Trivalent chromium plating bath

Abstract

The present invention relates to the use of a trivalent chrome plating bath composition for the electrodeposition of a hard chrome layer with a thickness of 1 µm or more, the composition comprising (A) trivalent chromium ions in an amount of 5-40 g / l, (B) at least one anion selected from sulfate, chloride, bromide and methanesulfonate, (C) at least one carboxylate ion selected from formate and acetate, (D) at least one buffer substance selected from ammonia, boric acid, carboxylic acids with 2 to 8 carbon atoms other than component (C) and salts thereof, and aluminium salts, and (E) 0.01 to 100 g / l of at least one specific compound which comprises a sulfonate group and a sulfide or disulfide group. Other aspects of the invention relate to a hard chrome plating process, a hard chrome plating layer and a hard chrome plated article.

Description

Field of the Invention

[0001] The present invention relates to the field of plating chromium layers of thickness >1 µm, often referred to as hard chrome plating. More particularly the invention relates to a trivalent chromium bath for depositing a hard-chrome plating on a metal substrate, a respective electroplating process and a hard-chrome plated metal substrate thus obtained.Background of the Invention

[0002] Chrome electroplating generally can be divided into two different fields of application: A) For decorative purposes, in which case the thickness of the chrome plating layer typically is <1 µm = "decorative chrome" or "bright chrome" (wherein "bright" means glossy B) For technical purposes, in which case the thickness of the chrome plating layer typically is >1 µm = "hard chrome"

[0003] The demands for these two types of chrome electroplating processes are quite different.

[0004] For decorative chrome layers, there typically is a focus on the optical appearance (brightness, colour). The thickness of bright chrome generally is <1µm. Typical applications for bright chrome are sanitary items (e.g., shower heads), decorative automotive parts (e.g., chrome plated mouldings) or consumer goods (e.g., housings for electrical shavers).

[0005] US 10,006,135 B2 describes an electrolyte for dark decorative chrome layers, which comprises a certain sulfur compound (D) as a colorant.

[0006] Hard chrome layers generally are applied in a thickness of >1 µm. Typical applications for hard chrome are, e.g., machine engineering parts (e.g., for polymer extruders), technical automotive parts (e.g., piston rods), gravure printing plates and hydraulic cylinders. Hard chrome layers are applied to improve wear resistance and tribological properties.

[0007] According to the state of the art, thick chromium layers, often referred to as hard chrome layers, can be electrochemically deposited from trivalent or hexavalent baths. An example of a bath which is capable of plating chromium layers with a thickness >1 µm and therefore is suitable for hard chrome plating is presented in EP 2 899 299 A1. Possible anodes according to the state of the art also are presented in this document.

[0008] To be able to plate high thicknesses, the compositions of baths for hard chrome plating differ from compositions for bright chrome. The deposition of hard chrome generally requires extremely high current densities. Hard chrome plating baths can plate fast at high current densities, but the achievable metal distribution of the plated chromium layer, and thus the ability to plate complexly shaped parts, is extremely limited. The prior art hard chrome baths are particularly suited for plating rotation symmetric parts (e.g., rods or shafts), but are more difficult to employ for plating complexly shaped parts or large flat parts (e.g., panels) having both, areas of locally very high current densities and areas of very low current densities. To overcome this shortcoming, it often is necessary for parts which have a complex shape or parts which have a large flat surface to apply so-called shielding in order to achieve an even metal distribution across a whole part and to prevent excessive plating or burning in areas of locally high current density, e.g., on the edges of a workpiece. Shields are made of either conductive or nonconductive materials, and the application of suitable shielding is time consuming and expensive. Especially when different workpiece geometries are to be plated, the shielding must be individually adjusted to every workpiece geometry. Moreover, in the case of applying metallic shielding, also known as cathode thieves, for the modification of current density on edges, chromium will be consumed and dragged-out, which adds more expense to the process (ASM, Vol 5, p. 195-196).

[0009] In terms of industrial electroplating, it is desirable to plate multiple parts by hanging them on a rack and plate them simultaneously. The challenging point in such a scenario is that the individual current densities can strongly vary for the different parts on a rack. In other words, the variation of current densities for multiple parts on a rack is much higher than that for a single part. Some reasons for this phenomenon are: Different distances between the parts and anodes. Parts which are positioned directly in front of an anode will experience higher current densities than parts that are located at a greater distance from the anode. Parts hanging at the edges of a rack typically experience higher local current densities than those hanging in the middle. Neighbouring parts can partially shield each other. This results in higher variances in the local current density in comparison to an individually hanging part.

[0010] Thus, there exists a need to develop a bath which can equalize such variations. Furthermore, it is desirable to improve the metal distribution of the plated chromium layer and to develop means for plating at lower current densities. This demand is addressed by the presently claimed invention.

[0011] The use of a trivalent chromium-based solution is favourable from the viewpoint of hexavalent chromium salts being very toxic, and European and worldwide legislation restricting the use and the availability of Cr(VI)-salts. In Europe, REACH legislation (CMD - Carcinogens and Mutagens Directive, Annex III - OELVs) restricts the use of Cr(VI)-salts. In view thereof, it is desirable to develop means and methods for carrying out hard chrome plating using trivalent chromium.

[0012] Existing methods of hard chrome plating from trivalent chromium-containing baths suffer from the drawback of the plated layer offering poorer corrosion resistance than layers plated from hexavalent chromium. According to the state of the art, it is therefore necessary to plate a nickel or nickel-phosphorous ("electroless nickel") sublayer under the hard chrome layer when it is deposited from trivalent chromium baths (see, for example, WOTECH 2021 / 02 / 07 "Hartverchromung aus dreiwertigen Elektrolyten" Hannah Betz et al.). In view thereof, it is another objective of the present application to provide a means for preparing hard chrome plating layers that are prepared from trivalent chromium-containing baths, and which exhibit improved corrosion resistance.

[0013] For some applications, there also is a special interest to minimize the dissolution of iron from steel substrates, when it is immersed into the plating solution for a longer period of time. This applies especially for applications where only a part of a substrate is plated with chromium whereas another part is not plated with chromium. One such example is the plating of the inside of steel tubes. In this case the outside of the substrate is constantly exposed to the plating bath. Generally iron impurities in the plating bath can cause dark deposition in the low current density. It is therefore important to control the concentration of iron in the bath. Usually, the plating bath is treated with ion exchange resins to remove the iron from the bath solution when a limit of 10 - 20 mg / l of iron is reached. It is desirable to reduce the frequency of such iron removal treatments by using a bath with reduced propensity for iron dissolution.Description of the Figures

[0014] Figure 1 is a picture of a Hull cell panel after carrying out plating using the standard bath (Example 2). Figure 2 shows the distribution of chromium thickness after using a standard bath to plate a flat sheet (Example 3). Figure 3 is a 3D schematic diagram showing the chromium thickness distribution on a flat sheet that was plated using a standard bath (Example 3). Figure 4 shows the distribution of chromium thickness after using a modified bath to plate a flat sheet (Example 4). Figure 5 is a 3D schematic diagram showing the chromium thickness distribution on a flat sheet that was plated using a modified bath (Example 4). Figure 6 shows the test piece ("Schlötter part") used for thickness distribution and throwing power tests (Example 5). Figure 7 shows a schematic drawing of a plating bath (top view) as employed in Examples 5, 8, 10 and 11. Figure 8 shows the measuring points on the Schlötter part used in Example 5 and Example 6. Figure 9 shows the thickness distribution of the plated chromium layers in Example 5. Figure 10 shows the thickness distribution of the plated chromium layers in Example 6. Figure 11 shows a comparison of the surface quality (number of cracks) of the chromium plates steel rods of Example 8 (left-hand: standard bath; right-hand: bath according to the present invention). Figure 12 shows the appearance of the rods of Example 8 after the salt spray test (left-hand: standard bath; right-hand: bath according to the present invention). Figures 13 and 14 show the appearance of the steel sheets of Example 9 after immersion for 20 h at 50°C in the unmodified standard bath and the bath according to the invention, respectively. Figures 15 to 17 are plots showing the concentration of the constituents of the hard chrome plated brass panels of Example 10 in depth direction for the unmodified standard bath and the baths according to the invention comprising 0.5 and 1 g / l of 3-mercapto-1-propanesulfonic acid, sodium salt, respectively, as measured by Glow Discharge Optical Emission Spectroscopy (GDOES). Figure 18 is a plot showing the coefficients of friction of the hard chromium plated steel substrates of Example 11. Summary of the Invention

[0015] To address the above-described problems of the state of the art, the present inventors have developed a trivalent Cr bath for introducing a uniform hard chrome layer on metallic surfaces, and in particular on the surfaces of metal parts.

[0016] In one aspect, the present invention therefore relates to the use of a trivalent chrome plating bath composition for the electrodeposition of a hard chrome layer with a thickness of 1 µm or more, the composition comprising: (A) trivalent chromium ions in an amount of 5 - 40 g / l, (B) at least one anion selected from sulfate, chloride, bromide and methanesulfonate, (C) at least one carboxylate ion selected from formate and acetate, and (D) at least one buffer substance selected from ammonia, boric acid, carboxylic acids with 2 to 8 carbon atoms other than component (C) and salts thereof, and aluminium salts, and (E) 0.01 to 100 g / l of at least one compound of the following formula (1):         X-S-R-SO 3 Y     (1) wherein: X represents H, C 1-6 -alkyl, R 1< -CZ 1< -, R 1< -Z 2< -CZ 1< - or a second moiety of the formula -S-R-SO 3 Y, wherein Z 1< is =O or =NR 2< ; Z 2< is -O-, -S- or -NR 3< -; R 1< , R 2< and R 3< independently are hydrogen or C 1-6 -alkyl, or two of R 1< , R 2< and R 3< are linked together so that X forms a mono- or bicyclic heterocycle with 5-10 ring atoms, which may be saturated or unsaturated; R is an alkylene group with 2 to 6 carbon atoms, Y is hydrogen, ammonium, an alkali metal or 0.5 equivalents of an alkaline earth metal; or the compound of formula (1) forms a zwitterion in which Y is a negative charge and one of Z 1< and Z 2< is =NR 2< H +< or -NR 3< H +< -, respectively.

[0017] Further aspects of the present invention are directed to a chromium electroplating process using the trivalent chrome plating bath composition to apply a hard chrome layer on a metal substrate, a hard chrome layer having a thickness of 1 µm or more, which is provided on a metal substrate and a steel article having the hard chrome plating. Yet another aspect relates to the use of the above-described compound (E) as an additive for a trivalent chrome plating bath.Effects of the Invention

[0018] With the trivalent Cr bath used in the present invention, it is possible to achieve a uniform Cr deposition even in the case of varying current density. The use of the bath of the present invention therefore allows complexly shaped parts to be fully covered with a hard Cr layer, especially at lower current density areas, without causing burning at high current density areas.

[0019] Surprisingly, it has also been found that chromium layers deposited by using the bath in accordance with the present invention exhibit superior corrosion resistance. Furthermore, the crack structure is drastically reduced in comparison to a layer plated from a bath according to the state of the art.

[0020] In the present invention, a uniform metal distribution of the plated chromium layer is achieved by improving the efficiency of the chrome plating at lower current densities and modifying the plating efficiency (reducing the plating speed) at higher current density. The plating capacity at the lowest current density areas is often referred to as "throwing power".

[0021] A good metal distribution of the plated chromium layer is favourable, because this means that less "over-plating" on a workpiece is required in order to achieve a minimum thickness in an area of low current density. It also allows the plating process to be more sustainable and economical, since less excess chrome metal is plated out from the bath as drag-out. Also, a good throwing power will reduce the plating times required for arriving at a required minimum plating thickness. This allows energy consumption to be reduced.

[0022] Moreover, the modification of the bath to plate at lower current densities makes the use of lower current densities possible, thereby preventing not only excessive plating but also burning in the areas of highest current density.

[0023] A better metal distribution of the plated chromium layer also serves to drastically reduce the need for the time consuming and expensive use of auxiliary equipment like shielding.

[0024] It was also found that by using the bath of the present invention for the plating of a steel substrate, the dissolution of iron from the steel substrate is suppressed. This helps to reduce the need and the frequency of iron removal treatments.

[0025] Furthermore, it also was found that the chromium layer obtained by using the trivalent Cr bath of the present invention comprises sulfur and has a reduced carbon content compared to layers obtained with a conventional trivalent Cr bath. This difference in composition leads to a significantly reduced coefficient of friction.

[0026] In a further aspect of the invention, chromium depositions from different plating baths can be combined to form multi-layer chromium depositions, e.g., when chromium layers from a plating bath according to the invention with a good metal distribution of the plated chromium layer and high corrosion resistance are plated onto or underneath a chromium layer plated according to the state of the art or in a series of layers from alternating chromium plating baths, in order to combine different layer properties and to achieve better overall layer properties in terms of tribology, corrosion resistance and proper coverage of a complexly shaped part.

[0027] One example for a multi-layer application would be the combination of a chromium sublayer which exhibits a crack-free, highly corrosion resistant structure, with a top layer exhibiting a crack-containing chromium layer. For some applications it is desirable to have a crack structure that works as reservoirs for lubricants in the top layer. The crack-free sublayer in this example ensures the corrosion performance of the layer composition.Detailed Description

[0028] The embodiments and features of the present invention will now be described in greater detail.Plating Bath Composition

[0029] The trivalent chrome plating bath composition used in the present invention comprises: (A) trivalent chromium ions in an amount of 5-40 g / l, (B) at least one anion selected from sulfate, chloride, bromide and methanesulfonate, (C) at least one carboxylate ion selected from formiate and acetate, (D) at least one buffer substance selected from ammonia, boric acid, carboxylic acids with 2 to 8 carbon atoms other than component (C) and salts thereof, and aluminium salts, and (E) 0.01 to 100 g / l of at least one sulfur-containing substance of the formula (1) as described below. Trivalent chromium ion (A)

[0030] The plating bath according to the invention comprises trivalent chromium ions in an amount of 5-40 g / l. Preferably, the concentration of trivalent chromium ions lies in the range of 10-30 g / l, more preferably 15-25 g / l.

[0031] The trivalent chromium ions are added to the bath composition in the form of chromium(III) salts, such as chromium(III) sulfate, basic chromium(III) sulfate, chromium(III) methanesulfonate, chromium(III) chloride, chromium(III) acetate, chromium(III) nitrate and chromium alums such as chromium(III) potassium sulfate dodecahydrate, with basic chromium(III) sulfate, chromium(III) potassium sulfate dodecahydrate and chromium(III) methanesulfonate being preferred, and basic chromium(III) sulfate being more preferred.

[0032] Depending on the counter ion, the chromium(III) salt also can at least partially contribute the anion (B) and / or the carboxylate ion (C).

