Method for passivating tin-coated steel strip and apparatus for producing said passivated tin-coated steel strip

Abstract

The present invention relates to a method for passivating tin-plated steel strip after electrodeposition of one or more tin layers or after optional reflow of one or more electrodeposited tin layers, and to an apparatus for producing said passivated tin-plated steel strip.

Description

Field of the Invention

[0001] The present invention relates to a method for passivating tin-plated steel strip after electrodeposition of one or more tin layers or after optional reflow of one or more electrodeposited tin layers, and to an apparatus for producing said passivated tin-plated steel strip. Background of the Invention

[0003] Tinplate is a light gauge cold rolled low carbon steel sheet or strip coated on both sides with commercially pure tin to protect the steel sheet from corrosion, and is used primarily in the packaging industry. The tin layer is usually deposited electrolytically, usually in a continuous production line.

[0004] Tin-plated steel combines in one material the strength and formability of steel with the corrosion resistance, weldability and good appearance of tin. Within this broad description, today there is an extremely wide range of products, tailored to meet the end-use requirements. The production of the steel substrate and its subsequent coating with tin are independent of each other, so that in theory any set of properties in the steel can be combined with any tin coating. Depending on the grade selected and the way it is processed and closely controlling the steel composition used for tin-plating steel, various types with different formability ("temper") can be produced. Tin-plated steel is sold in a range of steel thicknesses from about 0.10 mm to 0.49 mm. Steel can be coated with different thicknesses of tin. Even different thicknesses (differential coatings) can be produced on both sides to meet different conditions at the inner and outer surfaces of the container. Various surface finishes are also produced for different applications.

[0005] The tin is deposited as a whitish coating with a slight metallic sheen. When desired, it is reflowed by induction or resistance heating (or a combination) to produce a bright mirror-like finish. This reflow process enhances the corrosion resistance of the product by forming an inactive tin-iron alloy layer. Most DWI tinned steel (drawn wall ironed) is not reflowed, and this can be a significant portion of the output for many manufacturers.

[0006] Tin-plated steel and especially reflowed tin-plated steel have a thin tin oxide film on the surface which, if not treated, can grow in the thickness direction during storage. In order to improve corrosion resistance and adhesion to organic coatings, chemical or electrochemical passivation is applied to the strip. The most common form of passivation for decades involved cathodic treatment in dichromate or chromic acid solution containing dichromate at temperatures between 50 and 85 ° C. This treatment deposits a complex layer of chromium and its hydrated oxide, which inhibits the growth of tin oxides, prevents yellowing, improves paint adhesion and minimizes staining by sulfur compounds. Dichromate or chromic acid solution contains Cr 6+These solutions are increasingly being discouraged because they are harmful, especially in cases where the metal product is intended for the food industry. EU regulations (REACH) prohibit the use of these solutions if alternatives are available.

[0007] When tin-plated steel is used to prepare containers (cans) for preserving food, passivation must prevent the growth of the tin oxide layer from being too intense during the storage of the tin-plated steel or the food container made therefrom until it is coated with a protective layer and then until the contents of the storage are consumed. In addition, passivation should prevent the tin-plated steel surface from discoloring. Such discoloration occurs, for example, when a can containing sulfur-containing substances is sterilized, because if it is not fully passivated, sulfur reacts with the tin in the coated steel surface. Due to the fading discoloration (marbling) or golden discoloration of the packaging surface, consumers may have the idea that the contents are contaminated. Adhesion problems of the protective layer may also occur, and these can be avoided by passivation of the coated steel sheet. In addition, passivation must ensure that the metal container is resistant to the acid contained in the food after being filled with food. If the passivation of the tin-plated steel is insufficient, such acid anions in the can contents can cause the delamination of the protective layer in the container and corrode the tin-plated steel below.

[0008] EP2802688 discloses a method for passivating the surface of tin-plated steel, wherein after tin-plating the surface is anodized to form an oxide layer consisting essentially of tetravalent tin oxide, after which a liquid 0 solution of a chromium-free post-treatment agent is applied. The method in EP2802688 claims that the corrosion resistance and resistance to reaction with sulfur of the tin-plated steel can be greatly improved by anodizing before post-treatment with a chromium-free post-treatment agent. An oxide layer with a layer thickness in the nm range is produced on the surface of the tin-plated steel strip by anodizing. The oxide layer is essentially a tetravalent tin oxide (SnO2) layer. It is claimed that the thin surface layer of the chromium-free post-treatment agent deposited on this oxide layer protects the surface of the tin-plated steel strip completely and effectively against corrosion and reaction with sulfur.

[0009] Problems with prior art methods are that 1) oxidation of the strip is not uniform across the width of the strip, resulting in differences in protection of the tinned steel surface against corrosion and reaction with sulphur across the width of the strip, as well as differences in adhesion between the tin layer and the further protective layer on the steel strip, and that 2) a newly applied oxide layer forms on top of an already existing ill-defined oxide layer.

[0010] Purpose of the Invention

[0011] It is an object of the present invention to provide a method for passivating tin-plated steel strip after electrodeposition of one or more tin layers, which has an improved uniformity of the oxide layer over the width of the tin-plated steel strip.

[0012] It is also an object of the present invention to provide a method for passivating tin-plated steel strip after electrodeposition of one or more tin layers, which has an improved adhesion uniformity between the tin layer and the further protective layer on the steel strip.

