RS59292B1 - Method for manufacturing chromium-chromium oxide coated substrates - Google Patents

Description

Description

[0001] The present invention relates to the process for the production of substrates coated with chromium-chromium oxide and to the chromium-chromium oxide substrates thus produced. The present invention further relates to the use of chromium-chromium oxide coated substrates in packaging applications.

[0002] Electrodeposition is the process of depositing a metal coating on a substrate by passing an electric current through an electrolyte solution containing the metal to be deposited.

[0003] Traditionally, the electrodeposition of chromium has been achieved by passing an electric current through an electrolyte solution containing hexavalent chromium (Cr(VI)). However, the use of Cr(VI) electrolyte solutions will soon be banned due to the toxic and carcinogenic nature of Cr(VI) compounds. Research in recent years has therefore focused on finding suitable alternatives to electrolytes based on Cr(VI). One alternative is to provide an electrolyte based on trivalent chromium since such electrolytes are substantially less toxic and provide chromium coatings similar to those precipitated from Cr(VI) electrolyte solutions.

[0004] Despite the use of trivalent chromium electrolytes, one major problem is the possible oxidation of trivalent chromium to hexavalent chromium at the anode. In addition to water, some Cr(III) may also be oxidized unintentionally to Cr(VI) at the anode, because the electrode potential for the oxidation of water to oxygen and the oxidation of Cr(III) to Cr(VI) are very close.

[0005] US2010/0108532 discloses a process for chromium plating from a trivalent chromium plating bath. According to US2010/0108532, the electrolyte contains chromium metal added as basic chromium sulfate, sodium sulfate, boric acid or maleic acid. The electrolyte further contains manganese ions to reduce the formation of excessive amounts of hexavalent chromium. Although the formation of excessive amounts of hexavalent chromium has been avoided, hexavalent chromium is still produced.

[0006] In contrast to US2010/0108532, EP0747510 describes a process for the precipitation of chromium oxide from a solution of trivalent chromium that is without added buffer. Due to the absence of a buffer, the pH increases in the cathode film, which in turn allows the direct formation of chromium oxide at the cathode. According to EP0747510, the formation of hexavalent chromium on the anode can be prevented or reduced by choosing a suitable anode, e.g. platinum, platinized titanium, nickel-chromium or carbon, and using a depolarizer such as potassium bromide. However, the trivalent chromium electrolyte solution used in EP0747510 also contains potassium chloride, which is converted to chlorine during the electrodeposition process. Chlorine gas is potentially harmful to the environment and workers and is therefore undesirable. WO 2013/143928 also describes a process for the precipitation of chromium oxide from a solution of trivalent chromium containing chlorine.

[0007] The aim of the present invention is to provide a process for depositing a coating on a substrate from a trivalent chromium solution that avoids the formation of hexavalent chromium and has low porosity.

[0008] Another goal of the present invention is to provide a process for depositing a coating on a substrate from a trivalent chromium solution that avoids the formation of chlorine gas and has low porosity.

[0009] The first aspect of the invention relates to the process for the production of a substrate coated with chromium metal - chromium oxide according to patent claim 1.

[0010] This invention relates to the deposition of multiple layers of chromium and chromium oxide (Cr-CrOx) from a trivalent chromium electrolyte by electrolysis in a strip coating line. Conventionally, the chromium layer is deposited first and then the CrOx layer is produced over it in the second step of the process. In the process according to the invention, Cr and CrOx are formed simultaneously (ie in one step), denoted as Cr-CrOx layer. Chromium oxide is dispersed in a chromochromium oxide shell obtained from a one-step deposition process according to the invention. This is in contrast to the two-step process where the first Cr-layer is deposited, followed by the surface conversion of this Cr-layer to CrOx and which consequently leads to a layered structure. The next difference when two (or more) layers are deposited in a two-step process is that the later layers consist of chromium metal, and only after the last deposition of the Cr-layer would the conversion of the surface of this Cr-layer into CrOx be performed. So CrOx is present in the conventional layer, only over the last layer. In the coated substrate according to the invention, each individual layer contains CrOx distributed in each chromium-chromium oxide layer. The degree of porosity is reduced by depositing multiple (>1) Cr-CrOx plating layers one over the other on one or both sides of the electrically conductive substrate. Each individual Cr-CrOx layer is deposited in one step, and multiple individual layers are deposited in subsequent plating cells or even subsequent plating lines, or by passing through a single cell or plating line more than once. Between depositions of multiple layers, hydrogen bubbles must be removed from the tape surface. After depositing one or more layers on the substrate, the substrate with these one or more layers is understood to be a strip. The bubbles adhere to the outer surface of the coated substrate and it is necessary to remove the bubbles from this surface before depositing the next Cr-CrOx layer.