[0033] When the bath is operated using insoluble anodes, the deposition of chrome leads to a decrease of the chromium(III) concentration, hence component (A) needs to be replenished intermittently or continuously. The replenishment of component (A) simultaneously may lead to an increase of the anion (B) and / or the carboxylate ion (C) as the counter ion.Anion (B)

[0034] The plating bath according to the invention comprises at least one anion (B) selected from sulfate, chloride, bromide and methanesulfonate.

[0035] Alkali metal salts and ammonium salts, such as lithium chloride, lithium bromide, lithium sulfate, lithium methanesulfonate, sodium chloride, sodium bromide, sodium sulfate, sodium methanesulfonate, potassium chloride, potassium bromide, potassium sulfate, potassium methanesulfonate, ammonium chloride, ammonium bromide, ammonium sulfate, and ammonium methanesulfonate can serve as a source of the anion (B).

[0036] Among these, lithium chloride, lithium bromide, lithium sulfate, sodium chloride, sodium bromide, sodium sulfate, sodium methanesulfonate, potassium chloride, potassium bromide, potassium sulfate, ammonium chloride, ammonium bromide, and ammonium sulfate are preferred. More preferred are lithium chloride, lithium bromide, sodium chloride, sodium bromide, potassium chloride, potassium bromide, ammonium chloride, and ammonium bromide. Even more preferred are lithium bromide, sodium bromide, potassium bromide, and ammonium bromide, and still more preferred is sodium bromide and potassium bromide.

[0037] Among those, the ammonium salts simultaneously can supply at least a part of the buffer component (D). Besides, further to the above, the chromium(III) salt used for providing the trivalent chromium ions (A) can also at least partially provide the anionic ion (B), depending on the counterion.

[0038] Preferably, the concentration of the anion (B) lies in the range of from 50 to 500 g / l, more preferably in the range of from 100 to 450 g / l, even more preferably in the range of from 150 to 400 g / l, and still more preferably in the range of from 200 to 400 g / l.Carboxylate ion (C)

[0039] The inventive plating bath according to the invention comprises at least one carboxylate selected from formate and acetate.

[0040] Formic acid or acetic acid or respective salts thereof such as ammonium formate, ammonium acetate, sodium formate, sodium acetate, potassium formate and potassium acetate can serve as a source for the carboxylate ion (C). Among these, ammonium formate, ammonium acetate, sodium acetate, and potassium acetate are preferred, and ammonium formate and ammonium acetate are more preferred.

[0041] Again, ammonium formate and ammonium acetate can supply at least a part of the buffer component (D), and conversely, the carboxylate ion (C) can be at least partially supplied as counterions of the chromium(III) salt used for providing the trivalent chromium ions (A).

[0042] Preferably, the concentration of the carboxylate ion (C) lies in the range of from 30 to 400 g / l, more preferably in the range of from 40 to 350 g / l, even more preferably in the range of from 60 to 300 g / l.

[0043] If the below-described buffer substance does not comprise carboxylate groups, the concentration of the carboxylate ion (C) still more preferably is 100 to 300 g / l and yet still more preferably 150 to 300 g / mol.Buffer substance (D)

[0044] The inventive plating bath according to the invention comprises at least one buffer substance (D) selected from ammonia, boric acid, carboxylic acids with 2 to 8 carbon atoms other than component (C) and salts thereof, and aluminium salts.

[0045] Among those, boric acid, ammonia and salts thereof are preferable. Examples for the salts include sodium borate, ammonium chloride, ammonium formate or ammonium acetate.

[0046] The carboxylic acid with 2 to 8 carbon atoms other than component (C) specifically can be a di- or tricarboxylic acid, a hydroxycarboxylic acid, an oxocarboxylic acid, an aminocarboxylic acid, a sulfocarboxylic acid or an aromatic carboxylic acid, or may have a plurality of the aforementioned functional groups.

[0047] Examples for the carboxylic acid include oxalic acid, succinic acid, glutaric acid, adipic acid, fumaric acid and maleic acid, glycolic acid, lactic acid, malic acid, tartaric acid, citric acid, glyoxylic acid, pyruvic acid, glycine, alanine, 3-aminopropionic acid, 4-aminobutyric acid, 5-aminopentanoic acid, 6-aminohexanoic acid, glutamic acid, phthalic acid, sulfosuccinic acid, 2-hydroxy-5-sulfo-benzoic acid, methylglycindiacetic acid and salts thereof. Among those, glycine and salts thereof are preferable.

[0048] Further to their buffer function, the aforementioned carboxylic acids also can act as chelation agents useful for controlling the chromium deposition, thereby improving the deposition quality especially in high current density areas, thus preventing burnings or powdery deposition and improving the appearance. They can be used singly or in combination, or a combination of one or more of these acids with one or more further buffering substances such as ammonia or borate can be used.

[0049] Aluminium salts likewise can be used as buffer substance (D), preferably in the form of aluminium chloride or aluminium sulfate.

[0050] If the buffer substance (D) is used in the form of a salt, the counter ion may correspond to component (B), (C) or (D), respectively. As examples for the latter case, ammonium borate and ammonium salts of the aforementioned carboxylic acids may be mentioned.

[0051] The concentration of the buffer substance (D) typically lies in the range of from 15 to 300 g / l, preferably in the range of from 25 to 250 g / l, even more preferably in the range of from 50 to 200 g / l.Sulfur-containing substance (E)

[0052] The plating bath according to the invention comprises at least one sulfur-containing substance (E) of the following formula (1), at a total concentration of 0.01 to 100 g / l, preferably 0.1 to 10 g / l and more preferably 0.2 to 5 g / l:         X-S-R-SO 3 Y     (1)

[0053] In the formula (1), X represents H, C 1-6 -alkyl, R 1< -CZ 1< -, R 1< -Z 2< -CZ 1< - or a second moiety of the formula -S-R-SO 3 Y, wherein Z 1< is =O or =NR 2< ; Z 2< is -O-, -S- or -NR 3< -; R 1< , R 2< and R 3< independently are hydrogen or C 1-6 -alkyl, or two of R 1< , R 2< and R 3< are linked together so that X forms a mono- or bicyclic heterocycle with 5-10 ring atoms, which may be saturated or unsaturated; R is an alkylene group with 2 to 6 carbon atoms, and Y is hydrogen, ammonium, an alkali metal or 0.5 equivalents of an alkaline earth metal, or wherein the compound of formula (1) forms a zwitterion in which Y is a negative charge and one of Z 1< and Z 2< is =NR 2< H +< or - NR 3< H +< -, respectively.

[0054] Accordingly, the compound of the formula (1) is an alkylsulfonic acid or a salt thereof, in which the alkyl group has a sulfide substituent such as mercapto or a thioester group, or two mercapto groups are dimerized to form a disulfide.

[0055] In the formula (1), R preferably is propylene. Preferable examples of the compound (E) of formula (1) include the following: bis-(3-sulfopropyl)-disulfide or a salt thereof, e.g. the sodium salt:         NaO 3 S-CH 2 -CH 2 -CH 2 -S-S-CH 2 -CH 2 -CH 2 -SO 3 Na 3-mercapto-1-propanesulfonic acid or a salt thereof, e.g., the sodium salt:         H-S-CH 2 -CH 2 -CH 2 -SO 3 Na 3-(benzothiazolyl-2-mercapto)-propyl-sulfonic acid or a salt thereof, such as sodium salt 3-S-isothiuronium propyl sulfonate (zwitterion):

[0056] Among those, bis-(3-sulfopropyl)-disulfide and 3-mercapto-1-propanesulfonic acid or salt thereof or mixtures thereof are more preferable, and the sodium salts thereof are still more preferable.

[0057] In accordance with the present invention, the sulfur-containing substance (E) improves the uniformity of the deposition, especially under varying current densities, as occurring, e.g., when plating substrates of a complex shape, and also improves the quality of the obtained hard chrome layer, as described below.Formulation of the composition

[0058] The trivalent chrome plating bath composition of the present invention can be prepared by dissolving components (A) to (E) in water; components (A) to (D) can also be dissolved in advance, and component (E) can be included afterwards as an additive. The dissolution can be performed at a temperature corresponding to the envisaged operating temperature of the plating process and usually ranges from 20 to 70°C, preferably from 30 to 60°C, more preferably 40 to 60°C.

[0059] The molar ratio of (A) to the total mole number of carboxylate groups from component (C) and in given case also from component (D) preferably lies in the range of from 1:5 to 1:25, more preferably in the range of from 1:8 to 1:15.

[0060] The pH value of the bath preferably ranges from 2.5 to 6.5, more preferably from 3.0 to 6.0, even more preferably from 4.0 to 5.7 and most preferably from 4.5 to 5.5. The pH may be adjusted through the addition of a base or an acid as required. Examples for the base include sodium hydroxide, potassium hydroxide and ammonium hydroxide. Examples for the acid include sulfuric acid, methanesulfonic acid, formic acid, acetic acid and hydrochloric acid.Electroplating Process

[0061] Another aspect of the present invention relates to a chromium electroplating process using the trivalent chrome plating bath composition of the present invention as described above.

[0062] The process of the present invention is for the deposition of hard chrome layers on a metal substrate and generally comprises: Providing the above-described trivalent chrome plating bath composition of the present invention; Introducing an anode and a metal substrate to be coated as a cathode into the plating bath; Performing an electroplating to deposit a chrome layer having a thickness of >1µm on the metal substrate. Substrate

[0063] In the electroplating process of the present invention, the metal substrate to be coated is not particularly limited, and any metal article conventionally provided with hard chrome plating can be used.

[0064] Examples of the metal include steel, aluminium, nickel and alloys thereof, as well as copper alloys and zinc alloys such as brass. The process of the present invention is especially advantageous for steel substrates, since in addition to the formation of a uniform coating on articles with a complex shape, the plating bath of the invention also suppresses the undesired dissolution of iron which may occur with conventional baths and achieves a superior corrosion resistance of the plated product.

[0065] The substrate may be any metal article in need of protection against wear and / or corrosion, including tools and machine parts such as tooltips, bearings, shafts, pistons or engine elements. The process of the present invention is especially suitable for articles with a complex shape, which are difficult to coat in a uniform manner with a conventional trivalent chromium bath due to the position-dependent current variations.

[0066] The substrate may be pre-treated in a conventional manner, in order to provide a good adhesion of the electroplated layer to the substrate. Typical pre-treatment steps may comprise cleansing and degreasing processes and pickling or etching / activation processes, or electrolytic pretreatments which involve treatment steps in hydrochloric acid, sulfuric acid or fluoride containing solutions.

[0067] For steel substrates, especially for high alloyed steels, a nickel-strike coating may be applied, using a nickel-strike electrolyte (e.g., based on nickel chloride in hydrochloric acid) to apply a thin intermediate layer with a thickness of 1 µm or less, which may greatly increase the adhesion of the subsequent hard chrome plating.

[0068] It also is possible to apply interim nickel layers with higher thickness between the substrate and the hard chrome layer. The nickel layers can for example be deposited from sulfate, methanesulfonate or chloride containing electrolytes which can also be found in literature. The nickel layers can improve the adhesion, corrosion resistivity and tribological properties. The thickness of the nickel layer generally is more than 1 pm, and usually in the range of 10 to 50 µm in view of corrosion resistance.

[0069] Some known nickel sublayers include: "Watts" nickel, Multilayer Nickel, e.g., a combination of sulfur free and sulfur containing nickel layers = "double nickel", Electroless nickel, Nickel-phosphor-alloy which can be plated from electroless or electrolytic electrolytes, Nickel-sulfamate, or Porous or microcracked nickel layers.

[0070] After the plating of the hard chrome layer from the bath of the present invention, it also is possible to apply further post-treatments or finishing treatments. For example, one or more of the following chemical or electrochemical processes could be used: Chromating post treatment, Anodic or cathodic post treatment as described in US 11,214,881 B2, Applying pulse post treatment in different modes including on-off, on-on or reverse pulse as described in US 11,268,206 B2, Thin <1µm decorative chromium (SLOTOCHROM BC4130) Chrome free passivation for providing protection in non-plated areas (SLOTOPROTECT FE 1050) Anodes

[0071] The anodes used to perform the deposition are not particularly limited, and any anodes conventionally applied for trivalent chromium deposition can be used.

[0072] Preferably, the anodes are insoluble anodes, which means that they are inert and do not dissolve under the given plating conditions (as opposed to soluble anodes, which are designed to dissolve in order to supply the ions of the metal for the plating).

[0073] Preferred examples of the anodes include anodes made of graphite or MMO (mixed metal oxides from tantalum and iridium which are coated on titanium or niobium) or platinum which is coated on titanium or niobium or are membrane anodes.Plating Conditions

[0074] The plating conditions are not particularly limited, and can be suitably chosen in accordance with the conditions conventionally applied for hard chrome plating baths.

[0075] The bath temperature during the deposition preferably ranges from 20 to 70°C, more preferably from 30 to 60°C, most preferably 40 to 60°C.

[0076] The applied current density preferably ranges from 5 to 70 A / dm 2< , more preferably from 10 to 60 A / dm 2< , and most preferably from 15 to 50 A / dm 2< .

[0077] During the plating process, the bath can be continuously or intermittently be subjected to filtration and monitoring of the composition, and the concentration of the constituents and the pH value can be suitably adjusted as necessary.Hard chrome layer

[0078] A further aspect of the present invention is directed to a hard chrome layer which is characterized by a sulfur content of 1-7%, preferably 1-5% and a carbon content of 0.1-2.5%, preferably 0.1-2%, more 0.1-1%. The percentages are based on the mass of the coating layer and can be determined by Glow Discharge Optical Emission Spectroscopy (GDOES) in accordance with ISO 14707:2021.

[0079] Conventional trivalent chromium baths yield hard chrome layers comprising carbon originating from the organic bath constituents. The incorporation of carbon is an unintended and unavoidable side reaction.

[0080] Surprisingly, it has been found that the hard chrome layer of the present invention additionally comprises sulfur, which originates from the sulfur-containing component (E), and has a reduced carbon content as compared to layers obtained with a conventional bath.

[0081] It was also found that the hard chrome layer of the present invention has a very low number of cracks in comparison to layers obtained with a conventional bath, which is believed to result from the reduced carbon content. When investigated under a microscope, the chromium layer preferably has a crack density of <10 cracks / mm.

[0082] The hard chrome layer confers a surprising improvement of the tribological properties such as friction coefficient and wear resistance and improved corrosion resistance to the substrate, which both are believed to result from the improved coating quality and the reduced number of cracks. It was found that the inventive bath composition yields a hard chrome layer showing a significantly reduced coefficient of friction according to ASTM G133-22 standard in comparison to both, a conventional trivalent chromium bath and a hexavalent chromium bath.