[0013] It is also an object of the present invention to provide a method for passivating tin-plated steel strip after electrodeposition of one or more tin layers, which has improved corrosion resistance and marbling resistance over the width of the tin-plated steel strip.

[0014] It is also an object of the present invention to provide a method for passivating tinned steel strip after electrodeposition of one or more tin layers, which is an alternative to the use of dichromate or chromic acid solutions.

[0015] Description of the invention

[0016] The object of the invention is achieved by a method according to claim 1. Preferred embodiments are provided by the dependent method claims 2 to 13 .

[0017] Now we will use the special and Non-restrictive The method according to the present invention is further explained by the embodiments. Any range mentioned herein below has general applicability to the method according to the present invention, and is not limited to the following embodiments, and is also independently applicable.

[0018] In the first step according to an embodiment of the present invention, an electrolytic deposition of a tin layer on a cold rolled steel strip (black steel plate) is carried out in a continuous electrolytic tin plating line running at a speed of at least 50 m / min. After being coated with a tin layer on one or both sides, the black steel plate becomes tin-plated steel. Currently, a high-speed industrial electrolytic tin plating line can run up to a speed of about 750 m / min. After the tin layer is deposited, the tin-plated steel is heated to a temperature greater than the melting point of tin (232 ° C) in order to melt the tin layer. As a result of melting, tin forms an iron-tin FeSn2 intermetallic compound with the iron from the steel strip. The surface of the tin layer retains tin and becomes very shiny after solidification by quenching in water. A new oxide layer is immediately formed on the fresh surface, and this oxide layer keeps growing during storage and is defined as a pre-existing oxide layer in the context of the present invention. Although remelting tin as described is an optional feature, most tin-plated steels undergo such a remelting or soft melting step.

[0019] In the second step of the process according to the invention, the electrochemical treatment tank (II) contains an alkaline aqueous solution serving as electrolyte, which in this example is a sodium carbonate solution. Cathodic removal The tin-plated steel enters the electrochemical treatment tank in a downward direction (i.e. the entry pass is the downward pass) with the aid of a non-conductive guide roll (3). Near the bottom of the vertical tank is a non-conductive guide roll (sink roll) against which the tin-plated steel strip runs. Electrodes ( Figure 1 , reference numerals 6, 7) (e.g. stainless steel anodes). The strip (1) runs between the electrodes without touching them. An electric potential is applied by a rectifier between the electrode in the entry pass (anode, 6) and the electrode in the exit pass (cathode, 7). As a result, the strip assumes the opposite charge of the electrodes when passing through the electrodes. The strip thus becomes cathodic when passing through the anode (6) in the entry pass and becomes anodic when passing through the cathode (7) in the exit pass. After the oxide is removed, for the reasons explained herein below, the tin layer no longer has an oxide layer on its surface over the entire width of the tin-plated steel, i.e. the tin-plated steel surface is a pure (bare) tin surface.

[0020] After the direction of movement of the tin-plated steel is reversed by means of guide rollers the tin-plated steel starts an exit pass in an upward direction (i.e. the exit pass is an upward pass) and subsequently passes the cathode and becomes anodic. As a result, a new oxide layer grows on the steel strip under carefully controlled conditions on the pure (bare) tin surface resulting from the cathodic removal of the pre-existing tin oxide. The charge Q1 required to remove the pre-existing oxide is significantly lower than the charge Q2 required to grow the new oxide layer to the thickness required for the oxide layer to provide sufficient corrosion resistance and marbling resistance. The re-oxidized tin-plated steel leaves the electrochemical treatment cell (II) by means of non-conductive guide rollers (5).

[0021] It should be noted that the second step of the method according to the invention can also be carried out in an apparatus in which the strip travels between the anode and the cathode in a substantially horizontal direction. In this case there is no reversal of the direction of travel between the entry pass and the exit pass, but one or more non-conductive guide rollers may be present in the trough to guide and support the strip in its travel between the anode and the cathode.

[0022] In the method according to the invention it is essential that the guide means, such as guide rollers and deflection rollers, which guide the strip into, into and out of the electrochemical treatment tank are non-conductive, since no current can flow from the strip through the guide means to the ground. In order for the guide rollers and deflection rollers to be non-conductive, the rollers, which are usually made of metal, are covered with a rubber layer.

[0023] In the method according to the invention the charge applied during the entry pass is the same as during the exit pass. This means that the value of Q1 is greater than that required to remove the pre-existing oxide layer and that the tin-plated steel always has a pure and bare tin surface when anodic reoxidation begins. In this embodiment, only one rectifier is required if the anode and cathode work in pairs, or two rectifiers if the top (right) and bottom (left) cathode / anode work individually. In most cases a single rectifier will be sufficient. This makes the method both simple to control (because Q2 is dominant) and technically simpler (because Q2 is significantly greater than Q1 in all practical cases). In the embodiment according to the invention it is not necessary to check whether the pre-existing oxide has been completely removed.

[0024] The anodizing time corresponds to the residence time of the tin-plated steel strip in the electrochemical oxidation bath during the exit pass. This is determined by the anode length and the strip speed. For a typical low strip speed of about 50 m / min, it is in the range of about 2 to 2.5 seconds. For a high strip speed of about 750 m / min, the anodizing time is about 0.1 to 0.2 seconds. For most industrial production lines, the anodizing time is therefore between 0.1 s and 2.5 s, preferably in the range of 0.15 s to 1.5 s, more preferably at most 1.0 s, even more preferably at most 0.7 s and still more preferably at most 0.4 s.

