METHOD FOR PASSIVATING THE SURFACE OF A TINPLATE AND ELECTROLYSIS SYSTEM FOR CARRYING OUT THE METHOD

Abstract

In a method for passivating the surface of a tinplate using electrolytic deposition of a passivation layer containing chromium oxide/chromium hydroxide on the surface, the electrolytic deposition of the passivation layer is carried out at least partly from an electrolyte solution which contains a trivalent chromium compound, at least one salt for increasing the conductivity and at least one acid or one base for adjusting a desired pH value and is free from organic complexing agents and free from buffering agents. In order to increase the amount of chromium oxide in the passivation layer, after the electrolytic deposition of the passivation layer, the passivated tinplate is subjected to a thermal treatment in which the passivated tinplate is kept at a treatment temperature of 100° C. or more for a treatment time of at least 0.5 seconds.

Claims

1. A method for passivating the surface of a tinplate comprising electrolytic deposition of a chromium oxide/chromium hydroxide containing passivation layer on the surface, the electrolytic deposition of the passivation layer being effected at least partly from an electrolyte solution which contains a trivalent chromium compound, at least one salt for increasing the conductivity and at least one acid or one base for adjusting a desired pH value and is free from organic complexing agents and free from buffering agents, wherein, after the electrolytic deposition of the passivation layer, the passivated tinplate is subjected to a thermal treatment in which the passivated tinplate is kept at a treatment temperature of 100° C. or more for a treatment time of at least 0.5 seconds.

2. The method according to claim 1, wherein the treatment time is between 0.5 seconds and 30 minutes and the treatment temperature is between 150° C. and 232° C.

3. The method according to claim 1, wherein the thermal treatment comprises inductive heating of the passivated tinplate to the treatment temperature.

4. The method according to claim 1, wherein the thermal treatment is carried out immediately after the electrolytic deposition of the passivation layer.

5. The method according to claim 1, wherein the thermal treatment is carried out within an intermediate time of at most 10 seconds after the completion of the deposition of the passivation layer.

6. The method according to claim 1, wherein the electrolyte solution has an averaged temperature in the range from 20° C. to 80° C. and the passivated tinplate immediately after completion of the electrolytic deposition of the passivation layer has at least substantially a tinplate temperature equivalent to the averaged temperature of the electrolyte solution and in the thermal treatment is heated to the treatment temperature starting from the tinplate temperature.

7. The method according to claim 1, wherein the passivated tinplate is heated to the treatment temperature at a heating rate of 100 to 700 K/s immediately after completion of the electrolytic deposition of the passivation layer.

8. The method according to claim 1, wherein the electrolyte solution consists of the trivalent chromium compound, the at least one salt and the at least one acid or base and a solvent.

9. The method according to claim 1, wherein the passivation layer, besides unavoidable by-products comprising chromium sulfate, consists of at least one of trivalent chromium oxide and chromium hydroxide.

10. The method according to claim 1, wherein the passivation layer has a weight fraction of chromium oxide and/or chromium hydroxide of more than 95% and the remainder of the passivation layer is formed by unavoidable by-products comprising chromium sulfate.

11. The method according to claim 1, wherein the tinplate for electrolytic deposition of the passivation layer is passed at a speed in a running direction through at least one electrolytic tank or a plurality of electrolytic tanks arranged one behind the other in the running direction, the speed being at least 100 m/min.

12. The method according to claim 11, wherein the tinplate is passed through the plurality of electrolytic tanks arranged one behind the other in the running direction and wherein the electrolysis time, in which the tinplate is electrolytically effectively in contact with the electrolyte solution, in each of the plurality of electrolysis tanks is less than 1.0 seconds and/or the total electrolysis time during which the tinplate is in electrolytically effective contact with the electrolyte solution in all electrolyte tanks is between 0.5 seconds and 2.0 seconds.

