The present invention refers to an electroplating bath for depositing chromium which comprises at least one trivalent chromium salt, at least one complexing agent, at least one halogen salt and optionally further additives. Moreover, the invention refers to a process for depositing chromium on a substrate using the mentioned electroplating bath.

Chromium plating from trivalent chrome plating baths has been known for years and many documents in the prior art mention the ability to obtain chrome deposits from a trivalent chrome bath.

It is now very well established that uniform coatings of chromium of a thickness between 0.1 and 1 μιη can be produced from trivalent chrome electrolytes. These thicknesses are well suited for the so called decorative applications. However, there are many applications where thicker chromium layers are required, i.e. applications for high wear and/or corrosion resistance, like the plating of chrome on sanitary fittings, on exterior automotive parts, but also functional applications for plating on rods, pistons or landing gear components. The required thicknesses for these applications are between 0.1 and 300 μιη.

US 4,804,446 describes a process for electrodepositing hard smooth coatings of chromium. The bath includes chromium(lll) chloride as a source of chromium, citric acid to complex the chromium, and a wetting agent preferably Triton X 100. Bromide is also added to prevent production of hexavalent chromium at the anode. The pH of the bath is maintained at 4.0 and the temperature at approximately 35 °C. Moreover, the electrolyte further comprises boric acid to advance the reaction kinetics. However, due to the toxic and hazardous potential of boric acid it would be desirable to avoid its presence in the electroplating bath.

WO 2009/046181 discloses deposits of nanogranular crystalline or amorphous functional chromium alloys obtained from a trivalent chromium bath containing a carboxylic acid and comprising sources for divalent sulfur and of carbon, nitrogen and oxygen which are the alloying components. The deposits contain from 0.05 to 20 wt% of sulfur, and the electrodeposition baths used to plate these deposits contain the source(s) of divalent sulfur in a concentration range from about 0.0001 M and 0.05 M.

US2013/0220819 describes a process for producing a dense hard chrome coating from a trivalent chromium plating bath. The coatings have microhard- ness values between 804 KHN up to 1067 KHN. These properties are achieved by using a trivalent chromium electrolyte and a pulsed plating with a waveform of dedicated cycles. It has to be noted that the use of pulse current for electroplating hard chrome on complex and large surface parts requires some major modifications of the plating equipment. However, it would be desirable not to use a pulsed current to deposit the mentioned thick chrome layers.

Several publications describe the use and the effects of the pulse and pulse reverse current on the trivalent chromium process for the hard chrome application.

The publication Pulse and pulse reverse plating— Conceptual, advantages and applications, M.S. Chandrasekar, Malathy Pushpavanam Central Electrochemical Research Institute, Karaikudi 630006, TN, India Electrochimica Acta 53 (2008) 3313-3322 is a review on pulse and pulse reverse techniques for electrodeposition wherein the pulse electrodeposition (PED) of some metals and alloys is reported. The effects of mass transport, electrical double layer pulse parameters and current distribution on the surface roughness and on the morphology are presented. Applications, advantages and disadvantages of PC and PRC techniques are discussed along with theoretical aspects and mechanism. In Improving hardness and tribological characteristics of nanocrystalline Cr-C films obtained from Cr(lll) plating bath using pulsed electrodeposition, Int. Journal of Refractory Metals and Hard Materials 31 (2012) 281-283 the effect of pulsed electrodepostion on the nanocrystal size, composition, hardness, coefficient of friction, and wear resistance was investigated for the Cr-C elec- trodeposits obtained from a trivalent chromium bath. The electrodeposits were shown to contain about 9% of carbon. Pulsed electrodeposition does not significantly affect the carbon content. At the same time, an increase in the off-time duration leads to a decrease in the nanocrystals size. The hardness and wear parameters of the electrodeposits may be sufficiently improved when using pulsed current. For instance, at t _on = t _0ff = 1 s, the hardness reaches the values of ~1200÷1300 HV (while it is close to 850÷950 HV at a steady-state electrolysis).

Though there are several publications about trivalent chrome deposition there is still a need for a commercial system which allows to plate consistent thick chrome deposits of thicknesses between 0.1 and 300 μιη, with are dense and uniform, and show corrosion resistance, hardness and wear properties equivalent to a deposit made out of a Cr0 ₃ based electrolyte.

It was therefore an object of the present invention to provide an electroplating bath which provides chromium layers with a dense and uniform structure of a thickness which makes the layers usable for high wear and/or corrosion resistance.

This object has been solved by the electroplating bath with the features of claim 1 and the process for depositing chromium layers with the features of claim 13.

