ES2703595T3 - Method for the production of chrome and chromium oxide coatings applied to steel substrates for packaging applications - Google Patents

Abstract

Process for producing a coated steel substrate for packaging applications by depositing a chromium metal-chromium oxide coating on the substrate for packaging applications that contain 1. a conventional non-passivated electrolytic tin, optionally die-cast, or 2. a tin recovery annealed and cold rolled electrolytic, optionally die-cast, comprising electrolytically depositing on said substrate, said chromium metal-chromium oxide coating in a single process step of a plating solution comprising a mixture of a compound of trivalent chromium, a chelating agent, an optional conductivity enhancing salt, an optional depolarizer, an optional surfactant, to which an acid or base is optionally added to adjust the pH, in which the plating solution does not contain a buffering agent , and where a sufficiently high cathodic current density is being applied each to deposit chrome metal.

Description

DESCRIPTION

Method for the production of chrome and chromium oxide coatings applied to steel substrates for packaging applications

This invention relates to chromium and chromium oxide coatings (Cr-CrOx) applied to steel substrates for packaging applications and to a method for producing such coatings.

The tinplate line products include tinplate, Electrolytic Chrome Plated Steel (ECCS, also known as tin-free steel or TFS), and plaster, uncoated steel. Packaging steels are usually supplied as tinplate or as ECCS on which an organic coating can be applied. In the case of tinplate, this organic coating is usually a lacquer, while in the case of ECCS, polymer coatings such as PET or PP are increasingly used, as in the case of Protact®.

Tinplate is characterized by excellent resistance to corrosion and weldability. Tinplate is supplied within a range of coating weights, usually between 1.0 and 11.2 g / m2, which are generally applied by electrolytic deposition. Currently, most tinplate is subsequently treated with fluids containing hexavalent chromium, Cr (VI), using an immersion or electrolyte assisted application process. The objective of this subsequent treatment is to passivate the tin surface to stop or reduce the growth of tin oxides, since too thick oxide layers can eventually cause problems with respect to the adhesion of organic coatings, such as lacquers. It is important that the passivation treatment not only suppresses or eliminates the growth of tin oxide, but also can maintain or improve the adhesion levels of the organic coating. The passivated outer surface of the tinplate is extremely thin (less than 1 micron thick) and consists of a mixture of tin and chromium oxides.

ECCS consists of a palladium product that has been coated with a layer of metallic chromium superimposed with a chromium oxide film, both applied by electrolytic deposition. ECCS excels in adhering to organic coatings and retaining the integrity of the coating at temperatures that exceed the melting point of tin (232 ° C). In these cases you can not use tinned material. This is important to produce packaging steel coated with polymer because during the application process of the thermoplastic coating, the steel substrate can be heated to temperatures above 232 ° C, with the actual maximum temperature values depending on the type of thermoplastic coating applied. . This heat cycle is necessary to allow the initial thermal bonding / bonding of the thermoplastic to the substrate (preheating treatment) and is usually followed by a post-heat treatment to modify the properties of the polymer. It is considered that the chromium oxide layer is responsible for the excellent adhesion properties of thermoplastic coatings such as polypropylene (PP) or poly (polyester terephthalate) (PET) to ECCS. ECCS can also be supplied within a range of coating weights for both the Cr and CrOx coatings, typically between 20-110 and 2-20 mg / m2 respectively. The ECCS can be delivered with an equal coating specification for both sides of the steel strip, or with different coating weights per side, which is known 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).

At present, hexavalent chromium is considered a dangerous substance that is potentially harmful to the environment and constitutes a risk to the safety of workers. Therefore, there is an incentive to develop alternative metal coatings that can replace conventional tinplate and ECCS, without the need to resort to the use of hexavalent chromium during fabrication.

It is an object of the invention to provide an alternative to the use of hexavalent chromium for tinplate passivation.

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

It is also 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 a corrosion protection that is not based on the use of hexavalent chromium during manufacture.

One or more of these objects are achieved by providing a steel packaging substrate that contains:

1. a non-passivated electrolytic tin plate, optionally die-cast, optional (i.e., ETP), or

2. an electrolytic tin plate cold rolled and annealed for recovery, optionally pressure cast.

wherein one or both sides of the substrate are coated with a chromium oxide-chrome metal coating layer (Cr-CrOx) produced in a single coating step by the use of a trivalent chromium plating process

The steel packaging substrate is preferably provided in the form of a band.

