ES2644868T3 - Method for the production of tinplate and product produced with it - Google Patents

Description

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DESCRIPTION

Method for the production of tinplate and product produced with it

The invention relates to a method for forming a passivation layer on an article that has at least one flattened surface that has excellent resistance to sulfur staining and an article produced by said method.

Tin is used to protect the steel base against corrosion both externally (aerobic conditions) and internally when in contact with food (anaerobic). Under the expected anaerobic conditions within a can of internally processed simple food, the tin will normally behave as a sacrificial anode, which dissolves very slowly while protecting the steel base from corrosion, and creates a reducing environment in the can. It is this mechanism that has allowed the smooth tin can to maintain its long history and proven track record of providing healthy food on a round anus base and secure storage for long periods of time.

Some foods, especially meat rich in protein and fish and, to a lesser extent, vegetables (for example peas, beans, corn, etc.) contain natural sulfur compounds. These can react with a smooth surface of the tin to give a purple-black stain of the tin sulfide. Although the stain is harmless, it can be used to change the passivation of the tin surface, which, in turn, could alter the rate of tin absorption. Although a general increase in passivation is more likely to decrease tin absorption, localized areas of staining can have a detrimental effect, especially if a corrosion accelerator, such as oxygen, is also present. The degree of sulfide staining is also influenced by the pH, the time of the procedure, and the temperature and the presence of certain cations.

Passivation refers to the chemical treatment applied after the deposition of tin which stabilizes the surface characteristics of the tin by controlling the formation and growth of tin oxide. Passivation treatments can be electrochemical or chemical. Electrochemical treatments involve the use of an external electric current. At present, cathode dichromate (CDC) treatments are generally applied. A CDC treatment is an electrochemical passivation treatment. The chromates are based on hexavalent chromium and are currently considered hazardous substances potentially harmful to the environment and a risk in terms of worker safety. Consequently, intensive research efforts are underway to develop passivation treatments free of hexavalent chromium, also denoted as Cr (VI) free passivation treatments.

Recent developments in Cr (VI) free passivation treatments have resulted in renewed attention to combat sulfide staining because these new alternatives developed for cathode dichromate treatments struggle to offer the same performance in terms of resistance to staining with sulfides.

The most common method to improve resistance to staining with Cr (VI) free passivated tin sulphide is to increase the thickness of the passivation layer itself (for example, using procedural conditions that lead to thicker applied wet films, or using more concentrated treatment solutions, or using a longer treatment time, etc.). This leads to improved barrier properties, thus suppressing the formation of sulfur stains. However, the passivation layers are mechanically weak and the risk of cohesive failure within the passivation layer increases with thickness. The cohesive failure of the passivation layer (in particular during the heavy deformation found in, for example, manufacture of cans) leads to the loss of adhesion and delamination of organic coatings that are applied on the passivation layer.

WO2012097926 discloses a method for electrically passivating the tin by anodic polarization in an alkaline aqueous electrolyte comprising a water soluble silicate of composition M2O.nSiO2, in which M is an alkali metal ion or a quaternary ammonium ion and n is a natural number between 0.8 and 7.

US3616307 discloses a method of anodizing a ferrous article coated with tin by subjecting the article as an anode to an electrolysis in an aqueous electrolyte comprising a phosphate solution in which the phosphate radical is present in an amount between 2 and 90 g / Ly and a hypophosphite in whose radical hypophosphite is present in an amount between 0.5 and 50 g / l at a current density of 8 to 100 asf (86 to 1076 A / m2). The tin oxide layer must be additionally coated with a thin film of release agent and cooked at 450 ° F (232 ° C) for 2 hours to cure the release agent and stabilize the oxide film.

US 2010/181201 discloses an electrolyte to passivate stained steel consisting of a mixture of organic hydroxyl acids and organic phenol acids. The aim of the invention is to provide a method for improving the tincture resistance with sulfide of the tin which has been passivated using Cr (VI) free passivation systems.

Another object of the invention is to provide a method for improving tincture resistance with tin sulphide that has been passivated using Cr (VI) free passivation systems that can be integrated into existing electrolytic standaline lines.

