Field of the invention

The present invention concerns a passivation process of a steel tinplate. In particular, the present invention concerns an electrolytic passivation process, with low environmental impact, which allows a passivated steel tinplate to be obtained, on which it is possible to make a coating adhere, such as a layer of paint, a layer of ink or a polymeric film, with an excellent degree of adhesion.

The passivated steel tinplate obtained with the passivation process according to the present invention can be advantageously used to produce steel packaging, such as for example containers for packaging food products, chemical and cosmetic products.

Background of the Invention

The steel tinplate (hereinafter "tinplate") is a steel sheet obtained by cold rolling, generally having a thickness up to a maximum of about 0.5 mm, coated on both faces with a thin layer of tin, which has the function of protecting the steel from corrosion. The tin coating is generally made by electrolytic deposition of metallic tin on the steel sheet. Due to exposure to air, a thin layer of oxide is present on the surface of the tin coating.

The tinplate is mainly used to produce packaging, in particular cans for food products intended for human and animal consumption, packaging for chemical products, aerosol containers, beverage cans and to produce parts of said packaging, such as closures, lids, bottoms, etc.

In general, the tinplate has high resistance to corrosion and stability against acids and good processability. For some applications, for example for the production of packaging for food products, the surface of the tinplate is also provided with an additional coating, for example a layer of paint (e.g. epoxy, acrylic paints, etc.) or a laminated polymeric film (e.g. polyethylene terephthalate (PET), polypropylene (PP) film, etc.), to ensure greater protection of the surface of the container from corrosion with respect to the tin coating only.

Coating paints, inks and polymeric films are generally applied to the tinplate at the end of the production process thereof and before its use in the production of packaging. Therefore, in the packaging production process, these coatings undergo mechanical processing (e.g., drawing and stretching), which can cause deterioration or detachment (peel-off) from the surface of the tinplate, if the adhesion to the latter is not good. Furthermore, in some applications, after being filled with the contents, the packaging is subjected to heat treatments (e.g., pasteurization, sterilization), which can damage the coating, for example by forming bubbles (blistering), or can cause the detachment thereof.

In order to improve the resistance to attacks by aggressive chemicals and the adhesion of the coating to the tinplate, in the prior art it is known to subject the tinplate to a chromium passivation treatment, generally of the electrolytic type, by means of which a thin chromium layer is deposited on the surface of the tinplate.

In recent years, however, due to growing demands for limiting the use of environmentally hazardous substances, such as chromium and cadmium, the need is strongly felt to have treatment processes with a low environmental impact as an alternative to the chromium passivation processes, which still guarantees an adequate degree of adhesion of the coating to the tinplate.

In the state of the art some passivation processes are known which do not use chromium compounds, so-called "Cr-free" processes.

For example, EP 2180084 Al, EP 2557202 A1 and EP 3626862 Al describe electrochemical Cr-free passivation processes in which a passivating layer containing zirconium is deposited on the tinplate. These processes involve a first treatment step to partially remove the tin oxide layer originally present on the surface of the tinplate by means of a cathodic electrolytic treatment, followed by a second step of actual passivation treatment, in which the tinplate is subjected to a cathodic electrolytic treatment in an aqueous solution containing sulphate ions and zirconium ions to form a passivation layer containing zirconium mainly in the form of oxide and hydroxide.

While ensuring adequate adhesion of the coatings to the tinplate, the aforementioned known processes have the disadvantage that they require careful control of the amount of sulfate ions and carbonate ions present on the surface of the passivation layer containing zirconium, since the presence of such ions in relatively large quantities adversely affects the adhesion of the coatings to the passivation layer.

US 2015/010773 describes a Cr-free passivation process in which the tinplate is subjected to a preliminary step of anodic oxidation in an electrolyte solution containing a basic electrolyte followed by a step of application of a post-coating aqueous liquid composition and subsequent drying to form the passivation layer.

In one embodiment, the post-coating aqueous liquid composition comprises Ti and/or Zr complex ions for forming a passivating layer comprising the respective oxides and water-soluble or water-dispersible organic polymers (e.g., acrylic polymers) having the function of promoting the adhesion of the secondary coatings. This composition therefore has both a passivating and a priming function for the coating (hereafter also referred to as 'passivating primer').

This liquid composition can be applied to the tinplate by spraying, using coating rollers or dipping. After application, the excess liquid composition is removed from the surface of the tinplate by means of squeeze rollers and finally the tinplate is dried, so that a thin surface passivating film is obtained.

In industrial practice, the implementation of chemical passivation processes such as the one described in US 2015/010773 has several drawbacks. In these processes, for example, it is not easy to ensure an even distribution and application of the liquid passivating composition on the surface of the tinplate, e.g., in case of wear of the dispersing or squeeze rollers. Furthermore, as these processes are carried out in continuous treatment systems where the tinplate is advanced at very high speeds (e.g., 200 m/min or higher), evaporation of the water and solvents in the liquid passivating composition may be incomplete or foaming may occur on the surface of the tinplate due to the vibrations to which it is subjected, resulting in blistering and consequent coating adhesion problems. It is important to note that the above difficulties become more pronounced as the thickness of the applied layer of passivating liquid composition and the speed of the tinplate in the treatment system increase. However, it has been observed that the application of thin layers of liquid passivating composition does not impart a sufficient degree of protection and passivation to the tinplate.

The passivation processes known in the art based on the formation of Ti and/or Zr-containing passivating layers, whether electrolytically or chemically, must also be subject to legislative restrictions or practical requirements imposing an upper limit on the amount of Zr, Ti and total organic compounds that may be present in the outermost layer of the tinplate. In general, the maximum concentration limit to be observed for zirconium is 12 mg/m ² of passivated tinplate surface. In some cases, however, the concentration limit to be observed for zirconium is 6 mg/m ² of passivated tinplate surface, e.g., where the surface is intended to come into direct contact with foodstuffs, i.e., where no coating is applied (e.g., lacquering). Where a coating comprising organic compounds (e.g., passivating primer) is applied, the maximum permitted concentration of organic compounds is generally 40 mg/m ² expressed as Total Organic Carbon (TOC) or less. Compliance with the above regulatory or practical limits, however, requires the application to the tinplate of very thin passivating layers containing Zr and Ti, which, as mentioned, offer less resistance to attack by the aggressive agents contained in food or the additives used for its preservation with respect to thicker coatings.

