Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/34—Anodisation of metals or alloys not provided for in groups C25D11/04 - C25D11/32
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/48—After-treatment of electroplated surfaces
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12535—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.] with additional, spatially distinct nonmetal component
  • Y10T428/12611—Oxide-containing component

Definitions

• the invention pertains to a method for passivating the surface of tinned steel strips, which are moving at a strip speed through a coating system, by anodic oxidation and post-treatment of the tinned steel strip with a chromium-free post-treatment agent.
• the invention further concerns the use of agents with complexly bound transition metals or organic additives, which are normally used for dispersion or improvement of wetting, as chromium-free post-treatment agents in the production of tinned steel strips.
• a further object of the invention is a tinned steel strip or sheet with a support layer made of a fine or ultrafine sheet of steel, a tin layer deposited thereon, and a surface layer of a chromium-free post-treatment agent.
• Tinplate is a versatile, economical and ecologically advantageous material, which is used first of all in the packaging industry.
• Tinplate is a cold-rolled steel sheet with a thickness up to 0.5 millimeters, which is coated with a thin tin layer in order to protect the steel sheet from corrosion.
• the tin layer is deposited electrolytically, for example, on the sheet.
• the coated steel sheet is chemically or electrochemically passivated and then coated with lubricant in order to make the tinned steel sheet resistant to oxidation and in order to reduce the coefficient of friction so that the tinned steel sheet can be more readily worked in the subsequent processing, for example in the manufacture of tin packaging for food.
• Solutions containing Cr 6+ are usually used in the passivation.
• the use of chromium-containing agents in the treatment of metal products is environmentally harmful and therefore should be regulated by law, especially in the case of metal products that are intended for the food industry.
• DE4205819A discloses a water-free composition for production of corrosion protection layers on metal surfaces, which is based on silane compounds and compounds of tetravalent titanium or zirconium.
• EP1002143A discloses a method for alkali passivation of zinc plated and zinc alloy plated steel surfaces and of aluminum and its alloys in strip coating systems. Said passivation with an aqueous solution results in a corrosion protection layer that can serve as a base for subsequent coating with enamel.
• EP1270764A discloses a surface-treated tinplate comprising an alloy layer on a surface of a steel sheet, a tin layer that has been deposited on the alloy layer so that the alloy layer is exposed over a surface area of 3.0% or more and a film comprising P and Si with a film weight of 0.5 to 100 mg/m 2 or 250 mg/m 2 , is deposited on the exposed alloy layer and the tin layer.
• Chromium-free post-treatment agents for tinplate are described in U.S. 2009/0155621 A1 and the publications cited therein.
• the passivation in this case has to prevent too strong a growth of the tin oxide layer during the storage of the coated steel sheet or the food container made therefrom until it is coated with a protective enamel and furthermore until the preserved food is consumed.
• the passivation should prevent discolorations of the coated metal surface. Such discolorations arise, for example, when tin cans that contain sulfur-containing substances are sterilized, since the sulfur reacts with the tin in the coated steel surface if it is not sufficiently passivated.
• the consumer may get the idea that the contents are tainted. Enamel adhesion problems may also arise due to reaction with sulfur and these can be avoided by passivation of the coated steel sheet.
• the passivation moreover must guarantee the resistance of the enameled metal container, after being filled with foods, to the acids contained in the foods, such as mercaptoaminocarboxylic acid anions, for example cysteine and methionine. If passivation is not sufficient, such acid anions in the can contents can give rise to delamination of the inner coating of the container.
• the cold rolled fine or ultrafine sheet after tinning is first fused by heating to temperatures above the melting point of tin and then quenched in a water bath. This is followed by passivation by treating the tinned steel sheet with a chromate solution and finally rinsing it with demineralized water and drying it under heat. This is followed by electrostatic lubrication with dioctyl sebacate (DOS) or acetyl tributyl citrate (ATBC).
• DOS dioctyl sebacate
• ATBC acetyl tributyl citrate
• the chromate adsorbed on the surface is reduced to Cr 3+ by a reaction with the ⁇ Sn ⁇ O and ⁇ Sn—OH groups on the tin surface and also in some cases to chromium metal in the case of electrochemical cathodic passivation.
• the Cr 3+ precipitates as Cr 3+ hydroxide. After rinsing and drying the tinplate surface, the passivation layer no longer contains any Cr 6+ ions.
• the invention has the problem of making available a method for chromium-free passivation of the surface of steel strips that have been coated with a tin coating, which are moving at a strip speed through a coating system, said method enabling efficient passivation of the steel strip surface even at high strip speeds. Moreover, at the same time improved enamel coat adhesion and resistance to acids in foods, especially to amino acids that contain sulfur, should be achieved.
