Method of Producing a Metal Strip Coated with a Coating of Chromium and Chromium Oxide Using an Electrolyte Solution with a Trivalent Chromium Compound and Electrolysis System for Implementing the Method

Abstract

A method of producing a metal strip coated with a coating, said coating containing chromium metal and chromium oxide and being electrolytically deposited from an electrolyte solution that contains a trivalent chromium compound and at least one salt for increasing conductivity and at least one acid or one base for setting a desired pH value, onto the metal strip by bringing the metal strip into electrolytically effective contact with the electrolyte solution during an electrolysis time. The metal strip is successively passed at a predefined strip travel speed in a strip travel direction through a plurality of electrolysis tanks successively arranged in the strip travel direction. At least the first electrolysis tank, as viewed in the strip travel direction, or a front group of electrolysis tanks is filled with a first electrolyte solution and the last electrolysis tank, as viewed in the strip travel direction, or a rear group of electrolysis tanks is filled with a second electrolyte solution. The second 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 of producing a metal strip coated with a coating, the coating containing chromium metal and chromium oxide/chromium hydroxide and being electrolytically deposited from an electrolyte solution that contains a trivalent chromium compound and at least one salt for increasing conductivity and at least one acid or one base for setting a desired pH value, on the metal strip by bringing the metal strip during an electrolysis time into electrolytically effective contact with the electrolyte solution, the method comprising: successively passing the metal strip at a predetermined strip travel speed through a plurality of electrolysis tanks successively disposed in a strip travel direction, with at least the first electrolysis tank, as viewed in the strip travel direction, or a front group of electrolysis tanks being filled with a first electrolyte solution and with the last electrolysis tank, as viewed in the strip travel direction, or a rear group of electrolysis tanks being filled with a second electrolyte solution, with the second electrolyte solution, except for the trivalent chromium compound and the at least one salt and the at least one acid or base, containing no other constituents and being free of organic complexing agents and free of buffering agents.

2. The method of claim 1, wherein a layer containing at least for the most part of chromium oxide and/or chromium hydroxide is deposited in the last electrolysis tank or in the rear group of electrolysis tanks.

3. The method of claim 2, wherein the layer containing chromium oxide and/or chromium hydroxide has a weight portion of chromium oxide and/or chromium hydroxide of more than 90%.

4. The method of claim 2, wherein the composition of the first electrolyte solution differs from that of the second electrolyte solution, with the first electrolyte solution and the second electrolyte solution, in addition to the trivalent chromium compound, containing at least one salt and at least one acid or base and being free of chloride ions and free of buffering agents and with the first electrolyte solution containing organic complexing agents.

5. The method of claim 1, wherein the metal strip is successively passed at a predefined strip travel speed through the electrolysis tanks, with the strip travel speed being at least 100 m/min, and with the metal strip being brought into contact with the first electrolyte solution during a first electrolysis time and into contact with the second electrolyte solution during a second electrolysis time, with the total electrolysis time being in a range of 0.5 to 6.0 seconds, and with the first electrolysis time, during which the metal strip is electrolytically effectively in contact with the first electrolyte solution being shorter than 2.0 seconds, and with the second electrolysis time during which the metal strip is electrolytically effectively in contact with the second electrolyte solution being shorter than 2.0 seconds.

6. The method of claim 1, wherein the temperature of the first and the second electrolyte solution, averaged over the volume of the particular electrolysis tank, is in a range of 20 C. to 65 C.

7. The method of claim 1, wherein the first electrolyte solution and the second electrolyte solution contain each a trivalent chromium compound selected from the group comprising 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 that the first electrolyte solution and the second electrolyte solution each contain at least one salt for the purpose of increasing the conductivity, said salt containing at least one alkali metal sulfate, and wherein the first electrolyte solution and the second electrolyte solution are free of halides and free of chloride ions and bromide ions.

8. The method of claim 1, wherein the first electrolyte solution and/or the second electrolyte solution have/has a pH value (measured at a temperature of 20 C.) in a range of 2.3 to 5.0, with the pH value being set by adding at least one acid to the first or the second electrolyte solution.

9. The method of claim 1, wherein the concentration of the trivalent chromium compound in the first electrolyte solution and/or in the second electrolyte solution is at least 10 g/L.

10. The method of claim 1, wherein the total coating weight of chromium oxide and/or chromium hydroxide of the layer of chromium oxide/chromium hydroxide deposited from the second electrolyte solution is at least 3 mg/m.sup.2.

11. The method of claim 1, wherein after the coating has been electrolytically deposited, a cover coat of an organic material is applied to the coating.

