Classifications

• • 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
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/73—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process
  • C23C22/76—Applying the liquid by spraying
• • 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
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/30—Electroplating: Baths therefor from solutions of tin

Definitions

• the invention relates to a method for forming a passivation layer on an article having at least one tinplated surface having excellent sulphur staining resistance and to an article produced by said method.
• Tin is used to protect the steel base from corrosion both externally (aerobic conditions) and internally when in contact with foods (anaerobic). Under the anaerobic conditions expected inside an internally plain processed food can, tin will normally behave as the sacrificial anode, dissolving very slowly whilst protecting the steel base from corrosion and creating a reducing environment in the can. It is this mechanism that has enabled the plain tinplate can to maintain its long history and proven track record of providing wholesome food on a year round basis and safe storage for long periods of time.
• Some foods especially protein rich meat and fish and, to a lesser extent, vegetables (e.g. peas, beans, corn etc.) contain naturally occurring sulphur compounds. These can react with a plain tinplate surface to give a purple-black stain of tin sulphide. Although the stain is harmless, it may serve to change the passivation of the tinplate surface, which, in turn, could alter the rate of tin uptake. Whilst an overall increase in passivation is more likely to slow tin uptake, localised areas of staining can have a detrimental effect, especially if a corrosion accelerator such as oxygen is also present. Degree of sulphide staining is also influenced by pH, process time and temperature and the presence of certain cations.
• Passivation refers to the chemical treatment applied after tin deposition which stabilises the surface characteristics of tinplate by controlling tin oxide formation and growth.
• Passivation treatments can be electro-chemical or chemical. Electrochemical treatments involve the use of an external electric current. At present, cathodic dichromate (CDC) treatments are usually applied.
• a CDC treatment is an electrochemical passivation treatment. Chromates are based on hexavalent chromium and these are nowadays considered to be hazardous substances being potentially harmful to the environment and a risk in terms of worker safety. Consequently, intensive research efforts are being made towards developing passivation treatments free from hexavalent chromium, also denoted as Cr(VI)-free passivation treatments. Recent developments of Cr(VI)-free passivation treatments have resulted in renewed attention to combatting sulphide staining because these newly developed alternatives to cathodic dichromate treatments struggle to deliver the same performance in terms of sulphide staining resistance.
• the most common method to improve the sulphide staining resistance of Cr(VI)- free passivated tinplate is to increase the thickness of the passivation layer itself (e.g. by using process conditions leading to thicker applied wet films, or by using more concentrated treatment solutions, or by employing longer treatment time, etc.) This leads to enhanced barrier properties, thus suppressing the formation of sulphide stains.
• passivation layers are mechanically weak and the risk of cohesive failure within the passivation layer increases with thickness. Cohesive failure of the passivation layer (in particular during heavy deformation that is encountered in e.g. canmaking) leads to loss of adhesion and delamination of organic coatings that are applied over the passivation layer.
• the object of the invention is to provide a method to improve the sulphide staining resistance of tinplate that has been passivated using Cr(VI)-free passivation systems.
• Another object of the invention is to provide a method to improve the sulphide staining resistance of tinplate that has been passivated using Cr(VI)-free passivation systems that can be integrated into existing electolytic tinning lines.
• Still another object of the invention is to provide a tinplated article having excellent sulphide staining resistance that is at least similar to that of a cathodic dichromate passivation treatment.
• the invention solves the problem of poor sulphide staining resistance of tinplate by subjecting the tinplate to an anodic treatment in a suitable aqueous electrolyte which is carried out within certain boundaries of treatment time, current density and total charge passed, in order to be effective in achieving the required sulphur staining resistance.
• a very thin layer of tin oxide on the tinplate surface is formed by the electrochemical anodic treatment. This tin oxide layer provides the improvement in sulphide staining resistance and together with the subsequent chemical Cr(VI)-free passivation treatment provides a tinplate which delivers the same, or a better performance than the known CDC-treated tinplate.
