Trivalent chromium plating bath and method of electroplating an article

Abstract

A trivalent chromium plating bath comprising: solvent; trivalent chromium ions; complexing agent; and one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid.

Description

[0001] Trivalent chromium plating bath and method of electroplating an article

[0002] The invention relates to a trivalent chromium plating bath and method of electroplating an article.

[0003] Chromium electroplating is an established industrial process and is used to produce many items for applications including sanitary fittings and automotive trim, where chromium plating is often the preferred decorative finish.

[0004] Decorative chromium deposits can be electroplated from baths containing either hexavalent chromium or trivalent chromium. Baths containing hexavalent chromium are commonly used as they are relatively low-cost, simple to maintain and are generally very robust. Although electroplating from baths containing trivalent chromium has been known for a number of decades, for a number of reasons the practice of trivalent chromium plating has not been widely adopted and many chromium plating installations still use hexavalent chromium plating baths.

[0005] More recently there is increased environmental pressure to eliminate hexavalent chromium due to its toxicity and carcinogenic nature, and trivalent chromium plating baths have been developed that are now capable of producing chromium deposits that are indistinguishable from deposits produced from hexavalent chromium baths.

[0006] Unlike hexavalent chromium plating baths, the pH of trivalent chromium plating baths needs to be controlled and maintained. To achieve this a buffering compound is added to the plating bath. The buffer compound of universal choice is boric acid, and this compound is incorporated in many commercial decorative trivalent chromium baths. However, attempts have been made to eliminate boric acid due to environmental and toxicity concerns.

[0007] GB1488381 discloses a method of making an aqueous chromium electroplating bath which method comprises a) providing an aqueous solution having a pH of from 2.5 to 5 and containing dissolved trivalent chromium ions at a concentration of at least 0.1 molar and a dissolved weak complexing agent for trivalent chromium at a concentration at least sufficient to complex all the trivalent chromium, b) holding said aqueous solution at the pH of from 2.5 to 5 for a period of at least 5 minutes, and c) then adding acid to reduce the pH of the aqueous solution to a value in the range of 1 .0 to 4.0.

[0008] LIS4157945 discloses an electrolyte bath and a method for using such a bath comprising trivalent chromium ions dissolved in an aqueous solution containing sulphide. US4374007 discloses a chromium electroplating solution in which the source of chromium comprises an equilibrated aqueous solution of chromium (III) - thiocyanate complexes having supporting electrolyte consisting essentially of potassium sulphate or, preferably, a mixture of potassium and sodium sulphates. US4448648 discloses a trivalent chromium electroplating solution containing trivalent chromium ions, a complexant, a buffer and a sulphur species having S--0 or S--S bonds. US4448649 discloses a trivalent chromium electroplating solution containing trivalent chromium ions, a complexant, a buffer and a sulphur species selected from sulphites and dithionites. US4432843 discloses an aqueous acidic trivalent chromium electrolyte and process for increasing the tolerance of such electrolytes to the presence of deleterious contaminating metal ions. The composition contains controlled effective amounts of thiazole and benzothiazole compounds. US4472250 discloses a trivalent chromium electroplating solution containing trivalent chromium ions, a complexant, a buffer and thiocyanate ions. US4502927 discloses a trivalent chromium electroplating solution containing trivalent chromium ions, a complexant, a buffer and an organic compound having a -C=S group or a -C-S- group.

[0009] US4196063A discloses dark-coloured trivalent chromium deposits achieved by incorporating iron or cobalt. The disclosures of these documents are hereby incorporated by reference.

[0010] Currently, trivalent chromium plating baths for producing decorative chromium finishes are aqueous solutions typically containing the following ingredients:

[0011] • A source of trivalent chromium, e.g. one or more trivalent chromium salts

[0012] • One or more complexing agents

[0013] • One or more conducting salts

[0014] • A buffering compound (most typically boric acid)

[0015] • optionally, compounds such as organosulfur compounds, transition metal ions or wetting agents can be added.

[0016] During the electrodeposition process, the primary cathodic reaction is the generation of hydrogen which is readily visible in the form of gas bubbles. The creation of gaseous hydrogen leads to depletion of hydrogen ions in the plating bath where it is in direct contact with the parts and therefore the pH increases next to the parts, in what is known as the boundary layer, also called the cathode layer or cathode film. In areas of high current, this rapid local pH increase can cause plating defects such as dull and powdery deposits, or no deposit at all. To control this localised pH change, the buffer compound of choice is boric acid, and this compound is incorporated in many commercial decorative trivalent chromium plating baths. In addition to its excellent buffering properties, it is readily soluble in the plating bath and does not form any complexes with the dissolved chromium ions.

[0017] Some alternatives are disclosed in prior art, for example phosphate or amino acids as briefly disclosed in US20100243463A1 , EP2705176A2 and EP2886683A2, the disclosures of which are hereby incorporated by reference. A disadvantage of phosphate is that it does not have the same buffering capability as boric acid and most often leads to powdery deposits. The disadvantage of amino acids is that they can form complexes with chromium and therefore over time the bath performance can be affected by changes in the complexation of the chromium. Over-complexation of chromium can lead to reduced plating speed and in extreme cases, no deposit at all. This can lead to affected baths needing special treatments or even replacement in order to recover bath performance. Additionally, amino acids are known to affect the colour of the deposited metal, most often making it darker as demonstrated by their use specifically for this purpose in WO2017184380A1 and W02012150198A, the disclosures of which are hereby incorporated by reference. The colour of the chromium deposit is an important aspect of the finished parts and should vary as little as possible, therefore amino acids are not generally satisfactory in this respect as direct replacements for boric acid.

