EP4159896A2 - Method for passivating the surface of a white sheet and electrolysis system for carrying out the method - Google Patents

Abstract

The invention relates to a method for passivating the surface of a tinplate, comprising electrolytic deposition of a passivation layer containing chromium oxide/chromium hydroxide on the surface, the electrolytic deposition of the passivation layer taking place at least partially from an electrolyte solution (E) which contains a trivalent chromium compound, at least one salt for Increase in conductivity and contains at least one acid or one base to set a desired pH and is free from organic complexing agents and free from buffering agents. In order to increase the proportion 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 held at a treatment temperature of 100° C. or more for a treatment time of at least 0.5 seconds becomes.

Description

The invention relates to a method for passivating the surface of a tinplate by electrolytic deposition of a chromium oxide-containing passivation layer on the surface, and an electrolysis system for the electrolytic deposition of a chromium oxide-containing passivation layer on the surface of a tinplate.

For the production of packaging, steel sheets electrolytically coated with a passivation layer of chromium and chromium oxide/chromium hydroxide are known from the prior art, which are known as tin-free steel sheet ("Tin Free Steel", TFS) or as "Electrolytic Chromium Coated Steel (ECCS)". be called and represent an alternative to tinplate. These tin-free steel sheets are particularly characterized by good adhesion for paint or organic coatings (such as polymer coatings made of PP or PET). Despite the low thickness of the passivation layer of chromium and chromium oxide/chromium hydroxide, which is usually less than 20 nm, these chromium-coated steel sheets have good corrosion resistance and good processing properties in forming processes for the production of packaging, e.g. in deep-drawing and ironing processes .

Tinned steel sheets (tinplate) are usually provided with a passivation layer after electrolytic tinning in order to prevent oxidation of the tin surface by atmospheric oxygen. Chromium-containing layers which can be deposited electrolytically from a chromium(VI)-containing electrolyte on the tin surface of tinplate have proven to be suitable passivation layers. These chromium-containing passivation layers are composed of metallic chromium and chromium oxides. Chromium oxide is understood to mean all compounds of chromium and oxygen, including chromium hydroxides.

For the production of chromium-plated steel sheets (ECCS) and for the passivation of tinplate surfaces, electrolytic coating processes are known from the prior art, with which a metallic chromium and chromium oxide/ Chromium hydroxide-containing passivation layer can be applied to a strip-shaped substrate (uncoated sheet steel or tinplate). However, these coating processes have significant disadvantages due to the environmental and health-endangering properties of the chromium-VI-containing electrolytes used in the electrolysis process and must be replaced by alternative coating processes in the foreseeable future, since the use of chromium-VI-containing materials will be banned in the future.

For this reason, electrolytic coating processes have already been developed in the prior art, which can dispense with the use of electrolytes containing chromium VI. For example, from the WO 2015/177314-A1 and the WO 2015/177315-A1 a method for the electrolytic passivation of a strip-shaped steel sheet, in particular a strip-shaped black or tin sheet, with a chromium metal-chromium oxide (Cr-CrOx) layer is known, in which the steel sheet is connected as a cathode in a strip coating plant with high strip speeds of more than 100 m/ min is passed through a single electrolyte solution which contains a trivalent chromium compound (especially Cr-III-sulphate) as well as a complexing agent and a conductivity-increasing salt and is free of chlorides and buffering agents such as boric acid.

Organic substances, in particular formates and preferably sodium or potassium formate, are used as complexing agents. To set a preferred pH in the range from 2.5 to 3.5, the electrolyte solution can contain sulfuric acid. The passivation layer of chromium metal and chromium oxide can be deposited layer by layer in successive electrolysis tanks or in successively arranged coil coating systems, with the electrolysis tanks each being filled with the same electrolyte solution.

It was observed that the electrolytically deposited passivation layer can also contain chromium sulfates and chromium carbides in addition to the components chromium metal and chromium oxide/chromium hydroxide and that the proportions of these components in the total weight of the passivation layer depends very significantly on the current densities set in the electrolysis tanks. It was determined that three areas (regime I, regime II and regime III) form depending on the current density, with a first area having a low current density up to a first Current density threshold (regime I) no chromium-containing deposition has yet taken place on the steel substrate, in a second area with medium current density (regime II) there is a linear relationship between the current density and the weight of the deposited passivation layer and for current densities above a second current density threshold (regime III) there is a partial decomposition of the applied passivation layer takes place, so that the weight of the chromium of the passivation layer initially falls in this regime with increasing current density and then settles at a constant value at higher current densities. In the area with medium current density (regime II), essentially metallic chromium is deposited on the steel substrate with a weight proportion of up to 80% (based on the total weight of the passivation layer) and above the second current density threshold (regime III), the passivation layer contains a higher one Portion of chromium oxide that makes up between ¼ and 1/3 of the total weight of the passivation layer in the area of higher current densities. The values of the current density thresholds, which delimit the areas (regimes I to III) from one another, depend on the belt speed at which the steel sheet is moved through the electrolyte solution.

A high percentage of the Chromium oxide in the chromium-containing passivation layer advantageous.

From the EP 3 722 464 A1 a method is known for passivating the surface of a tinplate by depositing a passivation layer, which consists at least essentially only of chromium oxide and chromium hydroxide, using an aqueous solution of an electrolyte with a trivalent chromium compound that contains no organic substances and in particular no organic complexing agents contains. Due to the use of an aqueous electrolyte solution, the deposited passivation layer has a high proportion of chromium hydroxide in addition to chromium oxide. However, the chromium hydroxide content of the passivation layer enables atmospheric oxygen to diffuse through the passivation layer if the passivated tinplate is stored in an air atmosphere for a long period of time. Due to the diffusion of atmospheric oxygen through the Passivation layer A harmful layer of tin oxide forms on the tin surface of the tinplate, which essentially consists of divalent tin oxide (SnO).

The object of the present invention is therefore to provide an efficient, cost-effective and environmentally friendly and health-friendly electrolysis process for passivating the surface of a tinplate with a chromium oxide-containing passivation layer based on an electrolytic solution with a trivalent chromium compound, which causes a zinc oxide layer to grow on the tin surface of the tinplate in an oxygen- or air-containing atmosphere. In any case, the use of substances containing chromium VI, also as intermediate products of the electrolysis process, should be avoided in order to be able to fully meet the legal requirements regarding the ban on substances containing chromium VI. The tinplate coated according to the process should have the highest possible resistance to oxidation in an oxygen-containing environment, in particular in atmospheric oxygen, and should have a good adhesion base for organic coatings, such as e.g. for organic paints and for polymer layers, in particular for polymer films, e.g. made of PET, PE or PP, form.

This object is achieved by a method having the features of claim 1 and by an electrolysis system having the features of claim 11 and by a tinplate according to claim 12. Preferred embodiments of the method and of the tinplate can be found in the dependent claims.