Summary of the invention

In view of the above state of the art, the Applicant has therefore set itself the primary objective of providing a Cr-free tinplate passivation process that ensures that the chemical resistance and adhesion requirements of coatings generally required for the use of tinplate in the production of food packaging are met.

In particular, an object of the present invention is to obtain a tinplate having a Zr-containing passivation layer which meets the above requirements while having a maximum Zr content of 12 mg/m ² , preferably 6 mg/m ² .

Moreover, the Applicant has set itself the objective of providing a passivated tinplate to which a primer can be easily applied to promote adhesion of the coatings and, in particular for some specific applications, a primer in the form of a liquid composition containing Ti and/or Zr of the type generally used in chemical passivation processes (passivating primer), so as to be able to use the passivated tinplate also in plants in which a system for applying these primers is already provided without, however, the drawbacks encountered in the prior art.

The Applicant has now found that the above and other drawbacks of the state of the art, which will become more apparent in the following description, can be overcome, at least in part, by a Cr-free passivation process of a tinplate in which electrochemical deposition of a passivation layer containing zirconium in an amount less than or equal to 12 mg/m ² , preferably 6 mg/m ² , is followed by a chemical or electrochemical oxidation treatment of the zirconium passivated tinplate.

The oxidation treatment can be a chemical treatment, carried out for example by placing the tinplate in contact with an aqueous solution containing an oxidising reactant, or an electrochemical treatment, carried out for example by anodic electrolytic treatment of the tinplate in an electrolytic solution.

It has been observed that the aforementioned oxidation treatment makes it possible to improve the resistance to chemical attack of the Zr-containing passivation layer (measurable, for example, by the cysteine test and the sterilisation test), even when it has a reduced thickness equal to a maximum of 12 mg/m ² or less, preferably 6 mg/m ² or less. By means of the above-mentioned oxidation treatment, it is therefore possible to reduce the amount of Zr deposited on the surface of the tinplate, while still achieving the desired passivation effect for the passivation layer. In addition, the oxidation treatment allows for the proper adhesion of a passivating layer of primer.

Without wishing to refer to any particular theory, it is believed that the oxidation treatment leads to the formation of a surface layer of Zr and Sn oxides in which said metals are both predominantly in their respective higher oxidation states and therefore more inert to chemical attacks.

A primer can also be applied to the resulting passivated tinplate to aid the adhesion of a subsequent coating. In particular, the primer may be a passivating primer applied in the form of a liquid composition containing Ti and/or Zr ions, for example of the type used in known chemical passivation processes or the like. Since such liquid compositions are applied to a passivated tinplate, it is sufficient that these are applied in relatively low quantities, i.e., in low thicknesses (e.g., less than 1.2 mg/m ² of Ti, preferably less than 1 mg/m ² ), to effectively promote the adhesion of the coating. The composition of the passivating primer can also be adjusted according to the amount of Zr present in the electrolytically deposited passivation layer, so that the desired Ti/Zr weight ratio is achieved.

The lower amount of primer applied simplifies the application process, making it possible for the water and solvents contained in the drying primer to evaporate quickly and effectively, resulting in a homogeneous and uniform layer of primer. Furthermore, in view of the reduced thickness applied, the total content of organic compounds, Ti and Zr in the applied layer of primer ensures compliance with the concentration limits of these metals and organic substances as well as contributing to the passivating effect.

Passivated tinplate obtained as described herein may therefore be used in existing packaging manufacturing processes in which the application of a coating (enamel, lacquer or film) has been optimised for adhesion to a chemically obtained Ti and/or Zr-based passivating layer (applied, for example, by spray or coater systems), without implying any substantial changes to the process of applying the primer and subsequent coating.

According to a first aspect, the present invention therefore concerns a passivation process of a tinplate comprising the following steps in sequence: a. subjecting the tinplate to at least one cathodic electrolytic treatment in an aqueous solution containing at least one alkali metal sulphate to obtain a tin oxide surface layer having a thickness less than 6 mC/cm ² ; b. subjecting the tinplate coming from step a to at least one cathodic electrolytic treatment in an aqueous solution containing at least sulphate ions and zirconium ions to form a passivation layer containing zirconium on said tin oxide surface layer, the amount of zirconium in the passivation layer being 12 mg/m ² at most, preferably 6 mg/m ² at most; c. subjecting the passivated tinplate coming from step b to an oxidation treatment to form an oxidized passivation layer, said oxidation treatment comprising at least one step selected from: cl. contacting the tinplate with an oxidizing aqueous solution; c2. subjecting the tinplate to an anodic electrolytic treatment.

According to a second aspect, the present invention concerns a process for producing a coated tinplate which comprises the following steps:

A. providing a tinplate comprising an oxidised passivation layer, said tinplate being obtained by the above electrolytic passivation process;

B. applying at least one coating on said passivation layer containing zirconium selected from: layer of paint, layer of lacquer, layer of enamel, layer of ink and polymeric material film; optionally, after said step A and before said step B, applying a layer of a primer to promote the adhesion of the aforesaid coating.

In accordance with a third aspect, the present invention relates to a process for producing a tinplate coated with a primer comprising the following steps:

A1. providing a tinplate comprising an oxidised passivation layer, said tinplate being obtained by the above electrolytic passivation process;

B1. applying a layer of a primer, preferably a passivating primer, on said oxidised passivation layer, and wherein no coating is applied to said layer of primer, the coating being selected from: layer of paint, layer of lacquer, layer of enamel, layer of ink and polymeric material film.

This is the case, for example, where tinplate is intended to come into contact with plant foodstuffs (e.g., fruit).