[0011] A buffering agent is a weak acid or base used to maintain the acidity (pH) of a solution close to a selected value after the addition of the next acid or base. That is, the function of a buffering agent is to prevent a rapid change in pH when acids or bases are added to a solution. Boric acid is a buffering agent.

[0012] In the invention, hydrogen bubbles are removed from the surface of the tape using a plate pulse rectifier or by shaking.

[0013] Using a plate pulse rectifier or shaking the bubbles were removed and the following Cr-CrOx cladding layers were then deposited on the de-bubbled surface. A product coated on one or both sides with multiple individual layers of Cr-CrOx cladding layers passes all performance tests for packaging applications if the Cr-CrOx cladding steel substrate is provided with a polymer sheath. Its performance is similar or even better than conventional ECCS material (based on Cr(VI)!) with polymer coating. The deposition of CrOx is driven by an increase in surface pH as a consequence of the reduction of H<+> (more formally: H3O<+>) to H2(g) on the strip surface (which is the cathode). This means that bubbles of hydrogen are formed on the surface of the strip. Most of these bubbles are extruded during the plating process, but a smaller number stick to the substrate over time sufficient to cause minor plating at these points leading to porosity of the chromium and chromium oxide (Cr-CrOx) layer. This substrate with only one layer of chromium and chromium oxide (Cr-CrOx) passes all performance tests for packaging applications where a steel substrate with a Cr-CrOx cladding layer is provided with a polymer jacket. Its performance is thus similar to conventional ECCS material (based on Cr(VI)!) with a polymer coating. However, there is still a need to produce a coating on a substrate from a solution of trivalent chromium that avoids the formation of chlorine gas and hexavalent chromium that has even lower porosity. The inventors have found that the addition of multiple electrolyte plating layers and the process of the invention results in plating layers with very low or no porosity. There is no preference for hydrogen bubbles to form at the location of the former porosity, because the current density exchange of the substrate at the location of the porosity is similar to that of the chromium and chromium oxide layer. In this way bubbles will form at random points and not preferably at porosity. The resulting layer since two or more layers are deposited (ie, a larger number, which is 2 layers or more) is consequently significantly or completely pore-free and performs on par with Cr(VI)-based product rappers.

[0014] These inventors have found that regardless of the catalytic coating material (platinum, iridium oxide or mixed metal oxide), toxic chlorine gas is formed at the anode when a chromium-chromium oxide coating is electrolytically deposited from a chloride containing trivalent chromium electrolyte. Although a depolarizer such as bromide has been found to greatly suppress this harmful side reaction, it would not be possible to completely prevent the formation of chlorine gas. In order to prevent the release of chlorine gas at the anode, chloride-containing compounds, e.g. conductivity-enhancing salts such as potassium chloride are omitted from trivalent chromium-based electrolytes.

[0015] The boric acid buffer agent was initially omitted from the chromium-based trivalent electrolyte so that chromium oxide would preferentially form at the cathode, i.e. especially on the metal chromium. The absence of boric acid buffering agent in the electrolyte has the effect of making the anode very acidic:

2H2O → 4H+ O2(g) 4e-

As a result of the above reaction, it is understood that the oxidation of Cr(III) to Cr(VI) is avoided or at least suppressed:

Cr3+ 4H2O ⇔ HCrO -4+ 7H+ 3e-

However, when the electrodeposition of the chromium-chromium oxide coating is performed in the presence of the electrolyte according to the invention, i.e. electrolyte without chloride ions and without the buffering agent boric acid, sulfate containing salt to increase conductivity, and anode containing a catalytic platinum coating, a significant amount of hexavalent chromium was recorded at the anode. Surprisingly, hexavalent chromium formation was found to be avoided when the platinum catalyst liner was replaced with an iridium oxide or mixed metal oxide catalyst liner. However, when the boric acid buffering agent was reintroduced into the aforementioned chloride-free trivalent chromium-based electrolyte, a significant amount of hexavalent chromium was re-formed at the anode, even when the anode contained an iridium oxide or mixed metal oxide catalytic coating.

[0016] The omission of boric acid from the electrolyte and the choice of an anode coated with iridium oxide or mixed metal oxide has the additional advantage that it is not necessary to provide an electrolyte with additives, e.g. Mn2+ ions, in order to suppress or avoid the formation of hexavalent chromium.

