Process for preparing a nickel electrolyte and the use thereof in an electroplating process

Abstract

The invention relates to a nickel-plating bath. The invention further relates to the use of the nickel-plating bath to produce nickel plated substrates in an electroplating process, in particular for electrical applications such as batteries and electrolysers.

Description

[0001] PROCESS FOR PREPARING A NICKEL ELECTROLYTE AND THE USE THEREOF IN AN ELECTROPLATING PROCESS

[0002] Field of the invention

[0003] The invention relates to a nickel-plating bath comprising an aqueous nickel electrolyte and a process for preparing such an electrolyte. The invention further relates to the use of the electrolyte to produce nickel plated substrates, in particular for electrical applications such as batteries and electrolysers.

[0004] Background of the invention

[0005] The world is facing immense challenges in terms of environmental changes. To reduce climate change, a significant reduction of greenhouse gases is needed. One of these greenhouses gases is carbon dioxide and it is released during the combustion of fossil fuels. A major user of fossil fuel is the transportation sector and in particular the automotive sector. A way to reduce the consumption of fossil fuel is the electrification of the automotive sector and this would mean an enormous increase in power storage capacity for cars to have the desired range for making electric cars attractive as an alternative for cars with combustion engines. Another way to reduce the consumption is the switch to an economy based on hydrogen as a fuel, rather than fossil fuels.

[0006] Both options, the electrification of the transport sector and the hydrogen fuel-based economy requires access to significant volumes of nickel-plated steels. This nickel-plated steel is used in batteries either to protect the battery against corrosion or serves as a part of the electrical circuit in the battery. The nickel-plated steel is used in the production of hydrogen in electrolysers.

[0007] Depending on the application or on the type of battery, the steel substrate, which is usually provided in the form of a (coiled) strip or sheet, is provided on one, or on both major sides ("top" and "bottom") of the steel substrate. Most batteries have a cylindrical shape or a prismatic shape, including the components of batteries in the automobile sector, and the material for the battery can is usually a steel strip or sheet substrate with a nickel coating on one side or on both sides. This nickel coating is usually applied by an electroplating process in a continuous electroplating line.

[0008] In view of the environmental challenges above the demand for nickel-plated steel will increase. It is expected that the current production of nickel-plated steel will not be sufficient to meet this increasing demand so that additional production capacity is required. The scale of the current production lines for nickel-plated steel does not fit the expected demands and cannot easily be enhanced by increasing the existing capacity based on existing plating technology.

[0009] Nickel-plated steel strip or sheet is usually made by plating a nickel layer onto the strip or sheet from a sulphate based electroplating bath, a chloride electroplating bath or a sulphamate based electroplating bath. In an electroplating process the nickel ions are consumed at the cathode. This can be done by replenishing the nickel by adding fresh electrolyte to the plating cells. The continuous addition of fresh electrolyte is expensive and requires careful storage of large amounts of these chemicals. A further problem of nickel-plating baths is that during plating the maximum usable current density limits their usability for high-speed electroplating processes. An alternative is to produce the nickel ions for deposition at the anode, i.e., the anode is made from nickel in some metallic form, for example nickel pellets stacked in an inert titanium basket to assure good electric contact between each pellet. The dissolution of the nickel anode material into the plating bath is the limiting factor in this type of electroplating of nickel on a steel substrate, thereby limiting the production capacity of such plating lines. In EP0892087-A2 the electroplating of nickel layers is disclosed on the basis of nickel salts and also the option of using soluble nickel anodes is disclosed.

[0010] A problem with the use of all current standard nickel-plating electrolytes is that do not easily allow scaling up. Most electrolytes contain chloride which generates toxic chlorine at the anode, or the electrolytes have a limited conductivity which limits the deposition rate of nickel, or the electrolytes decompose such as sulphamate electrolytes.

[0011] Objectives of the invention

[0012] It is an objective of the invention to provide a process for preparing a nickel ion containing solution in water that can be used as an electrolyte in a nickel electroplating process

[0013] It is also an objective of the invention to provide an electrolyte that avoids the production of toxic or environmentally harmful by-products, such as chlorine, or undesired metal ions.

[0014] It is also an objective of the invention to provide a process for preparing a nickel ion containing solution in water that can be used as electrolyte in a continuous high-speed nickel electroplating process.

[0015] It is also an objective of the invention to a provide an apparatus for using the electrolyte in a continuous high-speed nickel electroplating process.

