Method for the Production of a Metal Strip Coated with a Coating of Chromium and Chromium Oxide Using an Electrolyte Solution with a Trivalent Chromium Compound

Abstract

A method for the production of a metal strip coated with a coating. The coating containing chromium metal and chromium oxide and is electrolytically deposited from an electrolyte solution that contains a trivalent chromium compound onto the metal strip by bringing the metal strip, which is connected as the cathode, into contact with the electrolyte solution. An effective deposition of the coating with a high chromium oxide portion is achieved by successively passing the metal strip at a predefined strip travel speed through a plurality of electrolysis tanks arranged successively in a strip travel direction. The first electrolysis tank is set to a low current density; a second electrolysis tank, which follows in the strip travel direction, is set to a medium current density; and a last electrolysis tank is set to a high current density, where the low current density is greater than 20 A/dm.sup.2.

Claims

1. A method for the production of a metal strip coated with a coating, said coating containing chromium metal and chromium oxide and being electrolytically deposited from an electrolyte solution, which contains a trivalent chromium compound, onto the metal strip by bringing the metal strip, which is connected as the cathode, into contact with the electrolyte solution, the method comprising: successively passing the metal strip at a predefined strip travel speed through a plurality of electrolysis tanks successively arranged in a strip travel direction, wherein the plurality of electrolysis tanks comprises a front group of electrolysis tanks including at least one front electrolysis tank; a middle group of electrolysis tanks including at least one middle electrolysis tank; and a rear group of electrolysis tanks including at least one rear electrolysis tank; wherein the front group of electrolysis tanks is at a low current density (j.sub.1); the middle group of electrolysis tanks, which follows the front group of electrolysis tanks in the strip travel direction is at a medium current density (j.sub.2); and the rear group of electrolysis tanks which follows the middle group of electrolysis tanks in the strip travel direction is at a high current density (j.sub.3); and wherein j.sub.1j.sub.2<j.sub.3 and the low current density is greater than 20 A/dm.sup.2.

2. The method as in claim 1, wherein the current densities in each of the plurality of electrolysis tanks are adjusted to the strip travel speed, wherein there is a substantially linear relationship between the strip travel speed and the respective current density.

3. The method as in claim 1, wherein in each electrolysis tank there is arranged at least one anode pair with two opposing anodes, wherein the metal strip is passed between and through the opposing arranged anodes of an anode pair.

4. The method as in claim 3, wherein the at least one rear electrolysis tank includes a last electrolysis tank and wherein the at least one anode pair of the last electrolysis tank has a shorter length that the at least one anode pairs of the preceding electrolysis tanks.

5. The method as in claim 1, wherein in each of the electrolysis tanks, the electrolysis time, during which the metal strip is in electrolytically effective contact with the electrolyte solution, is less than 2.0 seconds.

6. The method as in claim 1, wherein the total electrolysis time, during which the metal strip is in electrolytically effective contact with the electrolyte solution across the electrolysis tanks is less than 16 seconds.

7. The method as in claim 1, wherein the electrolysis tanks are filled with the electrolyte solution, wherein the electrolyte solution in the electrolysis tanks have at least substantially the same composition and/or temperature, where the mean temperature of the electrolyte solution in the electrolysis tanks is less than 40 C.

8. The method as in claim 1, wherein the mean temperature of the electrolyte solution in the rear group of electrolysis tanks is between 20 C. and 40 C.

9. The method as in claim 1, wherein the at least one rear electrolysis tank includes a last electrolysis tank, wherein the temperature of the electrolyte solution in the last electrolysis tank is less than 40 C. and wherein the temperature of the electrolyte solution in the electrolysis tanks preceding the last electrolysis tank is greater than 40 C.

10. The method as in claim 1, wherein the trivalent chromium compound of the electrolyte solution comprises basic Cr(III) sulfate (Cr.sub.2(SO.sub.4).sub.3).

11. The method as in claim 1, wherein the electrolyte solution, in addition to the trivalent chromium compound, comprises at least one complexing agent, wherein the ratio of the proportion by weight of the trivalent chromium compound to the proportion by weight of the complexing agents is between 1:1.1 and 1:1.4, and/or wherein in order to increase the conductivity, the electrolyte solution comprises an alkali metal sulfate, and/or is free of halides and free of a buffering agent.