[0018] US9689081 B2 relates to an electroplating bath and method for producing dark chromium layers. Examples 1 describes the use of 3-carbaminodoylsulfanylpropane- 1 -sulfonic acid, i.e. an aminoalkylthiosulfonic acid, together with boric acid. US2024271308A1 relates to a method for adjusting the brightness L* of an electroplated chromium layer using a bath comprising, inter alia, a sulfur-containing compound having a sulfur atom with an oxidation number of +5 or below, an example of which is 3-carbaminodoylsulfanylpropane-1 -sulfonic acid.

[0019] Boric acid is under increasing environmental scrutiny. It is classified as a substance of very high concern (SVHC) in Europe due to reprotoxic properties, and additionally discharges to the environment from plating plant wastewater systems are also a concern. Thus, there is a desire in the industry to eliminate boric acid from trivalent chromium plating baths, but industrially viable alternatives have limitations and are less effective than boric acid.

[0020] The present invention seeks to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto.

[0021] In a first aspect, the present invention provides a trivalent chromium plating bath comprising: solvent; trivalent chromium ions; complexing agent; and one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid.

[0022] Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any features indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous.

[0023] The inventors have surprisingly found that use of the plating bath may avoid the occurrence of plating defects, such as dull or powdery defects, or no plating at all. Without being bound by theory, it is considered that this results from the pH of the cathode film being controlled. This may be achieved without the use of boric acid, thereby reducing the environmental impact of the plating bath. Advantageously, the quality of the plating formed by the plating bath may be similar to, or superior to, that formed by a trivalent chromium plating bath comprising boric acid, or by a hexavalent chromium plating bath. The quality of the plating formed by the plating bath may be superior to that formed by a bath comprising phosphate. Specifically, the plating bath may produce even-coloured chromium deposits at a favourable plating rate over a wide current density range.

[0024] The plating bath comprises solvent. The solvent may be in amount sufficient to dissolve the other species of the plating bath, i.e. the trivalent chromium ions, the complexing agent and the one or mor of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid. The solvent is preferably an aqueous solvent, i.e. the solvent preferably comprises water. The solvent may comprise multiple solvents or may comprise a single solvent.

[0025] The plating bath comprises trivalent chromium ions, i.e. Cr(lll). The trivalent chromium ions are typically introduced into the plating bath by dissolving a soluble trivalent chromium salt in the solvent. Suitable trivalent chromium salts will be known to the skilled person. Suitable trivalent chromium salts may include, for example, chromium sulfate, chromium hydroxy sulfate (chrometan), chromium chloride, chromium methanesulfonate, chromium citrate, chromium acetate, chromium hydroxy acetate, chromium formate, chromium hydroxy formate, chromium carbonate and potassium chromium sulphate. The trivalent chromium salts may be used singly or in combination.

[0026] The plating bath comprises complexing agent. The term “complexing agent” as used herein may encompass a ligand. The complexing agent may be mono-dentate or bi- dentate (i.e. a chelating agent). The plating bath may comprise a single complexing agent or multiple complexing agents. The presence of the complexing agent may help to keep the trivalent chromium ions in solution, thereby preventing their premature precipitation or unwanted side reactions. The complexing agent is typically present in an amount to ensure that substantially all of the trivalent chromium is complexed. The plating bath comprises one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid. The plating bath preferably comprises an aminoalkanesulfonic acid. The plating bath may comprise a single aminoalkanesulfonic acid or multiple aminoalkanesulfonic acids, a single aminoalkenesulfonic acid or multiple aminoalkenesulfonic acids, a single aminoarylsulfonic acid or multiple aminoarylsulfonic acids. Such acids, in particular the aminoalkanesulfonic acid, may function as a buffer. During use of the plating bath, such acids, in particular the aminoalkanesulfonic acid, may serve to control the pH of the bulk plating bath to be in a desired range, for example from 1 to 5, typically from 2 to 4.5, more typically from 2.5 to 4. Additionally, it may control the pH of the plating bath in the cathode film and may reduce the occurrence of the plating defects discussed above. Advantageously, such acids, in particular the aminoalkanesulfonic acid, typically do not form complexes with trivalent chromium ions. Such acids, in particular the aminoalkanesulfonic acid, are typically readily soluble in typical plating bath solvents. Favourably, the presence of such acids, in particular the aminoalkanesulfonic acid, may not have a significant effect on the colour of the deposited chromium. Thus, the plating bath may not suffer the same disadvantages as those formulated with, for example, amino acid buffering compounds.

[0027] The plating bath comprises the recited species. By “comprising” it is meant that the plating bath may contain species in addition to those recited. The plating bath may consist essentially of the recited species, i.e. it may contain species other than those recited provided that they do not materially affect the essential characteristics of the plating bath. The plating bath may consist of the recited species, i.e. it may contain only the recited species.

[0028] As will be appreciated, during use the plating composition may have one or more gases dissolved therein, for example hydrogen gas generated at the cathode.

[0029] Preferably, the trivalent chromium plating bath is substantially devoid of boric acid and / or hexavalent chromium. This may reduce the environmental impact of the plating bath.

