METHOD FOR MANUFACTURING CHROMIUM OXIDE COATED TINPLATE

Abstract

A method for electroplating a steel strip with a plating layer and an improvement thereof.

Claims

1. A method for manufacturing chromium oxide coated tinplate comprising: electrolytically depositing a chromium oxide layer onto a tinplate 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 comprising a trivalent chromium compound provided by a water-soluble chromium(III) salt, wherein the steel 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.sup.3+-ions to Cr.sup.6+-ions, and wherein the electrolyte solution contains at least 50 mM and at most 1000 mM Cr.sup.3+-ions, a total of from 25 to 2800 mM of sodium sulphate or potassium sulphate, a pH of between 2.50 and 3.6 measured at 25° C., and wherein plating temperature is between 40 and 70° C. and wherein no other compounds are added to the electrolyte, except optionally sulphuric acid or sodium hydroxide or potassium hydroxide to adjust the pH to the desired value.

2. The method according to claim 1, wherein the pH is adjusted to a value of 2.55 or more, and preferably to a value of 3.25 or less.

3. The method according to claim 1, wherein the plating time, being the duration of the 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 amount of chromium deposited as chromium oxide is at least 5 mg/m.sup.2.

6. The method according to claim 1, wherein the electrolyte solution contains at most 10 mM of sodium formate (NaCOOH).

7. The method according to claim 1, wherein the electrolyte solution contains at least 210 mM and/or at most 8′15 mM of sodium sulphate.

8. The method according to claim 1, wherein the plating temperature is at least 50° C.

9. The method according to claim 1, wherein the line speed of the plating line is at least 100 m/min.

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

11. The method according to claim 1, wherein the steel substrate 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, or a thermoplastic multi-layer polymer.

12. The method according to claim 11, wherein the thermoplastic polymer coating is a polymer coating system comprising one or more layers comprising thermoplastic resins selected from polyesters, polyolefins, acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene type resins, ABS resins, chlorinated polyethers, ionomers, urethane resins and functionalised polymers; and/or copolymers thereof; and or blends thereof.

13. The method according to claim 12, wherein the thermoplastic polymer coating on the one or both sides of the coated blackplate is a multi-layer coating system, said coating system comprising at least an adhesion layer for adhering to the coated blackplate, a surface layer and a bulk layer between the adhesion layer and the surface layer, wherein the layers of the multi-layer coating system comprise or consist of polyesters, such as polyethylene terephthalate, Isophthalic acid-modified polyethylene terephthalate, cyclohexanedimethanol-modified polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, or copolymers or blends thereof.

14. The coated metal substrate obtainable by the process according to claim 1.

15. A process comprising producing containers for packaging purposes from the coated metal substrate of claim 14.

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

17. The method according to claim 1, wherein the amount of chromium deposited as chromium oxide is at least 6 mg/m.sup.2.

18. The method according to claim 1, wherein the amount of chromium deposited as chromium oxide is at least 7 mg/m.sup.2.

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

20. The method according to claim 1, wherein the electrolyte solution contains 210 mM to 845 mM of sodium sulphate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The invention will now be explained by means of the following, non-limiting figures.

[0061] FIG. 1 schematically summarises the process steps to obtain the coated product, starting from a hot-rolled strip. Before cold-rolling, the hot-rolled strip is usually pickled (not shown) to remove the hot-rolling scale and cleaned (not shown) to remove any contaminants from the strip.

[0062] FIG. 2: Amount of Cr-oxide as a function of current density in RCE-experiments for the experiments performed at pH=2.7.

[0063] FIG. 3: schematic drawing of tinplate producible with a top layer of CrOx deposited according to the invention:

[0064] a. tinplate (not reflown)

[0065] b. tinplate (reflown)

[0066] c. tinplate (reflown) with additional tin

[0067] d. FeSn

[0068] e. FeSn with tin