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Yet another objective of the invention is to provide a standalone article that has excellent resistance to sulfide staining that is at least similar to that of a cathode dichromate passivation treatment.

One or more of the objects of the invention are achieved by a method according to claim 1. Preferred embodiments are provided in the dependent claims.

The invention solves the problem of poor tincture resistance with tin sulphide by subjecting the tin to an anodic treatment in a suitable aqueous electrolyte which is carried out within certain limits of the treatment time, current density and past total charge , in order to be effective in achieving the required sulfur staining resistance. A very thin layer of tin oxide on the surface of the tin is formed by the electrochemical anodic treatment. This layer of tin oxide provides the improvement of resistance to staining with sulphide and together with the subsequent chemical free passivation treatment of Cr (VI) provides a tin that delivers the same, or better performance than the known tin treated with CDC

An essential element of the invention is the thickness of the layer of tin oxide (D), which is expressed in Coulomb / m2 and represents the total load necessary to reduce the layer to metallic tin. The thickness of the tin oxide layer is related to the anodic treatment time (t) and the current density (A) by D = E x A xt, where E is the efficiency of the electrochemical reaction, and in which D is at least 30 C / m2. The efficiency therefore represents the proportion of the thickness D of the oxide layer produced at the applied charge density (A x t), and can be estimated by representing D as a function of (A x t). Initially, at values of anodic load passed below 50 C / m2, the curve is more or less linear, but with the increase in past anodic load, efficiency E decreases which leads to a slower growth rate of layer of tin oxide, and therefore a slower increase in D. If D <30 C / m2, then the layer of tin oxide is too thin and is not effective to achieve the desired sulfide staining resistance. Therefore, a minimum thickness of 30 C / m2 is required.

The thickness of the tin oxide layer is determined using a coulometric method. The tin oxide layer is reduced by a small cathode current controlled in a 0.1% solution of bromfndric acid (HBr) that is released from oxygen by rubbing with nitrogen. The progress of the oxide reduction is monitored by monitoring the reduction potential, and the past charge (A * t) for the complete reduction serves as a measure of the thickness of the tin oxide layer. For the test, a cylindrical electrolysis cell having a circular opening of approx. 4 cm in diameter at one end. The other end of the cell contains a platinum counter electrode and an Ag / AgCl reference electrode. The test specimen covers the opening, which is sealed using an O-ring to make a waterproof connection of a well-defined area (13.69 cm2 in this case), and is tightened in place using an air pressure cylinder. The cell is connected to the electrolytic solution by a flexible tube so that it can be filled and emptied under a nitrogen atmosphere. A cathode current density of -0.40 A / m2 using a potentiostat-galvanostat is applied to the sample, and the potential is measured until the reduction is completed. A typical potential time curve is shown in Fig. 1, from which the thickness of the tin oxide layer is determined based on the inflection point of the potential coffee at the instant ti. The thickness D of the tin oxide layer, expressed in C / m2, is obtained from D [C / m2] = ti [s] * 0.4 [A / m2].

The only purpose of the electrolyte is to allow anodic treatment, not to deposit foreign species contained in the electrolyte on the surface of the substrate. The electrolyte pH may not be too low, not less than pH = 8, otherwise the efficiency of the electrochemical reaction becomes too low and then the process cannot be incorporated into the existing high productivity process lines. Also, the pH of the electrolyte may not be too high, that is, not higher than pH = 11.5 because this will cause the dissolution of the tin layer in the electrolyte. Preferably, the pH is not less than 8.5. A suitable maximum pH value is 11 or even 10.5.

According to the invention, the electrolyte comprises monoatomic cations of Group 1 or 2 of the periodic table or polyatomic cations, and polyatomic anions. The elements of Group 1 and Group 2, according to the new IUPAC numbering, comprise alkali metals and alkaline earth metals. It is important that the electrolyte does not contain mono-atomic halogen anions (Group 17) such as Cl-, F- because these anions prevent the tin oxide layer from forming.