For the production of ECCS, in general, three types of chroming processes are used throughout the world. The three processes are the "one-step vertical process" (V-1), the "two-step vertical process" (V-2) and the "one-step horizontal high-current density process" (HCD) and are based on electrolytes of Cr (VI). The ECCS specifications are standardized according to Euronorm EN 10202: 2001. The vertical two-step process uses a Cr (VI) electrolyte without sulfuric acid to apply the chromium oxide layer in the second step. Sulfuric acid is necessary for a good efficiency in the application of chromium metal and, therefore, is always used for the passage of chromium metal plating in these processes. The process of "one vertical step" and "one horizontal step of high current density (HCD)" always have sulphate in the oxide layer because the chromium metal and the chromium oxide are produced simultaneously in the same electrolyte (Boelen, thesis TU Delft 2009, page 8-9, ISBN 978-90-805661-5-6). In all cases, the ECCS consists of a layer of chromium oxide on the chromium metal.

In the process according to the invention, a coating layer comprising chromium metal and chromium oxide is deposited, not first depositing a chromium metal layer, and then providing a layer of chromium oxide at the top as a layer of chromium oxide. conversion. The Cr-CrOx layer should consist of a mixture of Cr oxide and Cr metal and the Cr oxide should not be present as a distinct layer on the outermost surface, but should be mixed throughout the Cr-CrOx layer. Of course, there can be more than one of these simple plating steps one after the other if, for example, a thicker coating layer comprising a layer of chromium oxide and chromium metal is deposited. Therefore, the phrase simple plating step is not limited to meaning that only one of these simple plating steps is used.

The steel substrate for packaging is generally provided in the form of a band of low carbon content (LC), extra low carbon (ELC) or ultra low carbon (ULC) with a carbon content, expressed as a percentage by weight, of between 0.05 and 0.15 (LC), between 0.02 and 0.05 (ELC) or below 0.02 (ULC) respectively. Alloy elements such as manganese, aluminum, nitrogen, but sometimes also elements such as boron are added to improve the mechanical properties (see also, for example, EN 10 202, 10 205 and 10 239). In one embodiment of the invention, the substrate consists of a low, very low or very low carbonless steel without interstices, such as steel without interstitial stabilized with titanium, stabilized with niobium or stabilized with titanium and niobium.

It was found that a chrome-chromium oxide (Cr-CrOx) metal coating produced from a trivalent chromium-based electroplating process provides excellent adhesion to organic coatings. In this regard, chromium oxide-chromium metal coating (Cr-CrOx) produced from a trivalent chromium electrodeposition process has very similar adhesion properties compared to conventional ECCS produced through an electrodeposition process of hexavalent chromium. By increasing the thickness of the Cr-CrOx coating layer, the porosity of the coating is reduced and its corrosion resistance properties improve.

The Cr-CrOx coating can be applied over conventional tinplate, not passivated, electrolytic and, optionally, with pressure cast (ETP, electrolytic tinplate). The Cr-CrOx layer ensures that the growth of tin oxides is suppressed, that is, it has a passivation function. With the increase of the Cr-CrOx thickness, it was unexpectedly found that the wet adhesion performance, ie the adhesion of the organic coating after sterilization, exceeds the tinplate passivated with conventional hexavalent chromium. In addition, the resistance to so-called sulfur staining, ie the brown discoloration of tinplate due to contact with sulfur-containing fillers, can be completely suppressed by applying a Cr-CrOx coating thick enough. The material according to the invention is, therefore, very suitable for the replacement of passivated tinplate with hexavalent chromium, which optionally exceeds the technical performance limits of standard tinplate. From the process point of view, the fact that the Cr-CrOx coating layer is applied in a single step of the process means that two steps of the process are combined, which is beneficial in terms of process economy and in terms of environmental impact.

Alternatively, the Cr-CrOx coating can also be applied directly on the sheet steel packaging substrate, without the prior application of a tin coating, that is, applied directly to the bare steel surface. According to Merriam Webster, palastrus is defined as steel sheet that has not yet been converted to tinplate when it is coated with tin or that is used uncoated where the protection afforded by tin is unnecessary. It was found that dry adhesion levels to organic coatings for both thermoset lacquers and thermoplastic coatings of this material can be approximated to those normally associated with the use of ECCS. The material according to the invention can be used to directly replace the ECCS for applications that require moderate resistance to corrosion.

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

In one embodiment, the Cr-CrOx coating layer applied over non-passivated tinplate contains at least 20 mg Cr / m 2, to create a passivation effect of tin oxide. This thickness is suitable for many purposes.