In the method according to the invention, a passivation layer containing chromium oxide is electrolytically removed from an electrolyte solution which contains a trivalent chromium compound and at least one salt to increase conductivity and at least one acid or one base to set a desired pH value and is free from organic complexing agents and free from Buffering agents is applied to a tinned steel strip (tin strip), whereby after the electrolytic deposition of the passivation layer, the passivated tin plate is subjected to a thermal treatment in which the passivated tin plate is heated to a treatment temperature of 100°C for a treatment time of at least 0.5 seconds °C or more is maintained. The chromium hydroxide components contained in the passivation layer are removed from the passivation layer by the thermal treatment of the passivated tinplate immediately after the electrolytic deposition of the passivation layer. This will make the Passivation layer impermeable to oxygen, which can prevent the growth of a tin oxide layer on the tin surface of the tinplate. After storage for at least four weeks in an oxygen-containing atmosphere, the tinplates which have been thermally treated according to the invention after the passivation layer has been deposited have a tin oxide layer with a tin oxide coating of less than 70 C/m ² under the passivation layer. After storage for at least four weeks, the coverage of the tin oxide layer is preferably less than 55 C/m ² and particularly preferably less than 40 C/m ² , in particular in the range from 20 C/m ² to 60 C/m ² . The entire coating of the tin oxide layer can consist of divalent tin oxide (SnO), which is produced by natural oxidation of the tin surface in atmospheric oxygen, and of tetravalent tin oxide (SnO ₂ ), which is produced by targeted anodic oxidation before the passivation layer is electrolytically deposited. put together. The proportion of the tetravalent tin oxide preferably has a coating of less than 40 C/m ² and particularly preferably less than 30 C/m ² .

Substances containing chromium VI are not used in the process according to the invention, not even as intermediate products, so that the process is entirely free of substances containing chromium VI and is therefore environmentally friendly and health-friendly when the process is carried out.

Due to the use of an electrolyte solution free of organic complexing agents, which in particular does not contain formates, for the electrolytic deposition of the passivation layer or at least an upper layer of the passivation layer and because of the subsequent thermal treatment of the tinplate provided with the passivation layer, the passivation layer consists, apart from unavoidable impurities ( which can in particular be residues of chromium hydroxide and chromium sulphate if Cr(III) sulphate is used as the trivalent chromium compound in the electrolytic solution), at least essentially of pure chromium oxide or it contains at least an upper layer of pure chromium oxide. The passivation layer consisting of pure chromium oxide or the upper layer of the passivation layer forms a very good barrier against the penetration of oxygen on the one hand and on the other hand represents a good adhesion base for organic coatings, such as organic paints or polymer films made of PET, PP or PE.

When speaking of chromium oxide, all (trivalent) oxide forms of chromium (CrO _x ) are meant. When speaking of chromium hydroxide, all hydrated forms of chromium oxide are meant, in particular chromium(III) hydroxide and chromium(III) oxide hydrate, as well as mixtures thereof.

After the thermal treatment according to the invention, the electrolytically applied passivation layer has the highest possible proportion by weight of chromium oxide. The proportion by weight of chromium oxide is preferably more than 95%. On the one hand, this ensures good passivation against oxidation of the surface of the tinplate and, on the other hand, offers a good adhesion base with good adhesion for organic coatings such as organic paints or polymer layers made of thermoplastics such as PET; PE or PP.

For the electrolytic deposition of the passivation layer, the tinplate strip, connected as a cathode, is brought into contact with the electrolyte solution in at least one electrolysis tank for a predetermined electrolysis period. The duration of the electrolysis is preferably in the range from 0.3 to 5.0 seconds and particularly preferably between 0.6 and 1.5 seconds. For this purpose, the tinplate strip is guided at a predetermined strip speed through at least one electrolysis tank or successively through several electrolysis tanks arranged one behind the other in the direction of strip travel, with the strip speed preferably being at least 100 m/min and particularly preferably between 200 m/min and 750 m/min. The high belt speeds ensure that the process is highly efficient.

The thickness or the weight applied to the electrolytically deposited passivation layer can be controlled via the duration of the electrolysis and thus via the belt speed. The duration of the electrolysis is preferably selected such that the deposited chromium oxide has a weight of at least 3 mg/m ² and preferably from 8 mg/m ² to 12 mg/m ² , based on the chromium. These preferred weight restrictions on the passivation layer give the tinplate sufficient oxidation and corrosion resistance for packaging applications and also provide a good adhesion base for organic coatings such as paints or thermoplastic films.

In order to improve the corrosion resistance and to form a barrier against sulphur-containing materials, in particular against sulphate or sulphite-containing fillings in packaging, after the electrolytic application of the passivation layer, a coating of an organic material, in particular a lacquer, which adheres well to the chromium oxide layer of the passivation layer, can be applied or a thermoplastic material, in particular a polymer film made of PET, PE, PP or a mixture thereof, by painting the surface of the passivation layer with an organic paint or by providing it with a plastic layer made of a thermoplastic material such as PET, PP and/or PE .

In order to ensure a process completely free of substances containing chromium VI, a suitable anode is suitably selected for the electrolytic deposition of the passivation layer and arranged in the or each electrolysis tank in order to oxidize chromium (III) from the trivalent chromium compound of the electrolyte solution Chromium(VI) too prevents. In particular, steel and stainless steel-free anodes with an outer surface or a passivation layer made of a metal oxide, in particular iridium oxide, or of a mixed metal oxide, in particular iridium-tantalum oxide, have proven to be suitable for this. The anode preferably contains neither stainless steel nor platinum. By using such anodes, coatings can be deposited on the tinplate which exclusively contain trivalent chromium oxides and/or chromium hydroxides, in particular Cr ₂ O ₃ and/or Cr(OH) ₃ .

• wenigstens einen mit einer ersten Elektrolytlösung gefüllten Elektrolysetank, oder mehrere hintereinander angeordnete Elektrolysetanks, von denen mindestens ein Elektrolysetank mit der ersten Elektrolytlösung gefüllt ist und die übrigen Elektrolysetanks mit der ersten Elektrolytlösung oder einer anderen Elektrolytlösung, welche eine dreiwertige Chromverbindung enthält und deren Zusammensetzung sich von der ersten Elektrolytlösung unterscheidet, gefüllt sind,
• wobei die erste Elektrolytlösung eine dreiwertige Chromverbindung sowie wenigstens ein Salz zur Erhöhung der Leitfähigkeit und wenigstens eine Säure oder eine Base zur Einstellung eines gewünschten pH-Werts enthält und frei von organischen Komplexbildnern sowie frei von Pufferungsmitteln ist,
• eine Transporteinrichtung zum Transport eines bandförmigen Weißblechs in einer Bandlaufrichtung mit einer vorgegebenen Bandgeschwindigkeit, welche bevorzugt mehr als 100 m/min beträgt, durch den wenigstens einen Elektrolysetank oder nacheinander durch die mehreren Elektrolysetanks, zur elektrolytischen Abscheidung einer Passivierungsschicht aus der ersten Elektrolytlösung und ggf. der anderen Elektrolytlösung auf der Oberfläche des Weißblechs,
• eine Heizeinrichtung, die in der Bandlaufrichtung nach dem Elektrolysetank oder nach den mehreren Elektrolysetanks angeordnet und zur Erwärmung des passivierten Weißblechs auf eine vorgegebene Behandlungstemperatur von mindestens 100°C für eine vorgegebene Behandlungszeit von mindestens 0,5 Sekunden ausgebildet ist.