[0025] Depending on the system, the spacing between the counter electrode and the tinned steel strip in the electrolyte bath is set, for example, in the range of 3 to 15 cm, preferably in the range of 5 to 10 cm, and in particular about 5 cm.

[0026] The temperature of the electrochemical oxidation bath is preferably in the range of 25 to 60°C, more preferably in the range of 25 to 50°C, and in particular about 35°C.

[0027] In the third step of the process, the anodically reoxidized tin-plated steel strip is rinsed, for example, with water or deionized or demineralized water and subsequently dried, for example, with hot air. However, other drying methods are also suitable, such as drying with a hydrophilic solvent followed by drying with a cold or hot air blower (hot air is preferred), drying with a drying system without convection air, such as IR radiators, induction heating or resistance heating, or drying only with a cold or hot air blower, preferably a hot air blower.

[0028] In the fourth step of the method, coating of the anodically reoxidized tin-plated steel strip with a post-treatment agent occurs. Although this step is optional, and the product produced up to and including the third step can already be considered as passivated tin-plated steel, the use of this additional fourth step is preferred for longer-term and consistent passivation. A solution of the post-treatment agent, preferably a solution in water or an organic solvent or a ready-to-use preparation of the post-treatment agent, is sprayed onto the steel strip moving at the strip speed. A 1.5 to 10% aqueous solution of the post-treatment agent has proven to be advantageous. The thickness of the post-treatment agent solution is preferably then homogenized and dried by a homogenizing roller. After drying, a thin film of the post-treatment agent remains on the surface of the coated steel strip, wherein the weight of the film is typically between 2 and 30 mg / m 2 Suitable application techniques for the post-treatment agent include: dipping, dipping with squeeze rollers, rotor-spray application, rotor-spray application supported by a smooth roller, spray application, one (or more) spray bars, spray-squeeze application, application by a roll coater system, application by slot coating, slot curtain coating, etc. If so, any excess treatment agent desired can be removed by the squeeze roller pair arranged in the direction of strip movement after the post-treatment agent is applied, and in some cases the excess post-treatment agent can be reused.

[0029] Suitable post-treatment agents that can be used in combination with the present invention are:

[0030] All organic systems such as organic acids (oleic acid, rosin acid);

[0031] All organic systems such as acrylates, polyurethane dispersions and other types of thin organic coatings;

[0032] Organic / inorganic coupling agents such as one-component and two-component siloxane systems;

[0033] Inorganic systems such as silicate-based systems;

[0034] • Inorganic systems in an organic matrix such as fluoro-titanate and zirconium-titanate combined with an organic polymer matrix.

[0035] Preference is given to using inorganic systems in an organic matrix such as fluorotitanates and zirconate titanates in combination with an organic polymer matrix. Such post-treatment agents are currently commercially available, for example as shown below.

[0036] After reoxidation and rinsing and drying, a post-treatment agent is applied to the anodically reoxidized tin-plated steel surface by the usual application techniques for such passivation systems. The post-treatment agent is preferably chromium-free, preferably a no-rinse / in-situ drying post-treatment agent. Such post-treatment agents may be based on zirconium, titanium, combinations of zirconium and titanium, phosphates, siloxanes, etc., such as acidic aqueous solutions of water-soluble inorganic compounds containing the elements Zr, Ti, Hf and / or Si as disclosed in US10011915. Examples are X4744, MM0705(Chemetall) or Z801(ADChemicals), M-NT1455, Bonderite M-NT1456, Bonderite M-NT10456 (Henkel). This was prepared to have a dry coverage range of 0.2 to 2 mg Ti / m on the surface of the tinned and reoxidized steel strip. 2 , more preferably 0.5 to 1.5 mg Ti / m 2 or 0.8 to 1.5 Ti / m 2 range, especially 1mg Ti / m 2 of solution.

[0037] The advantages of the no-rinse / in-situ drying post-treatment agent over the electrolytic system are that the solution is easy to apply, simple equipment is used in a compact application unit, allowing easy assembly on existing production lines, and more general chemicals are available. The post-treatment agent can be applied to the surface treated tin-plated steel surface by the usual application technology for such a passivation system. Suitable application techniques include: dipping, dipping with squeeze rollers, rotor spray application, rotor spray application supported by a smooth roller, spray application, one (or more) spray bars, spray-extrusion application, application by a roller coater system, application by slot coating, slot curtain coating, etc.

[0038] The inventors have also found that although it is preferred to tin-plating and optionally reflow in a continuous process immediatelyThe cathodic removal of the pre-existing oxide, the anodic reoxidation and the subsequent passivation steps with the post-treatment agent are carried out without interruption, but the method is also capable of processing tin-plated steel that has not been passivated cathodically and anodically immediately after tinning and optional reflow. The method according to the invention can also be used to process coils of tin-plated steel that have been produced previously. This situation occurs, for example, when there is a delay between the production and passivation of the tin-plated steel strip, for example after a period of storage of the tin-plated steel strip in the usual coiled form for whatever reason. Any growth of the oxide layer that occurs in the meantime can be easily handled by the cathodic removal of the pre-existing oxide, and the subsequent process is started, which results in a tin layer that is only as pure and bare as in the process that is carried out continuously and immediately after tinning. The method according to the invention is able to handle additional spontaneous growth of oxides, because Q2 is significantly greater than that required for the removal of the pre-existing oxides, and the excess charge is used to remove any additional spontaneously grown oxides.

