WO2019121582A1

WO2019121582A1 - Method for manufacturing chromium-chromium oxide coated blackplate

Info

Publication number: WO2019121582A1
Application number: PCT/EP2018/085335
Authority: WO
Inventors: Jacques Hubert Olga Joseph Wijenberg; Adrianus Jacobus Wittebrood; Mark Willem LITZ
Original assignee: Tata Steel Ijmuiden B.V.
Priority date: 2017-12-22
Filing date: 2018-12-17
Publication date: 2019-06-27
Also published as: EP3728701A1

Abstract

This invention relates to a method for electroplating uncoated blackplate with a plating layer and an improvement thereof. The invention also relates to a method wherein the coated blackplate is further coated on one or both sides by a film lamination step or a direct extrusion step, and to the use of said coated blackplate in a process to produce containers for packaging purposes.

Description

METHOD FOR MANUFACTURING CHROMIUM-CHROMIUM OXIDE COATED BLACKPLATE

This invention relates to a method for electroplating an uncoated steel strip with a plating layer and an improvement thereof.

In continuous steel strip plating, a cold-rolled steel strip is provided which is usually annealed after cold-rolling to soften the steel by recrystallisation annealing or recovery annealing. After the annealing and before plating the steel strip is first cleaned for removing oil and other surface contaminants. After the cleaning step, the steel strip is pickled in a sulphuric or hydrochloric acid solution for removing the oxide film. Between different treatment steps the steel strip is always rinsed with deionised water to prevent contamination of the solution used for the next treatment step with solution of the preceding treatment step. Consequently the steel strip is thoroughly rinsed after the pickling step. During rinsing and transport of the steel strip to the plating section a fresh thin oxide layer is formed instantly on the bare steel surface and needs to be protected quickly.

One such process used in electroplating is called electrodeposition. The part to be plated (the steel strip) is the cathode of the circuit. The anode of the circuit may be made of the metal to be plated on the part (dissolving anode, such as those used in conventional tinplating) or a dimensionally stable anode (which does not dissolve during plating). The anode and cathode are immersed in an electrolyte solution containing ions of the metal to be deposited onto the blackplate substrate.

Blackplate is a tin mill product which has not (yet) received any metallic coating during production. It is the basic material for the production of other tin mill products. Blackplate can be single reduced (SR) full-hard or annealed (recovery annealed or recrystallisation annealed) or double reduced (DR) in which case it has been subjected to a second cold rolling reduction after annealing. The SR or DR blackplate is usually provided in the form of a coiled strip and is the uncoated starting material for the coating process according to the invention. Figure 1 schematically summarises the process steps to obtain the coated product, starting from a hot-rolled strip. Before cold-rolling, the hot-rolled strip is usually pickled (not shown) to remove the hot-rolling scale and cleaned (not shown) to remove any contaminants from the strip.

In the production of packaging steels with an electroplated chromium coating from an electrolyte solution comprising a trivalent chromium compound on an uncoated steel strip (blackplate), as disclosed in WO2014202316-A1, it occurs occasionally that at lower line speeds, the resulting coated product has a stripy appearance. Although the stripes are very faint and cannot be detected with XPS and/or SEM, they are nevertheless visible with the naked eye. The pattern appears to be even more visible when a clear lacquer coating (thermosetting coating) or polymer coating (thermoplastic coating) is applied onto the coated product. Although the coated product performs just as well in terms of corrosion performance, coatability, adherence of the lacquer coating or polymer coating to the coated product and the can-making performance of the coated product, no adverse effects have been observed of the presence of the stripes, the stripy appearance is considered to be visually less appealing and therefore undesired.

Also, it was observed that for some applications, the adhesion of an organic coating was not completely satisfactory. So if the stripy appearance would occur, the lacquer used to cover these stripes would show insufficient adherence.

WO2015177314-A1 discloses a method for producing a steel substrate coated with a chromium metal-chromium oxide (Cr-CrOx) coating layer in a continuous high speed plating line, operating at a line speed of at least 100 m/min, wherein one or both sides of the electrically conductive substrate in the form of a strip, moving through the line, is coated with a chromium metal-chromium oxide (Cr-CrOx) coating layer from a single electrolyte by using a plating process. The chromium content in the electrolyte contained 120 g/l basic chromium sulphate (=0.385 M).