In accordance with a fourth aspect, the present invention relates to a passivated tinplate according to claim 16.

The passivated tinplate in accordance with the process according to the present invention is a substrate on which organic coatings, such as paints, lacquers, enamels, inks or polymeric films adhere very well. The degree of adhesion is comparable to that of the chrome- passivated tinplates of the known art.

The process according to the present invention, not using compounds containing chromium (Cr-free), is a process with low environmental impact. Furthermore, at least in one embodiment, the process does not even use fluorinated or nitrogen-based compounds which can give rise to nitrate compounds which, as is known, represent a problem from the point of view of the environmental impact.

Further features and advantages of the present invention will be apparent from the following detailed description.

The compositions according to the present invention may "comprise", "consist of" or "consist essentially of the" essential and optional components described in the present description and in the appended claims. For the purposes of the present description and the appended claims, the term "consist essentially of" means that the composition or the component may include additional ingredients, but only to the extent that the additional ingredients do not materially alter the essential characteristics of the composition or component.

The numerical limits and intervals expressed in the present description and appended claims also include the numerical value or numerical values mentioned. Furthermore, all the values and sub-intervals of a limit or numerical interval must be considered to be specifically included as though they had been explicitly mentioned.

Detailed description of the invention

The tinplate that can be treated with the passivation process according to the present invention does not present particular restrictions in terms of composition. In general, the tinplate can be a conventional tinplate, for example of the type used to produce packaging, such as containers (so-called cans) for food products intended for human and animal consumption, chemical products, containers for aerosol and to produce parts of said packaging, such as closures, lids, bottoms, etc.

The tin layer of the tinplate is preferably present in a mass per unit area in the range 0.5-15.2 g/m ² (expressed as metallic Sn; mass per unit area referring to each face of the tinplate).

The coating layer of metallic tin is coated superficially by a tin oxide layer, which inevitably forms following the exposure of the tinplate to air.

In the first step (step a) the tinplate is subjected to at least one cathodic electrolytic treatment (that is, an electrolytic treatment in which the tinplate acts as a cathode) using an aqueous solution containing at least one alkali metal sulphate as an electrolytic solution. The purpose of electrolytic treatment of step a is to reduce the thickness of the tin oxide layer on the non-passivated tinplate in order to substantially remove all of the oxide formed by tin in its highest oxidation state (SnCy).

The Applicant has observed that tin oxide layer thickness values of less than 6 mC/cm ² promote the adhesion of the passivation layer containing zirconium applied in step b. Preferably, the thickness of the tin oxide surface layer obtained at the end of the treatment of step a is less than or equal to 5.0 mC/cm ² , more preferably it is less than or equal to 4 mC/cm ² , even more preferably it is less than or equal to 2.5 mC/cm ² , even more preferably it is less than or equal to 1.5 mC/cm ² , even more preferably it is comprised in the range 0.1-2.5 mC/cm ² . Indeed, it was observed that a thickness of the tin oxide layer in the aforesaid ranges, in particular in the range 1.5-4 mC/cm ² , promotes a more uniform deposition of the passivation layer containing zirconium applied in step b.

For the purposes of the present invention, the thickness values of the tin oxide layer expressed in mC/cm ² (before or after step a) are considered to be determined by the method described in the Examples.

The aforesaid thickness values of the tin oxide can be obtained in step a by selecting the ion concentration in the electrolytic solution, the pH, the temperature and the current density applied in relatively wide ranges of values.

Preferably, the sulphate of an alkali metal of the electrolytic bath of step a is selected from sodium sulphate, potassium sulphate or mixtures thereof.

Preferably, the concentration of the alkali metal sulphate in the bath is in the range 10-70 g/1, more preferably in the range 20 - 50 g/1.

Preferably, the cathodic electrolytic treatment is carried out in the aforesaid solution in substantial absence of carbonate ions, that is in the absence of specially added carbonate and bicarbonate ions.

Preferably, the cathodic electrolytic treatment is carried out with an electrical current density in the range 2-50 A/dm ² , more preferably in the range 5 - 30 A/dm ² , even more preferably in the range 5 - 20 A/dm ² .

Preferably, the duration of the cathodic electrolytic treatment is in the range 0.2-2.0 seconds, more preferably in the range 0.3-1 seconds.

The cathodic electrolytic treatment is carried out with a direct electric current.

Preferably, the temperature of the electrolyte solution is in the range 20 - 80 °C, more preferably in the range 40 - 60 °C, even more preferably in the range 30 - 60 °C.

Preferably, the pH of the electrolyte solution used in step a is in the range 1.5 - 7, more preferably in the range 1.8 - 6, even more preferably in the range 1.8 - 5.5, even more preferably in the range 2 - 5. The pH of the solution can be adjusted to the desired value by the addition of a non-oxidising or reducing acid, preferably by the addition of sulphuric acid.

The Applicant has surprisingly observed that the aforementioned pH conditions ensure that in step a the surface of the tinplate is uniformly exposed to reducing or at least non-oxidizing conditions, which promote substantially complete removal of or otherwise reduction of the tin oxide SnCy and prevent re-oxidation of the underlying tin before it is coated with the zirconium- containing passivating layer in step b. At the aforesaid pH conditions, it also promotes rapid passivation of the surface by the electrolyte solution in step b.

This makes it possible to prevent the surface of the tinplate coming out of step a containing imperfectly treated areas, i.e., areas in which tin in the form of SnCt is still present, on which the deposition of zirconium in step b would be less effective, consequently giving rise to imperfections in the passivation layer.

In one embodiment, the above non-oxidising or mildly reducing conditions can also be achieved by the addition of an oxygen absorber or oxygen scavenger to the electrolyte solution of step a. As is well known, the oxygen absorber is a substance that can react with dissolved oxygen in the water to form soluble oxidised species and thus prevent undesirable oxygen reactions with the tin.