[0017] According to US6004448 two different electrolytes for ECCS production via trivalent Cr chemistry. Cr metal was deposited from the first electrolyte with boric acid buffer and then Cr oxide was deposited from the second electrolyte without boric acid buffer. According to this patent application, in a continuous high-speed line, the problem arises that boric acid from the first electrolyte will be increasingly introduced into the second electrolyte due to the extraction from the vessel containing the first electrolyte to the vessel containing the second electrolyte, and as a result, the deposition of Cr metal increases and the deposition of Cr oxide decreases or even ends. This problem is solved by adding a complexing agent to the second electrolyte which neutralizes the buffer introduced. The inventors of the present invention have discovered that only a simple, unbuffered electrolyte is required to produce ECCS via trivalent Cr chemistry. Even though this simple electrolyte does not contain a buffer, the inventors of the present invention surprisingly also found Cr metal precipitated from this electrolyte as a consequence of the partial reduction of Cr oxide to Cr metal. This discovery enormously simplifies the overall production of ECCS, because a buffered electrolyte for Cr metal deposition is not needed as wrongly assumed in US6004488, but only a simple unbuffered electrolyte, which also solves the contamination problem of this buffered electrolyte.

[0018] In a preferred embodiment, the electrolyte contains a salt to increase conductivity, preferably alkali metal sulfate, more preferably potassium sulfate. The inventors found that alkali metal sulfate-based conductivity salts were suitable substitutes for chloride-based conductivity salts in that good electrolyte conductivity was still obtained, albeit to a lesser degree. An additional advantage is that the use of such electrolytes in combination with iridium oxide coatings or mixed metal oxide anode coatings avoids the formation of harmful by-products such as hexavalent chromium and chlorine. It was found that electrolytes containing potassium sulfate as a conductivity increasing salt were very suitable for increasing the conductivity of the electrolyte. Chloride-free lithium, sodium or ammonia salts are also very suitable for increasing the conductivity of the electrolyte. Sodium sulfate is particularly preferred since the solubility of sodium sulfate is much higher than that of potassium sulfate. A higher salt concentration increases the kinematic viscosity of the electrolyte and allows the use of lower currents for the deposition of chromium-chromium oxide coatings. By lowering the current density, the risk of unwanted side reactions, e.g. oxidation of Cr(III) to Cr(VI), is reduced and the service life of the catalytic coating can be extended.

[0019] In a preferred embodiment, the chelating agent contains an alkali metal cation and a carboxylate. The benefit of using an alkali metal cation is that its presence greatly increases the conductivity of the electrolyte. Potassium or sodium cations are particularly preferred for this purpose, since compared to other alkali metal cations, they give the greatest increase in conductivity. Chelating agents containing carboxylate anions, preferably having between 1 and 6 carbon atoms, have been used to improve the coating characteristics of the chromium-chromium oxide coating. Suitable carboxylate anions include oxalate, malate, acetate, and formate, with formate being most preferred, since very good coating characteristics are obtained. The carboxylate anions mentioned above are weak chelating agents and can be used individually or in combination. These weak chelating agents destabilize the very stable hexaaqua complex, where L represents the ligand of the chelating agent:

[0020] When the electrolyte contains sodium sulfate, it is preferable to use sodium formate, for example instead of potassium formate, since this simplifies the composition of the electrolyte.

[0021] In a preferred embodiment, the electrolyte solution does not have a buffering agent. It was found that the absence of a buffering agent in the electrolyte allows the deposition of chromium oxide rather than chromium metal. In addition, the omission of the boric acid buffering agent from the electrolyte means that the oxidation of Cr(III) to Cr(VI) is prevented or at least suppressed when the electrolyte contains an alkali metal sulfate as a conductivity enhancing salt. By omitting the buffer from the electrolyte, the pH value of the cathode surface increases to between 6.5 and 11.5, so that chromium oxide will precipitate next to the chromium metal.

[0022] According to the invention, the trivalent chromium compound contains basic chromium(III) sulfate. Basic chromium sulfate is very suitable as an alternative to chloride containing chromium compounds such as chromium(III) chloride. By using basic chromium sulfate in the electrolyte instead of chloride containing a chromium compound, the risk of chlorine gas production at the anode was avoided. Other preferred trivalent chromium salts include chromium(III) formate, chromium(III) oxalate, chromium(III) acetate, chromium(III) potassium oxalate, and chromium(III) nitrate. The above salts, including basic chromium(III) sulfate may be provided individually or in combination.

[0023] In a preferred embodiment, the mixed metal oxide contains iridium and tantalum oxides. Typically, the anode is provided with a platinum-based electrocatalytic coating. However, the inventors have determined that hexavalent chromium is produced when this type of anode is brought into contact with a chloride-free trivalent chromium-based electrolyte. It was found that electrocatalytic coatings containing a mixture of iridium oxide and tantalum oxide did not cause the formation of hexavalent chromium on the anode when the anode was immersed in a chloride-free trivalent chromium electrolyte.