After the anodic treatment, the article is optionally rinsed and dried before passing it to the next stage, that is, the Cr (VI) free passivation treatment. The need to rinse the anodically treated tin depends on the exact nature of the Cr (VI) free passivation system, certain systems will be more susceptible than others to the contamination of the electrolyte that is present on the surface of the tin.

Finally, the Cr (VI) free passivation system is applied to the tin surface treated anodically by application techniques that are common for such passivation systems. Suitable application techniques include: immersion, immersion with runoff rollers, application by rotor atomization, application of rotor atomization supported by the use of a smoothing roller, application by atomization, application by atomization of runoff, application by means of runoff atomization roller coating systems, slot coating application, slot curtain coating, etc. After the passivation step, the strip is dried and passed for the final processing steps such as greasing, rolling, 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 / dm2, preferably A> 1.0 A / dm2) in combination with

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a short treatment time (t <1 s) is preferred in view of its processing capacity in a high speed tin line. The interchangeability of A and t in the anodic treatment, which is further demonstrated in Example 1 and in Figures 2 and 3, implies that the procedure can be operated at short treatment times, adjusting the current density applied accordingly. Therefore, the method according to the invention can be used in industrial tinning lines operating at linear speeds greater than 300 m / min at speeds of up to 1000 m / min. In addition, the treatment time t is determined not only by the speed v of the line but also by the effective length or the 'length of the anode' L of the treatment section, according to t = L / v, which indicates that the processing window can be further extended by the appropriate choice of the length of anode L. For example, to deposit a layer having a thickness of 50 C / m2 (assuming E = 1) on a line operating at 600 m / min (10 m / s), a current density A of 1000 A / m2 and a treatment length of 0.5 m will be required. It could also be done at a current density of 100 A / m2 if the treatment length is 5 m. This design and process flexibility is a great advantage of this method.

In one embodiment of the invention, the anodic treatment is carried out in line with or immediately after the electrolytic treatment and in which 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 interval and the most preferable intervals are consistent with high speed processing lines. In one embodiment, the anodic treatment is performed on the line with an industrial electrolytic staining line and in which the current density during the anodic treatment (A) is at least 10 A / m2, preferably at least 50 A / m2 and more preferably at least 100 A m2, and / or at most 4000 A / m2, preferably at most 2000 A / m2 or more preferably at most 1000 A / m2. This interval and the most preferable intervals are consistent with high speed processing lines.

The electrolyte to be used may be an aqueous solution of an acid, a base or a salt. The main function of the electrolyte is to withstand the electrochemical reaction provided by the anodic treatment while the ionic species present in the electrolyte do not take part in the electrochemical modification of the tin surface. Although there is considerable freedom in the selection of a suitable electrolyte, the following ionic species should be avoided:

• monoatomic anions such as fluorides, chlorides, bromides, etc., since these are harmful to form a suitable layer of tin oxide

• cations or containing transition metals, transition metals and post transition metals; Such elements typically exhibit multiple valence states and therefore can easily participate in electrochemical reactions, and can negatively affect the stability of the electrolyte by forming insoluble salts with the available anions, or by forming insoluble oxides.

Based on the above considerations, the preferred electrolyte contains cations of Group 1 (for example Na +, K +) or Group 2 (for example Mg2 +, Ca2 +) of the Periodic Table or polyatomic cations (for example NH4 +), and polyatomic anions (phosphates , borates, sulfates, carbonates and the like). In addition, the anion can be the conjugate base of an organic acid (for example acetates, citrates). Since it is important that the pH is maintained within certain limits, a buffered solution may be used. In addition, the electrolyte may contain other chemical additives, such as surfactants, wetting agents, antifoaming agents, etc. to withstand electrochemical treatment, provided that these additives do not adversely affect the formation of tin oxide.

The anodic treatment of the tinned surface converts the outer outer layer of the tin surface of the metal tin into tin oxide by electrochemical oxidation. The tin oxide layer produced as such (within a certain thickness range) provides a barrier against sulfide staining. However, the tin oxide layer is not sufficiently stable and / or passive in itself and, during prolonged storage in environmental and / or humid conditions, or during thermal treatments such as baking and cooking, will continue to grow until it becomes a thicker tin oxide layer with undesirable properties (poor wettability, yellowish appearance, poor lacquer adhesion).