In one embodiment, the Cr-CrOx coating layer applied over non-passivated tinplate contains at least 40 mg Cr / m2, preferably at least 60 Cr / m2, to create a tin oxide passivation effect and to prevent or eliminate Staining of sulfur. To prevent or eliminate the sulfur stain, it was found that a layer of 20 mg Cr / m2 was too thin. Starting at thicknesses of approximately 40 mg Cr / m2, the sulfur stain is already very reduced, while in a layer thickness of at least about 60 mg Cr / m2 practically eliminates the sulfur staining.

It was found that a suitable maximum thickness was 140 mg Cr / m2. Preferably, the Cr-CrOx coating layer applied over non-passivated tinplate contains at least 20 to 140 mg Cr / m2, more preferably at least 40 and / or at most 90 mg Cr / m2, and most preferably at least 60 and / or at most 80 mg Cr / m2.

These embodiments are intended to replace the passivated tinplate with hexavalent chromium. The main advantage, apart from the elimination of hexavalent chromium from manufacturing, is the possibility of creating a product with superior resistance to sulfur stains and better resistance to corrosion.

It was found that the color of the material changes with the increase of the thickness of the Cr-CrOx layer, with the product becoming darker (ie, lower L value) with the increase in the thickness of the coating. As the optical properties of packaging steels are very important to create an attractive aesthetic appearance of metal containers, such as aerosol cans, this could be considered a drawback of the invention for specific applications. However, one way to avoid these problems would be to use a differential coating, for example, using a low weight of the Cr-CrOx coating on one side of the material, while applying a heavier weight of the Cr-CrOx coating on the other side. The surface containing a thicker Cr-CrOx coating weight should be used for the interior of the container, to take advantage of the improved properties of corrosion resistance. In this case, the surface with the lowest Cr-CrOx coating weight is on the outside of the container, so the requirements for corrosion resistance are generally less severe, which guarantees optimal optical properties.

In one embodiment, the Cr-CrOx coating layer applied on a paddle is at least 20 mg Cr / m2, to create a material that approximates the ECCS functionality (eg, excellent adhesion to organic coatings in combination with moderate resistance to corrosion). Preferably, the Cr-CrOx coating layer applied on a paddle is at least 40 and more preferably at least 60 mg Cr / m2. It was found that a suitable maximum thickness was 140 mg Cr / m2. Preferably, the Cr-CrOx coating layer applied on a paddle contains at least 20 to 140 mg Cr / m2, more preferably at least 40 mg Cr / m2, and most preferably at least 60 mg Cr / m2. In one embodiment, an appropriate maximum is 110 mg Cr / m2.

Cr-CrOx-coated palladium aims to replace the ECCS. The main advantage, in addition to the elimination of hexavalent chromium from manufacturing, is the possibility of creating a product for applications for which the superior properties of tinplate corrosion resistance are not required. From the process point of view, the fact that the Cr-CrOx coating layer is applied in a single step of the process means that two steps of the process are combined, which is beneficial in terms of process economy and in terms of environmental impact.

The Cr-CrOx coating can also be applied to cold-rolled recovery and cold-rolled annealed plate, or to an annealed and cold-rolled annealed plate and, optionally, die-cast. These substrates have an annealed recovery substrate, instead of the single reduced recrystallized ETP or double reduced palastro or palastro. It was not found that the difference in the microstructure of the substrate materially affects the Cr-CrOx coating.

It was 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 lacquers (ie, for baking products, such as baking molds, or products with moderate requirements for corrosion resistance) or as a substitute for conventional tinplate for applications where the requirements in terms of corrosion resistance are moderate.

In one embodiment, the coated substrate is further provided with an organic coating, which consists of a thermosetting organic coating, or a single layer thermoplastic polymer coating, or a thermoplastic multilayer polymer coating. The Cr-CrOx layer provides excellent adhesion to the organic coating, similar to that obtained using conventional ECCS.

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, resins of fluorocarbon, polycarbonates, styrene-type resins, ABS resins, chlorinated polyethers, ionomers, urethane resins and functionalized polymers. To clarify:

Polyester is a polymer composed of dicarboxylic acid and glycol. Examples of suitable dicarboxylic acids include phthalic acid, isophthalic acid, naphthalene dicarboxylic acid and cyclohexanedicarboxylic 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 acid or glycol can be used together.

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

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, so-called Nylon 6, Nylon 66, Nylon 46, Nylon 610 and Nylon 11.

Polyvinyl chloride includes homopolymers and copolymers, for example with ethylene or vinyl acetate.