It is an object of the invention to improve the surface appearance of an electroplated chromium coating deposited from an electrolyte solution comprising a trivalent chromium compound on an uncoated (blackplate) steel strip.

It is also an object of the invention to improve the adhesion of an organic coating to the electroplated chromium coating deposited from an electrolyte solution comprising a trivalent chromium compound on an uncoated (blackplate) steel strip.

The object is reached with a method for manufacturing a chromium metal - chromium oxide coated blackplate by electrolytically depositing the chromium metal - chromium oxide coating on blackplate in a continuous high speed plating line operating at a line speed of at least 50 m/min from an electrolyte solution comprising a trivalent chromium compound, wherein the electrolyte solution is free of chloride ions and of a boric acid buffering agent, the electrically conductive substrate acts as a cathode and an anode comprising a catalytic coating of iridium oxide or a mixed metal oxide for reducing or eliminating the oxidation of Cr³⁺-ions to Cr⁶⁺-ions, wherein the electrolyte solution contains at most 250 mM Cr³⁺-ions, a complexing agent at a molar ratio of at least 1 : 1, 0 to 2800 mM of sodium sulphate (Na₂S0₄), a pH of between 1.5 and 3.0 measured at 25 °C, and wherein the plating temperature is between 30 and 70 °C, wherein a first chromium metal - chromium oxide coating is deposited onto the blackplate, the first chromium metal - chromium oxide coating having a first chromium oxide content, and wherein a second chromium metal - chromium oxide coating is deposited onto the first chromium metal - chromium oxide coating, wherein the first chromium oxide content is lower than the second chromium oxide content, obtained by depositing the first chromium metal - chromium oxide coating at a first current density ( i regime ii) and by depositing the second chromium metal - chromium oxide coating at a higher second current density ( i _regime iii) .

This means that the first current density is chosen in regime II and the second current density is chosen in regime III of the current density versus deposited Cr-metal curve. This curve is determined as explained herein below. The molar ratio is expressed as follows:

As the plating line usually consists of a plurality of individual plating cells, each plating cell usually comprising a plurality of separate anodes, the first chromium metal - chromium oxide coating may consist of several identical layers on top of each other. These layers are not discernible from each other. The same applies to the second chromium metal - chromium oxide coating. The first chromium metal - chromium oxide coating and the second chromium metal - chromium oxide coating are discernible from each other because the oxide content of the second chromium metal - chromium oxide coating is significantly higher than that of the first.

The effect of this method is that the resulting coated blackplate has a better adhesion to an organic coating and the surface is shiny and shows no stripy appearance from an electrolyte with a reduced amount of trivalent chromium compound. The ever increasing demands in terms of adhesion is thereby met.

The method according to the invention can be used in a plating line consisting one or more plating cells each comprising one or more anodes. The requirement is that the anodes can be controlled in terms of current. If the plating line consists of only one plating cell with a plurality of individually controllable anodes then the first chromium metal - chromium oxide coating will be deposited using the first anode(s) and the second chromium metal - chromium oxide coating will be deposited using later anode(s). The optional currentless part can be the part of the plating cells where the anodes are currentless, or it can be in between two plating cells, or it can be a currentless intermediate plating cell. The important part is that the first part of the plating process is executed under regime II conditions, and the second part under regime III (preferably regime Illb) conditions, with an optional currentless interruption in between.

In an embodiment wherein the continuous high speed plating line consists of a sequence of N plating cells (CI ..C_N) each of which has individual current control, wherein the deposition of the first chromium metal - chromium oxide coating at the first current density (i _regime n) is performed in one or more of the first anodes of the plating line, optionally followed by a currentless period, followed by the deposition of the second chromium metal - chromium oxide coating at the second current density (i _regime m) in the last anode or anodes of the plating line. It should be noted that this finding is not limited to the tested situation in which there are 5 plating cells each with one or more anodes, but that this finding is equally valid for plating lines with 3 or 4 and more than 5. It is important that the cells or the anodes in the cells can be controlled individually as to the imposed current.

In an embodiment the chromium oxide in the second chromium metal - chromium oxide coating is between 4 and 24 mg/m², preferably between 6 and 18 mg/m², more preferably between 6 and 15 mg/m², and most preferably between 8 and 15 mg/m².