If we consider the Cr (VI) free passivation system by itself, it will normally provide a stable passivation layer that protects the tin against uncontrolled growth of tin oxides and also provides good adhesion of organic coatings. However, the Cr (Vl) free passivation layer in almost all cases investigated has a poor resistance to sulfide staining.

Through the application of the present invention, a favorable combination of properties is achieved. First, a layer of tin oxide of the correct thickness is applied using anodic treatment under conditions of suitable procedures, and then the layer of tin oxide is passive and / or stabilized against further uncontrolled growth, applying a system of free passivation of Cr (VI) on top of this, through the use of a non-electrolytic application method.

The anodic treatment of the present invention should take place after canning and / or flow fusion and before the application of a Cr (VI) free passivation system.

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The Cr (VI) free passivation system before which the anodic treatment is applied must be a chemical passivation treatment, preferably a so-called non-rinsed procedure, for the application of a dry passivation system without rinsing. Anodic pretreatment is not expected to work in combination with a Cr (VI) free passivation system that applies itself electrolytically. In particular, the anodic pretreatment is not expected to work in combination with a Cr (VI) free passivation system that is applied by a cathode electrochemical procedure, since such a procedure will eliminate the anodic treatment layer by electrochemical reduction.

Appropriate non-rinse free Cr (VI) passivation systems that can be used in combination with the anodic treatment of this invention are, for example:

- all organic systems such as organic acids (oleic acid, abietic acid);

- all organic systems such as acrylates, polyurethane dispersions and other types of thin organic coatings;

- organic / inorganic coupling agents, such as single-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.

Other methods for applying a layer of tin oxide, and potentially to achieve the same technological effect, include chemical oxidation by strong oxidizing agents such as permanganates or peroxides and thermal oxidation. The former is not preferred because strong oxidizing agents are unpleasant for humans and the environment, and the latter method is very extensive and can take several minutes at very high temperatures to achieve a layer of tin oxide of suitable thickness.

The anodic treatment of the present invention is capable of applying the appropriate tin oxide layer on a very short time scale (up to 0.1 s), which uses environmentally friendly chemical solutions, and with very low energy consumption.

Although the method according to the invention also works with a thickness of the tin oxide layer of for example 150 or 200 C / m2, it is preferable that the thickness D of the tin oxide layer is at most 100 C / m2 A value above 100 is not possible to achieve economically in the high speed stamping process and also leads to reduced adhesion of organic coatings subsequently applied. The preferred range for thickness D of the tin oxide layer is therefore 30 to 100 C / m2.

A suitable minimum value of D is 40 C / m2. A maximum value suitable for D from a procedural efficiency point of view is 80 C / m2 or even 60 C / m2.

In a preferred embodiment, the polyatomic anion in the electrolyte is a phosphate, borate, sulfate or carbonate anion. In a preferable embodiment, the cation in the electrolyte is Na +, K + (Group 1) and / or Ca2 + (Group 2) and / or polyatomic, for example NH4 +.

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

In one embodiment, the article is also provided with an organic coating layer such as epoxy-phenolic gold lacquers, white epoxy-anhydrous lacquer, PVC or vinyl organosol coating, polyester lacquers, epoxy-amino or epoxy-technical coatings. amino-aqueous The excellent adhesion of organic coating layers to the passivated tinplate allows this product to be provided as a replacement for systems treated with CDC and subsequently coated with poftmero, thus avoiding the use of chromates completely.

The invention is explained below by means of the following Examples and Non-limiting Figures.

Example 1

This Example describes several laboratory scale tests that aim to demonstrate the anodic treatment using various electrolytes and process conditions with respect to the current density A and the treatment time t.