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

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

Mixtures of two or more resins can be used. In addition, the resin can be mixed with antioxidant, thermal stabilizer, UV absorber, plasticizer, pigment, nucleating agent, antistatic agent, release agent, antiblocking agent, etc. The use of such thermoplastic polymer coating systems has been shown to provide excellent performance in the manufacture and use of cans, such as shelf life.

According to a second aspect, the invention is embodied in a process for producing a coated steel substrate for packaging applications, the process comprising the electrodeposition of a chrome-chromium oxide metal coating on the substrate with the electrolytic deposition on said substrate of said coating of chromium metal and chromium oxide is produced in a single plating step of a plating solution comprising a trivalent chromium compound, an optional chelating agent, a salt for improving the optional conductivity, a depolarizer Optional, an optional surfactant and to which an acid or base can be added to adjust the pH.

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 salt improving the conductivity contains an alkali metal cation and the depolarizer comprises a bromide-containing salt.

In one embodiment, the cationic species in the chelating agent, the salt that improves the conductivity and the depolarizer are potassium. The advantage of using potassium is that its presence in the electrolyte greatly improves the electrical conductivity of the solution, more than any other alkali metal cation, which provides a maximum contribution to the cell voltage reduction required to boost the electro-deposition process.

In one embodiment of the invention, the electrolyte composition used for the deposition of Cr-CrOx was: 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 was adjusted to values between 2.3 and 2.8 measured at 25 ° C by the addition of sulfuric acid.

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 at least 45 ° C and / or at most 60 ° C.

Surprisingly, it was found that it is possible to electropositively deposit a coating layer of chromium metal and chromium oxide from this electrolyte in a single process step. From the prior art, the addition of a buffering agent to the electrolyte is followed, as, for example, boric acid is strictly necessary to allow electrodeposition of the chromium metal. In addition, it has been reported that it is not possible to deposit chromium metal and chromium oxide from the same electrolyte, due to this buffering effect (a buffering agent is required for the electrodeposition of the chromium metal but excluding the formation of chromium oxides). and vice versa). However, it was found that such addition of a buffering agent to deposit the chromium metal was not required, provided that a sufficiently high cathodic current density is being applied.

The XPS depth profiles were measured and the peaks measured are Fe2p, Cr2p, O1s, Sn3d, C1s. It was observed that the Cr layer consists of a mixture of Cr oxide and Cr metal and that 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 peak O that is present throughout the Cr layer. In all cases, the Cr-CrOx layer has a shiny metallic appearance.

It is considered that a certain threshold value for the current density must be exceeded for the chromium metal electrodeposition to occur, which is closely related to the pH on the surface of the band which reaches certain values as a result of the evolution of the hydrogen gas and the equilibrium of several polychrome hydroxide complexes (chelates). It was found that, after crossing this threshold value for the current density, the electrodeposition of the chromium-metal chromium oxide coating layer virtually increases linearly with increasing current density, as observed with the electrodeposition of conventional metals, following Faraday's law. The actual value for the threshold current density seems to be closely related to the mass transfer conditions at the surface of the band: it was observed that this threshold value increases with increasing mass transfer rates. This phenomenon can be explained by changes in the pH values at the surface of the band: by increasing the mass transfer rates, the supply of hydronium ions to the surface of the band increases, which requires an increase in density of cathodic current to maintain a specific pH level (obviously higher than the pH in volume) at the surface of the band under steady-state process conditions. The validity of this hypothesis is supported by the results obtained in experiments in which the pH of the electrolyte in volume varied between a value of 2.5 and 2.8: the threshold value for the current density decreases with increasing pH value.

With respect to the electrodeposition process of Cr-CrOx coatings from electrolytes based on trivalent chromium, it is important to prevent / minimize the oxidation of the trivalent chromium to its hexavalent state at the anode and an anode or suitable anode material should be selected. By using a hydrogen gas diffusion anode as described below, the formation of Cr (IV) can be prevented.

In one embodiment of the invention, the formation of Cr (IV) can be prevented by using one, more or only hydrogen gas diffusion anodes in which hydrogen gas (H2 (g)) is oxidized. The H + (protons) in an aqueous solution are bound to one or more water molecules, for example, as hydronium ions (H3O +). The oxidation of H2 (g) to H + (ac) prevents the appearance of undesirable oxidation reactions, such as the formation of Cr (IV), which occurs at a higher anodic overpotential when an anode is used in which water ( H2O) is oxidized to oxygen (O2 (g)).