12. The method of claim 1, wherein the use of a suitable anode during the electrolytic deposition of the coating prevents an oxidation of chromium(III) from the trivalent chromium compound of the electrolyte solution to chromium(VI).

13. The method of claim 1, wherein the anode is free of stainless steel and platinum.

14. The method of claim 13, wherein the anode has an outside surface or a coating of a metal oxide or of a mixed metal oxide.

15. The method of claim 1, wherein, for the purpose of preparing the second electrolyte solution, the trivalent chromium compound, which, except for unavoidable contaminants and residual constituents, has been liberated of organic residues, the at least one salt and the at least one acid or one base for the purpose of setting a pH value desired are dissolved in water, and the solution thus obtained is allowed to stand for at least 5 days for the purpose of complexation, and the pH value is then finely adjusted by adding an acid or a base.

16. An electrolysis system for electrolytically depositing a chromium- and chromium oxide- and/or chromium hydroxide-containing coating onto the surface of a metal strip, the electrolysis system comprising: at least one first electrolysis tank filled with a first electrolyte solution or a group of front electrolysis tanks which are each filled with a first electrolyte solution, a last electrolysis tank filled with a second electrolyte solution or a group of rear electrolysis tanks which are each filled with a second electrolyte solution, wherein the composition of the first electrolyte solution is different from that of the second electrolyte solution in that the first electrolyte solution contains a trivalent chromium compound and at least one salt for increasing the conductivity and at least one acid or one base for setting a desired pH value and organic complexing agents, and wherein the second electrolyte solution, except for a trivalent chromium compound and at least a salt for increasing the conductivity and at least one acid or one base for setting a desired pH value, contains no other constituents and is free of organic components and free of buffering agents, wherein the metal strip, for the purpose of electrolytically depositing the passivating layer, is passed at a predefined strip travel speed through the electrolysis tanks in a strip travel direction, which causes the surface to come into electrolytically effective contact with the first and the second electrolyte solution, as a result of which a layer of chromium metal and chromium oxide and, where applicable, additional chromium compounds is deposited in the first electrolysis tank or in the front group of electrolysis tanks, and a layer, which consists at least for the most part only of chromium oxide, is deposited in the last electrolysis tank or in the rear group of electrolysis tanks.

17. A steel strip with a coating produced on a surface of the strip by electrolytic deposition, the coating containing chromium and chromium oxide/chromium hydroxide, wherein the coating is composed of a first layer, which faces the surface of the strip, and a second layer lying on top, wherein the first layer contains metallic chromium and the second layer contains at least for the most part only of chromium oxide and/or chromium hydroxide.

18. The steel strip of claim 16, wherein the weight portion of chromium oxide and/or chromium hydroxide in the second layer is higher than 90%.

19. The steel strip of claim 16, wherein the weight portion of chromium oxide and/or chromium hydroxide in the second layer is greater than 95%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] 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:

[0038] FIG. 1: schematic depiction of a first embodiment example of a strip coating line for the performance of the method according to the disclosure, comprising three electrolysis tanks successively connected to each other in the strip travel direction v;

[0039] FIG. 2: schematic depiction of a second embodiment example of a strip coating line for the performance of the method according to the present disclosure, comprising eight electrolysis tanks successively connected to each other in the strip travel direction v;

[0040] FIG. 3: a sectional drawing of a metal strip coated by means of the method according to the present disclosure as described in the first embodiment example;

[0041] FIG. 4: GDOES spectrum of a layer which has been electrolytically deposited onto a steel strip, using an electrolyte solution containing a trivalent chromium substance (basic Cr(III) sulfate) and an organic complexing agent (sodium formate), which layer contains chromium metal, chromium oxide and chromium carbide, with the chromium oxide being located mainly on the surface of the layer.

DETAILED DESCRIPTION

[0042] 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 metal strip M, for example, a blackplate strip or a tinplate strip, is successively passed through the electrolysis tanks 1a-1c. To this end, using a conveyor device (not shown), the metal strip M is pulled at a predefined strip travel speed through the electrolysis tanks 1a-1c in a strip travel direction v. Disposed above the electrolysis tanks 1a-1c are conductor rollers S, by means of which the metal strip M is connected as the cathode. Also disposed in each electrolysis tank is a guide roller U, around which the metal strip M is guided and thereby moved into and out of the electrolysis tank.