12. The method as in claim 1, wherein the concentration of the trivalent chromium compound in the electrolyte solution is at least 10 g/L and/or wherein the pH value of the electrolyte solution measured at a temperature of 20 C. is between 2.0 and 3.0.

13. The method as in claim 1, wherein the metal strip is passed through the electrolysis tanks at a strip travel speed of at least 100 m/min.

14. The method as in claim 1, wherein the coating deposited from the electrolyte solution has a total coating weight of chromium of at least 40 mg/m.sup.2, wherein the proportion of chromium oxide contained in the total weight of deposited chromium is at least 5%.

15. The method as in claim 1, wherein the coating deposited from the electrolyte solution has a chromium oxide content with a deposited weight of chromium bound as chromium oxide of at least 3 mg of Cr per m.sup.2.

16. The method as in claim 1, wherein following the electrolytic deposition of the coating, a cover coat of an organic material is deposited on the coating of chromium metal and chromium oxide.

17. The method as in claim 1, wherein the metal strip is a tin-free steel strip or a steel strip coated with tin.

18. The method as in claim 1, wherein in the front group of electrolysis tanks, a chromium metal- and chromium oxide-containing coating with a proportion by weight of chromium oxide of more than 5% is deposited on a surface of the metal strip.

19. The method as in claim 1, wherein in the middle group of electrolysis tanks, a chromium metal- and chromium oxide-containing coating with a weight portion of chromium oxide of less than 5% is deposited on a surface of the metal strip.

20. The method as in claim 1, wherein in the rear group of electrolysis tanks, a chromium metal- and chromium oxide-containing coating with a proportion by weight of chromium oxide of more than 40% is deposited on a surface of the metal strip.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present disclosure will be described in greater detail with reference to the appended drawings and based on the following embodiment examples, which are merely intended to explain the disclosure by way of example, without in any way limiting the scope of protection defined by the following claims. The drawings show:

[0031] FIG. 1: a diagrammatic representation of a strip coating system for carrying out the method disclosed by the present disclosure in a first embodiment with three electrolysis tanks which are successively arranged in the strip travel direction v;

[0032] FIG. 2: a diagrammatic representation of a strip coating system for carrying out the method disclosed by the present disclosure in a second embodiment with eight electrolysis tanks which are successively arranged in the strip travel direction v;

[0033] FIG. 3: a sectional view of a metal strip coated by means of the method disclosed by the present disclosure in a first embodiment;

[0034] FIG. 4: a GDOES spectrum of a layer electrolytically deposited on a steel strip and containing chromium metal, chromium oxide and chromium carbides, where the chromium oxide is located on the layer surface.

DETAILED DESCRIPTION

[0035] FIG. 1 shows a diagrammatic representation of a strip coating system for carrying out the method disclosed by the present disclosure in a first embodiment. The strip coating system comprises three electrolysis tanks 1a, 1b, 1c which are arranged side by side or one after another and which are each filled with an electrolyte solution E. An initially uncoated metal strip M is passed successively through the electrolysis tanks 1a-1c. To this end, by means of a conveyor device (not shown), the metal strip M is pulled at a predefined strip travel speed through the electrolysis tanks 1a-1c in the 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.

[0036] Within 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 shown, two anode pairs AP arranged one after the other are disposed in each electrolysis tank 1a-1c. The metal strip M is passed through and between the opposing 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 last downstream 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, a higher current density can be generated with this last anode pair APc with application of the same amount of electric current.

[0037] The metal strip M involved can be an initially uncoated steel strip (tin-free steel strip) or a steel strip coated with tin (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, it is subsequently passed successively 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.

[0038] The electrolysis tanks 1a-1c, which are successively arranged in the strip travel direction v, 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. In addition to the trivalent chromium compound, the electrolyte solution preferably also contains at least one complexing agent, for example, a salt of formic acid, in particular potassium formate or sodium formate. The ratio of the proportion by weight of the trivalent chromium compound to the proportion by weight of the complexing agents, especially the formates, is preferably between 1:1.1 and 1:1.4 and is most preferably is 1:1.25. To increase conductivity, the electrolyte solution E may contain an alkali metal sulfate, for example, potassium sulfate or sodium sulfate. The concentration of the trivalent chromium compound in the electrolyte solution E is at least 10 g/L and most preferably is 20 g/L or more. The pH value of the electrolyte solution is adjusted to a preferred value between 2.0 and 3.0 and specifically to a pH=2.7 by adding an acid, for example, sulfuric acid.