[0024] In a preferred embodiment, the electrolyte solution does not have a depolarizer, preferably potassium bromide. According to EP0747510, the presence of a depolarizer such as bromide in an electrolyte based on trivalent chromium suppresses the oxidation of Cr(III) to Cr(VI). However, the inventors found that despite the absence of a depolarizer in the electrolyte, no hexavalent chromium was formed at the (platinum-coated) anode when the electrolyte was a trivalent chromium chloride based electrolyte. Instead, the depolarizer was found to suppress chlorine formation. The inventors have also found that when the trivalent chromium electrolyte of the invention contains a depolarizer and a sulfate-based conductivity salt, a significant amount of hexavalent chromium is formed at the platinum-coated anode. In addition, bromine gas was found to be formed when the depolarizer contained potassium bromide. Bromine gas is potentially harmful to the environment and workers and is therefore undesirable. The inventors have found that in order to avoid the formation of hexavalent chromium, it is not necessary to provide a depolarizer, e.g. potassium bromide when performing electrodeposition in the presence of a trivalent chromium-based electrolyte containing a sulfate-based conductivity-enhancing salt and a mixed metal oxide-coated anode. Hexavalent chromium is also formed on iridium oxide-coated anodes when a depolarizer is omitted from a trivalent chromium-based electrolyte.

[0025] In a preferred embodiment, the pH of the electrolyte solution is adjusted to between 2.6 and 3.4, preferably to between pH 2.8 and pH 3.0. It was found that the pH of the electrolyte affects the composition, appearance of the surface, e.g. the color and morphology of the chromium-chromium oxide coating surface. Regarding the effect of pH on the composition of the chromium-chromium oxide coating, it was found that the amount of chromium metal deposited on the cathode could be increased by providing an electrolyte based on trivalent chromium having a pH value between pH 2.6 and 3.0. On the other hand, if the pH of the electrolyte is adjusted to above pH 3.0, chromium oxide precipitates rather than chromium metal.

[0026] It is also clear that surface pH has an effect on the surface appearance of the deposited coating. In this sense, it was observed that the surface appearance of the chromium-chromium oxide coating changed from gray to brownish color because the pH value of the electrolyte increased. This is attributed to the composition of the chromium-chromium oxide coating containing more chromium metal (grey) at low pH and more chromium oxide (brown) at higher pH. Regarding the appearance of the surface, it is desirable to provide an electrolyte that has a pH value between 2.6 and 3.0 so that a chrome-chromium oxide coating is obtained that is predominantly gray in color.

[0027] The pH value of the electrolyte has a direct influence on the surface morphology of the chromachrome oxide coating. In this sense, the use of an electrolyte having a pH value above 3.0 resulted in a chromium-chromium oxide coating having a relatively open and coarse structure. In contrast, when the pH value was between 2.6 and 3.0, preferably between 2.8 and 3.0, the resulting chromium-chromium oxide coating was characterized by a more compact coating structure that exhibited reduced porosity compared to coatings deposited at pH values above 3.0. From the perspective of surface morphology, it is desirable to provide an electrolyte having a pH between 2.8 and 3.0 because it is possible to obtain a greater improvement in the passivation properties of the coating in terms of reduced porosity of such coatings.

[0028] It was also determined that the pH value of the electrolyte affects the rate at which the chromachrome oxide coating is deposited on the substrate. This can be understood by considering the mechanism of chromium oxide deposition. Chromium oxide deposition on the cathode occurs at pH values between 6.5 and 11.5 and is driven by the reduction of H<+>(H3O+) to H2(g). With this mechanism in mind, the use of an electrolyte that has an acidic pH value will increase the electrolysis time required to deposit the chromium-chromium oxide coating, since more H<+> must be reduced to raise the surface pH to between 6.5 and 11.5 so that the chromium oxide is deposited. Since an increase in electrolysis time will result in a more expensive production process, it is desirable to provide an electrolyte with a pH value of at least 3.4. However, in light of the effects mentioned above regarding the composition, appearance, and morphology of the deposited chromium-chromium oxide coating, an electrolyte pH of at least 2.8 is preferred.

[0029] It was found that the temperature of the electrolyte solution also affects the deposition reaction and surface appearance of the chromium-chromium oxide coating. It has been found that an electrolyte solution having a temperature between 30°C and 70°C is very suitable for the deposition of a chromium-chromium oxide coating with a good surface appearance. Preferably, the temperature of the electrolyte solution is between 40°C and 60°C as this leads to a more efficient deposition reaction. Within this temperature range, the electrolyte solution exhibits good conductivity, which means that less energy is required to deposit the chromium-chromium oxide coating.

[0030] In a preferred embodiment, an electrically conductive substrate is provided by electrolytic deposition of a tin coating on one or both sides of the steel substrate and subjects the tin-coated steel to a diffusion annealing treatment to form an iron-tin alloy on the steel.

