Method for passivating tinplate strip and apparatus for producing passivated tinplate strip

Abstract

The present invention relates to a method for passivating tinplate strip after electrodeposition of one or more tin layers, or optionally after flow-melting of one or more electrodeposited tin layers, and to an apparatus for producing passivated tinplate strip.

Description

[Technical Field]

[0001] The present invention relates to a method for passivating tinplate strip after electrodeposition of one or more tin layers or optionally after flow-melting of one or more electrodeposited tin layers, and to an apparatus for producing passivated tinplate strip. [Background technology]

[0002] Tinplate is a lightweight, cold-rolled, low-carbon steel sheet or strip that is 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.

[0003] Tinplate combines the strength and formability of steel with the corrosion resistance, solderability, and aesthetic appeal of tin in one material. Within this broad range, a wide variety of products exist today, tailored to meet end-use requirements. Because the manufacturing of the steel substrate and the subsequent tin coating are independent of each other, theoretically, all of the properties of steel can be combined with the tin coating. The composition of the steel used in tinplate is tightly controlled, and various types with different formability ("tempers") can be produced depending on the grade selected and its processing method. Tinplate is sold in a variety of steel thicknesses, ranging from approximately 0.10 to 0.49 mm. The steel can be coated with tin at different thicknesses. Different thicknesses (different coatings) can also be produced on the two sides to accommodate varying conditions on the interior and exterior surfaces of a container. A variety of surface finishes are also produced for various applications.

[0004] The tin is deposited as a whitish coating with a slight metallic luster. If desired, it can be flow-melted by induction or resistance heating (or a combination) to produce a brilliant mirror finish. This flow-melting process increases the corrosion resistance of the product by forming an inert tin-iron alloy layer. Most DWI tinplate (drawn and wall-ironed tinplate) is not flow-melted, and this can be a significant part of the output for many manufacturers.

[0005] Tinplate, especially flow-melted tinplate, has a thin film of tin oxide on its surface that, if left untreated, can grow in thickness during storage. Chemical or electrochemical passivation is applied to the strip to improve tarnish resistance and adhesion to organic coatings. For decades, the most common form of passivation involved cathodic treatment with dichromate or chromic acid solutions containing dichromate at temperatures between 50 and 85°C. This treatment deposits a complex layer of chromium and its hydrous oxides (which inhibits tin oxide growth) to prevent yellowing, improve paint adhesion, and minimize staining by sulfur compounds. The dichromate or chromic acid solution removes Cr 6+ These solutions are increasingly being rejected because they are hazardous, especially in the case of metal products intended for the food industry. EU regulations (REACH) prohibit the use of these solutions when alternatives are available.

[0006] When tinplate is used to manufacture containers (cans) for storing foodstuffs, passivation must prevent the overgrowth of a tin oxide layer during storage of the tinplate or food containers made from it until it is coated with a protective layer, and subsequently until the stored contents are consumed. Furthermore, passivation must prevent discoloration of the tinplate surface. For example, such discoloration occurs when cans containing sulfur-containing substances are sterilized because, if the can is not sufficiently passivated, sulfur chemically reacts with the tin on the coated steel surface. Matte discoloration (marbling) or gold discoloration on the surface of the packaging container may lead consumers to believe that the contents are contaminated. Adhesion problems with the protective layer may also occur, which can be avoided by passivating the coated steel sheet. Furthermore, passivation must ensure the resistance of the metal container to the acids contained in the foodstuff after it has been filled. If the tinplate is not properly passivated, such acidic anions in the contents of the can can cause the container's inner protective layer to peel off and corrode the underlying tinplate.

[0007] EP 2802688 describes a method for passivating the surface of tinplate, which involves anodizing the surface after tin plating to form an oxide layer consisting essentially of tetravalent tin oxide, followed by application of a liquid solution of a chromium-free after-treatment agent. The method of EP 2802688 claims that anodizing before post-treatment with the chromium-free after-treatment agent can significantly increase the resistance of tinplate to corrosion and reactions with sulfur. An oxide layer with a thickness in the nanometer range is produced on the surface of the tin-plated steel strip by anodization. The oxide layer is essentially a layer of tetravalent tin oxide (SnO). A thin surface layer of a chromium-free after-treatment agent deposited on this oxide layer is claimed to completely and effectively protect the surface of the tin-plated steel strip from corrosion and reactions with sulfur.

[0008] The problems with the prior art methods are that 1) oxidation of the strip is not uniform across the width of the strip, resulting in differences across the width of the strip in protecting the tinplate surface from corrosion and reaction with sulfur, as well as differences in adhesion between the additional protective layer and the tin layer on the steel strip, and 2) the newly applied oxide layer forms on top of an already existing, non-specific oxide layer. Summary of the Invention [Problem to be solved by the invention]

[0009] It is an object of the present invention to provide a method for passivating tinplate strip after electrodeposition of one or more tin layers, which method improves the uniformity of the oxide layer across the width of the tinplate strip.

