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This study investigates the deposition conditions of carbonate-based coatings on AZ31 magnesium alloy pretreated with an Mg(OH)₂ interlayer in the presence of ammonium ions. The effect of temperature and exposure time on phase formation, growth mechanism, and corrosion performance was systematically evaluated. At laboratory temperature, no continuous carbonate coating was even formed after 12 or 24 h of deposition. Instead, substrate corrosion occurred, accompanied by further Mg(OH)₂ and hydrotalcite formation and crack development within the surface layer. Increasing the temperature to 50 °C led after 6 h primarily to chemical destabilization and partial dissolution of the Mg(OH)₂ interlayer rather than controlled carbonation. The first nuclei appeared after 12 h of exposure, and after 24 h the surface was fully covered by the newly formed layer. However, EDS and XRD analyses revealed that the coating predominantly consisted of hydromagnesite rather than anhydrous magnesium carbonate. Increasing the temperature up to 90 °C under hydrothermal conditions, hydromagnesite remained the dominant phase. The resulting hydromagnesite coating was compact, homogeneous, and significantly improved the corrosion resistance of the system. The results demonstrate that increasing temperature in the presence of ammonium ions is insufficient to induce controlled carbonation of the Mg(OH)₂ interlayer. Hydrothermal conditions primarily enhance the crystallinity and protective performance of hydromagnesite-based coatings.
Keywords: AZ31 magnesium alloy, ammonium ions, carbonate coating, hydromagnesite, structure, corrosion© This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.