METHOD FOR PLATING A MOVING METAL STRIP AND COATED METAL STRIP PRODUCED THEREBY

Abstract

A method for producing a steel substrate coated with a chromium metal-chromium oxide (CrCrOx) coating layer in a continuous high speed plating line, operating at a line speed (v1) of at least 100 m.Math.min.sup.1, wherein one or both sides of the electrically conductive substrate in the form of a strip, moving through the line, is coated with a chromium metal-chromium oxide (CrCrOx) coating layer from a single electrolyte by using a plating process. A coated steel substrate and a packaging made thereof.

Claims

1. A method for producing a steel substrate coated with a chromium metal-chromium oxide (CrCrOx) coating layer in a continuous high speed plating line, operating at a line speed (v1) of at least 100 m.Math.min.sup.1, wherein one or both sides of the electrically conductive substrate in the form of a strip, moving through the line, is coated with a chromium metal-chromium oxide (CrCrOx) coating layer from a single electrolyte by using a plating process, wherein the substrate is a steel substrate which acts as a cathode and wherein the CrOx deposition is driven by the increase of the pH at the substrate/electrolyte interface due to the reduction of H.sup.+ to H.sub.2(g), and wherein the increase of pH is counteracted by a diffusion flux of H.sup.+-ions from the bulk of the electrolyte to the substrate/electrolyte interface and wherein this diffusion flux of H.sup.+-ions from the bulk of the electrolyte to the substrate/electrolyte interface is reduced by increasing the kinematic viscosity of the electrolyte, and/or by moving the strip and the electrolyte through the plating line in concurrent flow wherein the steel strip is transported through the plating line with a velocity (v1) and wherein the electrolyte is transported through the strip plating line with a velocity of v2, thereby reducing the current density to deposit CrOx and reducing the amount of H.sub.2(g) formed at the substrate/electrolyte interface.

2. The method for producing a coated steel substrate according to claim 1, wherein one or both sides of the electrically conductive substrate moving through the line is coated with a CrCrOx coating layer from a single electrolyte by using a plating process with an electrolytic solution based on a trivalent chromium electrolyte that comprises a trivalent chromium compound, a chelating agent and a conductivity enhancing salt.

3. The method according to claim 1, wherein the kinematic viscosity of the electrolyte is increased by using a suitable conductivity enhancing salt in a concentration for the electrolyte kinematic viscosity to be at least 1.Math.10.sup.6 m.sup.2.Math.s.sup.1 (1.0 cSt) when measured at 50 C.

4. The method according to claim 1, wherein the kinematic viscosity is increased by using sodium sulphate as the conductivity enhancing salt.

5. The method according to claim 1, wherein the kinematic viscosity is increased by using a thickening agent.

6. The method according to claim 2, wherein the chelating agent is sodium formate.

7. The method according to claim 1, wherein the strip and the electrolyte are moving through the plating line in concurrent flow wherein the ratio of (v1/v2) is at least 0.1 and/or at most 10.

8. The method according to claim 1, wherein a plurality (>1) of CrCrOx coating layers is deposited onto one or both sides of the electrically conductive substrate, wherein each layer is deposited in a single step in subsequent plating cells, in subsequent passes through the same plating line or in subsequent passes through subsequent plating lines.

9. The method according to claim 1, wherein the electrolyte consists of an aqueous solution of chromium (III) sulphate, sodium sulphate and sodium formate, unavoidable impurities and optionally sulphuric acid, the aqueous electrolyte having a pH at 25 C. of between 2.5 and 3.5.

10. The method according to claim 1, wherein the electrically conductive steel substrate prior to being coated with a chromium metal-chromium oxide (CrCrOx) coating layer is one of: tinplate, as deposited or flow-melted; tinplate, diffusion annealed with an iron-tin alloy consisting of at least 80% of FeSn (50 at. % iron and 50 at. % tin); cold-rolled full-hard blackplate, single or double reduced; cold-rolled and recrystallisation annealed blackplate; cold-rolled and recovery annealed blackplate, wherein the resulting coated steel substrate is intended for use in packaging applications.