[0030] Preferably, the trivalent chromium plating bath is substantially devoid of phosphate. The term “substantially devoid of’ as used herein may encompass where the concentration of the species in the plating bath is less than 0.1 M, typically less than 0.01 M, more typically less than 0.001 M, even more typically less than 0.0001 M.

[0031] The one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid is preferably according to the following structural formula: wherein:

[0032] R1and R2is H, alkyl, substituted alkyl, alkenyl, substituted alkenyl, aryl, substituted aryl, and may be joined in a cyclic structure that optionally includes heteroatoms; and

[0033] R3is an alkyl, substituted alkyl, alkenyl, substituted alkenyl, aryl or substituted aryl linking group.

[0034] More preferably, R1and R2is H, alkyl or substituted alkyl and may be joined in a cyclic structure that optionally includes heteroatoms; and / or R3is an alkyl or substituted alkyl linking group. Even more preferably, R1and R2is H, alkyl or substituted alkyl and may be joined in a cyclic structure that optionally includes heteroatoms; and R3is an alkyl or substituted alkyl linking group.

[0035] R1and R2may be joined in a cyclic structure that optionally includes heteroatoms. The cyclic structure is preferably cycloalkyl or cycloalkenyl, more preferably cycloalkyl. The cyclic structure may be substituted or unsubstituted. The heteroatoms preferably comprise oxygen or nitrogen.

[0036] As will be appreciated by the person skilled in the art, the precise form of the one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid in the plating bath will depend on factors such as, for example, the pH of the plating bath and the polarity of the solvent. Accordingly, the acid, or a portion of the acid, may, for example, be in zwitterionic form, i.e.

[0037] At particularly low pHs, the acid, or a portion of the acid, may, for example, be in protonated form, i.e.

[0038] While the nitrogen atom may be protonated and have a positive charge as a result of the bath conditions, the terms aminoalkanesulfonic acid, aminoalkenesulfonic acid and aminoarylsulfonic acid as used herein do not cover a compound where the nitrogen is permanently charged regardless of the pH, i.e. a quaternary ammonium alkanesulfonic acid. In other words, the only possible substituent on the nitrogen other than R1, R2and R3is hydrogen. As will be appreciated, as a result, when R1and R2are joined in a cyclic structure that optionally includes heteroatoms, the cyclic structure may not be aryl.

[0039] The source of the acid may be the acid itself. Alternatively, the source of the acid may be a corresponding salt, i.e. where M represents a cation and (q x n) = 1 such that the -1 charge of the sulfonic acid group is balanced. Suitable cations include but are not limited to metal ions including sodium and potassium. Other suitable cations could include ammonium ions. Once dissolved into the solvent, the salt will form the acid or zwitterionic species described above.

[0040] R1and R2are preferably both H.

[0041] Preferably, R1is H and R2is alkyl, preferably methyl, ethyl or cyclohexyl, which may be substituted or unsubstituted

[0042] R1and R2preferably form a morpholino ring with the nitrogen of the amino group, i.e.

[0043] The morpholino ring may be substituted or unsubstituted.

[0044] R1and R2preferably form a piperzine ring with the nitrogen of the amino group, i.e.

[0045] The piperzine ring may be substituted or unsubstituted.

[0046] R1and / or R2are preferably hydroxy substituted alkyl, more preferably tris(hydroxymethyl)methyl or hydroxyethyl.

[0047] R3preferably comprises methyl, ethyl or propyl, preferably ethyl.

[0048] In a particularly preferred embodiment, R3comprises 2-hydroxypropyl.

[0049] The aminoalkanesulfonic acid preferably comprises aminoalkaned / sulfonic acid, such as, for example, 2,2'-(Piperazine-1 ,4-diyl)di(ethane-1 -sulfonic acid). The one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid preferably comprises one or more of: 2- aminoethanesulfonic acid, 2-aminomethanesulfonic acid, 2- methylaminoethanesulfonic acid, 2-(cyclohexylamino)ethane-1 -sulfonic acid, 3- (cyclohexylamino)propane-l -sulfonic acid, N-cyclohexyl-2-hydroxyl-3- aminopropanesulfonic acid, 2-( / V-morpholino)ethanesulfonic acid, 2,2'-(piperazine- 1 ,4-diyl)di(ethane-1 -sulfonic acid), 3-morpholino-2-hydroxypropanesulfonic acid, 3- (morpholin-4-yl)propane-1 -sulfonic acid, 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethane- 1 -sulfonic acid, A / -tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid, 3-[N- tris(hydroxymethyl)methylamino]-2-hydroxypropanesulfonic acid, 2-[[1 ,3-dihydroxy-2- (hydroxymethyl)propan-2-yl]amino]ethanesulfonic acid and 2-(bis(2- hydroxyethyl)amino)ethane sulfonic acid, more preferably one or both of 2- (cyclohexylamino)ethane-l -sulfonic acid and N-cyclohexyl-2-hydroxyl-3- aminopropanesulfonic acid.

[0050] The common names and chemical structures of these acids are set out in Table 1 below.

[0051] Table 1 The solvent preferably comprises water. Water is low cost, readily available and is environmentally benign.

[0052] The concentration of trivalent chromium ions in the plating bath is preferably from 0.02 to 1 .0, preferably from 0.05 to 0.5 M, more preferably from 0.1 to 0.3 M. Lower concentrations may result in slow and / or inadequate chromium plating. Higher concentrations may increase the cost of the plating bath without any significant increase in plating performance.