[0039] During the cathodic removal of pre-existing tin oxide, hydrogen is formed in the cathode strip once the pre-existing oxide has been cathodically removed. Particularly if Q1 is much smaller than Q2 mode, it may be advantageous for safety reasons and for environmental reasons to capture the hydrogen by a hydrogen capture device. In the case of tinned steel treated after tinning either in-line (i.e. immediately) or using a very short time and using controlled storage between tinning and passivation treatments, Q1 is usually much smaller than Q2.

[0040] The sole purpose of the alkaline aqueous solution is to enable cathode and anode treatment, not to deposit foreign substances contained in the electrolyte onto the substrate surface. The pH of the alkaline aqueous solution must not be too low, not below pH=8.75, otherwise the efficiency of the electrochemical reaction becomes too low and then the method cannot be integrated into existing high-productivity production lines. In addition, the pH of the alkaline aqueous solution is not higher than 11.0, and preferably not higher than 10.5, because this will increasingly cause the tin layer to dissolve in the alkaline aqueous solution.

[0041] In the context of the present invention, the thickness (D) of the tin oxide layer is measured in coulombs / m 2 The thickness of the tin oxide layer is related to the anodic treatment time (t) and the current density (A) by D = E × A × t, where E is the efficiency of the electrochemical reaction and where D is at least 15 C / m 2 .

[0042] The efficiency therefore represents the ratio of the thickness D of the produced oxide layer to the applied charge density (A×t) and can be estimated by plotting D as a function of (A×t). Initially, the anodic charge value passed is less than 50 C / m 2, the curve is more or less linear, but with increasing anodic charge passed, the efficiency E decreases resulting in a slower growth rate of the tin oxide layer, and therefore a slower increase in D. If D < 15 C / m2, the tin oxide layer is too thin and is not effective for achieving the desired resistance to sulfide coloration. A minimum thickness of D of 15 C / m2 is therefore required.

[0043] The total D as specified above can be achieved by any combination of A and t, but considering its processability on high speed tinning lines, high current density (A>0.1A / dm 2 、Preferably A>1.0A / dm 2 ) is combined with a short treatment time (t<1s). The interchangeability of A and t in the anodic reoxidation treatment means that the method can work at short treatment times by adjusting the applied current density accordingly. Therefore, the method according to the present invention can be used in industrial tinning production lines running at production line speeds of more than 300m / min to speeds of up to 1000m / min. In addition, the treatment time t is determined not only by the production line speed v but also by the effective length of the treatment section or "anode length" L, according to t=L / v, which means that the processing window can be further expanded by appropriately selecting the anode length L. For example, in order to deposit a 50C / m on a production line running at 600m / min (10m / s), 2 A layer of thickness (assuming E = 1) would require 1000 A / m 2 The method of the invention is carried out with a current density A of 100 A / m2 and a treatment length of 0.5 m. If the treatment length is 5 m, it can also be carried out with a current density of 100 A / m2. This design and process flexibility are important advantages of this method. The advantage of the method according to the invention is that due to the required thickness of the reoxidized oxide layer, cathodic removal always requires less current than subsequent anodic reoxidation. When the pre-existing tin layer is completely cathodically removed, hydrogen is formed at the anode, but the pure and exposed tin layer remains uncontacted. For safety reasons and for environmental reasons, hydrogen can be advantageously captured by a hydrogen capture device.

[0044] In an embodiment of the invention, cathodic removal of anodizing is performed online immediately after and together with the electrolytic tin plating and optional reflowing step, wherein the anodic reoxidation treatment time (t) after the cathodic removal of the pre-existing tin oxide layer is at most 5 seconds, preferably at most 2 seconds, more preferably between 0.05 seconds and 1.5 seconds. This range and more preferred ranges are consistent with high-speed processing lines. In an embodiment, cathodic and anodic treatment is performed online using an industrial electrolytic tin plating line, and wherein the current density (A) during the anodic treatment is at least 10 A / m 2 , preferably at least 50A / m 2 and more preferably at least 100 A / m 2and / or up to 4000A / m 2 , preferably up to 2000A / m 2 or more preferably at most 1000A / m 2 This range and the more preferred ranges are consistent with high speed processing lines.

[0045] The main function of the alkaline aqueous solution is to support the electrochemical reactions used for the cathode and anode treatments while the ionic species present in the alkaline aqueous solution do not participate in the electrochemical modification of the tin-plated steel surface. The preferred alkaline aqueous solution contains ions from Group 1 of the periodic table (e.g. Na + , K + ) or Group 2 (e.g. Mg 2+ , Ca 2+ ) cations or polyatomic cations (such as NH4 + ) and polyatomic anions (phosphate, borate, sulfate, carbonate, etc.). In addition, the anion may be a conjugate base of an organic acid (e.g. acetate, citrate). Since it is important that the pH is maintained within certain boundaries, a buffer solution may be used. It is preferred that the alkaline aqueous solution does not contain monoatomic halogen anions (Group 17) such as Cl - 、F - .