11. A coated steel strip produced in accordance with the process of claim 1.

12. A packaging produced from the coated metal strip according to claim 11.

13. The method for producing a coated steel substrate according to claim 2, wherein the electrolyte solution is free of chloride ions

14. The method for producing a coated steel substrate according to claim 13, wherein the electrolyte solution is free of a buffering agent such as boric acid.

15. The method for producing a coated steel substrate according to claim 13, wherein the electrolyte solution is free of boric acid.

16. The method according to claim 1, wherein the kinematic viscosity is increased by using a polysaccharide.

17. The method according to claim 1, wherein the electrolyte consists of an aqueous solution of chromium (III) sulphate, sodium sulphate and sodium formate, unavoidable impurities and optionally sulphuric acid, the aqueous electrolyte having a pH at 25 C. of at least 2.7 and/or at most 3.1.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0065] FIG. 1 shows the concentration gradient of the H.sup.+-ions from at the electrode (c.sub.s) (the dashed block, at x=0) to the bulk concentration (c.sub.b). The indicated the stagnant layer (diffusion layer thickness) in the Nernst diffusion layer concept. Outside this layer, convection maintains the concentration uniform at the bulk concentration. Within this layer, mass transfer occurs only by diffusion. The thickness of is determined by the gradient of concentration at the electrode (c/x).sub.x=0.

[0066] FIG. 2 is a schematical representation of the mechanism of the deposition of Cr(OH).sub.3 on the substrate. Note that the H.sup.+-concentration profile is approximated by a straight line for simplicity. The again indicates the stagnant layer in the Nernst diffusion layer concept.

[0067] FIG. 3 shows how the required current density for the deposition of a fixed amount of Cr(OH).sub.3 increases when the speed of the strip moving through a plating line increases. For electrodeposition based on Me.sup.n+(aq)+n.Math.e.sup..fwdarw.Me(s) the increase of current density would be sufficient. For the mechanism based on deposition of Cr(OH).sub.3 the high speeds result in a thinner diffusion layer thickness, and therefore the unwanted diffusion of H.sup.+ to the electrode speeds up as well. Measurements have indicated that for a line speed of 100 m.Math.min.sup.1 a current density of 24.3 A.Math.dm.sup.2 is needed for depositing 60 mg.Math.m.sup.2 CrCrOx, whereas for 300 m/min 73 A.Math.dm.sup.2 is needed and for 600 m.Math.min.sup.1 nearly 150 A.Math.dm.sup.2 is needed.

[0068] FIG. 4 shows the CrCrOx vs. current density plots: a threshold value before CrCrOx deposition starts, a peak followed by a sudden, steep decline ending in a plateau.

[0069] FIG. 5 shows CrCrOx vs. current density plots for different electrolytes and for varying amounts of sodium phosphate.

[0070] FIG. 6 shows a cut-out from FIG. 5 which shows the current density for depositing 100 mg/m.sup.2 Cr, which is a suitable target value.

[0071] FIG. 7 plots the coating composition is vs. current density for 200 g/l Na.sub.2SO.sub.4 for a deposition time of 1 second, and in FIG. 8, the coating composition weight is plotted vs. deposition time for a current density of 20 A/dm.sup.2 and for 200 g/l Na.sub.2SO.sub.4. Beyond the maximum current density (Regime IIIas depicted in FIGS. 4 and 5, which for 200 g/l Na.sub.2SO.sub.4 is about 25 A/dm.sup.2) the amount of Cr-metal drops and the coating is increasingly composed of Cr-oxide with increasing current density. In the linear regime II towards the maximum the Cr-metal content increases with increasing electrolysis time mainly at the expense of Cr oxide. The amount of Cr-carbide is about the same for all deposition times in FIG. 8.