[0043] Inside each electrolysis tank 1a-1c, at least one anode pair AP is disposed below the fluid level of the electrolyte solution E. In the embodiment example illustrated, two anode pairs AP, which are successively connected to each other in the strip travel direction, are disposed in each electrolysis tank 1a-1c. The metal strip M is passed through between the oppositely disposed anodes of an anode pair AP. Thus, in the embodiment example of FIG. 1, two anode pairs AP are arranged in each electrolysis tank 1a, 1b, 1c such that the metal strip M is successively passed through these anode pairs AP. The downstream last anode pair APc of the last electrolysis tank 1c, as viewed in the strip travel direction v, has a shorter length when compared to the lengths of the other anode pairs AP. As a result, with this last anode pair APc, it is possible to generate a higher current density while applying the same amount of electric current.

[0044] The metal strip M involved can be an initially uncoated steel strip (blackplate strip) or a tin-plated steel strip (tinplate strip). In preparation for the electrolysis process, the metal strip M is first degreased, rinsed, pickled and rinsed again, and in this pretreated form is subsequently successively passed through the electrolysis tanks 1a-1c, with the metal strip M being connected as the cathode by supplying electric current via the conductor rollers S. The strip travel speed at which the metal strip M is passed through the electrolysis tanks 1a-1c is at least 100 m/min and may measure up to 900 m/min.

[0045] The front electrolysis tanks 1a and 1b, as viewed in the strip travel direction v, are each filled with the same electrolyte solution E1. This first electrolyte solution E1 contains a trivalent chromium compound, preferably basic Cr(III) sulfate, Cr.sub.2(SO.sub.4).sub.3. In addition to the trivalent chromium compound, the first electrolyte solution E1 contains at least one organic complexing agent, for example, a salt of formic acid, especially potassium or sodium format [sic; formate]. The ratio of the weight portion of the trivalent chromium compound to the weight portion of the complexing agents, especially the formates, is preferably between 1:1.1 and 1:1.4 and most preferably 1:1.25. To increase conductivity, the first electrolyte solution E1 contains a salt, especially an alkali metal sulfate, for example, potassium sulfate or sodium sulfate. The concentration of the trivalent chromium compound in the first electrolyte solution E1 is at least 10 g/L and most preferably 20 g/L or higher. The pH value of the first electrolyte solution E1 is set to a preferred value between 2.0 and 3.0 and especially to pH=2.7 by adding an acid, for example, sulfuric acid.

[0046] The temperature of the first electrolyte solution E1 is expediently equally high in the two front electrolysis tanks 1a, 1b and is preferably in a range of 25 C. to 70 C. However, the electrolyte solution in the two front electrolysis tanks 1a-1b can also be set to different temperatures. Thus, for example, the temperature of the electrolyte solution in the middle electrolysis tank 1b can be lower than in the front electrolysis tank 1a disposed upstream. The temperature of the electrolyte solution in the middle electrolysis tank 1b is preferably in a range of 25 C. to 37 C. and is especially 35 C., and the temperature of the first electrolyte solution E1 in the front electrolysis tank 1a is preferably in a range of 40 C. to 75 C. and is especially 55 C. Because of the lower temperature of the electrolyte solution E1, the deposition of a chromium/chromium oxide layer with a higher portion of chromium oxide in the middle electrolysis tank 1b is promoted.

[0047] The rear or, more specifically, last electrolysis tank 1c, as viewed in the strip travel direction v, is filled with a second electrolyte solution E2, the composition of which differs from that of the first electrolyte solution E1 at least in that the second electrolyte solution E2 contains no organic constituents and especially no complexing agents. In all other respects, the constituents of the second electrolyte solution E2 can be the same as those of the first electrolyte solution E1. More specifically, the second electrolyte solution E2 also contains a trivalent chromium compound, preferably basic Cr(III) sulfate, Cr.sub.2(SO.sub.4).sub.3, and at least one salt and one acid or base for setting a suitable pH value. The salt which, as in the first electrolyte solution E1, can be an alkali metal sulfate, for example, potassium sulfate or sodium sulfate, serves to increase the conductivity. The concentration of the trivalent chromium compound in the second electrolyte solution E2 is at least 10 g/L and most preferably 20 g/L or higher. The pH value of the second electrolyte solution E2 is set to a preferred value between 2.0 and 5.0 and especially to pH=4.0 by adding the acid or base, for example, sulfuric acid.

[0048] The temperature of the second electrolyte solution E2 in the rear electrolysis tank 1c is expediently in a range of 25 C. to 70 C., more preferably from 25 C. to 40 C., and is most preferably 35 C.