• the conduction of an anodic oxidation of the tin surface is provided.
• the invention proceeds from the knowledge that passivation of the tinned surface of the steel strip by post-treatment with a chromium-free post-treatment agent is not by itself sufficient to protect the surface completely and permanently against corrosion and discolorations (marbling). In particular, it has been shown that chromium-free post-treatment agents do not protect the tinned steel strip surface against a reaction with sulfur everywhere.
• an electrochemical tinning of a cold rolled steel strip takes place in a strip tinning system.
• the steel strip is moved through an electrolysis bath at a strip speed that usually lies in the range of 200 m/min to 750 m/min and is electrolytically coated with tin.
• the steel strip is heated to temperatures above the melting point of tin (232° C.) conductively or inductively (or even conductively and inductively) in order to fuse the tin coating onto it.
• the moved strip is quenched in a water bath.
• the tinned steel strip arrives with this fresh surface at the second step of the method, namely anodic oxidation of the tin surface.
• an anodic oxidation takes place in a soda solution, i.e., in a sodium carbonate solution.
• the tinned steel strip is moved along at the strip speed and is connected as the anode in the soda electrolyte bath.
• An aqueous soda solution serves as the electrolyte.
• the concentration of sodium carbonate in the soda solution is preferably 1 wt % to 10 wt %, particularly 2 wt % to 8 wt %, preferably 3 wt % to 7 wt %, above all 4 wt % to 6 wt %, especially about 5 wt %.
• the device for electrolytic anodic oxidation expediently comprises an electrolysis immersion bath with a vertical tank. Near the bottom in the vertical tank is a reversing roller over which the tinned steel strip reverses travel.
• the vertical tank is filled with the electrolyte.
• a potential is applied between the tinned steel strip and the counter electrode (for example a steel cathode) in the vertical tank.
• the charge Q is expediently between 0.2 C and 2 C, preferably between 0.2 C and 0.6 C at a current density of 1-3 A/dm 2 .
• the anodization time corresponds to the residence time of the tinned steel strip in the electrochemical oxidation bath (electrolyte bath). This is determined by the length of the electrolyte bath or its fill level and the anode length and the strip speed and for typical strip speeds it expediently lies in the range of 0.1 s to 1 s, especially between 0.1 s and 0.7 s, preferably in the range of 0.15 s to 0.5 s, and ideally around 0.2 s.
• the anodization time, in dependence on the strip speed, can be set via the fill level to values that are suitable in accordance with the invention.
• the spacing between the steel strip and the counter electrode in the electrolyte bath is set in dependence on the system. It lies, for example, in the range of 3 to 15 cm, preferably in the range of 5 to 10 cm, and especially around 10 cm.
• the temperature of the electrochemical oxidation bath preferably lies in the range of 30 to 60° C., more preferably in the range of 35 to 50° C., and especially around 45° C.
• the current density is set in the range of 1.0 to 3 A/dm 2 , preferably 1.3 to 2.8 A/dm 2 , more preferably 2 to 2.6 A/dm 2 , especially around 2.4 A/dm 2 .
• the total charge in this case varies between 0.2 C and 0.6 C and is preferably, for example, 0.48 C.
• the corresponding charge densities lie in the range of 0.2 C/dm 2 to 0.6 C/dm 2 .
• the third step of the method there takes place a thorough rinsing of the tinned and oxidized steel strip with distilled or fully demineralized water and a subsequent drying, which can take place, for example, with hot air.
• drying means are also suitable for this, such as drying with water-absorbing solvents followed by drying with a cold or hot air blower, hot air being preferred, drying with convection air-free drying systems like IR radiators, inductive heating, or resistance heating, or drying only with a cold or hot air blower, preferably a hot air blower.
• a coating of the tinned and oxidized steel strip surface with a post-treatment agent takes place.
• 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
• a solution of the post-treatment agent is sprayed onto the steel strip, which is moving at the strip speed.
• 1.5 to 10% aqueous solutions of the post-treatment agent proved to be expedient.
• the solution of the post-treatment agent is then stripped off by squeeze rollers and dried.
• a thin film of the post-treatment agent still remains on the surface of the coated metal strip after the squeezing and drying, where the weight of said thin film as a rule is between 2 and 30 mg/m 2 .
• the post-treatment agent is sprayed, for example via tubes that are arranged at a distance from the coated metal strip surface and transverse to the direction of strip travel and have holes or nozzles through which the post-treatment agent reaches the coated steel strip surface.
• tubes that are arranged at a distance from the coated metal strip surface and transverse to the direction of strip travel and have holes or nozzles through which the post-treatment agent reaches the coated steel strip surface.
• at least one tube with such holes is arranged on each side of the steel strip in order to splatter or spray both sides of the metal strip with the post-treatment agent.