To carry out the method according to the invention, an electrolysis system can be used which comprises:

• at least one electrolytic tank filled with a first electrolytic solution, or several electrolytic tanks arranged in series, of which at least one electrolytic tank is filled with the first electrolytic solution and the other electrolytic tanks with the first electrolytic solution or another electrolytic solution which contains a trivalent chromium compound and whose composition differs from the first electrolyte solution are filled,
• wherein the first electrolytic solution contains a trivalent chromium compound and at least one salt to increase the conductivity and at least one acid or one base adjustment of a desired pH value and is free from organic complexing agents and free from buffering agents,
• a transport device for transporting a strip-shaped tinplate in a direction of strip travel at a predetermined strip speed, which is preferably more than 100 m/min, through the at least one electrolysis tank or successively through the several electrolysis tanks, for the electrolytic deposition of a passivation layer from the first electrolyte solution and, if necessary, the other electrolytic solution on the surface of the tinplate,
• a heater disposed downstream of the electrolytic tank or plural electrolytic tanks in the direction of strip travel and configured 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.

In one embodiment, the electrolysis system according to the invention comprises a plurality of electrolysis tanks arranged one behind the other, of which at least the last electrolysis tank seen in the direction of strip travel is filled with the first electrolytic solution and the preceding electrolysis tanks are filled with another electrolytic solution which, in addition to the trivalent chromium compound, contains at least one salt to increase conductivity and contains at least one acid or one base for setting a desired pH and organic complexing agents. In particular, formates and preferably sodium or potassium formate can be used as complexing agents. The temperatures of the electrolyte solutions in the individual electrolysis tanks can also be chosen to be different. This embodiment makes it possible to deposit a passivation layer containing chromium oxide on the surface of the tinplate with several layers which differ in their composition.

In particular, with this embodiment of the electrolysis system, a passivation layer with a lower layer made of metallic chromium and chromium oxide/chromium hydroxide and possibly chromium carbides and an upper layer made of pure chromium oxide can be produced. The individual layers of the passivation layer are deposited on the tin surface of the tinplate in the individual electrolysis tanks of the electrolysis system arranged one behind the other. The different composition of the layers results from the differences in the composition and/or the temperature of the electrolyte solutions in the individual electrolysis tanks Passivation layer, which differ from each other in particular by the proportion by weight of chromium oxide.

With the heating device, the chromium hydroxide components of the passivation layer or at least the chromium hydroxide components in the upper layer of the passivation layer can be converted into chromium oxide, so that the passivation layer either consists entirely of pure chromium oxide or at least the upper layer of the passivation layer contains essentially only chromium oxide and in particular no metallic one Has chromium and possibly still unavoidable residual components of chromium hydroxide. The heating device is preferably arranged immediately after the electrolysis tank or after the last electrolysis tank of the plurality of electrolysis tanks, viewed in the direction of strip travel. This makes it possible for the thermal treatment of the passivation layer to be carried out immediately at a short time interval of, for example, less than 10 seconds after the end of the electrolytic application of the passivation layer.

The electrolysis system according to the invention can expediently be arranged immediately after an electrolytic tinning line in which the steel sheet substrate of the tinplate is electrolytically provided with a layer of tin. This means that the passivation layer can be applied to the tin surface of the tinplate immediately after the steel substrate has been electrolytically tinned, without the tinplate strip having to be wound up. The steel sheet substrate of the tinplate, which is fed through the tinning line at a predetermined line speed for tinning as a steel strip, can therefore be fed further through the inventive electrolysis system at the same line speed directly after tinning by means of the transport device. However, it is also possible to first wind the tinned steel strip into a coil and feed the coil to the transport device of the electrolysis system according to the invention for unwinding and transporting the tinplate strip.

The electrolysis system according to the invention can also contain a further heating device, which is arranged in front of the (first) electrolysis tank or in front of the several electrolysis tanks, viewed in the direction of strip travel. With this additional heating device, which is preferably designed as an induction heater, the tin layer applied to the tinplate in the tinning line can be at least partially melted by the tinplate strip being the further heating device is heated to temperatures above the melting point of tin.

The electrolyte solution preferably has an electrolyte temperature in the range from 20°C to 80°C and particularly preferably in the range from 30°C to 65°C and in particular between 40°C and 60°C. At these temperatures, the electrolytic deposition of the passivation layer containing chromium oxide is very efficient. When talking about the electrolyte temperature or the temperature of the electrolyte solution or the temperature in an electrolysis tank, what is meant in each case is the mean temperature that is averaged over the entire volume of the electrolysis tank. As a rule, there is a temperature gradient in the electrolysis tank with a temperature increase from top to bottom. If several electrolysis tanks are used, the electrolyte temperature in the individual electrolysis tanks can differ, which can influence the composition of the layers of the passivation layer deposited in the individual electrolysis tanks. For example, at electrolyte temperatures below 40°C, a layer with a higher proportion of chromium oxide is deposited compared to electrolysis tanks with a higher electrolyte temperature. It is therefore advantageous in the method according to the invention to set an electrolyte temperature of 40° C. or less in the last electrolysis tank in order to maximize the proportion of chromium oxide in the upper layer of the passivation layer.

The first electrolytic solution, which is filled in at least one and preferably in the last electrolysis tank seen in the direction of strip travel, contains, in addition to the trivalent chromium compound and the solvent, water, at least one salt that increases the conductivity and at least one acid or base for setting a suitable pH value and is preferably free of chloride ions and free of buffering agents, especially free of a boric acid buffer.

The trivalent chromium compound of the electrolyte solution is preferably selected from the group consisting of basic Cr(III) sulfate (Cr2(SO4)3), Cr(III) nitrate (Cr(NO3)3), Cr(III) oxalate (CrC2O4 ), Cr(III) acetate (C12H36ClCr3O22), Cr(III) formate (Cr(OOCH)3) or a mixture thereof. The concentration of the trivalent chromium compound in the electrolyte solution is preferably at least 10 g/l and particularly preferably more than 15 g/l and in particular 20 g/l or more.

To increase the conductivity, the electrolyte solution contains at least one salt, which is preferably an alkali metal sulphate, in particular potassium or sodium sulphate.

A very efficient deposition of a passivation layer containing chromium oxide and chromium hydroxide is achieved when the pH value (measured at a temperature of 20°C) of the electrolytic solution is in a range from 2.3 to 5.0 and preferably between 2.5 and 3. 5 lies. The desired pH can be adjusted by adding an acid or base to the electrolyte solution. When using basic Cr(III) sulfate as the trivalent chromium compound, sulfuric acid or an acid mixture containing sulfuric acid is particularly suitable for setting the desired pH.

Particularly preferred compositions of the electrolyte solution each include basic Cr(III) sulfate (Cr ₂ (SO ₄ ) ₃ ) as a trivalent chromium compound, and sodium sulfate as a conductivity-increasing salt and sulfuric acid to set a preferred pH in the range from 2.5 to 3. 5.

In addition to the trivalent, chromium-containing substance, the electrolyte solution contains at least one salt to increase the conductivity and the at least one acid or base to adjust the pH, and apart from the solvent water, no other components. This ensures a simple and inexpensive production of the electrolyte solution and leads to the deposition of a passivation layer, which consists at least essentially only of chromium oxide and chromium hydroxide and otherwise only contains unavoidable impurities such as chromium sulfate, if Cr (III) sulfate as a trivalent chromium compound is used.