[0039] The temperature of the electrolyte solution E is conveniently the same in all electrolysis tanks 1a-1c and is preferably between 25 C. and 70 C. However, in especially preferred embodiment examples of the method according to the present disclosure, it is possible to set the temperatures of the electrolyte solution in the electrolysis tanks 1a-1c to different settings. For example, the temperature of the electrolyte solution of the last electrolysis tank 1c can be lower than that of the electrolysis tanks 1a and 1b disposed upstream thereto. In this embodiment of the method, the temperature of the electrolyte solution in the last electrolysis tank 1c is preferably between 25 C. and 38 C. and most preferably measures 35 C. In this embodiment example, the temperature of the electrolyte solution in the first two electrolysis tanks 1a, 1b is preferably between 40 C. and 75 C. and most preferably measures 55 C. Because of the lower temperature of the electrolyte solution E in the electrolysis tank 1c, the deposition of a chromium/chromium oxide layer with a higher chromium oxide content is promoted.

[0040] The anode pairs AP disposed in the electrolysis tanks 1a-1c are supplied with electric direct current such that there is a different current density in each of the electrolysis tanks 1a, 1b, 1c. The first electrolysis tank 1a, located upstream as viewed in the strip travel direction v, has a low current density j.sub.1; the second electrolysis tank 1b, following in the strip travel direction, has a medium current density j.sub.2; and the last electrolysis tank 1c, as viewed in the strip travel direction, has a high current density j.sub.3, where j.sub.1<j.sub.2<j.sub.3 and the low current density j.sub.1>20 A/dm.sup.2.

[0041] Because of the current density set in each respective electrolysis tank, a chromium- and chromium oxide-containing layer is electrolytically deposited on at least one side of the metal strip M, thereby generating a layer B1, B2, B3 in each of the electrolysis tanks. Because of the different current densities j.sub.1, j.sub.2, j.sub.3 in the individual electrolysis tanks 1a, 1b, 1c, each electrolytically deposited layer B1, B2, B3 has a different composition, which differ in terms of the chromium oxide content.

[0042] FIG. 3 diagrammatically shows a sectional view of a metal strip M which has been electrolytically coated using the method according to the present disclosure. On one side of the metal strip M, a coating B has been deposited, which is composed of the individual layers B1, B2, B3. Each individual layer B1, B2, B3 is applied to the surface in one of the electrolysis tanks 1a, 1b, 1c.

[0043] The coating B, which is composed of the individual layers B1, B2, B3, contains metallic chromium (chromium metal) and chromium oxides (CrOx) as its major constituents, with the composition of the individual layers B1, B2, B3 relative to the respective proportions by weight of chromium metal and chromium oxide differing as a result of the different respective current densities j.sub.1, j.sub.2, j.sub.3 in the electrolysis tanks 1a, 1b, 1c.

[0044] The layer structure of the layers deposited on the metal substrate can be determined by means of GDOES spectra (Glow Discharge Optical Emission Spectroscopy). A metallic chromium layer with a thickness of 10-15 nm is first deposited on the metal strip substrate. The surface of this layer oxidizes and is present mainly as chromium oxide in the form of Cr.sub.2O.sub.3 or as a mixed oxide/hydroxide in the form of Cr.sub.2O.sub.2(OH).sub.2. This oxide layer is only a few nanometers thick. In addition, chromium carbon and chromium sulfate compounds, which are uniformly integrated through the entire layer, are formed as a result of the reduction of the organic complexing agent and the sulfate of the electrolyte solution. Typical GDOES spectra of the layers B1, B2, B3 that were deposited in the individual tanks show a considerable increase in the oxygen signal in the first nanometers of the layer, which leads to the conclusion that the oxide layer on the surface of the respective layer is concentrated (FIG. 4).