[0049] DC current is applied to the anode pairs AP arranged in electrolysis tanks 1a-1c 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 strip travel speed and amounts to about 15 to 20 A/dm.sup.2 at a (minimal) strip travel speed of 100 m/min. At higher strip travel speeds, the minimum current density required for electrolytic deposition of a chromium-containing layer increases.

[0050] Depending on the strip travel speed, the metal strip M, which is connected as the cathode and which is passed through the electrolysis tanks 1a-1c, is in electrolytically effective contact with the first electrolyte solution E1 in the two front electrolysis tanks 1a, 1b during an electrolysis time t1 and is thereafter in electrolytically effective contact with the second electrolyte solution E2 in the rear electrolysis tank 1c during an electrolysis time t2. At strip travel speeds between 100 and 700 m/min, the electrolysis time in each of the electrolysis tanks 1a, 1b, 1c is in a range of 0.5 to 2.0 seconds. The strip travel speeds are preferably set at such a high setting that the electrolysis time in each electrolysis tank 1a, 1b, 1c is shorter than 2 seconds and is especially in a range of 0.6 seconds to 1.8 seconds. The total electrolysis time tG=t1+t2+t3, during which the sheet B is in electrolytically effective contact with the first and the second electrolyte solution E1, E2 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 be adjusted, on the one hand, by means of the strip travel speed and, on the other hand, by means of dimensioning the electrolysis tanks 1a-1c.

[0051] If the current density in the respective electrolysis tanks 1a-1c is set higher than the minimum current density, a layer that contains chromium and chromium oxide/chromium hydroxide and chromium carbide and, if a sulfate-containing first electrolyte solution E1 is used, possibly also chromium sulfate, is deposited onto at least one surface of the metal strip M in the front electrolysis tank 1a and in the middle electrolysis tank 1b. In each of the two electrolysis tanks 1a, 1b, this leads to the formation of a layer B1 and B2, respectively, the composition of which layers B1, B2, especially with regard to the weight portion of chromium oxide/chromium hydroxide, can differ if the electrolysis parameters used, especially the current densities and temperatures, in the front electrolysis tank 1a differ from those in the middle electrolysis tank 1b.

[0052] In the rear electrolysis tank 1c, an upper layer B3 is deposited onto at least one surface of the metal strip M, which layer consists, at least for the most part, of pure chromium oxide and/or chromium hydroxide. The weight portion of chromium oxide/chromium hydroxide relative to the total coating weight of the upper layer B3 expediently accounts for 90%, preferably for more than 95%.

[0053] FIG. 3 shows a diagrammatic sectional drawing of a metal strip M which has been electrolytically coated using the method according to the present disclosure. One surface of the metal strip M is coated with a coating B which is composed of the individual layers B1, B2, B3.

[0054] Each individual layer B1, B2, B3 is applied to the surface in one of the electrolysis tanks 1a, 1b, 1c.

[0055] The two lower layers B1, B2 facing the surface of the metal strip M contain metallic chromium (chromium metal) and chromium oxides (CrOx)/chromium hydroxides and chromium carbides and, where applicable, chromium sulfates as main constituents, with the composition of the individual layers B1, B2, especially with regard to the weight portion of chromium metal and chromium oxide/chromium hydroxide contained in each, being the same or different, depending on whether the same or different electrolysis parameters were used in the two front electrolysis tanks 1a, 1b. The upper layer B2 facing away from the surface of the metal strip M for the most part contains only chromium oxides (CrOx) and/or chromium hydroxides and especially no chromium carbides and hardly any metallic chromium and chromium sulfates.

[0056] The layer buildup of the layers B1, B2, B3 deposited onto the metal substrate can be demonstrated by means of GDOES spectra (Glow Discharge Optical Emission Spectroscopy). In the two front electrolysis tanks 1a, 1b, first a metallic chromium layer with a thickness of 10-15 nm is deposited on the metal strip M. The surface of this layer oxidizes and consists mainly of chromium oxide in the form of Cr.sub.2O.sub.3 or of mixed oxide/hydroxide in the form of Cr.sub.2O.sub.2(OH).sub.2. This oxide layer is only few nanometers thick. In addition, chromium carbon and chromium sulfate compounds form as a result of the reduction of the organic complexing agent and the sulfate of the electrolyte solution, respectively, which compounds are uniformly distributed throughout the entire layer. In the first nanometers of the layer, typical GDOES spectra of the layers B1, B2 which have been deposited in the individual electrolysis tanks show a sharp increase in the oxygen signal, from which it is possible to conclude that the oxide layer is concentrated on the surface of the particular layer (FIG. 4).

