Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/04—Electroplating: Baths therefor from solutions of chromium
  • C25D3/06—Electroplating: Baths therefor from solutions of chromium from solutions of trivalent chromium
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/04—Electroplating: Baths therefor from solutions of chromium
  • C25D3/10—Electroplating: Baths therefor from solutions of chromium characterised by the organic bath constituents used
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces

Definitions

• the present invention relates to the field of plating chromium layers of thickness >1 ⁇ m, 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.
• the thickness of bright chrome generally is ⁇ 1 ⁇ m.
• sanitary items e.g., shower heads
• decorative automotive parts e.g., chrome plated mouldings
• consumer goods e.g., housings for electrical shavers.
• US 10,006,135 B2 describes an electrolyte for dark decorative chrome layers, which comprises a certain sulfur compound (D) as a colorant.
• Hard chrome layers generally are applied in a thickness of >1 ⁇ m. 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.
• thick chromium 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 ⁇ m 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.
• 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.
• 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.
• 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.
• 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.
• 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 ⁇ m or more, the composition comprising:
• 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 ⁇ m 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.
• 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.
• 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.
• 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".
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• a chromium sublayer which exhibits a crack-free, highly corrosion resistant structure
• a top layer exhibiting a crack-containing chromium layer.
• the crack-free sublayer in this example ensures the corrosion performance of the layer composition.
• the trivalent chrome plating bath composition used in the present invention comprises:
• the plating bath according to the invention comprises trivalent chromium ions in an amount of 5-40 g/l.
• concentration of trivalent chromium ions lies in the range of 10-30 g/l, more preferably 15-25 g/l.
• 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.
• chromium(III) salts such as chromium(III) sulfate, basic chromium(III) sulfate, chromium(III) methane
• the chromium(III) salt also can at least partially contribute the anion (B) and/or the carboxylate ion (C).
• component (A) 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.
• the plating bath according to the invention comprises at least one anion (B) selected from sulfate, chloride, bromide and methanesulfonate.
• 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).
• 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.
• the ammonium salts simultaneously can supply at least a part of the buffer component (D).
• 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.
• 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.
• the inventive plating bath according to the invention comprises at least one carboxylate selected from formate and acetate.
• 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).
• C carboxylate ion
• ammonium formate, ammonium acetate, sodium acetate, and potassium acetate are preferred, and ammonium formate and ammonium acetate are more preferred.
• 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).
• 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.
• 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.
• 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.
• D 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.
• boric acid ammonia and salts thereof are preferable.
• the salts include sodium borate, ammonium chloride, ammonium formate or ammonium acetate.
• 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.
• carboxylic acid examples 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.
• glycine and salts thereof are preferable.
• 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.
• Aluminium salts likewise can be used as buffer substance (D), preferably in the form of aluminium chloride or aluminium sulfate.
• the counter ion may correspond to component (B), (C) or (D), respectively.
• ammonium borate and ammonium salts of the aforementioned carboxylic acids may be mentioned.
• 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.
• 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)
• 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
• 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.
• R preferably is propylene.
• R preferably is propylene.
• Preferable examples of the compound (E) of formula (1) include the following:
• 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.
• 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.
• 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.
• 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.
• 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.
• the base include sodium hydroxide, potassium hydroxide and ammonium hydroxide.
• the acid include sulfuric acid, methanesulfonic acid, formic acid, acetic acid and hydrochloric acid.
• 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.
• the process of the present invention is for the deposition of hard chrome layers on a metal substrate and generally comprises:
• the metal substrate to be coated is not particularly limited, and any metal article conventionally provided with hard chrome plating can be used.
• the metal examples 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.
• 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.
• 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.
• 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 ⁇ m or less, which may greatly increase the adhesion of the subsequent hard chrome plating.
• a nickel-strike electrolyte e.g., based on nickel chloride in hydrochloric acid
• 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 ⁇ m in view of corrosion resistance.
• Some known nickel sublayers include:
• the anodes used to perform the deposition are not particularly limited, and any anodes conventionally applied for trivalent chromium deposition can be used.
• 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).
• 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.
• MMO mixed metal oxides from tantalum and iridium which are coated on titanium or niobium
• platinum which is coated on titanium or niobium or are membrane anodes.
• the plating conditions are not particularly limited, and can be suitably chosen in accordance with the conditions conventionally applied for hard chrome plating baths.
• 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.
• 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 .
• 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.
• 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.