[0053] The complexing agent preferably comprises one or both of: a carboxylic acid, preferably comprising one or more of formic acid, acetic acid, glycolic acid, citric acid, lactic acid and malic acid, more preferably malic acid; and an amino acid, preferably comprising one or more of glycine, glutamic acid and aspartic acid, preferably aspartic acid.

[0054] The complexing agent preferably comprises a carboxylic acid rather than an amino acid since amino acids may colour the plating.

[0055] Such species may be particularly suitable complexing agents and may be particular effective at keeping the trivalent chromium ions in solution. Such complexing agents may be the only complexing agents in the plating bath.

[0056] Preferably, the complexing agent comprises malic acid and / or aspartic acid. In a preferred embodiment, the complexing agent comprises malic acid and aspartic acid. In another preferred embodiment, the complexing agent comprises aspartic acid.

[0057] As noted above, the complexing agent is typically present in an amount such that substantially all of the trivalent chromium ions are complexed. The concentration of complexing agent in the plating bath is preferably from 0.01 to 2.0 M, more preferably from 0.02 to 1 .0 M, even more preferably from 0.05 to 0.5 M. Lower concentrations may be less effective at keeping substantially all of the trivalent chromium ions in solution. Higher concentrations may result in a slow plating speed or even no plating due to over-complexation of the chromium. The concentration of the one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid, typically the concentration of the aminoalkanesulfonic acid, in the plating bath is preferably from 0.1 M up to saturation, more preferably from 0.2 to 3.0 M, even more preferably from 0.3 to 1 .5 M. Lower concentrations may result in insufficient buffering. Higher concentrations may increase the cost of the plating bath without any significant increase in buffering performance.

[0058] The trivalent chromium plating bath preferably further comprises one or more conducting ions, the one or more conducting ions being different to trivalent chromium ions. The presence of the conducting ions may aid the flow of electric current through the plating bath. In other words, the conducting ions may serve to increase the electrical conductivity of the bath. The conducting ions are preferably inert ions, i.e. ions that will not react or degrade on the application of an electric current, thereby avoiding any degradation of the plating bath during use. As will be appreciated, all ions are electrically conducting. The conducting ions may alternatively be referred to as “electrical conductivity enhancing ions” or “nonchromium ions”.

[0059] The one or more conducting ions preferably comprise one or more of lithium ions, sodium ions, potassium ions, ammonium ions, chloride ions and sulfate ions, more preferably a combination of sodium ions, potassium ions and sulfate ions or a combination of sodium ions and sulfate ions. Such ions are particularly effective at aiding the flow of electric current through the plating bath and are suitably inert under typical plating conditions. Such ions may be the only conducting ions in the plating bath.

[0060] The conducting ions are typically introduced into the plating bath in the form of a salt such as, for example, one or more of sodium chloride, sodium sulfate, potassium chloride, potassium sulfate, ammonium chloride and ammonium sulfate.

[0061] The required concentration of conducting ions will vary depending on the identities and concentrations of the other species in the bath, which will affect the conductivity of the bath. However, the concentration of the conducting ions in the bath is preferably from 0.01 to 5.0 M, more preferably from 0.1 to 3.0 M. Lower concentrations may result in an unfavourably low conductivity of the bath. Higher concentrations may increase the cost of the plating bath without any significant increase in conductivity. In addition, higher concentrations may reach the solubility limit and leave insoluble particles in the bath, which may cause plating defects.

[0062] The trivalent chromium plating bath preferably has a pH of from 1 to 5, more preferably from 2 to 4.5, even more preferably from 2.5 to 4. Higher or lower pHs may lead to plating defects.

[0063] The trivalent chromium plating bath preferably (or optionally) further comprises: an organosulfur compound, preferably comprising one or more of a thiazole, an organosulfonate, a mercaptan and a thiocyanates; and / or a darkening agent, preferably comprising one or more of methionine, cysteine and cystine; and / or a colloid, preferably comprising one or more of colloidal silica, colloidal alumina and nanodiamond; and / or transition metal ions, preferably ions of one or more of nickel, iron, cobalt and manganese; and / or a wetting agent, preferably comprising one or more of a sulfosuccinate ester, an alkylsulfate, an alkylbenzenesulfonate, an alkylethersulfate and sodium diamylsulfosuccinate.

[0064] The organosulfur compound may increase the plating speed and / or enhance the deposit appearance. The darkening agent, colloid and transition metal ions may result in a darker finish. The transition metal may be co-deposited with the chromium to produce a coloured alloy. The wetting agent may desirably reduce the surface tension of the plating bath.

[0065] The concentration of the organosulfur compound in the plating bath is preferably from 0.005 to 0.1 , more preferably from 0.01 to 0.03 M. The concentration of the darkening agent in the plating bath is preferably from 0.0005 to 0.1 M, more preferably from 0.0008 to 0.1 M, even more preferably from 0.001 to 0.05 M. The concentration of the colloid in the plating bath is preferably from 0.0005 to 0.1 M, more preferably from 0.0008 to 0.1 M, even more preferably from 0.001 to 0.05 M. The concentration of the transition metal ions in the plating bath is preferably from 0.0005 to 0.1 M, more preferably from 0.0008 to 0.1 M, even more preferably from 0.001 to 0.05 M. The concentration of the wetting agent in the plating bath is preferably from 0.0001 to 0.001 M, more preferably from 0.0002 to 0.0008 M.