[0031] Preferably, the steel substrate comprises recrystallization annealed single-reduced steel or double-reduced steel that has undergone a recrystallization annealing treatment between the first rolling treatment and the second rolling treatment. A tin coating may be provided on one or both sides of the steel substrate in the tin electroplating step, wherein the weight of the tin coating is at most 1000 mg/m<2>and preferably between at least 100 and/or at most 600 mg/m<2>substrate area. By diffusion annealing the tin-galvanized substrate at a temperature of at least 513°C in a reducing atmosphere, the tin layer is converted into an iron-tin alloy containing at least 80 weight percent (wt.%) FeSn (50 at.% iron and 50 at.% tin). This substrate can then be rapidly cooled in an inert, non-oxidizing cooling medium, while the coated substrate is held in a reducing or inert gas atmosphere prior to cooling, so that a robust, stable surface oxide is obtained. The FeSn alloy layer provides corrosion protection to the underlying steel substrate. This is partly achieved by protecting the substrate, as the FeSn alloy layer is very dense and has very low porosity. In addition, the FeSn alloy itself is very corrosion resistant by nature.

[0032] According to the invention, the electrically conductive substrate contains a black sheet or a white sheet. The process of the present invention has been found to be very suitable for depositing a chromium-chromium oxide coating on black sheet (also known as uncoated steel) and white sheet, both of which are commonly used in the packaging industry.

[0033] In a preferred embodiment, the organic coating is provided on one or both sides of the chromium-chromium oxide metal coated substrate. It was found that organic coatings could be easily deposited on the chromium-chromium oxide coating, which itself acts as a passivation layer to protect the electrically conductive substrate. In the case of a white sheet or steel substrate with a layer of FeSn, a chromium-chromium oxide coating is provided to passivate the sheet surface to prevent or at least reduce the growth of tin oxide, which over time, can cause the applied organic coating to delaminate from the substrate. The chromium-chromium oxide coating also exhibited good adhesion to the electrically conductive substrate and to the subsequently applied organic coating. The organic coating can be provided as a varnish or as a thermoplastic polymer coating. Preferably, the thermoplastic polymer coating is a polymer coating system comprising one or more layers of thermoplastic resins such as polyesters or polyolefins, but may also include acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, 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 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. It is possible to use more than two types of dicarboxylic acid or glycol 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 esters, methacrylic acid esters or acrylamides.

• Polyamide resins include for example the 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 ethylenepropylene resin, polyvinyl fluoride and polyvinylidene fluoride.

• Functionalized polymers, for example by maleic anhydride grafting, include, for example, modified polyethylenes, modified polypropylenes, modified ethylene acrylate copolymers and modified ethylene vinyl acetates.

[0034] It is possible to use mixtures of two or more resins. In addition, the resin can be mixed with antioxidant, heat stabilizer, UV absorbent, plasticizer, pigment, nucleating agent, antistatic agent, release agent, anti-blocking agent, etc. The use of such thermoplastic polymer coating systems has been shown to provide excellent performance in can making and can use, such as shelf life.

[0035] The invention can be used to provide a substrate coated with chromium chromium oxide metal.

[0036] Chromium carbide was present in the chromium-chromium oxide coating in the chromium metal layer immediately adjacent to the cathode (not found in the chromium oxide layer). It is clear that the anion of the chelating agent, e.g. formate, can be a source of carbide. The presence of chromium carbide in chromium metal is believed to stimulate growth in an upward direction relative to the substrate.

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[0037] Organic carbon is predominantly found in the chromium oxide layer, but it is also found in the chromium metal layer, more specifically, between the chromium metal grains in the chromium metal layer. Chromium carbide would be found at the boundaries of these grains.

[0038] Chromium sulfate is also found in chromium-chromium oxide coating. More specifically, sulfate was present in the chromium oxide layer, indicating that sulfur is incorporated into (bonded to) the chromium oxide layer during its formation.

[0039] The invention will now be explained by means of some examples. These examples are intended to enable those skilled in the art to practice the invention and in no way limit the scope of the invention as defined in the claims.

[0040] A sample of packing steel (consisting of commonly used low carbon steel and temper) was cleaned in a commercial alkaline cleaner (Chela Clean KC-25 from Foster Chemicals), rinsed with deionized water, left in a 5% sulfuric acid solution at 25°C for 10 s, and rinsed again. The sample was coated with a tin coating (600 mg/m<2>) from an MSA (methanesulfonic acid) bath commonly used for the production of white sheet in a continuous coating line. A current density of 10 A/dm2 was applied for 1 s.

