Method and system for depositing zinc-nickel alloy on a substrate

Abstract

The invention relates to a method for depositing a zinc-nickel alloy on a substrate, the method comprising the steps of: (a) providing the substrate, (b) providing an aqueous zinc-nickel deposition bath as catholyte in a deposition compartment, wherein - the deposition compartment comprises at least one anode with an anolyte, and - the anolyte is separated from the catholyte by at least one membrane, and the catholyte comprises (i) nickel ions, (ii) at least one nickel ion complexing agent, and (iii) zinc ions, (c) contacting the substrate with the catholyte in the deposition compartment, such that the zinc-nickel alloy is electrolytically deposited onto the substrate and a zinc-nickel coated substrate is obtained thereby, wherein the concentration of the nickel ions in the catholyte is lower after step (c) than before step (c), (d) rinsing the zinc-nickel coated substrate in a rinsing compartment comprising water, such that a rinsed zinc-nickel coated substrate and a rinsing water are obtained, wherein the rinsing water comprises a part of the at least one nickel ion complexing agent and a part of the nickel ions, characterized in that (i) at least a part of the rinsing water and / or at least a part of the catholyte is treated in a first treatment compartment, such that water is separated from the at least one nickel ion complexing agent and the nickel ions, (ii) at least a part of the at least one complexing agent that is separated from the water is returned into the catholyte, and (iii) a source of nickel ions is added to the catholyte, with the proviso that the source of nickel ions does not comprise the at least one nickel ion complexing agent or any other nickel ion complexing agent.

Description

TECHNICAL FIELD

[0001] The present invention relates, according to a first aspect, to a method of depositing a zinc-nickel alloy on a substrate, in particular to a method of electrolytically depositing a zinc-nickel alloy on a substrate.

[0002] According to a second aspect, the present invention further relates to a system for depositing a zinc-nickel alloy on a substrate, in particular to a system for electrolytically depositing a zinc-nickel alloy on a substrate. BACKGROUND

[0003] Electrolytic deposition of metal alloys (sometimes also referred to as coatings) on other metals or metal-coated plastics (often referred to as substrates) is a well-established technology to increase the corrosion resistance of the substrates. The deposition is usually carried out using an anode and the substrate as cathode when applying an electric current in a respective electrolyte.

[0004] In some cases, the electrolyte is advantageously separated by means of a semi-permeable membrane into a catholyte compartment containing a catholyte (which is the electrolyte in the cathode space) and an anolyte compartment containing an anolyte (which is the electrolyte in the anode space). Usually, the anolyte is different from the catholyte. By applying an electric potential, an electric current flows through the membrane from the anolyte compartment to the catholyte compartment to start the electrolytic deposition on the substrate.

[0005] US 2011 / 031127 A1 to Hillebrand discloses such a basic electroplating bath for galvanizing zinc-nickel coatings having an anode and a cathode, wherein the anode is separated from the basic electrolyte by means of an ion exchange membrane.

[0006] US 2013 / 0264215 A1 to Umicore discloses an anode system configured in such a way that it is suitable for use in an electroplating unit for depositing an electrolytic coating since it is simply immersed in a catholyte, wherein after immersion in the catholyte, the catholyte is separated from the anode by means of a swollen polymer membrane, which is permeable for cations or anions, and which is in direct contact with the anode without contact with the cathode, wherein the membrane is fixed to the anode by means of an electrolyte-permeable holder and a pressing device by means of a multi-layer structure, which ensures a good contact of the membrane to the anode.

[0007] DE 20 2015 002 289 U1 discloses an electrodialysis cell with an anion and a cation exchange membrane for use as an anode in a basic zinc and zinc alloy electrolyte for electrodeposition in an electroplating system.

[0008] EP 1 533 399 A2 refers to a method for plating alkaline zinc nickel with reduced waste water.

[0009] Typically, zinc-nickel deposition baths are used continuously for extended periods of time, e.g. for weeks or even months, to allow for efficient deposition of zinc-nickel alloys on a plurality of different substrates. When using a zinc-nickel deposition bath for said extended periods of time, compounds that are typically not required, in particular degradation products of organic compounds, e.g. complexing agents including cyanide, start to accumulate in the zinc-nickel deposition bath over time. This typically significantly impairs the deposition process after a certain time and eventually can lead to the need to at least partially replace the zinc-nickel electrodeposition bath. In many cases, this is prevented by constantly removing at least a portion of the deposition bath, e.g. by squeegeeing, as waste water.

[0010] However, due to the inclusion of nickel ions and typically cyanide, a deep waste water treatment is required prior to waste water disposal. Thus, there is a continuous need to further improve existing deposition methods, in particular in view of environmental aspects. Due to tightening legal restrictions worldwide, in particular in view of nickel ions, there is an urgent need for more sustainable methods for depositing zinc-nickel alloys on substrates that provide less or no waste water, or at least have a lower contamination with critical metal ions. On the other hand, there is still a need that said methods can be operated economically and do not impair the corrosion protection as known so far.

[0011] Object of the invention

[0012] It is therefore an object of the present invention to provide a very environmentally friendly method and system for depositing zinc-nickel alloys on substrates that do not generate waste water or at least minimize the contamination with critical metal ions, e.g. nickel ions and cyanide ions, but at the same time can be operated economically over a long time. SUMMARY

[0013] According to a first aspect, the above-mentioned object is solved by a method for depositing a zinc-nickel alloy on a substrate, said method comprising the following steps:

[0014] (a) providing said substrate,

[0015] (b) providing an aqueous zinc-nickel deposition bath as catholyte in a deposition compartment, wherein

[0016] - said deposition compartment comprises at least one anode with an anolyte, and

[0017] - said anolyte is separated from said catholyte by at least one membrane, and

[0018] said catholyte comprises

[0019] (i) nickel ions,

[0020] (ii) at least one nickel ion complexing agent, and

[0021] (iii) zinc ions,

[0022] (c) contacting the substrate with the catholyte in the deposition compartment, so that the zinc-nickel alloy is electrolytically deposited onto the substrate and a zinc-nickel coated substrate is obtained therefrom, wherein

[0023] After step (c), the concentration of the nickel ions in the catholyte is lower than before step (c),

[0024] (d) rinsing the zinc-nickel coated substrate in a rinsing compartment comprising water, so that a rinsed zinc-nickel coated substrate and rinsing water are obtained, wherein

[0025] the rinsing water comprises a part of the at least one nickel ion complexing agent and a part of the nickel ions,

[0026] characterized in that

[0027] (i) at least a part (preferably all) of the rinsing water and / or at least a part of the catholyte is treated in a first treatment compartment, so that water is separated from the at least one nickel ion complexing agent and the nickel ions,

[0028] (ii) at least a part (preferably all) of the at least one complexing agent that is separated from the water is returned to the catholyte, and

[0029] (iii) a source of nickel ions is added to the catholyte, with the proviso that the source of nickel ions does not comprise the at least one nickel ion complexing agent or any other nickel ion complexing agent.