To achieve the best possible appearance of the first chromium metal - chromium oxide coating, the inventors found that it is preferable that the chromium oxide in the first chromium metal - chromium oxide coating is below 6 mg/m², preferably below 4 mg/m², more preferably below 3 mg/m², and most preferably below 2 mg/m².

The molar ratio is expressed as follows:

The deposition mechanism of the chromium layer from the electrolyte solution according to the invention is assumed to be based on a fast, stepwise deprotonation of the water ligands in the Cr³⁺-formate complex ion induced by a surface pH increase as a result of hydrogen evolution (2H⁺ + 2e H₂(g)) (See Figure 3 and 4A and 4B):

[Cr(HC00)(H₂0)₅]²⁺ + OH [Cr(HC00)(0H)(H₂0)₄]⁺ + H₂0 (regime I)

[Cr(HC00)(0H)(H₂0)₄] ⁺ + OH Cr(HC00)(0H)₂(H₂0) + H₂0 (regime II)

Cr(HC00)(0H)₂(H₂0) + OH [Cr(HC00)(0H)₃(H₂0)₂]- + H₂0 (regime III)

It is noted that the chromium metal - chromium oxide always also contains Cr-carbide. The amount was found to be relatively independent of the deposition process. It is believed that the origin of the carbon in the carbide is the formate ion complexing agent. So even though carbide is not mentioned explicitly herein above and below as a constuent of the chromium metal - chromium oxide, it is present. Values of between 5 and 25% of the total chromium content can be present as Cr-carbide. In addition, minute amounts of Cr-sulphate of up to several percents may be incorporated in the chromium metal - chromium oxide layer. It is important to note that the chromium oxide (and the Cr-carbide and Cr-sulphate (if present)) is more or less uniformly distributed in the Cr-metal. This applies to the first (regime II) layer(s) and to the second (regime III) layers.

In regime II (See Figure 4A), a mixed Cr-metal-carbide-oxide coating is deposited on the steel substrate. In regime I there is no deposition of chromium, and in regime III the amount of deposited chromium drops sharply. In figure 4B the regime III is split in two, indicated with the dashed lines which shows a sharp drop (regime Ilia) to a plateau value (regime Illb). From a plating and process control perspective the deposition of the second chromium metal - chromium oxide coating at a higher second current density is preferably performed in regime Illb.

The current density at which the desired chromium coating weight and composition are obtained, depends on the electrolyte composition, pH, temperature and mass transfer rate (strip speed in case of a strip plating line). In practice, the optimal current density is not a discrete value, but a range of values restricted by a lower and an upper limit. This current density range is called the 'plating window'. Within the plating window, the coating properties fall within certain specifications. From an operational perspective, a large plating window is desired, because this simplifies process control.

The method according to the invention is based on the combination of first depositing a Cr-metal-carbide-oxide coating on the blackplate substrate under the regime II conditions, followed by depositing a Cr-metal-carbide-oxide coating on the blackplate substrate under the regime III conditions.

So it is crucial that the shape of the curve of the current (I) against Cr-metal deposition, or alternatively the curve of the current density (i) against Cr-metal deposition is determined. This can be done easily by varying only the current density and keeping the other parameters equal and measuring the Cr-metal, Cr-oxide and Cr carbide in the deposited layer. The plot as shown in figure 4A will provide the information to determine the positions of the different regimes.

Preferable embodiments are provided in the dependent claims. For the sake of clarity it is noted that 1 mM means 1 millimole/l.

Firstly, it is noted that the process according to the invention is equally applicable to provide a chromium metal-chromium oxide coating on other metal substrates such as nickel plated steel strip.

Secondly, if the pH of the electrolyte solution becomes too high or too low, then sulphuric acid or sodium hydroxide may be added to adjust the pH to a value inside the desired range. Also different acids or bases may be used, but in view of the bath chemistry sulphuric acid and sodium hydroxide are preferable.

Thirdly, as blackplate any steel grade suitable for producing packaging steel may be used. By means of example, but not intended to be limited by this, reference is made to the steel grades for packaging applications in EN10202:2001.

