Addition compounds of sulfur dioxide and Schiff bases from primary aliphatic or araliphatic amines and aromatic aldehydes can be used for the sealing of the surface of phosphated mild steel. As a result of this the corrosion resistance of the steel and the adhesion of paint coatings on the treated steel are increased substantially. The addition compounds may have three different structures depending on the components used. They can be prepared easily by passing a stream of SO.sub.2 through the solution of the Schiff base.