[0030] The process of the present invention solves the above defined object very well, because it allows a closed loop operation in theory for an unlimited period of time, but at least for several weeks and in particular for one month. During said period of time, water is treated to be substantially free of nickel and cyanide ions (and thus not referred to as waste water).

[0031] During the closed loop operation, preferably only the nickel ions and the zinc ions that are deposited on the substrate during deposition have to be replenished. All other compounds that are comprised in the deposition bath, preferably in the catholyte, are recycled.

[0032] By returning at least a part, preferably all, of the at least one complexing agent separated from the water in the first treatment compartment to the catholyte (directly or indirectly), the concentration of the at least one nickel ion complexing agent in the catholyte is maintained at a constant concentration. As defined in the method of the present application, no or hardly any replenishment of the complexing agent is necessary. This is done by using at least one anode with at least one membrane. The membrane prevents anode degradation of organic compounds, such as complexing agents. The complexing agent dragged out to the rinse compartment is recycled through the first treatment compartment. This allows the replenished nickel ions to be free of any complexing agent.

[0033] In particular, when setting up an aqueous zinc-nickel deposition bath, preferably a catholyte, it is sufficient to provide an initial concentration of at least one nickel ion complexing agent, wherein no additional complexing agent has to be added during the deposition process.

[0034] Furthermore, when treating the rinse water in the first treatment compartment in order to separate the water, a very pure water is typically obtained which can be reused.

[0035] Furthermore, according to a second aspect, the above mentioned objects are solved by a system for depositing a zinc-nickel alloy on a substrate, the system comprising:

[0036] (I) optionally, a pre-rinse compartment for pre-rinsing the substrate,

[0037] (II) a deposition compartment for electrolytically depositing a zinc-nickel alloy on the substrate in a catholyte, such that a zinc-nickel coated substrate is obtained, wherein the deposition compartment comprises at least one anode with at least one membrane,

[0038] (III) a rinse compartment for rinsing the zinc-nickel coated substrate, such that a rinsed zinc-nickel coated substrate and a rinse water are obtained,

[0039] (IV) a first treatment compartment for treating the rinse water and a part of the catholyte, such that the water is separated from nickel ions and nickel ion complexing agents, and

[0040] (V) optionally, a second treatment compartment for treating the catholyte, such that dissolved anions are separated from the catholyte,

[0041] wherein

[0042] the first treatment compartment is adapted such that

[0043] - the separated water is returned into the pre-rinse compartment and / or the rinse compartment, and

[0044] - the separated nickel ions and the separated nickel ion complexing agents are returned into the deposition compartment, preferably via a mixing compartment. BRIEF DESCRIPTION OF DRAWINGS

[0045] In Figure 1 In the context of the present application, the term "at least one", "one or more than one" and / or "one or more" means "one, two, three or more than three" (and are interchangeable therewith). DETAILED DESCRIPTION

[0046] In the context of the present application, the term "at least one", "one or more than one" and / or "one or more" means "one, two, three or more than three" (and are interchangeable therewith).

[0047] In the context of the present application, the anolyte is typically the electrolyte in direct contact with the at least one anode, wherein the catholyte is the electrolyte or at least a portion of the electrolyte in contact with the cathode (i.e. the substrate) at least for the time the catholyte is located in the deposition compartment.

[0048] As already mentioned above, the main advantage achieved by the method of the present application is that no degradation products are formed due to the at least one membrane of the anode. This preferably means that the at least one anode and the at least one membrane are adapted to form an anolyte separated from the catholyte and the selective permeation of ions between the catholyte and the anolyte is only possible through the at least one membrane. The at least one membrane is adapted to not allow the at least one complexing agent to pass through said membrane (from the catholyte into the anolyte). This allows for said closed loop operation which constantly recycles the initial concentration of the at least one nickel ion complexing agent. Most preferably, the at least one membrane only allows the permeation of hydrogen ions (formed in the anolyte) into the catholyte.

[0049] Thus, the method of the present application is preferred, wherein the at least one nickel ion complexing agent is not in contact with the at least one anode, most preferably not in contact with any of the at least one anode.

[0050] Further, the method of the present application is preferred, wherein the catholyte only comprises the initial concentration of the at least one nickel ion complexing agent for at least one nickel ion turnover, more preferably for at least 2 nickel ion turnovers, even more preferably for at least 3 nickel ion turnovers, most preferably for the entire lifetime of the catholyte.

[0051] The at least one membrane preferably only allows the diffusion of protons between the anolyte and the catholyte, which ensures an efficient distribution of the electric charge between the anolyte and the catholyte.

[0052] During the process of the present invention, water is typically introduced into the catholyte, e.g. by means of a nickel ion source for replenishing nickel ions. However, in the first treatment compartment, excess water is separated and subsequently removed from the process of the present invention, such that a substantially constant volume of catholyte is maintained over time. If the excess water cannot be used in the process of the present invention, it is preferably readily disposable, since it is substantially free of nickel ions, and preferably also free of zinc ions; substantially no complexing agent is present.

[0053] In summary, the process of the present invention allows for an economic, sustainable, continuous operation over an extended period of time, i.e. over weeks or even months. During the extended period of time, no nickel-contaminated wastewater is produced, and also no valuable metal ions as well as complexing agent are lost due to being dragged out. Basically, only the amount of deposited nickel and zinc ions has to be replenished by the respective nickel and zinc ion sources.

[0054] With regard to the process of the present invention, most preferably at least a part of the rinse water (preferably all of it) and at least a part of the catholyte are treated in the first treatment compartment, such that water is separated from at least one nickel ion complexing agent and nickel ions. Also treating a part of the catholyte (in addition to the rinse water, preferably in addition to all of the rinse water) allows for maintaining a substantially constant volume of catholyte.

[0055] By separating at least one nickel ion complexing agent and nickel ions from water, the complexing agent and nickel ions thus recycled have the required concentration before being returned into the catholyte.

[0056] The process of the present invention is preferred, wherein the complexing agent separated from water is returned to the catholyte as a concentrated aqueous solution. More preferably, the complexing agent separated from water is returned to the catholyte as a concentrated aqueous solution directly or indirectly, most preferably the complexing agent separated from water is returned to the catholyte as a concentrated aqueous solution indirectly via a mixing unit.

[0057] The mixing unit is preferably used for mixing the separated complexing agent with, e.g., a nickel ion source and / or a zinc ion source, most preferably the mixing unit provides a freshly mixed aqueous zinc-nickel deposition bath ready for transfer into the deposition compartment for replenishing the catholyte.

[0058] By returning the complexing agent and thereby maintaining a substantially constant concentration of complexing agent, a continuous constant stabilization of nickel ions in the catholyte is achieved, which in turn provides for a good stability of the catholyte. When the complexing agent is returned to the catholyte indirectly via a mixing unit, the complexing agent is preferably used for complexing freshly introduced nickel ions from a nickel ion source into the mixing unit (see Figure 1 ).