Preferably, the oxygen absorber is selected from: SO3 ² - sulphite ions, HSC>3 hydrogen sulphite ions and mixtures thereof. These ions can be introduced into the electrolyte solution in the form of alkali metal salts, preferably in the form of sodium salts. Sulphite and hydrogen sulphite ions in water react with dissolved oxygen to form sulphate ions.

Preferably, the total concentration of sulphite and hydrogen sulphite ions in the electrolyte solution is in the range 0 - 20 g/1, more preferably in the range 0 - 10 g/1, even more preferably in the range 1 - 10 g/1.

It has also been observed that a partial re oxidation of the surface of the tinplate coming out of step a can also occur during the transit of the tinplate from the apparatus used for the implementation of step a to the apparatus used for the implementation of step b, due to the exposure of the tinplate to the air between the two apparatuses. Advantageously, the extent of such re-oxidation can be controlled or substantially eliminated by providing between step a and step b at least one step of washing the tinplate with water or an aqueous solution under non-oxidising or mildly reducing conditions.

The above non-oxidising or mildly reducing conditions may be obtained by using water or an aqueous solution having one or more of the following characteristics for washing:

- temperature in the range 50 °C - 90 °C;

- pH in the range 2 - 6, preferably 2 - 4;

- presence of at least one oxygen absorber in solution.

The pH can be controlled, for example, by adding sulphuric acid or phosphoric acid to the aqueous solution.

Preferably, the oxygen absorber is selected from: SO3 ² - sulphite ions, HSC>3 hydrogen sulphite ions and mixtures thereof.

Preferably, the total concentration of sulphite and hydrogen sulphite ions in the tinplate washing solution is in the range 0 - 10 g/1, more preferably in the range 0.5 - 8 g/1.

In a preferred embodiment of the process according to the present invention, the aforementioned tinplate washing step, after step a and before step b, is carried out when the pH of the aqueous solution in step a is in the range 5 - 7, more preferably in the range 6 - 7.

It is noted that, in general, washing the tinplate with water at the end of a treatment step helps to remove any residues of the solutions used for the treatments or other impurities present on the surface of the tinplate. However, considering that in the process according to the present invention also the electrolytic solution used in step b is based on sulphate anions, the possible entrainment of these anions from step a to step b does not represent a criticality and, therefore, can be eliminated. For example, washing of the tinplate between step a and step b may advantageously be omitted when the pH of the aqueous solution in step a is less than 5, preferably less than 4, more preferably in the range 2 - 4.

In step b the tinplate treated in step a is subjected to at least one cathodic electrolytic treatment in a bath formed by an aqueous solution containing at least sulphate ions and zirconium ions. The purpose of step b is to form a passivation layer containing zirconium on the tin oxide surface layer present on the tinplate after step a. The treatment of step b allows the deposition of a passivation layer containing zirconium mainly in the form of oxide or hydroxide.

After step b, the amount of zirconium in the passivation layer containing zirconium is less than 12 mg/m ² , preferably it is in the range 4-10 mg/m ² . In a preferred embodiment, the amount of zirconium in the passivation layer containing zirconium is less than 6 mg/m ² , preferably in the range 2-6 mg/m ² , more preferably in the range 4-6 mg/m ² . For the purposes of the present invention, the mass per unit area of the passivation layer containing zirconium is considered to be determined by X-ray fluorescence absorption spectroscopy.

In a first embodiment the electrolytic solution used in step b is an aqueous solution of zirconium sulphate.

In a second embodiment, the electrolytic solution used in step b is an aqueous solution containing zirconium sulphate and alkali metal sulphate, preferably sodium.

In both of the above embodiments, preferably the concentration of zirconium ions in the electrolyte solution is in the range 0.1-10.0 g/1, more preferably in the range 0.5-4.0 g/1, even more preferably in the range 0.5-2.0 g/1.

Preferably, the concentration of alkali metal sulphate in the electrolytic solution, where present, is in the range 5-60 g/1, more preferably in the range 10- 50 g/1.

It has been observed that the presence of sodium sulphate in the electrolytic solution of step b in addition to zirconium sulphate promotes the control of the zirconium deposition, thus being able to obtain a more uniform passivation layer, even in the presence of possible variations in the density of the electric current.

It was also observed that when steps a and b are carried out under the following conditions:

- the weight ratio of the alkali metal sulphate present in the solution of step a to the alkali metal sulphate present in the solution of step b is in the range from 0.5 to 2.5,

- the pH of the aqueous solution used in step a is in the range 1.8 - 5.5, preferably 2 - 5, re-oxidation of the tin surface in transit between steps a and b is effectively prevented, even without washing and with only squeezing the tape, due to the excellent compatibility of the solutions used in steps a and b.

Preferably, in a first embodiment the pH of the electrolyte solution used in step b is in the range 1.4 - 4, more preferably in the range 1.6 - 3. Preferably, in a second embodiment the pH of the electrolyte solution used in step b is in the range 1.4 - 2.6, more preferably in the range 1.6 - 2.5. The pH of the solution can be regulated, for example, by adding an aqueous solution of sulphuric or phosphoric acid.

Preferably, the temperature of the electrolytic solution used in step a is in the range 20 - 60°C, more preferably in the range 30 - 50°C.

Preferably, the cathodic electrolytic treatment of step b is carried out with an electric current density in the range 2-50 A/dm ² , more preferably in the range 5- 30 A/dm ² .

Preferably, the cathodic electrolytic treatment has a duration in the range 0.3-5.0 seconds, more preferably in the range 0.5-2.0 seconds.

The cathodic electrolytic treatment is carried out with a direct electric current.

At the end of step b, the passivated tinplate is subjected to step c of oxidation of the passivation layer to obtain an oxidised passivation layer. The aim of the treatment is to complete the oxidation of the tin, zirconium and zirconium oxides on the surface of the tinplate, bringing the Sn into its highest oxidation state (from Sn ²⁺ to Sn ⁴⁺ ), thus forming stable and poorly reactive oxides. The oxidation treatment also reduces the amount of tin hydroxides on the surface of the tinplate.