In the method according to the invention, the chromium hydroxide contained in the electrolytically deposited passivation layer is removed from the passivation layer by subjecting the passivated tinplate to a thermal treatment, the passivated tinplate being heated to a treatment temperature of 100° C. for a treatment time of at least 0.5 seconds or more is held. As a result, the water in the hydrated chromium oxides evaporates and the chromium hydroxide components are converted into chromium oxides.

The treatment time is preferably between 0.5 seconds and 30 minutes and particularly preferably between 1.0 seconds and 1 minute. The treatment temperatures are preferably between 150° C. and the melting temperature of tin (232° C.) in order to avoid melting the tin surface of the tinplate. The thermal treatment takes place in particular immediately after the electrolytic deposition of the passivation layer on the tinplate strip, in that the tinplate strip is guided through an oven at the strip speed at which the tinplate strip is passed through the electrolysis tank or tanks. The high strip speeds of preferably more than 100 m/min and up to 900 m/min result in very short treatment times of a few seconds or even less than one, depending on the length of the furnace in the direction of strip travel, which is preferably between 5 and 30 m Second. In the case of very short treatment times of 1 second or less, higher treatment temperatures are expediently selected, which can be between 170°C and 230°C, for example. Very short treatment times of one second or less are sufficient even at lower treatment temperatures of below 150°C if the temperature of the electrolyte in the last electrolysis tank seen in the direction of strip travel is relatively high, e.g. in the range of 50°C or more, because then the passivated tinplate strip has this temperature and only has to be heated to the (maximum) treatment temperature within a short time for thermal treatment.

The passivated tinplate can be heated particularly quickly and efficiently to the (maximum) treatment temperature by inductive heating in an induction coil. Heating rates of 100 K/s to 700 K/s can be achieved, with which the passivated tinplate is heated from the electrolyte temperature of the last electrolysis tank to the (maximum) treatment temperature.

However, it is also possible to carry out the thermal treatment of the passivated tinplate, optionally after the tinplate strip has been divided into panels, in an oven or by means of IR irradiation of the passivated tinplate. Longer treatment times of a few minutes can be selected here, with lower treatment temperatures in the range from 100° C. to 150° C. being able to be selected.

In order to prevent oxygen from diffusing through the still wet passivation layer after the passivation layer has been deposited, the thermal process is carried out Treatment preferably immediately after the electrolytic deposition of the passivation layer, i.e. immediately after leaving the last electrolysis tank. There is preferably an interval of at most 10 seconds between the end of the deposition of the passivation layer, ie leaving the last electrolysis tank, and reaching the treatment temperature.

In a preferred exemplary embodiment of the method, a tin oxide layer consisting essentially of tetravalent tin oxide (SnO ₂ ) is produced on the surface of the tinplate before the electrolytic deposition of the passivation layer, in that the tinplate is connected as an anode in an aqueous and, in particular, basic electrolyte which contains a phosphate, contains borate, sulfate or carbonate. The formation of a tetravalent tin oxide layer, which is more inert to further oxidation in an oxygen atmosphere compared to divalent tin oxide, minimizes unhindered growth of the oxide layer on the tin surface of tinplate in an oxygen-containing atmosphere and improves the marbling resistance of the tinplate to sulfur-containing substances, such as sulfur-containing filling goods of packaging made from the tinplate. Targeted anodic oxidation can be used to control the stoichiometry of the tin oxide layer and the ratio of divalent and tetravalent tin oxide, and the tin oxide layer is distributed homogeneously on the surface of the tinplate.

The anodic oxidation is expediently carried out in the electrolysis line in which the electrolytic application of the passivation layer takes place, i.e. the tinplate strip is passed through a first electrolysis tank before the electrolytic application of the passivation layer in the electrolysis system according to the invention, which is the electrolysis tank with the trivalent chromium electrolyte solution upstream and filled with the basic electrolyte. The tinplate strip is connected in the electrolysis tank with the basic electrolyte as the anode and in the subsequent electrolysis tanks with the trivalent chromium electrolytic solution as the cathode.

The basic electrolyte, which can be, for example, an aqueous sodium carbonate solution with a concentration in the range from 1% by weight to 10% by weight, preferably has a temperature in the range from 30 to 60° C. and a pH value in the range from 7 to 11 and in particular between 10 and 11 for sodium carbonate. The current density in first electrolysis tank in which the anodic oxidation takes place is preferably in the range of 0.1 to 10 A/dm ² and more preferably between 0.2 and 3 A/dm ² . The anodizing time, i.e. the time during which the tinplate is in electrolytically active contact with the basic electrolyte, is preferably less than 5 seconds and preferably in the range from 0.1 to 1.0 seconds and can be appropriately adapted to the strip speed. with which the tinplate strip is fed through the electrolysis system.

By anodic oxidation, a tin oxide layer (SnO ₂ ) is applied to the still unpassivated tin surface of the tinplate, which preferably has a coating in the range from 10 to 40 C/m ² and particularly preferably less than 30 C/m ² , in particular between 10 and 28 C/m ² , corresponding to a thickness of a few nm. The coating of the tin oxide layer on a sample can be determined coulometrically in a chrono-amperometric measuring method by measuring the applied tin oxide layer with a working electrode made of the material to be measured and previously degreased, a counter electrode consisting of a carbon rod and a Ag/AgCl reference electrode in a 47% hydrobromic acid (HBr) diluted to a 0.01 N solution (0.01 mol/l) in water and purged with an inert gas (e.g. N ₂ ) is completely deaerated, reduced by the tinplate sample and the amount of charge in coulombs required for a complete reduction of the tin oxide layer at a current density in the range of 0.5 to 0.7 A/m ² (determined from the product of the cathode current A and the reduction time t ) is recorded, whereby the coating per unit area in C/m ² results from the quotient of the amount of charge in Coulomb (C) and the area of the sample (in m ² ) (according to ANNEX D to the draft European standard EN 10202: 2001).

Before the anodic oxidation, the tin layer of the tinplate can optionally be partially melted by heating the tinplate to temperatures above the melting point of tin (232°C). The heating preferably takes place inductively, so that only part of the tin layer facing the steel substrate can be melted and metallic tin remains on the surface of the tin layer. The melted area of the tin layer forms an iron-tin alloy layer with the iron atoms of the steel substrate, which forms a barrier against corrosion.

• Stahlsubstrat (insbesondere ein einfach (SR) oder doppelt reduziertes (DR) kaltgewalztes Stahlblech mit einer Dicke von 0,5 mm oder weniger),
• optional: Eisen-Zinn-Legierungsschicht, insbesondere mit einer Gewichtsauflage im Bereich von 0,1 bis 2 g/m² bezogen auf das Zinn,
• freies (metallisches) Zinn, insbesondere mit einer Gewichtsauflage im Bereich von 0,5 bis 5 g/m²,
• Zinnoxid, insbesondere aus SnO₂ und insbesondere mit einer Auflage im Bereich von 10 bis 30 C/m²,
• chromoxidhaltige Passivierungsschicht, insbesondere mit einer Gewichtsauflage im Bereich von 3 bis 12 mg/m² bezogen auf das Chrom.

