Acidic chromium-free aqueous solution, the concentrate and a process for corrosion protection treatment of metal surfaces

Acidic chromium-free aqueous solution of a fluoro complex of at least one element M selected from the group B, Si, Ti, Zr and Hf, having a pH value in the range from 2 to 5.5 for treating metal surfaces, characterized in that it zusatzlicha ) both zinc ions and magnesium ions, b) one or more components selected from tin ions, bismuth ions, buffer systems for the pH range of 2.5 to 5.5, aromatic carboxylic acids having at least two groups containing donor atoms, or derivatives of such carboxylic acids c contains) copper ions and / or silver ions.