short chain aliphatic sulfonic acids their salts and or derivatives are added to the electrolyte in a concentration over 0 1 g l and an amount of 2 g l proved to be especially advantageous the addition of short chain aliphatic sulfonic acid its salts and or derivatives also makes it possible to operate with lower chromic acid concentrations in the electrolyte in comparison with the concentration of the isopolyanion forming metal the relevant isopolyanion forming metal is added to the electrolyte in amounts from about 1 g l up to the limit of solubility the solubility limit varies in dependence on the chromic acid content according to one embodiment molybdenum in the form of molybdic acid ammonium molybdate or an alkali molybdate is added to the electrolyte as the isopolyanion forming metal the ratio of chromic acid to the molybdenum compound is preferably about 2 1 the addition of 50 90 g l molybdic acid proved to be especially advantageous according to another embodiment vanadium is added to the electrolyte as polyanion forming metal preferably ammonium metavanadate vanadic acid or vanadium pentoxide is used to generate a vanadium containing electrolyte the ratio of chromic acid to the vanadium compound is preferably about 5 1 according to another embodiment of the method in accordance with the invention niobium is added to the electrolyte as isopolyanion forming metal niobium is chiefly added to the electrolyte in the form of niobic acid the ratio of chromic acid to.