METHOD FOR PASSIVATION OF THE SURFACE OF BLACKPLATE OR TINPLATE AND ELECTROLYSIS SYSTEM FOR PERFORMANCE OF THE METHOD

Abstract

A method for passivation of the surface of blackplate or tinplate by electrolytic deposition of a chromium oxide-containing passivation layer on the surface. The electrolytic deposition of the chromium-containing passivation layer occurs from an electrolyte solution that contains a trivalent chromium compound as well as at least one salt to increase the conductivity and at least one acid or a base to adjust a desired pH value. The electrolyte solution contains no additional components apart from the trivalent chromium compound as well as the at least one salt and the at least one acid or base and is especially free of organic complexing agents and free of buffering agents.

Claims

1. A method for passivation of the surface of blackplate or tinplate, the method comprising: electrolytic deposition of a chromium oxide-containing passivation layer on the surface, wherein the electrolytic deposition of the chromium-containing passivation layer occurs from an electrolyte solution that contains a trivalent chromium compound as well as at least one salt to increase conductivity and at least one acid or base to adjust a desired pH value, wherein the electrolyte solution contains no additional components apart from the trivalent chromium compound and the at least one salt and the at least one acid or base and is free of organic sequestering agents and free of complexing agents.

2. The method according to claim 1, wherein the passivation layer consists at least essentially of chromium oxide and/or chromium hydroxide.

3. The method according to claim 1, wherein the passivation layer has a weight fraction of chromium oxide and/or chromium hydroxide of more than 90%.

4. The method according to claim 1, wherein the blackplate or tinplate is connected as a cathode and is brought into contact with the electrolyte solution during an electrolysis time, wherein the electrolysis time lies in the range of 0.1 to 2.0 seconds.

5. The method according to claim 1, wherein the blackplate or tinplate is passed through at least one electrolysis tank or several electrolysis tanks arranged one behind the other in a sheet travel direction at a prescribed sheet speed, during which the sheet speed is at least 100 m/min.

6. The method according to claim 5, wherein a temperature of the electrolyte solution has a temperature averaged over a volume of the corresponding electrolysis tanks in the range of 20 C. to 65 C.

7. The method according to claim 5, wherein an electrolysis time, in which the blackplate or tinplate is in effective electrolytic contact with the electrolyte solution is less than 1.0 seconds in each of the tanks, wherein a total electrolysis time in which the blackplate or tinplate is in effective electrolytic contact with the electrolyte solution in all the tanks is between 0.5 seconds and 2.0 seconds.

8. The method according to claim 1, wherein the trivalent chromium compound is chosen from the group of basic Cr(III) sulfate (Cr.sub.2(SO.sub.4).sub.3), Cr(III) nitrate (Cr(NO.sub.3).sub.3), Cr(III) oxalate (CrC.sub.2O.sub.4), Cr(III) acetate (C.sub.12H.sub.36ClCr.sub.3O.sub.22), Cr(III) formate (Cr(OOCH).sub.3) or a mixture thereof, and wherein the salt of the electrolyte solution contains at least one alkali metal sulfate.

9. The method according to claim 1, wherein the electrolyte solution has a pH value (measured at a temperature of 20 C.) in the range of 2.3 to 5.0, wherein the pH value is adjusted by adding at least one acid to the electrolyte solution.

10. The method according to claim 1, wherein the concentration of trivalent chromium compound in the electrolyte solution is at least 10 g/L.

11. The method according to claim 1, wherein the passivation layer applied from the electrolyte solution has a total coating weight of chromium oxide and/or chromium hydroxide of at least 3 mg/m.sup.2.

12. The method according to claim 1, wherein by using an appropriate anode during electrolytic deposition of the passivation layer, oxidation of chromium(III) from the trivalent chromium compound of the electrolyte solution (E) to chromium(VI) is prevented.

13. The method according to claim 12, wherein the anode contains no stainless steel and no platinum.

14. The method according to claim 11, wherein the anode contains an outer surface or coating of a metal oxide, or wherein the anode consists of one of these materials.

15. The method according to claim 1, wherein to produce the electrolyte solution the trivalent chromium compound that has been freed of organic residues except for unavoidable contaminants, the at least one salt as well as the at least one acid or base to adjust the desired pH value is dissolved in water and the solution thereby obtained is allowed to stand for complexation for at least 5 days, and then fine adjustment of the pH value occurs by addition of an acid or base.