The oxidation treatment can be carried out, in a first embodiment, by placing the tinplate from step b in contact with an oxidising aqueous solution.

The oxidising aqueous solution comprises water and at least one oxidising substance capable of oxidising Zr and Sn. Preferably, the oxidising aqueous solution comprises an oxidising substance selected from gaseous oxygen, hydrogen peroxide, percarbonate salt, hydrogen- percarbonate salt, permanganate salt, persulphate salt and mixtures thereof.

Preferably, in a first embodiment the total concentration of oxidising substances in the oxidising aqueous solution is in the range 1 - 20 g/1, more preferably in the range 2 - 15 g/1.

Preferably, in a second embodiment, the total concentration of oxidising substances in the oxidising aqueous solution is in the range 3 - 20 g/1, more preferably in the range 4 - 15 g/1.

In the case of oxygen, this can be insufflated in gaseous form into the oxidising solution. This can be done using techniques and devices known to a person skilled in the art of steel surface treatments (e.g., pickling).

The oxidation treatment can be carried out by spraying the oxidising aqueous solution onto the surface of the passivated tinplate. Alternatively, the treatment can be carried out by immersing the passivated tinplate in the oxidising solution. Preferably, the immersion duration is in the range of 0.1-3 seconds.

Preferably, the aqueous solution used in step c contains sulphuric or phosphoric acid in addition to the oxidising substance.

In another embodiment, the aqueous solution used in step c is a buffer solution capable of keeping the pH as constant as possible at the interface between the aqueous solution and the tinplate. For example, the buffer solution may be a solution comprising the species H3PO4, NaH ₂ P0 ₄ and Na ₂ HP0 ₄ .

The pH of the aqueous solution of step c can also be kept stable by the presence in solution of one or more compounds selected from: at least one of: borate salt, phosphate salt, carbonate salt or mixtures thereof, said salts being preferably salts of an alkali metal.

If carbonate salts are used, it is advisable to reduce the presence of residual carbonate species on the surface of the passivated tinplate at the end of the oxidation step in order to facilitate the adhesion of any further coatings, for example by washing the surface with water.

In another embodiment, the oxidation step involves an anodic electrolytic treatment of the tinplate (i.e., an electrolytic treatment in which the tinplate acts as the anode).

The anodic electrolytic treatment is carried out in an aqueous, acidic, neutral or basic electrolytic solution.

In a preferred embodiment, the aqueous electrolytic solution comprises sulphate ions and alkali metal ions; in such a case, any sulphate ions and alkali metal ions entrained from step b would not substantially pollute the electrolytic solution.

Preferably, the electrolyte solution does not contain carbonate ions.

Preferably, the concentration of sulphate ions in the electrolyte solution is in the range 20 - 60 g/1.

Preferably, the concentration of alkali metal in the electrolyte solution is in the range 20 - 60 g/1.

In one embodiment, the electrolyte solution is a sodium sulphate solution.

Preferably, the pH of the electrolytic solution used in step b is in the range 2.5-11.5.

In one embodiment, the pH of this solution is preferably in the range 3-7, even more preferably in the range 4-7. It has also been observed that when oxidising solutions are used in step c, preferably the pH of the oxidising solution is in the range 3-10.5, more preferably in the range 3-7.

If step c is carried out by anodic oxidation, preferably the pH of the electrolyte solution of step c is in the range 7-11.

Preferably, the temperature of the electrolytic solution used in step c is in the range 30-60°C, more preferably in the range 40-50°C.

Preferably, the anodic electrolytic treatment of step c is carried out at an electrical current density greater than or equal to 0.1 A/dm ² and less than or equal to 25 A/dm ² . In one embodiment, the current density is preferably in the range 3-25 A/dm ² , more preferably in the range 4-20 A/dm ² .

In another embodiment, the current density is preferably in the range 0.1-20 A/dm ² , more preferably in the range 0.1-10 A/dm ² , even more preferably in the range 0.2-5 A/dm ² .

Preferably, the cathodic electrolytic treatment has a duration in the range 0.2-2 seconds, more preferably in the range 0.3-1.4 seconds.

The anodic electrolytic treatment is carried out with a direct current.

At the end of step c and, optionally, at the end of each of steps a and b the passivated tinplate is preferably washed with water, in particular to remove any sulphate ions and carbonate ions present on the surface, which may adversely affect the adhesion ability of the coatings (whether in paint or ink form or in film form) or may give rise to the appearance of stains on the surface of the tinplate.

The washing with water can be carried out by immersion of the passivated tinplate in water or with spray systems, preferably with hot water (e.g., up to 80°C). The duration of the washing is preferably in the range 0.4-5.0 seconds, preferably in the range 0.3-2.0 seconds. The washing is generally followed by a drying step, for example by exposure to ambient air or by heating.

It has been observed that, in general, the adhesion of the coating to the passivation layer is optimal when there is a residual amount of sulphate ions on the passivated surface of less than about 20 mg/m ² and a residual amount of carbonate ions (i.e., hydrogen carbonate ions + carbonate ions) of less than about 20 mg/m ² .

For the purposes of the present invention, a coating has a degree of adhesion suitable for most applications if the detachment strength is greater than or equal to 60 N/lOmm, determined with the T-peel strength test described in the examples.

For the purposes of the present invention, the concentration of sulphate and carbonate ions is understood to be measured by the method described in the Examples of EP 3626862 A1.

The surface of the passivated tinplate obtainable by the process according to the present invention has specific characteristics which can be highlighted by X- ray photoelectron spectroscopy (XPS) analysis. In the following, reference will be made to the figures below which show:

- Figure 1: XPS spectra of the valence band of: (A) SnC>2; (B) mixture of SnCt and SnO with predominant concentration of SnCp over SnO; (C) mixture of SnO and Sn0 ₂ with predominant concentration of SnO over Sn0 ₂ , as published in "Structural and electronic phase evolution of Tin dioxide" (S. Mahana, P. Sapkota et al., 6/2016);

- Figure 2: XPS spectrum of the oxygen Is orbital relative to tin oxides and related deconvolution (curve fitting), as published in "Spettroscopia elettronica delle superfici" (S. Kaciulis, 1/2005);

- Figure 3: XPS spectrum of the valence band of a passivated tinplate sample subjected to oxidation treatment in accordance with the present invention.