[0016] Description of the invention

[0017] This object is achieved by the process according to a process for preparing a nickel- plating bath comprising the steps that

[0018] (a) nickel (met.),

[0019] (b) methane sulfonic acid, and

[0020] (c) an oxygen releasing compound are combined in an aqueous solution. A nickel-plating bath is understood to be a nickel ion (Ni2+) containing aqueous solution for electroplating nickel onto a metallic substrate. A plating bath can also be referred to as an electrolyte or an electroplating bath. An example of a nickel-plating bath or electrolyte is an aqueous nickel methane sulfonate solution.

[0021] In the context of this invention an oxygen releasing compound is a chemical compound that can be dissolved in the aqueous solution. The compound releases oxygen in the compound. Without being bound to this theory, it is assumed that the oxygen that is released is so-called active oxygen or atomic oxygen, and not molecular oxygen (O2). The oxygen is then able to react with the other compounds in the aqueous solution to produce the nickel-plating bath. This reaction is preferably exothermic.

[0022] In a preferred embodiment of the invention, the process for preparing a nickel- plating bath comprises the steps that

[0023] (a) nickel (met.),

[0024] (b) methane sulfonic acid, and

[0025] (c) hydrogen peroxide and / or ozone are combined in an aqueous solution.

[0026] In another embodiment of the invention, the process for preparing a nickel-plating bath comprises the steps that

[0027] (a) nickel (met.),

[0028] (b) methane sulfonic acid, and

[0029] (c) ozone are combined in an aqueous solution.

[0030] In another embodiment of the invention, the process for preparing a nickel-plating bath comprises the steps that

[0031] (a) nickel (met.),

[0032] (b) methane sulfonic acid, and

[0033] (c) sodium perborate are combined in an aqueous solution.

[0034] A nickel methane sulfonate solution has the advantage in an electroplating process for preparing a nickel coating that the conductivity of the solution is quite high so that a high deposition rate is possible.

[0035] A simple process for preparing a nickel methane sulfonate solution would be the dissolution of nickel metal (nickel (met.)) in methane sulfonic acid. However, nickel is not soluble in methane sulfonic acid. Surprisingly however, nickel is oxidized to nickel ions (Ni2+) very fast and clean in an aqueous methane sulfonic acid solution by the active oxygen released by the oxygen releasing compounds such as hydrogen peroxide (H2O2), ozone (O3) or sodium perborate (NaH2BO4). It has surprisingly been found that an oxidation process with a suitable oxygen releasing compound thereby the preparation speed of nickel methane sulfonate in water from metallic nickel is very fast and clean.

[0036] The temperature of the solution or the water also influences the speed of dissolution of the nickel metal. It has been found that the temperature of the solution is preferably 20 °C - 80 °C. The dissolution is particularly fast at temperatures above 40 °C, for example between 50 and 70 °C. A suitable maximum dissolution temperature is 68 °C. Preferably the temperature is at between 55 °C and 65 °C. For the sake of avoiding misunderstanding it is noted that the temperatures of the solution or the water is the temperature during the preparation of the nickel methane sulfonate solution.

[0037] The following intermediate reaction takes place between hydrogen peroxide and nickel:

[0038] Ni (met.) + H2O2 NiO + H2O (1)

[0039] The NiO is an intermediate reaction product which immediately dissolves in methane sulfonic acid. The overall reaction is as follows:

[0040] Ni (met.) + H2O2 + 2 CH3SO3H Ni2++ 2 CH3SO3’ + 2 H2O (2)

[0041] The following reaction takes place between ozone and nickel:

[0042] Ni (met.) + O3 NiO + 02 1" (gas) (3)

[0043] The overall reaction is as follows:

[0044] Ni (met.) + O3+ 2 CH3SO3H Ni2++ 2 CH3SO3- + O2 J (gas) + H2O (4)

[0045] Sodium perborate undergoes hydrolysis in contact with water, producing hydrogen peroxide (H2O2) that reacts as described in eq. (1). As a result of the hydrolysis also boric acid (B(OH)s) may be formed.

[0046] As can be seen from eq. (2) above, only water is formed and from eq. (4) O2 and water is formed as a by-product. The reaction can be sped up by increasing the surface area of the metallic nickel, e.g. by using smaller pellets, thin strips, or even nickel powder and by increasing the temperature of the aqueous solution. An advantage of using hydrogen peroxide as the oxygen releasing compound is that it is available as a liquid and therefore easier to manage than the gaseous ozone. Ozone is preferably added as a gas, preferably by bubbling ozone gas through the aqueous solution.

