BR112015011731B1 - COATED STEEL SUBSTRATE FOR PACKAGING APPLICATIONS AND ITS PRODUCTION PROCESS - Google Patents

Abstract

chromium-chromium oxide coatings applied to steel substrates for packaging applications and a method for producing said coatings. The invention relates to a coated steel substrate for packaging applications, said substrate containing 1. an uncoated packaging steel sheet with single recrystallization annealing, or 2. an uncoated cold-rolled and annealed steel sheet. recovery process, where one or both sides of the substrate is coated with a layer of metallic chromium - chromium oxide coating produced in a single-step process using a trivalent chromium electrodeposition process, and a process to obtain the mentioned coated steel substrate.

Description

[001] The invention relates to chromium - dechromium oxide (Cr - CrOx) coatings applied to steel substrates for packaging applications and a method for producing the same.

[002] Products from the tinplate line include tinplate, electrolytic chromium coated sheet (ECCS, also referred to as tin free steel or TFS) and black sheet, the uncoated steel. Packaging steels are usually supplied as tinplate, or as ECCS to which an organic coating can be applied. In the case of tinplate, this organic coating is generally a varnish, while in the case of ECCS polymer coatings such as PET or PP are increasingly used, as in the case of Protact®.

[003] Tinplate is characterized by its excellent corrosion resistance and weldability. Tinplate is supplied within a range of coating weights, typically between 1.0 and 11.2 g/m2, which are generally applied by electrolytic deposition. Currently, most tinplate is post-treated with fluids containing hexavalent chromium, Cr(VI), using either a dip-assisted or electrolytically-assisted application process. The purpose of post-treatment is to passivate the tin surface to stop or reduce the growth of tin oxides, because very thick oxide layers can eventually lead to problems regarding the adhesion of organic coatings such as varnishes. It is important that the passivation treatment must not only suppress or eliminate the growth of tin oxides, but must also be able to retain or improve the adhesion levels of the organic coating. The passivated outer surface of tinplate is extremely thin (less than 1 micron thick), and consists of a mixture of tin and chromium oxides.

[004] ECCS consists of a black plate product that has been coated with a layer of metallic chromium covered with a film of chromium oxide, both applied by electrolytic deposition. ECCS excels at adhering to organic coatings and retaining coating integrity at temperatures exceeding the melting point of tin (232°C). In those cases tin coated material cannot be used. This is important for producing polymer coated packaging materials because during the thermoplastic coating application process the steel substrate can be heated to temperatures exceeding 232°C with the actual maximum temperature values used being dependent on the type of thermoplastic coating applied . This thermal cycle is necessary to allow the initial heat setting/bonding of the thermoplastic to the substrate (pre-heat treatment) and is often followed by a post heat treatment to modify the polymer properties. The chromium oxide layer is believed to be responsible for the excellent adhesion properties of thermoplastic coatings such as polypropylene (PP) or polyester terephthalate (PET) to the ECCS. ECCS can also be supplied within a range of coating weights for both Cr and CrOx coatings, typically ranging between 20-110 and 2-20 mg/m2 respectively. The ECCS can be delivered with the same coating specification for both sides of the steel strip, or with different coating weights per side, the latter case being referred to as differentially coated strip. The production of ECCS currently involves the use of solutions based on chromium in its hexavalent state, also known as hexavalent chromium or Cr(VI).

[005] Hexavalent chromium is currently considered a hazardous substance that is potentially harmful to the environment and poses a risk in terms of worker safety. There is, therefore, an incentive to develop alternative metallic coatings that are capable of replacing conventional tinplate and ECCS, without the need to resort to the use of hexavalent chromium during production.

[006] It is an object of the invention to provide an alternative to the use of hexavalent chromium for the passivation of tin sheets.

[007] It is an object of the invention to provide an alternative to conventional tinplate to improve product properties, for example, in terms of corrosion performance and resistance to sulfur stains.

[008] It is an object of the invention to provide an alternative substrate to tinplate and ECCS that provides excellent dry adhesion to organic coatings in combination with corrosion protection that is not based on the use of hexavalent chromium during production.

[009] One or more of these objectives are achieved by providing a steel substrate for packaging containing:

[0010] 1. - a steel substrate for packaging with annealing of single or double reduced recrystallization (i.e. black plate), or

[0011] 2. - a black plate cold-rolled and with recrystallization annealing,

[0012] where one or both sides of the substrate is coated with a metallic chromium - chromium oxide (Cr - CrOx) coating layer produced in a single coating step using a trivalent chromium electrodeposition process.

[0013] The steel substrate for packaging is preferably provided in the form of a strip.

[0014] For the production of ECCS, three types of chromium coating processes are generally in use around the world. The three processes are "one-step vertical process" (V-1), "two-step vertical process" (V-2) and the "one-step high current density horizontal process" (HCD) and based on Cr(VI) electrolytes. ECCS specifications are standardized under Euronorm EN 10202-2001. The two-step vertical process uses a sulfuric acid-free CR(VI) electrolyte to apply the chromium oxide layer in the second step. Sulfuric acid is required for good efficiency in metallic chromium application and is therefore always used for the metallic chromium coating step in these processes. The "one-step vertical process" and the "one-step high current density horizontal process (HCD)" always have sulfate in the oxide layer because metallic chromium and chromium oxide are produced simultaneously in the same electrolyte (Boelen, thesis TU Delft 2009, pp. 8-9, ISBN 978-90805661-5-6). In all cases the ECCS consists of a layer of chromium oxide on top of metallic chromium.

