)4H\({}_{2}\)O) layer and an outer CaCO\({}_{3}\) layer effectively protected the Al -containing alloy against corrosion [21; 22]. Several studies report porous carbonate coatings consisting of a variety of crystal shapes protruding upwards from the metal surface: apical aragonite [21; 23; 24] and double -layered hydroxides platelets [25; 26], coatings that, despite their porous external structure, provide some corrosion protection, which indicates that carbonate coated areas can be effectively protected from corrosion attack in solution. Carbonates have, therefore, great potential to be used for controlling the degradation rate of medical Mg devices in physiological fluids. However, there is a lack of studies on the application of carbonate coating for the surface modification of Mg alloys for medical applications.