13. The method according to claim 11, wherein the tinplate is passed through the plurality of electrolytic tanks arranged one behind the other in the running direction and wherein at least the last electrolytic tank, as viewed in the running direction, contains an electrolyte solution which consists of the trivalent chromium compound, the at least one salt and the at least one acid or base and a solvent.

14. 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 in terms of chromium.

15. The method according to claim 1, wherein a tin oxide layer consisting essentially of tetravalent tin oxide (SnO.sub.2) is developed on the surface of the tinplate prior to the electrolytic deposition of the passivation layer by placing the tinplate as anode in an aqueous electrolyte containing a phosphate, borate, sulfate or carbonate.

16. An electrolysis system for electrolytically passivating the surface of a tinplate by depositing a chromium oxide/chromium hydroxide containing passivation layer on a surface of the tinplate, the electrolysis system comprising: at least one electrolytic tank filled with a first electrolyte solution, or a plurality of electrolytic tanks arranged in series, of which at least one electrolytic tank is filled with the first electrolyte solution and the remaining electrolytic tanks are filled with the first electrolyte solution or another electrolyte solution which contains a trivalent chromium compound and whose composition differs from the first electrolyte solution, wherein the first electrolyte solution contains a trivalent chromium compound as well as at least one salt for increasing the conductivity and at least one acid or one base for adjusting a desired pH value and is free from organic complexing agents as well as free from buffering agents; a transport device for transporting a strip-shaped tinplate in a running direction at a predetermined speed through the at least one electrolytic tank or successively through the plurality of electrolytic tanks and for an electrolytic deposition of a passivation layer from the first electrolyte solution on the surface of the tinplate; a heating device arranged downstream of the electrolytic tank or downstream of the plurality of electrolytic tanks in the running direction and designed to heat the passivated tinplate to a predetermined treatment temperature of at least 100° C. for a predetermined treatment time of at least 0.5 seconds.

17. The electrolysis system according to claim 16, wherein the heating device is arranged directly after the electrolytic tank or directly after the last electrolytic tank of the plurality of electrolytic tanks, as viewed in the running direction.

18. A tinplate having a surface passivated by electrolytic deposition of a chromium oxide/chromium hydroxide containing passivation layer, the passivation layer consisting at least essentially of chromium oxide and/or chromium hydroxide with a chromium oxide and/or chromium hydroxide weight fraction of more than 95%, wherein, after storage for at least four weeks in an oxygen-containing atmosphere, the passivated tinplate has on the tin surface a tin oxide layer with a tin oxide coating of less than 70 C/m.sup.2.

19. The tinplate according to claim 18, wherein the tinplate has the following sequence of a layered structure: a steel substrate, a metallic tin layer, the tin oxide layer and the passivation layer.

20. The tinplate according to claim 18, wherein the passivation layer comprises at least a top layer consisting essentially, apart from unavoidable impurities or residuals, of trivalent chromium oxide, chromium hydroxide and residual chromium sulfate, with the weight fraction of trivalent chromium oxide in the top layer being at least 95%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Below, the invention is explained in more detail by means of embodiment examples with reference to the accompanying drawings, these embodiment examples explaining the invention by way of examples only and are not limiting it with respect to the scope of protection defined by the following claims. The drawings show:

[0061] FIGS. 1A and 1B: Schematic representation of two different embodiments of an electrolysis system according to the invention for carrying out the method according to the invention, wherein FIG. 1A shows a first embodiment with an electrolytic tank filled with a trivalent chromium electrolyte solution and FIG. 1B shows a second embodiment with two electrolytic tanks, each filled with a trivalent chromium electrolyte solution of different composition;

[0062] FIGS. 2A and 2B: Schematic sectional views of embodiments of tinplate according to the invention, where FIG. 2A shows an embodiment with a single-layer passivation layer and FIG. 2B shows an embodiment with a double-layer passivation layer;

[0063] FIG. 3: Shows the method and material parameters of the tested tinplate samples from the laboratory tests; and

[0064] FIG. 4: Summarizes the results of the plant test.