[0015] In the process according to the invention, a coating layer comprising metallic chromium and chromium oxide is deposited, and not by initially depositing the metallic chromium layer, and then providing a chromium oxide layer on top as a conversion layer. The Cr-CrOx layer should consist of a mixture of Cr oxide and metallic Cr and the Cr oxide should not be present as a distinct layer on the outermost surface, but mixed throughout the entire Cr-CrOx layer. Naturally there can be more than one of these single coating layers one after the other if, for example, a thicker coating layer comprising metallic chromium and chromium oxide is to be deposited. The single phase coating step is therefore not limited to only one of these single coating steps being used.

[0016] The steel substrate for packaging is generally supplied in the form of a low carbon (LC), extra low carbon (ELC) and ultra low carbon (ULC) strip with a carbon content, expressed as a percentage by weight, of between 0. 05e 0.15 (LC), between 0.02 and 0.05 (ELC) or below 0.02 (ULC) respectively. Binding elements such as manganese, aluminum, nitrogen, but also sometimes also elements such as boron, are added to improve the mechanical properties (see also, for example, EM 10 202, 10 205, and 10 239). In one embodiment of the invention the substrate consists of a low, extra-low or ultra-low carbon interstitial free steel, such as titanium stabilized interstitial free steel, niobium stabilized or niobium-titanium stabilized interstitial free steel.

[0017] It has been found that a metallic corm-chromium oxide (Cr-CrOx) coating produced from a trivalent chromium based electrodeposition process provides excellent adhesion to organic coatings. In this respect, the metallic chromium - chromium oxide (Cr-CrOx) coating produced from a trivalent chromium electrodeposition process has very similar adhesion properties compared to conventional ECCS produced through a hexavalent chromium electrodeposition process. By increasing the thickness of the Cr-CrOx coating layer, the porosity of the coating is reduced and its corrosion resistance properties are improved.

[0018] The Cr-CrOx coating can be applied on conventional, non-passivated, electrolytic, and optionally molten-fluid (ETP, electrolytic tinplate) tinplates. The Cr-CrOx layer ensures that the growth of tin oxides is suppressed, i.e. it has a passivation function. With increasing Cr-CrOx thickness, it was unexpectedly found that dry adhesion performance, ie. , the adhesion of the organic coating after sterilization, surpassed conventional hexavalent chromium passivated tinplate. In addition, resistance to so-called tin stain, i.e. the brown discoloration of the tinplate due to contact with sulfur-containing fillers, can be totally suppressed by applying a sufficiently thick Cr-CrOx coating. The material according to the invention is therefore quite suitable to replace the hexavalent chromium passivated tinplate, optionally exceeding the technical performance limits of the standard tinplate. From a process point of view, the fact that the Cr-CrOx coating layer is applied in a single-step process means that two process steps are combined which is beneficial in terms of process economy and in terms of impact environmental.

[0019] Alternatively, the Cr-CrOx coating can also be applied directly onto the steel substrate for black plate packaging. No prior application of a tin coating, ie applied directly to the bare steel surface. According to Merriam Webster, black plate is defined as a sheet of steel that has not yet been made into tinplate by being coated with tin or that is used uncoated where the protection provided by tin is unnecessary. It has been found that the dry adhesion levels for organic coatings for both thermosetting varnishes and thermoplastic coatings of this material can approach those normally associated with the use of ECCS. The material according to the invention can be used directly to replace ECCS for applications requiring moderate corrosion resistance.

[0020] The great advantage, both in terms of environmental impact and in terms of health and safety, is the fact that with this invention the use of hexavalent chromium is avoided, while it is possible to retain the performance properties of the product normally attributed to ECCS or to tin sheets.

[0021] In one embodiment, the Cr-CrOx coating layer applied to the unpassivated tinplate contains at least 20 mg Cr/m2 to create a tin oxide passivation effect. This thickness is suitable for many purposes.

[0022] In one embodiment, the Cr-CrOx coating layer applied to the unpassivated tinplate contains at least 40 mg Cr/m2, preferably at least 60 mg Cr/m2, to create a tin oxide passivation effect and avoid or eliminate tin stains. To avoid or eliminate tin stains, a layer of 20 mg Cr/m2 has been found to be very thin. Starting at thicknesses of around 40 mg Cr/m2 the tin stains are already very reduced, while at a layer thickness of around 60 mg Cr/m2 the sulfur stains are practically eliminated.

[0023] The maximum suitable thickness was found to be 140 mgCr/m2. Preferably, the Cr-CrOx coating layer applied to the unpassivated tinplate contains at least 20 to 140 mg Cr/m2, more preferably at least 40 and/or at most 90 mg Cr/m2, and more preferably at least 60 and /or at most 80 mg Cr/m2.

[0024] These modalities aim to replace the passive tinplate with hexavalent chromium. The main advantage beyond the elimination of hexavalent chromium from production is the potential to create a product with superior resistance to sulfur stains and improved corrosion resistance.