[0083] Furthermore, it was also found that on a steel substrate, the hard chrome layer of the present invention achieves a surprising improvement of the corrosion resistance even in the absence of an interim nickel layer with a thickness of ≥1 pm, which is conventionally provided between the substrate and the hard chrome layer as discussed above.

[0084] The thickness of the hard chrome layer of the present invention is generally 1 µm or more, preferably 2 µm or more, and more preferably 5 µm or more, in view of the improvement of the tribological properties such as friction coefficient and wear resistance.

[0085] For the improvement of the corrosion resistance, the thickness preferably is 10 µm or more and more preferably 20 µm or more. If the thickness is 20 µm or more, the hard chrome layer of the present invention confers superior corrosion protection even on steel substrates which do not have an interim nickel layer of 1 µm or more.

[0086] The maximum thickness is not specifically limited and generally may be up to 100 µm or even up to 150 pm, depending on the requirements. For economic reasons, however, the thickness usually is 20 µm or less for substrates which do not require corrosion protection and 50 µm or less for substrates in need of corrosion protection.Hard chrome plated article

[0087] Another aspect of the present invention relates to a hard chrome plated article, which comprises the hard chrome layer of the present invention on a metal substrate as described above.

[0088] One advantageous aspect of the present invention is the possibility to achieve superior corrosion protection on a steel substrate without requiring an interim nickel layer of ≥1µm below the hard chrome layer, as conventionally applied in the state of the art.

[0089] Thus, in a preferred embodiment, the hard chrome plated article comprises a steel substrate plated with the above-described hard chrome layer of the present invention with a thickness of 20 µm or more, which does not have an interim nickel layer with a thickness of ≥1µm between the substrate and the hard chrome layer, and which has a corrosion resistance of <5% base metal (steel substrate) corrosion after 24 h in SST according to DIN EN ISO 9227 NSS.Examples

[0090] The present invention will be explained in more detail by means of the following examples, although it is not limited to such examples. Through the examples, the throwing power of the inventive bath, as well as the obtained metal distribution of the plated chromium layer were investigated. The throwing power was measured by Hull cell test panels and the metal distribution of the plated chromium layer was investigated through practical parts.Example 1 (Reference)

[0091] To examine the "throwing power" of the inventive bath, Hull cell tests were conducted according to DIN 50957-1. In a Hull cell as specified in DIN 50957-1, the cathode is a metal panel arranged at an inclined angle relative to the anode, so that there is a proximal edge which has the smallest distance to the anode and thus is exposed to the highest local current density, and a distal edge having a larger distance to the anode and thus being exposed to a lower local current density. Accordingly, at a fixed average cathodic current density, the plating thickness will be highest at the proximal edge and lowest at the distal edge.

[0092] When the plating conditions (including current density, chromium amount, temperature and coating time) are selected such that the proximal edge is plated and the distal edge remains uncoated, then there will be a transition line between these edges, which separates the plated and the uncoated region. The "throwing power" is measured as the distance x between the proximal edge and the transition line.

[0093] In order to evaluate the effect of component (E) on the throwing power, it is firstly required to provide a standard bath comprising components (A) to (D) serving as a reference, and to suitably adjust the test conditions (current density, time and temperature) such that the transition line for the standard bath is approximately in the middle of the test panel, which corresponds to a throwing power of about 5.5 cm. Subsequently, it can be assessed how the throwing power changes when component (E) is added. The standard bath contained 259 g / l ammonium formiate, 20 g / l chromium obtained from 250 g basic chromium sulfate solution [8.5% Cr(III), 32% Cr 2 (SO 4 ) 3 ] and 21 g / l sodium bromide. For making up the standard bath, all salts were mixed with DI water and then heated for about 2 hours at 60°C. In the next step, the temperature was adjusted to 56°C and the pH was adjusted to 5.1 with aqueous ammonium hydroxide solution (25%). The bath composition in terms of the constituent ions is summarized in Table 1. Table 1. Composition of standard bath.(A)Chromium(III)20.00 g / l(B)Sulfate55.42 g / lBromide16.31 g / l(C)Formiate181.91 g / l(D)Ammonium81.1 g / l**74.1 g / l from NH 4 HCOO and 7.0 g / l from NH 4 OH

[0094] In the Hull cell tests, the temperature and pH of the bath were 56 °C and 5.1 respectively. Polished brass panels (7,5×10 cm 2< ) were used as cathode. For anode, meshed type MMO was used. The Hull cell tests were conducted according to DIN 50957 part 1. The test conditions are summarized in Table 2. Table 2. Conditions for plating with standard bath.pH5.1Current7.5 ATime3 minTemperature56 °CAnodeMMO (Mixed Metal Oxide)

[0095] The standard bath under the given test conditions achieved a throwing power of 5.5 cm, as can be seen from the respective Hull cell test panel shown in Figure 1.Example 2

[0096] Different types of compound (E) were added to the standard bath at different addition levels, and the throwing power was determined in the same way as described for Example 1. The respective bath formulations and the throwing power are shown in Table 3, wherein the "initial throwing power" refers to the result for the standard bath without additive serving as a reference, and the "final throwing power" refers to the bath including the additive (E). Table 3. Hull cell tests results.AdditiveConcentration (g / l)Initial throwing power (cm)Final throwing power (cm)Standard bath (without additive)--5.5Bis-(sodium sulfopropyl)-disulfide0.55.58.33-Mercapto-1-propanesulfonic acid, sodium salt0.35.58.23-(Benzothiazolyl-2-mercapto)-propyl-sulfunic acid, sodium salt0.55.56.5(O-Ethyldithiocarbonato)-s-(3-sulfopropyl)-ester, potassium salt0.055.56.5

[0097] As can be seen, an increase of the throwing power was achieved with all additives (E). The largest increase was found for bis-(sodium sulfopropyl)-disulfide and 3-mercapto-1-propanesulfonic acid, sodium salt, which increased the throwing power up to 8.3 and 8.2 cm, respectively, and also yielded a coated product with the best appearance.Example 3 (Reference)

[0098] The standard bath described in Example 1 was used to apply a chromium layer on the surface of a flat sheet made of Ni (25×30 cm 2< ). The flat sheet was used as cathode and a MMO mesh type anode was used. After pre-treatment including degreasing, activation and Ni strike, the part was plated in the standard chromium bath.

[0099] The temperature and pH of the bath were adjusted to 56 °C and 5.1, respectively. During the plating time the bath was filtered continuously. The current density was 22.5 A / dm 2< and the plating time was 1 hour. The thickness distribution of the plated chromium layer is shown in Figure 2 and schematically represented in 3D in Figure 3.

[0100] As the data in Figure 2 shows, the thickness of the plated chromium layer was about 0.01 µm at the center of the flat sheet, and the thickness reached a maximum of about 9.12 µm at the edges. Thus, the ratio of maximum to minimum thickness is about 912. Thus, it is evident that plating a uniform chromium layer on the surface of a sample with flat geometry would be challenging when using a standard bath and extra equipment would be required to manage the issue.Example 4

[0101] The plating of a flat sheet made of Ni (25×30 cm 2< ) was performed in the same way as described in Example 3, except that 0.5 g / l 3-mercapto-1-propanesulfonic acid, sodium salt were added to the standard bath. The thickness distribution of the plated chromium layer is shown in Figure 4 and schematically represented in 3D in Figure 5.

[0102] As can be seen, the thickness of the chrome layer is higher than 3.4 µm in all areas, and there are no regions with a thickness of less than 1 pm, as it was the case for the middle region in Example 3. Besides, the ratio of the highest and the lowest thickness value is merely about 1.66, which is much lower than the ratio of 912 in Example 3.

[0103] This indicates that an improvement in uniformity of the thickness of the plated chromium layer can be achieved by modifying the standard bath. Furthermore, since the chromium layer is plated uniformly on the entire part, a waste of chromium is greatly reduced when using the modified bath, rather than the standard bath.Example 5

[0104] To evaluate the throwing power and metal distribution achieved for chromium layers plated on substrates with a complex shape, tests were conducted on a shaped steel part developed by Dr. Ing. Max. Schlötter GmbH, as shown in Figure 6 (in the following referred to as "Schlötter part").

[0105] The working area of the part was 2 dm 2< and contained bent areas with holes by which distribution of the deposited layer in different current density areas could be evaluated. For doing so, two set-ups of 80 liters of the bath were prepared.

[0106] The first setup comprised the unmodified standard bath. In the second setup, the standard bath was modified by adding 1 g / l of 3-mercapto-1-propanesulfonic acid, sodium salt was added. The pH and working temperature of the baths were 5.1 and 56 °C, respectively. Also, the baths were filtered continuously. The schematic of the set-up is presented in Figure 7.

[0107] The plating time was 30 minutes and two different current densities, 20 and 40 A / dm 2< , were applied to the parts. After plating, the thickness of the plated chromium layer was measured at the points indicated in Figure 8. The measurement results are shown in Figure 9.

[0108] As can be seen, the unmodified standard bath at 20 A / dm 2< leads to a maximum thickness of more than 12 µm at point 1, and a thickness of less than 1 µm at points 2, 3 and 4. The ratio between the highest and the lowest value was found to be as high as 36.86. At 40 A / dm 2< , there are no more points with a thickness below 1 pm, but the unifomity still is poor, with a ratio of the highest and the lowest value being 4.66.

[0109] The modified bath according to the present invention achieves a much more uniform distribution even at both 20 A / dm 2< and 40 A / dm 2< , the ratio between highest and lowest thickness being 2.75 and 2.41, respectively, and no points with a coating thickness of less than 1 µm can be found.

[0110] This confirms the improved distribution of the plated chromium layer, especially when employing a lower current density plating mode. It can also be seen that the effect of the applied current density on the metal distribution of the plated chromium layer is much more pronounced when using the standard bath. In fact, the metal distribution of the plated chromium layer remains almost the same when increasing the current density in the case of the modified bath.Example 6

[0111] A "Schlötter part" was coated in the same way as in Example 5, except that the standard bath was modified by adding 1.7 g / l 3-mercapto-1-propanesulfonic acid, and the coating time was 2 hours at a current density and pH of 40 A / dm 2< and 5.1, respectively.

[0112] The thickness distribution of the plated chromium layer is shown in Figure 10. As can be seen, the distribution is highly uniform, and the ratio of maximum to minimum thickness of the plating layer was found to be 1.11. This confirms the uniformity of the deposited layer obtained when using the inventive bath.Example 7

[0113] A first bath (reference) was prepared using a chromium(III)-methanesulfonate solution (50 wt.%) comprising 7.6 wt.% Cr(III) and having a density of 1.41 g / cm 3< . The chromium concentration in the bath was 20.51 g / l. The composition or the first bath is shown in Table 4. Table 4. Composition of the reference bath (reference).Ammonium formate259 g / LAmmonium acetate2.5 g / lChromium(III)-methanesulfonate solution (50wt.%)263 g / lSodium bromide21 g / LAmmonium chloride25 g / lBoric acid50 g / l

[0114] Furthermore, a second bath in accordance with the present invention was prepared, which otherwise had the same composition as the reference bath, except that bis-(sodium sulfopropyl)-disulfide and 3-Mercapto-1-propanesulfonic acid, sodium salt were added as component (E). The composition of the second bath is shown in Table 5. Table 5. Composition of the second bath (invention).Ammonium formate259 g / LAmmonium acetate2.5 g / lChromium(III)-methanesulfonate solution (50%)263 g / lPotassium bromide21 g / LAmmonium chloride25 g / lBoric acid50 g / lBis-(sodium sulfopropyl)-disulfide0.5 g / l3-Mercapto-1-propanesulfonic acid, sodium salt0.5 g / l

[0115] For making up the baths, all salts were mixed with DI water and then heated for about 2 hours at 60°C. In the next step, the temperature was adjusted to 56°C and the pH was adjusted to 5.1 with aqueous ammonium hydroxide solution (25%).

[0116] In a first test, the "initial state" Hull cell was prepared and the throwing power on the Hull cell panel was measured for the first bath (reference). In a second test, the throwing power for the second bath (invention) was determined as the final throwing power.

[0117] Throughout the tests the temperature and pH of the bath were 56 °C and 5.1, respectively. A polished brass panel (7.5×10 cm 2< ) was used as cathode. Graphite was used as anode. The Hull cell tests were conducted according to DIN 50957 part 1 and the test result is shown in Table 6. Table 6. Throwing power result.Initial throwing power (cm)Final throwing power (cm)5.68.5 Example 8

[0118] To evaluate corrosion resistance, 1 cm in diameter steel rods (composition according to production code S235JR (1.0038)) were plated with trivalent chromium. Within the layer composition, no Ni-containing sublayer was deposited, only a chromium layer was deposited on the steel surface.

[0119] Before the plating, the rods were cleaned and pre-treated in accordance with a standard pre-treatment process for steel as described in Table 7. Table 7. Pre-treatment procedure for steel.Stage Process Bath composition Parameter 1Hot degreasing- Deionized water- Temperature: 65 °C- Degreaser salt SLOTOCLEAN AK 161: 30 g / l-Wetting agent additive RV111: 10 ml / l- Treatment time: 10 min2Pickling-Deionized water- Temperature: 25 °C- Pickle degreaser additive SLOTOCLEAN BEF 30: 40 ml / l-Hydrochloric acid, conc., (36%): 50% vol- Treatment time: 10 min3Anodic degreasing- Deionized water- Temperature: 40 °C- Treatment time: 3 min- Degreaser salt SLOTOCLEAN EL DCG: 120 g / l- Current density: 6 A / dm 2< 4Activation-Deionized water- Temperature: 25 °C-Hydrochloric acid, conc., (36%): 10% vol- Treatment time: 2 min

[0120] SLOTOCLEAN AK 161, SLOTOCLEAN BEF 30 and SLOTOCLEAN EL DCG are products of Dr.-Ing. Max Schlötter GmbH & Co. KG.

[0121] After the pre-treatment, the steel rods were placed into a plating tank, in the middle between the anodes. The working area of the part was 0.2 dm 2< . The volume of the bath set-up was 80 liters. The plating was performed with the unmodified standard bath as described in Example 1 serving as a reference, or with a modified bath, to which 1 g / l of 3-mercapto-1-propanesulfonic acid sodium salt was added.

[0122] The pH and working temperature of the baths were 5.1 and 56 °C, respectively. Also, the baths were filtered continuously. The schematic of the set-up is shown in Figure 7.

[0123] The applied current density was 40 A / dm 2< . The plating time was 30 minutes in the standard bath without 3-mercapto-1-propanesulfonic acid, sodium salt and 120 minutes in the modified bath with 1 g / l of 3-mercapto-1-propanesulfonic acid sodium salt. The different plating times were necessary to obtain plating layers of approximately the same thickness of about 20 µm at the given current density.

[0124] After plating, the properties of the chromium layer were examined under the microscope using top and cross section views. The top view of the deposited layers showed >10 cracks / mm for the layer prepared using the standard bath and <10 cracks / mm for the layer prepared using the modified bath. The results are shown in Figure 11.