Figures 1 and 2 show that by XPS spectroscopic analysis of tin oxide samples, in particular of the valence band and the Ols orbital, it is possible to distinguish the different oxidised forms of Sn present in the material. In particular, the literature data allow us to assign, in the valence band, the signal with a maximum peak around 2.5 eV to the Sn ²⁺ species (SnO) and the signals with a maximum peak around 4.5 eV, 7.5 eV and 10.6 eV to the Sn ⁴⁺ species (Sn0 ₂ ).

These two types of oxidised Sn are less easily distinguished, however, in the electron spectrum of the Ols orbital (Fig. 2), as the Sn0 ₂ peak centred at about 530.5 eV is very close to that of SnO (about 530 eV). The spectrum of the Ols orbital, however, allows detection of the Sn(OH)2 hydroxide species, represented by the B peak centred at approximately 531.9 - 532 eV. The deconvolution (curve fitting) of the peaks of the XPS signal (Fig. 2) also makes it possible to distinguish a peak centred at approximately 533.5 eV (peak C) corresponding to H2O species adsorbed on the surface of the material.

It has been observed that the passivated tinplate according to the present invention shows a characteristic XPS spectrum. The XPS spectrum of the valence band comprises at least one signal having a maximum peak centred at about 2.5 eV, preferably in the range 2 - 3.5 eV, which, according to literature data, can be associated with SnO species, and two signals having a maximum peak centred, respectively, at about 4.5 eV, preferably in the range 4 - 6 eV, and 10.6 eV, preferably in the range 9.5 - 12 eV, which can be associated with SnCy species.

It was also observed that SnO species were present in lower amounts than Sn0 ₂ species, as detected by XPS analysis. Preferably, on XPS analysis, the area subtended by the curve of the valence band spectrum in the range 0 - 4.5 eV is less than 40%, of the area subtended in the range 0 - 15 eV.

It was also observed that the surface of the tinplate according to the present invention has a relatively low amount of hydroxide species detectable by XPS. Preferably, the overall XPS signal related to the binding energy of the electron in the oxygen Is orbital due to oxides (peak A), hydroxides (peak B) and adsorbed H2O (peak C) has a maximum peak centred in the range 530.0 eV - 531.0 eV. In addition, deconvolution of the above signal shows that the area of the Sn hydroxide peak (peak B) is less than 50% of the area of the oxide peak (peak A).

The tinplate passivated with the coating layer comprising zirconium obtained with the passivation process according to the present invention is a suitable support for the application of a coating, such as a layer of paint, a layer of lacquer, a layer of enamel, a layer of ink or a polymeric material film.

Examples of paints that can be used as coatings are: epoxy paints, phenol-epoxy paints, vinyl paints, acrylic paints and polyester paints.

Examples of polymeric films that can be applied as a coating are: polyethylene terephthalate (PET), polypropylene (PP) film.

Before applying any of the above coatings, it is preferable to apply a layer of primer to the passivated and oxidised tinplate and facilitate subsequent adhesion of the coating.

The layer of primer can be made by applying compositions known to a person skilled in the art and generally used to promote the adhesion of paints and coatings to metal surfaces. Typically, the layer of primer is obtained by application of a liquid composition followed by thermal drying to evaporate the water and solvents contained therein.

In a preferred embodiment, the liquid composition comprises titanium ions and/or zirconium ions and is therefore a passivating primer. For this purpose, passivating primers of the type generally used in chemical passivation processes of tinplate known in the art or the like may be used.

In general, the aforesaid type of primer is an aqueous liquid composition comprising titanium and/or zirconium anionic complexes (e.g., TiF ₆ ² hexafluorotitanate anions and ZrF ₆ ² hexafluorozirconate anions) and at least one organic, water-soluble or water- dispersible polymer capable of increasing the wettability and/or adhesion of the aforesaid coatings to metal surfaces, such as for example acryl (co)polymers, acrylamide (co)polymers, polymethylsiloxanes, vinyl phenols, etc. Typically, these aqueous liquid compositions also include phosphoric acid and/or boric acid.

The above liquid compositions comprising anionic complexes of Ti and Zr ions and organic polymers are commercially available, such as the products in the Granodine®, Gardobond®, Bonderite® and Alodine® series. Further information about the chemical composition of the aforesaid compositions used in the art for the chemical passivation of tinplates and, for the purposes of the present invention, usable primarily as primers for the application of coatings, can be found in US 2015/010773 and in I. Milosev and G. S. Frankel, J. Electrochem . Soc., 165 (3) C127-C144 (2018).

Preferably, the liquid composition is applied in such concentrations and amounts that the applied primer layer comprises one or more of the following characteristics :

- titanium in a total maximum amount of 1.4 mg/m ² of metallic Ti, preferably equal to a maximum of 1.2 mg/m ² , more preferably in the range 0.5 - 1.2 mg/m ² , even more preferably in the range 0.5 -1.0 mg/m ² ;

- zirconium in the range 0 - 0.7 mg/m ² , the amount being selected according to the amount of zirconium deposited in step b;

- organic compounds in a total amount less than or equal to 40 mg/m ² of Total Organic Carbon (TOC).

The tinplate passivation process according to the present invention can be carried out using techniques and equipment known to a person skilled in the art, including continuous treatment systems in which the tinplate is advanced at very high speeds (e.g., 200 m/min or higher).

Embodiments of the present invention are provided below solely by way of illustrative example, which must not be considered limiting to the scope of protection defined by the appended claims.

EXAMPLES

The characterisation of the materials described in the present patent application was carried out with the following methods.