For all the experimental results presented here, non-passivated tinplate with molten flow was used. The material was produced on an industrial electrolysis line (ETL) line. The applied tin coating weight is approximately 2.8 g / m2. After the fusion of the flow, the amount of free tin is approximately 1.9 g / m2 and the amount of FeSn2 alloy is approximately 1.1 g / m2, which corresponds to approximately 0.9 g / m2 of tin

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in the alloy layer. After canning and melting the flow, the tin strip was passed over the winder while passing through the passivation section of chromate. In this way, un passivated tin is obtained without contamination with chromium residues.

Two electrolytes were used:

• Phosphate solution, which contains 3.56 g / l of KH2PO4 and 7.22 g / l of Na2HPO4.2H2O in deionized water, pH = 7.1;

• Solution of sodium carbonate decahydrate, which contains approx. 1 g / l Na2CO3.10H2O in deionized water, pH = 10.3.

Obviously, while sodium carbonate decahydrate was used in these experiments, the results are equally achievable using anhydrous sodium carbonate or a sodium carbonate hydrate with a different number of water molecules of 10, provided the appropriate pH is obtained. . Before the study, the "natural" tin oxide layer was removed in a so-called cathode soda treatment. The sample was placed in the sodium carbonate solution of pH = 10.3 described above, at a solution temperature of 50 ° C. The tin sample was then connected as a cathode to a galvanostat, and a current density of - 1.0 A / dm2 was applied for 1 second. The sample was then rinsed with deionized water and used immediately for the anodic treatment experiment ("wet to wet" application).

In the anodic treatment experiments, the tin sample was placed in the electrolyte at 50 ° C, and connected as an anode to a galvanostat. Various current densities (in the range of 0.2 - 1.4 A / dm2) and treatment times (in the range of 0.4 - 9.0 s) were used. After treatment, the electrolyte sample was removed, rinsed with deionized water and dried at room temperature. The thickness of the tin oxide layer was then determined as described above.

Figure 1 shows a typical reduction potential curve (V) as a function of time (t) in a tin oxide reduction experiment.

Figure 2 shows the thickness of the tin oxide layer (in C / m2) as a function of the past charge (A * t) in the anodic treatment using the phosphate solution for different current densities (see legend) and time combinations The line of dots serves as a grin for the eye.

Figure 3 shows the thickness of the tin oxide layer (in C / m2) as a function of the past charge (A * t) in the anodic treatment using the sodium carbonate solution for different current densities and combinations of weather. For example, a current density of 120 A / m2 for 0.4 seconds produced the same layer thickness as a current density of 60 A / m2 for 0.8 seconds. The dashed lines serve as a grna for the eye. From the different current densities (see legend) and the associated differences in the treatment time it is clearly visible that the different combinations of time (t) and current density (A) result in the same thickness of the tin layer.

For a given electrolyte, the amount of tin oxide produced in the anodic treatment is a unique function of the past charge. For a given load, it does not matter if this was achieved, for example, by a low current density and a long treatment time or by a high current density and a correspondingly short treatment time. Therefore, the tin oxidation process can be controlled very easily over the total past load and, in an industrial process, it can be made independent of, for example, the speed line. For the sodium carbonate electrolyte with pH = 10.3 (Fig. 3), the relationship between the thickness of the tin oxide layer and the total past charge shows two different regimes. When the past load is low, that is, less than approx. 50 C / m2, a steep linear relationship is observed. The slope of the regression line in this regime is 0.92, which indicates that the efficiency (E) of the oxidation process is 92%. With a higher total past load, that is, greater than 50 C / m2, the oxidation process still progresses with increasing load, but at a much slower rate.

In the phosphate electrolyte with pH = 7.1 (Fig. 2), there is also a linear relationship between the thickness of the tin oxide layer and the total past charge (regardless of current density), but it takes much more charge compared with the pH = 10.3 of sodium carbonate electrolyte to achieve the same thickness of the tin oxide layer. The slope of the regression line for the phosphate electrolyte corresponds to 0.19, which shows the much lower efficiency of the oxidation reaction in this case.