[0125] In addition, the corrosion resistivity was examined with a salt spray test according to DIN EN ISO 9227 NSS. The sample prepared using the standard bath showed significant corrosion (more than 40%) after 24 h in the salt spray test, whereas the sample prepared using the modified bath showed almost no corrosion (0-5%) after 24 h in the salt spray test. The results are shown in Figure 12.Example 9

[0126] In the following example, the dissolution of iron from a steel substrate exposed to the plating bath for a prolonged period of time is determined.

[0127] The unmodified standard bath of Example 1 served as a reference, and a modified bath was prepared by adding 1g / l of 3-mercapto-1-propanesulfonic acid, sodium salt to the standard bath.

[0128] Additionally, two steel panels of each 0.1 dm 2< made of DC04 steel was degreased with isopropanol and weighted. The weight of the steel panel No. 1 was 1.1779 g, the weight of the steel panel No.2 was 1.1791 g.

[0129] Steel panel No. 1 was placed into a polypropylene bottle filled with 200 ml of the unmodified standard bath, whereas steel panel No. 2 was placed into a polypropylene bottle filled with 200 ml of the modified bath.

[0130] The lids of the bottles were closed, and the bottles were then stored for 20 h at 50°C in an oven. Then, the steel panels were removed from the bottles, rinsed with water and dried with pressurized air. Finally, the steel panels were weighted a second time.

[0131] Panel No. 1, which was in the standard bath had a weight of 1,0589g, which means that the weight loss after 20 h storage at 50°C was 0,119 g. Panel No. 1 also showed a dark grey discoloration, as shown in Figure 13.

[0132] Panel No. 2, which was in the modified bath including 3-mercapto-1-propanesulfonic acid, sodium salt, had a weight of 1,1683 g, which means that the weight loss after 20 h storage at 50°C was only 0,0108g. Accordingly, with the modified bath comprising 3-mercapto-1-propanesulfonic acid, sodium salt according to the present invention, the iron dissolution by the bath was only 9,1% of the dissolution of the standard bath. Furthermore, panel No. 2 did not show any discoloration, as shown in Figure 14.Example 10

[0133] In the following example, the composition of the chromium layer is investigated by Glow Discharge Optical Emission Spectroscopy (GDOES). The unmodified standard bath of Example 1 served as a reference, and two modified baths according to the invention were 0.5 g / l and 1 g / l of 3-mercapto-1-propanesulfonic acid, sodium salt as component (E) to the standard bath.

[0134] The chromium layer was deposited on polished brass panels. Before the plating, the panels were cleansed and pre-treated in accordance with a standard pre-treatment process for brass as summarized in the following Table 8. Table 8. Pre-treatment procedure for brass.Stage Process Bath composition Parameter 1Hot degreasing- Deionized water- Temperature: 65 °C- Degreaser salt SLOTOCLEAN AK 341: 30 g / l- Treatment time: 10 min- Wetting agent additive RV111: 10 ml / l2cathodic degreasing- Deionized water- Temperature: 40 °C- Degreaser salt SLOTOCLEAN EL 130: 50 g / l- Treatment time: 1 min- Current density: 6 A / dm 2< 3Activation-Deionized water- Temperature: 25 °C-Sulfuric acid, conc., (96%): 2,5% vol- Treatment time: 1 min

[0135] SLOTOCLEAN AK 341, Wetting additive RV111 and SLOTOCLEAN EL 130 are products of Dr.-Ing. Max Schlötter GmbH & Co. KG.

[0136] The pH and working temperature of the baths were 5.1 and 56 °C, respectively. Also, the baths were filtered continuously. The schematic of the set-up is the same as shown in Figure 7.

[0137] The applied current density was 40 A / dm 2< . The plating time was 30 minutes in the standard bath without 3-mercapto-1-propanesulfonic acid, sodium salt and 60 minutes in the modified bath with 0.5 or 1 g / l of 3-mercapto-1-propanesulfonic acid sodium salt.

[0138] Because of the lower coating uniformity, on the other hand, the reference bath yields regions with high thickness even at short coating times, which could be used for the investigation. The modified baths of the present invention, on the other hand, yield a more uniform coating thickness regardless of the current density variations, hence a longer coating time was used to obtain an appropriate thickness of the investigated region.

[0139] The coated panels were investigated by means of GDOES. The results are shown in Figures 15 (Reference), 16 (0.5 g / l) and 17 (1 g / l), respectively. The x-axis refers to the depth direction, wherein the origin corresponds to the surface of the coating layer. The y-axis refers to the concentration of the elements, including Cr (coating), Cu and Zn (substrate), C, S and O. As also indicated in the legend to the Figures, the scaling of the y-axis is 100% for the metals (Cr, Zn, Cu), 10% for O and 5% for C and S.

[0140] The layer thickness was more than 10 µm for each panel. As can be seen in Figure 15, the reference coating exhibited a high incorporation of carbon of about 3%. For the coatings according to the present invention, on the other hand, Figures 16 and 17 show the presence of about 3% of sulfur, but the incorporated carbon amount is substantially reduced to about 0.5%. The precise values at a depth of 5 µm are also shown in Table 9 below. Table 9. The GDOES analysis results.Standard bath (Reference)Bath with 0,5 g / l of 3-mercapto-1-propanesulfonic acid sodium saltBath with 1,0 g / l of 3-mercapto-1-propanesulfonic acid sodium saltSulfur content0.0%3.23%3.449%Carbon content2.985%0.634%0.55%

[0141] The results show that the addition of 3-mercapto-1-propanesulphonic acid sodium salt increases the sulfur content of the coating and at the same time reduces the incorporation of carbon.Example 11

[0142] In the following example, the friction coefficient of the chromium layer is investigated. The unmodified standard bath of Example 1 served as a reference, and two modified baths according to the invention were 0.5 g / l and 1 g / l of 3-mercapto-1-propanesulfonic acid, sodium salt as component (E) to the standard bath. As a further reference, a chromium(VI)-based hard chrome plating bath was used (SLOTOCHROM S, available from Dr.-Ing. Max Schlötter GmbH & Co. KG.).

[0143] The test was performed with 1 × 5 cm DC04 steel panels, which were cleansed and activated in the same way as described in Example 9 (see Table 7). The chromium layers was deposited directly onto the steel surface, without an interim Ni-containing sublayer. After the pre-treatment, the steel panels were placed into a plating tank, in the middle between the anodes. The working area of the part was 0.1 dm 2< . The volume of the bath set-up was 80 liters. The pH and working temperature of the baths were 5.1 and 56 °C, respectively. Also, the baths were filtered continuously. The schematic of the set-up is the same as shown in Figure 7.

[0144] The applied current density was 40 A / dm 2< . The plating time was 15 minutes in the standard bath without 3-mercapto-1-propanesulfonic acid, sodium salt and in the SLOTOCHROM S bath and 60 minutes in the modified bath with 0.5 or 1.0 g / l of 3-mercapto-1-propanesulfonic acid sodium salt. The different plating times were necessary to obtain plating layers of approximately the same thickness of about 10 µm at the given current density.

[0145] The coefficient of friction was tested according to ASTM G133-22 "Linearly Reciprocating Ball-on-Flat Sliding Wear" against a counter material which was a ball of 3 mm diameter made of tungsten carbide. The normal force for the test was 5 N and the friction was tested for 1000 cycles of back-and-forth movement with a stroke length of 2mm, back-and-forth one cycle equals 4mm. 1000 cycles equal a combined stroke length of 4 m. The frequency of the oscillation was 0,2 Hz, for a test duration of 5000 s. Further test conditions were as followed: Temperature: 25°C, air humidity: 55%, no lubrication was applied. All samples were tested simultaneously on a friction test stand according to ASTM G133-22 (WECO-X by iChemAnalytics GmbH). The results are shown in Figure 18, and the numerical value of the coefficient of friction at 100 cycles is also shown in Table 10below: Table 10. Coefficient of friction values after 100 cycles.Standard bathBath with 0.5 g / lof 3-mercapto-1-propanesulfonic acid sodium saltBath with 1.0 g / lof 3-mercapto-1-propanesulfonic acid sodium saltReference SLOTOCHROM S0.310.220.200.29

[0146] As can be seen, the trivalent chromium baths comprising 3-mercapto-1-propanesulfonic acid yielded hard chrome layers with significantly lower coefficients of friction as compared to both, the trivalent chromium standard bath and the hexavalent chromium bath SLOTOCHROM S. A lower coefficient of friction can be beneficial for wear applications.Example 12

[0147] In the following Example, the use of component (E) in an ammonia- and borate-free plating bath was investigated by means of Hull cell tests in accordance with DIN 50957-1. A reference bath comprising sulfate, formate and glycine as components (B), (C) and (D), respectively, was formulated by mixing the components as shown in Table 11 below. Table 11. Composition of the Boric and Ammonium free bath.Chromium(III) potassium sulfate dodecahydrate384.53 g / lFormic acid 98%69.045 g / lGlycine75.07 g / l

[0148] The bath contained 384.53 g / l Chromium(III) potassium sulfate dodecahydrates by which 40 g / l chromium was obtained. For making up the bath, all salts were mixed with DI water and then heated for about 2 hours at 60°C. Then pH was adjusted to 3.8 with potassium hydroxide. In the next step, the temperature was adjusted to 30°C.

[0149] In the Hull cell tests, the temperature and pH of the bath were 30°C and 3.8 respectively. Polished brass panels (7,5×10 cm 2< ) were used as cathode. For anode, meshed type MMO was used. The Hull cell tests were conducted according to DIN 50957 part 1. The test conditions are summarized in Table 12. Table 12. Conditions for plating with standard bath.pH3.8Current1.0 ATime10 minTemperature30 °CAnodeMMO (Mixed Metal Oxide)

[0150] The reference bath under the given test conditions achieved a throwing power of 3.1 cm.

[0151] Furthermore, a second bath in accordance with the present invention was prepared, which otherwise had the same composition as the reference bath, except that 3-Mercapto-1-propanesulfonic acid, sodium salt was added as component (E). The composition of the second bath is shown in Table 13. Table 13. Composition of the Boric and Ammonium free bath.Chromium(III) potassium sulfate dodecahydrate384.53 g / lFormic acid 98%69.045 g / lGlycine75.07 g / l3-Mercapto-1-propanesulfonic acid, sodium salt1 g / l

[0152] The makeup and test condition were in the same way as reference bath. The throwing power was improved up to 5.2 cm. This confirms that component (E) also is useful for borate- and ammonia-free baths.

Examples

example 1 (

Example 1 (Reference)

[0091]To examine the "throwing power" of the inventive bath, Hull cell tests were conducted according to DIN 50957-1. In a Hull cell as specified in DIN 50957-1, the cathode is a metal panel arranged at an inclined angle relative to the anode, so that there is a proximal edge which has the smallest distance to the anode and thus is exposed to the highest local current density, and a distal edge having a larger distance to the anode and thus being exposed to a lower local current density. Accordingly, at a fixed average cathodic current density, the plating thickness will be highest at the proximal edge and lowest at the distal edge.

[0092]When the plating conditions (including current density, chromium amount, temperature and coating time) are selected such that the proximal edge is plated and the distal edge remains uncoated, then there will be a transition line between these edges, which separates the plated and the uncoated region. The "throwing power" is meas...

example 10

[0133]In the following example, the composition of the chromium layer is investigated by Glow Discharge Optical Emission Spectroscopy (GDOES). The unmodified standard bath of Example 1 served as a reference, and two modified baths according to the invention were 0.5 g / l and 1 g / l of 3-mercapto-1-propanesulfonic acid, sodium salt as component (E) to the standard bath.

[0134]The chromium layer was deposited on polished brass panels. Before the plating, the panels were cleansed and pre-treated in accordance with a standard pre-treatment process for brass as summarized in the following Table 8.

Table 8. Pre-treatment procedure for brass.

Stage Process Bath composition Parameter

1Hot degreasing- Deionized water- Temperature: 65 °C

- Degreaser salt SLOTOCLEAN AK 341: 30 g / l

- Treatment time: 10 min

- Wetting agent additive RV111: 10 ml / l

2cathodic degreasing- Deionized water- Temperature: 40 °C

- Degreaser salt SLOTOCLEAN EL 130: 50 g / l- Treatment time: 1 min

- Current density: 6 A / dm 2

3Activatio...

Field of the Invention

[0001] The present invention relates to the field of plating chromium layers of thickness >1 µm, often referred to as hard chrome plating. More particularly the invention relates to a trivalent chromium bath for depositing a hard-chrome plating on a metal substrate, a respective electroplating process and a hard-chrome plated metal substrate thus obtained.Background of the Invention

[0002] Chrome electroplating generally can be divided into two different fields of application: A) For decorative purposes, in which case the thickness of the chrome plating layer typically is <1 µm = "decorative chrome" or "bright chrome" (wherein "bright" means glossy B) For technical purposes, in which case the thickness of the chrome plating layer typically is >1 µm = "hard chrome"

[0003] The demands for these two types of chrome electroplating processes are quite different.

[0004] For decorative chrome layers, there typically is a focus on the optical appearance (brightness, colour). The thickness of bright chrome generally is <1µm. Typical applications for bright chrome are sanitary items (e.g., shower heads), decorative automotive parts (e.g., chrome plated mouldings) or consumer goods (e.g., housings for electrical shavers).

[0005] US 10,006,135 B2 describes an electrolyte for dark decorative chrome layers, which comprises a certain sulfur compound (D) as a colorant.

[0006] Hard chrome layers generally are applied in a thickness of >1 µm. Typical applications for hard chrome are, e.g., machine engineering parts (e.g., for polymer extruders), technical automotive parts (e.g., piston rods), gravure printing plates and hydraulic cylinders. Hard chrome layers are applied to improve wear resistance and tribological properties.

[0007] According to the state of the art, thick chromium layers, often referred to as hard chrome layers, can be electrochemically deposited from trivalent or hexavalent baths. An example of a bath which is capable of plating chromium layers with a thickness >1 µm and therefore is suitable for hard chrome plating is presented in EP 2 899 299 A1. Possible anodes according to the state of the art also are presented in this document.