[0047] Further, it has surprisingly been found that the addition of boric acid and / or taurine significantly enhances the maximum current density that can be used in a nickel-plating process when using these nickel plating baths. The use of sodium perborate as an oxygen releasing compound may already provide boric acid to the solution. From practical perspective the use of hydrogen peroxide and / or ozone is preferred because it produces a pure nickel methane sulfonate solution and it produces no boric acid to the solution, which is an advantage if a nickel methane sulfonate solution with the addition of just taurine (and no boric acid) is opted for.

[0048] Various compounds can be added to the nickel methane sulfonate solution to prepare a suitable nickel-plating bath for nickel electroplating. These compounds can be, but are not limited to, buffers, brighteners, stress control agents etc. Their addition, the effects of the additions, and the appropriate amounts to be added are well known in the art.

[0049] The reaction of hydrogen peroxide or ozone with nickel has an efficiency of 100 % since both react to water or oxygen and no undesired, toxic or environmentally harmful by-products, such as chlorine, are formed. The process according to the invention results in a nickel plating bath that is free from compounds that may cause the production of toxic or environmentally harmful by-products, such as chlorine or bromine, or undesired metal ions.

[0050] Further, the dissolution of nickel in an acidic aqueous solution of methane sulfonic acid with hydrogen peroxide and / or ozone is very fast so that the process is useful in combination with high-speed continuous electroplating processes.

[0051] The process according to the invention has the big advantage that the amount of nickel needed for the production nickel methane sulfonate solution of the electrolyte in terms of volume is much less than for the alternatives.

[0052] For instance, 1000 kg metallic nickel represents a volume of 112,2 dm3, while an equivalent amount of nickel from nickel carbonate represents a volume of 460.6 dm3. A 4,5 time larger volume has to be handled when using nickel carbonate as a nickel source. Also, the handling, transportation and storage of salts is more challenging than sorting metallic nickel from a health and safety perspective and from a logistical perspective. The compounds have been identified as potentially carcinogenic and toxic compounds (see e.g. www.ilo.org for nickel carbonate and nickel(II)oxide). Therefore the costs and the safety hazards associated with this method are higher than for the method according to the invention.

[0053] The mechanism for producing the nickel methane sulfonate solution, with the proper safety precautions, on the basis of nickel carbonate (NiCOs), nickel hydroxide (Ni(OH)2) or NiO is as follows:

[0054] NiCO3+ 2CH3SO3H Ni2++ 2CH3SO3- + H2CO3(5) followed by H2CC>3 in H2O + CO2(g) (5a)

[0055] Ni(OH)2+ 2CH3SO3H Ni2+ + 2CH3SO3’ + 2H2O (6) NiO + 2CH3SO3H Ni2++ 2CH3SO3’ + H2O (7)

[0056] However, compared to the process according to the invention this is more cumbersome from a health and safety perspective, from a handling perspective and from a cost perspective.

[0057] In an embodiment the process for preparing a nickel-plating bath (nickel methane sulfonate solution in water) according to the invention comprises the steps that

[0058] (a) nickel (metal),

[0059] (b) methane sulfonic acid and

[0060] (c) hydrogen peroxide and / or ozone are combined in an aqueous solution to obtain a nickel methane sulfonate solution in water.

[0061] That means nickel, methane sulfonic acid and hydrogen peroxide or, alternatively, nickel, methane sulfonic acid and ozone or, alternatively, nickel, methane sulfonic acid, hydrogen peroxide and ozone are combined in an aqueous solution for reaction, i.e. the components react in an aqueous solution.

[0062] The aqueous solution is preferably water or an aqueous solution of one or two of the other components (a), (b) and (c). The water can be distilled water or de-mineralized water or simply tap water. All sequences of addition of components (a), (b) and (c) are possible. That means, for example, methane sulfonic acid and hydrogen peroxide are added to water or methane sulfonic acid and hydrogen peroxide are combined as aqueous solutions and nickel is then added to the resulting aqueous solution. Alternatively, as an example, nickel is added to water or one of the components (b) or (c) and the other component(s) is / are added to the resulting aqueous solution. When components (a), (b) and (c) are combined, they react with each other. Nickel is oxidized to Ni2+ions by hydrogen peroxide and / or ozone and the anion of methane sulfonic acid forms the counter ion to the Ni2+ions.

[0063] In case sodium perborate is used the conditions of the aqueous solution must be favourable to the hydrolysis of the perborate into hydrogen peroxide and boric acid. This hydrolysis is favoured by a higher temperature of the aqueous solution. Sodium perborate releases oxygen rapidly at temperatures over 55°C. To make it active at lower temperatures (40-55 °C) a suitable activator may be used.