• An essential element of the invention is the thickness of the tin oxide layer (D), which is expressed in Coulomb/m 2 and represents the total charge needed to reduce the layer to metallic tin.
• the efficiency thus represents the ratio of the thickness D of the produced oxide layer to the applied charge density (A x t), and can be estimated by plotting D as a function of (A x t).
• the efficiency E decreases leading to a slower growth rate of the tin oxide layer, and thus in a slower increase in D. If D ⁇ 30 C/m 2 , then the tin oxide layer is too thin and is not effective in achieving the desired sulphide staining resistance. A minimum thickness of 30 C/m 2 is therefore required.
• the tin oxide layer thickness is determined using a coulometric method. The tin oxide layer is reduced by a controlled small cathodic current in a 0.1% solution of hydrobromic acid (HBr) that is freed from oxygen by scrubbing with nitrogen.
• HBr hydrobromic acid
• the progress of the reduction of the oxide is monitored by measuring the reduction potential, and the charge passed (A*t) for the complete reduction serves as a measure of the tin oxide layer thickness.
• A*t charge passed
• a cylindrical electrolysis cell having a circular aperture of ca. 4 cm diameter on one end.
• the other end of the cell contains a platinum counter electrode and an Ag/AgCI reference electrode.
• the test specimen covers the aperture, which is sealed using an O-ring to make a water-tight connection of well defined area (13.69 cm 2 in this case), and is tightened into place using an air-pressure cylinder.
• the cell is connected to the electrolyte solution by a flexible tube so that it can be filled and emptied under nitrogen atmosphere.
• a cathodic current density of -0.40 A/m 2 is applied to the sample using a potentiostat-galvanostat, and the potential is measured until the reduction is complete.
• a typical potential-time curve is shown in Fig . 1, from which the tin oxide layer thickness is determined based on the inflection point of the potential drop at time t,.
• the sole purpose of the electrolyte is to enable the anodic treatment, not to deposit foreign species contained in the electrolyte onto the substrate surface.
• the pH is not lower than 8.5 and/or not higher than 11.5.
• a suitable maximum pH value is 11 or even 10.5.
• the electrolyte comprises monoatomic cations from Group 1 or 2 from the periodic table or polyatomic cations, and poly-atomic anions.
• Group 1 and Group 2 elements, according to the new IUPAC numbering comprise the alkali metals and the alkali earth metals. It is important that the electrolyte does not contain mono-atomic halogen anions (Group 17) such as CI " , F " because these anions prevent the tin-oxide layer from forming.
• the article is optionally rinsed and dried before it is passed on to the subsequent step, i.e. the Cr(VI)-free passivation treatment.
• the need to rinse the anodically treated tinplate depends on the exact nature of the Cr(VI)-free passivation system, certain systems will be more susceptible than others to contamination of electrolyte being present on the tinplate surface.
• the Cr(VI)-free passivation system is applied to the anodically treated tinplate surface by application techniques that are common for such passivation systems. Suitable application techniques include: dipping, dipping with squeegee rolls, rotor-spray application, rotor-spray application supported by the use of a smoothing roll, spray application, spray-squeegee application, application by means of a roll coater systems, application by slot coating, slot curtain coating, etc.
• the strip is dried and passed on for final processing steps such as oiling, winding, cutting, etc.
• the total D as specified above can be achieved by any combination of A and t, but a combination of a high current density (A > 0.1 A/dm 2 , preferably A > 1.0 A/ dm 2 ) in combination with a short treatment time (t ⁇ 1 sec) is preferred in view of its processability on a high-speed tinning line.
• a and t in the anodic treatment which is further demonstrated in Example 1 and Figures 2 and 3, implies that the process can be operated at short treatment times, by adjusting the applied current density accordingly.
• the method according to the invention can be employed in industrial tinning lines running at line speeds in excess of 300 m/min to speeds of up to 1000 m/min.
