The invention relates to a method for forming a passivation layer on a strip of packaging steel for producing a packaging container or a can, said strip having at least one tinplated surface having excellent sulphide staining resistance. The method comprises the steps of: (i) subjecting said tin-plated surface to an anodic treatment at a pH of from 8 to 12 in a suitable aqueous electrolyte, wherein the suitable electrolyte is a phosphate solution, borate solution, sulphate solution or a carbonate solution, wherein the electrolyte for the anodic treatment does not contain mono-atomic halogen anions (Group 17) such as CI-, F-, wherein the thickness of the tin oxide layer (D), expressed in Coulomb/m2 and representing the total charge needed to reduce the layer to metallic tin is related to anodic treatment time (t), current density (A) by D = E x A x t, where E is the efficiency of the electrochemical reaction, and wherein D is at least 30 C/m2; wherein the electrolyte comprises monoatomic cations from Group 1 or 2 from the periodic table or polyatomic cations, and polyatomic anions; (ii) optionally rinsing and drying the anodically treated tin-plated surface - subjecting the anodically treated tin-plated surface to a chemical passivation treatment free from using hexavalent chromium compounds; (iii) drying of the passivated anodically treated tin-plated surface; (iv) final processing such as oiling, winding, cutting etc.