[0046] Preferably, the aqueous buffer solution contains sodium carbonate and preferably does not contain borates, phosphates, sulfates, etc. The concentration of sodium carbonate in the aqueous buffer solution is at least 0.25% by weight, preferably at least 0.5% by weight, preferably 1% to 10% by weight, in particular 2% to 8% by weight, preferably 3% to 7% by weight, especially 4% to 6% by weight, especially about 5% by weight. 1% by weight of sodium carbonate corresponds to about 10 g / l of sodium carbonate in the electrolyte.

[0047] In addition, the alkaline aqueous solution may contain other chemical additives such as surfactants, wetting agents, defoamers, etc. to support the electrochemical treatment, provided that these additives do not adversely affect the removal of the pre-existing tin oxide layer and the reformation of the tin oxide layer.

[0048] The anodic treatment of tin-plated surface is converted into tin oxide from metallic tin by electrochemical oxidation by the outermost layer of tin surface.The tin oxide layer (within certain thickness range) produced in this way provides the barrier to sulfide coloring.But, the tin oxide layer itself is not stable enough and / or passivation, in long-term storage process under environment and / or humid conditions, or will continue to grow into the thicker tin oxide layer with undesirable property (poor wettability, yellowish outward appearance, poor paint adhesion) in thermal treatment such as baking and drying process.Thereby post-treatment agent will provide stable passivation layer usually by itself to protect the uncontrolled growth of tin-plated steel anti-oxidation tin and also provide the good adhesion of organic coating.But, passivation layer has poor resistance to anti-sulfide coloring under the situation of nearly all investigations.Through application of the present invention, realize favourable property combination. Firstly, by applying a tin oxide layer of the correct thickness and the correct composition, i.e. consisting mainly of SnO (i.e. consisting predominantly of SnO), preferably consisting only of SnO, using cathodic and anodic treatment under suitable process conditions, and then by applying a post-treatment agent passivation system on top of the tin oxide layer, the tin oxide layer is passivated and / or stabilized against further uncontrolled growth by using a non-electrolytic application method to produce a post-treated tin-plated steel.

[0049] Passivation systems that do not contain Cr(VI) must be chemically passivated before the anodic treatment is applied, preferably the so-called no-rinse process for applying a no-rinse, dry-in-place passivation system.

[0050] The inventors found that the thickness of the tin oxide layer on the strip must be between 15 and 100 C / m 2 Preferably, the thickness D of the tin oxide layer is at most 100 C / m 2 Values ​​above 100 are not only economically unattractive in high-speed tinning processes, but also lead to reduced adhesion of subsequently applied organic coatings, especially at the edges of the tinned steel due to the presence of increasing SnO2 in the oxide layer. Even if no additional spontaneous growth of the oxide layer occurs, at least 15 C / m 2 The value of is considered to be the minimum required to reliably remove pre-existing oxides. 2 The inventors have found that the thickness of the tin oxide layer on the strip is preferably at least 25 C / m 2 From the perspective of process efficiency and tin oxide species, the appropriate maximum value for D is 80C / m 2 , preferably 70C / m 2 Or even 60C / m 2 When the thickness of the strip is between 30 and 60C / m 2 Good results are obtained with a minimum value of 35 or even 40 C / m 2 Delivers excellent results.

[0051] Assuming an efficiency E=1, the above values ​​also reflect the settings of the rectifier for the anodic reoxidation step. However, it should be noted that as a result of spontaneous growth during storage, the oxide layer can grow and thus result in a thicker oxide layer than expected based on the rectifier settings. However, assuming that no spontaneous growth occurs, the value of E can be easily determined by measuring the thickness on the strip with the method described herein and correlating the results with the rectifier settings. In this way, the settings on the rectifier can be "translated" into the oxide layer thickness on the strip and the values ​​are given at 15 and 100 C / m 2 Preferred values ​​between 0.05 and 0.10 are reliably and reproducibly obtained.

[0052] In an embodiment, the article is a packaging steel strip provided with a tin layer on at least one side (for typical chemical compositions, see for example EN 10202-2001 or ASTM 623M). The strip is produced in a known manner, for example by cold rolling and annealing and optionally temper rolling of a steel strip of suitable composition, followed by electrolytic tinning.

[0053] Cathodic and anodic treated tinplate steel, optionally treated with a post-treatment agent, can be wound up for storage and transport and unwound afterwards. The passivated tinplate steel produced according to the invention can be further provided with an organic coating, such as epoxy-phenolic gold paint, epoxy-anhydride white paint, PVC or vinyl organosol coating, polyester paint, epoxy-amino or epoxy acrylic-amino waterborne coating. The excellent adhesion of the organic coating to the passivated tinplate steel enables the product to be offered as an alternative to the system of CDC treatment and subsequent polymer coating, thereby completely avoiding the use of chromates.

[0054] Alternatively, the cathodically and anodically treated tinplate steel, optionally treated with a post-treatment agent, may be transferred to a laminating device where the laminated layers are laminated in-line onto the tinplate steel.