[0066] In a further aspect, the present invention provides a trivalent chromium plating bath comprising, a) at least one source of trivalent chromium ions, b) at least one complexing agent, c) at least one conducting salt, and d) at least one buffering compounds as described in structure (I), (II) or (III) where R1and R2can be H, alkyl or substituted alkyl and may be joined for example in a cyclic structure that can include heteroatoms, and R3is an alkyl or substituted alkyl linking group; and where M represents a cation and (q x n) = 1 such that the -1 charge of the sulfonic acid group is balanced.

[0067] The advantages and preferable features of the first aspect apply equally to this aspect.

[0068] The at least one source of trivalent chromium may comprise one or more of the trivalent chromium salts discussed above. The conducting salt may comprise one or more of the conducting ions discussed above, e.g. one of the conducting ions discussed above and a counter ion and / or two of the conducting ions discussed above of different polarity.

[0069] In a further aspect, the present invention provides a method of electroplating an article, the method comprising: providing an article to be electroplated; contacting the article with the trivalent chromium plating bath described herein; and applying an electrical current to the article to cause chromium to be deposited onto the article.

[0070] The advantages and preferable features of the first aspect apply equally to this aspect.

[0071] The method may result in a plated article with a finish of a similar or better quality to that provided by a hexavalent chromium plating bath or a boric acid-containing trivalent chromium bath. However, in contrast to methods using such baths, the method of this aspect of the present invention is more environmentally friendly. In contrast to methods using a phosphate-containing trivalent chromium bath, the method may result in a plated article with a finish of a better quality.

[0072] The article to be electroplated is typically pre-plated with copper and / or nickel. This may improve the visual appearance and quality of the chromium plating. The article may comprise metal and / or plastic.

[0073] Contacting the article with the trivalent chromium plating bath typically comprises immersing the article in a vessel containing the trivalent chromium plating bath, typically completely immersing.

[0074] The method typically comprises the use of an electrochemical cell comprising a cathode, an anode and an electrolyte therebetween. The cathode typically comprises the article. The anode typically comprises, for example, graphite or a metal oxide coated on a substrate, for example a titanium substrate. The electrolyte typically comprises the plating bath.

[0075] Applying an electrical current to the article typically comprises applying a potential difference between the cathode and the anode.

[0076] Applying an electrical current to the article typically results in trivalent chromium ions present in the plating bath being reduced to metallic chromium and thereby depositing on the article.

[0077] Contacting the article with the trivalent chromium plating bath is preferably carried out at a temperature of from 0 to 100 °C, preferably from 20 to 70°C, more preferably from 30 and 60 °C. Lower temperatures may result in an unfavourably slow plating rate. Higher temperatures may result in damage to the article and / or unfavourably high levels of evaporation of the solvent and / or unfavourably fast plating rate and / or degradation of the plating bath.

[0078] Applying an electrical current to the article preferably comprises applying an electrical current at a current density of from 2 to 20 A / dm2. This may favourably produce chromium deposits with a thickness in the order of 0.1 to 0.5 pm in a plating time of 3 to 12 minutes. Lower current densities may result in an unfavourably slow plating rate or an unfavourably thin thickness. Higher current densities may increase the cost of the method without any significant increase in plating rate and may cause plating defects.

[0079] The article to be electroplated preferably comprises a sanitary fitting (e.g. a shower fitting, a tap or faucet, a bath fitting, a urinal fitting, or a plug fitting) or an automotive part (e.g. a headlight, a bumper, a doorhandle, a car badge, or a hubcap) or white goods trims (e.g. oven door handles, washing machine door handles or knobs) or furniture fittings and fixtures (e.g. switch plates, lighting surrounds, door handles, chair legs).

[0080] In a further aspect, the present invention provides an electroplated article, the electroplated article being electroplated according to the method described herein. The advantages and preferable features of the first aspect apply equally to this aspect.

[0081] In a further aspect, the present invention provides use of one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid as a buffer in a trivalent chromium plating bath.

[0082] The advantages and preferable features of the first aspect apply equally to this aspect.

[0083] In a further aspect, the present invention provides a trivalent chromium plating bath comprising: solvent; trivalent chromium ions; complexing agent; and an aminoalkanesulfonic acid, wherein: the aminoalkanesulfonic acid is according to the following structural formula:

[0084] R1and R2is H, alkyl, substituted alkyl, alkenyl, substituted alkenyl, aryl, substituted aryl, and may be joined in a cyclic structure that optionally includes heteroatoms; and

[0085] R3is an alkyl or substituted alkyl linking group.

[0086] The advantages and preferable features of the first aspect apply equally to this aspect.

[0087] Preferably, the bath comprises less than 0.1 M boric acid. The invention will now be demonstrated by the following non-limited examples.

[0088] Examples

[0089] Trivalent chromium plating solutions were prepared by dissolving in water the compounds listed according to Table 2.

[0090] Table 2: basic solution composition for examples

[0091] To solutions prepared according to Table 2, various buffering compounds were added and plating evaluations were carried out using a Hull cell under the test conditions shown in Table 3.