[0025] It has been found that the color of the material changes with increasing thickness of the Cr-0CrOx layer, with the product becoming darker (ie, lower L-value) as the coating thickness increases. As the optical properties of packaging steels are very important to create an attractive aesthetic appearance of metallic containers, such as aerosol cans, this must be considered a disadvantage of the invention for specific applications. However, one way to avoid these problems would be to use differential coatings, for example using a lightweight Cr-COx coating on one side of the material while applying a thicker coating on the other side. The surface containing a thicker coating weight should be used for the interior of the container to take advantage of the improved corrosion resistance properties. In this case, the surface with the least weight of Cr-CrOx coating is on the outside of the container for which corrosion resistance requirements are generally less stringent, ensuring optimum optical properties.

[0026] In one embodiment the Cr-CrOx coating layer applied to the black sheet is at least 20 mg Cr/m2 to create a material that approximates ECCS functionality (eg excellent adhesion to organic coatings in combination with strength to moderate corrosion). Preferably the Cr-CrOx coating layer applied on the black sheet is at least 40 and more preferably at least 60 mg Cr/m2. A suitable maximum thickness was found to be 140 mg Cr/m2. Preferably, the Cr-CrOx coating layer applied to the black sheet contains at least 20 to 140 mg Cr/m2, more preferably at least 40 mg Cr/m2, and more preferably at least 60 mg Cr/m2. In one modality, the most suitable is 110 mg Cr/m2 .

[0027] Cr-CrOx coated black sheet aims to replace ECCS. The main advantage beyond the elimination of hexavalent chromium from production is the potential to create a product for applications for which superior corrosion resistance properties of tinplate are not required. From a process point of view, the fact that the Cr-CrOx coating layer is applied in a single process step means that two process steps are combined, which is beneficial in terms of process economy and in terms of environmental impact.

[0028] The Cr-CrOx coating can also be applied to a black plate cold rolled and with recovery annealing, or to a cold rolled tin plate and with an electrolytic recovery annealing, and optionally molten fluid. These substrates have a recovery annealed substrate, rather than single reduction recrystallized ETP or black plate or double reduction black plate. The difference in substrate microstructure was not found to materially affect the Cr-CrOx coating.

[0029] It has been found that the material according to the invention can be used in combination with thermoplastic coatings, but also for applications where ECCS is traditionally used in combination with varnishes (i.e., for baking instruments such as baking tins, or products with moderate requirement for corrosion resistance) or as a substrate for conventional tinplate for applications where requirements in terms of corrosion resistance are moderate.

[0030] In one embodiment, the coated substrate is also provided with an organic coating, consisting of either a thermosetting organic coating, a single-layer thermoplastic polymer coating, or a multi-layer thermoplastic polymer coating. The Cr-CrOx layer provides excellent adhesion to the organic coating similar to that achieved using conventional ECC S.

[0031] In a preferred embodiment the thermoplastic polymer coating is a polymer coating system comprising one or more layers comprising the use of thermoplastic resins such as polyesters or polyolefins, but may also include acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, resins ABS, polycarbonates, styrene resins, ABS resins, chlorinated polyethers, ionomers, urethane resins and functionalized polymers. For clarification:

[0032] Polyester is a polymer composed of dicarboxylic acid and glycol. Examples of suitable dicarboxylic acids include terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, and cyclohexane dicarboxylic acid. Examples of suitable glycols include ethylene glycol, propane diol, butane diol, hexane diol, cyclohexane diol, cyclohexane dimethanol, neopentyl glycol, etc. More than two types of dicarboxylic acids or glycols can be used together.

[0033] Polyolefins include, for example, polymers or copolymers of ethylene, propylene, 1-butene, 1-pentene, 2-hexene or 1-octene.

[0034] Acrylic resins include, for example, polymers or copolymers of acrylic acid, methacrylic acid, acrylic acid ester, methacrylic acid ester or acrylamide.

Polyamide resins include, for example, the so-called Nylon 6, Nylon 66, Nylon 46, Nylon 610 and Nylon 11.

[0036] Polyvinyl chlorides include homopolymers or copolymers, for example, with ethylene or vinyl acetate.

[0037] Fluorocarbon resins include, for example, tetrafluorinated polyethylene, monochlorinated trifluorinated polyethylene, hexafluorinated ethylene-propylene resin, polyvinyl fluoride and polyvinylidene fluoride.

Polymers functionalized, for example, by grafting maleic anhydride, include, for example, modified polyethylenes, modified polypropylenes, modified ethylene acrylate copolymers, and modified ethylene vinyl acetate.

[0039] Mixtures of two or more resins can be used. Aresin can be mixed with antioxidant, heat stabilizer, UV absorber, plasticizer, pigment, nucleating agent, antistatic agent, antiblocking agent, etc. the use of such thermoplastic polymer coating systems has been shown to provide excellent performance in can production and can usage, such as shelf exposure time.

[0040] According to a second aspect, the invention is configured in a process for producing a coated steel substrate for packaging applications, the process comprising the electrodeposition of metallic chromium coating - chromium oxide on the substrate with electrolytic deposition on the said substrate of said metallic chromium coating - chromium oxide occurring in a single coating step from a coating solution comprising a trivalent chromium compound, an optional chelating agent, an optional salt to increase conductivity, an optional depolarizer, an optional surfactant to which an acid or base may be added to adjust the pH.

[0041] In one embodiment the electrodeposition of the Cr-CrOx coating is achieved using an electrolyte in which the chelating agent comprises a formic acid anion, the conductivity enhancing salt contains an alkali metal cation and the depolarizer comprises a salt containing bromide.