1. Measurement of the thickness of the tin oxide layer

The thickness of the tin oxide layer was determined by means of a coulometric method. According to this method, the tin oxide layer is reduced by applying a constant and controlled cathodic current, in an aqueous solution of 0.1% hydrobromic acid (HBr) which is deprived of the present oxygen by insufflation of gaseous nitrogen. The progress of oxide reduction is monitored by measuring the reduction potential. The delivered electric charge (current density * treatment time) to reach the complete reduction is used as a measurement of the thickness of the tin oxide layer.

The test is carried out in an electrolytic cell with a platinum counter-electrode and an Ag/AgCl reference electrode. A cathodic current density of -0.40 A/m ² is applied to the sample and the potential is measured until the potential of the metallic tin is reached, indicating that the reduction has been completed. The measured values are displayed in a graph showing potential v. treatment time, which typically shows a sharp decrease in the potential with a point of inflection at which the treatment time (ti) corresponding to the complete reduction of the tin oxide layer is determined. The thickness value of the tin oxide layer is calculated by means of the equation D [mC/cm ² ] = 0.1 * ti [seconds] * 0.40 [A/m ² ].

2. Characterisation of the surface passivation layer by X-ray Photoelectron Spectroscopy (XPS)

The samples were analysed by XPS technique to determine the position of the valence band binding energy peak and the spectrum of the surface tin Ols electron. The following measurement conditions were adopted:

• ThermoFisher Scientific instrument - Theta Probe

• source = Aik alpha (1486.6 eV) monochromatic source

• analysis spot = 15pm - 400 pm.

The sample is irradiated with a monochromatic X-ray source. Photons enter the material; an electron is ejected from the material with kinetic energy related to the binding energy thereof.

By measuring the kinetic energy of the ejected electron, its binding energy, which is indicative of the chemical element concerned, can be determined according to the formula:

K=hV -EB - F where EB is the binding energy, hv is the energy of the incident photons, K is the kinetic energy of the electron and F is the work function of the spectrometer that can be compensated for and thus the eliminated term.

The XPS photoelectron spectrum provides the number of electrons emitted by the sample following X-ray irradiation as a function of their kinetic energy (measured by the analyser) and hence the binding energy (calculated from the relationship written above). The spectrum consists of a background with overlapping peaks corresponding to the core levels (the innermost levels of the atoms of the elements in the sample) and valence band levels: electrons with low binding energy (0-20 eV) or involved in chemical (valence) bonds.

The technique allows the first atomic layers to be analysed at a depth corresponding to the field of interest for the oxides studied (0-6 nm).

3. Test of staining with Cysteine

The resistance of a passivated tinplate to prolonged food contact was assessed by the cysteine stainability test, carried out both on uncoated (unlacquered) tinplate in order to provide an immediate comparative indication with reference samples passivated with traditional chromate passivation, and on coated tinplate (as described in Section 4).

The test involves immersing square samples (40x40 mm) of passivated sheet in a solution containing, for uncoated and coated samples respectively, 1 g/1 and 3 g/1 of cysteine hydrochloride neutralised to pH 7 with 0.2 M Na3PC>4 and left to boil for 1 hour.

The samples, drilled with a 4 mm hole and inserted into a glass rod at intervals with 15 mm spacers, are introduced into a 1000 ml bottle, made of glass resistant to sterilisation, filled with the Cysteine solution.

The bottle is placed in a suitable pressure vessel which is heated to 110°C and remains there at temperature for 30 and 120 minutes for uncoated and coated samples, respectively. After cooling, washing and drying, the surface appearance of the samples is compared with standards at different levels of stains:

• Level 1 = surface free from stains and discolouration, or blistering in the case of a coated strip

• Level 2 = surface free from stains but with very weak colouring or surface with a few small stains of weak colouring, or sporadic blistering phenomena in the case of a coated band, evenly distributed

• Level 3 = surface with widespread blistering or little widespread blistering in the case of a coated band

• Level 4 = completely stained surface or widespread blistering in the case of coated strip (such as non-passivated tinplate).

4. Coating of the passivated tinplate The tinplate passivated according to the present invention was coated, using a bar coater, with a white epoxy enamel for interior use or a BPANI white polyester enamel for interior use, in both cases applying a thickness of about 13-15 g/m ² (dry); since no significant differences were found in the tests with the two different enamels, it was decided to report the results for the samples coated with epoxy enamel in Table 3. After application, the coating was thermally treated at 200°C for 10 minutes and then allowed to cool to room temperature by exposure to air. The completion of the crosslinking of the coating was verified by the ASTM D 5402 method.

For comparison, the same coating was applied to a sample of a tinplate passivated with commercially available chromium ("Reference").

5. Evaluation of the adhesion strength of the coating on the passivated tinplate

The adhesion strength of the coating applied on the passivated tinplate as described in the previous point 4 was evaluated with the following tests.

5.1 Dry adhesion test

The dry adhesion strength of the coating was evaluated with the ASTM D3359 B method. The method provides for the application of a strip of adhesive tape on the surface of the coating, on which a grid was previously engraved, and after 15 minutes of contact for its removal by a quick tear. The extent of the defects visible to the naked eye caused by the tear was evaluated using a scale of values from 0 to 5, which were assigned based on the percentage of the damaged surface area (0 = no visible defect; 5 = more than 50% of the surface area has visible defects).

5.2 Cathodic wet adhesion test

The wet adhesion strength of the coating was evaluated by subjecting a sample of a passivated and coated tinplate on which a grid in an area of 4 cm x 4 cm was previously engraved, to immersion in a solution containing citric acid (0.1 M, pH = 3) with application of a cathodic polarisation of -2 V, for 30 minutes, at 25°C. The sample was then washed with distilled water and dried. The adhesion strength of the coating was assessed using the ASTM D3359 B method described in point 5.1 (0 = no visible defects; 5 = more than 65% of the surface area has visible defects).

5.3. Coating detachment strength ("T-peel test") The detachment strength of the coating was determined by a comparative test derived from the ASTM D1876-08 method, which was modified as described below.