16. An electrolysis system for electrolytic passivation of the surface of a blackplate or tinplate by deposition of a chromium oxide-containing passivation layer on the surface, the electrolysis system comprising: at least one electrolysis tank filled with an electrolyte solution, or several electrolysis tanks arranged one behind the other, each of which is filled with the same electrolyte solution, wherein the electrolyte solution contains no other components apart from the trivalent chromium compound, as well as at the least one salt to increase conductivity and the at least one acid or base to adjust the desired pH value and is especially free of organic components and free of complexing agents, and the blackplate or tinplate for electrolytic deposition of the passivation layer is passed through the at least one electrolysis tank or through the several electrolysis tanks in succession with a stipulated sheet speed in a sheet travel direction so that the surface is electrolytically in effective contact with the electrolyte solution and a passivation layer consisting at least essentially of just chromium oxide and/or chromium hydroxide is deposited on the surface.

17. The electrolysis system according to claim 15, wherein an anode with an outer surface or a coating of a metal oxide or of a mixed metal oxide is arranged in the or each electrolysis tank.

18. A blackplate or tinplate with a surface passivated by electrolytic deposition of a chromium oxide containing passivation layer, wherein the passivation layer consists at least essentially of just chromium oxide and/or chromium hydroxide and has a weight fraction of chromium oxide and/or chromium hydroxide of more than 90%.

19. The blackplate or tinplate according to claim 17, wherein the passivation layer includes at least of a first layer facing the surface of the blackplate or tinplate and a second layer forming the surface of the passivated blackplate or tinplate, the first layer containing metallic chromium and a second layer consisting of pure chromium oxide and/or chromium hydroxide.

20. The blackplate or tinplate according to claim 17, wherein the passivation layer has a total coating weight of chromium oxide and/or chromium hydroxide of at least 3 mg/m.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The disclosure is further explained below by means of embodiment examples with reference to the accompanying drawings, where the aforementioned embodiment examples explain the disclosure only in exemplary terms and do not restrict it with reference to the scope of protection defined in the subsequent claims. The drawings show:

[0036] FIG. 1: schematic depiction of a strip coating line for performance of the method according to the disclosure;

[0037] FIG. 2: schematic sectional view of a blackplate or tinplate sheet coated with the strip coating line of FIG. 1 in a method according to the disclosure;

[0038] FIG. 3: GDOES spectrum of a layer deposited electrolytically, using electrolyte solution from a trivalent chromium substance (basic Cr(III) sulfate) and an organic complexing agent (sodium formate) on a steel sheet, which contains chromium metal, chromium oxide and chromium carbides;

[0039] FIG. 4: GDOES spectrum of a layer deposited on a steel sheet electrolytically, using electrolyte solution from a trivalent chromium substance (basic Cr(III) sulfate) without organic complexing agent, which consists essentially of pure chromium oxide.

DETAILED DESCRIPTION

[0040] A strip coating line for performance of the method according to the disclosure is shown schematically in FIG. 1. The strip coating line includes three electrolysis tanks 1a, 1b, 1c arranged one next to or behind the other, each of which is filled with an electrolyte solution E. An initially uncoated blackplate sheet or tinplate sheet (subsequently referred to as sheet B) is passed through electrolysis tanks 1a-1c in succession. Sheet B is pulled for this purpose by a transport device (not shown here) in a sheet travel direction v at a prescribed sheet speed through the electrolysis tanks 1a-1c. Current rolls S are arranged above the electrolysis tanks 1a-1c, through which the sheet B is connected as cathode. A deflection roll U is also arranged in each electrolysis tank around which the sheet B is guided and in so doing directed into or from the corresponding electrolysis tank.

[0041] At least one anode pair AP is arranged within each electrolysis tank 1a-1c beneath the liquid level of the electrolyte solution E. In the depicted embodiment example, two anode pairs AP arranged one behind the other in the sheet travel direction are provided in each electrolysis tank 1a-1c. The sheet B is then guided between the opposite anodes of an anode pair AP. In the embodiment example of FIG. 1, two anode pairs AP are therefore arranged in each electrolysis tank 1a, 1b, 1c, so that the sheet B is guided through these anode pairs AP in succession. The last anode pair APc of the last electrolysis tank 1c, as viewed in the sheet travel direction v in the downstream direction, then has a shortened length in comparison to the other anode pairs AP. A higher current density can thus be generated with this last anode pair APc when an equally high electric current is applied.