The reaction H2 (g) ^ 2H + (ac) 2e_ occurs at an anode potential of 0.00 V (SHE). The reaction 2H2O ^ 4H + (ac) O2 (g) 4e- occurs at an anode potential of 1.23 V (SHE). When an anode is used in which water is oxidized to oxygen, then reactions that would not have been possible when using an anode in which hydrogen gas is oxidized are possible.

One of these undesirable oxidation reactions is the oxidation of Cr (III) to Cr (VI) and this oxidation reaction can be completely excluded using a hydrogen gas diffusion anode (GDA) in which H2 (g) is oxidizes to H +.

In one embodiment of the method, the H2 (g) is oxidized at the gas diffusion anode to H + (ac) with a current efficiency of at least 99%, preferably 100%. The greater the efficiency of the current, the lower the probability of undesirable side reactions. Therefore, it is preferable that the efficiency of the current is at least 99%, and preferably 100%. On the basis of thermodynamic and kinetic considerations, it can be argued that the use of a hydrogen gas diffusion anode completely eliminates the risk of Cr (III) oxidation, since the anode operation potential is too low for it to be produce the oxidation of Cr (III).

Thermodynamically, under standard conditions (ie 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 + (ac), while a potential of electrode of> 1.23 V is required to oxidize H2O to O2 (g). Cr (III) can only be oxidized to Cr (VI) when the potential of the electrode is> 1.35 V.

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

The prevailing equilibrium potential (zero current) can be calculated from the Nernst equation by completing the appropriate temperature, pressure and activities of the electro-active species. The operating potential of the anode (non-zero current) necessary to generate a specific anodic current is determined by the activation superpotential (that is, the potential difference required to drive the electrode reaction) and the concentration overpotential (ie, the potential difference required to compensate for the concentration gradients). of electro-active species in the electrode).

Due to the low anode overpotential required for the oxidation of H2 (g) to H + (ac), the operating potential of the anode will always remain well below the value at which the oxidation of Cr (III) can take place (see Figure 4 where the current is plotted against the potential of the anode in SHE). First, this results in lower consumption of energy of the electrodeposition process. Secondly, at an anode potential less than about 1.35V, oxidation of Cr (III) to Cr (VI) (indicated by the crossed arrow) is not possible.

In one embodiment, depolarizer is not added to the electrolyte. When an anode of hydrogen gas diffusion is used, it is no longer necessary to add a depolarizer to the electrolyte.

The use of an anode for hydrogen gas diffusion has the additional advantage that the use of an electrolyte containing chloride becomes possible without the risk of chlorine formation. This chlorine gas is potentially harmful to the environment and to workers and, therefore, is undesirable. This means that in the case of a Cr (III) electrolyte, the electrolyte could be partially or totally chlorides-based. The advantage of using a chloride-based electrolyte is that the conductivity of the electrolyte is much higher compared to a sulphate-based electrolyte, which leads to a lower cell voltage that is required to perform the electrodeposition, which results in in a lower energy consumption.

The oxidation reaction of dissolved hydrogen in an electrocatalyst active surface is a very fast process. Since the solubility of hydrogen in a liquid electrolyte is usually low, this oxidation reaction can be easily controlled by mass transfer limitations. The porous electrodes have been specifically designed to overcome the limitations of mass transfer. An anode of hydrogen gas diffusion is a porous anode containing a three-phase interface of hydrogen gas, the electrolytic fluid and a solid electrocatalyst (e.g., platinum) that has been applied to the porous electrically conductive matrix (e.g. a porous metal foam). The main advantage of using such a porous electrode is that it provides a very large internal surface area for the reaction contained in a small volume combined with a very small length of the diffusion path from the gas-liquid interface to the reactive sites . Through this design, the mass transfer rate of hydrogen is greatly improved, while the true local current density is reduced to a given global electrode current density, resulting in a lower electrode potential.

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 feed chamber 1 of a current collector 2 and a diffusion anode gas, which consists of a hydrophobic porous gas diffusion transport layer 3 combined with a hydrophilic reaction layer 4 (see Figure 5). The latter is formed by a network of micropores that drown (in part) with 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 in the liquid electrolyte by volume in the Inside the anode gas diffusion, since these compounds can potentially poison electrocatalyst sites, causing degradation in electrocatalytic activity.

The main function of the gas supply chamber is to supply hydrogen gas uniformly to the hydrophobic rear side of the hydrogen gas diffusion anode. The gas supply chamber needs two connections: one to feed hydrogen gas and another to allow the purging of a small amount of hydrogen gas to avoid the accumulation of gaseous phase contaminations potentially present in minimal quantities in the supplied hydrogen gas. The gas feed chamber often contains a channel type structure to ensure that the hydrogen gas is evenly distributed on the hydrophobic back side.