[0042] In one embodiment, the cationic species in the chelating agent, the conductivity-enhancing salt, and the depolarizer are potassium. The benefit of using potassium is that its presence in the electrolyte increases the electrical conductivity of the solution more than any other alkali metal cation, thus making a maximum contribution to lowering the cell voltage needed to drive the electrodeposition process.

[0043] In an embodiment of the invention, the composition of the electrolyte used for the deposition of Cr-CrOx was: 120 g/l of basic chromium sulfate, 2150 g/l of potassium chloride, 15 g/l of potassium bromide and 51.2 g/l of potassium formate. The pH was adjusted to values between 2.3 and 2.8 measured at 25°C by the addition of sulfuric acid.

[0044] According to the invention, the chromium-containing coating is preferably deposited from the trivalent chromium-based electrolyte at a bath temperature of between 40 and 70°C, preferably of at least 45°C and/or at most 60 °C.

[0045] Surprisingly, it has been found that it is possible to electrodeposit a metallic chromium coating layer - chromium oxide from that electrolyte in a single step process. From the prior art it follows that the addition of a buffering agent to the electrolyte, such as boric acid, is strictly necessary to allow metallic chromium electrodeposition to occur. In addition, it has been reported that it is not possible to deposit metallic chromium and chromium oxide from the same electrolyte, due to this dampening effect (with the buffering agent being necessary for the electrodeposition of metallic chromium, but excluding the formation of chromium oxides and vice versa). However, it has been found that none of these additions of buffering agent were required to deposit metallic chromium, as long as a sufficiently high cathodic current density is being applied.

[0046] XPS depth profiles were measured and the peaks that are measured are Fe2p, Cr2p, O1s, Sn3d, C1s. It was observed that the Cr layer consists of a mixture of Cr oxide and metallic Cr and that the Cr oxide is not present as a distinct layer on the outermost surface, but is mixed throughout the entire layer. This is also indicated by the O-peak which is present throughout the Cr layer. In all cases the Cr-CrOx layer has a shiny metallic appearance.

[0047] It is believed that a certain minimum value for the resulting density must be exceeded for the electrodeposition of metallic chromium to occur, which is closely linked to the pH of the strip surface to reach certain values as a result of the evolution of hydrogen gas and equilibrium of various complexes (chelates) of chromium hydroxide. It was found that after crossing this minimum value for current density that the electrodeposition of the metallic chromium-chromium oxide layer increases virtually linearly with increasing current density, as observed with conventional electrodeposition of metals, according to Faraday's Law. The actual value for the minimum current density value appears to be closely linked to the mass transfer conditions at the surface of the strip. It has been observed that this minimum value increases with increasing mass transfer rates. The phenomenon can be explained by changes in pH values on the strip surface: at increasing rates of mass transfer, the supply of hydronium ions to the strip surface is increased, necessitating an increase in the cathodic current density to maintain a level of specific pH (obviously greater than mass pH) on the strip surface under steady-state process conditions. The validity of this hypothesis is supported by the results obtained in experiments in which the pH of the mass electrolyte was varied between a value of 2.5 and 2.8: the minimum value for the current density decreases with an increase in the pH value.

[0048] In relation to the electrodeposition process of Cr-CrOx coatings from trivalent chromium-based electrolytes, it is important to avoid/minimize the oxidation of trivalent chromium to its hexavalent state at the anode and an anode or anode material must be selected adequate. By using a hydrogen gas diffusion anode as described below and in copending application EP12193623, the formation of Cr(IV) can be avoided.

[0049] In one embodiment of the invention, the formation of Cr(IV) can be prevented by using one or more hydrogen gas diffusion anodes in which hydrogen gas (H2(g)) is oxidized. H+ (protons) in an aqueous solution bind to one or more water molecules, for example as hydronium ions (H3O+). The oxidation of H2(g) to H+ (aq) prevents the occurrence of undesirable oxidation reactions, such as the formation of Cr(IV), which occurs at a higher anode superpotential when using an anode in which water (H2O) it is oxidized to oxygen (O2(g)).

[0050] The relation H2(g) - 2H+ + 2e occurs at a 1.23 V anode potential (SHE). When using an anode in which water is oxidized to oxygen, then reactions are possible that were not possible when using an anode in which hydrogen gas is oxidized.

[0051] One of such undesirable oxidation reactions is the oxidation of Cr(III) to Cr(IV) and this oxidation reaction can be completely excluded by using hydrogen gas diffusion anode (GDA) in which H2(g) is oxidized to H+.

[0052] In one embodiment of the method, H2(g) is oxidized at the gas diffusion anode to H+(aq) and a current efficiency of at least 99%, preferably 100%. The greater the current efficiency, the lesser the probability of undesirable side reactions. It is, therefore, preferable that the current efficiency is at least 99%, and preferably 100%. Based on thermodynamic and kinetic considerations, it can be argued that using a hydrogen gas diffusion anode completely eliminates the risk of Cr(III) oxidation since the anode operating potential is too low for Cr( III).

[0053] Thermodynamically, under standard conditions (ie, at a temperature of 25°C and a pressure of 1 atm) an electrode potential of > 0 V is already sufficient to oxidize H2(g) to H+(aq), while an electrode potential of > 1.23 V is required to oxidize H2O to O2(g). Cr(III) can only be oxidized to Cr(VI) when the electrode potential is > 1.35 V.