DETAILED DESCRIPTION

[0065] FIGS. 1A and 1B schematically show various embodiments of electrolysis systems for carrying out the method according to the invention. The electrolysis system of FIG. 1A comprises three tanks 1a, 1b, 1c arranged next to each other or one behind the other in a strip running direction, the first tank 1a being filled with a basic electrolyte BE, the middle tank 1b being filled with a rinsing solution Sp and the last tank 1c being filled with a first electrolyte solution E1.

[0066] The basic electrolyte BE consists of an aqueous soda solution (sodium carbonate solution with a concentration of 1 to 10 wt. % and a pH value of 10 to 11). The rinsing solution Sp consists of distilled or demineralized water.

[0067] The first electrolyte solution E1 is an aqueous solution of a trivalent chromium compound, which furthermore comprises a salt for increasing conductivity and an acid for adjusting a desired pH value between 2.5 and 3.5 and is free from organic complexing agents and buffering agents. In a preferred embodiment, the first electrolyte solution E1 consists of the trivalent chromium compound, in particular Cr-(III) sulfate, the salt (e.g. potassium sulfate or sodium sulfate), the acid (e.g. sulfuric acid) and water as solvent and otherwise has no other components. The first electrolyte solution E1 contains in particular no organic components, in particular no organic complexing agents such as formates and no buffering agents such as boric acid, and is free of halides. An example of the composition of the first electrolyte solution E1 is given in Table 1. The concentration of the trivalent chromium compound in the first electrolyte solution E1 is preferably at least 10 g/L and particularly preferably 20 g/L or more. The temperature of the first electrolyte solution E1 is preferably between 25° C. and 70° C.

[0068] A cathode pair KP is arranged in the first tank 1a and an anode pair AP is arranged in the last tank 1c. The anode pair AP is free of stainless steel and platinum and contains a coating of a metal oxide such as iridium oxide or a mixed metal oxide such as tantalum-iridium oxide. The anodes of the AP anode pair may also be made entirely of a metal oxide or a mixed metal oxide. Electric current may be applied to the cathodes of cathode pair KP and the anodes of anode pair AP.

[0069] A tinned steel strip (tinplate strip, hereinafter also referred to as strip B) is successively fed through the tanks 1a-1c. The strip B is drawn through the tanks 1a-1c by a transport device not shown here in a strip running direction v at a predetermined strip speed of preferably more than 100 m/min and in particular in the range from 100 to 750 m/min. Current rollers S are arranged above the tanks 1a-1c, via which the belt B can be switched as anode or cathode. In each electrolytic tank and above tanks 1a-1c, deflection rollers U are also arranged, around which the strip B is guided and thereby fed through tanks 1a-1c. The strip B is guided between the opposing cathodes of the cathode pair KP and between the two anodes of the anode pair AP.

[0070] A first heating device H1 is arranged downstream of the last tank 1c and a second heating device H2 is arranged upstream of the first tank. The heating devices H1 and H2 can each be a continuous furnace in which the strip B is heated to a predefinable temperature and held at this temperature for a holding time. The holding time is determined by the strip speed and the length of the continuous furnace. The heating devices H1 and H2 can preferably also contain induction coils for inductive heating of the strip. The first heating device H1 is arranged for rapid heating of the strip B to temperatures between 100° C. and 232° C. for a treatment time of at least 0.5 seconds. The second heating device H2 is set up for heating the strip B to temperatures above the melting point of tin (232° C.).

[0071] In preparation for the electrolysis method, strip B is first degreased, rinsed, pickled and rinsed again and then passed first through the second heating device H2, then successively through tanks 1a-1c and finally through the first heating device H1.