[0059] Therefore, preferred is the process of the present application, wherein the source of nickel ions is added directly or indirectly to the catholyte, preferably indirectly via the mixing unit, preferably as described above.

[0060] Preferred is the process of the present application, wherein the source of zinc ions is added directly or indirectly to the catholyte, preferably indirectly via the mixing unit, preferably as described above. More preferably, the zinc ions are obtained by dissolving metallic zinc in sodium hydroxide to obtain a zinc hydroxyl complex, which allows for an efficient stabilization of the zinc ions in the catholyte.

[0061] By adding the sources of nickel and zinc ions to the catholyte, the nickel and zinc ions are replenished. Preferably, the sources of nickel and zinc ions are added indirectly via the mixing unit, such that a sufficiently mixed composition is prepared before it is transferred to the deposition compartment.

[0062] Preferred is the process of the present application, wherein the anolyte is water, preferably water comprising sulfuric acid, most preferably water comprising 5 vol.-% to 40 vol.-% sulfuric acid.

[0063] Preferred is the process of the present application, wherein the catholyte comprises more than 50 vol.-% water, more preferably 75 vol.-% or more water, even more preferably 85 vol.-% or more water, most preferably 92 vol.-% or more water, based on the total volume of the catholyte. Preferably, the water is the only solvent in the catholyte.

[0064] Preferred is the process of the present application, wherein the source of nickel ions is an aqueous solution comprising water and a nickel salt dissolved therein. Preferred is the process of the present application, wherein the nickel salt is an inorganic salt. This preference means that the nickel salt does not comprise a carboxylate anion, more preferably does not comprise an organic acid anion, most preferably does not comprise an organic anion.

[0065] By excluding organic anions, in particular carboxylate anions, the potential accumulation of potentially detrimental organic anions in the catholyte over time can be prevented. Furthermore, potential complexing agents for the nickel ions are thus essentially excluded.

[0066] Preferred is the process of the present application, wherein the nickel salt comprises nickel sulfate, preferably nickel sulfate hexahydrate.

[0067] Preferred is the process of the present application, wherein the nickel salt does not comprise nickel chloride. By excluding nickel chloride, the concentration of chloride ions in the catholyte can be minimized or most preferably even eliminated, thus eliminating the necessity to remove excess chloride from the catholyte during the process of the present application (which is often difficult due to the high solubility of chloride salts).

[0068] The process of the present invention is preferred, wherein the nickel salt does not comprise nickel nitrate. By excluding nickel nitrate, the concentration of nitrate ions in the catholyte is prevented. In many cases, nitrate interferes with the overall electrolytic deposition and is highly undesirable.

[0069] The source of nickel ions is most preferably an aqueous solution comprising water and dissolved therein nickel sulfate, preferably nickel sulfate hexahydrate. Said preferred source of nickel ions is excellently suited to replenish nickel ions. As for any accumulation of sulfate anions, see below.

[0070] The process of the present invention is preferred, wherein the concentration of nickel ions in the source of nickel ions, based on the total volume of the source of nickel ions, is in the range of 70 g / L to 140 g / L, preferably 80 g / L to 120 g / L, more preferably 90 g / L to 110 g / L, even more preferably 95 g / L to 105 g / L.

[0071] As mentioned above, the source of nickel ions does not comprise said at least one nickel ion complexing agent or any other nickel ion complexing agent. This means that the at least one nickel ion complexing agent is not replenished by means of the source of nickel ions. Most preferably, the at least one nickel ion complexing agent is not replenished at all. Furthermore, no complexing agent different from the at least one nickel ion complexing agent is added to the catholyte, e.g. a complexing agent used for initially setting up the aqueous zinc-nickel deposition bath. It is therefore preferred that the process of the present invention, wherein the catholyte comprises only one nickel ion complexing agent (and thus not a mixture of two or more complexing agents). This is helpful for monitoring the total amount of complexing agent in the catholyte over time.

[0072] The process of the present invention is preferred, wherein the source of nickel ions is essentially free of or does not comprise tetraethylenepentamine, preferably essentially free of or does not comprise a diamine, most preferably essentially free of or does not comprise an amine. This is most preferred, because said compounds are typically used as nickel ion complexing agents in aqueous zinc-nickel deposition baths (for further details on complexing agents, see below). Thus, in particular, said compounds are not needed in the source of nickel ions to prevent their accumulation.

[0073] The process of the present invention is preferred, wherein the source of nickel ions is essentially free of or does not comprise an amine having one or more than one, preferably two, primary amine groups and one or more than one secondary amine group.

[0074] In the process of the present invention, the catholyte comprises at least one, preferably one, nickel ion complexing agent.

[0075] The method of the present application is preferred, wherein in the catholyte at least one nickel ion complexing agent comprises a chelate complexing agent, wherein preferably the chelate complexing agent is the only nickel ion complexing agent in the catholyte. By using a chelate complexing agent, an efficient stabilization of the nickel ions in the catholyte is ensured. In particular, when setting up the aqueous zinc-nickel deposition bath initially, the at least one complexing agent has to be provided only once, without having to add additional complexing agents afterwards.

[0076] The method of the present application is preferred, wherein in the catholyte at least one nickel ion complexing agent comprises an amine, preferably a diamine, most preferably tetraethylenepentamine. The amine, the diamine and the tetraethylenepentamine, respectively, as nickel ion complexing agent allow for an excellent stability of the nickel ions in the catholyte, in particular at alkaline pH.

[0077] The method of the present application is preferred, wherein the amine, preferably the diamine, most preferably tetraethylenepentamine, is the only nickel ion complexing agent in the catholyte.

[0078] The method of the present application is preferred, wherein the diamine is selected from the group consisting of ethylenediamine, diethylenetriamine, triethylenetetramine and tetraethylenepentamine.

[0079] Generally, the method of the present application is preferred, wherein in the catholyte at least one nickel ion complexing agent comprises an amine having one or more than one, preferably two, primary amine groups and one or more than one secondary amine group.

[0080] The method of the present application is preferred, wherein the amine having one or more than one, preferably two, primary amine groups and one or more than one secondary amine group is the only nickel ion complexing agent in the catholyte.

[0081] The method of the present application is preferred, wherein the nickel ions of the nickel ion source added to the catholyte are not complexed prior to contact with an alkaline environment, preferably an environment having a pH in the range of 10.0 to 14.0, more preferably 11.0 to 13.3, even more preferably 11.5 to 13.0, still even more preferably 12.0 to 12.9, most preferably 12.3 to 12.8. In other words, the nickel ions of the nickel ion source added to the catholyte are preferably complexed for the first time upon contact with an alkaline environment, preferably an environment having a pH as defined above, which is most preferably the catholyte.