[0054] The electrode potential is measured against the standard dehydrogen electrode. The standard hydrogen electrode (abbreviated as SHE) is a redox electrode that forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be 4.44 ± 0.02 V at 25°C, but to form a basis for comparison with all other electrode reactions, the standard hydrogen electrode potential (E0) is declared to be zero at all temperatures. Potentials of any other electrodes are compared with that of the standard hydrogen electrode at the same temperature.

[0055] The equilibrium potential that prevails (zero current) can be calculated from the Nernst Equation filling in the appropriate temperature, pressure and activities of the electroactive species. The operating anode potential (non-zero current) required to generate a specific anodic current is determined by the activation superpotential (i.e., the potential difference necessary to drive the electrode reaction) and the concentration superpotential (i.e., the potential difference necessary to compensate for the concentration gradients of the electroactive species in the electrode).

[0056] Due to the low anode superpotential required for the oxidation of H2(g) to H+(aq), the operating potential of the anode will always be well below the value at which the oxidation of Cr(III) can occur (see Figure 4 where the current is plotted against the anode potential in the SHE). Firstly, this results in less energy consumption in the electrodeposition process. Second, at an anode potential below 1.35 V oxidation of Cr(III) to Cr(VI) is not possible (indicated with arrow).

[0057] In one modality no depolarizer is added to the electrolyte. When a hydrogen gas diffusion anode is used, then the addition of a depolarizer to the electrolyte is no longer necessary.

[0058] The use of a hydrogen gas diffusion anode has the additional advantage that the use of a chloride containing electrolyte becomes possible without the risk of chlorine formation. Chlorine gas is potentially harmful to the environment and workers and is therefore undesirable. This means that in a case of a Cr(III) electrolyte the electrolyte must be partially or fully based on chlorides. The advantage of using a chloride-based electrolyte is that the conductivity of the electrolyte is much higher compared to a sulfate-only electrolyte, which leads to a lower battery voltage that is needed to perform the electrodeposition, which results in in a lower power consumption.

[0059] The oxidation reaction of hydrogen dissolved in a surface of the active electrocatalyst is a very fast process. Since the solubility of hydrogen in a liquid electrolyte is often low, this oxidation reaction can easily become controlled by mass transfer limitations. Porous electrodes are specifically designed to overcome mass transfer limitations. A hydrogen gas diffusion anode is a porous anode containing a three-phase interface of hydrogen gas, electrolytic fluid and a solid electrocatalyst (eg, platinum) that has been applied to the porous electrolytically conductive matrix (eg, porous carbon or a foam porous metal). The main advantage of using this porous electrode is that it provides a very large internal surface area for the reaction contained in a small volume combined with a greatly reduced diffusion path length from the gas-liquid interface to the reactive sites. Through this design the hydrogen mass transfer rate is greatly increased, while the actual local current density is reduced at a given total electrode current density, resulting in a lower electrode potential.

[0060] A gas diffusion anode assembly to be used in the proposed electrodeposition method typically comprises the use of the following functional components (see Figure 5): A gas supply chamber 1, a current collector 2, and an anode of gas diffusion, which consists of a hydrophobic porous gas diffusion transport layer 3 combined with a hydrophilic reaction layer 4 (see Figure 5). The latter is made up of a network of micropores that are (partially) drowned in liquid electrolyte. Optionally, the reaction layer is provided with a proton exchange membrane on the outside 5, such as a Nafion® membrane, to prevent the diffusion of chemical species (such as anions or large neutral molecules) present inside the liquid electrolyte mass_ inside the anode of gas diffusion. Since these compounds can potentially be the electrocatalyzed sites, causing the degradation of the electrocatalytic activity.

[0061] The main function of the gas supply chamber is to supply hydrogen gas regularly to the hydrophobic rear side of the hydrogen gas diffusion anode. The hydrogen gas supply chamber needs two connections: one to supply hydrogen gas and one to allow cleaning of a small amount of hydrogen gas to prevent the development of gas phase contamination potentially present in negligible amounts in the supplied hydrogen gas. The gas supply chamber often contains a channel-like structure to ensure that the hydrogen gas is distributed evenly throughout the hydrophobic backside.

[0062] The electric current collector 2 is (generally) connected to the hydrophobic rear side 3 of the hydrogen gas diffusion anode to allow the transport of the electric current generated within the anode to a rectifier (not shown in Figure 5). This current collector plate must be designed to allow the hydrogen gas to contact the rear of the hydrogen gas diffusion anode so that it can be transported to the reactive side within the gas diffusion anode. This is usually done using an electrically conductive sheet with a large number of holes, a mesh or an expanded metal sheet made, for example, of titanium.

[0063] The functionalities of the gas supply channels and the electric current collector can also be combined into a single component, which is then pressed against the hydrophobic rear side of the gas diffusion anode.

[0064] Since hydrogen gas diffuses through the hydrophobic rear side of the hydrogen gas diffusion anode, it comes into contact with the electrolyte, which is present in the hydrophilic part of the anode, ie, the reaction layer (see Figure 5, right side). At the gas-liquid interface (between 3 and 4) hydrogen gas dissolves in the electrolyte and is transported by diffusion to the active electrocatalytic sites of the hydrogen gas diffusion anode. Platinum is generally used as an electrocatalyst, but also other materials such as platinum-ruthenium or platinum-molybdenum alloys can be used. At the electrocatalytic sites the dissolved hydrogen is oxidized, the electrons that are generated are transported through the conducting matrix of the gas diffusion anode (usually a carbon matrix) to the current collector 2, while the hydrogen ions (H+) diffuse through the proton exchange membrane in the electrolytes.