[0072] In the second heating device H2, the tin coating of the tinplate strip B is at least partially melted by heating to temperatures above the melting point of tin. The melting of the tin coating produces a dense iron-tin alloy layer at the interface of the steel sheet substrate and the tin coating of the tinplate, the composition of which depends on the temperature and which may contain FeSn and FeSn.sub.2 or a mixture thereof. Preferably, the tin coating is only partially melted, leaving a layer of free metallic tin on the surface. This can be anodically oxidized in the first tank 1a.

[0073] For this purpose, the strip B in the first tank 1a is connected as anode and a current density in the range of 0.1 to 10 A/dm.sup.2 and preferably between 0.2 and 3 A/dm.sup.2 is generated by the cathode pair KP, depending on the strip speed. At a corresponding current density, a tin oxide layer consisting at least essentially of tetravalent tin oxide (SnO.sub.2) forms on the tin surface of the tinplate due to electrolytic interaction with the basic electrolyte BE. The thickness of the tin oxide layer electrolytically generated in the first tank 1a depends on the strip speed and the current density. The first tank 1a can also be passed through without current, so that no (tetravalent) tin oxide layer is formed on the tin surface of the tinplate strip B.

[0074] Subsequently, the strip B is passed through the central tank 1b with the rinsing solution Sp in order to rinse the strip. This is followed by drying by means of a drying device not shown here.

[0075] In the following tank 1c, strip B is connected as a cathode and a current density of more than 15 A/dm.sup.2, in particular in the range from 20 A/dm.sup.2 to 40 A/dm.sup.2, is generated by means of the anodes of the anode pair AP. At this current density, a passivation layer P containing chromium oxide is deposited on the (oxidized) surface of the tinplate strip B, which may contain chromium hydroxide and unavoidable impurities of chromium sulfate in addition to chromium oxide. The weight of the chromium oxide-containing layer can be controlled by the electrolysis time in the last tank 1c, which in turn can be controlled by the strip speed and the current density. With higher strip speeds, the minimum current density required for electrolytic deposition of a chromium oxide-containing layer increases. The electrolysis time in the last tank is between 0.5 and 2.0 seconds, depending on the strip speed. Preferably, in the last tank 1c a chromium oxide-containing passivation layer P is deposited on the (oxidized) surface of the tinplate strip B with a chromium-related coating weight of 3 to 12 mg/m.sup.2.

[0076] The electrodeposited passivation layer P consists essentially of chromium oxide and chromium hydroxide and, in particular, has a proportion by weight of the chromium oxide and chromium hydroxide in terms of the total coating weight of the passivation layer of at least 90%, preferably more than 95%. In addition to chromium oxide and chromium hydroxide, the passivation layer may still contain unavoidable impurities, such as residual chromium sulfate, if Cr-(III) sulfate has been used as the chromium compound in the first electrolyte solution E1.

[0077] To minimize the amount of chromium hydroxide in the passivation layer, the passivated strip B is passed through the first heating device H1 and kept therein at treatment temperatures above 100° C., in particular in the range from 100° C. to 230° C., for a treatment time of at least 0.5 seconds. The treatment temperature should not exceed the melting point of tin (232° C.) to prevent melting of the tin layer. The treatment time depends on the strip speed and the length of the first heating device H1, which can be in the range of 3 m to 30 m. If induction heating is used, the length of the heating device can also be shorter. After the thermal treatment in the first heating device, the weight fraction of the chromium oxide in terms of the total coating weight of the passivation layer is preferably at least 95% and even more preferred exceeds 98%.

[0078] FIG. 2A schematically shows a sectional view of a tinplate strip B which can be produced with the electrolysis system of FIG. 1A. On one side of the strip B, which is composed of the steel sheet substrate S, the iron-tin alloy layer (FeSn/FeSn.sub.2), the metallic tin layer (Sn) and the (tetravalent) tin oxide layer (SnO.sub.2), the passivation layer P is applied, which consists essentially of pure chromium oxide The strip B can also be provided with a corresponding passivation layer P on both sides.