[0082] Further, herein relates to an alternative method of depositing a zinc-nickel alloy on a substrate, the method comprising the following steps:

[0083] (a) providing the substrate,

[0084] (b) providing an alkaline aqueous zinc-nickel deposition bath as catholyte in a deposition compartment, wherein

[0085] - the deposition compartment comprises at least one anode with an anolyte, and

[0086] - the anolyte is separated from the catholyte by at least one membrane, and

[0087] the catholyte comprises

[0088] (i) nickel ions,

[0089] (ii) at least one nickel ion complexing agent, and

[0090] (iii) zinc ions,

[0091] (c) contacting the substrate with the catholyte in the deposition compartment, such that the zinc-nickel alloy is electrolytically deposited onto the substrate and a zinc-nickel coated substrate is obtained thereby, wherein

[0092] after step (c), the concentration of the nickel ions in the catholyte is lower than before step (c),

[0093] (d) rinsing the zinc-nickel coated substrate in a rinsing compartment comprising water, such that a rinsed zinc-nickel coated substrate and a rinsing water are obtained, wherein

[0094] the rinsing water comprises a part of the at least one nickel ion complexing agent and a part of the nickel ions,

[0095] characterized in that

[0096] (i) at least a part of the rinsing water and / or at least a part of the catholyte is treated in a first treatment compartment, such that water is separated from the at least one nickel ion complexing agent and the nickel ions,

[0097] (ii) at least a part of the at least one complexing agent separated from water is returned to the catholyte, and

[0098] (iii) nickel ions are added to the catholyte from a source of nickel ions to replenish nickel ions, wherein the nickel ions of the source of nickel ions added to the catholyte have not been complexed with a complexing agent prior to contact with an alkaline environment, preferably a pH in the range of 10.0 to 14.0, more preferably 11.0 to 13.3, even more preferably 11.5 to 13.0, still even more preferably 12.0 to 12.9, most preferably 12.3 to 12.8.

[0099] The features of the method of the application as defined throughout this text, including preferred features and the like, preferably apply also to the alternative methods, if technically applicable.

[0100] The method of the present invention is preferred, wherein step (a) comprises the following steps before step (c)

[0101] (a-1 ) pre-rinsing the substrate in a pre-rinsing compartment comprising water, such that a pre-rinsed substrate and pre-rinsing water are obtained.

[0102] By pre-rinsing the substrate in a pre-rinsing compartment, potential contaminants on the substrate are removed before the substrate is transferred to the deposition compartment. Preferably, the pre-rinsing compartment comprises an aqueous sodium hydroxide solution as pre-rinsing solution.

[0103] In the method of the present invention, the zinc-nickel coated substrate is rinsed in a rinsing compartment in step (d).

[0104] The method of the present invention is preferred, wherein the rinsing compartment comprises 2 to 5 fluidly connected rinsing sub-compartments, thereby forming a rinsing cascade.

[0105] The rinsing cascade is particularly effective in rinsing, because the ion concentration rinsed off from the zinc-nickel coated substrate is effectively stepwise reduced, such that the most downstream rinsing sub-compartments comprise significantly lower ion concentrations compared to the most upstream rinsing sub-compartments of the rinsing cascade.

[0106] In the deposition compartment, there is at least one anode and at least one membrane, wherein the at least one membrane separates the anolyte from the catholyte. Most preferably, the at least one membrane is a semi-permeable membrane. This means that the at least one membrane is selectively permeable.

[0107] The method of the present invention is preferred, wherein the at least one membrane is a cation exchange membrane. By using a cation exchange membrane, any unwanted permeation of the at least one complexing agent from the catholyte into the anolyte is effectively prevented.

[0108] The method of the present invention is preferred, wherein in the deposition compartment, the at least one anode is an insoluble anode, preferably an insoluble mixed metal oxide anode, most preferably an insoluble iridium / tantalum oxide on a titanium anode.

[0109] The method of the present invention is preferred, wherein the distance of the at least one anode to the at least one membrane is in the range of 0.5 mm to 5.0 mm, preferably 0.75 mm to 4 mm, more preferably 1.0 mm to 3.0 mm. This advantageously allows to keep the anolyte volume low, which in turn leads to a low amount of waste water from the anolyte.

[0110] In the method of the present invention, at least a portion of the rinsing water and / or at least a portion of the catholyte is treated in a first treatment compartment, such that water is separated from the at least one nickel ion complexing agent and the nickel ions.

[0111] The method of the present invention is preferred, wherein the first treatment compartment comprises an evaporator, preferably a vacuum evaporator.

[0112] The method of the present invention is preferred, wherein a vacuum in the range of 1 mbar to 100 mbar, preferably 5 mbar to 70 mbar, more preferably 10 mbar to 50 mbar, most preferably 15 mbar to 35 mbar is applied in the evaporator.

[0113] The method of the present invention is preferred, wherein water is separated in the first treatment compartment, preferably in the evaporator, most preferably in the vacuum evaporator, at a temperature in the range of 18°C to 50°C, more preferably 23°C to 46°C, more preferably 28°C to 42°C, most preferably 31°C to 40°C.

[0114] By using an evaporator, preferably a vacuum evaporator, efficient evaporation of water can be achieved, in particular by reducing the atmospheric pressure, thereby allowing efficient separation of water from nickel ions and from the at least one complexing agent. Efficient separation of water is achieved since the boiling point of water is significantly lower than the boiling point of the at least one complexing agent, nickel and / or zinc ions.

[0115] By operating the vacuum evaporator at a temperature between 18°C and 50°C, unwanted heating or even thermal degradation of the at least one complexing agent is prevented.

[0116] The method of the present invention is preferred, wherein the vacuum evaporator is operated and controlled based on density measurements of the concentrated aqueous solution, preferably the density of the concentrated aqueous solution is in the range of 1.08 kg / L to 1.30 kg / L, more preferably 1.10 kg / L to 1.26 kg / L, more preferably 1.15 kg / L to 1.24 kg / L, most preferably 1.20 kg / L to 1.23 kg / L based on the total volume of the concentrated aqueous solution. Control based on density measurements is extremely well suited for automatic operation of the first treatment compartment, preferably the evaporator, most preferably the vacuum evaporator. The density ranges mentioned above are most preferred. However, in some cases a higher maximum density is acceptable as long as the concentrated aqueous solution does not form a phase separation. This can include a maximum density of e.g. 1.28 kg / L, 1.30 kg / L, in some cases even 1.32 kg / L. Phase separation also typically depends on the total amount of e.g. sulphates, carbonates and hydroxides (e.g. sodium and / or potassium), which changes over time.

[0117] The concentrated aqueous solution is aqueous as defined above. Thus, the method of the present invention is preferred, wherein the concentrated aqueous solution is homogenous. This preferably means that the concentrated aqueous solution forms only a single phase; in other words, the concentrated aqueous solution preferably does not form a phase separation. Most preferably, the concentrated aqueous solution does not comprise an organic phase that is phase separated from water.

[0118] Therefore, even more preferred is the process according to the application, wherein the concentrated aqueous solution is fully aqueous.

[0119] By not exceeding the maximum density mentioned above (or even more, as mentioned above), preferably of 1.26 kg / L, phase separation is preferably avoided.

