Method for manufacturing chromium oxide coated tinplate

1 . 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 Cr 3+ -ions to Cr 6+ -ions, and

wherein the electrolyte solution contains at least 125 mM and at most 1000 mM Cr 3+ -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.

2 . The method according to claim 1 , wherein the pH is in a range of 2.55 to 3.25,

wherein the aqueous electrolyte solution consists of:

water,

the water-soluble chromium(III) salt,

the conductivity enhancing salt selected from the group consisting of sodium sulphate and potassium sulphate,

at most 5 mM of the complexing agent, and

optionally the sulphuric acid or sodium hydroxide or potassium hydroxide.

3 . The method according to claim 1 ,

wherein the electrolyte solution contains at most 5 mM of the complexing agent, and

wherein the plating time of the plating, being the duration of application of electrical current to the cathode, is at most 1000 ms.

4 . The method according to claim 1 , wherein the water-soluble chromium(III) salt is basic chromium(III)sulphate.

5 . The method according to claim 1 ,

wherein the chromium oxide layer is deposited in an amount of at least 5 mg/m 2 .

6 . The method according to claim 1 ,

wherein the electrolyte solution contains at most 2 mM of the complexing agent, wherein the complexing agent is sodium formate (NaCOOH).

7 . The method according to claim 1 , wherein the conductivity enhancing salt is the sodium sulphate,

wherein the electrolyte solution consists of:

at least 210 mM of the sodium sulphate,

water,

the basic chromium(III) sulphate,

at most 5 mM of the complexing agent, wherein the complexing agent is sodium formate (NaCOOH), and

optionally the sulphuric acid or sodium hydroxide or potassium hydroxide.

8 . The method according to claim 1 , wherein the plating temperature is between 5° and 70° C., wherein the plating deposits the chromium oxide layer to have an absence of chromium carbide, and

wherein the electrolyte solution contains at most 5 mM of the complexing agent.

9 . The method according to claim 1 , wherein the line speed of the plating line is at least 100 m/min, wherein the electrolyte solution contains no complexing agent.

10 . The method according to claim 1 , wherein the aqueous electrolyte solution consists of the basic chromium(III) sulphate, the sodium sulphate and optionally the sulphuric acid or sodium hydroxide in an amount sufficient to adjust the pH of the aqueous electrolyte solution to the desired value and unavoidable impurities.

11 . The method according to claim 1 , wherein the tinplate strip substrate coated with the chromium oxide layer that has an absence of chromium metal is further coated on one or both sides by a lacquering step, a film lamination step or a direct extrusion step with an organic coating consisting of a lacquer, a thermoplastic single layer polymer, or a thermoplastic multi-layer polymer.

12 . The method according to claim 1 , wherein the tinplate strip substrate coated with the chromium oxide layer that has an absence of chromium metal is further coated on one or both sides by a film lamination step or a direct extrusion step with an organic coating consisting of a thermoplastic polymer coating which is a polymer coating system comprising one or more layers comprising thermoplastic resins selected from at least one member of the group consisting of polyesters, polyolefins, acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene resins, ABS resins, chlorinated polyethers, ionomers, urethane resins, functionalised polymers, copolymers thereof and blends thereof.

13 . The method according to claim 1 , wherein the tinplate strip substrate coated with the chromium oxide layer that has an absence of chromium metal is further coated on one or both sides by a film lamination step or a direct extrusion step with an organic coating consisting of a thermoplastic polymer coating which is a multi-layer polymer coating system, said multi-layer polymer coating system comprising at least an adhesion layer for adhering to the tinplate strip substrate, a surface layer and a bulk layer between the adhesion layer and the surface layer, wherein the layers of the multi-layer polymer coating system comprise polyesters selected from the group consisting of polyethylene terephthalate, isophthalic acid-modified polyethylene terephthalate, cyclohexanedimethanol-modified polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, copolymers thereof and blends thereof.

14 . The method according to claim 1 , wherein the pH is adjusted to a value of 2.55 to 3.25.

15 . The method according to claim 1 , wherein the chromium oxide layer is deposited in an amount of at least 6 mg/m 2 .

16 . The method according to claim 2 , wherein the plating deposits the chromium oxide layer to have an absence of chromium carbide.

17 . The method according to claim 1 , wherein the electrolyte solution contains at most 845 mM of the sodium sulphate.

18 . The method according to claim 1 , wherein the conductivity enhancing salt is the sodium sulphate, wherein the electrolyte solution contains 210 mM to 845 mM of the sodium sulphate.

19 . The method according to claim 1 , wherein the tinplate strip substrate coated with the chromium oxide layer that has an absence of chromium metal is further coated on one or both sides by a film lamination step or a direct extrusion step with an organic coating consisting of a thermoplastic polymer coating which is a multi-layer polymer coating system, said multi-layer polymer coating system comprising at least an adhesion layer for adhering to the tinplate strip substrate, a surface layer and a bulk layer between the adhesion layer and the surface layer, wherein the layers of the multi-layer polymer coating system comprise polyesters.

20 . The method according to claim 1 , wherein said electrolytically depositing the chromium oxide layer onto the tinplate strip substrate deposits a closed layer of chromium oxide, wherein a surface of the chromium oxide layer has 0.5 wt. % to 10 wt. % sulphate.

21 . The method according to claim 1 , wherein the plating deposits the chromium oxide layer to have an absence of chromium carbide.

22 . The method according to claim 1 , wherein the plating line passes the tinplate strip substrate into the electrolyte solution for an immersion time to be coated with the chromium oxide layer that has an absence of chromium metal during the immersion time, and then the plating line passes the tinplate strip substrate coated with the chromium oxide layer that has an absence of chromium metal moves out of the electrolyte solution.