[0079] After thermal treatment in the first heating device H1, the strip B provided with the dried passivation layer P can be rinsed, dried and oiled (for example with DOS). After this, the passivated strip B can additionally be provided with an organic coating. The organic coating is applied to the surface of the chromium oxide passivation layer in a known manner, for example by lacquering or laminating a plastic film. The chromium oxide surface of the passivation layer provides a good adhesive base for the organic material of the organic coating. The organic coating can be, for example, an organic lacquer or a polymer film made of thermoplastic polymers such as PET, PE, PP or mixtures thereof. The organic coating can be applied, for example, in a coil coating method or in a sheet method, wherein the coated strip is first divided into sheets in the sheet method, which are then coated with the organic coating or laminated with a polymer film.

[0080] FIG. 1B shows a second embodiment of an electrolysis system containing four tanks 1a, 1b, 1c, 1d arranged one behind the other in the running direction of the strip. The two front tanks 1a, 1b, viewed in the running direction of the strip, correspond to tanks 1a and 1b of the embodiment of FIG. 1A and are filled with the basic electrolyte BE and the rinsing solution Sp. Downstream of the second tank 1b is a third tank 1c, which is filled with a second electrolyte solution E2. Adjacent to the third tank 1c in the strip running direction v is a fourth tank 1d, which is filled with the first electrolyte solution E1. Examples of the composition of the first electrolyte solution E1 and the second electrolyte solution E2 are given in Table 1.

TABLE-US-00001 TABLE 1 Component Concentration Chromium sulfate 120 g/L Sodium sulfate 100 g/L diluted sulfuric acid 96%  7 ml/L Deionized water Rest

[0081] The composition of the first and second electrolyte solutions E1, E2 differs in that the first electrolyte solution E1 (as in the embodiment of FIG. 1A) is free of organic components and in particular free of organic complexing agents, whereas the second electrolyte solution E2 contains organic complexing agents in addition to the trivalent chromium compound, a conductivity-increasing salt, an acid and water as the solvent. In particular, formates, for example sodium or potassium formate, are used as organic complexing agents.

[0082] Due to the different composition of the electrolyte solutions E1 and E2 filled in the two downstream tanks 1c and 1d, layers containing chromium oxide are electrolytically deposited on the surface of the tinplate strip B in these last two tanks 1c and 1d, which differ from each other in terms of their composition. Here, in tank 1c, a lower layer L1 of a passivation layer P is deposited from the second electrolyte solution E2, and in the last tank 1d, an upper layer L2 is deposited from the first electrolyte solution E1. The lower layer L1 deposited in tank 1c from the second electrolyte solution E2 contains chromium oxide/chromium hydroxide as well as metallic chromium and chromium carbides. The upper layer L2, on the other hand, consists essentially of chromium oxide/chromium hydroxide. The proportion by weight of chromium oxide and chromium hydroxide is therefore lower in the lower layer L1, which is deposited in the upstream tank 1c, than in the upper layer L2, which is deposited on the surface of the tinplate strip B in the last tank 1d, viewed in the strip running direction. The passivation layer P deposited in the last two tanks 1c, 1d is therefore composed of a lower layer L1 facing the steel sheet substrate S and an upper layer L2 deposited thereon, the composition of the lower and upper layers differing in terms of the weight fraction of chromium oxide and chromium hydroxide and of metallic chromium and chromium carbide. In particular, the upper layer L2 has a higher weight fraction of chromium oxide/chromium hydroxide and does not contain metallic chromium. In the lower layer, a weight fraction of 10% to 50% can be attributed to metallic chromium and the remainder to chromium oxide/chromium hydroxide and chromium carbide.

[0083] FIG. 2A schematically shows a sectional view of a tinplate strip B with a passivation layer P which can be produced with the electrolysis system of FIG. 1B. Compared with the embodiment of FIG. 2A, the passivation layer P on the surface of the passivated tinplate strip is composed of two layers, namely the lower layer L1 and the upper layer L2, which differ from one another in terms of composition and, in particular, the proportion by weight of chromium metal and chromium oxide/chromium hydroxide, with a higher proportion by weight of chromium oxide/chromium hydroxide in the upper layer L2.