The electric current collector 2 is (generally) attached 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 in such a manner as to allow the hydrogen gas to come into contact with the back of the hydrogen gas diffusion anode so that it can be transported to the reactive side within the gas diffusion anode. Generally, this is achieved by the use of an electrically conductive plate with a large number of holes, a mesh or a sheet of expanded metal made of, for example. titanium.

The functionality 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 back of the gas diffusion anode.

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

In one embodiment, the coated substrate is additionally 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 thermoplastic multilayer polymer by a lamination step of film or direct extrusion step.

In one 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, resins of fluorocarbon, polycarbonates, styrene-type resins, ABS resins, chlorinated polyethers, ionomers, urethane resins and functionalized polymers; and / or their copolymers; and / or mixtures thereof.

Preferably, the substrate is cleaned before the Cr-CrOx electrodeposition by immersing the substrate in a sodium carbonate solution containing between 1 and 50 g / l of Na 2 CO 3 at a temperature between 35 and 65 ° C, and where the density of cathodic current between 0.5 and 2 A / dm2 are applied for a period between 0.5 and 5 seconds.

Preferably, the sodium carbonate solution containing at least 2 and / or at most 5 g / l of Na 2 CO 3.

The invention is now explained in more detail by means of the following non-limiting examples and figures.

Example 1: Conventional non-passivated molten tinplate sheets (common steel grade and quenching), with a tin coating weight of 2.8 g Sn / m2 on both sides, first received an electrolytic pretreatment to minimize the thickness of the tin oxide layer. This was done by immersing the leaves in a sodium 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 rinsing with deionized water, the samples were immersed in a trivalent chromium electrolyte maintained at 50 ° C composed of: 120 g / l of basic chromium sulfate, 250 g / l of potassium chloride, 15 g / l of 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 coating of Cr-CrOx containing between 21 25 mg of Cr / m2 (measured by XRF) was applied on the surface applying a cathodic current density of 10 A / dm2 for approximately 1 second, using a titanium anode plated as counter electrode . The samples thus produced showed a shiny metallic appearance.

The study analyzed the passivating action of the Cr-CrOx thin coating on tin, the samples were subjected to a long-term storage test at 40 ° C at a static humidity level of 80% RH. The amount of tin oxide developed on the tinplate surface during storage is 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 (indicated as "0 weeks"). The determination of the thickness of the tin oxide layer is carried out by a coulometric method, as described in SC Britton, "Tin vs. corrosion", ITRI Publication No. 510 (1975), Chapter 4. The tin oxide layer is reduces by means of a small controlled cathodic current in a 0.1% solution of hydrobromic acid (HBr) that is released from oxygen by means of nitrogen washing. The progress of the oxide reduction is followed by the measurement of potential and the past charge for the complete reduction (expressed as Coulomb / m2 or C / m2) serves as a measure of the thickness of the tin oxide layer. The results for the sample according to Example 1 are presented in Table 1, which includes the performance of the reference material, which is the same tinplate material that was passivated using hexavalent chromium, ie the so-called tinplate 311 passivated.

Table 1: Thickness of tin oxide layer (in C / m2)

The results show that the non-passivated tinplate treated in accordance with the present invention to obtain a light coating of Cr-CrOx shows a perfect stability in the tin oxide growth and is completely comparable in performance to the traditional passivated tinplate 311.

Example 2: Conventional non-passivated molten tinplate sheets (common steel and temper grade), with a tin coating weight of 2.8 g Sn / m2 on both sides, first received an electrolytic pretreatment to minimize oxide tin. thickness of the layer. This was done by immersing the leaves in a sodium 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 rinsing with deionized water, the samples were immersed in a trivalent chromium electrolyte maintained at 50 ° C composed of: 120 g / l of basic chromium sulfate, 250 g / l of potassium chloride, 15 g / l of 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 65-75 mg of Cr / m2 (measured by XRF) was applied on the surface by applying a cathodic current density of 15 A / dm2 for about 1 second, using a titanium anode platinum as counter electrode. All Samples thus produced showed a shiny metallic appearance. A typical SEM image is shown in Figures 1 and 2, which shows the deposition of very fine grains of chrome-chromium oxide metal on the surface of the tin.

The sheets were subsequently lacquered, applying an epoxy-anhydride lacquer system available on the market (Vitalure ™ 120 supplied by AkzoNobel). Subsequently, the lacquered sheets were locally deformed by Erichsen's cupping.