[0008] To be able to plate high thicknesses, the compositions of baths for hard chrome plating differ from compositions for bright chrome. The deposition of hard chrome generally requires extremely high current densities. Hard chrome plating baths can plate fast at high current densities, but the achievable metal distribution of the plated chromium layer, and thus the ability to plate complexly shaped parts, is extremely limited. The prior art hard chrome baths are particularly suited for plating rotation symmetric parts (e.g., rods or shafts), but are more difficult to employ for plating complexly shaped parts or large flat parts (e.g., panels) having both, areas of locally very high current densities and areas of very low current densities. To overcome this shortcoming, it often is necessary for parts which have a complex shape or parts which have a large flat surface to apply so-called shielding in order to achieve an even metal distribution across a whole part and to prevent excessive plating or burning in areas of locally high current density, e.g., on the edges of a workpiece. Shields are made of either conductive or nonconductive materials, and the application of suitable shielding is time consuming and expensive. Especially when different workpiece geometries are to be plated, the shielding must be individually adjusted to every workpiece geometry. Moreover, in the case of applying metallic shielding, also known as cathode thieves, for the modification of current density on edges, chromium will be consumed and dragged-out, which adds more expense to the process (ASM, Vol 5, p. 195-196).

[0009] In terms of industrial electroplating, it is desirable to plate multiple parts by hanging them on a rack and plate them simultaneously. The challenging point in such a scenario is that the individual current densities can strongly vary for the different parts on a rack. In other words, the variation of current densities for multiple parts on a rack is much higher than that for a single part. Some reasons for this phenomenon are: Different distances between the parts and anodes. Parts which are positioned directly in front of an anode will experience higher current densities than parts that are located at a greater distance from the anode. Parts hanging at the edges of a rack typically experience higher local current densities than those hanging in the middle. Neighbouring parts can partially shield each other. This results in higher variances in the local current density in comparison to an individually hanging part.

[0010] Thus, there exists a need to develop a bath which can equalize such variations. Furthermore, it is desirable to improve the metal distribution of the plated chromium layer and to develop means for plating at lower current densities. This demand is addressed by the presently claimed invention.

[0011] The use of a trivalent chromium-based solution is favourable from the viewpoint of hexavalent chromium salts being very toxic, and European and worldwide legislation restricting the use and the availability of Cr(VI)-salts. In Europe, REACH legislation (CMD - Carcinogens and Mutagens Directive, Annex III - OELVs) restricts the use of Cr(VI)-salts. In view thereof, it is desirable to develop means and methods for carrying out hard chrome plating using trivalent chromium.

[0012] Existing methods of hard chrome plating from trivalent chromium-containing baths suffer from the drawback of the plated layer offering poorer corrosion resistance than layers plated from hexavalent chromium. According to the state of the art, it is therefore necessary to plate a nickel or nickel-phosphorous ("electroless nickel") sublayer under the hard chrome layer when it is deposited from trivalent chromium baths (see, for example, WOTECH 2021 / 02 / 07 "Hartverchromung aus dreiwertigen Elektrolyten" Hannah Betz et al.). In view thereof, it is another objective of the present application to provide a means for preparing hard chrome plating layers that are prepared from trivalent chromium-containing baths, and which exhibit improved corrosion resistance.

[0013] For some applications, there also is a special interest to minimize the dissolution of iron from steel substrates, when it is immersed into the plating solution for a longer period of time. This applies especially for applications where only a part of a substrate is plated with chromium whereas another part is not plated with chromium. One such example is the plating of the inside of steel tubes. In this case the outside of the substrate is constantly exposed to the plating bath. Generally iron impurities in the plating bath can cause dark deposition in the low current density. It is therefore important to control the concentration of iron in the bath. Usually, the plating bath is treated with ion exchange resins to remove the iron from the bath solution when a limit of 10 - 20 mg / l of iron is reached. It is desirable to reduce the frequency of such iron removal treatments by using a bath with reduced propensity for iron dissolution.Description of the Figures

[0014] Figure 1 is a picture of a Hull cell panel after carrying out plating using the standard bath (Example 2). Figure 2 shows the distribution of chromium thickness after using a standard bath to plate a flat sheet (Example 3). Figure 3 is a 3D schematic diagram showing the chromium thickness distribution on a flat sheet that was plated using a standard bath (Example 3). Figure 4 shows the distribution of chromium thickness after using a modified bath to plate a flat sheet (Example 4). Figure 5 is a 3D schematic diagram showing the chromium thickness distribution on a flat sheet that was plated using a modified bath (Example 4). Figure 6 shows the test piece ("Schlötter part") used for thickness distribution and throwing power tests (Example 5). Figure 7 shows a schematic drawing of a plating bath (top view) as employed in Examples 5, 8, 10 and 11. Figure 8 shows the measuring points on the Schlötter part used in Example 5 and Example 6. Figure 9 shows the thickness distribution of the plated chromium layers in Example 5. Figure 10 shows the thickness distribution of the plated chromium layers in Example 6. Figure 11 shows a comparison of the surface quality (number of cracks) of the chromium plates steel rods of Example 8 (left-hand: standard bath; right-hand: bath according to the present invention). Figure 12 shows the appearance of the rods of Example 8 after the salt spray test (left-hand: standard bath; right-hand: bath according to the present invention). Figures 13 and 14 show the appearance of the steel sheets of Example 9 after immersion for 20 h at 50°C in the unmodified standard bath and the bath according to the invention, respectively. Figures 15 to 17 are plots showing the concentration of the constituents of the hard chrome plated brass panels of Example 10 in depth direction for the unmodified standard bath and the baths according to the invention comprising 0.5 and 1 g / l of 3-mercapto-1-propanesulfonic acid, sodium salt, respectively, as measured by Glow Discharge Optical Emission Spectroscopy (GDOES). Figure 18 is a plot showing the coefficients of friction of the hard chromium plated steel substrates of Example 11. Summary of the Invention

[0015] To address the above-described problems of the state of the art, the present inventors have developed a trivalent Cr bath for introducing a uniform hard chrome layer on metallic surfaces, and in particular on the surfaces of metal parts.

[0016] In one aspect, the present invention therefore relates to the use of a trivalent chrome plating bath composition for the electrodeposition of a hard chrome layer with a thickness of 1 µm or more, the composition comprising: (A) trivalent chromium ions in an amount of 5 - 40 g / l, (B) at least one anion selected from sulfate, chloride, bromide and methanesulfonate, (C) at least one carboxylate ion selected from formate and acetate, and (D) at least one buffer substance selected from ammonia, boric acid, carboxylic acids with 2 to 8 carbon atoms other than component (C) and salts thereof, and aluminium salts, and (E) 0.01 to 100 g / l of at least one compound of the following formula (1):         X-S-R-SO 3 Y     (1) wherein: X represents H, C 1-6 -alkyl, R 1< -CZ 1< -, R 1< -Z 2< -CZ 1< - or a second moiety of the formula -S-R-SO 3 Y, wherein Z 1< is =O or =NR 2< ; Z 2< is -O-, -S- or -NR 3< -; R 1< , R 2< and R 3< independently are hydrogen or C 1-6 -alkyl, or two of R 1< , R 2< and R 3< are linked together so that X forms a mono- or bicyclic heterocycle with 5-10 ring atoms, which may be saturated or unsaturated; R is an alkylene group with 2 to 6 carbon atoms, Y is hydrogen, ammonium, an alkali metal or 0.5 equivalents of an alkaline earth metal; or the compound of formula (1) forms a zwitterion in which Y is a negative charge and one of Z 1< and Z 2< is =NR 2< H +< or -NR 3< H +< -, respectively.

[0017] Further aspects of the present invention are directed to a chromium electroplating process using the trivalent chrome plating bath composition to apply a hard chrome layer on a metal substrate, a hard chrome layer having a thickness of 1 µm or more, which is provided on a metal substrate and a steel article having the hard chrome plating. Yet another aspect relates to the use of the above-described compound (E) as an additive for a trivalent chrome plating bath.Effects of the Invention

[0018] With the trivalent Cr bath used in the present invention, it is possible to achieve a uniform Cr deposition even in the case of varying current density. The use of the bath of the present invention therefore allows complexly shaped parts to be fully covered with a hard Cr layer, especially at lower current density areas, without causing burning at high current density areas.

[0019] Surprisingly, it has also been found that chromium layers deposited by using the bath in accordance with the present invention exhibit superior corrosion resistance. Furthermore, the crack structure is drastically reduced in comparison to a layer plated from a bath according to the state of the art.

[0020] In the present invention, a uniform metal distribution of the plated chromium layer is achieved by improving the efficiency of the chrome plating at lower current densities and modifying the plating efficiency (reducing the plating speed) at higher current density. The plating capacity at the lowest current density areas is often referred to as "throwing power".

[0021] A good metal distribution of the plated chromium layer is favourable, because this means that less "over-plating" on a workpiece is required in order to achieve a minimum thickness in an area of low current density. It also allows the plating process to be more sustainable and economical, since less excess chrome metal is plated out from the bath as drag-out. Also, a good throwing power will reduce the plating times required for arriving at a required minimum plating thickness. This allows energy consumption to be reduced.

[0022] Moreover, the modification of the bath to plate at lower current densities makes the use of lower current densities possible, thereby preventing not only excessive plating but also burning in the areas of highest current density.

[0023] A better metal distribution of the plated chromium layer also serves to drastically reduce the need for the time consuming and expensive use of auxiliary equipment like shielding.

[0024] It was also found that by using the bath of the present invention for the plating of a steel substrate, the dissolution of iron from the steel substrate is suppressed. This helps to reduce the need and the frequency of iron removal treatments.

[0025] Furthermore, it also was found that the chromium layer obtained by using the trivalent Cr bath of the present invention comprises sulfur and has a reduced carbon content compared to layers obtained with a conventional trivalent Cr bath. This difference in composition leads to a significantly reduced coefficient of friction.

[0026] In a further aspect of the invention, chromium depositions from different plating baths can be combined to form multi-layer chromium depositions, e.g., when chromium layers from a plating bath according to the invention with a good metal distribution of the plated chromium layer and high corrosion resistance are plated onto or underneath a chromium layer plated according to the state of the art or in a series of layers from alternating chromium plating baths, in order to combine different layer properties and to achieve better overall layer properties in terms of tribology, corrosion resistance and proper coverage of a complexly shaped part.

[0027] One example for a multi-layer application would be the combination of a chromium sublayer which exhibits a crack-free, highly corrosion resistant structure, with a top layer exhibiting a crack-containing chromium layer. For some applications it is desirable to have a crack structure that works as reservoirs for lubricants in the top layer. The crack-free sublayer in this example ensures the corrosion performance of the layer composition.Detailed Description

[0028] The embodiments and features of the present invention will now be described in greater detail.Plating Bath Composition

[0029] The trivalent chrome plating bath composition used in the present invention comprises: (A) trivalent chromium ions in an amount of 5-40 g / l, (B) at least one anion selected from sulfate, chloride, bromide and methanesulfonate, (C) at least one carboxylate ion selected from formiate and acetate, (D) at least one buffer substance selected from ammonia, boric acid, carboxylic acids with 2 to 8 carbon atoms other than component (C) and salts thereof, and aluminium salts, and (E) 0.01 to 100 g / l of at least one sulfur-containing substance of the formula (1) as described below. Trivalent chromium ion (A)

[0030] The plating bath according to the invention comprises trivalent chromium ions in an amount of 5-40 g / l. Preferably, the concentration of trivalent chromium ions lies in the range of 10-30 g / l, more preferably 15-25 g / l.

[0031] The trivalent chromium ions are added to the bath composition in the form of chromium(III) salts, such as chromium(III) sulfate, basic chromium(III) sulfate, chromium(III) methanesulfonate, chromium(III) chloride, chromium(III) acetate, chromium(III) nitrate and chromium alums such as chromium(III) potassium sulfate dodecahydrate, with basic chromium(III) sulfate, chromium(III) potassium sulfate dodecahydrate and chromium(III) methanesulfonate being preferred, and basic chromium(III) sulfate being more preferred.

[0032] Depending on the counter ion, the chromium(III) salt also can at least partially contribute the anion (B) and / or the carboxylate ion (C).

[0033] When the bath is operated using insoluble anodes, the deposition of chrome leads to a decrease of the chromium(III) concentration, hence component (A) needs to be replenished intermittently or continuously. The replenishment of component (A) simultaneously may lead to an increase of the anion (B) and / or the carboxylate ion (C) as the counter ion.Anion (B)

[0034] The plating bath according to the invention comprises at least one anion (B) selected from sulfate, chloride, bromide and methanesulfonate.

[0035] Alkali metal salts and ammonium salts, such as lithium chloride, lithium bromide, lithium sulfate, lithium methanesulfonate, sodium chloride, sodium bromide, sodium sulfate, sodium methanesulfonate, potassium chloride, potassium bromide, potassium sulfate, potassium methanesulfonate, ammonium chloride, ammonium bromide, ammonium sulfate, and ammonium methanesulfonate can serve as a source of the anion (B).

[0036] Among these, lithium chloride, lithium bromide, lithium sulfate, sodium chloride, sodium bromide, sodium sulfate, sodium methanesulfonate, potassium chloride, potassium bromide, potassium sulfate, ammonium chloride, ammonium bromide, and ammonium sulfate are preferred. More preferred are lithium chloride, lithium bromide, sodium chloride, sodium bromide, potassium chloride, potassium bromide, ammonium chloride, and ammonium bromide. Even more preferred are lithium bromide, sodium bromide, potassium bromide, and ammonium bromide, and still more preferred is sodium bromide and potassium bromide.

[0037] Among those, the ammonium salts simultaneously can supply at least a part of the buffer component (D). Besides, further to the above, the chromium(III) salt used for providing the trivalent chromium ions (A) can also at least partially provide the anionic ion (B), depending on the counterion.

[0038] Preferably, the concentration of the anion (B) lies in the range of from 50 to 500 g / l, more preferably in the range of from 100 to 450 g / l, even more preferably in the range of from 150 to 400 g / l, and still more preferably in the range of from 200 to 400 g / l.Carboxylate ion (C)

[0039] The inventive plating bath according to the invention comprises at least one carboxylate selected from formate and acetate.

[0040] Formic acid or acetic acid or respective salts thereof such as ammonium formate, ammonium acetate, sodium formate, sodium acetate, potassium formate and potassium acetate can serve as a source for the carboxylate ion (C). Among these, ammonium formate, ammonium acetate, sodium acetate, and potassium acetate are preferred, and ammonium formate and ammonium acetate are more preferred.

[0041] Again, ammonium formate and ammonium acetate can supply at least a part of the buffer component (D), and conversely, the carboxylate ion (C) can be at least partially supplied as counterions of the chromium(III) salt used for providing the trivalent chromium ions (A).

[0042] Preferably, the concentration of the carboxylate ion (C) lies in the range of from 30 to 400 g / l, more preferably in the range of from 40 to 350 g / l, even more preferably in the range of from 60 to 300 g / l.

[0043] If the below-described buffer substance does not comprise carboxylate groups, the concentration of the carboxylate ion (C) still more preferably is 100 to 300 g / l and yet still more preferably 150 to 300 g / mol.Buffer substance (D)

[0044] The inventive plating bath according to the invention comprises at least one buffer substance (D) selected from ammonia, boric acid, carboxylic acids with 2 to 8 carbon atoms other than component (C) and salts thereof, and aluminium salts.