[0010] It is also an object of the present invention to provide a method for passivating tinplate strip after electrodeposition of one or more tin layers, which method results in improved uniformity of adhesion between the further protective layer and the tin layer on the steel strip.

[0011] It is also an object of the present invention to provide a method for passivating tinplate strip after electrodeposition of one or more tin layers, which provides improved corrosion resistance and marbling resistance across the width of the tinplate strip.

[0012] It is also an object of the present invention to provide a method for passivating tinplate strip after electrodeposition of the tin layer or layers which is an alternative to the use of dichromate or chromic acid solutions. [Means for solving the problem]

[0013] The object of the present invention is achieved by a method according to claim 1.

[0014] Preferred embodiments are provided by dependent method claims 2 to 13. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram showing an apparatus for the method according to the invention. [Figure 2] FIG. 2 is a schematic diagram illustrating the cathodic removal of a pre-existing tin oxide layer. [Figure 3] FIG. 3 shows the difference in Vt response between the cathodic removal of a pre-existing tin oxide layer based on SnO2 and that based on SnO. [Figure 4] FIG. 4 is a schematic diagram showing the various stages during the method according to the invention based on blackplate strip as raw material. [Figure 5] FIG. 5 is a schematic diagram showing various configurations of the layers shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0016] The method according to the present invention is further described below by certain non-limiting embodiments. Any ranges mentioned herein below are generally applicable to the method according to the present invention and are not limited to the following embodiments, but may also be independently applicable.

[0017] In a first step of an embodiment according to the present invention, the electrodeposition of a tin layer onto cold-rolled steel strip (blackplate) is carried out in a continuous electrolytic tinning line operating at a speed of at least 50 m / min. Once coated on one or both sides with a tin layer, the blackplate becomes tinplate. Current industrial high-speed electrolytic tinning lines can operate at speeds up to approximately 750 m / min. After the tin layer is deposited, the tinplate is heated to a temperature above the melting point of tin (232°C) to melt the tin layer. As a result of the melting, the tin combines with the iron from the steel strip to form an iron-tin FeSn intermetallic compound. The surface of the tin layer remains tin and becomes very shiny when quenched in water and solidified. A new oxide layer immediately forms on the fresh surface; this oxide layer continues to grow during storage and is defined in the context of the present invention as a pre-existing oxide layer. Although the remelting of tin described is an optional feature, most tinplate is subjected to such a remelting or flow melting process.

[0018] In the second step of the method according to the invention, the pre-existing oxide layer on the tin is completely cathodically removed in an electrochemical treatment tank (II) containing a basic aqueous solution, in this example an aqueous sodium carbonate solution, which serves as the electrolyte. The tinplate enters the electrochemical treatment tank in a downward direction by means of non-conductive guide rollers (3) (i.e., the entry pass is the down-pass). Near the bottom of the vertical tank, there is a non-conductive sink roll across which the tinplate strip moves in the reverse direction. Electrodes (see FIG. 1, reference numbers 6 and 7) (e.g., stainless steel anodes) are provided in the tank (II) containing the electrolyte (8). The strip (1) moves between the electrodes without touching them. A potential is applied by a rectifier between the electrode (anode, 6) in the entry pass and the electrode (cathode, 7) in the exit pass. As a result, the strip acquires the opposite charge of the electrodes as it passes between them. Thus, the strip becomes cathodic when it passes over the anode (6) in the inlet passage, and becomes anodic when it passes over the cathode (7) in the outlet passage. After oxide removal, the tin layer no longer has an oxide layer on its surface, i.e., the tinplate surface is a pure (bare) tin surface across the entire width of the tinplate, for reasons explained below.

[0019] After reversing the direction of travel of the tinplate by the sink roll (4), it leaves the exit pass in an upward direction (i.e., the exit pass is the up-pass) and then passes through a cathode where it becomes anodic. As a result, a new oxide layer grows on the steel strip under carefully controlled conditions on the pure (bare) tin surface obtained by 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 required thickness to provide sufficient corrosion resistance and marbling resistance. The reoxidized tinplate leaves the electrochemical treatment tank (II) by means of non-conductive guide rollers (5).

[0020] It should be noted that the second step of the method according to the invention may also be carried out in an arrangement in which the strip moves substantially horizontally between the anode and cathode, in which case the direction of movement does not reverse between the inlet and outlet passages, but one or more non-conductive guide rollers may be present within the tank to guide and support the strip in its movement between the anode and cathode.

[0021] In the method according to the invention, it is essential that the guide means for guiding the strip into, through and out of the electrochemical treatment tank, e.g., the guide rollers and sink rolls, are electrically non-conductive, since current cannot flow from the strip through the guide means to earth. In the case of guide rollers or sink rolls which must be electrically non-conductive, the rollers are usually made of metal and covered with a rubber layer.