[0042] For preparation of the electrolysis process, the sheet B is initially degreased, rinsed, pickled and rinsed again and passed through the electrolysis tanks 1a-1c in succession in this pretreated form, during which sheet B is connected as cathode by supplying electric current via current rolls S. The sheet speed with which the sheet B is passed through the electrolysis tanks 1a-1c is at least 100 m/min and can be as high as 900 m/min.

[0043] The electrolysis tanks 1a to 1c arranged one behind the other in the sheet travel direction are each filled with the same electrolyte solution E. The electrolyte solution E contains a trivalent chromium compound, preferably basic Cr(III) sulfate, Cr.sub.2(SO.sub.4).sub.3. To increase the conductivity, the electrolyte solution E also contains a salt, especially an alkali metal sulfate, for example, potassium or sodium sulfate, as well as an acid or base to adjust an appropriate pH value. The pH value of the first electrolyte solution E is adjusted by adding the acid or base to a preferred value between 2.0 and 5.0. When basic Cr(III) sulfate is used as trivalent chromium compound, sulfuric acid has proven to be an appropriate acid for pH value regulation. The concentration of trivalent chromium compound in the electrolyte solution E preferably lies at at least 10 g/L and with particular preference at 20 g/L or more.

[0044] The temperature of the electrolyte solution E is expediently equally high in electrolysis tanks 1a, 1b, 1c, and preferably lies between 25 C. and 70 C. However, different temperatures of the electrolyte solution can also be set in the electrolysis tanks 1a, 1b, 1c. For example, the temperature of the electrolyte solution in the center electrolysis tank 1b can be lower than in the front electrolysis tank 1a arranged upstream. The temperature of the electrolysis solution in the center electrolysis tank 1b then lies, for example, between 25 C. and 37 C. and especially at 35 C. and the temperature of the electrolyte solution E in the front electrolysis tank 1a lies between 40 C. and 75 C. and especially at 55 C.

[0045] The electrolyte solution E contains no organic components and especially no complexing agent. The electrolyte solution E is also free of halides and buffering agents, such as boric acid.

[0046] The anode pairs AP arranged in electrolysis tanks 1a-1c are exposed to electrical direct current so that a sufficiently high current density is present in the electrolysis tanks 1a, 1b, 1c in order to produce electrolytic deposition of a chromium-containing (especially a Cr(III)-containing) layer. The minimum current density required for this purpose is then dependent on the sheet speed and amounts to about 15 to 20 A/dm.sup.2 at a (minimal) sheet speed of 100 m/min. At higher sheet speeds, the minimum current density required for electrolytic deposition of a chromium-containing layer increases.

[0047] Depending on the sheet speed, the sheet B connected as cathode and passed through electrolysis tanks 1a-1c is effectively in electrolytic contact with the electrolyte solution E during an electrolysis time t1, t2 and t3 in the electrolysis tanks 1a, 1b, 1c, respectively. At sheet speeds between 100 and 700 m/min, the electrolysis time t1, t2, t3 in each of the electrolysis tanks 1a, 1b, 1c lies between 0.5 and 2.0 seconds. High sheet speeds are preferably set so that the electrolysis time in each electrolysis tank 1a, 1b, 1c is less than 2 seconds and especially between 0.6 seconds and 1.8 seconds. The entire electrolysis time tG=t1+t2+t3, in which the sheet B is electrolytically in contact with electrolyte solution E over all electrolysis tanks 1a-1c, is accordingly between 1.8 and 5.4 seconds. The electrolysis time in the individual electrolysis tanks 1a, 1b, 1c can then be adjusted, on the one hand, by the sheet speed, and, on the other hand, by dimensioning of the electrolysis tanks 1a-1c.

[0048] When a current density in the corresponding electrolysis tanks 1a-1c that is greater than the minimum current density is adjusted, a layer is deposited on at least one side of the sheet B in each electrolysis tank 1a, 1b, 1c that consists at least essentially of chromium oxide and/or chromium hydroxide and optionally contains chromium sulfates, when a sulfate-containing electrolyte solution E is used. A layer B1, B2, B3 is then produced in each of the electrolysis tanks 1a, 1b, 1c, in which the composition of layers B1, B2, B3 is at least essentially the same if the same electrolyte solution E is contained in the electrolysis tanks 1a-1c and the same electrolysis parameters, especially the same current densities and temperatures, are used.

[0049] The weight fraction of chromium oxide/chromium hydroxide in the coating weight of the layers B1, B2 and B3 and also the total coating weight of the coating that consists of these layers B1, B2 and B3 then expediently lies at at least 90%, preferably at more than 95%.