[0120] Due to the treatment in the first treatment compartment, very pure water and a concentrated aqueous solution are obtained.

[0121] The process according to the application is preferred, wherein at least part of the separated water obtained in the first treatment compartment is returned into the pre-rinse compartment and / or the rinse compartment. Preferably, at least part of the separated water obtained in the first treatment compartment is returned into the rinse compartment, more preferably into a rinse sub-compartment of the rinse cascade.

[0122] By returning the very pure separated water, water is recycled and waste water is avoided, since the separated water is substantially free of complexing agent, nickel ions and zinc ions. Therefore, it is fully suitable for being used again in the pre-rinse and rinse compartments. By means of said loop, preferably no fresh water is needed for rinsing over a relatively long period of time.

[0123] The process according to the application is preferred, wherein the water is separated from the at least one nickel ion complexing agent and the nickel ions in such a way that the catholyte has a substantially constant volume, preferably a constant volume, in the deposition compartment. This is especially achieved if in the first treatment compartment at least part of the catholyte is additionally treated besides the rinse water. Typically, more water is introduced into the catholyte (for example by adding a nickel ion source and by hydrogen ions formed at the anode in the catholyte) than is separated from the rinse water.

[0124] The process according to the application is preferred, wherein at least part of the at least one complexing agent separated from the water (preferably all) and at least part of the nickel ions separated from the water (preferably all) are returned into the catholyte, preferably as a concentrated aqueous solution (preferably as described throughout this text) into the catholyte. Preferably, the concentrated aqueous solution is returned directly or indirectly, preferably indirectly via a mixing unit.

[0125] Typically, the rinse water also comprises zinc ions. Therefore, the process according to the application is preferred, wherein the rinse water comprises part of the zinc ions.

[0126] The process according to the application is preferred, wherein in the first treatment compartment the water is separated from the nickel ions, the at least one nickel ion complexing agent and the zinc ions.

[0127] The process of the present invention is preferred, wherein the nickel ions, the zinc ions and the at least one nickel ion complexing agent are returned together, preferably as a concentrated aqueous solution, preferably as described throughout this text, to the cathode electrolyte.

[0128] As mentioned above, it is preferred that the nickel ion source comprises nickel sulfate. This means that sulfate anions are introduced into the cathode electrolyte, which usually accumulate over time. Furthermore, the cathode electrolyte usually has a tendency to form and accumulate carbonate anions. Both anions are usually well soluble in the cathode electrolyte. Although a certain concentration can be tolerated, an excessive accumulation of said anions is to be prevented. Therefore, the process of the present invention is preferred, wherein the process comprises the step

[0129] (e) treating at least a portion of the cathode electrolyte in a second treatment compartment, preferably by precipitation and / or ion exchange, most preferably by precipitation, such that the dissolved anions are separated from the cathode electrolyte.

[0130] The process of the present invention is preferred, wherein the dissolved anions comprise sulfate, carbonate and / or chloride, preferably at least sulfate and carbonate.

[0131] By applying step (e) in addition to steps (a) to (d), the concentration of dissolved anions in the cathode electrolyte is significantly reduced and an excessive accumulation is avoided. Therefore, the process of the present invention can be operated for a very long time. Preferably, step (e) is applied when the dissolved anions, individually or in total, reach an undesirable concentration. Preferably, step (e) comprises the most preferred precipitation for removing one or more than one of said anions from the cathode electrolyte by reducing the temperature of at least a portion of the cathode electrolyte in the second treatment compartment and thereby reducing the solubility of the respective salt.

[0132] Therefore, preferably, the sulfate and carbonate anions are separated from the cathode electrolyte by precipitation of a salt comprising sulfate anions and carbonate anions.

[0133] Most preferably, the treatment in step (e) forms a solid precipitate. If the solid precipitate co-precipitates other cathode electrolyte components, a supplement of the same (e.g. at least one nickel ion complexing agent) is recommended. In some cases, said co-precipitation seems to be unavoidable.

[0134] The process of the present invention is less preferred, wherein in the second treatment compartment the dissolved anions are separated by ion exchange. Usually, ion exchange is not specific enough for said dissolved anions.

[0135] The process of the present application is preferred, wherein the precipitation is carried out at a temperature in the range of -5°C to 11.0°C, preferably in the range of 0.5°C to 10.0°C, more preferably in the range of 1.0°C to 8.0°C, even more preferably in the range of 1.5°C to 6°C, most preferably in the range of 2.0°C to 4.0°C. As mentioned above, by significantly lowering the temperature in the second treatment compartment, typically a low-solubility anion-containing salt is formed, thereby at least partially removing the anion from the catholyte. Most preferably, the low-solubility anion-containing salt is a sodium salt. Alternative preferred temperatures are in the range of -3°C to 5°C, more preferably -2.5°C to 4°C, most preferably -2°C to 3°C.

[0136] Thus, the process of the present application is preferred, wherein the dissolved anions comprise at least sulfate anions, and wherein the sulfate anions are preferably separated by precipitated sodium sulfate.

[0137] Further, the process of the present application is preferred, wherein the dissolved anions comprise at least sulfate anions and carbonate anions, and wherein the sulfate anions and carbonate anions are preferably separated by precipitated sodium sulfate and sodium carbonate, respectively.

[0138] Sodium salts are particularly preferred, because sodium hydroxide is preferably used to maintain the pH of the catholyte. Due to the constant anodic formation of hydrogen ions (resulting in chemically formed water), hydroxide is constantly replenished, which also introduces a significant amount of sodium. Thus, sodium is removed by the treatment in the second treatment compartment.

[0139] The process of the present application is preferred, wherein the catholyte is alkaline, preferably having a pH in the range of 10.0 to 14.0, more preferably 11.0 to 13.3, even more preferably 11.5 to 13.0, still even more preferably 12.0 to 12.9, most preferably 12.3 to 12.8.

[0140] As mentioned above, due to the separation of the anolyte from the catholyte by the at least one anode and the at least one separator, the formation of degradation products in the catholyte is substantially avoided. This includes the substantial absence of formation of unwanted cyanide in the catholyte. Thus, the process of the present application is preferred, wherein the catholyte comprises cyanide ions in the range of 0 mg / L to 2.5 mg / L, preferably 0 mg / L to 1.5 mg / L, more preferably 0 mg / L to 1 mg / L, most preferably 0 mg / L to 0.5 mg / L, based on the total volume of the catholyte. Most preferably, the catholyte is substantially free of cyanide ions, i.e. 0.001 mg / L to 0.05 mg / L; even most preferably, it does not comprise cyanide ions.

[0141] The process of the present application is preferred, wherein the catholyte comprises oxalate ions in a range of 0 mg / L to 2.5 mg / L, preferably 0 mg / L to 1.5 mg / L, more preferably 0 mg / L to 1 mg / L, most preferably 0 mg / L to 0.5 mg / L, based on the total volume of the catholyte. Most preferably, the catholyte is essentially free of oxalate ions, i.e. 0.001 mg / L to 0.05 mg / L; even most preferred, the catholyte does not comprise oxalate ions. Oxalate ions are also typical degradation products, which are essentially avoided in the process of the present application.