According to the present invention an electroplating bath for depositing chromium is provided which comprises: a) 100 to 400 g/L of at least one trivalent chrome salt b) 100 to 400 g/L of at least one complexing agent, c) 1 to 50 g/l of at least one halogen salt d) 0 to 10 g/L of additives,

Moreover, the electroplating bath has a pH from 4 to 7. It is essential for the present invention that the electroplating bath is substantially free of divalent sulphur compounds and boric acid and/or its salts and derivatives.

It was surprisingly found that with the inventive electroplating bath layers with a dense and uniform structure can be provided. As the layers are provided with thickness of 10 to 400 μιη the layers can be used for high wear and/or corrosion resistance applications.

The trivalent chromium salt is preferably selected from the group consisting of chromium(lll) sulphate, in acidic or alkaline form, chromium(lll)chloride, chromium(lll) acetate, chromium(lll) hydroxyacetate, chromium(lll) formate, chromium(lll) hydroxy formate, chromium(lll) carbonate, chromium(lll) methanesulfonate, potassium chromium(lll) sulphate, and mixtures thereof.

It is preferred that the trivalent chromium salt is present in an amount of 100 to 400 g/L, in particular in an amount of 120 to 160 g/L.

A major drawback associated with the electrolytes described in the prior art refers to the accumulation of the counterion of the trivalent chromium salt. The consumption of Cr(lll) in such baths can be very high, in particular if the targeted thicknesses are in the upper range > 10 μιη. The counterion associated with the trivalent chromium cation will then accumulate in the electrolyte and create some drawbacks like increase of the bath density and risks of precipitation. The dry content of the bath can increase up to a point where further dissolution of trivalent chromium salts is impossible due to the solubility limit.

It is therefore one preferred embodiment of the present invention to select a counterion for the trivalent chromium salt contains a "temporary", i. e.

electrolytically consumable anion which will not accumulate in the electrolyte to the same extent as "permanent" anions (like sulphate). Among these temporary anions, formate, acetate, propionates, glycolates, oxalates, carbonate, citrates, and combinations thereof are preferred.

The inventive electroplating bath preferably comprises an alloy former selected from the group consisting of vanadium, manganese, iron, cobalt, nickel, molybdenum, tungsten, and indium. The organic components of the bath and ammonia are sources for carbon, nitrogen and oxygen taken up by the alloy during its deposition. Urea as an additive is also particularly efficient. Preferably, the electroplating bath comprises ammonia, especially in a molar concentration which is less than or equal to the molar concentration of the at least one complexing agent. Most preferably, ammonia is comprised in a concentration of 70 g/L to 110 g/L

The presence of salts of metals not codeposited in the alloy, like aluminium and/or gallium, is also advantageous owing to the formation of mixed-metal complexes with chromium(lll) in the bath influencing the kinetics and mechanism of the deposition. However, the electroplating bath may also be free of said salts of metals (e.g. free of aluminium salts.

According to the present invention, the complexing agent is preferably selected from the group consisting of carboxylic acids and carboxylate salts, preferably formic acid, acetic acid, propionic acid, glycolic acid, lactic acid, oxalic acid, malic acid, citric acid, tartaric acid, succinic acid, gluconic acid, glycine, aspartic acid, glutamic acid, and mixtures thereof, or their salts and mixtures thereof.

The complexing agent is preferably present in an amount of 100 to 300 g/L, more preferably 150 to 250 g/L. The molar ratio of the complexing agent to the trivalent chromium salt is from 8:1 to 15:1, preferably 10:1 to 13:1 which allows the operation of the bath in the mentioned pH range and ensures deposition of chromium and not chromite. The halogen salt present in the electroplating bath acts as a suppressor for the generation of hexavalent chromium in the bath. The halogen salt is preferably selected from the group consisting of bromide, chloride, iodide, fluoride salts and mixtures thereof. The bromide salts are more preferred, in particular potassium bromide, sodium bromide, ammonium bromide and mixtures thereof. The halogen salt is preferably present in an amount of 5 to 50 g/L.

The additives of the electroplating bath may be selected from the group consisting of brighteners, such as a polyamine or a mixture of polyamines including quaternary ammonium compounds (which are the preferred brightening agents for the application like the ones cited in US 7964083 patent) and wetting agents like electroneutral, cationic and amphoteric surfactants.

It is particularly preferred that the electroplating bath is (substantially) free of chloride ions and/or (substantially) free of aluminium ions, but the bath may contain fluoride which - as at least one further complexing agent (ligand) and/or as at least one further halogen salt - assists in the ligand exchange of the chromium(lll) complexes in the bath.

