An aqueous solution of 2.0 to 12.0 gm/l hexavalent chromium and 0.7 to 4.0 gm/l of sodium or ammonium bifluoride, adjusted to a pH of 2.7 to 5.5, is applied to coated (eg. Zn) ferrous substrates, metered to result in a final thickness of 0.8 to 2.0 mg of Cr per square foot of substrate surface, and thereafter dried to remove physically bound water.