A stripy appearance of a deposited surface is usually associated with a certain inhomogeneity in the electroplating process. A local difference in coverage or local differences in the composition of the coating layer may be the cause of the stripy appearance. It would be obvious to the skilled person to attempt to solve this problem by increasing the amount of deposited material by either increasing the amount of metal ions in the electrolyte, or by increasing the current density. WO2014202316-A1, WO2015177314-A1 and WO2015177215-A1 disclose a Cr³⁺ electrolyte using 120 g/l (= 385 mM) basic chromium(III)sulphate. This resulted in the aforementioned stripy surface under some conditions, such as a low line speed. Surprisingly, the inventors found that by increasing the chromium(III) content in the electrolyte, the appearance of the coated strip worsened, and that the stripy appearance persisted. Surprisingly and counter-intuitively the inventors found that decreasing the amount of metal ions in the electrolyte resulted in a decrease in the stripy surface and that the surface becomes even and shiny when the coating was deposited in accordance with the process of the invention.

The reduction of the pH of the electrolyte also appeared to have a beneficial effect on the surface appearance of the coated product. This also is counter-intuitive, because a lower pH decreases the efficiency of this particular plating process. The higher the pH of the electrolyte, the lower the current density that is needed to deposit a certain amount of chromium. An increase of 2.7 to 2.85 already results in a considerable increase in current density required for a certain plating thickness (as expressed in mg/m²). This effect is independent of the line speed, although the magnitude of the current density required for a certain plating thickness increases with increasing line speed. So also at higher line speeds, a lower pH results in a less efficient plating process. And, reducing the pH has a positive effect on the appearance of the coated blackplate in that the stripy appearance is absent.

The effect of a lower chromium content in the electrolyte solution is shown in Figure 2A. The lower the chromium content, the lower the current density needed for a certain plating thickness. In Figure 2B, the Cr weight is plotted as a function of current density. All curves show the typical features that can be associated with the 3 different regimes as outlined in the introduction. When the Cr content is lowered from 20 to 10 g/l the same Cr coating weight is obtained at a lower current density (regime II is shifted to the left), because less OH- is required for converting less [Cr(HC00)(H₂0)₅]²⁺ to Cr(HC00)(0H)₂(H₂0)₃. When the pH is lowered the same Cr coating weight is obtained at a higher current density, because more OH- is required for achieving the same surface pH increase. The maximum Cr coating weight (transition from regime II to III) is about halved when the Cr content is lowered from 20 to 10 g/l. The maximum Cr coating weight strongly increases when the pH is lowered. Assumably, the various Cr(III) formate complex ion species as depicted in Figure 3 will coexist at a given pH. The pH gradient along the diffusion boundary layer will be steeper for a lower bulk pH. Consequently, the distribution of the various species not only depends on the surface pH, but also on the bulk pH.

In a preferred embodiment the complexing agent is formate (HCOO ), added to the electrolyte solution preferably as sodiumformate (HCOONa). Other complexing agents that can be used instead of formate, or in addition thereto are oxalate-ions, and acetate-ions.

In an embodiment wherein the Cr³⁺-ions are provided by a water soluble chromium(III) salt and wherein the water soluble chromium(III) salt preferably is one or more of the following water soluble chromium(III) salts:

• basic chromium(III)sulphate

• chromium(III)formate

• chromium(III)oxalate

• chromium(III)acetate.

These salts have proven to work well in the eletrolyte as claimed. The use of basic chromium(III)sulphate and/or chromium(III)formate is preferable from the point of view of keeping the bath chemistry as simple as possible. The addition of these compounds does not introduce additional ion-types to the electrolyte. The use of the chromium(III)oxalate and/or chromium(III)acetate instead of, or in addition to, chromium(III)formate may be desired if a different complexing agent is needed.

In an embodiment the electrolyte solution contains at most 225 mM of Cr³⁺-ions and/or at least 100 mM of Cr³⁺-ions, preferably at least 125 mM of Cr³⁺-ions. This preferred range provides good results with regard to appearance, particularly relevant for the first chromium metal - chromium oxide coating (deposited in regime II) and good adherence with regard to lacquers particularly relevant for the second chromium metal - chromium oxide coating (deposited in regime III).

In an embodiment the pH of the electrolyte solution is at most 2.8 (i.e. £ 2.8), preferably at most 2.6 or 2.4, more preferably at most 2.2. Although the lower pH results in a less efficient plating process, the surface quality is much improved in that it shows no stripes.