[0084] As in the embodiment of FIG. 1A, in the electrolysis system of FIG. 1B, after the passivation layer P has been applied to the surface of the tinplate strip B, a thermal treatment is carried out in the first heating device H1 in order to remove the chromium hydroxides from the passivation layer P and in particular from the upper layer L2 by drying and conversion into chromium oxides. After the thermal treatment, at least the upper layer L2 consists essentially of pure chromium oxide, apart from unavoidable impurities, the percentage by weight of chromium oxide in terms of the total coating weight of the passivation layer P being at least 95% and preferably more than 98%.

EXAMPLES

Example 1

[0085] In order to determine the growth of tin oxide on the tin surface of passivated tin sheets which have been electrolytically coated with a passivation layer containing chromium oxide on the basis of an electrolyte solution containing a trivalent chromium compound, tin sheets were passivated in the laboratory by electrolytic deposition of a passivation layer containing chromium oxide and then subjected to a thermal treatment in accordance with the invention. Subsequently, the samples were stored in an oxygen-containing atmosphere (air) in a climate chamber at 40° C. and a humidity of 80% for a period of 6 weeks. Before the beginning and during the storage period, the amount of tin oxide layer formed on the tin surface of the tinplate samples due to oxidation with atmospheric oxygen was recorded. The amount of tin oxide layer formed by oxidation to atmospheric oxygen was determined coulometrically.

[0086] For comparison, the same samples without a thermal treatment were exposed to the same storage conditions in the climate chamber after the electrolytic deposition of the passivation layer P, and the amount of tin oxide formed on the tin surface of the tinplate samples by oxidation with atmospheric oxygen before and during storage was also recorded coulometrically on these comparative samples.

[0087] The method and material parameters of the tested tinplate samples from the laboratory tests are shown in Table 2 (FIG. 3).

[0088] To prepare the specimens, partially melted, unpassivated tinplate specimens with a tin coating of 1.4 g/m.sup.2 on both sides were cathodically degreased after cathodic degreasing for an anodizing time of 30 seconds at a current density of 2.5 A/dm′ in a 5-percentage soda solution and then rinsed with fully demineralized water. After cathodic degreasing at a current density of 2 A/dm.sup.2, the tinplate samples were then electrolytically provided with a chromium-oxide containing passivation layer P by applying to the tinplate samples from the first electrolyte E1 from Tab. 1, the pH of which was adjusted to pH=3.2 by the addition of sulfuric acid, at a temperature of 35° C. for an electrolysis time of 0.5 to 1.0 seconds. The coating weight in terms of chromium of the electrolytically deposited passivation layer P is given in Table 2 as “chromium support (Cr)”.

[0089] The tinplate samples provided with the passivation layer P were then subjected to a temperature of 200° C. in a furnace for 600 seconds for a thermal treatment in accordance with the invention. This thermal treatment was not carried out on the comparative specimens (specimen nos. 1a, 1b, 2a, 2b, 3a and 4a) in Table 2.

[0090] Thereafter, the samples thermally treated according to the invention and the comparison samples were stored for 6 weeks in the climate chamber in the presence of atmospheric oxygen at 40° C. and 80% humidity. At the beginning of storage and at 2-week intervals, the amount of tin oxide layer (SnO.sub.2) formed on the tin surface of the tinplate samples during the respective storage period was determined coulometrically. Table 2 shows the initial amounts of tin oxide layer (SnO.sub.2) that were present on the surface before the samples were placed in the storage and the amounts of tin oxide layer (SnO.sub.2) that were recorded after storage periods of 2, 4, and 6 weeks. The last column of Table 2 shows the difference between the amount of tin oxide layer (SnO.sub.2) after six weeks of storage in the climate chamber and the initial amount of tin oxide layer (SnO.sub.2).