In the following Examples, un passivated tin material, with molten flow, as described above, was subjected to various treatments in a coil-to-coil process line. The line consists of the following sections: unwinder, electrochemical treatment tank, rinsing tank, drying unit with hot air, draining spray application section, second drying unit with hot air and winding. All experiments were performed at a 50 m / min line speed using a 300 mm wide tin coil. In all experiments, the electrolyte was the sodium carbonate solution that had a pH of 10.3 as described above that was maintained at 60 ° C.

In some of the Examples, a Cr (VI) free passivation treatment was applied to the tin using an atomization and runoff application unit. The passivation solution was a commercial product, called

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Granodine ™ 1456, from Henkel AG & Co. KGaA (Dusseldorf, Germany) which includes, among other things, fluoro-titanates, fluoro-zirconates and organic polymers. In the experiments a passivation solution containing 150 g / l of Granodine ™ 1456 in deionized water was used. The passivation solution was atomized on one side of the strip using an Ahlbrandt IQ-140 Rotor Atomization system. The strip was then passed through a pair of rollers (not driven) consisting of a stainless steel coating roller and a polyurethane coated reinforcement roller, in which the applied wet film was homogenized and the applied Kquido in excess of the strip. After passing through the runoff rollers, the strip was heated to 70 ° C using an inductive heater and subsequently passed through an air dryer operated at an air temperature of 90 ° C.

The newly received univated tinplate coils were first passed through the line and given cathode treatment in sodium carbonate using the electrochemical treatment tank. This is done to remove the 'natural' tin oxide layer that has developed on the unpasivated tin material during storage. A total cathode current density of 1.11 A / dm2 was used for 0.72 s, giving a total load density of 80 C / m2. Next, the treated coil was placed again on the unwinder and subjected to the following treatments:

A (Comparative Example 1) - The coil was passed through the line, which includes the electrochemical treatment tank, but no current was applied. No passivation treatment was applied.

B (Comparative Example 2) - The coil was passed through the electrochemical treatment tank, but no current was applied to the electrochemical treatment tank. In the passivation section, approx. 5 ml / m2 of the passivation solution was applied to the strip. After passing through the set of draining rollers and drying unit, the strip is rolled. The homogeneity and thickness of the passivation layer applied was determined using X-ray fluorescence by measuring the surface concentration of the Ti element.

C (Comparative Example 3) - The coil was passed through the electrochemical treatment tank, filled with the sodium carbonate solution as described above, and an anodic current density of 0.83 A / dm2 was used for 0.72 sec, giving a total load density of 60 C / m2. No passivation treatment was applied.

D (Example of the Invention 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 / dm2 was used during 0.72 s, giving a total load density of 20 C / m2. The passivation of Granodine 1456 was applied as described above.

E (Example of Invention 2) - Like the Example of Invention 1 but using an anodic current density of 0.83 A / dm2 for 0.72 s, giving a total load density of 60 C / m2.

F (Example of Invention 3) - Like Example 1 of the Invention but using an anodic current density of 1.39 A / dm2 for 0.72 s, giving a total charge density of 100 C / m2.

The conditions of the various treatments are summarized in Table 1.

Table 1. Condition of different treatments used.

 Treatment

 Anodic pretreatment Anodic charge density 2 (C / m) D (C / m2) Passivation Treatment Thickness of 2 passivation layer (mg Ti / m)

 TO

 Comp example 1 No - 28 No -

 B

 Comp example 2 No - 27 Yes 1.3

 C

 Comp example 3 Yes 60 54 No -

 D

 Example inv. 1 Yes 20 31 Yes 1.4

 AND

 Example inv. 2 Yes 60 44 Yes 1.3

 F

 Example inv. 3 Yes 100 50 Yes 1.3

The yield of several treated tin samples was tested with respect to three properties that are of utmost importance with respect to the final use of tin, such as food cans, can ends, etc .:

• Oxidation of the tin surface under environmental conditions

• Resistance to sulfide staining

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• Adhesion of lacquers after sterilization in food media

To determine the rate of oxidation of the tin surface, the amount of tin oxide present on the treated surface was measured directly after preparing the samples, using the coulometric method described above. Then, the samples were stored at room temperature in dry air for 14 weeks, after which the thickness of the tin oxide layer was determined again. If the increase in the thickness of the tin oxide layer during this period is 10 C / m2 or less, the stability of the tin oxide layer is considered adequate.