[0142] Since neither cyanide ions nor oxalate ions are formed in the catholyte, no special waste water treatment is required to address said ions.

[0143] As mentioned above, the zinc ions in the catholyte are replenished by means of a zinc ion source. The process of the present application is preferred, wherein in the catholyte the zinc ions are present as hydroxo complexes. Preferably, the zinc ion source comprises water, hydroxide ions (preferably sodium hydroxide) and metallic zinc. If the metallic zinc is dissolved under alkaline conditions, the hydroxo complexes are preferably obtained.

[0144] The process of the present application is preferred, wherein in the catholyte the zinc ions do not form complexes with at least one nickel ion complexing agent, preferably do not form complexes with diamines, more preferably do not form complexes with organic complexing agents. Most preferably, the zinc ions in the catholyte are strongly stabilized as hydroxo complexes, such that no complexes of zinc ions with at least one nickel ion complexing agent are observed under alkaline conditions.

[0145] The process of the present application is preferred, wherein in the catholyte the concentration of zinc ions is below 10 g / L, preferably in a range of 5.0 g / L to 9.0 g / L, more preferably 5.2 g / L to 8.5 g / L, even more preferably 5.4 g / L to 8.0 g / L, still even more preferably 5.7 g / L to 7.5 g / L, most preferably 5.9 g / L to 7.3 g / L.

[0146] The process of the present application is preferred, wherein in the catholyte the concentration of nickel ions is below 2.0 g / L, preferably in a range of 0.5 g / L to 1.9 g / L, more preferably 0.6 g / L to 1.7 g / L, even more preferably 0.7 g / L to 1.6 g / L, still even more preferably 0.8 g / L to 1.5 g / L, most preferably 0.9 g / L to 1.4 g / L.

[0147] Advantageously, in the process of the present application the concentrations of nickel and zinc ions as defined above are generally lower than the concentrations which are common in processes known in the art. Since in the process of the present application nickel ions and preferably zinc ions are recycled, no significant amounts of nickel and zinc ions, respectively, are wasted.

[0148] As already mentioned above, excess water (which is extremely pure) is separated and removed from the process of the present application.

[0149] The process of the present application is preferred, wherein at least a part of the separated water obtained in the first treatment compartment is disposed, wherein the disposed water comprises nickel ions in a concentration in the range of 0 mg / L to 1.0 mg / L, preferably 0 mg / L to 0.5 mg / L, even more preferably 0.01 mg / L to 0.11 mg / L, and most preferably 0.01 mg / L to 0.1 mg / L, based on the total volume of the disposed water.

[0150] The process of the present application is preferred, wherein at least a part of the separated water obtained in the first treatment compartment is disposed, wherein the disposed water comprises zinc ions in a concentration in the range of 0 mg / L to 1.0 mg / L, preferably 0 mg / L to 0.5 mg / L, more preferably 0.01 mg / L to 0.11 mg / L, and most preferably 0.01 mg / L to 0.1 mg / L, based on the total volume of the disposed water.

[0151] Preferably, only the pH needs to be adapted before disposing of the excess water.

[0152] In other cases, it is extremely preferred to use the waste water (preferably the excess water) for a pre-rinse, i.e. a rinsing step performed prior to steps (b) and (c). This preferably means that the water is wasted (or disposed) into a pre-rinse compartment. This is most preferred. In this case, no water is wasted, but used to the maximum extent possible.

[0153] It is also preferred that the process of the present application, wherein the waste water (preferably the excess water) is used for a further pre-treatment step prior to steps (b) and (c), more preferably for a cleaning step, most preferably for one or more than one degreasing step (e.g. a soak cleaning step, an electro cleaning step, etc.).

[0154] It is also preferred that the process of the present application, wherein the waste water (preferably the excess water) is used for one or more than one further post-treatment step, preferably for passivating the zinc-nickel coated substrate in a passivation step.

[0155] By using the excess water in one or more than one of the above mentioned applications, water is used to the best extent and waste water is reduced to the best extent possible.

[0156] According to a second aspect, the present application provides a system for depositing a zinc-nickel alloy on a substrate, the system comprising:

[0157] (I) optionally, a pre-rinse compartment for pre-rinsing the substrate,

[0158] (II) a deposition compartment for electrolytic deposition of a zinc-nickel alloy in a catholyte on a substrate, such that a zinc-nickel coated substrate is obtained, wherein the deposition compartment comprises at least one anode with at least one membrane,

[0159] (III) a rinsing compartment for rinsing the zinc-nickel coated substrate, such that a rinsed zinc-nickel coated substrate and rinsing water are obtained,

[0160] (IV) a first treatment compartment for treating the rinsing water and a part of the catholyte, such that water is separated from nickel ions and nickel ion complexing agents, and

[0161] (V) optionally, a second treatment compartment for treating the catholyte, such that dissolved anions are separated from the catholyte,

[0162] wherein

[0163] the first treatment compartment is adapted such that

[0164] - the separated water is returned into the pre-rinsing compartment and / or the rinsing compartment, and

[0165] - the separated nickel ions and the separated nickel ion complexing agents are returned into the deposition compartment, preferably via the mixing compartment.

[0166] As to (I), (II), (III), (IV) and (V) of the system of the invention, the above mentioned as to the method of the invention preferably apply equally. Thus, preferably, the above mentioned as to the method of the invention, preferably described as preferred, apply equally to the system of the invention.

[0167] The invention is described in detail by the following non-limiting examples.

[0168] Examples

[0169] Test plating setup (according to the invention)

[0170] In a test plating setup according to the invention, a zinc-nickel deposition bath was set up as catholyte in a deposition compartment (about 20.000 L) to deposit a zinc-nickel alloy on small metal parts (e.g. screws; about 40 kg load / barrel).

[0171] The catholyte initially comprised 0.9 g / L to 1.4 g / L nickel (II) ions, 5.9 g / L to 7.3 g / L zinc (II) ions and as chelate complexing agent for the nickel ions an additional diamine with at least one secondary amine group. The pH was strongly alkaline of about 12.5 and adjusted with sodium hydroxide.

[0172] A variety of insoluble iridium / tantalum oxides on titanium anodes with cation exchange membranes are utilized. The distance between the anode and the respective membrane is below 5 mm for each anode. Each anode electrolyte comprising water and sulfuric acid is separated from the cathode electrolyte by the membrane so that the complexing agent is never in contact with the anode.

[0173] The metal part is contacted with the cathode electrolyte in the deposition compartment (at about 25°C) and a current density of less than 1 A / dm 2 is applied for the electrolytic deposition of varying times between 130 min and 170 min.

[0174] The test plating setup is utilized for 4 months and the water, chemical compound consumption and water disposal are closely monitored.