• the anodic treatment is performed in-line with or immediately after electrolytic tinning, and wherein the anodic treatment time (t) is at most 5 seconds, preferably at most 2 seconds, more preferably between 0.05 seconds and 1.5 seconds. This range and the more preferable ranges are consistent with high speeds processing lines.
• the anodic treatment is performed in-line with an industrial electrolytic tinning line, and wherein the current density during the anodic treatment (A) is at least 10 A/m 2 , preferably at least 50 A/m 2 and more preferably at least 100 A/m 2 , and/or at most 4000 A/m 2 , preferably at most 2000 A/m 2 or more preferably at most 1000 A/m 2 .
• the electrolyte to be employed can be an aqueous solution of an acid, a base or a salt.
• the main function of the electrolyte is to support the electrochemical reaction intended by the anodic treatment while the ionic species present in the electrolyte do not take part in the electrochemical modification of the tinplate surface.
• the preferred electrolyte contains cations from Group 1 (e.g . Na + , K + ) or Group 2 (e.g . Mg 2+ , Ca 2+ ) from the Periodic Table or polyatomic cations (e.g. NH 4 + ), and polyatomic anions (phosphates, borates, sulphates, carbonates and the like).
• Group 1 e.g . Na + , K +
• Group 2 e.g . Mg 2+ , Ca 2+
• polyatomic cations e.g. NH 4 +
• polyatomic anions phosphates, borates, sulphates, carbonates and the like.
• the anion may be the conjugate base of an organic acid (e.g. acetates, citrates). Since it is of importance that the pH be maintained within certain boundaries, a buffered solution could be used.
• the electrolyte may contain other chemical additives, such as surfactants, wetting agents, anti-foaming agents etc. to support the electrochemical treatment, provided these additives do not adversely affect the formation of the tin oxide.
• the anodic treatment of the tin-plated surface converts the extreme outer layer of the tin surface from metallic tin into tin oxide by electrochemical oxidation.
• the tin oxide layer produced as such provides a barrier against sulphide staining.
• the tin oxide layer is, however, not sufficiently stable and/or passive in itself and will, during prolonged storage under ambient and/or humid conditions, or during heat treatments such as baking and stoving, continue to grow into a thicker tin oxide layer with undesirable properties (poor wettability, yellowish appearance, poor lacquer adhesion).
• the Cr(VI)-free passivation system If we consider the Cr(VI)-free passivation system on its own, it usually will provide a stable passivation layer protecting the tinplate against uncontrolled growth of tin oxides and furthermore providing good adhesion of organic coatings. However, the Cr(VI)-free passivation layer in almost all investigated cases has a poor resistance against sulphide staining.
• a tin oxide layer of the correct thickness is applied by employing the anodic treatment under proper process conditions, and then the tin oxide layer is passivated and/or stabilised against further uncontrolled growth, by applying a Cr(VI)-free passivation system on top of it, by using a non-electrolytic application method.
• the anodic treatment of the present invention must take place after tinning and/or flow melting and before the application of a Cr(VI)-free passivation system.
• the Cr(VI)-free passivation system before which the anodic treatment is applied must be a chemical passivation treatment, preferably a so-called no-rinse process, for the application of a no-rinse, dry-in-place passivation system.
• the anodic pre-treatment is not expected to work in combination with a Cr(VI)-free passivation system that itself is applied electrolytically.
• the anodic pre-treatment is not expected to work in combination with a Cr(VI)-free passivation system that is applied by a cathodic electrochemical process, since such a process will remove the anodic treatment layer through electrochemical reduction.
• Suitable no-rinse Cr(VI)-free passivation systems that can be used in combination with the anodic treatment of this invention are e.g. :
• organic acids oleic acid, abietic acid
• organic/inorganic coupling agents such as one-component and two- component siloxane systems
• inorganic systems such as silicate-based systems
• inorganic systems in an organic matrix such as fluoro-titanates and zirconium- titanates in combination with an organic polymer matrix.