[0041] To form the iron-tin alloy on the steel, a tin-coated steel sample was annealed in a reducing gas atmosphere, using HNX containing 5% H 2 (g). The sample was then heated from room temperature to 600°C at a heating rate of 100°C/s. Immediately after the sample reached its maximum temperature of 600°C, the sample was cooled in 1 s to a temperature of 80°C using rapid water cooling. The iron-tin alloy layer that was formed contained more than 90% of the FeSn alloy phase.

[0042] A steel sample with a layer of FeSn alloy is provided in a rectangular plating cell with grooves along the side walls to hold the sample and the anode. The chromium-chromium oxide coating was precipitated from an electrolyte containing 120 g/l basic chromium sulfate, 80 g/l potassium sulfate, and 51 g/l potassium formate. This electrolyte solution is chloride-free, buffering agent, e.g. boric acid, and depolarizers such as potassium bromide. The pH value of this electrolyte was approximately 3.85. The temperature of the electrolyte solution was 50°C.

[0043] According to the following embodiment, a chromium-chromium oxide coating is deposited from an electrolyte for the deposition of a Cr-CrOx layer consisting of an aqueous solution of chromium (III) sulfate, sodium sulfate and sodium formate and optionally sulfuric acid, an aqueous electrolyte solution having a pH at 25°C of between 2.5 and 3.5, preferably at least 2.7 and/or at most 3.1. Preferably the electrolyte contains between 80 and 200 g·l<-1>chromium (III) sulfate, preferably between 80 and 160 g·l<-1>chromium (III) sulfate, between 80 and 320 g·l<-1>sodium sulfate, preferably between 80 and 320 g·l<-1>sodium sulfate and between 30 and 80 g·l<-1>sodium formate.

[0044] In order to determine the effect of pH on the electrolysis time, current density and color in the deposition of chromium-chromium oxide coatings, the pH of the electrolyte was gradually adjusted from pH 3.85 to 3.4, 3.2, 3.0, 2.8 and 2.6 respectively by adding sulfuric acid (98 wt%). At each pH, the electrolysis time was determined to deposit a total Cr coating weight of ~60 mg/m<2>, as determined by X-ray fluorescence (XRF) analysis using a SPECTRO XEPOS XRF spectrometer with a Si-Drift detector.

[0045] Similarly, the current density was determined at a fixed electrolysis time of 1 s. In each of these experiments the color of the chromium-chromium oxide coating was determined using a Minolta CM-2002 spectrophotometer according to the well-known CIELab system. The CIELab system uses three color values L*, a* and b* to describe colors, which are calculated from the so-called tristimulus values X, Y and Z. L* represents the hue of the color (L* = 0 gives black and L* = 100 means diffuse white). The a* value represents the green-red chromatic axis in the CIELab color space. The b* value represents the blue-yellow chromatic axis. The results of the deposition and color measurement experiments are shown in Table 1.

[0046] The results showed that either a longer electrolysis time or a higher current density is required to deposit the same amount of chromium when the electrolyte becomes more acidic. It can also be seen from the color measurement that as the pH value increases, the color of the deposited chromium-chromium oxide changes from pure gray to brownish. From the above experiments it appears that using an electrolyte having a pH of approximately 3.0 gives the best compromise between deposition rate and appearance. In applications where the appearance of the coating is less important, it follows that the pH of the electrolyte can be increased to a pH value that is more basic so that the electrolysis time or current density is reduced. In doing so, a more economical production process will be obtained.

[0047] Experiments to examine the effect of pH on surface morphology were also performed using a Zeiss-Ultra 55 FEG-SEM (Field Emission Gun - Scanning Electron Microscope). For optimal image resolution on the outer surface of the samples, a low acceleration voltage of 1 kV was used, in combination with a short working distance and a small aperture.

[0048] A change in the surface morphology of the chromium-chromium oxide layer was noted after adjusting the pH of the electrolyte. In this regard, a relatively open and rough coating structure was obtained when the pH of the electrolyte was set above 3.0. In contrast, when the pH value of the electrolyte was adjusted to between 2.6 and 3.0, a relatively compact, non-porous coating was obtained that exhibited good passivation properties.

[0049] To obtain chemical information on these samples, energy dispersive X-ray (EDX) analysis was performed with a standard accelerating voltage of 15 kV, standard working distance and aperture. These settings resulted in a dead time between 0 - 35%. For all samples, an average EDX spectrum was collected on an area of 1000 µm x 750 µm for 50 s.

Table 1

[0050] The obtained EDX spectra showed that the amount of oxygen in the chromium-chromium oxide coating increased with increasing pH, indicating that chromium oxide precipitates before chromium metal because the electrolyte becomes less acidic. EDX spectra also revealed the presence of chromium sulfate in the chromium-chromium oxide coating.