The tinplate produced using the method according to the invention then has the following layer structure (in this order):

• Steel substrate (particularly a single (SR) or double reduced (DR) cold-rolled steel sheet with a thickness of 0.5 mm or less),
• optional: iron-tin alloy layer, in particular with a weight in the range of 0.1 to 2 g/m ² based on the tin,
• Free (metallic) tin, in particular with a weight in the range from 0.5 to 5 g/m ² ,
• Tin oxide, in particular made of SnO ₂ and in particular with a coating in the range from 10 to 30 C/m ² ,
• Chromium oxide-containing passivation layer, in particular with a weight requirement in the range from 3 to 12 mg/m ² based on the chromium.

The chromium oxide-containing passivation layer can be a pure chromium oxide layer, apart from unavoidable impurities. However, the passivation layer can also be made up of several layers with different compositions arranged one on top of the other, with the individual layers containing metallic chromium, chromium oxides and/or chromium hydroxides and possibly chromium carbides and differing from one another in their proportion of chromium oxide. In particular, the passivation layer can contain a lower layer of metallic chromium and chromium oxide/chromium hydroxide and optionally chromium carbides and an upper layer of pure chromium oxide. The individual layers of the passivation layer can be deposited on the tin surface of the tinplate in the individual electrolysis tanks arranged one behind the other in the electrolysis system according to the invention, with the individual electrolysis tanks being filled with an electrolyte solution of different composition and/or having different electrolyte temperatures. The different compositions of the individual layers of the passivation layer result from the differences in the composition and/or the temperature of the electrolyte solutions in the individual electrolysis tanks.

The upper layer of the electrolytically deposited passivation layer contains expediently only chromium oxides and chromium hydroxides, the chromium hydroxides being converted into chromium oxides in the process according to the invention by the thermal treatment, so that the upper layer of the tinplate according to the invention apart of unavoidable impurities and residual components of chromium hydroxides, consists at least essentially of pure chromium oxide after the thermal treatment.

The method according to the invention can therefore be used to produce tinplate with a passivation layer containing chromium oxide, the passivation layer having at least one upper layer which consists essentially only of trivalent chromium oxides and, in particular, contains no chromium hydroxides apart from residual components. At least the upper layer of the passivation layer preferably has a chromium oxide content by weight of more than 95%, particularly preferably more than 98%. The passivation layer or its upper layer preferably contains at least essentially only compounds of chromium and oxygen in which the chromium is present in trivalent form, in particular as Cr ₂ O ₃ and/or Cr(OH) ₃ .

The tinplates according to the invention are characterized by high corrosion resistance, good marbling resistance to sulphur-containing materials and good adhesion for organic coatings such as paints or polymer layers.

In addition to chromium oxides and residues of chromium hydroxide, the passivation layer or its upper layer may contain, apart from unavoidable impurities, residual components of chromium sulphate (as the starting chromium compound of the electrolytic deposition process, if e.g. Cr(III) sulphate is used as a trivalent chromium compound in the electrolyte solution). be.

In an advantageous embodiment, the passivation layer of the tinplate according to the invention consists of at least one lower layer facing the surface of the tinplate and an upper layer forming the surface of the tinplate, the lower layer containing metallic chromium as well as chromium oxide/chromium hydroxide and optionally chromium carbides and the upper layer consists of pure chromium oxide, apart from the mentioned residual components of chromium hydroxide and chromium sulphate and unavoidable impurities.

Good corrosion resistance of the tinplate according to the invention can be achieved if the passivation layer has a total weight requirement based on chromium of at least 3 mg/m ² , preferably from 5 mg/m ² to 15 mg/m ² .

Figur 1:

schematische Darstellung von zwei verschiedenen Ausführungsformen eines erfindungsgemäßen Elektrolysesystems zur Durchführung des erfindungsgemäßen Verfahrens, wobei Figur 1A eine erste Ausführugsform mit einem Elektrolysetank, der mit einer dreiwertigen Chrom- Elektrolytlösung gefüllt ist und Figur 1B eine zweite Ausführungsform mit zwei Elektrolysetanks zeigt, die jeweils mit einer dreiwertigen Chrom-Elektrolytlösung mit unterschiedlicher Zusammensetzung gefüllt sind;

Figur 2:

Schematische Schnittdarstellungen von Ausführungsbeispielen erfindungsgemäßer Weißbleche, wobei Figur 2A ein Ausführungsbeispiel mit einer einlagigen Passivierungsschicht und Figur 2B ein Ausführungsbeispiel mit einer zweilagigen Passivierungsschicht zeigt.

The invention is explained in more detail below using exemplary embodiments with reference to the accompanying drawings, these exemplary embodiments merely explaining the invention by way of example and not limiting it with respect to the scope of protection defined by the following claims. The drawings show:

Figure 1:

schematic representation of two different embodiments of an electrolysis system according to the invention for carrying out the method according to the invention, wherein Figure 1A a first embodiment with an electrolysis tank filled with a trivalent chromium electrolytic solution and Figure 1B shows a second embodiment with two electrolytic tanks each filled with a trivalent chromium electrolytic solution of different composition;

Figure 2:

Schematic sectional views of exemplary embodiments of tinplate according to the invention, where Figure 2A an embodiment with a single-layer passivation layer and Figure 2B shows an embodiment with a two-layer passivation layer.

In the Figures 1A and 1B various embodiments of electrolysis systems for carrying out the method according to the invention are shown schematically. The electrolysis system of Figure 1A comprises three tanks 1a, 1b, 1c arranged next to one another or one behind the other in the direction of strip travel, the first tank 1a being filled with a basic electrolyte BE, the middle tank 1b with a rinsing solution Sp and the last tank 1c with a first electrolyte solution E1.

The basic electrolyte BE consists of an aqueous soda solution (sodium carbonate solution with a concentration of 1 to 10% by weight and a pH of 10 to 11). The rinsing solution Sp consists of distilled or deionized water.

The first electrolytic solution E1 is an aqueous solution of a trivalent chromium compound, which also has a salt to increase conductivity and an acid to set a desired pH between 2.5 and 3.5 and is free from organic complexing agents and free of buffering agents. In a preferred embodiment, the first electrolytic solution E1 consists of the trivalent one Chromium compound, in particular Cr (III) sulfate, the salt (e.g. potassium or sodium sulfate), the acid (e.g. sulfuric acid) and water as a solvent and otherwise has no other components. The first electrolytic 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 from halides. An example of the composition of the first electrolytic solution E1 is given in Table 1. The concentration of the trivalent chromium compound in the first electrolytic solution E1 is preferably at least 10 g/l and particularly preferably 20 g/l or more. The temperature of the first electrolytic solution E1 is preferably between 25°C and 70°C.

A pair of cathodes KP is arranged in the first tank 1a and a pair of anodes AP is arranged in the last tank 1c. The pair of anodes 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 anode pair AP can also consist entirely of a metal oxide or a mixed metal oxide. Electric current can be applied to the cathodes of the cathode pair KP and the anodes of the anode pair AP.

A tinned steel strip (tin plate strip, also referred to below as strip B) is passed through the tanks 1a-1c in succession. For this purpose, the strip B is pulled 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 strip B can be connected as an anode or as a cathode. In each electrolysis tank and above the tanks 1a-1c there are also deflection rollers U around which the strip B is guided and thereby guided through the tanks 1a-1c. The tape B is passed between the opposing cathodes of the cathode pair KP and between the two anodes of the anode pair AP.