In an embodiment the formate/Cr³⁺ molar ratio is at most 2.5: 1. The formate-ion is needed as a complexing agent and the ratio of at most 2.5: 1 has proven to be sufficient in most cases. More preferably the molar ratio is at most 2.0: 1, even more preferably 1.75: 1. Preferably the molar ratio is at least 1.1 : 1, more preferably 1.25: 1.

In an embodiment the electrolyte solution contains at least 75 mM and/or at most 600 mM of sodium formate. When using only sodium formate as the addition of formate, and no chromium(III)formate as the water soluble chromium salt, then at least 75 mM should be added, preferably at least 100 mM and even more preferably 200 mM. The maximum is preferably at most 600 mM of sodium formate. If also chromium(III)formate is added to the electrolyte solution as the water soluble chromium salt then the formate added this way needs to be subtracted from the sodium formate additions as given herein above. For example, if 50 mM of formate is added as chromium(III)formate, then the values for sodium formate become at least 25 mM, preferably at least 50 mM and even more preferably 150 mM. The maximum is preferably at most 550 mM of sodium formate.

In an embodiment the electrolyte solution contains at least 210 mM and/or at most 845 mM of sodium sulphate.

In a preferred embodiment the plating temperature is at least 40 °C, preferably at least 50 °C, more preferably at least 55 °C.

In an embodiment the line speed of the plating line is at least 100 m/min, more prefebrably at least 200 m/min.

In an embodiment the coated blackplate is further coated on one or both sides by a film lamination step or a direct extrusion step, with an organic coating consisting of a thermoplastic single layer, or a thermoplastic multi-layer polymer, 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 functionalised polymers; and/or copolymers thereof; and or blends thereof.

According to a second aspect of the invention the blackplate provided with a chromium metal - chromium oxide coating obtained by the process according to the invention has a shiny coating is shiny and shows no stripy appearance. The invention is also embodied in the use of the chromium metal - chromium oxide coated blackplate obtainable by the process according to the invention in a process to produce containers for packaging purposes.

In a preferred embodiment an organic coating is provided on one or both sides of the chromium metal - chromium oxide coated blackplate substrate. It was found that organic coatings could be readily applied on to the chromium-chromium oxide coating, which itself acts a passivation layer to protect the electrically conductive substrate. The chromium-chromium oxide coating also exhibited good adhesion the subsequently applied organic coating. The organic coating may be provided as a lacquer or as a thermoplastic polymer coating. Preferably the thermoplastic polymer coating is a polymer coating system that comprises one or more layers of thermoplastic resins such as polyesters or polyolefins, but can also include acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene type resins, ABS resins, chlorinated polyethers, ionomers, urethane resins and functionalised polymers. For clarification:

Polyester is a polymer composed of dicarboxylic acid and glycol. Examples of suitable dicarboxylic acids include therephthalic acid, isophthalic acid, naphthalene dicarboxylic acid and cyclohexane dicarboxylic acid. Examples of suitable glycols include ethylene glycol, propane diol, butane diol, hexane diol, cyclohexane diol, cyclohexane dimethanol, neopentyl glycol etc. More than two kinds of dicarboxylic acid or glycol may 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.

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

Mixtures of two or more resins can be used. Further, the resin may be mixed with anti-oxidant, heat stabiliser, UV absorbent, plasticiser, pigment, nucleating agent, antistatic agent, release agent, anti-blocking agent, etc. The use of such thermoplastic polymer coating systems have shown to provide excellent performance in can-making and use of the can, such as shelf-life.

A wide range of lacquers can be applied, for food and non-food applications. All show excellent adhesion, when enough chromium oxide is present on the surface of the coated blackplate according to the invention. Chromium oxide levels of 4 mg/m² or more give satisfactory lacquer adhesion.

The application process of the thermoplastic polymer coating is preferably performed by laminating a polymer film onto the coated blackplate by means of extrusion coating and lamination, wherein a polymer resin is melted and formed into thin hot film, which is coated onto the moving substrate. The coated substrate then usually passes between a set of counter-rotating rolls, which press the coating onto the substrate to ensure complete contact and adhesion. The alternative is film lamination, where a film of the polymer is supplied and coated onto a heated substrate and pressed onto the substrate by and between a set of counter-rotating rolls to ensure complete contact and adhesion.