[0022] In the method according to the invention, the charge applied in the inlet passage is the same as the charge applied in the outlet passage. This means that the value of Q1 is greater than that required to remove any pre-existing oxide layer and that the tinplate always has a pure, bare tin surface once anodic reoxidation begins. In this embodiment, only one rectifier is required if the anode and cathode operate in pairs, or two rectifiers are required if the upper (right) and lower (left) anode / cathode operate separately. In most cases, a single rectifier is sufficient. This makes the process easier to control, since Q2 is leading, and also technically simple, since in all practical cases Q2 is significantly greater than Q1. In the embodiment according to the invention, there is no need to check whether the pre-existing oxide has been completely removed.

[0023] The anodizing time corresponds to the residence time of the tin-plated steel strip in the electrochemical oxidation bath in the outlet passage. This is determined by the anode length and the strip speed. For a typical slow strip speed of about 50 m / min, the anodizing time is about 2 to 2.5 seconds. For a fast strip speed of about 750 m / min, the anodizing time is about 0.1 to 0.2 seconds. Therefore, in most industrial lines, the anodizing time is 0.1 to 2.5 seconds, preferably 0.15 to 1.5 seconds, more preferably up to 1.0 second, even more preferably up to 0.7 seconds, and even more preferably up to 0.4 seconds.

[0024] The distance between the tinplate strip and the counter electrode in the electrolytic bath is set according to the system, and is, for example, 3 to 15 cm, preferably 5 to 10 cm, in particular about 5 cm.

[0025] The temperature of the electrochemical oxidation bath is preferably 25 to 60°C, more preferably 25 to 50°C, and especially about 35°C.

[0026] In the third step of the method, the anodized reoxidized tin strip is rinsed, for example with water or deionized or demineralized water, and then dried, for example by hot air. However, other drying means are also suitable, such as drying with a water-absorbing solvent followed by drying with a cold or hot air blower (hot air is preferred), drying systems that do not use convection air, for example drying with an IR radiator, induction heating or resistance heating, drying with a cold or hot air blower, preferably a hot air blower only.

[0027] The fourth step of the process involves coating the anodized tinplate strip with a post-treatment agent. This step is optional; the product produced up to the third step can already be considered passivated tinplate, but the use of this additional fourth step is preferred for long-term and consistent passivation. A solution of the post-treatment agent, preferably an aqueous or organic solvent solution, or a ready-to-use preparation of the post-treatment agent, is sprayed onto the steel strip moving at the strip speed. A 1.5-10% aqueous solution of the post-treatment agent has proven convenient. The thickness of the solution of the post-treatment agent is then homogenized by a homogenization roller and preferably dried. A thin film of the post-treatment agent remains on the surface of the coated metal strip after drying; the weight of this film is generally 2-30 mg / m². 2 Suitable post-treatment application techniques include immersion, immersion with a squeegee roll, rotor-spray application, rotor-spray application supported by the use of a smoothing roll, spray application, spray bar, spray-squeegee application, application with a roll coater system, application by slot coating, application by slot curtain coating, etc. If necessary, excess treatment can be removed after application of the post-treatment by a pair of squeeze rollers positioned in the direction of movement of the strip, and in some cases the excess post-treatment can be reused.

[0028] Suitable post-treatment agents that can be used in conjunction with the present invention include: The entire organic system, e.g., organic acids (oleic acid, abietic acid), Totally organic systems, e.g. acrylates, polyurethane dispersions and other types of thin organic coatings, Organic / inorganic coupling agents, such as one- and two-component siloxane systems, inorganic systems, e.g. silicate systems, Inorganic systems in organic matrices, such as fluorotitanate or zirconium titanate in combination with an organic polymer matrix is.

[0029] It is preferred to use an inorganic system in an organic matrix, such as fluorotitanate or zirconium titanate in combination with an organic polymer matrix. Such post-treatments are currently commercially available, as shown below.

[0030] After reoxidation, rinsing, and drying, a post-treat is applied to the anodically reoxidized tinplate surface by application techniques common to such passivation systems. The post-treat is preferably a chromium-free post-treat, preferably a chromium-free, no-rinse / dry-in-place post-treat. This post-treat may be based on zirconium, titanium, combinations of zirconium and titanium, phosphates, siloxanes, etc., or may be based on acidic aqueous compositions containing water-soluble inorganic compounds of the elements Zr, Ti, Hf, and / or Si, as described, for example, in US 10,011,915. Examples include Gardobond® X4744, Oxsilan® MM0705 (Chemetall) or Primecoat® Z801 (AD Chemicals), Bonderite® M-NT1455, Bonderite M-NT1456, Bonderite M-NT10456 (Henkel), which have a dry coverage of 0.2-2 mg Ti / m on tin-plated and reoxidized steel strip surfaces. 2 , more preferably 0.5 to 1.5 mg Ti / m 2 or 0.8 to 1.5 mg Ti / m 2 , especially 1 mg Ti / m 2 It is prepared as a solution in which

[0031] The advantages of no-rinse / dry-in-place post-treats over electrolytic systems are the ease of application of the solution, the use of simple equipment with a compact application unit that can be easily retrofitted into existing lines, and the availability of more versatile chemistries. The post-treat can be applied to the surface-treated tinplate by application techniques that are common to such passivation systems. Suitable application techniques include immersion, squeegee roll immersion, rotor-spray application, rotor-spray application supported by the use of a smoothing roll, spray application, spray bar, spray-squeegee application, application by roll coater system, application by slot coating, application by slot curtain coating, etc.