A first heater H1 is arranged downstream of the last tank 1c, and a second heater 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 predeterminable temperature and is kept at this temperature for a holding time. The holding time is determined by the belt speed and the length of the continuous furnace determined. The heating devices H1 and H2 can preferably also contain induction coils for inductively heating the strip. The first heating device H1 is set up to rapidly heat 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 designed to heat the strip B to temperatures above the melting point of tin (232° C.).

To prepare for the electrolysis process, the strip B is first degreased, rinsed, pickled and rinsed again and first passed through the second heating device H2, then successively through the tanks 1a-1c and finally through the first heating device H1.

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 layer creates 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 ₂ or a mixture thereof. The tin coating is preferably only partially melted, so that a layer of free metallic tin remains on the surface. This can be anodically oxidized in the first tank 1a.

For this purpose, the strip B in the first tank 1a is connected as an anode and a current density in the range from 0.1 to 10 A/dm ² and preferably between 0.2 and 3 A/dm is generated by the pair of cathodes KP, depending on the strip speed ² generated. With a corresponding current density, a tin oxide layer, which consists at least essentially of tetravalent tin oxide (SnO 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 produced electrolytically 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 forms on the tin surface of the tinplate strip B.

Then, the band B is passed through the middle tank 1b with the rinsing solution Sp to rinse the band. This is followed by drying by means of a drying device not shown here.

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

The electrolytically deposited passivation layer P consists essentially of chromium oxide and chromium hydroxide and in particular has a proportion by weight of chromium oxide and chromium hydroxide of at least 90%, preferably more than 95%, of the total weight of the passivation layer. In addition to chromium oxide and chromium hydroxide, the passivation layer can also contain unavoidable impurities such as residual components of chromium sulphate if Cr(III) sulphate has been used as the chromium compound in the first electrolytic solution E1.

In order to minimize the proportion of chromium hydroxide in the passivation layer, the passivated strip B is passed through the first heating device H1 and therein over a treatment time of at least 0.5 seconds to treatment temperatures above 100° C., in particular in the range from 100° C. to 230 °C. The treatment temperature should not exceed the melting point of tin (232°C) to prevent the tin layer from melting. The treatment time depends on the belt speed and the length of the first heating device H1, which can be in the range from 3 m to 30 m. When using induction heating, the length of the heating device can also be shorter. After thermal treatment in the first Heating device, the proportion by weight of the chromium oxide in the total weight of the passivation layer is preferably at least 95% and particularly preferably more than 98%.

In Figure 2A is a schematic sectional view of one with the electrolysis system Figure 1A producible tinplate strip B shown. The passivation layer P is on one side of the strip B, which consists of the sheet steel substrate S, the iron-tin alloy layer (FeSn/FeSn ₂ ), the metallic tin layer (Sn) and the (tetravalent) tin oxide layer (SnO ₂ ). applied, which essentially consists of pure chromium oxide. The strip B can also be provided with a corresponding passivation layer P on both sides.

After the 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 that, the passivated strip B can also be provided with an organic layer. The organic layer is applied in a known manner, for example by painting or laminating a plastic film onto the surface of the chromium oxide passivation layer. The chromium oxide surface of the passivation layer offers a good adhesion base for the organic material of the coating. The organic coating can be, for example, an organic paint or polymer films made from thermoplastic polymers such as PET, PE, PP or mixtures thereof. The organic coating can be applied, for example, in a "coil coating" process or in a panel process, with the coated strip being first divided into panels in the panel process, which are then coated with an organic paint or with a polymer film be coated.

In Figure 1B a second embodiment of an electrolysis system is shown, which contains four tanks 1a, 1b, 1c, 1d arranged one behind the other in the strip running direction v. The two front tanks 1a, 1b seen in the belt running direction correspond to the tanks 1a and 1b of the embodiment of FIG Figure 1A and are filled with the basic electrolyte BE and the rinsing solution Sp. In the downstream direction after the second tank 1b, a third tank 1c is arranged, which is filled with a second electrolytic solution E2. A fourth tank 1d, which is filled with the first electrolytic solution E1, adjoins the third tank 1c in the strip travel direction v. Examples of the composition of the first electrolytic solution E1 and the second electrolytic solution E2 are given in Table 1. Table 1: component concentration chromium sulfate 120g/L sodium sulfate 100g/L diluted sulfuric acid 96% 7ml/L VE water rest

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

Due to the different composition of the electrolytic solutions E1 and E2, which are filled in the two downstream tanks 1c and 1d, in these last two tanks 1c and 1d, layers containing chromium oxide are electrolytically deposited on the surface of the tinplate strip B, which differ from each other in terms of their composition differentiate. A lower layer L1 of a passivation layer P is deposited in the tank 1c from the second electrolytic solution E2, and an upper layer L2 is deposited from the first electrolytic solution E1 in the last tank 1d. In addition to chromium oxide/chromium hydroxide, the lower layer L1 deposited from the second electrolytic solution E2 in the tank 1c also contains metallic chromium and chromium carbides. In contrast, the upper layer L2 essentially consists of chromium oxide/chromium hydroxide. Therefore, the weight fraction of chromium oxide and chromium hydroxide is lower in the lower layer L1 deposited in the upstream tank 1c than in the upper layer L2 deposited on the surface of the tinplate strip B in the last tank 1d 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, which faces the sheet steel substrate S, and an upper layer L2 deposited thereon, the Composition of the lower and the upper layer differs in terms of the proportion by weight of chromium oxide and chromium hydroxide and of metallic chromium and chromium carbide. In particular, the upper layer L2 has a higher proportion by weight of chromium oxide/chromium hydroxide and contains no metallic chromium. In the lower layer, 10% to 50% by weight can be metallic chromium and the remainder chromium oxide/chromium hydroxide and chromium carbides.

In Figure 2A is a schematic sectional view of one with the electrolysis system Figure 1B producible tinplate strips B shown with a passivation layer P. Compared to the embodiment of Figure 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 one Weight percentage of chromium oxide/ chromium hydroxide in the upper layer L2.

As in the embodiment of Figure 1A is also used in the electrolysis system Figure 1B after the application of the passivation layer P on 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 converting them into chromium oxides. After the thermal treatment, at least the upper layer L2 consists essentially of pure chromium oxide, apart from unavoidable impurities, the proportion by weight of chromium oxide in the total weight of the passivation layer P being at least 95% and preferably more than 98%.

Examples: EXAMPLE 1

To determine the growth of tin oxide on the tin surface of passivated tinplate, which has been electrolytically coated on the basis of an electrolyte solution with a trivalent chromium compound with a passivation layer containing chromium oxide, tinplate panels were in the laboratory by electrolytic deposition of a passivated chromium oxide-containing passivation layer and then subjected to a thermal treatment according to the invention. The samples were then stored in an oxygen-containing atmosphere (air) in a climatic cabinet at 40° C. and a humidity of 80% for a period of 6 weeks. Before and during the storage period, the amount of tin oxide layer that had formed on the tin surface of the tinplate samples as a result of oxidation with atmospheric oxygen was recorded. The amount of tin oxide layer formed by oxidation in atmospheric oxygen was determined coulometrically.

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

The process and material parameters of the tinplate samples examined from the laboratory tests can be found in Table 2.