To determine the resistance of the tinplate treated to sulfide staining, the prepared samples were subjected to immersion in a solution containing sulfur. Based on extensive research within our laboratories, we have found that the chemical procedure that leads to the formation of tinplate sulfide stains proceeds as follows. Organically bound sulfur, such as is present in, for example, proteins, can be converted, for example, by sterilization procedures for inorganic sulphides, such as the S2- anion. Inorganic sulfides can destabilize the surface of tin oxide and accelerate the dissolution of tin oxides in food media. Once the tin oxide layer dissolves from the surface and the metal tin is exposed to the sulfide-containing medium, the chemical reaction between the tin and the sulfide can take place, which leads to the formation of tin sulfide. In addition, if the reaction continues, a chemical reaction can take place between the sulfide and the exposed iron of the underlying steel substrate, which leads to the formation of iron sulfide. Tin sulphides and possibly iron sulfides present on the surface of the tin are seen as unpleasant dark spots known as sulfide spots.

Our test method for determining resistance to sulfide staining is based on the measurement of the tin oxide dissolution rate during exposure to a sulfide solution. It has been found that the rate of dissolution of tin oxides correlated strongly with the development of sulfide staining in real-life assays. The advantage of our method is that the results can be easily quantified, and it offers a better and faster way to investigate sulfide staining compared to the more commonly used visual observation. A particularly convenient method employs a solution containing 5 g / l of NaOH and 5 g / l of Na2S * 3H2O maintained at 30 ° C. A test specimen (without lacquer) is immersed in this solution for 6 minutes, and then the thickness of the tin oxide layer is determined using the coulometric test method described above. If the thickness of the tin oxide layer after the above-mentioned exposure test is greater than 30 C / m2, the resistance to sulfide staining is adequate.

To determine the performance of lacquer adhesion in various food simulator media, the samples were washed with a white internal lacquer for food cans, epoxy-anhydrous. This lacquer is critical with respect to adhesion and very discriminating in sterilization tests and is commonly used in the evaluation of strict product. The lacquer was applied at a dry coating weight of 10-12 g / m2. The lacquered panels were sterilized at 121 ° C for 60 minutes, in the following media:

1) deionized water

2) solution containing 22.5 g / l of lactic acid in deionized water

3) solution containing 18.7 g / l of sodium chloride and 30 g / l of acetic acid in deionized water

After sterilization, the panels were allowed to cool and dry, after which the lacquer adhesion was immediately tested in a so-called weft and ribbon crossover test in accordance with ASTM D3359 - 09e2 "Standard Test Methods for Measuring Adhesion by Tape Test ". This essay gives a classification in

a scale of 0 (excellent adhesion) to 5 (very poor adhesion) and was done using Scotch No. 610 adhesive tape.

The results of the performance tests described above are given in Tables 2 and 3.

Table 2. Statin oxide stability and resistance to sulfide staining

 Treatment

 Thickness of tin oxide at t = 0 wks (C / m2) Thickness of tin oxide at t = 14 wks (C / m2) Increase in thickness (C / m2) Thickness of tin oxide after the sulfide exposure test (C / m2)

 TO

 Comp example 1 28 47 19 (poor) 20 (poor)

 B

 Comp example 2 27 25 -2 (good) 19 (poor)

 C

 Comp example 3 54 68 14 (poor) 59 (good)

 D

 Example inv. 1 31 39 8 (good) 38 (good)

 AND

 Example inv. 2 45 47 2 (good) 43 (good)

 F

 Example inv. 3 50 57 7 (good) 58 (good)

 Treatment

 Solution 1 (Deionized water) Solution 2 (Lactic acid) Solution 3 (Salt and Acid)