[0032] The inventors have also found that, although it is preferable to perform cathodic removal of pre-existing oxides, anodic reoxidation, and subsequent passivation steps with a treating agent in a continuous process without interruption immediately after tin plating and any optional flow melting, this method can also be used to treat tinplate that has not been cathodically and anodically treated or passivated immediately after tin plating and any optional flow melting. The method according to the present invention can also be used to treat pre-manufactured coils of tinplate. This situation occurs, for example, when there is a delay between the manufacture of tinplate strip and passivation, such as after storing tinplate strip, usually in coiled form, for a period of time. Any oxide layer growth that occurs during this time can be easily addressed by cathodic removal of the pre-existing oxide, initiating a subsequent process that results in a tin layer as pure and bare as if the process had been performed continuously immediately after tin plating. The method according to the present invention can also deal with additional naturally grown oxide because Q2 is significantly greater than the charge required to remove the pre-existing oxide, and the excess charge is used to remove the additional naturally grown oxide.

[0033] During the cathodic removal of pre-existing tin oxide, hydrogen is evolved at the cathode strip as the pre-existing oxide is cathodically removed. In particular, when Q1 is much smaller than Q2, it may be beneficial for safety and environmental reasons to trap the hydrogen by a hydrogen trapping means. Q1 is usually much smaller than Q2 when the tinplate after tinning is processed in-line (i.e., immediately) or in a very short time and with controlled storage between tinning and passivation.

[0034] The sole purpose of the basic aqueous solution is to enable cathodic and anodic treatment, not to deposit foreign substances contained in the electrolyte on the substrate surface. The pH of the basic aqueous solution cannot be too low (below pH=8.75), otherwise the efficiency of the electrochemical reaction will be too low and the process will not be able to be integrated into existing high-productivity process lines. The pH of the basic aqueous solution is also 11.0 or less, preferably 10.5 or less, because this makes the tin layer more soluble in the basic aqueous solution.

[0035] In the context of the present invention, the thickness (D) of the tin oxide layer is expressed in coulombs / m 2 and represents the total charge required to reduce the layer to metallic tin. The thickness of the tin oxide layer is given by D = E × A × t Anodizing related to time (t) and current density (A), where E is the efficiency of the electrochemical reaction and D is at least 15 C / m 2 is.

[0036] The efficiency therefore represents the ratio of the thickness D of the oxide layer produced to the applied charge density (A × t) and can be evaluated by plotting D as a function of (A × t). Initially, the value of the passed anodic charge is 50 C / m 2 Below 15 C / m, the curve is approximately linear, but as the passed anodic charge increases, the efficiency E decreases, the growth rate of the tin oxide layer slows, and therefore the increase in D slows. 2In this case, the tin oxide layer is too thin and ineffective to achieve the desired sulfide staining resistance. Therefore, the minimum thickness of D is 15 C / m 2 is necessary.

[0037] The total D specified above can be achieved by any combination of A and t, but considering the processability in a high-speed tin plating line, a high current density (A>0.1A / dm) combined with a short processing time (t<1 sec) is recommended. 2 , preferably A>1.0A / dm 2 ) combination is preferred. The interchangeability of A and t in the anodizing reoxidation treatment means that the process can be operated with short treatment times by appropriately adjusting the applied current density. Therefore, the method according to the invention can be used in industrial tin plating lines operating at line speeds of over 300 m / min up to 1000 m / min. Furthermore, the treatment time t is determined not only by the line speed v but also by the effective length or "anode length" L of the treatment section according to t = L / v. This means that the process window can be further extended by appropriate selection of the anode length L. For example, in a line operating at 600 m / min (10 m / s), a thickness of 50 C / m 2 To deposit a layer with a current density of 1000 A / m (assuming E=1), 2 A treatment length of 0.5 m is required. If the treatment length is 5 m, a current density of 100 A / m 2 This design and process flexibility is a major advantage of this method. An 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. Once the pre-existing tin layer has been completely cathodic removed, hydrogen is produced at the anode, while the pure bare tin layer remains. For safety and environmental reasons, it may be beneficial to capture the hydrogen by a hydrogen capture means.