To analyze the performance of tinplate coated with chromium and chromium oxide, several sterilization tests were performed to evaluate the performance of wet adhesion in flat and deformed material. In total, 5 different sterilization media were used during these tests, as shown in Table 2.

Table 2: Conditions of sterilization tests

After the sterilization, the level of adhesion of the lacquer of the panels was evaluated (by the cross-sectional test and tape (ISO 2409: 1992 (E)), blistering (size and number of blisters) and visual discoloration. The overall results are presented in Table 3, which includes the performance of the reference material, which is the same tinplate material that was passivated using hexavalent chromium, that is, the passivated tin called 311. The performance ranking is on a scale from 0 to 5, 0 being an excellent performance and 5 a very poor performance.The results are averaged over a series of observations, which leads to scores with a decimal value.

Table 3: Results of lacquer adhesion tests

The inventors found that the tinplate variant manufactured according to the invention behaved consistently equal to or better than standard tinplate which is passivated using hexavalent chromium (ie, the reference). Surprising is the fact that no sulfur staining was found for the material according to the invention, which is difficult to achieve with conventional passivated tinplate and is notoriously difficult to achieve with alternative passivations for tinplate that are free of hexavalent chromium.

Example 3 (not part of the invention): A paddle coil (common steel grade and quenching), which did not contain any metallic coating, was treated in a processing line that operated at a line speed of 20 m / min . The processing sequence began with the alkaline cleaning of the steel by running 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 maintained at 60 ° C. During the cleaning of the band an anode current density of 1.3 A / dm2 was applied. After rinsing with deionized water, the steel strip was passed through an acid solution for about 10 seconds, to activate the surface. The acid solution consisted of 50 g / l of H2SO4, which was maintained at 25 ° C. After rinsing with deionized water, the steel band was transferred to an electroplating tank containing the electrolyte based on trivalent chromium maintained 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 set of anodes consisting of platinum titanium. During the band processing, a cathodic current density of approximately 17 A / dm2 was applied for a little over 1 second for electro depositing a chromium oxide and chromium oxide coating of 60-70 mg Cr / m2 (measured by XRF ) on the surface of the palastro. All the samples thus produced showed a shiny metallic appearance. An SEM image Typical is shown in Figures 1 and 2, which shows the deposition of very fine grains of chrome-chromium oxide metal on the surface of the steel.

The material thus produced was passed through a coating line to apply a commercially available 20 micron thick PET film by thermal sealing. After the lamination of the film, the coated strip was subsequently heated to temperatures above the melting point of the PET, and subsequently stopped in water at room temperature, according to a usual processing method for the metal rolling with PET. The same procedure was followed for the manufacture of the reference material, using a commercially produced ECCS coil.

Laminated materials were used to produce standard food DRD cans (211 * 400). In all cases, the dry adhesion of the PET film to the wall of the can was excellent. This was confirmed by measuring the T-peel forces of the PET film on the can wall, which showed similar values for the PET film applied to both the material according to the invention and the commercial ECCS (~ 7 N / 15 mm).

The DRD cans were subsequently filled with different media, closed and exposed to a sterilization treatment. Some cans containing a scratch made on the wall of the can were processed to simulate and observe the effect of the incidental damage of the coating. A general description of the type of sterilization tests performed is presented in Table 4.

Table 4: Conditions of sterilization tests

After the sterilization treatment, the DRD cans were cooled to room temperature, emptied, rinsed and dried for one day. The bottom and wall of the can were visually evaluated for the presence of corrosion points and blisters. The results, as presented in Table 5, show that the sterilization performance of the material according to the invention is generally somewhat lower compared to the ECCS reference. The material seems especially more susceptible to corrosion / delamination of the coating after damage of the coating. However, these sterilization tests are quite severe, so in practice the material according to the invention can be used in specifically selected applications involving sterilization.

The performance ranking is on a scale of 0 to 5, with 0 being excellent performance and 5 a very bad performance.

Table 5: Results of sterilization tests

Example 4: (not part of the invention) A paddle coil (common steel grade and quenching), which did not contain any metallic coating, was treated in a processing line identical to that described in the previous example to apply a coating Cr-CrOx.

The cut sheets of this coil were subsequently lacquered, applying a commercially available epoxy-phenol lacquer system (Vitalure ™ 345 supplied by AkzoNobel). Subsequently, the lacquered sheets were locally deformed by Erichsen's cupping.