[0057] The composition of the layers can be determined according to the EURO Standard DIN EN 10202 (Cr oxides, photometric: (European standard), step 1: 40 mL of NaOH (330 g/L), reaction at 90 C. for 10 minutes, oxidation with 10 mL of 6% H.sub.2O.sub.2, photometric@370 nm).

[0058] Following the electrolytic deposition of the coating, the metal strip M coated with the coating B is rinsed, dried and oiled (for example, with DOS oil). Subsequently, an organic cover coat can be applied to the metal strip M which has been electrolytically coated with the coating B.

[0059] The organic cover coat is applied by conventional means, for example, by painting or laminating a plastic film onto the surface of the coating B, i.e., onto the top layer B3 of chromium oxide/chromium hydroxide. The top layer B3 of chromium oxide/chromium hydroxide provides an excellent adhesive base for the organic material of the cover coat. The cover coat can be, for example, an organic paint or polymer films of thermoplastic polymers, such as PET, PE, PP or mixtures thereof. The organic top coat can be applied, for example, in a coil coating process or in a plate process, in which the coated metal strip in the plate coating process is initially divided into plates that are then painted with an organic paint or coated with a polymer film.

[0060] FIG. 2 shows a second embodiment example of a strip coating line with eight electrolysis tanks 1a1-h successively disposed one after the other in the strip travel direction v. The electrolysis tanks 1a1-h are arranged in three groups, i.e., a front group comprising the first two electrolysis tanks 1a, 1b, a middle group comprising the electrolysis tanks 1c-1f following downstream in the strip travel direction, and a rear group comprising the two last electrolysis tanks 1g and 1h.

[0061] The electrolysis tanks 1a, 1b of the front group and the electrolysis tanks 1c, 1d, 1e, 1f of the middle group are each filled with the first electrolyte solution E1 which contains organic complexing agents, especially formates. The electrolysis tanks 1g, 1h of the rear group are filled with the second electrolyte solution E2 which is free of organic substances and especially free of complexing agents.

[0062] With a strip coating line configured as shown in FIG. 2, it is possible to create the same layer buildup with the layers B1, B2 and B3 as with the strip coating line shown in FIG. 1. However, by arranging the electrolysis tanks 1a to 1h in groups, the overall electrolysis time can be increased. This makes it possible to operate at a higher strip travel speed or to increase the coating weights of the layers B1, B2, B3 while maintaining the same strip travel speed.

[0063] To ensure a sufficiently high corrosion resistance for packaging applications, the coatings B preferably have a total coating weight of chromium of at least 40 mg/m.sup.2 and most preferably of 70 mg/m.sup.2 to 180 mg/m.sup.2. When summed up over the overall weight of the deposited coating B, the weight portion of chromium oxide/chromium hydroxide relative to the total coating weight of chromium accounts for at least 15% and is preferably in a range of 20% to 40%. The coating B preferably has a total chromium oxide portion with a coating weight of chromium bound in the form of chromium oxide and/or chromium hydroxide of at least 3 mg of chromium per m.sup.2 and especially in a range of 3 to 15 mg/m.sup.2. When summed up over the overall weight of the deposited coating B, the coating weight of chromium bound in the form of chromium oxide and/or chromium hydroxide accounts for at least 5 and preferably for more than 7 mg of chromium per m.sup.2. Good adhesive strength for organic paints or thermoplastic polymer materials on the surface of the coating B can be achieved with coating weights of chromium oxide/chromium hydroxide of up to approximately 15 mg/m.sup.2. Consequently, a preferred coating weight of chromium oxide/chromium hydroxide in the coating B is in a range of 5 to 15 mg/m.sup.2.

[0064] The thickness and the respective 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 means of the electrolysis times t1, t2, t3 and the current density in the electrolysis tanks. As soon as a sufficiently high current density has been set in the electrolysis tanks, the thickness and the respective coating weight of the deposited layers B1, B2, B3 no longer depends on the current density but (with the electrolyte solution being maintained at the same the temperature) only on the electrolysis time t1, t2, during which the metal strip M is in electrolytically effective contact with the first and the second electrolyte solution E1, E2.

[0065] Thus, the weight portion of chromium oxide/chromium hydroxide relative to the total coating weight of the coating B can be determined by setting the electrolysis time t2 during which the metal strip M in the rear electrolysis tank 1c or in the rear group of electrolysis tanks 1g, 1h is in electrolytically effective contact with the second electrolyte solution E2. This electrolysis time t2 in turn depends on the dimensions of the rear electrolysis tank 1c and 1g, 1h and on the strip travel speed.