[0064] Nickel is added as a metal and can be in any form. To enhance the dissolution of nickel in water through the oxidation with hydrogen peroxide and / or ozone, the surface of the nickel should be as large as possible. In a preferred embodiment of the invention, the nickel is in the form of metallic pellets or a metallic powder. The average size of the pellets or the powder particles is preferably less than 10 mm, more preferably less than 5 mm, furthermore preferably less than 1 mm, even more preferably less than 0.1 mm and most preferably 0.001 - 0.1 mm. When using thin strips of nickel the thickness of the strips is preferably less than 1 mm, even more preferably less than 0.1 mm and most preferably 0.001 - 0.1 mm. The advantage of being able to use nickel as a metal is that the storage of the raw materials is simple and requires little space. The storage of nickel metal does not pose any safety issues.

[0065] The invention is also embodied in the nickel-plating bath or electrolyte, obtainable by the inventive process as described herein above.

[0066] In an embodiment the nickel-plating bath comprises

[0067] (a) 26 - 316 g / l nickel methane sulfonate,

[0068] (b) 10 - 100 g / l boric acid and / or 10 - 70 g / l taurine in water.

[0069] In a preferred embodiment is a nickel-plating bath comprising

[0070] (a) 26 - 285 g / l nickel methane sulfonate,

[0071] (b) 10 - 100 g / l boric acid and / or 10 - 70 g / l taurine in water.

[0072] The temperature of the solution or the water also influences the speed of dissolution of the nickel metal. It has been found that the temperature of the solution is preferably 20 °C - 80 °C. The dissolution is particularly fast at temperatures above 40 °C, for example between 50 and 70 °C., A suitable maximum dissolution temperature is 68 °C. Preferably the temperature is at between 55 °C and 65 °C.

[0073] The exact dissolution rate of metallic nickel in the solution is dependent on the exact composition of the solution and the exact temperature thereof and can be easily determined using simple laboratory means. Values for the dissolution rate of nickel were found to range between about 0.05 and about 0.2 kg / m2.min when adding nickel to a solution at 60 °C, wherein the solution is made from 80 g / l 30% H2O2 and 129 g / l 70% methane sulfonic acid solution. Assuming, for the sake of argument, a dissolution rate of nickel of 0.1 kg / m2.min, then the dissolution time for 1 kg of nickel pellets with an average size of around 5 mm is about 150 min. With an average particle size of 0.5 mm, the dissolution time is about 15 min and with an average particle size of 0.01 mm, the dissolution time for 1 kg of the particles is about 20 seconds.

[0074] The concentration of hydrogen peroxide in the solution is preferably 4 - 15 wt.-% H2O2, more preferably 7 - 12 wt.-% H2O2, most preferably 8 - 10 wt.-% H2O2, based on the total weight of the solution. The total weight of the solution includes water, hydrogen peroxide and methane sulfonic acid, but not the nickel metal. The concentration of methane sulfonic acid is preferably 15 - 35 wt.-%, more preferably 20 - 30 wt.-% methane sulfonic acid, based on the total weight of the solution. These concentrations give a high dissolution rate of nickel. In a preferred embodiment of the invention, the molar ratio of methane sulfonic acid to hydrogen peroxide in the solution is 2 : (0.90 - 1.10), preferably 2 : (0.95 - 1.05), more preferably 2 : (0.98 - 1.02), most preferably 2 : (0.995 - 1.005).

[0075] Nickel can be added in excess, but it is added preferably in the stochiometric amount. The molar ratio of nickel : methane sulfonic acid : hydrogen peroxide is preferably (0.90 - 1.10) : 2 : (0.90 - 1.10), preferably (0.95 - 1.05) : 2 : (0.95 - 1.05), more preferably (0.98 - 1.02) : 2 : (0.98 - 1.02), most preferably (0.995 - 1.005) : 2 : (0.995 - 1.005). Ozone is added as gas until the above amounts given for hydrogen peroxide have reacted.

[0076] Even if the molar ratio of nickel : methane sulfonic acid : hydrogen peroxide or ozone is exactly 1 : 2 : 1, a chemical reaction is never complete so that the solution may still contain traces of hydrogen peroxide or ozone. However, the inventor found that these trace amounts do not significantly affect the plating properties nor the properties or composition of the nickel-plating layer.

[0077] It has been found that the nickel-plating bath according to the invention is particularly useful for preparing an electrolyte to be used in nickel electroplating processes, in particular high-speed nickel electroplating processes.

[0078] Therefore, the invention also relates to a nickel-plating bath, obtainable by the process of the invention, the process comprising the steps that

[0079] (a) nickel,

[0080] (b) methane sulfonic acid and

[0081] (c) oxygen releasing compound such are combined in water or an aqueous solution to obtain a nickel methane sulfonate solution in water. The oxygen releasing compound may be hydrogen peroxide or ozone or sodium perborate or combinations thereof.