• the distance between the tubes and the tinned and anodically oxidized steel strip is adjusted and the position of the holes or nozzles with respect to the direction of travel of the steel strip selected so that the exiting liquid treatment agent strikes the steel strip perpendicular to its surface or at least within an angle range of ⁇ 45°, preferably within an angle range of ⁇ 15° about the normal (perpendicular) to the surface of the steel strip.
• two squeeze roller pairs are arranged after the tubes in the direction of strip travel.
• the spacing of the first squeeze roller pair from the tubes in the direction of strip travel is, for instance, between 20 and 100 cm.
• the solution stripped off by the squeeze rollers is collected in a supply tank, from which the excess post-treatment solution is optionally sent by pump to a preparation step and reused.
• the agents defined below can be used as post-treatment agents.
• the application of a Ti/Zr-containing post-treatment agent is described as representative of this.
• the substance obtainable from Henkel KGaA under the trade name Granodine® 1456, for example, is used as a Ti/Zr containing post-treatment agent. It is applied to the tinned and oxidized steel strip surface as a solution with a dry layer weight in the range of 0.5 to 2 mg Ti/m 2 , preferably 0.8 to 1.5 mg Ti/m 2 , especially around 1 mg Ti/m 2 .
• a drying step again takes place, where the drying temperature (strip temperature) lies in the range of 30 to 95° C. and preferably between 35 and 60° C.
• the drying time is matched to the strip speed.
• the drying devices indicated in step 3 can be used in this case.
• the described method yields a tinned steel sheet with a layer structure composed as follows: at the bottom there is, as support, a cold rolled steel sheet, in particular a fine or ultrafine sheet with a thickness of 0.5 mm to 3 mm (fine sheet) or less than 0.5 mm (ultrafine sheet).
• the tin layer for example electrolytically deposited, follows as the next layer on the steel sheet.
• the amount of tin as a rule is 0.1 to 11.2 g/m 2 , but in individual cases it can even be less than 0.1 g/m 2 or more than 11.2 g/m 2 . Any alloy layers of support material and tin are ignored here as special intermediate layers.
• the tin layer there then follows the oxide layer produced by anodic oxidation with a thickness of a few nm, which substantially consists of tetravalent tin oxide.
• the thickness of the tin oxide layer varies roughly in the range of 2 to 10 nm.
• the post-treatment agent layer which was deposited on the thin oxide layer by the post-treatment and subsequent drying, follows on the oxide layer as the surface layer.
• the weight of the post-treatment agent in the dry state is expediently between 2 and 30 mg/m 2 .
• the sheet produced and post-treated in accordance with the invention will either be cut into sheet panels or rolled up as a coil.
• the end user for example the maker of tin cans, will as a rule then provide the sheet with an enamel layer, for example with a protective can enamel or an epoxide resin enamel.
• the enameled sheet will then be processed by deep drawing into molded objects, for example tin cans.
• the layer structure produced through the treatment in accordance with the invention contributes to an improved enamel adhesion and reduced flaking of the enamel, for example because of the presence of mercapto group-containing amino acids.
• the substances that are suitable for use in the method in accordance with the invention for post-treatment of the surface of anodically oxidized tinplate must have properties such that they can adhere to the tin surface and at the same time allow wetting of the enamel layer that is to be applied in the subsequent enameling of the tin surface.
• the bonding between the functional group on the anodically oxidized tin surface and the functional groups of the enamel surface with their adhesion mediating molecules must be strong enough after drying the enamel film that they are [sic; it is] not disrupted in the cysteine test (sterilization of the enameled tinplate for 90 min at 121° C.
• the substances used for post-treatment should be chromium free and usable without the addition of organic solvents, or be soluble in distilled water, since the use of dissolving aids produces high concentrations of solvent in the exhaust air and thus would necessitate expensive purification systems to remove the solvent.
• Such agents are chosen from copolymers of acrylates, polymethylsiloxanes with polyether side chains, acid polyethers, and polymers with heterocyclic groups.
• substances that are used as corrosion protection agents for sheet metal for example in the automotive industry, for example bonderizing or Parkerizing agents can also be advantageously used.
• Such substances are chosen from acidic, aqueous, liquid compositions that contain complex metal-fluoride anions with di- to tetravalent cations and polymer substances.
• the substance (d) is an acidic, aqueous, liquid composition that contains the following components in addition to water:
• the substance d) is an acidic, aqueous, liquid composition that is substantially free of hexavalent chromium and/or ferricyanide, for treatment of metal surfaces, which contains no more than 1.0% and preferably no more than 0.0002% of both hexavalent chromium and ferricyanide and that otherwise contains the following components in addition to water:
• the component d) comprises dihydrogen hexafluorotitanate (2-) and organic polymers. Especially preferably is the component Granodine 1456.