[0051] X-ray photoelectron spectroscopy (XPS) was also used to characterize the samples (Table 2). XPS spectra and depth profiles were recorded on a Kratos Axis Ultra using Mg Kα X-rays at 1253.6 eV. The measured spot size was 700 µm X 300 µm. Depth profiles were recorded using 4 keV Ar<+>ions creating a 3 mm X 3 mm scatter crater. The sputtering rate was calibrated using a BCR standard of 30 nm Ta2O5na Ta and was 2.15 nm/min. The scattering rate for Cr species is expected to be similar to Ta2O5.

[0052] According to SEM/EDX analysis, the amount of chromium oxide deposited increases significantly when the pH of the electrolyte is above pH 3.0. XPS measurements also showed that at higher pH, the increase in the amount of deposited chromium oxide was greater when a constant current density was used compared to a varied current density and the electrolysis time remained constant. These same trends were noted when analyzing the sulfate content of the deposited coating and that sulfate was present throughout the chromium oxide layer, indicating that the sulfate was bound to the chromium oxide and not simply dispersed within it. This was confirmed when the samples were subsequently washed in deionized water and no significant reduction in sulfate content was noted. Chromium oxide was also found to be formed

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during deposition, and not after that when the samples are exposed to the atmosphere, ie. by oxidation of chromium metal with air.

[0053] It can also be seen that both chromium metal and chromium carbide are co-precipitated and that the chromium metal content is reduced as the pH becomes less acidic, especially at pH above 3.0. In addition, chromium carbide is predominantly found in the chromium metal layer next to the iron-tin alloy. When the chelating agent was omitted from the electrolyte, no chromium carbide was noted in the chromium layer, indicating that the chelating agent, in this case potassium formate, was the source of the carbide. Organic carbon, i.e. carbon that is not in the form of carbides is found in the chromium oxide layer.

[0054] The porosity of the coatings was also measured by integrating the atomic percentage (as determined by XPS) of Sn Fe/Cr over the outermost 3.2 nm of the coating. Each coating consisting of one coating layer, with the exception of the outer layer at pH 2.6, exhibited a porosity of less than 3.0%. It is clear from Table 2 that the degree of porosity is dramatically reduced already after 2 layers, which is therefore generally considered sufficient. The thickness of the 2-layer lining in Table 2 is twice the thickness of the single-layer lining, but the reduction in the degree of porosity is independent of the thickness of the two layers. Consequently, in a practical case, the total thickness of a single-layer cladding and a double-layer cladding will be similar. The total thickness of a layer consisting of a plurality of individual layers (ie 2 or more) is preferably between 20 and 150 mg/m<2>as expressed as total Cr, more preferably between 25 and 100 mg/m<2>as expressed as total Cr, even more preferably at least 40 and/or at most 85 mg/m<2>. The thickness of the lining layer is expressed in mg/m<2> as it is expressed in total Cr. This is therefore also a measure of the weight of the lining as expressed in total Cr. The thickness of the chromium-chromium oxide coating layer corresponding to 25 mg/m2 is equivalent to 3.5 nm using the specific density of Cr which is 7150 kg/m<3>(25 mg/m<2>= 2.5·10<-2>g/m<2>= 2.5·10<-5>kg/m<2>so therefore → 2.5·10<-5>kg/m<2> divided 7150 kg/m<3>results in a thickness of 3.5·10<-9>m = 3.5 nm.The thickness of the coating layer of 100 mg/m2 as expressed in total Crje is therefore 14 nm.

Table 2

Table 2 (continued)

[0055] Research was also carried out in order to understand under what circumstances hexavalent chromium and/or other harmful side products were formed on the anode. Each electrolyte contained 120 g/l basic chromium sulfate. The electroactive specific surface area of the anode was 122 mm x 10 mm. The anode current density was 60 A/dm2. Ambient air above the solution was analyzed using chlorine 0.2/a Dräger-tubes®. The concentration of Cr(VI) in the Cr(III) electrolyte was analyzed using differential pulse polarography (DPP). The results of the research after 5 hours of electrolysis are shown in Table 3 below.

[0056] The results (Table 3) show that when the electrolyte contains chloride ions (Test No. 1 and No. 2), chlorine gas is produced at the anode and the presence of a depolarizer such as bromide in the electrolyte strongly suppresses, but does not eliminate, this harmful side reaction (Test No. 1). The results also show that the presence of bromide in the electrolyte has no role in preventing the formation of hexavalent chromium at the anode when the electrolyte contains chloride ions (see Test No. 1 and Test No. 2).