[0055] The method of applying the laminated layer to the tinplate steel is preferably carried out by extrusion coating and lamination, wherein the polymer is melted and formed into a thin hot film in a flat (co)extrusion die, wherein the extruded polymer film is subsequently directed onto a casting or cooling roll and then laminated to the heated tinplate steel substrate to form the laminated tinplate steel. The laminated tinplate steel then typically passes through a roll-nip assembly which presses the laminated layer firmly onto the substrate to ensure complete contact and adhesion.

[0056] An alternative is film lamination, where a solid laminate is supplied and applied to the preheated tinplate steel and pressed against the tinplate steel by a nip roller assembly to ensure complete contact and adhesion of the laminate to the preheated tinplate steel.

[0057] Suitable single or multi-layer polymers comprise or consist of one or more of polyethylene terephthalate (PET), isophthalic acid-modified polyethylene terephthalate (IPA-PET), cyclohexanedimethanol-modified polyethylene terephthalate (CHDM-PET), polybutylene terephthalate, polyethylene naphthalate, or copolymers or blends thereof, or polycondensates such as polyethylene (PE) or polypropylene (PP). Example

[0058] Tinned steel is produced in various states ranging from TS245-TS290 and TH415-TH620. Table 1 shows an overview of the states and examples of use. The tin layer thickness also varies depending on the intended use and ranges from 1.4-11.2 on one side and 1.7-5 on the other side. The results of the passivation according to the invention prove to be independent of the state and the tin layer thickness. Most tinned steels undergo a reflow step.

[0059] Table 1: Overview of states and usage examples

[0060]

[0061]

[0062] After tinning and optionally reflow the tinned steel is subjected to the method steps according to the invention.

[0063] The inventors have discovered a tin oxide substance and that the thickness of the oxide layer found on the strip after anodic reoxidation is very consistent across the width of the strip. Earlier experiments conducted in accordance with the prior art disclosed in EP2802688 revealed differences in the oxide type in terms of oxide layer thickness and width. The prior art discloses that it is preferred that the oxide type is SnO2. Due to the prior art method, this oxide layer is deposited on top of a pre-existing layer because there is no cathodic removal of the pre-existing oxide in the prior art. The inventors have discovered that this cathodic removal is critical to obtaining a pure and bare tin surface, which is independent of the previous treatment of the tin-plated steel (soft melting or not, long storage period or not, storage conditions are favorable or not, etc.), and subsequent deposition on this pure and bare surface of a new oxide layer of uniform thickness across the width of the tin-plated steel strip, and consisting mainly of SnO rather than SnO2. The inventors have discovered that this surface forms an ideal tin oxide surface for further processing to package tin-plated steel strips for use in container production.

[0064] The thickness of the tin oxide layer is determined using the coulometric method. The tin oxide layer is reduced by a controlled small cathode current in a 0.01M hydrobromic acid (HBr) solution, and the hydrobromic acid is free of oxygen by washing with nitrogen. The reduction process of the oxide is monitored by measuring the reduction potential, and the charge (A*t) passed for complete reduction is used as a measure of the thickness of the tin oxide layer. For the test, a cylindrical battery with a circular hole of about 4 cm diameter at one end and an Ag / AgCl reference electrode is used. The other end of the battery contains a platinum counter electrode. The test sample covers the hole, and the hole is sealed using an O-ring to form a watertight connection with a clearly defined area, and the test sample is fastened in place using a pneumatic cylinder. The battery is connected to the electrolyte solution by a flexible tube so that it can be filled and emptied under a nitrogen atmosphere. A cathode current density of -0.50A / m2 is applied to the sample using a potentiostat-galvanostat, and the potential is measured until the reduction is complete. Figure 2 The inventors have also found that these curves can be used to distinguish the types of tin oxide in the tin oxide layer (see Figure 3 ).

[0065] By measuring and comparing samples taken over the width of a passivated tin-plated steel strip, the inventors found that in the case of tin-plated steel with a tin oxide layer consisting mainly of SnO, the composition of the tin oxide is identical at the edges, whereas this is not the case for samples in which the tin oxide layer consists mainly of SnO 2. The potential difference at 25 s (as measured at different positions over the width of the tin-plated steel with a tin oxide layer consisting mainly of SnO) is less than 0.025 V, including the edges, at a voltage level of about -0.52 V, whereas for tin-plated steel with SnO 2 present in the tin oxide layer, the difference over the width is considerably higher and reaches a value of -0.045 at a voltage level of -0.60 V.

[0066] When testing adhesion and resistance to sulfur staining, it was found that the samples without SnO2 had better adhesion to the organic coating. Importantly, the samples with the tin oxide layer consisting mainly of SnO also showed better adhesion and resistance to sulfur staining at the edges of the strip. So the resulting passivated tinplate steel not only showed good adhesion and resistance to sulfur staining in the center of the tinplate steel strip, but also at the edges.

[0067] The experimental results of the Q1=Q2 mode show that the dry adhesion of the critical epoxy white paint in the Gitterschnitt test is good both in the center and at the edge (=5 cm from the edge of the strip). Values ​​of 0 and 1 are considered good results. Intermediate values ​​between the edge and the center give equivalent results.

[0068] Table 2: Classification of Gitterschnitt results

[0069]

[0070] For this test, 7.5×7.5 cm plates were cut from flat sheets. According to the method described in ISO 2409:1992, 2nd edition, 4×5 mm cross hatching was applied to the flat portion of the plate, after which the tape was applied. Thereafter, delamination was evaluated using a Gitterschnitt scale ranging from 0 (excellent) to 5 (poor) (Table 3). All tests were performed in triplicate on each side of each metal-laminated variant in Table 2. The scores were then averaged for the three results. A value of 0 / 1 shows that one of the three samples had a Gitterschnitt value of 1 and two had a Gitterschnitt value of 0.