[0082] The nickel methane sulfonate solution in water, obtainable by the process of the invention, preferably comprises a nickel methane sulfonate solution in water, containing IO-6- 0.1 wt.-% hydrogen peroxide and / or IO-6- 0.01wt.-% ozone, preferably IO-5- 0.05 wt.-% hydrogen peroxide and / or IO-5- 0.005 wt.-% ozone. The pH is preferably between 1 and 4. More preferably the pH is below 3. Even more preferably the pH is between 1.5 and 2.5 and most preferably between 1.6 and 2.2.

[0083] The invention further relates to the use of the nickel-plating bath according to the invention in a nickel electroplating process. This use is a use of the nickel-plating bath as electrolyte in a process for preparing a nickel coating, in particular a process for preparing a nickel coating by electroplating of an article, preferably a steel plate. Electroplating is also known as electrodeposition or galvanic coating.

[0084] In a preferred embodiment of such use, the nickel methane sulfonate solution in water is used to prepare an electrolyte for high-speed nickel electroplating, preferably an electroplating with dimensional stable anodes. Such dimensionally stable anodes are preferably made of metal oxides, more preferably mixed metal oxides, for example on a titanium support. The metal oxides are preferably iridium oxide, osmium oxide, ruthenium oxide, rhodium oxide, palladium oxide, platinum oxide, or mixtures thereof.

[0085] When using a nickel methane sulfonate solution in water as nickel plating bath, a maximum current density of about 100 A / dm2(ampere per square decimetre) is possible. Within the investigations of the present invention, it has surprisingly been found that the addition of boric acid (H3BO3) and / or taurine (2-amino ethyl sulfonic acid) enhances the maximum current density that is usable up to 140 A / dm2, whereas nickel methane sulfonate solutions in water without adding boric acid or taurine have the above mentioned maximum current density of about 100 A / dm2. This improves the efficiency and the usability of this nickel-plating bath for high-speed plating processes.

[0086] The present invention therefore also relates to a nickel-plating bath for electroplating nickel comprising

[0087] (a) 26 - 316 g / l nickel methane sulfonate

[0088] (b) 10 - 100 g / l boric acid and / or 10 - 70 g / l taurine in water.

[0089] This means that the nickel-plating bath preferably comprises 26 - 316 g / l nickel methane sulfonate as prepared by the method according to the invention, 10 - 100 g / l boric acid and / or 10 - 70 g / l taurine and the rest water. The nickel-plating bath can additionally contain typical electrolysis additives.

[0090] In a preferred embodiment is a nickel-plating bath comprising

[0091] (a) 26 - 285 g / l nickel methane sulfonate,

[0092] (b) 10 - 100 g / l boric acid and / or 10 - 70 g / l taurine in water.

[0093] The nickel-plating bath according to the invention comprises 26 - 316 g / l nickel methane sulfonate, preferably 26 - 285 g / l nickel methane sulfonate, more preferably 80

[0094] - 200 g / l nickel methane sulfonate, even more preferably 105 - 160 g / l nickel methane sulfonate. The given amounts of nickel methane sulfonate correspond to about 10 - 120 g / l nickel dissolved in the form of ions, preferably about 10 - 110 g / l, more preferably about 30 - 75 g / l and even more preferably about 40 - 60 g / l nickel.

[0095] The amount of boric acid in the nickel-plating bath according to the invention is 10

[0096] - 100 g / l boric acid, preferably 30 - 70 g / l boric acid and more preferably 40 - 60 g / l boric acid. The amount of taurine according to a preferred embodiment is 10 - 70 g / l taurine, preferably 30 - 60 g / l taurine and more preferably 40 - 60 g / l taurine.

[0097] The above amounts can be combined in any combination. This means a preferred embodiment of the invention is, for example, a nickel-plating bath comprising

[0098] (a) 26 - 316 g / l nickel methane sulfonate

[0099] (b) 30 - 70 g / l boric acid and / or 30 - 60 g / l taurine in water, or

[0100] (a) 80 - 285 g / l nickel methane sulfonate

[0101] (b) 10 - 100 g / l boric acid and / or 10 - 70 g / l taurine in water, or

[0102] (a) 80 - 200 g / l nickel methane sulfonate

[0103] (b) 10 - 100 g / l boric acid and / or 10 - 70 g / l taurine in water.

[0104] Boric acid and taurine are both acting as buffers and complexation agents. Taurine is biodegradable and therefore, from an environmental point of view, preferred. A preferred embodiment of the invention comprises as component (b) taurine.

