METHOD FOR PASSIVATING A TINPLATE STRIP AND APPARATUS FOR PRODUCING SAID PASSIVATED TINPLATE STRIP

Abstract

A method for passivating a tinplate strip after electrodepositing the tin layer or tin layers, or after an optional flow-melting of the electrodeposited tin layer or tin layers, and an apparatus for producing the passivated tinplate strip.

Claims

1. A method for passivating a tinplate strip in a continuous process wherein after electrodepositing the tin layer or layers or after an optional flow-melting of the electrodeposited tin layer or layers, the tinplate strip enters a basic aqueous solution in an electrochemical treatment tank in an entry-pass and exits the basic aqueous solution in an exit-pass, wherein any pre-existing tin-oxide layer on the tinplate surface is cathodically removed from the tinplate surface during the entry-pass and wherein the tin surface is subsequently immediately anodically re-oxidised during the exit pass, wherein the charge for cathodically removing the pre-existing tin-oxide layer from the tinplate surface is Q1, and wherein the charge for anodically re-oxidising the tinplate is Q2 and wherein Q1<Q2, wherein the imposed charge density for the anodic re-oxidation and the cathodic removal of the pre-existing tin-oxide layer is identical and equal to Q2 and is at least 15 C/m.sup.2 and wherein the anodically re-oxidised tinplate is rinsed and dried after exiting the basic aqueous solution.

2. The method according to claim 1, wherein the imposed charge density for the anodic re-oxidation is at most 100 C/m2.

3. The method according to claim 1, wherein the anodically re-oxidised tinplate is covered with an oxide layer having a thickness D expressed in C/m.sup.2 and representing the total charge needed to reduce the oxide layer to metallic tin, which is related to the re-oxidation time t and the current density A by D=E×A×t, where E is the efficiency of the electrochemical reaction, and wherein D is between 15 and 100 C/m.sup.2.

4. The method according to claim 1, wherein a liquid solution of a chromium-free post-treatment agent is applied to the rinsed and dried anodically oxidised tinplate surface to produce a post-treated tinplate, wherein the chromium-free post-treatment agent is selected from copolymers of acrylates, polymethyl siloxanes with polyether side chains, acid polyethers, polymers with heterocyclic groups and acid, aqueous, liquid compounds which contain complex metal fluoride anions with divalent to tetravalent cations and polymeric substances.

5. The method according to claim 1, wherein the current density during anodic oxidation A is at least 10 A/m.sup.2.

6. The method according to claim 1, wherein the current density during anodic oxidation A is at most 4000 A/m.sup.2.

7. The method according to claim 1, wherein the basic aqueous solution is chosen from an alkali metal or alkaline earth metal hydroxide or carbonate, a basic alkali metal phosphate, and a basic organic alkali metal or alkaline earth metal salt.

8. The method according to claim 1, wherein the basic aqueous solution has a pH of between 8.75 and 10.5.

9. The method according to claim 1, wherein the anodic re-oxidation time t is at between 0.05 seconds and 1.5 seconds.

10. The method according to claim 1, wherein the tinplate is led into the basic aqueous solution immediately after the deposition of the tin layer or layers on the steel strip, or immediately after the flow-melting of the deposited tin layer or layers.

11. The method according to claim 1, wherein the tin-oxide layer after anodic re-oxidation consists mainly of SnO.

12. The method according to claim 1, wherein a thermoplastic polymer coating is applied directly on the rinsed and dried re-oxidised tinplate, wherein the tin-oxide layer after anodic re-oxidation consists mainly of SnO.

13. The method according to claim 4, wherein a thermoplastic polymer coating is applied directly on the post-treated tinplate, wherein the tin-oxide layer after anodic re-oxidation consists mainly of SnO.

14. A passivated tinplate produced according to claim 1, wherein the tin-oxide layer after anodic re-oxidation consists mainly of SnO and wherein the tin-oxide layer after rinsing and drying is subjected to a subsequent treatment with a no-rinse/dry-in-place post-treatment agent based on titanium or a combination of titanium and zirconium which is prepared as a solution with a dry coverage in the range of 0.2 to 2 mg Ti/m.sup.2.

15. An apparatus for producing passivated tinplate according to the method of claim 1, comprising an electrolytic tinning line, optionally provided with means for melting the tin layer; an electrochemical treatment tank for holding, in use, an aqueous basic solution; non-conductive means for guiding the cathodic tinplate into the electrochemical treatment tank, such as a non-conductive guide-roller, past the anodes during the entry-pass; a non-conductive sink roll for directing the tinplate from the entry-pass to the exit-pass guiding the anodic tinplate past the cathodes during the exit-pass; means for applying potential between the tinplate strip and the counter electrodes for the cathodic removal of the pre-existing oxide layer and the anodic re-oxidation of the tinplate strip, non-conductive means for guiding the tinplate from the electrochemical treatment tank to means for rinsing (III) and drying the tinplate, such as a non-conductive guide-roller, and optionally means for applying a liquid solution of a chromium-free post-treatment agent.