 TO

 Comp example 1 5 4 3

 B

 Comp example 2 1 1 1

 C

 Comp example 3 5 2 4

 D

 Example inv. 1 1 1 1

 AND

 Example inv. 2 3 2 1

 F

 Example inv. 3 1 1 1

The Comp example It shows that no treatment on the entire surface of the tin leads to poor stability to tin oxide, poor resistance to sulfide staining and poor lacquer adhesion. The Comp example 2 shows that a Cr (VI) free passivation treatment stabilizes the surface of the tin oxide, provides excellent lacquer adhesion, but has low resistance to sulfide staining. The Comp example 3 shows that the anodic treatment of the present invention provides a much better resistance to sulfide staining, but since it is applied without further passivation treatment, the tin oxide layer is not stable and the lacquer adhesion is poor.

Examples inv. 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 sulfide staining resistance, a stable layer of tin oxide and good adhesion of the lacquer.

Claims (9)

5 10 fifteen twenty 25 30 35 40 Four. Five 1. A method for forming a passivation layer on a strip of packaging steel provided with at least one tinned surface to form a layer of tin oxide on said stained surface to provide excellent resistance to sulfide staining of the strip of packaging steel, to produce a packaging container or a can for food packaging, where said method comprises the steps of: - subjecting said stained surface to an anodic treatment at a pH of 8 to 11.5 in a suitable aqueous electrolyte, in which the suitable electrolyte is a phosphate solution, borate solution, sulfate solution or a carbonate solution, in which The electrolyte for the anodic treatment does not contain monoatomic halogen anions (Group 17) such as Cl-, F- - in which the thickness (D) of the tin oxide layer, expressed in Coulomb / m2 and representing the charge Total necessary to reduce the layer to metallic tin is related to the time of anodic treatment (t), the current density (A) by means of D = E x A xt, where E is the efficiency of the electrochemical reaction, and in that D is at least 30 C / m2 and at most 100 C / m2; - in which the electrolyte comprises monoatomic cations of Group 1 or 2 of the periodic table or polyatomic cations, and polyatomic anions; - said electrolyte does not comprise a water soluble silicate of the composition M2OnSiO2, in which M is an alkali metal ion or a quaternary ammonium ion and n is a natural number between 0.8 and 7; - in which the anodic treatment is carried out in an industrial tinning line that runs at a line speed greater than 300 m / min; - in which the anodic treatment is carried out in line with or immediately after the electrolytic staining, and in which the time (t) of anodic treatment is at most 5 seconds; - optionally rinse and dry the anodically treated stained surface - subject the treated anodically treated surface to a chemical passivation treatment free from the use of hexavalent chromium compounds - drying of the anodically treated passivated surface - final processing such as greasing, winding, cutting, etc. 2. The method according to claim 1, wherein D is at most 80 C / m2. 3. The method according to any one of the preceding claims wherein the pH value of the electrolyte for the anodic treatment is not less than 8.5. 4. The method according to any one of the preceding claims wherein the pH value of the electrolyte for the anodic treatment is not greater than 11.0, preferably not greater than 10.5. 5. The method according to any one of the preceding claims wherein the electrolyte for the anodic treatment contains one or more of Na +, K +, Mg2 +, Ca2 +, NH4 +. 6. The method according to any one of the preceding claims wherein the electrolyte for the anodic treatment does not contain • monoatomic anions such as fluorides, chlorides, bromides and / or not • cations of or containing internal transition metals, transition metals and post transition metals. 7. The method according to any one of the preceding claims wherein the electrolyte for the anodic treatment consists mainly of an aqueous solution of sodium carbonate, preferably at a pH value between 10 and 11.5. 8. The method according to any one of the preceding claims wherein the time (t) of anodic treatment is at most 2 seconds, preferably between 0.05 seconds and 1.5 seconds. 9. The method according to any one of the preceding claims in which the anodic treatment is carried out in line with an industrial electrolytic staining line, and in which the current density during the anodic treatment (A) is at least 10 A / m2, preferably at least 50 A / m2 and more preferably at least 100 A / m2 and / or at most 4000 A / m2, preferably at most 2000 A / m2 or more preferably at most 1000 A / m2.