[0065] In one embodiment the coated substrate is also provided on one or both sides with an organic coating, consisting of a thermosetting organic coating by a lacquer step, or a single thermoplastic layer, or a multilayer thermoplastic polymer by a film lamination step or by a direct extrusion step.

[0066] In one embodiment, thermoplastic polymer coating is a polymer coating system comprising one or more layers comprising the use of thermoplastic resins such as polyesters or polyolefins, but may also include acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, resins styrene type, ABS resins, chlorinated polyethers, ionomers, urethane resins, and functionalized polymers; and/or its copolymers; and/or mixtures thereof.

[0067] Preferably, the substrate is cleaned before the Cr-CrOx electrodeposition by dipping the substrate in a sodium carbonate solution containing from 1 to 50 g/l of Na2CO3 at a temperature of between 35 and 65°C, and where a cathode current density of between 0.5 and 2 A/dm2 is applied for a period of between 0.5 and 5 seconds.

[0068] Preferably, the sodium carbonate solution contains at least 2 and/or at most 5 g/l of Na2Co3.

[0069] The invention will now also be explained by means of the following non-limiting examples and figures.

[0070] Example 1 (not part of the invention): Was given to conventional, non-passivated, fluid molten tinplate sheets (common steel grade and temper), with a tin coating weight of 2.8 g Sn /m2 on both sides, initially an electrolytic pretreatment to minimize the thickness of the tin oxide layer. This was done by immersing the plates in a sodium carbonate solution (3.1 g of Na2CO3, temperature of 50°C) and applying a cathodic current density of 0.8 A/dm2 for 2 seconds. After washing with ionized water, the samples were immersed in a trivalent chromium electrolyte kept at 50-°C composed of: 120 g/l basic chromium sulfate, 250 g/l potassium chloride, 15 g/l bromide of potassium, and 51.2 g/l of potassium formate. The pH of this solution was adjusted to 2.3 measured at 25°C by the addition of sulfuric acid. A Cr-CrOx coating containing between 21 - 25 mg Cr/m2 (measured by XRF) was deposited onto the surface by applying a cathodic current density of 10 A/dm2 for approximately 1 second using a platinum titanium anode as a counter-electrode. The samples thus produced showed a shiny metallic appearance.

[0071] The study of the passivating action of the thin Cr-CrOx coating on the tinplate, the samples were subjected to a long-term storage test at 40°CX at a static humidity level of 80% RH. The amount of oxide developed on the surface of the tinplate during storage is then measured after 2 weeks and after 4 weeks of exposure, and compared to the amount of tin oxide present in the sample before the storage test (denoted as "o weeks" ). The determination of the tin oxide layer thickness is done using a coulombmetric method as described in SC Britton, "Tin vs corrosion", ITRI Publication No. 510 (1975), Chapter 4. The tin oxide layer is reduced by a small controlled cathodic current in a 0.1% solution of hydrobromic acid (HBr) which is released from oxygen by friction with nitrogen. The progress of oxide reduction is followed by measuring the potential and charge that has passed to full reduction (expressed in Coulomb/m2 or C/m2) and serves as a measure of the thickness of the tin oxide layer. The results of the sample as per Example 1 are shown in Table 1, including the performance of the reference material, which is the same tinplate material that was passivated using hexavalent chromium, i.e., the so-called tinplate with passivation 311.Table 1 - Thickness of the tin oxide layer (in C/m2)

[0072] The results show that the non-passivated tinplate treated according to the present invention to obtain a lightweight Cr-CrOx coating shows perfect stability in tin oxide growth and is fully comparable in performance to traditional passivated tinplate 311.

[0073] Example 2 (not part of the invention): An electrolytic pretreatment was given to conventional, unpassivated, molten fluid, tinplate sheets (common temper and steel grade), with a coating weight of zinc 2.8 g Sn/m2 on both sides to minimize tin layer thickness. This was done by immersing the plates in a carbonate solution (3.1 g/l of Na2CO3, temperature of 50°C) and applying a cathodic current density of 0.8 A/dm2 for 2 seconds. After washing with deionized water, the samples were immersed in a trivalent chromium electrolyte held at 50°C composed of: 120 g/l basic chromium sulfate, 250 g/l potassium chloride, 15 g/l bromide potassium, and 51.2 g/l of potassium formate. The pH of this solution was adjusted to 2.3 measured at 25°C by the addition of sulfuric acid. A Cr-CrOx coating containing between 65 - 75 mg Cr/m2 (measured by XRF) was deposited onto the surface by applying a cathodic current density of 15 A/dm2 for approximately 1 second using a platinum titanium anode as against electrode. All samples thus produced showed a shiny metallic appearance. A typical SEM image is shown in Figure 1 & 2, which shows the deposition of very fine grains of metallic chromium - chromium oxide on the tin surface.

[0074] The sheets were subsequently varnished by applying a commercially available epoxy-anhydride varnish system (Vitalure® 120 supplied by AkzoNobel). Subsequently, the varnished sheets were deformed locally by Erichsen bleeding.