[0045] 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 electrolyte solution E during an electrolysis time t.sub.E. At strip travel speeds between 100 and 700 m/min, the electrolysis time in each of the electrolysis tanks 1a, 1b, 1c measures from 0.5 to 2.0 seconds. Preferably, the strip travel speeds are set sufficiently high that the electrolysis time t.sub.E in each electrolysis tank 1a, 1b, 1c is less than 2 seconds and, in particular, is between 0.6 seconds and 1.8 seconds. Accordingly, the total electrolysis time, during which the metal strip M is in electrolytically effective contact with the electrolyte solution E across all electrolysis tanks 1a-1c, is between 1.8 and 5.4 seconds.

[0046] Due to the low current density j.sub.1 in the first electrolysis tank 1a, the layer B1 deposited in the first electrolysis tank 1a, in comparison with the layer B2 deposited in the second (middle) electrolysis tank 1b, has a higher oxide content, since at lower current densities, which occur in Regime II, lead to higher oxide levels in the coating. In the last electrolysis tank 1c, a current density j.sub.3 is set, which is present in Regime III, in which the chromium oxide content generated in the coating is increased, which is preferably greater than 40 wt % and most preferably greater than 50 wt %.

[0047] By way of an example, Table 1 lists suitable current densities j.sub.1, j.sub.2, j.sub.3 in the individual tanks electrolysis tanks 1a, 1b, 1c at different strip travel speeds. As Table 1 indicates, the current densities j.sub.1 in the first electrolysis tank 1a are slightly lower than the current densities j.sub.2 in the second electrolysis tank 1b, and are above a lower limit value of j.sub.0=20 A/dm.sup.2. The current densities j.sub.1, j.sub.2 in the first two electrolysis tanks 1a, 1b are the current densities of Regime II in which there is a linear relationship between current density and the amount of electrolytically deposited chromium (or coating weight of chromium in the deposited coating). The current density j.sub.1 used in the first electrolysis tank 1a is preferably such that it is close to the first current density threshold, which separates Regime I (in which a deposition of chromium does not yet occur) from Regime II. At these low current densities j.sub.1, a chromium metal/chromium oxide coating (layer B1) is deposited on the surface of the metal strip M with a higher chromium oxide content than at the higher current densities of Regime II. Therefore, the layer B1, which is deposited in the first electrolysis tank 1a, has a higher chromium oxide content than the coating B2, which is deposited in the second electrolysis tank 1b.

[0048] In the last electrolysis tank 1a, the current density j.sub.3 is set that it is above the second current density threshold, which separates Regime II from Regime III. The current density j.sub.3 of the last electrolysis tank 1c is thus in Regime III, in which a partial decomposition of the chromium metal/chromium oxide coating takes place and a considerably higher proportion chromium oxide is deposited than at the current densities in Regime II. Therefore, the coating B3, which is deposited in the last electrolysis tank 1c, has a high chromium oxide content which is greater than the chromium oxide content of the coatings B1 and B2.

[0049] After 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 surface of the coating B on the metal strip M which has been electrolytically coated with the coating B. The organic cover coat may be, for example, an organic paint or polymer films of thermoplastic polymers, such as PET, PP or mixtures thereof. The organic cover coat can be applied by means of a coil coating method or a panel coating method, with the coated metal strip in the panel coating method first being divided into panels which are subsequently painted with an organic paint or coated with a polymer film.

[0050] FIG. 2 shows a second embodiment of a strip coating system with eight electrolysis tanks 1a-1h, which are successively connected in the strip travel direction v. The electrolysis tanks 1a-1h are arranged in three groups, i.e., a front group with the two first electrolysis tanks 1a, 1b, a middle group with the electrolysis tanks 1c-1f that follow in the strip travel direction, and a rear group with the two last electrolysis tanks 1g and 1h. The groups of electrolysis tanks have different current densities j.sub.1, j.sub.2, j.sub.3, wherein the front group of electrolysis tanks 1a, 1b has a low current density j.sub.1, the middle group of electrolysis tanks 1c-1f has a medium current density j.sub.2, and the rear group of electrolysis tanks 1g, 1h has a high current density j.sub.3, where j.sub.1<j.sub.2<j.sub.3 and the low current density j.sub.1>20 A/dm.sup.2.