[0045] Among those, boric acid, ammonia and salts thereof are preferable. Examples for the salts include sodium borate, ammonium chloride, ammonium formate or ammonium acetate.

[0046] The carboxylic acid with 2 to 8 carbon atoms other than component (C) specifically can be a di- or tricarboxylic acid, a hydroxycarboxylic acid, an oxocarboxylic acid, an aminocarboxylic acid, a sulfocarboxylic acid or an aromatic carboxylic acid, or may have a plurality of the aforementioned functional groups.

[0047] Examples for the carboxylic acid include oxalic acid, succinic acid, glutaric acid, adipic acid, fumaric acid and maleic acid, glycolic acid, lactic acid, malic acid, tartaric acid, citric acid, glyoxylic acid, pyruvic acid, glycine, alanine, 3-aminopropionic acid, 4-aminobutyric acid, 5-aminopentanoic acid, 6-aminohexanoic acid, glutamic acid, phthalic acid, sulfosuccinic acid, 2-hydroxy-5-sulfo-benzoic acid, methylglycindiacetic acid and salts thereof. Among those, glycine and salts thereof are preferable.

[0048] Further to their buffer function, the aforementioned carboxylic acids also can act as chelation agents useful for controlling the chromium deposition, thereby improving the deposition quality especially in high current density areas, thus preventing burnings or powdery deposition and improving the appearance. They can be used singly or in combination, or a combination of one or more of these acids with one or more further buffering substances such as ammonia or borate can be used.

[0049] Aluminium salts likewise can be used as buffer substance (D), preferably in the form of aluminium chloride or aluminium sulfate.

[0050] If the buffer substance (D) is used in the form of a salt, the counter ion may correspond to component (B), (C) or (D), respectively. As examples for the latter case, ammonium borate and ammonium salts of the aforementioned carboxylic acids may be mentioned.

[0051] The concentration of the buffer substance (D) typically lies in the range of from 15 to 300 g / l, preferably in the range of from 25 to 250 g / l, even more preferably in the range of from 50 to 200 g / l.Sulfur-containing substance (E)

[0052] The plating bath according to the invention comprises at least one sulfur-containing substance (E) of the following formula (1), at a total concentration of 0.01 to 100 g / l, preferably 0.1 to 10 g / l and more preferably 0.2 to 5 g / l:         X-S-R-SO 3 Y     (1)

[0053] In the formula (1), X represents H, C 1-6 -alkyl, R 1< -CZ 1< -, R 1< -Z 2< -CZ 1< - or a second moiety of the formula -S-R-SO 3 Y, wherein Z 1< is =O or =NR 2< ; Z 2< is -O-, -S- or -NR 3< -; R 1< , R 2< and R 3< independently are hydrogen or C 1-6 -alkyl, or two of R 1< , R 2< and R 3< are linked together so that X forms a mono- or bicyclic heterocycle with 5-10 ring atoms, which may be saturated or unsaturated; R is an alkylene group with 2 to 6 carbon atoms, and Y is hydrogen, ammonium, an alkali metal or 0.5 equivalents of an alkaline earth metal, or wherein the compound of formula (1) forms a zwitterion in which Y is a negative charge and one of Z 1< and Z 2< is =NR 2< H +< or - NR 3< H +< -, respectively.

[0054] Accordingly, the compound of the formula (1) is an alkylsulfonic acid or a salt thereof, in which the alkyl group has a sulfide substituent such as mercapto or a thioester group, or two mercapto groups are dimerized to form a disulfide.

[0055] In the formula (1), R preferably is propylene. Preferable examples of the compound (E) of formula (1) include the following: bis-(3-sulfopropyl)-disulfide or a salt thereof, e.g. the sodium salt:         NaO 3 S-CH 2 -CH 2 -CH 2 -S-S-CH 2 -CH 2 -CH 2 -SO 3 Na 3-mercapto-1-propanesulfonic acid or a salt thereof, e.g., the sodium salt:         H-S-CH 2 -CH 2 -CH 2 -SO 3 Na 3-(benzothiazolyl-2-mercapto)-propyl-sulfonic acid or a salt thereof, such as sodium salt 3-S-isothiuronium propyl sulfonate (zwitterion):

[0056] Among those, bis-(3-sulfopropyl)-disulfide and 3-mercapto-1-propanesulfonic acid or salt thereof or mixtures thereof are more preferable, and the sodium salts thereof are still more preferable.

[0057] In accordance with the present invention, the sulfur-containing substance (E) improves the uniformity of the deposition, especially under varying current densities, as occurring, e.g., when plating substrates of a complex shape, and also improves the quality of the obtained hard chrome layer, as described below.Formulation of the composition

[0058] The trivalent chrome plating bath composition of the present invention can be prepared by dissolving components (A) to (E) in water; components (A) to (D) can also be dissolved in advance, and component (E) can be included afterwards as an additive. The dissolution can be performed at a temperature corresponding to the envisaged operating temperature of the plating process and usually ranges from 20 to 70°C, preferably from 30 to 60°C, more preferably 40 to 60°C.

[0059] The molar ratio of (A) to the total mole number of carboxylate groups from component (C) and in given case also from component (D) preferably lies in the range of from 1:5 to 1:25, more preferably in the range of from 1:8 to 1:15.

[0060] The pH value of the bath preferably ranges from 2.5 to 6.5, more preferably from 3.0 to 6.0, even more preferably from 4.0 to 5.7 and most preferably from 4.5 to 5.5. The pH may be adjusted through the addition of a base or an acid as required. Examples for the base include sodium hydroxide, potassium hydroxide and ammonium hydroxide. Examples for the acid include sulfuric acid, methanesulfonic acid, formic acid, acetic acid and hydrochloric acid.Electroplating Process

[0061] Another aspect of the present invention relates to a chromium electroplating process using the trivalent chrome plating bath composition of the present invention as described above.

[0062] The process of the present invention is for the deposition of hard chrome layers on a metal substrate and generally comprises: Providing the above-described trivalent chrome plating bath composition of the present invention; Introducing an anode and a metal substrate to be coated as a cathode into the plating bath; Performing an electroplating to deposit a chrome layer having a thickness of >1µm on the metal substrate. Substrate

[0063] In the electroplating process of the present invention, the metal substrate to be coated is not particularly limited, and any metal article conventionally provided with hard chrome plating can be used.

[0064] Examples of the metal include steel, aluminium, nickel and alloys thereof, as well as copper alloys and zinc alloys such as brass. The process of the present invention is especially advantageous for steel substrates, since in addition to the formation of a uniform coating on articles with a complex shape, the plating bath of the invention also suppresses the undesired dissolution of iron which may occur with conventional baths and achieves a superior corrosion resistance of the plated product.

[0065] The substrate may be any metal article in need of protection against wear and / or corrosion, including tools and machine parts such as tooltips, bearings, shafts, pistons or engine elements. The process of the present invention is especially suitable for articles with a complex shape, which are difficult to coat in a uniform manner with a conventional trivalent chromium bath due to the position-dependent current variations.

[0066] The substrate may be pre-treated in a conventional manner, in order to provide a good adhesion of the electroplated layer to the substrate. Typical pre-treatment steps may comprise cleansing and degreasing processes and pickling or etching / activation processes, or electrolytic pretreatments which involve treatment steps in hydrochloric acid, sulfuric acid or fluoride containing solutions.

[0067] For steel substrates, especially for high alloyed steels, a nickel-strike coating may be applied, using a nickel-strike electrolyte (e.g., based on nickel chloride in hydrochloric acid) to apply a thin intermediate layer with a thickness of 1 µm or less, which may greatly increase the adhesion of the subsequent hard chrome plating.

[0068] It also is possible to apply interim nickel layers with higher thickness between the substrate and the hard chrome layer. The nickel layers can for example be deposited from sulfate, methanesulfonate or chloride containing electrolytes which can also be found in literature. The nickel layers can improve the adhesion, corrosion resistivity and tribological properties. The thickness of the nickel layer generally is more than 1 pm, and usually in the range of 10 to 50 µm in view of corrosion resistance.

[0069] Some known nickel sublayers include: "Watts" nickel, Multilayer Nickel, e.g., a combination of sulfur free and sulfur containing nickel layers = "double nickel", Electroless nickel, Nickel-phosphor-alloy which can be plated from electroless or electrolytic electrolytes, Nickel-sulfamate, or Porous or microcracked nickel layers.

[0070] After the plating of the hard chrome layer from the bath of the present invention, it also is possible to apply further post-treatments or finishing treatments. For example, one or more of the following chemical or electrochemical processes could be used: Chromating post treatment, Anodic or cathodic post treatment as described in US 11,214,881 B2, Applying pulse post treatment in different modes including on-off, on-on or reverse pulse as described in US 11,268,206 B2, Thin <1µm decorative chromium (SLOTOCHROM BC4130) Chrome free passivation for providing protection in non-plated areas (SLOTOPROTECT FE 1050) Anodes

[0071] The anodes used to perform the deposition are not particularly limited, and any anodes conventionally applied for trivalent chromium deposition can be used.

[0072] Preferably, the anodes are insoluble anodes, which means that they are inert and do not dissolve under the given plating conditions (as opposed to soluble anodes, which are designed to dissolve in order to supply the ions of the metal for the plating).

[0073] Preferred examples of the anodes include anodes made of graphite or MMO (mixed metal oxides from tantalum and iridium which are coated on titanium or niobium) or platinum which is coated on titanium or niobium or are membrane anodes.Plating Conditions

[0074] The plating conditions are not particularly limited, and can be suitably chosen in accordance with the conditions conventionally applied for hard chrome plating baths.

[0075] The bath temperature during the deposition preferably ranges from 20 to 70°C, more preferably from 30 to 60°C, most preferably 40 to 60°C.

[0076] The applied current density preferably ranges from 5 to 70 A / dm 2< , more preferably from 10 to 60 A / dm 2< , and most preferably from 15 to 50 A / dm 2< .

[0077] During the plating process, the bath can be continuously or intermittently be subjected to filtration and monitoring of the composition, and the concentration of the constituents and the pH value can be suitably adjusted as necessary.Hard chrome layer

[0078] A further aspect of the present invention is directed to a hard chrome layer which is characterized by a sulfur content of 1-7%, preferably 1-5% and a carbon content of 0.1-2.5%, preferably 0.1-2%, more 0.1-1%. The percentages are based on the mass of the coating layer and can be determined by Glow Discharge Optical Emission Spectroscopy (GDOES) in accordance with ISO 14707:2021.

[0079] Conventional trivalent chromium baths yield hard chrome layers comprising carbon originating from the organic bath constituents. The incorporation of carbon is an unintended and unavoidable side reaction.

[0080] Surprisingly, it has been found that the hard chrome layer of the present invention additionally comprises sulfur, which originates from the sulfur-containing component (E), and has a reduced carbon content as compared to layers obtained with a conventional bath.

[0081] It was also found that the hard chrome layer of the present invention has a very low number of cracks in comparison to layers obtained with a conventional bath, which is believed to result from the reduced carbon content. When investigated under a microscope, the chromium layer preferably has a crack density of <10 cracks / mm.

[0082] The hard chrome layer confers a surprising improvement of the tribological properties such as friction coefficient and wear resistance and improved corrosion resistance to the substrate, which both are believed to result from the improved coating quality and the reduced number of cracks. It was found that the inventive bath composition yields a hard chrome layer showing a significantly reduced coefficient of friction according to ASTM G133-22 standard in comparison to both, a conventional trivalent chromium bath and a hexavalent chromium bath.

[0083] Furthermore, it was also found that on a steel substrate, the hard chrome layer of the present invention achieves a surprising improvement of the corrosion resistance even in the absence of an interim nickel layer with a thickness of ≥1 pm, which is conventionally provided between the substrate and the hard chrome layer as discussed above.

[0084] The thickness of the hard chrome layer of the present invention is generally 1 µm or more, preferably 2 µm or more, and more preferably 5 µm or more, in view of the improvement of the tribological properties such as friction coefficient and wear resistance.

[0085] For the improvement of the corrosion resistance, the thickness preferably is 10 µm or more and more preferably 20 µm or more. If the thickness is 20 µm or more, the hard chrome layer of the present invention confers superior corrosion protection even on steel substrates which do not have an interim nickel layer of 1 µm or more.

[0086] The maximum thickness is not specifically limited and generally may be up to 100 µm or even up to 150 pm, depending on the requirements. For economic reasons, however, the thickness usually is 20 µm or less for substrates which do not require corrosion protection and 50 µm or less for substrates in need of corrosion protection.Hard chrome plated article

[0087] Another aspect of the present invention relates to a hard chrome plated article, which comprises the hard chrome layer of the present invention on a metal substrate as described above.

[0088] One advantageous aspect of the present invention is the possibility to achieve superior corrosion protection on a steel substrate without requiring an interim nickel layer of ≥1µm below the hard chrome layer, as conventionally applied in the state of the art.

[0089] Thus, in a preferred embodiment, the hard chrome plated article comprises a steel substrate plated with the above-described hard chrome layer of the present invention with a thickness of 20 µm or more, which does not have an interim nickel layer with a thickness of ≥1µm between the substrate and the hard chrome layer, and which has a corrosion resistance of <5% base metal (steel substrate) corrosion after 24 h in SST according to DIN EN ISO 9227 NSS.Examples

[0090] The present invention will be explained in more detail by means of the following examples, although it is not limited to such examples. Through the examples, the throwing power of the inventive bath, as well as the obtained metal distribution of the plated chromium layer were investigated. The throwing power was measured by Hull cell test panels and the metal distribution of the plated chromium layer was investigated through practical parts.Example 1 (Reference)

[0091] To examine the "throwing power" of the inventive bath, Hull cell tests were conducted according to DIN 50957-1. In a Hull cell as specified in DIN 50957-1, the cathode is a metal panel arranged at an inclined angle relative to the anode, so that there is a proximal edge which has the smallest distance to the anode and thus is exposed to the highest local current density, and a distal edge having a larger distance to the anode and thus being exposed to a lower local current density. Accordingly, at a fixed average cathodic current density, the plating thickness will be highest at the proximal edge and lowest at the distal edge.

[0092] When the plating conditions (including current density, chromium amount, temperature and coating time) are selected such that the proximal edge is plated and the distal edge remains uncoated, then there will be a transition line between these edges, which separates the plated and the uncoated region. The "throwing power" is measured as the distance x between the proximal edge and the transition line.