To produce the samples, partially melted, unpassivated tinplate samples with a tin coating of 1.4 g/m ² on both sides were cathodic degreasing for an anodizing time of 30 seconds at a current density of 2.5 A/dm ² in a 5 percent Soda solution cathodically degreased and then rinsed with deionized water. After cathodic degreasing at a current density of 2 A/dm ² , the tinplate samples were then electrolytically provided with a passivation layer P containing chromium oxide by applying to the tinplate samples from the first electrolyte E1 from Table 1, whose pH value was adjusted to pH = 3.2 was set, a passivation layer containing chromium oxide and chromium hydroxide was applied electrolytically at a temperature of 35° C. during an electrolysis period of 0.5 to 1.0 seconds. The weight of chromium of the electrodeposited passivation layer P is shown in Table 2 as "chromium layer (Cr)".

The tinplate samples provided with the passivation layer P were then subjected to a thermal treatment according to the invention in an oven for 600 seconds exposed to a temperature of 200 °C. In the case of the comparative samples (samples nos. 1a, 1b, 2a, 2b, 3a and 4a) in Table 2, this thermal treatment was not carried out.

The samples thermally treated according to the invention and the comparison samples were then stored for 6 weeks in a climate chamber in atmospheric oxygen at 40° C. and 80% humidity, the amount of the tin oxide layer (SnO ₂ ) being determined coulometrically at the beginning of storage and at 2-week intervals , which has formed on the tin surface of the tinplate samples in the respective storage period. Table 2 lists the initial amounts of tin oxide (SnO ₂ ) layer present at the surface prior to sample placement and the amounts of tin oxide (SnO ₂ ) layer detected after 2, 4 and 6 weeks of storage . The last column of Table 2 shows the difference in the amount of the tin oxide layer (SnO ₂ ) after six weeks of storage in the climatic cabinet and the initial amount of the tin oxide layer (SnO ₂ ).

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 was carried out after the electrolytic deposition of the passivation layer P, exhibit significantly less growth of tin oxide , in comparison to the comparative samples (samples 1a, 1b, 2a, 2b, 3a and 4a) in which no thermal treatment has taken place. The thermal treatment according to the invention of the passivated tinplate samples accordingly leads to a significant reduction in the tin oxide growth in an oxygen-containing atmosphere when the tinplate samples are stored for a longer period of time. In comparison, the samples treated according to the invention show an inhibition of tin oxide growth of more than 50% compared to the comparison samples.

EXAMPLE 2

By an electrolysis system according to the embodiment of FIG Figure 1A tinplate strips with a tin coating of 2.4 g/m ² on both sides were run at a strip speed of 300 m/min in a plant test. The electrolysis tanks were filled with the electrolyte E1 from Table 1. The tin layer was partially melted in the second heating device H2 and the first tank 1a was passed through without current, ie 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 weight layer (chromium layer) of approximately 9 mg/m ² based on the chromium. After the passivation layer had been deposited, the tinplate samples according to the invention (samples nos. 2 to 5 from Table 3) were cooled to room temperature and thermally treated in the first heating device H1 for different treatment times between 10 seconds and 120 seconds at a treatment temperature of 187°C. This thermal treatment was not carried out on a comparative sample (sample no. 1 from Table 3). Thereafter, the tinplate strip was cut into sheets and the initial amount of tin oxide on the tin surface of the samples thus produced was coulometrically determined. The tinplate samples were stored in air for 4 weeks in a climatic chamber at 40° C. and a humidity of 80%, and the amount of tin oxide that accumulated during storage due to oxidation with atmospheric oxygen on the tin surface of the tinplate was measured at 2-week intervals Samples formed, recorded coulometrically.

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

In contrast, the tinplate samples thermally treated according to the invention (samples nos. 2 to 5 from Table 3) already have a lower tin oxide layer at the beginning of storage in the climatic chamber, and the growth of the tin oxide layer during storage in the climatic cabinet falls significantly in the samples according to the invention lower, with the inhibition of tin oxide growth being 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) or 120 seconds (sample no. 5), exhibit a Tin oxide layer of less than 40 C / m ² . Such tin oxide coatings of less than 40 C/m ² are preferred both optically and with regard to paint adhesion and paintability. Tinplate with tin oxide coatings between 41 C/m ² and 69 C/m ² have sufficient adhesion for organic coatings, but have a pale yellowish surface and are therefore not ideal. With tin oxide occupancies above 69 C/m ² it can lead to a complete failure of the material and in particular detachment of the organic layer due to poor adhesion to the surface of the passivated tinplate.

With the method according to the invention, the tin oxide growth on the tin surface of tinplate which has been provided with a passivation layer electrolytically from a trivalent chromium electrolyte can be significantly reduced, resulting in better adhesion of organic coatings and a pleasing visual appearance of the surface can. Table 2 electrolysis thermal treatment Climatic cabinet (KS) (40°C, 80%) sample no j [A/dm ² ] t [sec] Cr [mg/m ² ] T [°C] τ [sec] SnO ₂ [C/m ² ] before KS SnO ₂ [C/m ² ] after 2 weeks SnO ₂ [C/m ² ] after 4 weeks SnO ₂ [C/m ² ] after 6 weeks Δ SnO ₂ [C/m ² ] (6 weeks) 1a (comparative sample) 2 0.5 4 - - 19 64 65 70 51 1b (comparative sample) e) 2 0.5 3 - - 18 60 54 48 30 1c (according to the invention) 2 0.5 5 200 600 29 42 44 47 18 2a (comparative sample) e) 2 1 11 - - 19 37 77 59 40 2b (comparative sample) 2 1 11 - - 13 34 32 38 25 2c (according to the invention) 2 1 10 200 600 14 20 20 25 11 3a (comparative sample) 2 0.8 15 - - 8th 49 76 108 100 3b (according to the invention) 2 0.8 11 200 600 8th 26 37 38 30 4a (comparative sample) e) 2 0.8 8th - - 15 53 54 74 59 4b (according to the invention) 2 0.8 9 200 600 19 28 37 44 25 j: current density, t: electrolysis time, Cr: chromium plating;
T: treatment temperature of the thermal treatment, τ: treatment time of the thermal treatment; SnO ₂ before KS: tin oxide coating before being placed in the climatic chamber;
SnO ₂ after 2/4/6 weeks: tin oxide coating 2/4/6 weeks after being placed in the climatic cabinet;
Δ SnO ₂ : tin oxide growth during storage in a climatic cabinet electrolysis thermal treatment Climatic cabinet (KS) (40°C, 80%) sample no j [A/dm ² ] t [sec] Cr [mg/m ² ] T [°C] τ [sec] SnO ₂ [C/m ² ] before KS SnO ₂ [C/m ² ] after 2 weeks SnO ₂ [C/m ² ] after 4 weeks Δ SnO ₂ [C/m ² ] (4 weeks) 1 (comparison sample) 37 0.2 9 - - 11 44 60 49 2 (according to the invention) 37 0.2 9 187 10 9 38 55 46 3 (according to the invention) 37 0.2 9 187 30 7 28 46 39 4 (according to the invention) 37 0.2 9 187 60 8th 26 39 31 5 (according to the invention) 37 0.2 9 187 120 9 21 37 28 j: current density, t: electrolysis time, Cr: chromium plating;
T: treatment temperature of the thermal treatment, τ: treatment time of the thermal treatment; SnO ₂ before KS: tin oxide coating before being placed in the climatic chamber;
SnO ₂ after 2/4 weeks: tin oxide coating 2/4 weeks after being placed in the climatic cabinet;
Δ SnO ₂ : tin oxide growth during storage in a climatic cabinet