EXAMPLES

An electrolyte was prepared having a sodium formate concentration of 20 g/l (294 mM), a sodium sulphate concentration of 80 g/l (563 mM) and a pH of 2.6, 2.15 and 2.0, and a chromium concentration of 10 g/l (192 mM). The formate/Cr³⁺ ratio = 1.53. At each of these pH-values the appearance is not stripy, with the better appearance being obtained with the lower pH-values. Stripe free and shiny surfaces were obtained at line speeds of 100 and 200 m/min or higher. Experiment 1

Experiments were performed in a plating line comprising 5 plating cells (C1..C5) operated at line speed v of 100 m/min. Each plating cell contains an anode of 300 mm long, and the current density can therefore be calculated on the basis of the strip width (which for this experiment was also 300 mm, so I = 240 A results in i = 240/9 A/dm²)

Of all settings a sample was taken out for analysis. Total Cr was determined by XRF. Figure 5A shows the total amount of chromium deposited at different anode currents for two line speeds. Figure 5 only shows part of regme III for the line speed of 200 m/min. Higher currents are required to move to regime III.

The amount of chromium oxide can be quantified according to EN 10202:2001, E 2.5.2. First the total amount of chromium is measured by XRF. Then the sample is placed in a 30wt% NaOH solution kept at 90 °C for 10 minutes. This dissolves the chromium present as chromium oxide or hydroxide and the chromium carbide and chromium metal stays behind. After thoroughly rinsing again the total amount of chromium is determined again with XRF. The difference between the 2 measurements is amount of chromium present as chromium oxide or hydroxide on the surface according to the following formula (the factor 1.46 comes from the ratio of the molar weight of Cr₂C>₃ and Cr ((2x52+3xl6)/(2x52):

(XRF1-XRF2)*1.46= chromium oxide (mg/m²)

Samples produced in regime II showed values of 1-3 mg/m² chromium oxide at the surface. Experiment 2

The next experiment was carried out at 40 °C at 150 m/min. At a line speed of 150m/min and an anode length of 300mm, the plating time per anode is 0.12 s. A current of 500 A is well into Regime Illb (see figure 5B, showing regime III, the peak at 400 A and regime Ilia in the drop to 425 A and the stable regime Illb at 425 A and higher. The table shows the active anodes (X = active, 0 = currentless).

The results are shown in Figure 6 indicating that the amount of deposited chromium is more or less stable around a value of 50-60 mg/m2 and the amount of oxide reduces linearly with the amount of current passed. It is remarkable that the difference between using one anode or two anodes results in a strong decrease of Cr- total, indicative of the build-up of the pH in the deposition mechanism which is apparently not yet complete after one anode pass, but more or less complete after two.

Experiment 3

Another combination of anodes was used in the next experiment where a current of 550 A was used.

It appears that the situation CC0CC delivers a combination of 92.5 mg/m² total Cr (i.e. metal, oxide and carbide) and of 10.6 mg/m² Cr-oxide. The currentless episode is apparently beneficial for the formation of Cr-oxide and Cr-metal. For those cases where less Cr-oxide and more Cr-metal is desired, another combination can be chosen. The XOXOX combination results in much lower amount of oxide (3.2) and about the same total Cr (90.5).

Experiment 2 and 3 relate to the deposition of chromium metal - chromium oxide coating with an enlarged amount of oxide (regime Illb). So in order to produce a coated blackplate this needs to be combined with the deposition of a chromium metal - chromium oxide coating in regime II. Figure 5C shows a part of Figure 5B with regime II represented as a straight dashed line. Depending on the choice of I, the thickness of the first chromium metal - chromium oxide coating layer can be controlled. A subsequent deposition of the second chromium metal - chromium oxide coating layer can then be achieved by increasing the current into the regime III region, i.e. 450 or higher as demonstrated in Figure 5B. It is important to determine the graph comparable to Figure 5B first for the chosen process conditions and plating line, because this determines the position of regime II and regime Ilia and Illb for the particular process conditions and plating line. Once this graph is known, the method according to the invention is easily applicable by properly selecting of the currents for regime II and regime III and by properly selecting the plating cells to be provided with current in regime II, which can optionally be left currentless and which cells operate in regime III.