• the polymer (D) can be a polymer compound that comprises a copolymer material, where at least a part of the copolymer has the structure
• Such a polymer can be prepared, for example, as follows:
• a slurry of 263.3 g N-methylglucamine in 400 [sic] deionized water is added and the mixture is heated to 60-65° C. while stirring. Then 100.2 mL 37% formaldehyde is added over one to one and a half hours. The mixture is then heated to 90° C. and held for 6 hours. After cooling, the mixture is diluted to 9.6 wt % solids with deionized water.
• the pH of the prepared solution is 9.1 and the solution comprises an N-methylglucamine derivative.
• Such a polymer can be prepared, for example, as follows:
• a resin of the above formula with an average molecular weight of about 2400 in which R 1 , R 13 , R 14 , and W 2 are H, R 2 is —CH 3 , and Y 2 is an alkylamine residue or an alkylammonium residue (with reference to the Formula I given in EP 0 319 016 A2 and its definitions), are slowly dissolved in 160 mL Propasol P (a propoxylated propanol solvent obtainable from Union Carbide Corp., Danbury, Conn.) in a 1 liter plastic flask using a high speed mixer. The plastic flask is fitted with a paddle stirrer, a reflux condenser, and a nitrogen flush device.
• Propasol P a propoxylated propanol solvent obtainable from Union Carbide Corp., Danbury, Conn.
• an amine oxide which does not require an additional neutralization step, is formed by adding 0.75 mol 30% H 2 O 2 (85 g) to the reaction mixture. The reaction mixture is stirred overnight and then diluted with 960 mL deionized water. The result of this optional step is a water-soluble amine oxide resin, which does not require neutralization for water stability.
• Suitable post-treatment agents with complexly bound transition metals are, for example, products of the Granodine® series, which are aqueous solutions containing complexly bound transition metals (Ti, Mn, Zr) and optionally a silane.
• Titanium- and/or zirconium-containing post-treatment agents proved to be especially suitable, for example products of the Granodine® series (manufacturer: Henkel) or Gardobond® series (manufacturer: Chemetall), especially the products “Granodine® 1456” and “Gardobond® X 4707.”
• These titanium- and zirconium-containing products are expediently metered as post-treatment agents for the method in accordance with the invention so that after the squeezing out and drying, a titanium weight (dry weight) of 0.5 m g/m 2 to 2 mg/m 2 and especially about 1.0 mg/m 2 is present on the surface of the treated tinplate.
• EFKA 3570 The following products from BASF SE are preferred: EFKA 3570, EFKA 3580, EFKA 4560, and EFKA 8512. EFKA 4560 is especially preferred.
• Acrylate-containing agents with N-heterocyclic groups proved to be especially suitable for the post-treatment agents, for example EFKA 4560. These agents correspond to the above defined components (c).
• the solids content was then adjusted to 40 wt % with butyl acetate.
• the solids content was then adjusted to 40 wt % with butyl acrylate.
• these substances are sprayed in undiluted form or as an aqueous solution onto the tinned steel strip and optionally then squeezed off and dried. Dry film weights in the range of 2 to 15 mg/m 2 and preferably between 2 and 10 mg/m 2 are suitable as application weights for, for example, the substance EFKA 4560.
• the (further) zinc oxide buildup on the coated metal strip surface during its storage before being coated with lubricant can be highly reduced.
• the treatment of the anodically oxidized, tinned steel strip surface with said post-treatment agents in accordance with the invention there is an improvement of enamel adhesion.
• the tinplate surfaces treated in accordance with the invention proved to be very readily coatable with enamel.
• tinplate samples treated in accordance with the invention in some cases have clearly lower sliding friction than untreated tinplate, and with individual substances it is even possible to obtain lower coefficients of friction than the traditional treatment of the tinplate with DOS.
• the substances EFKA 3580, EFKA 4560, EFKA 8512, and EFKA 3570 produce very good results with regard to the properties low coefficients of friction, resistance to oxidation (low tin oxide buildup), and resistance to marbling (sulfide blocking activity).
• EFKA 4560 turned out to be the preferred post-treatment agent.
• the titanium-containing post-treatment agents of the Granodine® series (especially Granodine® 1456) and Gardobond® series (especially Gardobond® X 4707) show very advantageous results.
• other chromium-free post-treatment agents in some cases already known from the prior art, are also suitable for conducting the method in accordance with the invention.
• the method in accordance with the invention can be employed both in the case of tinplate with a fused-on tin layer as well as in the case of a tinplate in which the tin layer has not been fused on.

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• 2012
  • 2012-01-12 DE DE102012000414.1A patent/DE102012000414B4/de active Active
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