e is at least partially covered with an iron oxide, wherein iron pentacarbonyl is oxidized to iron oxide in a fluidized bed of the metallic substrates with oxygen at above 100??C. In wet-chemical preparation methods, metal oxide containing layers can be applied by hydro lytic reaction of appropriate metal salts, e.g. iron(III) salts such as iron(III) chloride and sulfate, or hydrolysable organomet