To analyze the performance of the chromium coated plaster and chromium oxide, several sterilization tests were performed to evaluate the performance of the wet adhesion in flat and deformed material. In total, 5 different sterilization media were used during these tests, as shown in Table 6.

Table 6: Conditions of sterilization tests

After sterilization, the panels were evaluated with respect to the level of adhesion of the lacquer (by the cross-sectional test and tape (ISO 2409: 1992 (E)), blistering (size and number of blisters) and visual discoloration The overall results are presented in Table 7, including the performance of the reference material, for which commercially available ECCS was used.The performance ranking is on a scale of 0 to 5, with 0 an excellent performance and 5 a performance very bad.

Table 7: Results of sterilization tests

The inventors found that the Cr-CrOx-coated palladium material manufactured in accordance with the invention behaved in a manner consistent with the conventional ECCS.

Brief description of the drawings.

Figures 1 and 2 show typical SEM images, showing the deposition of very fine grains of chrome-chromium oxide metal on the surface. Figure 1 refers to a tinplate substrate and Figure 2 refers to a plaster substrate.

Figure 3 shows an overview of several packaging applications. On the X axis are the packing steel grades, and on the Y axis a typical thickness range is shown for these applications for which the steel packaging substrate according to the invention could be used.

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

Claims (12)

1. Process to produce a coated steel substrate for packaging applications by depositing a chromium metal-chromium oxide coating on the substrate for packaging applications containing 1. a conventional non-passivated electrolytic tin plate, optionally die-cast, or 2. an annealed and cold-rolled recovery annealed electrolytic sheet, optionally die-cast, comprising depositing electrolytically on said substrate, said chrome metal coating - chromium oxide in a single process step of a plating solution comprising a mixture of a trivalent chromium compound, a chelating agent, an optional conductivity enhancing salt, an optional depolarizer, an optional surfactant, to which is added optionally an acid or base for adjusting the pH, in which the plating solution does not contain a buffering agent, and where a sufficiently high cathodic current density applied to deposit chromium metal is being obtained. Process according to claim 1, wherein the chelating agent comprises a formic acid anion, the salt improving the conductivity contains an alkali metal cation and the depolarizer comprises a bromide-containing salt. 3. Process according to any one of claims 1 or 2, wherein the cationic species in the chelating agent, the salt that improves the conductivity and the depolarizer is potassium. Process according to any one of claims 1 to 3, wherein the coated substrate is further provided on one or both sides with an organic coating, consisting of a thermosetting organic coating by a coating step, or a coating single thermoplastic, or a thermoplastic multiple polymer layer by a film extrusion step or direct extrusion step, 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, chlorinated polyethers, ionomers, urethane resins and functionalized polymers; and / or their copolymers; and or mixtures thereof. Process according to any one of claims 1 to 4, wherein an anode is chosen that reduces or eliminates the oxidation of Cr (III) ions to Cr (VI) ions during the plating step, such as an anode of diffusion of hydrogen gas. Process according to any one of claims 1 to 5, wherein the tin-coated substrate for packaging applications is electrolytically pre-treated to minimize the thickness of the tin oxide layer before coating one or both sides with chrome-chromium oxide metal coating. Process according to claim 6, wherein the electrolytic pretreatment consists in immersing the tin-coated substrate in a sodium carbonate solution and applying a cathodic current density. Process according to claim 7, in which the sodium carbonate solution consists of between 2 to 5 g / l of Na 2 CO 3 at a temperature of between 35 and 65 ° C, and where the current density is applied cathodic between 0.5 and 2 A / dm2 for a period between 0.5 and 5 seconds. Process according to any one of the preceding claims, in which the electrolytic deposition deposits a chromium metal chromium oxide layer on the non-passivated tinplate 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. Process according to any one of the preceding claims, in which the electrolytic deposition deposits a chromium oxide-chromium metal layer on the non-passivated tinplate containing a total chromium content of at most 140 mg / m2, preferably at at most 90 mg / m2 and more preferably at most 80 mg / m2. 11. Process according to any one of claims 1 to 9, wherein the electrolytic deposition deposits a layer of chrome-chromium oxide metal on the annealed electrolytic tin plate by recovery and cold-rolled, optionally die-cast, 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. Process according to any one of claims 1 to 9 or 11, in which the electrolytic deposition deposits a layer of chrome-chromium oxide metal on the annealed electrolytic tin plate by recovery and cold-rolled, optionally die-cast, which contains a total content of at most 140 mg / m2 chrome, preferably at most 90 mg / m2 and more preferably at most 80 mg / m2.