[0057] When the conductivity-enhancing salt contains sulfates instead of chlorides, significant amounts of hexavalent chromium are formed at the anode when the anode contains a platinum catalytic coating (see Test No. 3 and No. 4). It can even be seen that the presence of bromide in the sulfate containing electrolyte

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increases the formation of hexavalent chromium. However, when the platinum catalyst coating was replaced with a mixed metal oxide catalyst coating of tantalum oxide and iridium oxide, no hexavalent chromium was formed at the anode (Test No. 5 and No. 6). The presence of potassium bromide in the electrolyte (Test No. 5) does not seem to play a role in preventing the formation of hexavalent chromium. The formation of hexavalent chromium on the anode was also avoided when the anode contained a catalytic coating of iridium oxide (Test No. 7 and No. 8). However, when the chloride-free electrolyte contained sulfates and boric acid, hexavalent chromium at the anode was once again noted (Test No. 9). The results suggest that when the electrolyte is free of chloride ions (to avoid chlorine formation at the anode) and alkali metal sulfate is used as a salt to increase conductivity, the electrolyte should be free of boric acid buffering agent and the anode should not contain platinum or a platinum-based catalytic coating (to avoid hexavalent chromium formation at the anode).

Table 3

[0058] Experiments were also performed to examine the composition of chromium-chromium oxide coatings that were (i) deposited according to the process of the subject invention (one-step process) or (ii) deposited according to the process of EP0747510 (two-step process). The use of a one-step or two-step deposition process was found to affect the composition of the deposited chromium-chromium oxide coating. Specifically, chromium-chromium oxide coatings obtained from the two-step process contained less chromium oxide than chromium-chromium oxide coatings obtained from the one-step process. In addition, when the two-step deposition process was used, a greater proportion of chromium oxide was concentrated on the surface of the chromium-chromium oxide coating, while the chromium oxide was more evenly distributed in the chromium-chromium oxide coating obtained from the one-step deposition process. It was also found that the chromium carbide content was significantly higher for

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chromium-chromium oxide coatings obtained from the two-step process compared with those

obtained from a one-step procedure.

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Claims (11)

Patent claims 1. Process for the production of a substrate coated with metal chromium - chromium oxide - chromium carbide - chromium sulfate by electrolytic deposition of a coating layer containing a number of coating layers of metal chromium - chromium oxide - chromium carbide - chromium sulfate on an electrically conductive substrate made of black sheet or white sheet for packaging applications from an electrolyte solution containing a trivalent chromium compound containing basic chromium (III) sulfate, and chelating means, wherein the electrolyte solution is free of chloride ions and without the buffering agent of boric acid, wherein the electrically conductive substrate acts as the cathode, and wherein the anode containing a catalytic coating of iridium oxide or mixed metal oxide is selected to reduce or eliminate the oxidation of Cr(III)-ions to Cr(VI)-ions to avoid the formation of chlorine gas and hexavalent chromium, wherein during the deposition of each chromium metal plating layer - chromium oxide - chromium carbide - chromium sulfate formed hydrogen bubbles on the surface of the strip, and wherein between the deposition of the coating layers of chromium metal - chromium oxide - chromium oxide - chromium sulfate, the hydrogen bubbles were removed from the surface of the strip, wherein the chromium oxide was distributed in each coating layer and wherein the hydrogen bubbles were removed from the surface of the strip using a plate pulse rectifier or shaking. 2. The method according to claim 1, characterized in that the electrolyte contains a salt to increase conductivity, preferably alkali metal sulfate, more preferably potassium sulfate or sodium sulfate. 3. The method according to any of the preceding patent claims, characterized in that the chelating agent contains an alkali metal carboxylate, preferably potassium formate or sodium formate. 4. The method according to any of the preceding patent claims, characterized in that the mixed metal oxide contains iridium and tantalum oxides. 5. The method according to any of the preceding patent claims, characterized in that the electrolyte solution is without potassium bromide. 6. The method according to any of the preceding claims, characterized in that the pH of the electrolyte solution is set between pH 2.6 and pH 3.4, preferably between pH 2.8 and pH 3.0. 7. A method according to any of the preceding claims, characterized in that the organic coating is provided on one or both sides of the substrate coated with chromium metal - chromium oxide - chromium carbide - chromium sulfate. 8. The method according to patent claim 7, characterized in that the organic coating provided on one or both sides of the substrate coated with metal chromium - chromium oxide - chromium carbide - chromium sulfate contains one or more layers of polyester or polyolefin. 9. The method according to patent claim 7, characterized in that the organic coating is varnish. 10. The method according to any of the preceding patent claims, characterized in that the weight of the cladding layer consisting of a number of individual layers is between 20 and 150 mg/m<2>, preferably between 25 and 100 mg/m<2> as expressed in total Cr. 11. The method according to claim 10, characterized in that the weight of the coating layer consisting of a large number of individual layers is at least 40 and/or at most 85 mg/m<2> as expressed in total Cr. Published and printed by: Institute for Intellectual Property, Belgrade, Kneginje Ljubice 5 1