Method for manufacturing chromium oxide coated tinplate

A method for electroplating a tinplate strip substrate to manufacture a chromium oxide coated tinplate strip substrate comprising a plating process comprising:

• plating by electrolytically depositing a chromium oxide layer having an absence of chromium metal onto the tinplate strip substrate in a continuous high-speed plating line operating at a line speed of at least 50 m/min from a halide-ion free aqueous electrolyte solution wherein there is a trivalent chromium compound in the aqueous electrolyte solution, wherein the trivalent chromium compound is provided by a water-soluble chromium(III) salt selected from at least one member of the group consisting of basic chromium (III) sulphate, chromium (III) sulphate and chromium (III) nitrate,
• wherein the tinplate strip substrate acts as a cathode, and
• wherein an anode comprises a catalytic coating of i) iridium oxide or ii) a mixed metal oxide comprising iridium oxide and tantalum oxide, for reducing or eliminating the oxidation of Cr3+-ions to Cr6+-ions, and
• wherein the electrolyte solution contains at least 125 mM and at most 1000 mM Cr3+-ions from the trivalent chromium compound, a total of from 25 to 2800 mM of a conductivity enhancing salt selected from the group consisting of sodium sulphate and potassium sulphate, and at most 10 mM of a complexing agent, wherein the electrolyte solution has a pH of between 2.50 and 3.6 measured at 25° C., and
• wherein plating temperature is between 4° and 70° C., and
• wherein no other compounds are added to the electrolyte solution, except optionally sulphuric acid or sodium hydroxide or potassium hydroxide to adjust the pH to a desired value,
• wherein the method does not deposit chromium metal, and
• wherein in the method the only application of an electrical current to the tinplate strip substrate is during the plating by electrolytically depositing step.