• the thickness of the tin oxide layer D it is preferable for the thickness of the tin oxide layer D to be at most 100 C/m 2 .
• a value above 100 is not possible to achieve economically in the high speed tinplating process and it also leads to a reduced adhesion of subsequently applied organic coatings.
• the preferred range for the tin oxide layer thickness D is therefore 30 to 100 C/m 2 .
• a suitable minimum value of D is 40 C/m 2 .
• a suitable maximum value for D from a process efficiency point of view is 80 C/m 2 or even 60 C/m 2 .
• the polyatomic anion in the electrolyte is a phosphate, a borate, a sulphate, or a carbonate anion.
• the cation in the electrolyte is Na + , K + (Group 1) and/or Ca 2+ (Group 2) and/or polyatomic e.g. NH4 + .
• the article is a strip of packaging steel provided with a tin layer on at least one side (for typical chemical compositions see e.g. EN10202-2001 or ASTM 623M).
• This strip is produced in a known way, e.g. by cold rolling and annealing and optionally temper rolling a steel strip of suitable composition, followed by electrolytic tinplating.
• the article is further provided with an organic coating layer such as epoxy-phenolic gold lacquers, epoxy-anhydride white lacquers, PVC or vinyl organosol coatings, polyester lacquers, epoxy-amino or epoxy-acrylic-amino waterborne coatings.
• an organic coating layer such as epoxy-phenolic gold lacquers, epoxy-anhydride white lacquers, PVC or vinyl organosol coatings, polyester lacquers, epoxy-amino or epoxy-acrylic-amino waterborne coatings.
• the tinplate sample was placed in the electrolyte at 50°C, and connected as the anode to a galvanostat.
• Various current densities in the range 0.2 - 1.4 A/dm 2
• treatment times in the range 0.4 - 9.0 sec
• the sample was removed from the electrolyte, rinsed with deionised water and dried at ambient temperature.
• the tin oxide layer thickness was then determined as described above.
• Figure 1 gives a typical curve of reduction potential (V) as a function of time (t) in a tin oxide reduction experiment.
• Figure 2 gives the tin oxide layer thickness (in C/m 2 ) as a function of the charge passed (A*t) in the anodic treatment using the phosphate solution for different current densities (see legend) and time combinations.
• the dashed line serves as a guide to the eye.
• Figure 3 gives the tin oxide layer thickness (in C/m 2 ) as a function of the charge passed (A*t) in the anodic treatment using the sodium carbonate solution for different current densities and time combinations. For instance, a current density of 120 A/m 2 for 0.4 seconds resulted in the same layer thickness as a current density of 60 A/m 2 for 0.8 seconds.
• the dashed lines serve as guide to the eye.
• a Cr(VI)-free passivation treatment was applied to the tinplate using a spray-and-squeegee application unit.
• the passivation solution was a commercial product, denoted GranodineTM 1456, from Henkel AG & Co. KGaA (Dusseldorf, Germany) containing, among other things, fluoro-titanates, fluoro-zirconates and organic polymers.
• a passivation solution containing 150 g/l of GranodineTM 1456 in deionised water was used in the experiments.
• the passivation solution was sprayed onto one side of the strip using an Ahlbrandt IQ- 140 Rotorspray system.
• the strip was then passed through a (non-driven) pair of rollers consisting of a stainless steel coater roll and a polyurethane-coated backing roll, in which the applied wet film is homogenised and the excess liquid is squeezed off from the strip.
• a (non-driven) pair of rollers consisting of a stainless steel coater roll and a polyurethane-coated backing roll, in which the applied wet film is homogenised and the excess liquid is squeezed off from the strip.
• the strip is heated to 70°C using an inductive heater and subsequently passed through an air dryer operated at 90°C air temperature.