[0175] During the 4 months process period, the nickel ions are replenished with a nickel ion source which is an aqueous solution comprising dissolved nickel sulfate without any nickel ion complexing agent and having a nickel ion concentration of about 100 g / L. The zinc is replenished from a metal zinc dissolved under alkaline pH conditions. No additional zinc ion complexing agent is used due to the formation of zinc hydroxide complexes under alkaline conditions.

[0176] After deposition of the zinc-nickel alloy, the metal part is rinsed with water in a rinsing compartment comprising five fluidly connected rinsing sub-compartments forming a 5-step rinsing cascade. Portions of the rinsing water are repeatedly combined with portions of the cathode electrolyte and transferred into a vacuum evaporator (40°C, about 50 mbar, capacity: about 150 L / h) to separate the water from the complexing agent, the nickel ions and the zinc ions, respectively. A portion of the separated water is returned into the rinsing cascade. The excess water (nickel and zinc concentration below 0.1 mg / L) is used for disposal or other industrial purposes, in particular for the pre-rinsing step as used in this example. In each case, the separated water has an electrical conductivity of less than 200 μS / cm. The nickel ions, the zinc ions and the complexing agent are enriched as concentrated aqueous solutions (density between 1.20 kg / L and 1.23 kg / L; completely aqueous without any phase separation) and returned into the cathode electrolyte. During the about 4 months operating time, about less than 500 L / week of excess water (<200 μS / cm) is disposed, preferably for pre-rinsing.

[0177] Even after the 4 months operating time, the cathode electrolyte does not contain decomposition products such as cyanide ions and oxalate ions. This confirms that the complexing agent does not decompose in the deposition compartment and in the vacuum evaporator. This is the basis for the repeated use of water.

[0178] After an operating time of about 4 months, a part of the catholyte was treated in the second treatment compartment (freezing unit) at a temperature between 2°C and 4°C or between -2°C and 2°C in order to precipitate at least a part of the sulphate and carbonate anions. However, even after 4 months, the critical concentration of carbonate and sulphate in the catholyte was not reached.

[0179] During the operating time of 4 months, no complexing agent was added to the catholyte. In contrast, the complexing agent concentration in the catholyte was kept constant with variations of + / - 2.5% due to measurement inaccuracies and volume changes of the catholyte. Nickel ions and zinc ions were added in such a way that the concentration remained within the initially set range. Furthermore, no nickel contaminated water was produced for disposal.

[0180] In addition, the cathode current efficiency (CCE) was about 15% to 30% higher than in the comparative test plating setup (see below).

[0181] Comparative test plating setup (not according to the invention):

[0182] In the comparative test plating setup (not according to the invention), a deposition bath was set up which was essentially identical to the catholyte used in the test plating setup according to the invention (also similar in volume). However, the anode was not separated by a membrane. Thus, the complexing agent was at least partially decomposed at the anode and thus had to be replenished together with the nickel ions. Although the rinse water (i.e. the waste water) was subjected to a vacuum evaporator treatment to reduce the volume before waste water disposal, the waste water contained a large amount of decomposition products including cyanide. This required cost-intensive and specialized disposal. The volume of the (concentrated) waste water amounted to about 1000 L / week with a nickel concentration of at least 1 g / L, a zinc concentration of at least 8 g / L, a cyanide concentration of at least 0.1 g / L and a large amount of complexing agent. Thus, a large amount of nickel and zinc was lost which had to be replenished to the deposition bath. Furthermore, the complexing agent had to be added to the deposition bath on a regular basis.

[0183] In contrast, the method of the invention (see example according to the invention) not only reduces the amount of water to be disposed of. The disposed water is additionally substantially free of nickel and zinc ions. Those ions which are transferred via rinsing are recycled back into the catholyte together with the complexing agent. Thus, the method of the invention is a very environmentally friendly and cost-effective method and is a strong improvement over the prior art.

[0184] System for depositing zinc-nickel alloy on a substrate (according to the invention):

[0185] In Figure 1 , a schematic representation of a system 1 for depositing a zinc-nickel alloy on a substrate is shown, wherein an aqueous zinc-nickel deposition bath is provided as catholyte 3-1 in a deposition compartment 3.

[0186] The system 1 optionally comprises a pre-rinse compartment 2 for pre-rinsing the substrate. As the substrate to be coated is often contaminated with unwanted contaminants, it is generally recommended to pre-rinse the substrate in the pre-rinse compartment 2 with e.g. an alkaline pre-rinse solution. However, if the substrate is already clean, it is preferred to omit the pre-rinse.

[0187] The system 1 further comprises a deposition compartment 3 for electrolytic deposition of a zinc-nickel alloy on the substrate in a catholyte 3-1. The catholyte provided in the deposition compartment comprises nickel ions, at least one nickel ion complexing agent and zinc ions. At least one anode 3-2 having at least one membrane separating the catholyte from an anolyte is provided in the deposition compartment 3. The volume of the anolyte is defined by the space formed by the at least one anode having at least one membrane.

[0188] When the substrate, preferably the pre-rinsed substrate, is transferred into the catholyte 3-1 in the deposition compartment 3 and an electric current is applied, the zinc-nickel alloy is electrolytically deposited on the substrate, so that a zinc-nickel coated substrate is obtained.

[0189] The system 1 further comprises a rinse compartment 4 for rinsing the zinc-nickel coated substrate, so that a rinsed zinc-nickel coated substrate and rinse water is obtained. By rinsing the zinc-nickel coated substrate, the remaining catholyte is removed, so that the obtained rinse water comprises a part of the catholyte, which in turn comprises nickel ions, at least one nickel ion complexing agent and zinc ions.

[0190] The rinse water is transferred, preferably pumped, from the rinse compartment 4 through a rinse water line 4-1 to a first treatment compartment 5 of the system 1 for treating the rinse water. In addition, a part of the catholyte is transferred, preferably pumped, from the deposition compartment 3 through a catholyte removal line 3-3 to the first treatment compartment 5. Said catholyte removal line needs to maintain a constant volume of catholyte.

[0191] The treatment compartment 5 is preferably an evaporator, more preferably a vacuum evaporator, which allows for an efficient separation of water by evaporation.

[0192] At least a part of the separated, preferably evaporated, water is returned from the first treatment compartment 5 to the rinse compartment 4 through a water return line 4-2. In addition, and optionally, another part of the water is returned to the pre-rinse compartment (not shown). Excess water is disposed through a water disposal line 5-2 and is preferably used for other industrial purposes, as this water is very pure.

[0193] After the separation of the water from the nickel ions, from the at least one nickel ion complexing agent and from the zinc ions in the first treatment compartment 5, the separated nickel ions, the separated at least one nickel ion complexing agent and the separated zinc ions are directly or as Figure 1The concentrated aqueous solution is indirectly returned into the deposition compartment 3, preferably by transferring it from the first treatment compartment 5 through a separation line 5-1 to an optional mixing unit 6.