[0105] Another preferred embodiment of the invention is the use of the nickel-plating bath according to the invention in a nickel electroplating process.

[0106] The invention also relates to the use of the nickel-plating bath according to the invention in a nickel electroplating process, preferably a nickel electroplating on steel, more preferably nickel electroplating on a steel sheet. A preferred embodiment of the invention is the use of the nickel-plating bath according to the invention for preparing a battery case or an electrolyser part. This process comprises nickel electroplating on a steel sheet and then using the resulting steel sheets with a nickel layer on the surface for preparing battery cases. The use inter alia comprises known steps such as cutting the nickel coated steel sheets and forming the sheets.

[0107] The nickel-plated steel sheet produced thusly can also be used for other applications and is not necessarily limited to batteries or electrolyser parts.

[0108] In the use of the nickel-plating bath according to the invention, i.e. an electroplating process, the temperature of the nickel plating bath in the nickel electroplating process is preferably 40 - 70 °C, more preferably 50 - 60 °C. The pH is preferably 1 - 4, more preferably 1 - 3. The pH is preferably between 1 and 4. More preferably the pH is below 3. Even more preferably the pH is between 1.5 and 2.5 and most preferably between 1.6 and 2.2.

[0109] The invention also relates to a nickel electroplating process, wherein nickel is electrodeposited on an article, preferably a steel sheet or strip in a nickel-plating bath produced according to the invention. The current density is preferably 40 to 140 A / dm2 and the temperature of the plating bath is as described herein above.

[0110] The substrate used for applications of nickel-plated metal substrates in batteries and electrolysers is usually a low carbon steel strip. This strip is produced by cold rolling. The strip is usually provided in the form of a coil, but the strip may also be cut into smaller portions, referred to as sheet. A wide coil may be slit into narrower strips and re-coiled or cut into sheets afterwards.

[0111] For the plating process it is preferable to plate a strip rather than individual sheets. The productivity of the plating process is much higher and can be executed in a continuous plating line. The steel strip is very hard after cold rolling and must usually be annealed to soften or recrystallise the microstructure of the steel to enable further processing like deep drawing to produce batteries. The annealing can be performed before or after the nickel plating. When the plating is performed after the annealing, then the metal substrate will be provided with a nickel layer on top of the steel substrate. If the plating is performed before the annealing, then the nickel will start diffusing into the steel base and iron will start diffusing into the nickel layer. Depending on the nickel layer thickness, the annealing temperature, the annealing time and the annealing type (batch or continuous annealing) the degree of interdiffusion may differ. The degree of diffusion may be very limited (so called shallow annealing) if the annealing temperature is not very high or the annealing time is very short. This way the surface of the nickel layer will still be nickel only, but a diffusion layer will have formed at the nickel-steel interface. The longer the annealing, the thinner the nickel plate and the higher the annealing temperature, the more nickel and iron will intermix up to a point that iron starts moving to the surface of the plated layer. For some applications this is very important and values of Ni:Fe on the surface of 1: 1 are often reported. The choice of the annealing temperature and time is also dependent on the desired microstructure of the steel substrate after annealing. Post-processing of the annealed and plated strip (in whatever order) may be performed, e.g. temper rolling or tension levelling. This heat treatment can be done in several ways, like batch annealing, continuous annealing, normalizing and flash annealing. All heat treatments will give the heat-treated material certain desirable mechanical properties. The main differences between the heat treatments are the heat-up rate, the maximum temperature and the duration at this maximum temperature. Heating can be done by gas fire, electrical heating or any other method to raise the temperature of the nickel-plated steel base in a controlled way.

[0112] Figure 8 shows a series of nickel-plated steel (Fe) substrates where on the left-hand side the structures in cross section are schematically shown and on the right-hand side schematic drawings of the nickel concentration where the left hand side of the graph is the surface. Figure 8a shows a nickel-plated steel strip without any diffusion. This is a steel substrate plated with a nickel layer either after annealing of the steel substrate or without any annealing of the substrate. Figure 8b shows a nickel-plated steel strip which has been subjected to a shallow annealing. The diffusion of nickel into the steel substrate and of iron into the nickel layer is minimal and the iron has not yet reached the surface. The surface of the nickel-plating layer is still 100% nickel. Figure 8c shows a nickel-plated steel strip which has been subjected to a more significant annealing. The diffusion of nickel into the steel substrate and of iron into the nickel layer is quite extensive and the iron has reached the surface. The surface of the plating layer is about 70 % Ni and 30 % Fe. The more extensive the annealing (in time or temperature) or the thinner the nickel layer, the higher the Fe content at the surface. By controlling the annealing and the plating layer thickness the composition of the surface can be controlled.