[0075] To analyze the performance of the tinplate coated with chromium - chromium oxide, several sterilization tests were carried out to evaluate the adhesion performance on the flat and deformed material. In total, 5 different sterilization media were used in these tests, as shown in Table 2.Table 2 - Sterilization Test Conditions

[0076] After sterilization the level of adhesion of the varnish to the panels was evaluated (by the cross-cut test and adhesive tape (ISO 2409:1992(E)) the formation of bubbles (size and number of bubbles) and visual discoloration. The results are shown in Table 3, including the performance of the reference material, which is the same tinplate material that has been passivated using hexavalent chromium, ie, the so-called 311 passivation tinplate. a scale from 0 to 5, with 0 being excellent performance and 5 being very poor performance. The results are averaged over a number of observations, leading to a decimal value count.Table 3 - Varnish Adhesion Test Results

[0077] The inventors found that variant tinplate produced in accordance with the invention consistently performed equal to or better compared to standard tinplate that is passivated using hexavalent chromium (ie, the reference). It is striking that no sulfur stains were discovered for the material according to the invention, which is difficult to achieve with conventional passivated tinplate and notoriously difficult to achieve with alternative passivations for tinplate that are free of hexavalent chromium.

[0078] Example 3: A coil of black plate (common steel grade and temper), not containing any metallic coating, was treated on a processing line passing at a line speed of 20 m/min. The processing sequence began with an alkaline cleaning of the steel by passing the strip for approximately 10 seconds through a solution containing 30 ml/l of a commercial cleaner (Percy P3) and 40 g/l of NaOH, which was kept at 60 °C. During strip cleaning an anodic current density of 1.3 A/dm2 was applied. After cleaning with deionized water, the steel strip was passed through an acidic solution for approximately 10 seconds to activate the surface. The acidic solution consisted of 50 g/l H2SO4 which was kept at 25°C. After cleaning with deionized water, the steel strip was passed into an electroplating tank containing the trivalent chromium-based electrolyte held at 50°C. This electrolyte consisted of 120 g/l of basic chromium sulfate, 250 g/l of potassium chloride, 15 g/l of potassium bromide, and 51.2 g/l of potassium formate. The pH of this solution was adjusted to 2.3 measured at 25°C by the addition of sulfuric acid. The electroplating tank contained a group of anodes consisting of platinum titanium. During strip processing a cathodic current density of approximately 17 A/dm2 was applied for just over 1 second to deposit a 60 - 70 mg Cr/m2 chromium oxide coating (measured by XRF) on the strip surface. black plate. All samples thus produced showed a shiny metallic appearance. A typical SEM image is shown in Figure 1 and 2 which shows the deposition of very fine grains of chromium - chromium oxide on the steel surface.

The material thus produced was passed through a coating line to apply a commercially available PET film 20 microns thick by heat sealing. After film lamination,. The coated strip was post-heated to temperatures above the melting point of PET, and subsequently cooled in water at room temperature, as in the usual processing method for laminating PET from metals. The same procedure was followed for the production of the reference material, using a commercially produced EXXS coil.

[0080] The laminated materials were used to produce standard DRD food cans (211 x 400). In all cases the dry adhesion of the PET film to the can wall was excellent. This was confirmed by measuring the peel strengths T of the PET film on the can wall, which showed similar values for the PET film applied to both materials as per the invention and commercial ECCS (~7 N/15 mm).

[0081] The DRD cans were subsequently filled with different media, closed and exposed to a sterilization treatment. Some cans that had a scratch made on the can wall were processed to simulate and observe the effect of incidental damage to the coating. An overview of the type of sterilization tests performed is presented in Table 4. Table 4 - Sterilization Test Conditions

[0082] After the sterilization treatment the DRD cans were cooled to room temperature, emptied, washed and dried for one day. The bottom and walls of the cans were visually judged for the presence of pitting and blisters. The results, as shown in Table 5, show that the sterilization performance of the material according to the invention is generally somewhat lower compared to the reference ECCS. The material appears especially more susceptible to corrosion/delamination of the coating after damage to the coating. coating. However, these sterilization tests are quite stringent, so in practice the material according to the invention can be used in specifically selected applications involving sterilization.

* Símbolos entre parênteses se referem a latas DRD com umarranhão na parede da lata[0083] A performance that varies on a scale of 0 to 5, with 0 being excellent performance and 5 being very poor performance. Table 5 - Results of sterilization tests

* Symbols in parentheses refer to DRD cans with a scratch on the can wall

[0084] Example 4: A coil of black plate (common steel grade and temper), not containing any metallic coating, was treated in a processing line identical to that described in the previous example for applying the Cr-CrOx coating.

The sheets cut from this coil were subsequently varnished by applying a commercially available epoxy phenol varnish (Vitalure® 345 supplied by AkzoNobel). Subsequently, the varnished sheets were deformed locally by Erichsen bleeding.