[0092] Plating evaluation using a Hull cell is a technique that is well known to those skilled in the art and involves placing a test panel (cathode) in a small trapezoidal plating cell, with the test panel angled diagonally opposite the anode. The test panels are 100mm long x 75mm deep and are immersed in the plating solution to a depth of 50mm such that a plating area of 100mm x 50mm is subject to the cell current whilst the immersed anode area is 50mm x 50mm. The angled geometry of the cell allows a wide range of current density to be represented on the test panel and is a useful indicator for industrial viability.

[0093] Table 3: Hull cell test conditions

[0094] After chromium plating, the panels were assessed for the degree of chromium deposit coverage and visual appearance. The useful chromium deposit coverage on the panel was determined and expressed as a percentage. For industrial viability the effective panel coverage should ideally be more than 70%. The visual appearance should be an even colour, and streaking or patchiness should be absent.

[0095] Table 4 shows the buffer compounds used in the examples and the plating evaluation results. In order to fully demonstrate the invention, the examples included trivalent chromium plating baths that contained no buffering agent, boric acid as an example of the state of the art, alternative buffers from prior art and examples of non-inventive buffers in addition to buffers of the invention. Where the chromium deposit was streaky and with intermittent coverage or variable colour, this was excluded from the effective panel coverage assessment.

[0096]

[0097]

[0098]

[0099]

[0100]

[0101] Examples 1 to 19 are not within the scope of the present invention because they do not contain one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid. For example, 2-hydroxyethanesulfonic acid has no amino group, sulfamic acid and cyclamate have no alkane group, and pyridinium propylsulfobetaine and pyridinium hydroxypropylsulfobetaine are quaternary ammonium alkanesulfonic acids, i.e. the nitrogen is permanently quaternised.

[0102] The inventors believe that the examples demonstrate the usefulness of the invention in allowing the elimination of boric acid from trivalent chromium plating electrolytes and retaining a good degree of effective deposit coverage and even colour. The baths of the invention allow the chromium plating of industrial articles without the use of boric acid.

[0103] The invention will now be further described with reference to the following numbered clauses:

[0104] 1 . A trivalent chromium plating bath comprising: solvent; trivalent chromium ions; complexing agent; and aminoalkanesulfonic acid.

[0105] 2. The trivalent chromium plating bath of clause 1 , wherein the trivalent chromium plating bath is substantially devoid of boric acid and / or hexavalent chromium.

[0106] 3. The trivalent chromium plating bath of clause 1 or clause 2, wherein the aminoalkanesulfonic acid is according to the following structural formula: wherein:

[0107] R1and R2is H, alkyl, substituted alkyl, alkenyl, substituted alkenyl, aryl, substituted aryl, and may be joined in a cyclic structure that optionally includes heteroatoms; and

[0108] R3is an alkyl, substituted alkyl, alkenyl, substituted alkenyl, aryl or substituted aryl linking group.

[0109] 4. The trivalent chromium plating bath of clause 3, wherein:

[0110] R1and R2is H, alkyl or substituted alkyl and may be joined in a cyclic structure that optionally includes heteroatoms; and

[0111] R3is an alkyl or substituted alkyl linking group.

[0112] 5. The trivalent chromium plating bath of clause 3 or clause 4, wherein R1and R2are both H.

[0113] 6. The trivalent chromium plating bath of clause 3 or clause 4, wherein R1is H and R2is alkyl, preferably methyl, ethyl or cyclohexyl.

[0114] 7. The trivalent chromium plating bath of clause 3 or clause 4, wherein R1and R2together form substituted or unsubstituted morpholino ring or a substituted or unsubstituted piperzine ring.

[0115] 8. The trivalent chromium plating bath of clause 3 or clause 4, wherein R1and / or R2are hydroxy substituted alkyl, preferably tris(hydroxymethyl)methyl or hydroxyethyl.

[0116] 9. The trivalent chromium plating bath of any of clauses 3 to 8, wherein R3comprises methyl, ethyl or propyl, preferably ethyl, or wherein R3comprises 2- hydroxypropyl.

[0117] 10. The trivalent chromium plating bath of any preceding clause, wherein the aminoalkanesulfonic acid comprises aminoalkaned / sulfonic acid.

[0118] 11 . The trivalent chromium plating bath of any preceding clause, wherein the aminoalkanesulfonic acid comprises one or more of: 2-aminoethanesulfonic acid, 2- aminomethanesulfonic acid, 2-methylaminoethanesulfonic acid, 2- (cyclohexylamino)ethane-l -sulfonic acid, 3-(cyclohexylamino)propane-1 -sulfonic acid, N-cyclohexyl-2-hydroxyl-3-aminopropanesulfonic acid, 2-( / V- morpholino)ethanesulfonic acid, 2, 2'-(piperazine-1 ,4-diyl)di(ethane-1 -sulfonic acid), 3-morpholino-2-hydroxypropanesulfonic acid, 3-(morpholin-4-yl)propane-1 -sulfonic acid, 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethane-1 -sulfonic acid, N- tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid, 3-[N- tris(hydroxymethyl)methylamino]-2-hydroxypropanesulfonic acid, 2-[[1 ,3-dihydroxy-2- (hydroxymethyl)propan-2-yl]amino]ethanesulfonic acid and 2-(bis(2- hydroxyethyl)amino)ethane sulfonic acid, preferably one or both of 2- (cyclohexylamino)ethane-l -sulfonic acid and N-cyclohexyl-2-hydroxyl-3- aminopropanesulfonic acid.

[0119] 12. The trivalent chromium plating bath of any preceding clause, wherein the solvent comprises water.