[0071] Table 3: Classification of Gitterschnitt results

[0072]

[0073]

[0074] Sulfur coloration testing using the epoxy gold standard paint after retort at 130°C for 1 hour resulted in acceptable values ​​with some outliers that were independent of the position (edge ​​or center) in the strip.

[0075] BRIEF DESCRIPTION OF THE DRAWINGS

[0076] The invention will now be explained by way of the following non-limiting drawings.

[0077] Figure 1 A schematic diagram showing an apparatus for use in the method according to the invention is shown.

[0078] Figure 2 Schematic diagram showing cathodic removal of a pre-existing tin oxide layer.

[0079] Figure 3 The difference between the Vt response of a pre-existing tin oxide layer based on SnO2 and a cathodic removal of a pre-existing tin oxide layer based on SnO is shown.

[0080] Figure 4 A schematic representation of the individual stages during the method according to the invention based on black steel sheet strip as raw material is shown.

[0081] Figure 5 Display as Figure 4 Schematic representation of the various stackings of layers shown in FIG. The thickness of the black steel plate and the thickness of the layers shown are not to scale.

[0082] Figure 1An embodiment of the invention for carrying out the method according to the invention is shown. A tinning bath (I) is shown, in which a strip (1) is introduced into a plating solution (2) as cathode to be plated to produce tin-plated steel. After tinning and optional reflow (not shown) in one or more such tinning baths, the tin-plated steel is introduced into an electrochemical treatment bath (II) containing an alkaline aqueous solution (8). The tin-plated steel enters the bath (II) via a non-conductive guide roller (3) in the entry pass (downward pass) as cathode and passes an anode (6) for cathodic removal of pre-existing oxides and producing a bare and pure tin surface. After being redirected by a non-conductive counter-guide roller (4), the tin-plated steel begins the exit pass (upward pass) and changes from cathode to anode. During the exit pass, the tin-plated steel passes a cathode (7) for applying a fresh tin oxide layer to the bare and pure tin surface. After leaving the bath through a non-conductive guide roller (5), the strip optionally enters a rinsing bath (III) and is dried (not shown). In section IV tinned steel strip, a post-treatment agent (11) is applied to the tinned steel strip by means of an application device (10). The strip can then be dried if necessary (not shown). The guide rollers 3 and 5 need to be non-conductive guide rollers. The term non-conductive in the general context of the present invention means that the rollers are not conductive. electricity .

[0083] Figure 2 A typical potential-time curve is shown in FIG. 1 , from which the thickness of the tin oxide layer is determined based on the time at which the tangent line of the curve at -0.7 V intersects the tangent line of the curve at about -0.85 V, and is calculated as C / m 2 The basis for calculating the thickness of the tin oxide layer. Figure 2 In the example, the time is about 190s×0.50=95C / m 2 . Tin oxide layer thickness D, in C / m 2 It is represented by D[C / m 2 ]=t[s]*0.50[A / m 2 ]get.

[0084] exist Figure 3 In the graph, at t = 25, a clear difference can be observed between the curves for thick layers compared to thinner layers, both of which are produced on fresh tin surfaces (i.e., cathodes and completely removed pre-existing oxides). The drop at t = 25 s is consistent with the thicker layers and is associated with the presence of SnO2 in the tin oxide layer. The other two curves have shapes consistent with tin oxide layers consisting primarily of SnO.

[0085] exist Figure 4 Reproduced in Figure 1 The letters A to G represent different stages of forming each layer on the black steel plate.

[0086] exist Figure 5 The letters in the text represent the following:

[0087] A: Black steel strip raw materials;

[0088] B: tinned steel (i.e., black steel plate coated with a tin layer in a tinning bath I);

[0089] C1: Tinned steel with pre-existing oxide and uninterrupted processing (no additional oxide growth);

[0090] C2: tinned steel with pre-existing oxide and additional oxide layer due to prolonged storage and / or storage under conditions that lead to growth of additional oxide layer;

[0091] D: Pure and bare tin layer on black steel plate after removal of the pre-existing oxide layer;

[0092] E: anodic reoxidized tin-plated steel;

[0093] F: Cleaned and rinsed anodic reoxidized tinned steel;

[0094] G: Anodic reoxidized tin-plated steel provided with a post-treatment agent.

Claims

1. A method for passivating a tinned steel strip (1) in a continuous process, wherein after electrodepositing one or more tin layers or after optionally reflowing one or more electrodeposited tin layers, the tinned steel strip (1) enters an alkaline aqueous solution (8) in an electrochemical treatment tank (I) in an inlet pass and leaves the alkaline aqueous solution in an outlet pass, wherein any pre-existing tin oxide layer on the tinned steel surface is cathodically removed from the tinned steel surface during the inlet pass and wherein the tinned steel surface is subsequently anodically re-oxidized immediately during the outlet pass, wherein the charge for cathodically removing the pre-existing tin oxide layer from the tinned steel surface is Q1, and wherein the charge for anodically re-oxidizing the tinned steel surface is Q2, and wherein Q1 < Q2, wherein the charge density applied for anodic re-oxidation and cathodic removal of the pre-existing tin oxide layer is the same and equal to Q2, and is at least 15 C / m 2 , and wherein the anodically re-oxidized tinned steel is rinsed and dried after leaving the alkaline aqueous solution.