[0038] In one embodiment of the present invention, the cathodic stripping and anodizing are performed inline with and immediately after the electrolytic tin plating and optional flow melting process. The process time (t) for the anodizing reoxidation after the cathodic stripping of the pre-existing tin oxide layer is up to 5 seconds, preferably up to 2 seconds, more preferably 0.05 to 1.5 seconds. This range and more preferred ranges are compatible with high-speed process lines. In one embodiment, the cathodic and anodizing are performed inline with an industrial electrolytic tin plating line, and the current density (A) during the anodizing is at least 10 A / m 2 , preferably at least 50 A / m 2 , more preferably at least 100 A / m 2 , and / or up to 4000A / m 2 , preferably up to 2000 A / m 2 , more preferably up to 1000 A / m 2 This range and the more preferred range are compatible with high speed process lines.

[0039] The main function of the basic aqueous solution is to support the electrochemical reactions intended by the cathodic and anodic treatments, but the ionic species present in the basic aqueous solution do not participate in the electrochemical modification of the tin surface. A preferred basic aqueous solution contains cations from Group 1 of the periodic table (e.g., Na + , K. + ) or cations from Group 2 (e.g., Mg 2+ , Ca 2+ ) or polyatomic cations (e.g., NH4 + ), and polyatomic anions (phosphate, borate, sulfate, carbonate, etc.). The anion may also be the conjugate base of an organic acid (e.g., acetate, citrate). Because it is important to maintain the pH within certain boundaries, buffer solutions may be used. Basic aqueous solutions contain monoatomic halogen anions (Group 17), e.g., Cl. - , F - It is preferable that it does not contain

[0040] Preferably, the aqueous buffer solution contains sodium carbonate, preferably without borate, phosphate, sulfate, 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, particularly 2% to 8% by weight, preferably 3% to 7% by weight, more particularly 4% to 6% by weight, and especially about 5% by weight. 1% by weight of sodium carbonate corresponds to about 10 g / L of sodium carbonate in the electrolyte.

[0041] Additionally, the basic aqueous solution may contain other chemical additives, such as surfactants, wetting agents, and anti-foaming agents, to support the electrochemical process, provided that they do not adversely affect the removal of the pre-existing tin oxide layer and the reformation of the tin oxide layer.

[0042] Anodizing a tin-plated surface converts the outermost layer of the tin surface from metallic tin to tin oxide by electrochemical oxidation. The tin oxide layer thus produced (within a certain thickness range) forms a barrier against sulfide contamination. However, the tin oxide layer itself is not sufficiently stable and / or passivating. During long-term storage under ambient and / or humid conditions, or during heat treatments such as baking and stoving, the tin oxide layer continues to grow into a thicker tin oxide layer with undesirable properties (poor wetting, yellowish appearance, poor lacquer adhesion). Post-treatment agents usually form a stable passivating layer by themselves to protect the tinplate from uncontrolled tin oxide growth and also provide good adhesion of organic coatings. However, the passivating layer in almost all cases investigated has poor resistance to sulfide contamination. By applying the present invention, a desirable combination of properties can be achieved. First, a tin oxide layer of suitable thickness and suitable composition, i.e. consisting primarily of (i.e. consisting predominantly of) SnO, preferably consisting only of SnO, is applied by using cathodic and anodic treatments under suitable process conditions, and then the tin oxide layer is passivated and / or stabilized against further uncontrolled growth by applying a post-treatment passivation system thereon to produce post-treated tinplate using an electroless application method.

[0043] The Cr(VI)-free passivation system before anodization is applied should be a chemical passivation treatment for the application of a no-rinse, dry-in-place passivation system, preferably a so-called rinse-free process.

[0044] The inventors have found that the thickness of the tin oxide layer on the strip is 15 to 100 C / m 2 The thickness D of the tin oxide layer must be 100 C / m 2Values ​​above 100 are not only economically unattractive in high speed tin plating processes, but also lead to poor adhesion of subsequently applied organic coatings, especially at the edges of the tinplate, due to the increased presence of SnO2 in the oxide layer. 2 A value of 15 C / m is considered to be the minimum required to ensure the removal of pre-existing oxide without further spontaneous growth of the oxide layer. 2 At values ​​below this, oxide residues are observed. The inventors have determined that the thickness of the tin oxide layer on the strip must be at least 25 C / m 2 It has been found that the appropriate maximum value of D from the viewpoint of process efficiency and tin oxide species is 80 C / m 2 , preferably 70 C / m 2 or 60C / m 2 The thickness on the strip is 30~60C / m 2 Good results were obtained with a minimum value of D of 35 C / m 2 or 40C / m 2 is.