• the as-received, non-passivated tinplate coils were first passed through the line and were given a cathodic treatment in sodium carbonate using the electrochemical treatment tank. This is done to remove the 'natural' tin oxide layer that has developed on the non-passivated tinplate material during storage. A total cathodic current density of 1.11 A/dm 2 was employed during 0.72 sec, giving a total charge density of 80 C/m 2 . Then, the treated coil was placed back on the uncoiler and subjected to the following treatments:
• D (Inventive Example 1) - The coil was passed through the electrochemical treatment tank, filled with the sodium carbonate solution as described above, and an anodic current density of 0.28 A/dm 2 was employed during 0.72 sec, giving a total charge density of 20 C/m 2 .
• the Granodine 1456 passivation was applied as described above.
• E (Inventive Example 2) - As Inventive Example 1 but using an anodic current density of 0.83 A/dm 2 during 0.72 sec, giving a total charge density of 60 C/m 2 .
• the rate of oxidation of the tin surface was measured directly after preparing the samples, using the coulometric method described above. Then, the samples were stored at ambient temperature in dry air during 14 weeks, after which the tin oxide layer thickness was determined again. If the increase in tin oxide layer thickness during this period is 10 C/m 2 or less, the stability of the tin oxide layer is considered to be adequate.
• the as-prepared samples were subjected to immersion in a sulphide-containing solution. Based on extensive research within our laboratories, we have found that the chemical process leading to the formation of sulphide stains on tinplate proceeds as follows.
• Organically-bound sulphur such as present in e.g. proteins, can be converted by e.g. sterilisation processes to inorganic sulphides, such as the S 2" anion.
• Inorganic sulphides can destabilise the tin oxide surface and accelerate the dissolution of tin oxides in food media. Once the tin oxide layer is dissolved from the surface and metallic tin is exposed to the sulphide-containing medium, the chemical reaction between tin and the sulphide, leading to the formation of tin-sulphide, can take place. Also, if the reaction proceeds further, a chemical reaction between the sulphide and exposed iron from the underlying steel substrate can take place leading to the formation of iron-sulphide. Tin- sulphides and possibly iron-sulphides present on the tinplate surface are observed as unsightly dark stains known as sulphide stains.
• Our test method to determine sulphide staining resistance is based on measuring the dissolution rate of tin oxide during exposure to a sulphide solution. It is found that the rate of dissolution of tin oxides correlated strongly with the development of sulphide staining in real-life testing.
• the advantage of our method is that the results can be easily quantified, and offers a better and faster way to investigate sulphide staining as compared to the more commonly used visual observation.
• a particularly convenient method employs a solution containing 5 g/l NaOH and 5 g/l Na 2 S*3H 2 0 maintained at 30°C.
• the samples were lacquered with a white-coloured, epoxy-anhydride internal food can lacquer.
• This lacquer is critical regarding adhesion and very discriminating in sterilisation tests and is commonly used in stringent product evaluation.
• the lacquer was applied at 10 - 12 g/m 2 dry coating weight.
• the lacquered panels were sterilised at 121°C during 60 minutes, in the following media :
• Comp. Ex. 1 shows that no treatment at all of the tinplate surface leads to poor tin oxide stability, poor sulphide staining resistance and poor lacquer adhesion.
• Comp. Ex. 2 shows that a Cr(VI)-free passivation treatment stabilises the tin oxide surface, provides excellent lacquer adhesion, but has a poor resistance to sulphide staining.
• Comp. Ex. 3 shows that the anodic treatment of the present invention provides much improved sulphide staining resistance, but since it is applied without further passivation treatment, the tin oxide layer is not stable and lacquer adhesion is poor.
• Inventive Examples 1 - 3 show that the combination of anodic treatment in combination with a Cr(VI)-free passivation treatment leads to the desired combination of improved sulphide staining resistance, a stable tin oxide layer and good lacquer adhesion.

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• 2013
  • 2013-07-02 RS RS20171072A patent/RS56562B1/sr unknown
  • 2013-07-02 WO PCT/EP2013/063912 patent/WO2014006031A1/fr not_active Ceased
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