A medical implant comprises a biodegradable magnesium-based alloy of which at least a part of its surface layer comprises a magnesium carbonate. A method for the manufacture of a biocompatible, corrosion-inhibiting protective surface layer on a medical implant comprising a magnesium-based alloy, comprises: providing an implant comprising a magnesium-based alloy to be coated; placing the implant into a reactor chamber; exposing at least part of the surface of said implant to an atmosphere comprising humid carbon dioxide to produce a coating on the surface of the implant comprising a magnesium carbonate of the formula x MgCO3.y Mg (OH)2, whereby x+y=1; removing the implant from the reactor chamber; and drying the surface of the implant.