[0045] Assuming an efficiency E = 1, the above values ​​also reflect the rectifier setting for the anodizing reoxidation process. However, it should be noted that an oxide layer may grow as a result of natural growth during storage, thus resulting in a thicker oxide layer than would be expected based on the rectifier setting. However, assuming no natural growth occurs, the value of E can be readily determined by measuring the strip thickness using the method described herein and correlating the results with the rectifier setting. In this way, the rectifier setting can be "translated" into the oxide layer thickness of the strip, which can range from 15 to 100 C / m. 2 The preferred values ​​of are reliably and reproducibly obtained.

[0046] In one embodiment, the article is a strip of packaging steel provided with a tin layer on at least one side (for typical chemical compositions see, for example, EN10202-2001 or ASTM 623M), which is produced by known methods, for example by cold rolling, annealing and optionally temper rolling a steel strip of suitable composition, followed by electrolytic tin plating.

[0047] The cathodically and anodically treated tinplate, optionally treated with a post-treatment agent, can be wound up for storage and transportation and later unwound. The passivated tinplate produced according to the present invention can further be provided with an organic coating layer, such as an epoxy phenolic gold lacquer, an epoxy anhydride white lacquer, a PVC or vinyl organosol coating, a polyester lacquer, an epoxy amino or an epoxy acryl amino aqueous coating. The excellent adhesion of the organic coating layer to the passivated tinplate allows this product to be offered as an alternative to CDC treatment and subsequent polymer coating systems, thus completely avoiding the use of chromates.

[0048] Alternatively, the cathodic and anodized tinplate, optionally treated with a post-treatment agent, can be transferred to a lamination unit where a laminate layer is laminated in-line to the tinplate.

[0049] The process of applying the laminate layer to the tinplate is preferably carried out by extrusion coating and lamination. The polymer is melted and formed into a thin hot film in a flat (co)extrusion die. The extruded polymer film is then guided through a casting or chill roll and then laminated to a heated tinplate substrate to form a laminated tinplate. The laminated tinplate is then typically passed through a roll-nip assembly, which firmly presses the laminate layer against the substrate to ensure complete contact and adhesion.

[0050] Another method is film lamination, in which a solid laminate layer is supplied, coated onto preheated tinplate, and pressed onto the tinplate by a roll nip device to ensure complete contact and adhesion of the laminate layer to the preheated tinplate.

[0051] Suitable single-layer 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]

[0052] Tinplate was produced in various tempers, from TS245 to TS290 and from TH415 to TH620. Table 1 shows an overview of the tempers and examples of their use. The thickness of the tin layer varied depending on the intended use, ranging from 1.4 to 11.2 on one side and 1.7 to 5 on the other side. The results of passivation according to the present invention proved to be independent of the temper and the thickness of the tin layer. Most of the tinplate was subjected to a flow melting process.

[0053] [Table 1]

[0054] After tin plating and optional flow melting, the tinplate was subjected to the process steps according to the invention.

[0055] The inventors have found that the tin oxide species and oxide layer thickness found on the strip after anodizing are remarkably consistent across the width of the strip. Previous experiments conducted according to the prior art described in EP 2802688 revealed differences in oxide layer thickness and oxide species across the width. The prior art describes a preference for the oxide species to be SnO2. Since there is no cathodic removal of pre-existing oxide in the prior art, this oxide layer is deposited on top of the pre-existing layer by the prior art process. The inventors have found that this cathodic removal is important for obtaining a pure, bare tin surface, regardless of the prior treatment of the tinplate (flow melt or not, long-term storage or not, preferred storage conditions or not, etc.), and the subsequent deposition on this pure, bare surface of a new oxide layer that is uniform in thickness across the width of the tinplate strip and is primarily composed of SnO rather than SnO2. The inventors have found that this surface forms an ideal tin oxide surface for further processing of the tinplate strip for packaging applications in the manufacture of containers.

[0056] The thickness of the tin oxide layer is determined using a coulometric method. The tin oxide layer is reduced by a controlled, low cathodic current in a 0.01 M hydrobromic acid (HBr) solution from which oxygen has been removed by scrubbing with nitrogen. The progress of oxide reduction is monitored by measuring the reduction potential. The charge (A x t) passed for complete reduction is a measure of the thickness of the tin oxide layer. The test uses a cylindrical cell with a circular opening approximately 4 cm in diameter at one end and an Ag / AgCl reference electrode. The other end of the cell is equipped with a platinum counter electrode. The test specimen covers the opening and seals it using an O-ring to establish a watertight connection with a clear area, then clamps in place using a pneumatic cylinder. The cell is connected to the electrolyte solution by flexible tubing so that it can be filled and emptied under a nitrogen atmosphere. A potentiostat-galvanostat is used to measure a current of -0.50 A / m. 2 A cathodic current density of 1000 kJ / s is applied to the sample and the potential is measured until the reduction is complete. . Noriyoshi Typical potential time curves are shown in Figure 2. The inventors have also found that these curves can be used to distinguish between tin oxide species in the tin oxide layer (see Figure 3).