According to the invention also a process for depositing chromium on a sub- strate is provided including the following steps:

• providing the above-described electroplating bath,

• immersing a substrate in the electroplating bath and

• applying an electrical current to deposit the chromium on the substrate. The temperature during deposition is preferably from 20 to 60 °C, more preferably from 30 to 50 °C.

The electroplating bath can be separated from the anode by a membrane, preferably by an anionic or cationic exchange membrane or a porous membrane, more preferably by a cationic exchange membrane. A cationic exchange membrane has the advantage that the migration of sulphate in the catholyte is prevented.

The anodes used to perform the deposit will be made of an insoluble material like graphite or mixed oxides materials like titanium covered with oxides of Tantalum and Iridium.

In one specific embodiment of the invention, the anodes can be surrounded by an appropriate material defining an anolyte and a catholyte to prevent certain components of the electroplating bath from coming into contact with the anode and to keep undesirable oxidation breakdown products in confinement.

Undesirable species are for example Cr(VI) originating from the anodic oxidation of Cr(lll), but also the products of the oxidation of the complexing agents at the anode.

Another benefit linked to the use of a barrier material to isolate the anodic region from the bath is to avoid the accumulation of species that are not elec- trodeposited and will accumulate in the catholyte like sulfate, for example upon replenishment with chromium(lll) sulfate.

The barriers can be any material selected from the class of ion exchange membranes. They can be anionic exchange membranes, e.g. the Sybron IONAC material MA 3470. Also cationic exchange membranes can be used, e.g. Nafion membranes from (Du Pont). One preferred cationic exchange membrane is the N424 membrane. Moreover, porous membranes, e.g. as described in EP 1 702 090, can also be considered as appropriate materials to define an anodic compartment separated from the remainder of the electrolyte. The anodic compartment can be filled with any conducting substance compatible with the electrolyte. It can be acidic or alkaline. Due to the slight acidic pH of the parent catholyte, an acidic pH will also be preferred for the anolyte. Formic acid, acetic acid, propionic acid, glycolic acid, citric acid but also mineral acids like H ₂ S0 ₄ , H ₃ P0 ₄ can be employed. A liquid solution of chromium (III) sulfate can also be used as the anolyte. Alternatively, sodium hydroxide, potassium hydroxide, lithium hydroxide or any kind of alkaline solution free of CMR properties can be used as anolyte in the process of the invention.

The current applied in the electrolyte can be a direct current or alternatively a pulsed current. The use of a pulsed current sequence provides the ability to plate deposits that are less sensitive to the formation of cracks due to hydrogen accumulation at the interface.

The pulsed sequence can be composed of a cathodic phase followed by a T off to help for the removal of hydrogen from the interface or eventually an anodic phase can be imposed to oxidize hydrogen at the interface.

The present invention is further illustrated by the following Figures and Examples. However, the present invention is not limited to these specific embodiments.

Fig. 1 shows a schematic illustration of the anodic setup according to one embodiment of the present invention.

Fig. 2 shows a diagram illustrating the development of the sulphate concentration for different electroplating systems

The inventive embodiment 1 illustrated in Fig. 1 uses an anolyte 7 that can serve as a reservoir of Cr(lll) ions. A solution of a trivalent chromium salt such as chromium sulphate or any other chromium salt comprising 10-50 g/L of trivalent chromium and 30-140 g/L of sulfate anions or other anions is used as a component of the anolyte 7 in the Fig. 1. The ion exchange membrane 3 may be included in or bound to a carrier 2 and will preferably be selected as a cation exchange membrane like Nafion N424 mentioned above. The catholyte 5 is composed of the trivalent chrome electrolyte of the invention as described in the following Example 2. The anode 6 is made of graphite material. A sample part to be plated is placed as cathode 4. The replenishment of chromium salt in the form of chromium(lll) sulphate is carried out in the anolyte.

In Fig. 2, the diagram demonstrates the time-dependence of the sulphate concentration in different electroplating systems. While the sulphate concentration for the electroplating system based on a bath with Cr(lll) sulphate and without a membrane rapidly increases, the concentrations for the first embodiment according to the present invention using a "temporary" anion and for the second embodiment according to the present invention using a membrane separation stay substantially constant for the measurement period.

In Table 1 shows the compositions of the electroplating baths of the inventive Examples 1-4 and of a reference example based on Cr(VI) together with the operation parameters for each electroplating bath.

Table 1

Cathodic duty 96% 96% 96% cycle

Frequency 6.5Hz 6.5Hz 6.5Hz

Magnetic induc300°C- 2sec 500°C- 2sec tion

DC : Direct current

PRC : Pulse Reverse Current

The resulting properties of the deposits obtained from the electroplating baths in table 1 are shown in table 2.

Table 2