In addition to the improvement of surface appearance by the reduction in pH there are additional advantages as a result of the lower Cr³⁺ in comparison to the prior art of WO2014202316-A1 (Cr³⁺ = 385 mM, pH 2.6). The higher current efficiency has already been mentioned and shown in Figure 2A and 2B. In addition, because of the lower concentrations in the electrolyte, the degree of dragout losses will also be reduced. It is also believed that the edge build-up is reduced, and the lower sulphate content of the electrolyte is likely to result in lower sulphate contents in the deposited coating layer, which is beneficial for lacquer adherence.

Experiments were performed for investigating the influence of the chromium and formate concentration on the plating window.

TABLE 1 : RCE-EXPERIMENTS

TOC=Total Organic Carbon

For the electrodeposition experiments titanium anodes comprising a catalytic coating or mixed metal oxide of iridium oxide and tantalum oxide are chosen. The rotational speed of the RCE was kept constant at 776 RPM (W^{0 7} = 6.0 s^{0 7}). The substrate was a 0.183 mm thick cold rolled blackplate material and the dimensions of the cylinder were 113.3 mm x ø 73 mm. The cylinders were cleaned and activated under the following conditions prior to plating.

TABLE 2 : PRETREATMENT OF THE SUBSTRATE

In figure 2A the results of coating trials with these compositions are given.

The invention has been explained by means of the following, non-limiting figures.

Figure 1 : Schematic process route of starting material for the coating process according to the invention.

Figure 2A: Deposition curve as a function of i (A/dm²) for electrolyte solutions with a different chromium(III)concentration (18.1 g/l = 349 mM, 21.2 g/l = 408 mM, 24.9 g/l = 479 mM).

Figure 2B: Deposition curve as a function of i (A/dm²) for electrolyte solutions with a different chromium(III)concentration (10 g/l (192 mM) and 20 g/l = 385 mM) for pH 2.0 and 2.6, molar ratio formate/Cr(III) = 1.5.

Figure 3 : Schematic principle of the chromium deposition mechanism

Figure 4A: Chromium coating weight as a function of current density showing the 3 different deposition regimes. Figure 4B as Figure 4A with explanation of regime Ilia and Illb.

Figure 5A: I-Cr curve for plating line speed of 100 and 200 m/min. Figure 5B: I-Cr curve for plating line speed of 150 m/min at 40°C, 192 mM Cr(III), 288 mM HCOONa (20 g/l), 80 g/l sodium sulphate.

Figure 5C: as Figure 5B for regime II and linear interpolation.

Figure 6: Results of Experiment 2

Figure 7: Results of Experiment 2

Figure 8: Schematic presentation of 5 cell plating line with 10 anode pairs and some possible switching options. II is current in regime II, III is current in regime III (preferably Illb) and 0 is no current. The table shows which of the 10 anode pairs is operated in which regime.

Figure 9: Schematic presentation of 1 cell plating line with 7 anode pairs and some possible switching options. II is current in regime II, III is current in regime III (preferably Illb) and 0 is no current. The table shows which of the 7 anode pairs is operated in which regime.

The switching options and the configurations shown in figure 8 and 9 are only used as illustration and by no means limitative.

Claims

1. Method for manufacturing a chromium metal - chromium oxide coated blackplate by electrolytically depositing the chromium metal - chromium oxide coating on blackplate in a continuous high speed plating line operating at a line speed of at least 50 m/min from an electrolyte solution comprising a trivalent chromium compound, wherein the electrolyte solution is free of chloride ions and of a boric acid buffering agent, the electrically conductive substrate acts as a cathode and an anode comprising a catalytic coating of iridium oxide or a mixed metal oxide for reducing or eliminating the oxidation of Cr³⁺-ions to Cr⁶⁺-ions, wherein the electrolyte solution contains at most 250 mM Cr³⁺-ions, a complexing agent at a

molar ratio of at least 1 : 1, 0 to 2800 mM of sodium sulphate (Na₂S0₄), a pH of between 1.5 and 3.0 measured at 25 °C, and wherein the plating temperature is between 30 and 70 °C, wherein a first chromium metal - chromium oxide coating is deposited onto the blackplate, the first chromium metal - chromium oxide coating having a first chromium oxide content, and wherein a second chromium metal - chromium oxide coating is deposited onto the first chromium metal - chromium oxide coating, wherein the first chromium oxide content is lower than the second chromium oxide content, obtained by depositing the first chromium metal - chromium oxide coating at a first current density and by depositing the second chromium metal - chromium oxide coating at a higher second current density, wherein the first current density is chosen in regime II and the second current density is chosen in regime III of the current density versus deposited Cr- metal curve determined according to the methodology in the description.