Claims (15)

Process 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 taking place at least partially from an electrolyte solution (E) which contains a trivalent chromium compound, at least one salt to increase conductivity and contains at least one acid or one base to set a desired pH value and is free from organic complexing agents and free from buffering agents, characterized in that after the electrolytic deposition of the passivation layer, the passivated tinplate is subjected to a thermal treatment in which the passivated tinplate during a treatment time of at least 0.5 seconds at a treatment temperature of 100°C or more. Method according to claim 1, characterized in that the treatment time is between 0.5 seconds and 30 minutes, preferably between 1.0 second and 60 seconds and/or that the treatment temperature is between 150°C and 232°C, preferably between 170°C and 230°C. Method according to one of the preceding claims, characterized in that the thermal treatment is carried out immediately after the electrolytic deposition of the passivation layer, there being preferably an interval of at most 10 seconds between the end of the deposition of the passivation layer and the reaching of the treatment temperature, the passivated tinplate immediately after completion of the electrolytic deposition of the passivation layer, preferably at a heating rate is heated from 100 to 700 K/s and in particular inductively to the treatment temperature. Method according to one of the preceding claims , characterized in that the electrolyte solution (E) has an average temperature in the range from 20°C to 80°C and preferably in the range from 30°C to 65°C and particularly preferably between 40°C and 60 °C, the passivated tinplate having at least essentially the average temperature of the electrolyte solution (E) immediately after the end of the electrolytic deposition of the passivation layer and, for the thermal treatment, starting from this temperature, is heated to the treatment temperature. Method according to one of the preceding claims, characterized in that the electrolyte solution (E) consists of the trivalent chromium compound, which is preferably chromium(III) sulfate, the at least one salt and the at least one acid or base and a solvent, in particular water . Method according to one of the preceding claims, characterized in that the passivation layer consists at least essentially of trivalent chromium oxide and/or chromium hydroxide and preferably has a weight proportion of chromium oxide and/or chromium hydroxide of more than 95%, particularly preferably more than 98% and the Rest of the passivation layer is formed by unavoidable by-products, which include in particular chromium sulfate. Method according to one of the preceding claims, characterized in that the tinplate for the electrolytic deposition of the passivation layer is guided at a predetermined strip speed (v) in a direction of strip travel through at least one electrolysis tank (1) or a plurality of electrolysis tanks (1a, 1b, 1c) arranged one behind the other in the direction of strip travel is, wherein the belt speed (v) is at least 100 m / min and preferably between 200 m/min and 750 m/min and the electrolytic time (t) in which the tinplate is in electrolytically effective contact with the electrolytic solution (E) in each of the electrolytic tanks (1, 1a - 1c) is less than 1.0 second and/or the total electrolysis time (t _G ), in which the tinplate in all electrolysis tanks (1, 1a - 1c) is in electrolytically effective contact with the electrolyte solution (E), is between 0.5 seconds and 2.0 seconds, in particular between 1.0 seconds and 1.8 seconds, preferably at least in the last electrolysis tank (1c) viewed in the direction of strip travel, an electrolyte solution (E) is contained which consists of the trivalent chromium compound, the at least one salt and the at least one acid or base and a solvent, especially water. Method according to one of the preceding claims, characterized in that the passivation layer applied from the electrolyte solution (E) has a total weight of chromium oxide and/or chromium hydroxide of at least 3 mg/m ² , preferably from 5 mg/m ² to 20 mg/m ² and particularly preferably from 8 mg/m ² to 12 mg/m ² , based in each case on the chromium. Method according to one of the preceding claims, characterized in that a tin oxide layer consisting essentially of tetravalent tin oxide (SnO ₂ ) is produced on the surface of the tinplate before the electrolytic deposition of the passivation layer, by the tinplate being connected as an anode in an aqueous electrolyte which is a contains phosphate, borate, sulfate or carbonate. Electrolysis system for the electrolytic passivation of the surface of a tinplate by depositing a passivation layer containing chromium oxide/chromium hydroxide on a surface of the blackplate or tinplate, the electrolysis system comprising: - At least one electrolysis tank (1) filled with a first electrolytic solution (E), or several electrolysis tanks arranged one behind the other (1a, 1b, 1c), of which at least one electrolysis tank (1c) is filled with the first electrolytic solution (E) and the remaining electrolysis tanks (1a, 1b) with the first electrolytic solution (E) or another electrolytic solution (E'), which contains a trivalent chromium compound and whose composition differs from the first electrolytic solution (E), are filled, - wherein the first electrolytic solution (E) contains a trivalent chromium compound and at least one salt to increase the conductivity and at least one acid or one base to set a desired pH value and is free from organic complexing agents and free from buffering agents, - a transport device for transporting a strip-shaped tinplate in a strip running direction at a predetermined strip speed (v), which is preferably more than 100 m/min, through the at least one electrolysis tank (1) or successively through the several electrolysis tanks (1a, 1b, 1c) , for the electrolytic deposition of a passivation layer from the first electrolytic solution (E) and, if necessary, the other electrolytic solution (E') on the surface of the tinplate, - a heating device, which is arranged in the direction of strip travel after the electrolysis tank (1) or after the plurality of electrolysis tanks (1a, 1b, 1c) and for heating the passivated tinplate to a predetermined treatment temperature of at least 100°C for a predetermined treatment time of at least 0, 5 seconds is formed. Electrolysis system according to Claim 10, characterized in that the heating device is arranged immediately after the electrolysis tank (1) or after the last electrolysis tank (1c) of the plurality of electrolysis tanks (1a, 1b, 1c), viewed in the direction of strip travel, the heating device preferably being in the form of a continuous furnace with a Is formed heating section of at least 3 m, in particular in the range of 5 to 30 m and / or preferably comprises an induction heater. Tinplate with 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 and preferably having a weight proportion of chromium oxide and/or chromium hydroxide of more than 95%, particularly preferably more than 98% %, characterized in that after at least four weeks of storage in an oxygen-containing atmosphere, the passivated tinplate has a tin oxide layer on the tin surface with a tin oxide coating of less than 70 C/m ² , in particular in the range from 20 C/m ² to 60 C/m ² and preferably less than 55 C/m ² and more preferably less than 40 C/m ² . Tinplate according to claim 12, wherein the tinplate has the following layer structure: a steel substrate, optionally an iron-tin alloy layer, a tin layer made of metallic tin and the tin oxide layer and the passivation layer. Tinplate according to claim 12 or 13, wherein the tin oxide layer consists of tetravalent tin oxide (SnO ₂ ) or contains at least tetravalent tin oxide (SnO ₂ ) and preferably has a coating of less than 40 C/m ² , particularly preferably less than 30 C/m ² and in particular in the range from 10 to 28 C/m ² . Tinplate according to any one of claims 12 to 14, wherein the passivation layer contains at least one upper layer which, apart from unavoidable impurities or residual components of chromium hydroxide and residual components of chromium sulfate, consists essentially of trivalent chromium oxide, the proportion by weight of the chromium oxide being preferred in the upper layer is at least 95%, the passivation layer preferably having a total weight coverage of chromium oxide based on the chromium of at least 3 mg/m ² and more preferably from 8 mg/m ² to 12 mg/m ² .

Images