2. The method according to claim 1, wherein the charge density applied for anodic reoxidation is at most 100 C / m 2 .

3. The method according to claim 1 or 2, wherein the anodic reoxidized tin-plated steel is covered with a 2 represents and represents the total charge required to reduce the oxide layer to metallic tin, the thickness D of the oxide layer, which is related to the reoxidation time t and the current density A by D = E × A × t, where E is the efficiency of the electrochemical reaction, and where D is between 15 and 100 C / m 2 between.

4. A method according to claim 1 or 2, wherein a liquid solution of a chromium-free post-treatment agent is applied to the rinsed and dried anodized tin-plated steel surface to produce post-treated tin-plated steel, wherein the chromium-free post-treatment agent is selected from the group consisting of copolymers of acrylates, polymethylsiloxanes with polyether side chains, acid polyethers, polymers with heterocyclic groups and acids, aqueous liquid compounds containing complex metal fluoride anions with divalent to tetravalent cations, and polymeric substances.

5. The method of claim 1 or 2, wherein a liquid solution of a chromium-free post-treatment agent is applied to the rinsed and dried anodized tin-plated steel surface to produce post-treated tin-plated steel, wherein the chromium-free post-treatment agent comprises fluorotitanate and zirconate titanate.

6. The method according to claim 1 or 2, wherein the current density A during anodization is at least 10 A / m 2 .

7. The method according to claim 1 or 2, wherein the current density A during anodization is at least 50 A / m 2 .

8. The method according to claim 1 or 2, wherein the current density A during anodization is at least 100 A / m 2 .

9. The method according to claim 1 or 2, wherein the current density A during anodization is at most 4000 A / m 2 .

10. The method according to claim 1 or 2, wherein the current density A during anodization is at most 2000 A / m 2 .

11. The method according to claim 1 or 2, wherein the current density A during anodization is at most 1000 A / m 2 .

12. The method according to claim 1 or 2, wherein the alkaline aqueous solution is selected from alkali metal or alkaline earth metal hydroxides or carbonates, basic alkali metal phosphates and basic organic alkali metal or alkaline earth metal salts.

13. The method according to claim 1 or 2, wherein the alkaline aqueous solution contains sodium carbonate.

14. The method according to claim 1 or 2, wherein the alkaline aqueous solution has a pH between 8.75 and 10.

5.

15. The method according to claim 1 or 2, wherein the anodic reoxidation time t is between 0.05 seconds and 1.5 seconds.

16. A method according to claim 1 or 2, wherein the tinned steel is introduced into the alkaline aqueous solution immediately after deposition of one or more tin layers on the steel strip, or immediately after reflow of the deposited one or more tin layers.

17. The method according to claim 1 or 2, wherein the tin oxide layer after anodic reoxidation consists mainly of SnO.

18. The method according to claim 1 or 2, wherein the thermoplastic polymer coating is applied directly to the rinsed and dried reoxidized tin-plated steel, wherein the tin oxide layer after anodic reoxidation consists predominantly of SnO.

19. The method according to claim 4, wherein the thermoplastic polymer coating is applied directly to post-treated tin-plated steel, wherein the tin oxide layer after anodic reoxidation consists predominantly of SnO.

20. Passivated tin-plated steel produced by a process according to any one of claims 1 to 19, wherein the tin oxide layer after anodic reoxidation consists essentially of SnO and wherein after rinsing and drying the tin oxide layer undergoes a subsequent treatment with a no-rinse / in-situ drying post-treatment agent based on titanium or a combination of titanium and zirconium, said post-treatment agent being prepared to have a relative humidity of between 0.2 and 2 mg Ti / m 2 Solutions with dry coverage in a range of.

21. Apparatus for producing passivated tin-plated steel (1) according to the method according to any one of claims 1 to 19, comprising Electrolytic tinning line (I), optionally provided with means for melting the tin layer; an electrochemical treatment cell (II) for containing, in use, an alkaline aqueous solution (8); Non-conductive means for guiding the cathode tinned steel into the electrochemical treatment cell (II) past the anode (6) during the entry pass; Non-conductive guide rollers (4) for guiding the tin-plated steel from the entry pass to the exit pass, which guide the anode tin-plated steel past the cathode (7) during the exit pass; Apparatus for applying an electrical potential between the tin-coated steel strip and a counter electrode for cathodically removing a pre-existing oxide layer and anodically reoxidizing the tin-coated steel strip; non-conductive means for conducting the tin-plated steel from the electrochemical treatment tank to the means for rinsing (III) and drying the tin-plated steel, and Optionally, means (10) for applying a liquid solution of a chromium-free post-treatment agent (11).

22. Apparatus according to claim 21, wherein the non-conductive means for guiding the cathodic tinned steel into the electrochemical treatment cell (II) past the anode (6) during the entry pass are non-conductive guide rollers.

23. Apparatus according to claim 21, wherein the non-conductive means for guiding the tin-plated steel from the electrochemical treatment tank to the means for rinsing (III) and drying the tin-plated steel are non-conductive guide rollers.

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