[0091] From the last column of Table 2, it can be seen that the samples according to the invention (samples 1c, 2c, 3b and 4b), in which the thermal treatment according to the invention has been carried out after the electrolytic deposition of the passivation layer P, show a significantly lower growth of tin oxide compared to the reference samples (samples 1a, 1b, 2a, 2b, 3a and 4a), in which no thermal treatment has taken place. Accordingly, the thermal treatment of the passivated tinplate samples according to the invention leads to a significant reduction in tin oxide growth in an oxygen-containing atmosphere when the tinplate samples are stored for a longer period of time. In comparison, an inhibition of tin oxide growth of more than 50% can be seen in the samples treated according to the invention compared with the reference samples.

Example 2

[0092] In a plant test, tinplate strips with a tin coating of 2.4 g/m.sup.2 on both sides were passed through an electrolysis system of the type shown in FIG. 1A at a strip speed of 300 m/min. The electrolyte tanks were filled with the electrolyte E1 from Table 1. In the second heating device H2, the tin layer was partially melted and the first tank 1a was passed through without current, i.e. no anodic oxidation of the tin surface was carried out. In the last tank 1c, a passivation layer P containing chromium oxide was deposited electrolytically on the tin surface of the tinplate strip with a coating weight (chromium support) of about 9 mg/m.sup.2 in terms of chromium. After deposition of the passivation layer, the tinplate samples according to the invention (samples Nos. 2 to 5 from Table 3 (FIG. 4)) were cooled to room temperature and thermally treated in the first heating device H1 for different treatment times ranging from 10 seconds to 120 seconds at a treatment temperature of 187° C. The first heating device H1 was used to heat the tinplate samples. This thermal treatment was not carried out on a comparative sample (Sample No. 1 from Table 3). The tinplate strip was then cut into sheets and the initial amount of tin oxide on the tin surface of the samples thus produced was recorded coulometrically. The tinplate samples were stored for 4 weeks in a climate chamber at 40° C. and 80% humidity in air and the amount of tin oxide formed on tin surface of the samples during storage due to oxidation with atmospheric oxygen was recorded coulometrically at 2-week intervals.

[0093] Table 3 summarizes the results of the plant test. The comparative sample (sample no. 1) shows a tin oxide (SnO.sub.2) occupancy of 11 C/m.sup.2 on the tin surface at the beginning of the climate chamber storage and this oxide occupancy has increased to 44 C/m.sup.2 after 2 weeks and to 60 C/m.sup.2 after 4 weeks.

[0094] In contrast, the tinplate samples thermally treated according to the invention (samples Nos. 2 to 5 from Table 3) show a lower tin oxide layer already at the beginning of the climate chamber storage and the growth of the tin oxide layer during storage in the climate chamber is significantly lower for the samples according to the invention, whereby the inhibition of the tin oxide growth is higher with longer treatment time. For example, Samples Nos. 4 and 5, which have been thermally treated at 187° C. for a treatment time of 60 seconds (Sample No. 4) and 120 seconds (Sample No. 5), respectively, exhibit a tin oxide overlay of less than 40 C/m.sup.2 after four weeks of storage in the climate chamber. Such tin oxide overlays of less than 40 C/m.sup.2 are preferred both visually and in terms of lacquer adhesion and paintability. Tinplate with tin oxide coatings between 41 C/m.sup.2 and 69 C/m.sup.2 exhibit sufficient adhesion for organic coatings, but have a pale yellowish discolored surface and are therefore not optimal. Tin oxide coatings above 69 C/m.sup.2 can lead to complete failure of the material and in particular to detachment of the organic coating due to insufficient adhesion to the surface of the passivated tinplate.

[0095] The method according to the invention can significantly reduce tin oxide growth on the tin surface of tinplate that has been electrolytically passivated from a trivalent chromium electrolyte, resulting in better adhesion of organic coatings and a pleasing visual appearance of the surface.