[0057] By measuring and comparing samples taken across the width of the passivated tinplate strip, the inventors found that the composition of the tin oxide in the case of tinplate with a tin oxide layer consisting predominantly of SnO is the same at the edges, whereas this is not the case for samples in which the tin oxide layer consists predominantly of SnO. The difference in potential at 25 seconds measured at various positions across the width (including the edges) of tinplate with a tin oxide layer consisting predominantly of SnO was less than 0.025 V at a voltage level of about -0.52 V, whereas for tinplate with SnO present in the tin oxide layer the difference across the width was significantly larger, reaching a value of -0.045 at a voltage level of -0.60 V.

[0058] When tested for adhesion and sulfur stain resistance, it was found that the SnO2-free specimens had better adhesion to organic coatings. Importantly, specimens with a tin oxide layer consisting predominantly of SnO also exhibited better adhesion and sulfur stain resistance at the edges of the strip. Thus, the resulting passivated tinplate not only exhibited good adhesion and sulfur stain resistance in the center of the tinplate strip, but also at the edges.

[0059] The experimental results for the Q1 = Q2 mode show that the dry adhesion of the critical white epoxy lacquer in the Gitterschnitt test works well both in the center and at the edge (5 cm from the edge of the strip). Values ​​of 0 and 1 are recognized as good results. Values ​​intermediate between the edge and center give comparable results.

[0060] [Table 2]

[0061] For this test, 7.5 x 7.5 cm panels were cut from the flat sheet. A 4 x 5 mm crosshatch was applied to the flat portion of the panel, and then adhesive tape was applied, according to the method described in ISO 2409:1992, 2nd Edition. The peel was then evaluated using a Gitterschnitt scale ranging from 0 (excellent) to 5 (poor) (Table 3). All tests were performed three times on each side of each metal-laminate variant in Table 2. The scores from the three results were then averaged. A value of 0 / 1 indicates that one of the three samples achieved a Gitterschnitt value of 1 and two achieved a Gitterschnitt value of 0.

[0062] [Table 3]

[0063] After retorting at 130°C for 1 hour, sulfur contamination tests using epoxy gold standard lacquer give acceptable results with some outliers, regardless of the strip position (edge ​​or center).

[0064] BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustrated by the following non-limiting figures.

[0065] FIG. 1 shows a schematic diagram of an apparatus for the method according to the invention. FIG. 2 shows a schematic diagram of the cathodic removal of a pre-existing tin oxide layer. FIG. 3 shows the difference in Vt response between the cathodic removal of a pre-existing tin oxide layer based on SnO2 and that based on SnO. FIG. 4 shows a schematic diagram of the various stages during the process according to the invention based on blackplate strip as raw material. Figure 5 shows a schematic diagram of various configurations of the layers shown in Figure 4. The thickness of the blackplate and the thickness of the various layers shown are not to scale.

[0066] FIG. 1 shows an embodiment of the invention for carrying out the method according to the invention. A tinning cell (I) for producing tinplate is shown, in which a strip (1) is introduced into a plating solution (2) as a cathode to be plated. After tinning in one or more such tinning cells and optional flow melting (not shown), the tinplate is introduced into an electrochemical treatment tank (II) containing an aqueous basic solution (8). The tinplate enters the tank (II) as a cathode in the inlet pass (down pass) via a non-conductive guide roller (3) and passes through an anode (6) for cathodic removal of pre-existing oxides, producing a bare, pure tin surface. After being redirected by a non-conductive counter-sink roll (4), the tinplate leaves the outlet pass (up pass) and changes from a cathode to an anode. In the outlet pass, the tinplate passes through a cathode (7) to apply a fresh tin oxide layer to the bare, pure tin surface. After leaving the bath through the non-conductive guide roller (5), the strip optionally enters a rinse bath (III) and is dried (not shown). In section IV, a post-treatment agent (11) is applied to the tinplate strip by application means (10). The strip can then be dried, if desired (not shown). Guide rollers 3 and 5 must be non-conductive guide rollers. The term non-conductive in the general context of the present invention means that the rollers do not conduct electricity.

[0067] A typical potential-time curve is shown in Figure 2. From this, the intersection of the tangent of the curve at -0.7 V and the tangent of the curve around -0.85 V is expressed in C / m 2 The thickness of the tin oxide layer is determined based on the time taken as a basis for calculating the thickness of the tin oxide layer at 190 s. In the example of FIG. 2, the time is approximately 190 s x 0.50 = 95 C / m. 2 C / m 2 The thickness D of the tin oxide layer is expressed as D [C / m 2 ]=t[sec]×0.50[A / m 2 ] can be obtained from

[0068] In Figure 3, there is a clear observable difference at t=25 seconds between the curves for the thin and thick layers. Both layers are formed on a fresh tin surface (i.e., any pre-existing oxide has been completely cathodically removed). The dip at t=25 seconds for the thick layer is consistent and is associated with the presence of SnO2 in the tin oxide layer. The other two curves have shapes consistent with a tin oxide layer composed primarily of SnO.