16. The method according to claim 4, wherein the chromium-free post-treatment agent comprises fluoro-titanates and zirconium-titanates

17. The method according to claim 5, wherein the current density during anodic oxidation A is at least 50 A/m.sup.2.

18. The method according to claim 5, wherein the current density during anodic oxidation A is at least 100 A/m.sup.2.

19. The method according to claim 7, wherein the basic aqueous solution contains sodium carbonate.

20. The method according to claim 6, wherein the current density during anodic oxidation A is at most 2000 A/m.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The invention will now be explained by means of the following, non-limiting figures.

[0064] FIG. 1 shows a schematic drawing of an apparatus for the method according to the invention.

[0065] FIG. 2 shows a schematic drawing of the cathodic removal of the pre-existing tin-oxide layer.

[0066] FIG. 3 shows the difference between the V-t response of the cathodic removal of a pre-existing tin-oxide layer based on SnO.sub.2 and of a pre-existing tin-oxide layer based on SnO.

[0067] FIG. 4 shows a schematic representation of the various stages during the process according to the invention on the basis of a blackplate strip as feed stock.

[0068] FIG. 5 shows a schematic representation of the various build-ups of the layers as presented in FIG. 4. The thickness of the blackplate and the thicknesses of the various layers shown are not to scale.

[0069] FIG. 1 shows an embodiment of the invention to execute the method according to the invention. A tinning cell (I) is shown in which a strip (1) is led in the plating solution (2) as a cathode to be plated to produce tinplate. After tinning in one or more of such tinning cells the tinplate, and the optional flow-melting (not shown) is led into the electrochemical treatment tank (II) containing the basic aqueous solution (8). The tinplate enters the tank (II) via the non-conductive guide roller (3) in the entry pass (down-pass) as a cathode and passes past the anodes (6) for cathodically removing the pre-existing oxide and produce a bare and pure tin surface. After being redirected by the non-conductive counter-sink roll (4) the tinplate starts the exit pass (up-pass) and changes from cathode into an anode. During the exit pass the tinplate passes past the cathodes (7) for applying a fresh tin oxide layer onto the bare and pure tin surface. After exiting the bath past the non-conductive guide roller (5) the strip optionally enters a rinsing bath (III) and is dried (not shown). In section IV the tinplate strip a post-treatment agent (11) is applied to the tinplate strip by means of application means (10). The strip may subsequently be dried is necessary (not shown). The guide rollers 3 and 5 need to be non-conductive guide rollers. The term non-conductive in the general context of this invention means that the rollers do not conduct electricity.

[0070] A typical potential time curve is shown in FIG. 2, from which the tin oxide layer thickness is determined based on the time where the tangent of the curve at −0.7 V and the tangent of the curve around −0.85 V cross is taken as the basis for the calculation of the tin oxide layer thickness in C/m.sup.2. In the example in FIG. 2 the time is about 190 s×0.50=95 C/m.sup.2. The tin oxide layer thickness D, expressed in C/m.sup.2, is obtained from D [C/m.sup.2]=t [s]*0.50 [A/m.sup.2].

[0071] In FIG. 3 there is a distinct difference observable at t=25 between the curve with thick layer compared to the thinner layer, both produced on a fresh tin surface (i.e. the pre-existing oxide is cathodically and fully removed). The dip at t=25 s is consistent for thicker layers and is associated with the presence of SnO.sub.2 in the tin oxide layer. The other two curves have a shape that is consistent with a tin oxide layer that predominantly consists of SnO.

[0072] In FIG. 4 the set-up of FIG. 1 is reproduced, and the letters A to G represent various stages of the development of the layers on the blackplate.

[0073] In FIG. 5 the letters represent the following: [0074] A: Blackplate strip feed-stock; [0075] B: Tinplate (i.e. blackplate coated with a tin layer in tin plating cell I); [0076] C1: Tinplate with a pre-existing oxide and uninterrupted processing (no additional oxide growth); [0077] C2: Tinplate with a pre-existing oxide and an additional oxide layer due to extended storage and/or storage under conditions leading to additional oxide layer growth; [0078] D: Pure and bare tin layer on blackplate after removal of the pre-existing oxide layer; [0079] E: Anodically re-oxidised tinplate; [0080] F: Cleaned and rinsed anodically re-oxidised tinplate; [0081] G: Anodically re-oxidised tinplate provided with post-treatment agent