[0051] In the front group of electrolysis tanks 1a, 1b, a chromium- and chromium oxide-containing layer B1, in the second group of electrolysis tanks 1c-1f, a second layer B2, and in the rear group of electrolysis tanks 1g, 1h, a third layer B3 is electrolytically deposited on the metal strip M. As in the embodiment example of FIG. 1, because of the different current densities j.sub.1, j.sub.2, j.sub.3 in the successively arranged electrolysis tanks, the layers B1, B2, B3 have different compositions, with the layer B1 containing a higher chromium oxide content than the second layer B2, and with the third layer B3 containing a higher chromium oxide content than the two layers B1 and B2.

[0052] Like Table 1, Table 2 lists exemplary and suitable current densities j.sub.1, j.sub.2, j.sub.3 in the individual electrolysis tanks 1a to 1h at different strip travel speeds v, wherein the electrolysis tanks 1a, 1b of the front group are set to a low current density j.sub.1, the electrolysis tanks 1c to 1f of the middle group are set to a medium current density j.sub.2, and the electrolysis tanks 1g, 1h of the last group are set to a high current density j.sub.3, where j.sub.1<j.sub.2<j.sub.3.

[0053] Thus, the coating B produced on the surface of the metal strip M by means of the method disclosed by the present disclosure in the strip coating system of FIG. 2 has essentially the same composition and structure as shown in FIG. 3.

[0054] Since the strip coating system of FIG. 2 comprises a larger number of electrolysis tanks, which is necessarily associated with an increase in the total electrolysis time, during which the metal strip, which is connected as the cathode, is in electrolytically effective contact with the electrolyte solution E, it is possible for the coatings B to be produced with higher coating weights.

[0055] To achieve a sufficiently high corrosion resistance, the total weight of chromium deposited in the coating B is preferably at least 40 mg/m.sup.2 and more preferably between 70 mg/m.sup.2 and 180 mg/m.sup.2. The proportion of chromium oxide contained in the total weight of deposited chromium, averaged across the total weight of the coating B, is at least 5% and is preferably between 10% and 15%. Overall, the coating B preferably has a chromium oxide content with a deposited weight of chromium bound as chromium oxide of at least 3 mg of chromium per m.sup.2 and particularly 3 to 15 mg/m.sup.2.

[0056] The deposited weight of chromium bound as chromium oxide, averaged across the total surface area of the coating B, is at least 7 mg of chromium per m.sup.2. Good adhesion of organic paints or thermoplastic polymer materials to the surface of the coating B can be achieved with chromium oxide weights of up to approximately 15 mg/m.sup.2. Therefore, a preferred range for the coating weight of chromium oxide in the coating B is between 5 and 15 mg/m.sup.2.

[0057] In the embodiment example of FIG. 2, the total electrolysis time, during which the metal strip M is in electrolytically effective contact with the electrolyte solution E, averaged across all electrolysis tanks 1a-1h, is preferably less than 16 seconds and more specifically between 4 and 16 seconds.

TABLE-US-00001 TABLE 1 Current densities j.sub.1, j.sub.2, j.sub.3 in the individual electrolysis tanks of the first embodiment example (with 3 electrolysis tanks 1a-1c) at different strip travel speeds V: Tank 1a 1b 1c v J.sub.1/ J.sub.2/ J.sub.3/ [m/min] [A/dm.sup.2] [A/dm.sup.2] [A/dm.sup.2] 100 25 29 75 150 41 45 91 200 57 61 107 300 73 77 133 400 89 93 149 500 105 109 165

TABLE-US-00002 TABLE 2 Current densities j.sub.1, j.sub.2, j.sub.3 in the individual electrolysis tanks of the second embodiment example (with 8 electrolysis tanks 1a-1h which are arranged in three groups) at different strip travel speeds v: Tank 1a 1b 1c 1d 1e 1f 1g 1h v J.sub.1/[A/ J.sub.1/[A/ J.sub.2/[A/ J.sub.2/[A/ J.sub.2/[A/ J.sub.2/[A/ J.sub.3/[A/ J.sub.3/[A/ [n/min] dm.sup.2] dm.sup.2] dm.sup.2] dm.sup.2] dm.sup.2] dm.sup.2] dm.sup.2] dm.sup.2] 100 25 25 29 29 29 29 75 75 150 41 41 45 45 45 45 91 91 200 57 57 61 61 61 61 107 107 300 73 73 77 77 77 77 133 133 400 89 89 93 93 93 93 149 149 500 105 105 109 109 109 109 165 165