[0120] 13. The trivalent chromium plating bath of any preceding clause, wherein the concentration of trivalent chromium ions in the plating bath is from 0.02 to 1.0, preferably from 0.05 to 0.5 M, more preferably from 0.1 to 0.3 M.

[0121] 14. The trivalent chromium plating bath of any preceding clause, wherein the complexing agent comprises one or both of: a carboxylic acid, preferably comprising one or more of formic acid, acetic acid, glycolic acid, citric acid, lactic acid and malic acid; and an amino acid, preferably comprising one or more of glycine, glutamic acid and aspartic acid.

[0122] 15. The trivalent chromium plating bath of any preceding clause, wherein the concentration of complexing agent in the plating bath is from 0.01 to 2.0 M, preferably from 0.02 to 1 .0 M, more preferably from 0.05 to 0.5 M.

[0123] 16. The trivalent chromium plating bath of any preceding clause, wherein the concentration of aminoalkanesulfonic acid in the plating bath is from 0.1 M up to saturation, preferably from 0.2 to 3.0 M, more preferably from 0.3 to 1 .5 M. 17. The trivalent chromium plating bath of any preceding clause further comprising one or more conducting ions, the one or more conducting ions being different to trivalent chromium ions.

[0124] 18. The trivalent chromium plating bath of clause 17, wherein: the one or more conducting ions comprise one or more of lithium ions, sodium ions, potassium ions, ammonium ions, chloride ions and sulfate ions; and / or the concentration of the conducting ions in the plating bath is from 0.01 to 5.0 M, preferably from 0.1 to 3.0 M.

[0125] 19. The trivalent chromium plating bath of any preceding clause having a pH of from 1 to 5, preferably from 2 to 4.5, more preferably from 2.5 to 4.

[0126] 20. The trivalent chromium plating bath of any preceding clause, further comprising: an organosulfur compound, preferably comprising one or more of a thiazole, an organosulfonate, a mercaptan and a thiocyanates; and / or a darkening agent, preferably comprising one or more of methionine, cysteine and cystine; and / or a colloid, preferably comprising one or more of colloidal silica, colloidal alumina and nanodiamond; and / or transition metal ions, preferably ions of one or more of nickel, iron, cobalt and manganese; and / or a wetting agent, preferably comprising one or more of a sulfosuccinate ester, an alkylsulfate, an alkylbenzenesulfonate, an alkylethersulfate and sodium diamylsulfosuccinate.

[0127] 21 . A trivalent chromium plating bath comprising, a) at least one source of trivalent chromium, b) at least one complexing agent, c) at least one conducting salt, and d) at least one buffering compounds as described in structure (I), (II) or (III) where R1and R2can be H, alkyl or substituted alkyl and may be joined for example in a cyclic structure that can include heteroatoms, and R3is an alkyl or substituted alkyl linking group; and where M represents a cation and (q x n) = 1 such that the -1 charge of the sulfonic acid group is balanced.

[0128] 22. A method of electroplating an article, the method comprising: providing an article to be electroplated; contacting the article with the trivalent chromium plating bath of any preceding clause; and applying an electrical current to the article to cause chromium to be deposited onto the article.

[0129] 23. The method of clause 22, wherein contacting the article with the trivalent chromium plating bath is carried out at a temperature of from 0 to 100 °C, preferably from 20 to 70°C, more preferably from 30 and 60 °C.

[0130] 24. The method of clause 22 or clause 23, wherein applying an electrical current to the article comprises applying an electrical current at a current density of from 2 to

[0131] 20 A / dm2.

[0132] 25. The method of any of clauses 22 to 24, wherein the article to be electroplated comprises a sanitary fitting, an automotive part, a white goods trim, a furniture fitting or a furniture fixing. The foregoing detailed description has been provided by way of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

Claims

Claims1 . A trivalent chromium plating bath comprising: solvent; trivalent chromium ions; complexing agent; and one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid.

2. The trivalent chromium plating bath of claim 1 , wherein the trivalent chromium plating bath is substantially devoid of boric acid and / or hexavalent chromium.

3. The trivalent chromium plating bath of claim 1 or claim 2, wherein the one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid is according to the following structural formula:R10\ 3 HN— R— S— OH / R20 wherein:R1and R2is H, alkyl, substituted alkyl, alkenyl, substituted alkenyl, aryl, substituted aryl, and may be joined in a cyclic structure that optionally includes heteroatoms; andR3is an alkyl, substituted alkyl, alkenyl, substituted alkenyl, aryl or substituted aryl linking group.

4. The trivalent chromium plating bath of claim 3, wherein:R1and R2is H, alkyl or substituted alkyl and may be joined in a cyclic structure that optionally includes heteroatoms; andR3is an alkyl or substituted alkyl linking group.

5. The trivalent chromium plating bath of claim 3 or claim 4, wherein R1and R2are both H.

6. The trivalent chromium plating bath of claim 3 or claim 4, wherein R1is H and R2is alkyl, preferably methyl, ethyl or cyclohexyl.

7. The trivalent chromium plating bath of claim 3 or claim 4, wherein R1and R2together form substituted or unsubstituted morpholino ring or a substituted or unsubstituted piperzine ring.

8. The trivalent chromium plating bath of claim 3 or claim 4, wherein R1and / or R2are hydroxy substituted alkyl, preferably tris(hydroxymethyl)methyl or hydroxyethyl.