[0194] The optional mixing unit 6 is fluidly connected to a source of nickel ions 7-1, which is preferably an aqueous solution comprising water and nickel sulphate dissolved therein, and a source of zinc ions 7-2, preferably as described above in the method of the invention. In the mixing unit 6, the replenished nickel ions and zinc ions are thoroughly mixed with the concentrated aqueous solution before being returned to the deposition compartment 3 through a return line 6-1, thereby closing the loop. Thus, the nickel ions, zinc ions and at least one nickel ion complexing agent are maintained at a substantially constant concentration in the catholyte.

[0195] The system 1 further comprises an optional second treatment compartment 8 for treating the catholyte 3-1 such that dissolved anions, such as sulphate anions and carbonate anions, are separated from the catholyte 3-1. When the system is operated for a long period of time, such as several months, the concentration of dissolved anions reaches an undesirable limit, such that at least part of said anions are removed in the second treatment compartment, preferably by precipitation. Said precipitated anions are removed through an anion disposal line 8-1.

[0196] Reference symbols

[0197] 1 System for depositing zinc-nickel alloy on a substrate

[0198] 2 Pre-rinse compartment

[0199] 3 Deposition compartment

[0200] 3-1 Space for catholyte

[0201] 3-2 At least one anode having at least one membrane

[0202] 3-3 Catholyte removal line

[0203] 4 Rinse compartment

[0204] 4-1 Rinse water line

[0205] 4-2 Water return line

[0206] 5 First treatment compartment

[0207] 5-1 Separation line

[0208] 5-2 Water disposal line

[0209] 6 Mixing unit

[0210] 6-1 Return line

[0211] 7-1 Source of nickel ions

[0212] 7-2 zinc ion source

[0213] 8 second treatment compartment

[0214] 8-1 anion disposal line

Claims

1. A method for depositing a zinc-nickel alloy on a substrate, the method comprising the steps of: (a) providing the substrate, (b) providing an aqueous zinc-nickel deposition bath as a catholyte in a deposition compartment, wherein - the deposition compartment comprises at least one anode with an anolyte, and - the anolyte is separated from the catholyte by at least one membrane, and - the catholyte comprises (i) nickel ions, (ii) at least one nickel ion complexing agent, and (iii) zinc ions, (c) contacting the substrate with the catholyte in the deposition compartment, so that the zinc-nickel alloy is electrolytically deposited onto the substrate and a zinc-nickel coated substrate is obtained thereby, wherein - the concentration of the nickel ions in the catholyte after step (c) is lower than before step (c), (d) rinsing the zinc-nickel coated substrate in a rinsing compartment comprising water, so that a rinsed zinc-nickel coated substrate and a rinsing water are obtained, wherein - the rinsing water comprises a portion of the at least one nickel ion complexing agent and a portion of the nickel ions, characterized in that (i) at least a portion of the rinsing water and at least a portion of the catholyte are treated in a first treatment compartment, so that water is separated from the at least one nickel ion complexing agent and the nickel ions, (ii) at least a portion of the at least one complexing agent separated from water is returned into the catholyte, and (iii) a source of nickel ions is added directly or indirectly to the catholyte, with the proviso that the source of nickel ions does not comprise the at least one nickel ion complexing agent or any other nickel ion complexing agent.

2. The method according to claim 1, wherein the at least one nickel ion complexing agent is not in contact with the at least one anode.

3. The method according to claim 1 or 2, wherein the source of nickel ions is an aqueous solution comprising water and a nickel salt dissolved therein.

4. The method according to claim 1 or 2, wherein the source of nickel ions is free of or does not comprise tetraethylenepentamine.

5. The method according to claim 1 or 2, wherein in the catholyte the at least one nickel ion complexing agent comprises an amine.

6. The method according to claim 1 or 2, wherein step (a) comprises the following step before step (c) (a-1) pre-rinsing the substrate in a pre-rinsing compartment comprising water, so that a pre-rinsed substrate and a pre-rinsing water are obtained.

7. The method according to claim 1 or 2, wherein the at least one anode is at a distance of 0.5 mm to 5.0 mm from the at least one membrane.

8. The method according to claim 1 or 2, wherein the first treatment compartment comprises an evaporator.

9. The method according to claim 1 or 2, wherein at least a portion of the separated water obtained in the first treatment compartment is returned into the pre-rinsing compartment and / or the rinsing compartment.

10. The method according to claim 1 or 2, comprising the step of (e) treating at least a portion of the catholyte in a second treatment compartment, so that dissolved anions are separated from the catholyte. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 11. The method according to claim 10, wherein the precipitation is carried out at a temperature in the range of -5 °C to 11.0 °C.

12. The method according to claim 10, wherein the dissolved anions comprise at least sulfate anions.

13. The method according to claim 1 or 2, wherein in the catholyte the zinc ions are present as hydroxo complexes.

14. The method according to claim 1 or 2, wherein at least a portion of the separated water obtained in the first treatment compartment is treated, wherein the treated water comprises nickel ions in a concentration in the range of 0 mg / L to 1.0 mg / L based on the total volume of the treated water.

15. A system (1) for carrying out a method for depositing a zinc-nickel alloy on a substrate according to any one of claims 1-14, the system (1) comprising: (I) optionally, a pre-rinse compartment (2) for pre-rinsing the substrate, (II) a deposition compartment (3) for electrolytically depositing the zinc-nickel alloy on the substrate in a catholyte (3-1) such that a zinc-nickel coated substrate is obtained, wherein the deposition compartment (3) comprises at least one anode (3-2) having at least one membrane, (III) a rinse compartment (4) for rinsing the zinc-nickel coated substrate such that a rinsed zinc-nickel coated substrate and rinse water is obtained, (IV) a first treatment compartment (5) for treating a portion of the rinse water and the catholyte (3-1) such that water is separated from nickel ions and nickel ion complexing agents, and (V) optionally, a second treatment compartment (8) for treating the catholyte such that dissolved anions are separated from the catholyte, wherein the first treatment compartment (5) is adapted such that - the separated water is returned into the pre-rinse compartment (2) and / or the rinse compartment (4), and - the separated nickel ions and the separated nickel ion complexing agents are returned into the deposition compartment (3).

16. The system according to claim 15, wherein the system (1) further comprises a second treatment compartment (8) for treating the catholyte such that dissolved anions are separated from the catholyte.

17. The system according to claim 15 or 16, wherein the system (1) further comprises a third treatment compartment (9) for treating the separated water such that nickel ions are separated from the separated water.

18. The system according to any one of claims 15-17, wherein the system (1) further comprises a fourth treatment compartment (10) for treating the separated water such that nickel ion complexing agents are separated from the separated water.

19. The system according to any one of claims 15-18, wherein the system (1) further comprises a fifth treatment compartment (11) for treating the separated water such that nickel ions and nickel ion complexing agents are separated from the separated water.

20. The system according to any one of claims 15-19, wherein the system (1) further comprises a sixth treatment compartment (12) for treating the separated water such that nickel ions and nickel ion complexing agents are separated from the separated water.

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