[0050] A sectional view of a sheet B electrolytically coated with the method according to the disclosure is schematically shown in FIG. 2. A passivation layer P consisting of the individual layers B1, B2, B3 is applied to one side of sheet B. Each individually layer B1, B2, B3 is then applied to the surface in one of the electrolysis tanks 1a, 1b, 1c.

[0051] The layer structure of the layers B1, B2, B3 deposited on the sheet can be demonstrated by GDOES spectra (Glow Discharge Optical Emission Spectroscopy).

[0052] Comparative experiments have demonstrated that when, according to the disclosure, known prior-art electrolyte solutions with organic complexing agents such as formates are not used, a metallic chromium layer with a thickness of 10-15 nm can be deposited on sheet B in electrolysis tanks 1a, 1b, 1c as a function of electrolysis time. The surface of these layers oxidizes after deposition and is mostly present as chromium oxide in the form Cr.sub.2O.sub.3 or as a mixed oxide-hydroxide in the form Cr.sub.2O.sub.2(OH).sub.2. This oxide layer is a few nanometers thick. In addition, chromium-carbon and chromium sulfate compounds that are formed from reduction of the organic complexing agent or the sulfate of the electrolyte solution are formed, uniformly incorporated through the complete layer. Typical GDOES spectra of layers B1, B2, B3 deposited in the individual electrolysis tanks exhibit a strong increase in oxygen signal in the first nanometers of the layer, from which it can be concluded that the oxide layer is concentrated on the surface of the corresponding layer (FIG. 3).

[0053] A GDOES spectrum of a sheet B is shown in FIG. 4, which was passivated with the method according to the disclosure, using an electrolyte solution E, in which the electrolyte solution E did not contain organic complexing agents, such as formates. It is shown that a layer (passivation layer) is then deposited on sheet B that consists at least essentially ofjust chromium oxide/chromium hydroxide and contains possibly limited fractions of chromium sulfate.

[0054] The composition of the passivation layer can be determined according to the EURO standard DIN EN 10202 (Cr oxides photometric: (Euro norm) step 1: 40 mL NaOH (330 g/L), reaction at 90 C. for 10 minutes, oxidation with 10 mL 6% H.sub.2O.sub.2, photometric @ 370 nm).

[0055] After electrolytic deposition of the passivation layer, the sheet B provided with the passivation layer can be rinsed, dried and oiled (for example, with DOS). The sheet B electrolytically coated with the passivation layer can then additionally be provided with an organic coating. The organic coating is then applied in a known manner, for example, by painting or lamination of a plastic film onto the surface of the passivation layer, i.e., onto the upper layer B3 of chromium oxide. The chromium oxide surface of layer B3 then provided a good adhesive base for the organic material with a coating. The organic coating, for example, can be an organic paint or a polymer film from thermoplastic polymers such as PET, PE, PP or their mixtures. The organic coating can be applied, for example, in a strip coating process or in a plate process in which the coated sheet in the plate process is initially divided into plates that are then painted with an organic paint or coated with a polymer film.

[0056] To achieve a resistance to oxidation and corrosion sufficient for packaging applications, the passivation layers applied by means of the method according to the disclosure preferably have a total coating weight of chromium oxide/chromium hydroxide of at least 3 mg/m.sup.2, and preferably at least 5 mg/m.sup.2. Good adhesion of organic paints or thermoplastic polymer materials on the surface of the passivation layer B could be achieved at coating weights of chromium oxide/chromium hydroxide up to 15 mg/m.sup.2. A preferred range for the coating weight of chromium oxide/chromium hydroxide in the passivation layer therefore lies between 3 and 15 mg/m.sup.2, and a particularly preferred range lies between 5 and 15 mg/m.sup.2.

[0057] The thickness and the coating weight of the individual layers B1, B2, B3 can be adjusted in the depicted embodiment example of the method according to the disclosure by the electrolysis times t1, t2, t3 and the current density in the electrolysis tanks. As soon as a sufficiently high current density is selected for the electrolysis tanks, the thickness and coating weight of the deposited layers B1, B2, B3 will be a linear function of the current density and (at equivalent temperature of the electrolyte solution) of the electrolysis time t1, t2, t3 in the electrolysis tanks, in which the sheet B is effectively in electrolytic contact with the electrolyte solution E.

[0058] The coating weight of the passivation layer can therefore be adjusted by the electrolysis time and/or the current density in which sheet B is effectively in electrolytic contact with electrolyte solution E. The electrolysis time t is again dependent on the dimensioning of the electrolysis tanks and the sheet speed.