[0113] Examples

[0114] The following examples provide preferred embodiments according to the invention and further exemplify the invention.

[0115] Example 1. A nickel methane sulfonate solution (nickel plating bath) was prepared from a 50 x 50 mm nickel sheet which was immersed in a mixture of 80 g 30 wt.-% H2O2 and 192 g 70 wt.-% methane sulfonic acid solution at 60 °C. The nickel dissolves at a rate of about 0.01 g / cm2per minute which corresponds with 0.1 kg / m2.min.

[0116] The nickel sheet was removed upon reaching 50 g / l Ni and the resulting solution contains only nickel methane sulfonate and traces of H2O2. The solution was used in a nickel electroplating process with a rotating cylinder electrode on blackplate (low carbon, cold-rolled steel). This rotating cylinder approach allows for an accurate simulation of an industrial plating process and industrial plating conditions.

[0117] The experimental conditions were as follows:

[0118] • Current density: 10, 20, 40, 60, 80 and 100 A / dm2

[0119] • Plating time: 10 seconds

[0120] • Rotation speed cylinder: 400 rpm (rotations per minute) which corresponds with a line speed of 100 m / min

[0121] • Temperature 50 and 60 °C

[0122] • Gap anode / cathode 15 mm

[0123] The above conditions gave very efficient nickel coatings on steel (efficiency of over 90% can be reached). The experiments show that the solution according to the invention can be efficiently used in a high-speed nickel electroplating process.

[0124] Example 2. The maximum current density that can be used with the nickel-plating baths of Example 1 is about 100 A / dm2. To the plating baths of Example 1 boric acid was added in amounts of 15, 30 and 45 g / l and electroplating was conducted at T = 60 °C, 400 rpm and t = 10 s. A current density of up to 140 A / dm2was possible and the obtained nickel layer on the steel sheet had an excellent appearance over the whole range of current densities. Example 3. Adhesion testing

[0125] Nickel electroplating was conducted with the above nickel-plating baths with and without boric acid at temperatures of 35, 40, 50, 60 and 70 °C at line speeds of 75 m / min up to 150 m / min at plating times of 5, 10 and 20 s with hydrogen peroxide concentrations of 0, 0.1 and 0.5 wt.-%. Current density was 100 A / dm2. The samples were subjected to an adhesion test (NEN EN ISO 2409:2013, Erichsen Dome Test, 5 mm, followed by adhesive tape (Gitterschnitt test)). None of the samples showed any delamination.

[0126] Example 4. Taurine addition

[0127] The maximum current density that can be used with the nickel-plating baths of Example 1 is about 100 A / dm2. To the plating baths of Example 1 30 g / l taurine was added and electroplating was conducted at T = 60 °C, 400 rpm and t = 10 s. A current density of up to 140 A / dm2was possible and the obtained nickel layer on the steel sheet had an excellent appearance over the entire range of current densities.

[0128] Brief description of the drawings

[0129] The invention will now be explained by means of the following, non-limiting figures.

[0130] Figure 1 shows the relation between the dissolution times (in minutes) of 1 kg of nickel pellets as a function of the nickel pellet diameter (in mm) based on an assumed dissolution rate of nickel of 0.1 kg / m2.min.

[0131] Figure 2 shows the relation between the rotations per minute of the rotating cylinder electrode (RCE) and the line speed of an industrial plating line.

[0132] Figure 3 shows the thickness of the nickel layer (in pm) deposited from a nickel methane sulfonate solution with 100 g / l of Ni at 60 °C at 400 rpm of the RCE and as a function of the current density (in A / dm2) for different deposition times (5 s (lower curve), 10 s and 20 s (upper curve)).

[0133] Figure 4 shows the nickel layer thickness in pm for different rotation speeds of the RCE (i.e. for different coating line speeds) as a function of the current density (in A / dm2) for a Ni=50 g / l, 30 g / l boric acid nickel methane sulfonate solution at 60 °C for a 10 s plating time. The

[0134] Figure 5 shows the effect of the boric acid concentration on the plating layer thickness at 400 rpm9 for 15 g / l (top curve), 30 g / l and 45 g / l boric acid (lower curve) as a function of the current density (in A / dm2) for a 10 s plating time.