[0086] To analyze the performance of the black plate coated with chrome - chromium oxide, several sterilization tests were carried out to evaluate the adhesion performance of the walls on the flat and deformed material. In total, 5 different sterilization media were used during these tests, as shown in Table 6.Table 6 - Sterilization Test Conditions

[0087] After sterilization, the panels were evaluated in relation to the level of adhesion of the varnish (by the cross-cut test and adhesive tape (ISO 2409:1992(E))), blister formation (size and number of blisters) and visual discoloration . The total results are shown in Table 7, including the performance of the reference material, for which a commercially available ECCS was used. The performance rating is on a scale of 0 to 5, with 0 being excellent performance and 5 being very poor performance. Table 7 - Sterilization Test Results

[0088] The inventors found that the black sheet material produced pursuant to the invention consistently acted similarly to conventional ECCS. Brief description of the drawings:

[0089] Figure 1 and 2 show typical SEM images, which show the deposition of very fine grains of chromium - chromium oxide on the surface. Figure 1 refers to a tinplate substrate and Figure 2 refers to a black plate substrate.

[0090] Figure 3 shows an overview of various packaging applications. On the x-axis are the package steel grades, and on the y-axis the range of typical thicknesses is shown for those applications for which the steel package substrate according to the invention can be used.

[0091] Figure 4 shows where the current is plotted against the anode potential in the SHE and Figure 5 shows a schematic drawing of a gas diffusion anode.

Claims (13)

1. Process for producing a coated steel substrate for packaging applications by depositing a metallic chromium - chromium oxide coating on the substrate for packaging applications, containing 1) a black steel plate for packaging with recrystallization annealing with simple or double reduction, or2) a black plate cold-rolled and with recovery annealing, characterized by the fact that it comprises electrolytically depositing said metallic chromium coating - chromium oxide on said substrate in a single-step process from a solution of coating comprising a mixture of a trivalent chromium compound, a chelating agent, an optional salt to increase conductivity, an optional depolarizer, an optional surfactant, to which an acid or base is optionally added to adjust the pH, wherein the solution coating is free of a buffering agent, and in which a sufficiently high cathodic current density is being applied to deposit metallic chromium. 2. Process according to claim 1, characterized in that the chelating agent comprises a formic acid anion, the conductivity enhancing salt contains an alkali metal cation and the depolarizer comprises a bromide-containing salt. 3. Process according to claim 1 or 2, characterized in that the cationic species of the chelating agent, the conductivity enhancing salt, the chelating agent, the conductivity enhancing salt and the depolarizer is potassium. 4. Process according to any one of claims 1 to 3, characterized in that the electrolytic deposition deposits a layer of metallic chromium - chromium oxide on the black plate containing a total chromium content of at least 20 mg/m2, preferably at least 40 mg/m2 and more preferably at least 60 mg/m2. 5. Process according to any one of claims 1 to 4, characterized in that the electrolytic deposition deposits a layer of metallic chromium - chromium oxide on the black plate containing a total chromium content of at most 140 mg/m2, preferably maximum 110 mg/m2. 6. Process according to any one of claims 1 to 5, characterized in that the coated substrate is also provided on one or both sides with an organic coating, consisting of a thermosetting organic coating by a varnishing step, or a single thermoplastic layer, or a multilayer thermoplastic polymer by a film lamination step or a direct extrusion step, preferably where the thermoplastic polymer coating is a polymer coating system comprising one or more layers comprising thermoplastic resins such as polyesters or polyolefins, acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene-type resins, ABS resins, chlorinated polyethers, ionomers, urethane resins, and functionalized polymers; and/or its copolymers; and/or mixtures thereof. 7. Process according to any one of claims 1 to 6, characterized in that an anode is chosen that reduces or eliminates the oxidation of Cr(III) ions to Cr(VI) during the coating step, such as an hydrogen gas diffusion anode. 8. Coated steel substrate for packaging applications, obtained by the process as defined in claim 1, characterized in that it contains 1) a black steel plate for packaging with single or double reduced recrystallization annealing, or 2) a laminated black plate cold and recovery annealing, in which one or both sides of the substrate is coated with a layer of metallic chromium - chromium oxide produced in a single-step process using a trivalent chromium electrodeposition process, in which the metallic chromium layer - chromium oxide contains a total chromium content of at least 40 mg/m2, and wherein the layer consists of a mixture of metallic chromium and chromium oxide, where chromium oxide is not present as a distinct layer in the outermost surface, but is blended through the entire layer. 9. Coated substrate for packaging applications, according to claim 8, characterized in that the metallic chromium layer - chromium oxide contains a total chromium content of at least 60 mg/m2. 10. Coated substrate for packaging applications according to claim 8 or 9, characterized in that the coated substrate is also provided with an organic coating, consisting of a single-layer thermoplastic polymer coating or a multi-layer thermoplastic polymer coating layers, preferably wherein the thermoplastic polymer coating is a polymer coating system comprising one or more layers comprising thermoplastic resins such as polyesters or polyolefins, acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene type resins, ABS resins, polyethers chlorinated, ionomers, urethane resins, and functionalized polymers. 11. Coated substrate for packaging applications according to any one of claims 8 to 10, characterized in that the organic coating consists of a single thermoplastic coating or a multilayer polymer, where the thermoplastic polymer coating is a coating system polymer comprising one or more layers comprising polyesters and/or their copolymers and/or mixtures thereof. 12. Coated substrate for packaging applications according to claim 8 or 9, characterized in that the organic coating is further provided with an organic coating consisting of a thermosetting organic coating. 13. Coated substrate for packaging applications according to any one of claims 8 to 12, characterized in that the metallic chromium - chromium oxide layer contains a total chromium content of at most 140 mg/m2, preferably at most 110 mg/m2.

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