Two passivated and coated tinplate sheets having dimensions of 100 mm x 10 mm were glued together by means of an epoxy structural adhesive (3M - EC 923 B/A). The adhesive was applied to the sheets for a length of 30 mm, so as to leave free two 70 mm long ends (bent at 90° to each other so as to form a "T") to be fixed to the traction machine.

During the test, the tensile load applied by the machine at the two ends necessary to obtain a separation speed of the two sheets equal to 10 mm/min was recorded. The result is expressed in the N/10 mm unit. For each material, the test was carried out on three samples, extrapolating a final average value that was compared with that relative to a sheet sample passivated with industrial chrome. In addition to the load it was evaluated whether the detachment was of the "cohesive" (within the layer of structural adhesive) or "adhesive" type (between the coating and tinplate). If the breakage is cohesive, it can be concluded that the test has measured the average adhesive breakage force over the entire detachment range and that the adhesion of the coating is above this value. Considering 60 N/lOmm as a threshold value for a satisfactory adhesion, an adhesive with a greater adhesion value was selected.

The same test was repeated by gluing two passivated and coated tinplate sheets measuring 100 mm x 95 mm and then trimming them to 25 mm width. The choice of larger sheets, trimmed after gluing and drying, aims to minimise possible influences on the measurement of the applied tensile load due to the uneven application of the glue on smaller sheets (10 mm). In these tests, the tensile load applied by the machine to the two ends required to achieve a separation speed of 10 mm/min of the two sheets was recorded, thus expressing the result in the unit N/25 mm.

6. Passivation and oxidation of tinplate

6.1 Step a - Removing tin oxide

A first series of samples (no. 1-6, 8) of a (not passivated) tinplate having a layer of tin with mass per unit area 2.8 g/m ² on both faces was passivated using the process according to the present invention. The tinplate was subjected to a cathodic electrolytic treatment to partially remove the tin oxide layer (step a). The operating conditions of step a adopted for each sample are shown in Table 1 below.

For comparative purposes, Table 1 shows the operating conditions of a comparative sample (No. 7) prepared as described in EP 3626862 A1.

Table 1 - Removal of tin oxide (step a)

(i): thickness of the tin oxide layer on the tinplate at the end of step a

*: comparative sample The results in Table 1 show that under the operating conditions shown, it is possible to achieve tin oxide layer thicknesses on the tinplate in the range from 0.25 mC/cm ² to 2.3 mC/cm ² .

5.2 Step b and c - Deposition of the passivation layer comprising Zr and oxidation

Samples 2, 3, 5, 6, 7 and 8 of Table 1 were then subjected to a subsequent cathodic electrolytic treatment for zirconium deposition to form the passivation layer (step b) in a bath containing zirconium sulphate and sodium sulphate.

Before step b, some samples underwent an intermediate wash (L) by immersion in demineralised water (T = 80°C, duration = 0.4 s), others were only squeezed (S) by passing them through squeeze rollers. Samples 3, 5 and 6 were divided into two parts, one of which (samples 3.2, 5.2 and 6.2) was subjected to the oxidation treatment in accordance with the present invention. Samples 3.2 and 5.2 underwent chemical oxidation by immersion in an aqueous solution acidified to pH 3.5 using sulphuric acid and containing hydrogen peroxide, while sample 6.2 underwent anodic oxidation treatment in an electrolyte containing 50 g/1 of sodium hydrogen phosphate buffer at pH 8 and a current of 5 A/dm ² . Sample 8 was subjected to an oxidation treatment by immersion for 0.4 seconds in water containing dissolved oxygen obtained by air insufflation. The step b and c treatment conditions adopted for each sample are shown in Table 2 below.

Table 2 - Cathodic electrodeposition of the passivation layer comprising zirconium and oxidation (step b and c)

*: comparative sample (L): washing by immersion in water at T = 80°C

(L*): washing by immersion in water at T = 50°C (S): squeezing of the tape

The results of Table 2 show that by varying the operating conditions of step b it is possible to obtain a zirconium-based passivation layer with mass per unit area in an optimum range with different solution concentrations, different current densities. 7. Characterisation of passivated tinplate

The samples in Table 2 and the 'Reference' sample (chromium passivated tinplate) were tested with cysteine. After being coated with epoxy enamel as described in point 4, the same samples were subjected to the dry adhesion test, to the cathodic wet adhesion test and to the T-peel test. The same tests were repeated after application of a polyester enamel instead of an epoxy enamel and the results were comparable.

Samples 3.2, 5.2 and 6.2 were also characterised by XPS spectroscopy.

The results of the characterisation tests are shown in Table 3 below.

Table 3 - Cysteine test, coating adhesion test and XPS analysis

⁽¹⁾ C = cohesive detachment

⁽²⁾ ratio of the area subtended by the valence band spectrum curve in the range 0 - 4.5 eV to the area subtended in the range 0-15 eV;

5 ⁽³⁾ ratio of the area of peak B relative to Sn hydroxides to the area of peak A relative to Sn oxides.

The data in Table 3 show that the coating applied to the tinplate passivated with a passivation layer containing Zr in an amount less than 6 mg/m ² , in the absence of oxidation treatment, has a slightly lower resistance to cysteine than the reference sample.

Coatings applied to passivated materials in accordance with the present invention (samples 3.2, 5.2 and 6.2), i.e., in the presence of oxidation treatment, instead show a resistance to cysteine equivalent to that of the reference sample.

The data in Table 3 also show that the coating applied to the passivated materials in accordance with the present invention exhibits optimal adhesion for most tinplate applications, the T-peel test having shown cohesive detachment (detachment of the adhesive and no detachment of the coating) in all cases. The adhesion is also comparable to that of traditional chrome tinplates.

The materials passivated in accordance with the process according to the present invention also have a greater resistance to the detachment of the coating with respect to the reference sample when subjected to the cathodic wet adhesion test and a resistance comparable to the reference sample in the case of the dry adhesion test.

The results of the XPS analysis indicate the prevalence of Sn ⁴⁺ species over Sn ²⁺ species.