[0135] Figure 6 shows the plating efficiency of the 50 g / l nickel methane sulfonate solution with 30 g / l boric acid as a function of the pH. The plating efficiency increases steeply from values below 50% at a pH of below 1 to values above 70% above pH values of 1. As of pH values of 1.5 and higher efficiencies of over 90% are obtained (for the sake of clarity: a higher pH value means that the solution is less acidic). Figure 7 gives an impression of the quality of the plating layers obtainable at a plating temperature of 60 °C for a plating time of 10 seconds for a nickel methane sulfonate solution obtained with the process according to the invention at different current densities. All show an excellent visual appearance. Figure 8 shows a series of nickel-plated steel (Fe) substrates at various stages of diffusion annealing as described herein above.

[0136] Figure 9 shows an apparatus consisting of one or more dissolution tanks (D) for dissolving metallic nickel in a methane sulfonic acid. One or more buffer tanks (B) can be used to store the nickel methane sulphonate bath for simple storage or for fine tuning the composition of the bath by adding boric acid or taurine if so desired and any other required additives. From these buffer tanks the solution can be pumped into the plating cells (only two cells are drawn here, but there may be many more. In a continuous plating process a coil of metal substrate can be uncoiled and fed to the plating cells and plated, after which the plated substrate is coiled again for further processing. Pre- or post-treatment of the plated strip like cleaning or annealing of the cold-rolled strip (before or after plating) is not shown. Piping, pumps and plating peripherals are also not shown. Also, the supply means for the nickel metal and the other required chemicals is not shown.

Claims

CLAIMS1. Process for preparing a nickel-plating bath comprising the steps that(a) nickel (met.),(b) methane sulfonic acid, and(c) an oxygen releasing compound are combined in an aqueous solution.

2. Process according to claim 1 comprising the steps that(a) nickel (met.),(b) methane sulfonic acid, and(c) hydrogen peroxide and / or ozone are combined in an aqueous solution.

3. Process according to claim 2 wherein the following intermediate reaction takes place: a) Ni (met.) + H2O2 NiO + H2O (1) wherein the NiO is an intermediate reaction product which immediately dissolves in the methane sulfonic acid and wherein the overall reaction is as follows:Ni (met.) + H2O2 + 2 CH3SO3H Ni2++ 2 CH3SO3- + 2 H2O (2), or b) Ni (met.) + O3 -» NiO + O2 J (gas) (3) wherein the NiO is an intermediate reaction product which immediately dissolves in the methane sulfonic acid and wherein the overall reaction is as follows:Ni (met.) + O3+ 2 CH3SO3H Ni2++ 2 CH3SO3- + O2 J (gas) + H2O (4), thereby producing an aqueous nickel methane sulfonate solution.

4. Process according to one of the preceding claims, characterized in that the temperature of the aqueous solution is between 20 °C to 80 °C, preferably 50 °C to 70 °C.

5. Process according to one of the preceding claims, characterized in that the concentration of hydrogen peroxide is 4 - 15 wt.-%, based on the total weight of the solution and / or the concentration of methane sulfonic acid is 15 - 25 wt.-%, based on the total weight of the solution.

6. Process according to one of the preceding claims, characterized in that methane sulfonic acid and / or hydrogen peroxide are used in the form of an aqueous solution.

7. Process according to one of the preceding claims, characterized in that nickel (met.) is used in the form of metallic pellets or a metallic powder.

8. Process according to claim 7, characterized in that the average size of the pellets or powder particles is less than 10 mm, preferably 0.001 - 1 mm.

9. Process according to any of claims 1 to 8, characterized in that the molar ratio of methane sulfonic acid : hydrogen peroxide is 2 : (0.95 - 1.05), preferably 2 : (0.98 - 1.02).

10. Nickel plating bath, obtainable by a process according to any of claims 1 to 9.

11. Nickel plating bath according to claim 10, characterized in that it additionally contains 10 - 100 g / l boric acid and / or 10 - 70 g / l taurine.

12. Nickel plating bath according to claim 10 or 11, comprising26 - 316 g / l nickel methane sulfonate,10 - 100 g / l boric acid and / or 10 - 70 g / l taurine in water.

13. Nickel plating bath according to claim 12 comprising80 - 200 g / l nickel methane sulfonate,30 - 70 g / l boric acid and / or 30 - 60 g / l taurine in water.

14. Nickel plating bath according to claim 12 or 13 comprising80 - 200 g / l nickel methane sulfonate and30 - 60 g / l taurine in water.

15. Use of a nickel-plating bath according to any of claims 9 - 14 in a nickel electroplating process, preferably a process of electroplating nickel on a steel strip or sheet, preferably in a continuous electroplating process.

16. Use according to claim 15 to produce nickel plated substrates, in particular for electrical applications such as batteries and electrolysers.

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