• 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.
• 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.
• the chromium layer preferably has a crack density of ⁇ 10 cracks/mm.
• 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.
• 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.
• the thickness of the hard chrome layer of the present invention is generally 1 ⁇ m or more, preferably 2 ⁇ m or more, and more preferably 5 ⁇ m or more, in view of the improvement of the tribological properties such as friction coefficient and wear resistance.
• the thickness preferably is 10 ⁇ m or more and more preferably 20 ⁇ m or more. If the thickness is 20 ⁇ m 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 ⁇ m or more.
• the maximum thickness is not specifically limited and generally may be up to 100 ⁇ m or even up to 150 pm, depending on the requirements. For economic reasons, however, the thickness usually is 20 ⁇ m or less for substrates which do not require corrosion protection and 50 ⁇ m or less for substrates in need of corrosion protection.
• 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.
• 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 ⁇ 1 ⁇ m below the hard chrome layer, as conventionally applied in the state of the art.
• 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 ⁇ m or more, which does not have an interim nickel layer with a thickness of ⁇ 1 ⁇ m 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.
• the present invention will be explained in more detail by means of the following examples, although it is not limited to such 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.
• 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.
• the plating conditions including current density, chromium amount, temperature and coating time
• the proximal edge is plated and the distal edge remains uncoated
• 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.
• component (E) 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.
• 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 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.
• 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 .
• the thickness of the plated chromium layer was about 0.01 ⁇ m at the center of the flat sheet, and the thickness reached a maximum of about 9.12 ⁇ m at the edges.
• the ratio of maximum to minimum thickness is about 912.
• 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 .
• the thickness of the chrome layer is higher than 3.4 ⁇ m 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.
• 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.
• 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.
• the first setup comprised the unmodified standard bath.
• 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 .
• 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 .
• the unmodified standard bath at 20 A/dm 2 leads to a maximum thickness of more than 12 ⁇ m at point 1, and a thickness of less than 1 ⁇ m 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.
• 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 ⁇ m can be found.
• 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.
• 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.
• a first bath 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 formate 259 g/L Ammonium acetate 2.5 g/l Chromium(III)-methanesulfonate solution (50wt.%) 263 g/l Sodium bromide 21 g/L Ammonium chloride 25 g/l Boric acid 50 g/l
• 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 formate 259 g/L Ammonium acetate 2.5 g/l Chromium(III)-methanesulfonate solution (50%) 263 g/l Potassium bromide 21 g/L Ammonium chloride 25 g/l Boric acid 50 g/l Bis-(sodium sulfopropyl)-disulfide 0.5 g/l 3-Mercapto-1-propanesulfonic acid, sodium salt 0.5 g/l
• the "initial state" Hull cell was prepared and the throwing power on the Hull cell panel was measured for the first bath (reference).
• the throwing power for the second bath (invention) was determined as the final throwing power.
• SLOTOCLEAN AK 161, SLOTOCLEAN BEF 30 and SLOTOCLEAN EL DCG are products of Dr.-Ing. Max Schlötter GmbH & Co. KG.
• 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.
• 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 .
• 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 ⁇ m at the given current density.
• the dissolution of iron from a steel substrate exposed to the plating bath for a prolonged period of time is determined.
• Example 1 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.
• 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.
• 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.
• 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 .
• 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 .
• the composition of the chromium layer is investigated by Glow Discharge Optical Emission Spectroscopy (GDOES).
• GDOES Glow Discharge Optical Emission Spectroscopy
• 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.
• 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 1 Hot 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 2 cathodic degreasing - Deionized water - Temperature: 40 °C - Degreaser salt SLOTOCLEAN EL 130: 50 g/l - Treatment time: 1 min - Current density: 6 A/dm 2 3 Activation -Deionized water - Temperature: 25 °C -Sulfuric acid, conc., (96%): 2,5% vol - Treatment time: 1 min
• SLOTOCLEAN AK 341, Wetting additive RV111 and SLOTOCLEAN EL 130 are products of Dr.-Ing. Max Schlötter GmbH & Co. KG.
• 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 .
• 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.
• 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 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.
• 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.
• the layer thickness was more than 10 ⁇ m for each panel.
• the reference coating exhibited a high incorporation of carbon of about 3%.
• 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 ⁇ m are also shown in Table 9 below. Table 9.
• Example 2 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.
• a chromium(VI)-based hard chrome plating bath was used (SLOTOCHROM S, available from Dr.-Ing. Max Schlötter GmbH & Co. KG.).
• 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.
• 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 .
• 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 ⁇ m at the given current density.
• 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.
• 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.
• 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 dodecahydrate 384.53 g/l Formic acid 98% 69.045 g/l Glycine 75.07 g/l
• the bath contained 384.53 g/l Chromium(III) potassium sulfate dodecahydrates by which 40 g/l chromium was obtained.
• 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.
• the reference bath under the given test conditions achieved a throwing power of 3.1 cm.
• 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 dodecahydrate 384.53 g/l Formic acid 98% 69.045 g/l Glycine 75.07 g/l 3-Mercapto-1-propanesulfonic acid, sodium salt 1 g/l

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