2. Method according to claim 1 wherein the continuous high speed plating line consists of a sequence of N plating cells (C_I..C_N) each of which has individual current control, wherein the deposition of the first chromium metal - chromium oxide coating at the first current density ( i _regime n) is performed in one or more of the first anodes of the plating line, optionally followed by a currentless period, followed by the deposition of the second chromium metal - chromium oxide coating at the second current density ( i _regime iii) in the last anode or anodes of the plating line.

3. Method according to claim 1 wherein the chromium oxide in the second chromium metal - chromium oxide coating is between 6 and 24 mg/m², preferably between 6 and 18 mg/m², more preferably between 6 and 15 mg/m², and most preferably between 8 and 15 mg/m².

4. Method according to claim 1 or 2 wherein the complexing agent is formate (HCOO ), preferably added to the electrolyte solution as sodium formate (HCOONa).

5. Method according to any one of the preceding claims wherein the Cr³⁺-ions are provided by a water soluble chromium(III) salt and wherein the water soluble chromium(III) salt preferably is one or more of the following water soluble chromium(III) salts:

• basic chromium(III)sulphate

• chromium(III)formate

• chromium(III)oxalate

• chromium(III)acetate.

6. Method according to any one of the preceding claims wherein the electrolyte solution contains at most 225 mM of Cr³⁺-ions, and/or at least 100 mM of Cr³⁺- ions, preferably at least 125 mM of Cr³⁺-ions.

7. Method according to any one of the preceding claims wherein the pH of the electrolyte solution is at most 2.8 (pH £ 2.8), preferably at most 2.6, more preferably at most 2.2.

8. Method according to any one of the preceding claims wherein the formate/Cr³⁺ molar ratio is at most 2.5: 1.

9. Method according to any one of the preceding claims wherein the electrolyte solution contains at least 75 mM and/or at most 600 mM of sodium formate.

10. Method according to any one of the preceding claims wherein the electrolyte solution contains at least 210 mM and/or at most 845 mM of sodium sulphate.

11. Method according to any one of the preceding claims wherein the plating temperature is at least 40 °C, preferably at least 50 °C, more preferably at least 55 °C.

12. Method according to any one of the preceding claims wherein the coated blackplate is further coated on one or both sides by a film lamination step or a direct extrusion step, with an organic coating consisting of a thermoplastic single layer, or a thermoplastic multi-layer polymer.

13. Method according to claim 12 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 functionalised polymers; and/or copolymers thereof; and or blends thereof.

14. Method according to claim 12 wherein the thermoplastic polymer coating on the one or both sides of the coated blackplate is a multi-layer coating system, said coating system comprising at least an adhesion layer for adhering to the coated blackplate, a surface layer and a bulk layer between the adhesion layer and the surface layer, wherein the layers of the multi-layer coating system comprise or consist of polyesters, such as polyethylene terephthalate, IPA-modified polyethylene terephthalate, CHDM-modified polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, or copolymers or blends thereof.

15. Blackplate provided with a chromium metal - chromium oxide coating system obtainable by the process according to any one of claims 1 to 11, wherein the coating system comprises two chromium metal - chromium oxide coating layers and wherein the first chromium metal - chromium oxide coating was deposited onto the blackplate and the second chromium metal - chromium oxide coating onto the first chromium metal - chromium oxide coating, and wherein the chromium oxide content of the first chromium metal - chromium oxide coating is lower than that of the second chromium metal - chromium oxide coating wherein the chromium oxide in the second chromium metal - chromium oxide coating is between 6 and 24 mg/m², preferably between 6 and 18 mg/m², more preferably between 6 and 15 mg/m², and most preferably between 8 and 15 mg/m².