[0093] In order to evaluate the effect of component (E) on the throwing power, it is firstly required to provide a standard bath comprising components (A) to (D) serving as a reference, and to suitably adjust the test conditions (current density, time and temperature) such that the transition line for the standard bath is approximately in the middle of the test panel, which corresponds to a throwing power of about 5.5 cm. Subsequently, it can be assessed how the throwing power changes when component (E) is added. The standard bath contained 259 g / l ammonium formiate, 20 g / l chromium obtained from 250 g basic chromium sulfate solution [8.5% Cr(III), 32% Cr 2 (SO 4 ) 3 ] and 21 g / l sodium bromide. For making up the standard bath, all salts were mixed with DI water and then heated for about 2 hours at 60°C. In the next step, the temperature was adjusted to 56°C and the pH was adjusted to 5.1 with aqueous ammonium hydroxide solution (25%). The bath composition in terms of the constituent ions is summarized in Table 1. Table 1. Composition of standard bath.(A)Chromium(III)20.00 g / l(B)Sulfate55.42 g / lBromide16.31 g / l(C)Formiate181.91 g / l(D)Ammonium81.1 g / l**74.1 g / l from NH 4 HCOO and 7.0 g / l from NH 4 OH

[0094] In the Hull cell tests, the temperature and pH of the bath were 56 °C and 5.1 respectively. Polished brass panels (7,5×10 cm 2< ) were used as cathode. For anode, meshed type MMO was used. The Hull cell tests were conducted according to DIN 50957 part 1. The test conditions are summarized in Table 2. Table 2. Conditions for plating with standard bath.pH5.1Current7.5 ATime3 minTemperature56 °CAnodeMMO (Mixed Metal Oxide)

[0095] The standard bath under the given test conditions achieved a throwing power of 5.5 cm, as can be seen from the respective Hull cell test panel shown in Figure 1.Example 2

[0096] Different types of compound (E) were added to the standard bath at different addition levels, and the throwing power was determined in the same way as described for Example 1. The respective bath formulations and the throwing power are shown in Table 3, wherein the "initial throwing power" refers to the result for the standard bath without additive serving as a reference, and the "final throwing power" refers to the bath including the additive (E). Table 3. Hull cell tests results.AdditiveConcentration (g / l)Initial throwing power (cm)Final throwing power (cm)Standard bath (without additive)--5.5Bis-(sodium sulfopropyl)-disulfide0.55.58.33-Mercapto-1-propanesulfonic acid, sodium salt0.35.58.23-(Benzothiazolyl-2-mercapto)-propyl-sulfunic acid, sodium salt0.55.56.5(O-Ethyldithiocarbonato)-s-(3-sulfopropyl)-ester, potassium salt0.055.56.5

[0097] As can be seen, an increase of the throwing power was achieved with all additives (E). The largest increase was found for bis-(sodium sulfopropyl)-disulfide and 3-mercapto-1-propanesulfonic acid, sodium salt, which increased the throwing power up to 8.3 and 8.2 cm, respectively, and also yielded a coated product with the best appearance.Example 3 (Reference)

[0098] The standard bath described in Example 1 was used to apply a chromium layer on the surface of a flat sheet made of Ni (25×30 cm 2< ). The flat sheet was used as cathode and a MMO mesh type anode was used. After pre-treatment including degreasing, activation and Ni strike, the part was plated in the standard chromium bath.

[0099] The temperature and pH of the bath were adjusted to 56 °C and 5.1, respectively. During the plating time the bath was filtered continuously. The current density was 22.5 A / dm 2< and the plating time was 1 hour. The thickness distribution of the plated chromium layer is shown in Figure 2 and schematically represented in 3D in Figure 3.

[0100] As the data in Figure 2 shows, the thickness of the plated chromium layer was about 0.01 µm at the center of the flat sheet, and the thickness reached a maximum of about 9.12 µm at the edges. Thus, the ratio of maximum to minimum thickness is about 912. Thus, it is evident that plating a uniform chromium layer on the surface of a sample with flat geometry would be challenging when using a standard bath and extra equipment would be required to manage the issue.Example 4

[0101] The plating of a flat sheet made of Ni (25×30 cm 2< ) was performed in the same way as described in Example 3, except that 0.5 g / l 3-mercapto-1-propanesulfonic acid, sodium salt were added to the standard bath. The thickness distribution of the plated chromium layer is shown in Figure 4 and schematically represented in 3D in Figure 5.

[0102] As can be seen, the thickness of the chrome layer is higher than 3.4 µm in all areas, and there are no regions with a thickness of less than 1 pm, as it was the case for the middle region in Example 3. Besides, the ratio of the highest and the lowest thickness value is merely about 1.66, which is much lower than the ratio of 912 in Example 3.

[0103] This indicates that an improvement in uniformity of the thickness of the plated chromium layer can be achieved by modifying the standard bath. Furthermore, since the chromium layer is plated uniformly on the entire part, a waste of chromium is greatly reduced when using the modified bath, rather than the standard bath.Example 5

[0104] To evaluate the throwing power and metal distribution achieved for chromium layers plated on substrates with a complex shape, tests were conducted on a shaped steel part developed by Dr. Ing. Max. Schlötter GmbH, as shown in Figure 6 (in the following referred to as "Schlötter part").

[0105] The working area of the part was 2 dm 2< and contained bent areas with holes by which distribution of the deposited layer in different current density areas could be evaluated. For doing so, two set-ups of 80 liters of the bath were prepared.

[0106] The first setup comprised the unmodified standard bath. In the second setup, the standard bath was modified by adding 1 g / l of 3-mercapto-1-propanesulfonic acid, sodium salt was added. The pH and working temperature of the baths were 5.1 and 56 °C, respectively. Also, the baths were filtered continuously. The schematic of the set-up is presented in Figure 7.

[0107] The plating time was 30 minutes and two different current densities, 20 and 40 A / dm 2< , were applied to the parts. After plating, the thickness of the plated chromium layer was measured at the points indicated in Figure 8. The measurement results are shown in Figure 9.

[0108] As can be seen, the unmodified standard bath at 20 A / dm 2< leads to a maximum thickness of more than 12 µm at point 1, and a thickness of less than 1 µm at points 2, 3 and 4. The ratio between the highest and the lowest value was found to be as high as 36.86. At 40 A / dm 2< , there are no more points with a thickness below 1 pm, but the unifomity still is poor, with a ratio of the highest and the lowest value being 4.66.

[0109] The modified bath according to the present invention achieves a much more uniform distribution even at both 20 A / dm 2< and 40 A / dm 2< , the ratio between highest and lowest thickness being 2.75 and 2.41, respectively, and no points with a coating thickness of less than 1 µm can be found.

[0110] This confirms the improved distribution of the plated chromium layer, especially when employing a lower current density plating mode. It can also be seen that the effect of the applied current density on the metal distribution of the plated chromium layer is much more pronounced when using the standard bath. In fact, the metal distribution of the plated chromium layer remains almost the same when increasing the current density in the case of the modified bath.Example 6

[0111] A "Schlötter part" was coated in the same way as in Example 5, except that the standard bath was modified by adding 1.7 g / l 3-mercapto-1-propanesulfonic acid, and the coating time was 2 hours at a current density and pH of 40 A / dm 2< and 5.1, respectively.

[0112] The thickness distribution of the plated chromium layer is shown in Figure 10. As can be seen, the distribution is highly uniform, and the ratio of maximum to minimum thickness of the plating layer was found to be 1.11. This confirms the uniformity of the deposited layer obtained when using the inventive bath.Example 7

[0113] A first bath (reference) was prepared using a chromium(III)-methanesulfonate solution (50 wt.%) comprising 7.6 wt.% Cr(III) and having a density of 1.41 g / cm 3< . The chromium concentration in the bath was 20.51 g / l. The composition or the first bath is shown in Table 4. Table 4. Composition of the reference bath (reference).Ammonium formate259 g / LAmmonium acetate2.5 g / lChromium(III)-methanesulfonate solution (50wt.%)263 g / lSodium bromide21 g / LAmmonium chloride25 g / lBoric acid50 g / l

[0114] Furthermore, a second bath in accordance with the present invention was prepared, which otherwise had the same composition as the reference bath, except that bis-(sodium sulfopropyl)-disulfide and 3-Mercapto-1-propanesulfonic acid, sodium salt were added as component (E). The composition of the second bath is shown in Table 5. Table 5. Composition of the second bath (invention).Ammonium formate259 g / LAmmonium acetate2.5 g / lChromium(III)-methanesulfonate solution (50%)263 g / lPotassium bromide21 g / LAmmonium chloride25 g / lBoric acid50 g / lBis-(sodium sulfopropyl)-disulfide0.5 g / l3-Mercapto-1-propanesulfonic acid, sodium salt0.5 g / l

[0115] For making up the baths, all salts were mixed with DI water and then heated for about 2 hours at 60°C. In the next step, the temperature was adjusted to 56°C and the pH was adjusted to 5.1 with aqueous ammonium hydroxide solution (25%).

[0116] In a first test, the "initial state" Hull cell was prepared and the throwing power on the Hull cell panel was measured for the first bath (reference). In a second test, the throwing power for the second bath (invention) was determined as the final throwing power.

[0117] Throughout the tests the temperature and pH of the bath were 56 °C and 5.1, respectively. A polished brass panel (7.5×10 cm 2< ) was used as cathode. Graphite was used as anode. The Hull cell tests were conducted according to DIN 50957 part 1 and the test result is shown in Table 6. Table 6. Throwing power result.Initial throwing power (cm)Final throwing power (cm)5.68.5 Example 8

[0118] To evaluate corrosion resistance, 1 cm in diameter steel rods (composition according to production code S235JR (1.0038)) were plated with trivalent chromium. Within the layer composition, no Ni-containing sublayer was deposited, only a chromium layer was deposited on the steel surface.

[0119] Before the plating, the rods were cleaned and pre-treated in accordance with a standard pre-treatment process for steel as described in Table 7. Table 7. Pre-treatment procedure for steel.Stage Process Bath composition Parameter 1Hot degreasing- Deionized water- Temperature: 65 °C- Degreaser salt SLOTOCLEAN AK 161: 30 g / l-Wetting agent additive RV111: 10 ml / l- Treatment time: 10 min2Pickling-Deionized water- Temperature: 25 °C- Pickle degreaser additive SLOTOCLEAN BEF 30: 40 ml / l-Hydrochloric acid, conc., (36%): 50% vol- Treatment time: 10 min3Anodic degreasing- Deionized water- Temperature: 40 °C- Treatment time: 3 min- Degreaser salt SLOTOCLEAN EL DCG: 120 g / l- Current density: 6 A / dm 2< 4Activation-Deionized water- Temperature: 25 °C-Hydrochloric acid, conc., (36%): 10% vol- Treatment time: 2 min

[0120] SLOTOCLEAN AK 161, SLOTOCLEAN BEF 30 and SLOTOCLEAN EL DCG are products of Dr.-Ing. Max Schlötter GmbH & Co. KG.

[0121] After the pre-treatment, the steel rods were placed into a plating tank, in the middle between the anodes. The working area of the part was 0.2 dm 2< . The volume of the bath set-up was 80 liters. The plating was performed with the unmodified standard bath as described in Example 1 serving as a reference, or with a modified bath, to which 1 g / l of 3-mercapto-1-propanesulfonic acid sodium salt was added.

[0122] The pH and working temperature of the baths were 5.1 and 56 °C, respectively. Also, the baths were filtered continuously. The schematic of the set-up is shown in Figure 7.

[0123] The applied current density was 40 A / dm 2< . The plating time was 30 minutes in the standard bath without 3-mercapto-1-propanesulfonic acid, sodium salt and 120 minutes in the modified bath with 1 g / l of 3-mercapto-1-propanesulfonic acid sodium salt. The different plating times were necessary to obtain plating layers of approximately the same thickness of about 20 µm at the given current density.

[0124] After plating, the properties of the chromium layer were examined under the microscope using top and cross section views. The top view of the deposited layers showed >10 cracks / mm for the layer prepared using the standard bath and <10 cracks / mm for the layer prepared using the modified bath. The results are shown in Figure 11.

[0125] In addition, the corrosion resistivity was examined with a salt spray test according to DIN EN ISO 9227 NSS. The sample prepared using the standard bath showed significant corrosion (more than 40%) after 24 h in the salt spray test, whereas the sample prepared using the modified bath showed almost no corrosion (0-5%) after 24 h in the salt spray test. The results are shown in Figure 12.Example 9

[0126] In the following example, the dissolution of iron from a steel substrate exposed to the plating bath for a prolonged period of time is determined.

[0127] The unmodified standard bath of Example 1 served as a reference, and a modified bath was prepared by adding 1g / l of 3-mercapto-1-propanesulfonic acid, sodium salt to the standard bath.

[0128] Additionally, two steel panels of each 0.1 dm 2< made of DC04 steel was degreased with isopropanol and weighted. The weight of the steel panel No. 1 was 1.1779 g, the weight of the steel panel No.2 was 1.1791 g.

[0129] Steel panel No. 1 was placed into a polypropylene bottle filled with 200 ml of the unmodified standard bath, whereas steel panel No. 2 was placed into a polypropylene bottle filled with 200 ml of the modified bath.

[0130] The lids of the bottles were closed, and the bottles were then stored for 20 h at 50°C in an oven. Then, the steel panels were removed from the bottles, rinsed with water and dried with pressurized air. Finally, the steel panels were weighted a second time.

[0131] Panel No. 1, which was in the standard bath had a weight of 1,0589g, which means that the weight loss after 20 h storage at 50°C was 0,119 g. Panel No. 1 also showed a dark grey discoloration, as shown in Figure 13.

[0132] Panel No. 2, which was in the modified bath including 3-mercapto-1-propanesulfonic acid, sodium salt, had a weight of 1,1683 g, which means that the weight loss after 20 h storage at 50°C was only 0,0108g. Accordingly, with the modified bath comprising 3-mercapto-1-propanesulfonic acid, sodium salt according to the present invention, the iron dissolution by the bath was only 9,1% of the dissolution of the standard bath. Furthermore, panel No. 2 did not show any discoloration, as shown in Figure 14.Example 10

[0133] In the following example, the composition of the chromium layer is investigated by Glow Discharge Optical Emission Spectroscopy (GDOES). The unmodified standard bath of Example 1 served as a reference, and two modified baths according to the invention were 0.5 g / l and 1 g / l of 3-mercapto-1-propanesulfonic acid, sodium salt as component (E) to the standard bath.

[0134] The chromium layer was deposited on polished brass panels. Before the plating, the panels were cleansed and pre-treated in accordance with a standard pre-treatment process for brass as summarized in the following Table 8. Table 8. Pre-treatment procedure for brass.Stage Process Bath composition Parameter 1Hot degreasing- Deionized water- Temperature: 65 °C- Degreaser salt SLOTOCLEAN AK 341: 30 g / l- Treatment time: 10 min- Wetting agent additive RV111: 10 ml / l2cathodic degreasing- Deionized water- Temperature: 40 °C- Degreaser salt SLOTOCLEAN EL 130: 50 g / l- Treatment time: 1 min- Current density: 6 A / dm 2< 3Activation-Deionized water- Temperature: 25 °C-Sulfuric acid, conc., (96%): 2,5% vol- Treatment time: 1 min

[0135] SLOTOCLEAN AK 341, Wetting additive RV111 and SLOTOCLEAN EL 130 are products of Dr.-Ing. Max Schlötter GmbH & Co. KG.

