The present invention relates to a process for producing a coating on the surface of a substrate by plasma-electrolytic oxidation. Improved corrosion protection for lightweight metals, in particular for magnesium or magnesium alloys, is achieved by the process. Furthermore, biocompatible protective layers can also be produced on these materials, with the option of controlling degradation of the substrate. The layers are amorphous. They are produced by plasma-electrolytic oxidation in which the substrate is dipped as electrode together with a counterelectrode into an electrolyte liquid and a sufficient electric potential for generating spark discharges at the surface of the substrate is applied, wherein the electrolyte comprises clay particles dispersed therein. Substrates can therefore be any machine components, automobile components, railroad components, aircraft components, ships' components, etc., or bioimplants such as bone replacement materials or medical bone screws made of a lightweight metal such as magnesium or a magnesium alloy.