[0069] In Figure 4 the setup of Figure 1 is reproduced, with letters A to G representing the various stages of layer development on the blackplate.

[0070] In Figure 5, the letters are A: Tinplate strip raw material, B: Tinplate (i.e., black tin plate coated with a tin layer in tin plating cell I), C1: Tinplate with pre-existing oxide and uninterrupted processing (no additional oxide growth); C2: Tinplate with pre-existing oxide and additional oxide layers due to prolonged storage and / or storage under conditions that result in the growth of additional oxide layers; D: Pure bare tin layer on blackplate after removal of pre-existing oxide layer; E: Anodized reoxidized tinplate, F: cleaned and rinsed anodized tinplate; G: Anodized reoxidized tinplate with post-treatment applied Represents.

Claims

1. A method for passivating tinplate strip (1) in a continuous process, comprising: After electrodepositing one or more tin layers or after flow-melting the electrodeposited one or more tin layers, the tinplate strip (1) is introduced into a basic aqueous solution (8) in an electrochemical treatment tank (II) in an inlet passage and withdrawn from the basic aqueous solution in an outlet passage, wherein a pre-existing tin oxide layer on the surface of the tinplate is cathodically removed from the surface of the tinplate in the inlet passage, and then the surface of the tinplate is immediately anodically reoxidized in the outlet passage, the charge density required for cathodically removing the pre-existing tin oxide layer from the surface of the tinplate is Q1, the charge density required for anodically reoxidizing the tinplate is Q2, Q1<Q2, the charge density applied for the anodically reoxidizing and the charge density applied for the cathodic removal of the pre-existing tin oxide layer are equal to Q2 and are at least 15 C / m ２ and The method wherein the anodized tinplate is rinsed and dried after being removed from the basic aqueous solution.

2. The charge density applied for anodic reoxidation is up to 100 C / m ２ The method of claim 1, wherein

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

4. 4. The method of claim 1, wherein a liquid solution of a chromium-free post-treating agent is applied to the surface of the rinsed and dried anodized tinplate to produce a post-treated tinplate, and the chromium-free post-treating agent is selected from acrylate copolymers; polymethylsiloxanes with polyether side chains; acidic polyethers; polymers with heterocyclic groups; and acidic aqueous liquid compounds containing complex metal fluoride anions and polymeric materials with divalent to tetravalent cations.

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

6. The method according to any one of claims 1 to 5, wherein the current density A during anodization is at most 4000 A / m2.

7. 7. The method of any one of claims 1 to 6, wherein the basic 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.

8. The method according to any one of claims 1 to 7, wherein the pH of the basic aqueous solution is from 8.75 to 10.

5.

9. 9. The method according to claim 1, wherein the anodic reoxidation time t is between 0.1 and 1.5 seconds.

10. 10. A method according to any one of the preceding claims, wherein the tinplate is introduced into the basic aqueous solution immediately after electrodeposition of the one or more tin layers onto a steel strip or immediately after flow-melting the electrodeposited one or more tin layers.

11. The method according to any one of claims 1 to 10, wherein the tin oxide layer after anodic reoxidation consists mainly of SnO.

12. 12. A method according to any one of claims 1 to 3 or 5 to 11, wherein a thermoplastic polymer coating is applied directly to the rinsed and dried reoxidized tinplate, and wherein the tin oxide layer after anodizing reoxidation consists primarily of SnO.

13. 12. A method according to any one of claims 4 to 11, wherein a thermoplastic polymer coating is applied directly to the post-treated tinplate, and the tin oxide layer after anodizing reoxidation consists mainly of SnO.

14. Passivated tinplate, the tinplate comprising a tin oxide layer on the tin surface of the tinplate, the tin oxide layer consisting mainly of SnO, the tin oxide layer being coated with a post-treatment agent based on titanium or a combination of titanium and zirconium, the dry coverage of the post-treatment agent being 0.2 to 2 mg Ti / m2.

15. An apparatus for producing passivated tinplate (1) by the method according to any one of claims 1 to 13, comprising: an electrolytic tinning line (I) optionally equipped with means for melting the tin layer, an electrochemical treatment tank (II) for holding the basic aqueous solution (8) during use; non-conductive means for guiding the cathode tinplate into said electrochemical treatment tank (II) past the anode (6) in the inlet passage; a non-conductive sink roll (4) for directing the tinplate from the inlet passage to the outlet passage and guiding the anode tinplate past the cathode (7) in the outlet passage; means for applying a potential between said tinplate strip and a counter electrode for the cathodic removal of any pre-existing oxide layer and for the anodic reoxidation of the tinplate strip; and non-conductive means for guiding the tinplate from the electrochemical treatment tank to the means for rinsing the tinplate (III) and to the means for drying the tinplate; The device comprising:

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