[0136] The pH and working temperature of the baths were 5.1 and 56 °C, respectively. Also, the baths were filtered continuously. The schematic of the set-up is the same as shown in Figure 7.

[0137] The applied current density was 40 A / dm 2< . The plating time was 30 minutes in the standard bath without 3-mercapto-1-propanesulfonic acid, sodium salt and 60 minutes in the modified bath with 0.5 or 1 g / l of 3-mercapto-1-propanesulfonic acid sodium salt.

[0138] Because of the lower coating uniformity, on the other hand, the reference bath yields regions with high thickness even at short coating times, which could be used for the investigation. The modified baths of the present invention, on the other hand, yield a more uniform coating thickness regardless of the current density variations, hence a longer coating time was used to obtain an appropriate thickness of the investigated region.

[0139] The coated panels were investigated by means of GDOES. The results are shown in Figures 15 (Reference), 16 (0.5 g / l) and 17 (1 g / l), respectively. The x-axis refers to the depth direction, wherein the origin corresponds to the surface of the coating layer. The y-axis refers to the concentration of the elements, including Cr (coating), Cu and Zn (substrate), C, S and O. As also indicated in the legend to the Figures, the scaling of the y-axis is 100% for the metals (Cr, Zn, Cu), 10% for O and 5% for C and S.

[0140] The layer thickness was more than 10 µm for each panel. As can be seen in Figure 15, the reference coating exhibited a high incorporation of carbon of about 3%. For the coatings according to the present invention, on the other hand, Figures 16 and 17 show the presence of about 3% of sulfur, but the incorporated carbon amount is substantially reduced to about 0.5%. The precise values at a depth of 5 µm are also shown in Table 9 below. Table 9. The GDOES analysis results.Standard bath (Reference)Bath with 0,5 g / l of 3-mercapto-1-propanesulfonic acid sodium saltBath with 1,0 g / l of 3-mercapto-1-propanesulfonic acid sodium saltSulfur content0.0%3.23%3.449%Carbon content2.985%0.634%0.55%

[0141] The results show that the addition of 3-mercapto-1-propanesulphonic acid sodium salt increases the sulfur content of the coating and at the same time reduces the incorporation of carbon.Example 11

[0142] In the following example, the friction coefficient of the chromium layer is investigated. The unmodified standard bath of Example 1 served as a reference, and two modified baths according to the invention were 0.5 g / l and 1 g / l of 3-mercapto-1-propanesulfonic acid, sodium salt as component (E) to the standard bath. As a further reference, a chromium(VI)-based hard chrome plating bath was used (SLOTOCHROM S, available from Dr.-Ing. Max Schlötter GmbH & Co. KG.).

[0143] The test was performed with 1 × 5 cm DC04 steel panels, which were cleansed and activated in the same way as described in Example 9 (see Table 7). The chromium layers was deposited directly onto the steel surface, without an interim Ni-containing sublayer. After the pre-treatment, the steel panels were placed into a plating tank, in the middle between the anodes. The working area of the part was 0.1 dm 2< . The volume of the bath set-up was 80 liters. The pH and working temperature of the baths were 5.1 and 56 °C, respectively. Also, the baths were filtered continuously. The schematic of the set-up is the same as shown in Figure 7.

[0144] The applied current density was 40 A / dm 2< . The plating time was 15 minutes in the standard bath without 3-mercapto-1-propanesulfonic acid, sodium salt and in the SLOTOCHROM S bath and 60 minutes in the modified bath with 0.5 or 1.0 g / l of 3-mercapto-1-propanesulfonic acid sodium salt. The different plating times were necessary to obtain plating layers of approximately the same thickness of about 10 µm at the given current density.

[0145] The coefficient of friction was tested according to ASTM G133-22 "Linearly Reciprocating Ball-on-Flat Sliding Wear" against a counter material which was a ball of 3 mm diameter made of tungsten carbide. The normal force for the test was 5 N and the friction was tested for 1000 cycles of back-and-forth movement with a stroke length of 2mm, back-and-forth one cycle equals 4mm. 1000 cycles equal a combined stroke length of 4 m. The frequency of the oscillation was 0,2 Hz, for a test duration of 5000 s. Further test conditions were as followed: Temperature: 25°C, air humidity: 55%, no lubrication was applied. All samples were tested simultaneously on a friction test stand according to ASTM G133-22 (WECO-X by iChemAnalytics GmbH). The results are shown in Figure 18, and the numerical value of the coefficient of friction at 100 cycles is also shown in Table 10below: Table 10. Coefficient of friction values after 100 cycles.Standard bathBath with 0.5 g / lof 3-mercapto-1-propanesulfonic acid sodium saltBath with 1.0 g / lof 3-mercapto-1-propanesulfonic acid sodium saltReference SLOTOCHROM S0.310.220.200.29

[0146] As can be seen, the trivalent chromium baths comprising 3-mercapto-1-propanesulfonic acid yielded hard chrome layers with significantly lower coefficients of friction as compared to both, the trivalent chromium standard bath and the hexavalent chromium bath SLOTOCHROM S. A lower coefficient of friction can be beneficial for wear applications.Example 12

[0147] In the following Example, the use of component (E) in an ammonia- and borate-free plating bath was investigated by means of Hull cell tests in accordance with DIN 50957-1. A reference bath comprising sulfate, formate and glycine as components (B), (C) and (D), respectively, was formulated by mixing the components as shown in Table 11 below. Table 11. Composition of the Boric and Ammonium free bath.Chromium(III) potassium sulfate dodecahydrate384.53 g / lFormic acid 98%69.045 g / lGlycine75.07 g / l

[0148] The bath contained 384.53 g / l Chromium(III) potassium sulfate dodecahydrates by which 40 g / l chromium was obtained. For making up the bath, all salts were mixed with DI water and then heated for about 2 hours at 60°C. Then pH was adjusted to 3.8 with potassium hydroxide. In the next step, the temperature was adjusted to 30°C.

[0149] In the Hull cell tests, the temperature and pH of the bath were 30°C and 3.8 respectively. Polished brass panels (7,5×10 cm 2< ) were used as cathode. For anode, meshed type MMO was used. The Hull cell tests were conducted according to DIN 50957 part 1. The test conditions are summarized in Table 12. Table 12. Conditions for plating with standard bath.pH3.8Current1.0 ATime10 minTemperature30 °CAnodeMMO (Mixed Metal Oxide)

[0150] The reference bath under the given test conditions achieved a throwing power of 3.1 cm.

[0151] Furthermore, a second bath in accordance with the present invention was prepared, which otherwise had the same composition as the reference bath, except that 3-Mercapto-1-propanesulfonic acid, sodium salt was added as component (E). The composition of the second bath is shown in Table 13. Table 13. Composition of the Boric and Ammonium free bath.Chromium(III) potassium sulfate dodecahydrate384.53 g / lFormic acid 98%69.045 g / lGlycine75.07 g / l3-Mercapto-1-propanesulfonic acid, sodium salt1 g / l

[0152] The makeup and test condition were in the same way as reference bath. The throwing power was improved up to 5.2 cm. This confirms that component (E) also is useful for borate- and ammonia-free baths.

Examples

example 1 (

Example 1 (Reference)

[0091]To examine the "throwing power" of the inventive bath, Hull cell tests were conducted according to DIN 50957-1. In a Hull cell as specified in DIN 50957-1, the cathode is a metal panel arranged at an inclined angle relative to the anode, so that there is a proximal edge which has the smallest distance to the anode and thus is exposed to the highest local current density, and a distal edge having a larger distance to the anode and thus being exposed to a lower local current density. Accordingly, at a fixed average cathodic current density, the plating thickness will be highest at the proximal edge and lowest at the distal edge.

[0092]When the plating conditions (including current density, chromium amount, temperature and coating time) are selected such that the proximal edge is plated and the distal edge remains uncoated, then there will be a transition line between these edges, which separates the plated and the uncoated region. The "throwing power" is meas...

example 10

[0133]In the following example, the composition of the chromium layer is investigated by Glow Discharge Optical Emission Spectroscopy (GDOES). The unmodified standard bath of Example 1 served as a reference, and two modified baths according to the invention were 0.5 g / l and 1 g / l of 3-mercapto-1-propanesulfonic acid, sodium salt as component (E) to the standard bath.

[0134]The chromium layer was deposited on polished brass panels. Before the plating, the panels were cleansed and pre-treated in accordance with a standard pre-treatment process for brass as summarized in the following Table 8.

Table 8. Pre-treatment procedure for brass.

Stage Process Bath composition Parameter

1Hot degreasing- Deionized water- Temperature: 65 °C

- Degreaser salt SLOTOCLEAN AK 341: 30 g / l

- Treatment time: 10 min

- Wetting agent additive RV111: 10 ml / l

2cathodic degreasing- Deionized water- Temperature: 40 °C

- Degreaser salt SLOTOCLEAN EL 130: 50 g / l- Treatment time: 1 min

- Current density: 6 A / dm 2

3Activatio...

Claims

1. Use of a trivalent chrome plating bath composition for the electrodeposition of a hard chrome layer with a thickness of 1 µm or more, the composition comprising: (A) trivalent chromium ions in an amount of 5-40 g / l, (B) at least one anion selected from sulfate, chloride, bromide and methanesulfonate, (C) at least one carboxylate ion selected from formate and acetate, and (D) at least one buffer substance selected from ammonia, boric acid, carboxylic acids with 2 to 8 carbon atoms other than component (C) and salts thereof, and aluminium salts, and (E) 0.01 to 100 g / l of at least one compound of the following formula (1):         X-S-R-SO3Y     (1) wherein: X represents H, C1-6-alkyl, R1-CZ1-, R1-Z2-CZ1- or a second moiety of the formula -S-R-SO3Y, wherein Z1 is =O or =NR2; Z2 is -O-, -S- or -NR3-; R1, R2 and R3 independently are hydrogen or C1-6-alkyl, or two of R1, R2 and R3 are linked together so that X forms a mono- or bicyclic heterocycle with 5-10 ring atoms, which may be saturated or unsaturated; R is an alkylene group with 2 to 6 carbon atoms, Y is hydrogen, ammonium, an alkali metal or 0.5 equivalents of an alkaline earth metal; or the compound of formula (1) forms a zwitterion in which Y is a negative charge and one of Z1 and Z2 is =NR2H+ or -NR3H+-, respectively.

2. The use according to claim 1, wherein component (E) is one or more selected from the group consisting of - bis-(sodium sulfopropyl)-disulfide         NaO3S-CH2-CH2-CH2-S-S-CH2-CH2-CH2-SO3Na - 3-mercapto-1-propanesulfonic acid sodium salt         H-S-CH2-CH2-CH2-SO3Na - 3-(benzothiazolyl-2-mercapto)-propyl-sulfonic acid sodium salt - 3-S-isothiuronium propyl sulfonate 3. The use according to claim 2, wherein component (E) is bis-(sodium sulfopropyl)-disulfide or 3-mercapto-1-propanesulfonic acid or salt thereof or mixtures thereof, and is comprised in a concentration of 0.1 to 10 g / l, preferably 0.2 to 5 g / l.

4. The use according to any one of claims 1 to 3, wherein component (D) is at least one of ammonia, boric acid, glycine and salts thereof.

5. The use according to any one of claims 1 to 4, wherein the total concentration of the anion (B) in the composition is from 50 to 500 g / l, more preferably in the range of from 100 to 450 g / l, even more preferably in the range of from 150 to 400 g / l, and still more preferably in the range of from 200 to 400 g / l.

6. The use according to any one of claims 1 to 5, wherein the total concentration of the carboxylate ion (C) in the composition is 30 to 400 g / l, preferably 50 to 350 g / l and more preferably in the range of from 60 to 300 g / l.

7. The use according to any one of claims 1 to 6, wherein the total concentration of the buffer (D) in the composition is from 15 to 300 g / l, preferably in the range of from 25 to 250 g / l, even more preferably in the range of from 50 to 200 g / l in terms of ammonium ions and boric acid, respectively.

8. The use according to any one of claims 1 to 7, wherein the composition has a pH of 2.5 to 6.5, preferably 3.0 to 6.0, more preferably from 4.0 to 5.7 and still more preferably from 4.5 to 5.5.

9. A chrome electroplating process, comprising: - Providing the trivalent chrome plating bath composition according to any one of claims 1 to 8 as a plating bath; - Introducing an anode and a metal substrate to be coated as a cathode into the plating bath; - Performing an electroplating to deposit a chrome layer having a thickness of >1µm on the metal substrate.

10. The chrome electroplating process according to claim 9, wherein the substrate is made of a metal selected from steel, aluminium, nickel or alloys thereof, copper alloys and zinc alloys.

11. Hard chrome layer having a thickness of 1 µm or more, which has a sulfur content of 1 - 7.5 mass%, preferably 1 - 5 mass%, and a carbon content of 0.1-2.5 mass%, preferably 0.1-2 mass%, more preferably 0.1-1 mass%, as measured by Glow Discharge Optical Emission Spectroscopy (GDOES).

12. Hard chrome layer according to claim 11, which has a crack density of less than 10 cracks / mm.

13. Hard chrome plated article, which comprises the hard chrome layer according to claim 11 or 12 plated on a metal substrate.

14. The article according to claim 13, comprising a steel substrate plated with the hard chrome layer with a thickness of 20 µm or more, which does not have an interim nickel layer with a thickness of ≥1µm between the substrate and the hard chrome layer, and which has a corrosion resistance of <5% base metal (steel substrate) corrosion after 24 h in SST according to DIN EN ISO 9227 NSS.

15. Use of a compound (E) of the following formula (1) as an additive for a trivalent chrome plating bath for the improvement of the uniformity of deposition:         X-S-R-SO3Y     (1) wherein: X represents H, C1-6-alkyl, R1-CZ1-, R1-Z2-CZ1- or a second moiety of the formula -S-R-SO3Y, wherein Z1 is =O or =NR2; Z2 is -O-, -S- or -NR3-; R1, R2 and R3 independently are hydrogen or C1-6-alkyl, or two of R1, R2 and R3 are linked together so that X forms a mono- or bicyclic heterocycle with 5-10 ring atoms, which may be saturated or unsaturated; R is an alkylene group with 2 to 6 carbon atoms, Y is hydrogen, ammonium, an alkali metal or 0.5 equivalents of an alkaline earth metal; or the compound of formula (1) forms a zwitterion in which Y is a negative charge and one of Z1 and Z2 is =NR2H+ or -NR3H+-, respectively.

Images