9. The trivalent chromium plating bath of any of claims 3 to 8, wherein R3comprises methyl, ethyl or propyl, preferably ethyl, or wherein R3comprises 2- hydroxypropyl.

10. The trivalent chromium plating bath of any preceding claim, wherein the aminoalkanesulfonic acid comprises aminoalkaned / sulfonic acid.11 . The trivalent chromium plating bath of any of claims 1 to 9, wherein the one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid comprises one or more of: 2-aminoethanesulfonic acid, 2- aminomethanesulfonic acid, 2-methylaminoethanesulfonic acid, 2- (cyclohexylamino)ethane-l -sulfonic acid, 3-(cyclohexylamino)propane-1 -sulfonic acid, N-cyclohexyl-2-hydroxyl-3-aminopropanesulfonic acid, 2-( / V- morpholino)ethanesulfonic acid, 2, 2'-(piperazine-1 ,4-diyl)di(ethane-1 -sulfonic acid), 3-morpholino-2-hydroxypropanesulfonic acid, 3-(morpholin-4-yl)propane-1 -sulfonic acid, 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethane-1 -sulfonic acid, N- tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid, 3-[N- tris(hydroxymethyl)methylamino]-2-hydroxypropanesulfonic acid, 2-[[1 ,3-dihydroxy-2- (hydroxymethyl)propan-2-yl]amino]ethanesulfonic acid and 2-(bis(2- hydroxyethyl)amino)ethane sulfonic acid, preferably one or both of 2- (cyclohexylamino)ethane-l -sulfonic acid and N-cyclohexyl-2-hydroxyl-3- aminopropanesulfonic acid.

12. The trivalent chromium plating bath of any preceding claim, wherein the solvent comprises water.

13. The trivalent chromium plating bath of any preceding claim, wherein the concentration of trivalent chromium ions in the plating bath is from 0.02 to 1.0, preferably from 0.05 to 0.5 M, more preferably from 0.1 to 0.3 M.

14. The trivalent chromium plating bath of any preceding claim, wherein the complexing agent comprises one or both of: a carboxylic acid, preferably comprising one or more of formic acid, acetic acid, glycolic acid, citric acid, lactic acid and malic acid; and an amino acid, preferably comprising one or more of glycine, glutamic acid and aspartic acid.

15. The trivalent chromium plating bath of any preceding claim, wherein the concentration of complexing agent in the plating bath is from 0.01 to 2.0 M, preferably from 0.02 to 1 .0 M, more preferably from 0.05 to 0.5 M.

16. The trivalent chromium plating bath of any preceding claim, wherein the concentration of the one or more of an aminoalkanesulfonic acid, an aminoalkenesulfonic acid and an aminoarylsulfonic acid in the plating bath is from 0.1 M up to saturation, preferably from 0.2 to 3.0 M, more preferably from 0.3 to 1 .5 M.

17. The trivalent chromium plating bath of any preceding claim further comprising one or more conducting ions, the one or more conducting ions being different to trivalent chromium ions.

18. The trivalent chromium plating bath of claim 17, wherein: the one or more conducting ions comprise one or more of lithium ions, sodium ions, potassium ions, ammonium ions, chloride ions and sulfate ions; and / or the concentration of the conducting ions in the plating bath is from 0.01 to 5.0 M, preferably from 0.1 to 3.0 M.

19. The trivalent chromium plating bath of any preceding claim having a pH of from 1 to 5, preferably from 2 to 4.5, more preferably from 2.5 to 4.

20. The trivalent chromium plating bath of any preceding claim, further comprising: an organosulfur compound, preferably comprising one or more of a thiazole, an organosulfonate, a mercaptan and a thiocyanates; and / or a darkening agent, preferably comprising one or more of methionine, cysteine and cystine; and / or a colloid, preferably comprising one or more of colloidal silica, colloidal alumina and nanodiamond; and / or transition metal ions, preferably ions of one or more of nickel, iron, cobalt and manganese; and / or a wetting agent, preferably comprising one or more of a sulfosuccinate ester, an alkylsulfate, an alkylbenzenesulfonate, an alkylethersulfate and sodium diamylsulfosuccinate.21 . A trivalent chromium plating bath comprising, a) trivalent chromium ions, b) at least one complexing agent, c) at least one conducting salt, and d) at least one buffering compounds as described in structure (I), (II) or (III)where R1and R2can be H, alkyl or substituted alkyl and may be joined for example in a cyclic structure that can include heteroatoms, and R3is an alkyl or substituted alkyl linking group; and where M represents a cation and (q x n) = 1 such that the -1 charge of the sulfonic acid group is balanced.

22. A method of electroplating an article, the method comprising: providing an article to be electroplated; contacting the article with the trivalent chromium plating bath of any preceding claim; and applying an electrical current to the article to cause chromium to be deposited onto the article.

23. The method of claim 22, wherein contacting the article with the trivalent chromium plating bath is carried out at a temperature of from 0 to 100 °C, preferably from 20 to 70°C, more preferably from 30 and 60 °C.

24. The method of claim 22 or claim 23, wherein applying an electrical current to the article comprises applying an electrical current at a current density of from 2 to 20 A / dm2.

25. The method of any of claims 22 to 24, wherein the article to be electroplated comprises a sanitary fitting, an automotive part, a white goods trim, a furniture fitting or a furniture fixing.