GB1592761A - Electroplating baths - Google Patents

Description

(54) ELECTROPLATING BATHS (71) We, ALBRIGHT & WILSON LIMITED, a British Company of P.O. Box 3, Oldbury, Warley, West Midlands, England do hereby declare the invention and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to chromium electroplating from solutions of trivalent chromium and is a modification of the invention described in our U.K. Patent Specification 1455580.

Our aforesaid Application describes and claims a chromium electroplating bath consisting of an aqueous solution containing trivalent chromium, formate or acetate, bromide and ammonia, together, preferably with borates and conductivity salts such as alkali metal chlorides or sulphates. One problem that is sometimes encountered with baths of this type is a tendency for chromium to precipitate out.

We have discovered that a bath having superior stability is obtained if formate and acetate are used in admixture in certain proportions.

Our invention provides an aqueous chromium electroplating bath comprising from 0.1 to 1.2 moles per litre of trivalent chromium, from 0.5 to 3 moles of formate per mole of trivalent chromium, from 0.1 to 0.5 moles acetate per mole of formate, at least 0.01 moles per litre of bromide and at least 0.05 moles per litre ammonium.

Preferably the bath contains at least 0.1 moles per litre borate, and also, preferably, at least 0.5 moles per litre of alkali metal such as sodium or potassium. Preferably the bath additionally contains from 0.5 to 5 mols of sulphate and/or from 0.5 to 5 moles chloride. The bath may also contain fluoride and, preferably, a wetting agent.

The constituents of the bath may be summarised as follows: (A) Trivalent Chromium This is an essential ingredient of the solution of the invention. Proportions of less than 0.1 molar or more than 1.2 molar trivalent chromium result in significant loss of covering power, and the concentration should therefore be maintained within these limits, and preferably from 0.2 to 0.6 molar. Preferably the solution is substantially free from hexavalent chromium, and preferably the chromium in the solution is substantially all present as trivalent chromium before plating.

(B) Bromide This is an essential ingredient. The concentration of bromide should be maintained above 0.01 molar, to avoid formation of hexavalent chromium, and lowering of the plating rate.

The maximum concentration is not critical but is typically less than 4 molar and preferably less than 1 molar. Iodide functions in a similar fashion to bromide, but suffers the disadvantage that free iodide, which would be formed during plating, is only soluble to the extent of 0.03% w/w in water compared with 4% for bromine. Consequently attempts to use iodide in place of bromide lead to unacceptable precipitation of iodine. Iodide is, moreover, too expensive to use economically in place of bromide. However, it is possible, in principle, to replace a minor part of the bromide with iodide, and references herein to bromide do not exclude bromide containing traces of iodide.

(C) Formate This is an essential ingredient. Typically the proportion of formate to chromium should not exceed 3 : 1 and is preferably not over 2.5 : 1 on a molar basis, to avoid unacceptably severe precipitation of the corresponding chromium salts. If the proportion is less than 0.5 1 the covering power is undesirably reduced. Preferably the total molar proportion of formate and acetate to chromium is between 3 1 and 1:1 most preferably between 2.5 1 and 1 : 1 e.g. about 2 : 1.

(D) Acetate This is an essential constituent of the bath. The presence of acetate in addition to formate increases the stability of the bath. Less than about 0.1 moles of acetate per mole of formate results in an undesirably high risk of chromium precipitating from the bath. Too much acetate tends to lower the plating rate. The proportion of acetate must not exceed 0.5 moles per mole of formate. However, for satisfactory commercial plating rates, the proportion of acetate is preferably from 0.2 to 0.4 mole per mole of formate, most preferably 0.2 to 0.3 e.g. 0.25 moles per mole of formate.

(E) Ammonium The presence of ammonium is an essential feature of our invention. Generally if the concentration of ammonium is less than 0.1 molar there is loss of covering power at high current density. The upper limit is not critical and ammonium may be present in amounts up to saturation, i.e. about 4 molar. Preferably the ammonium is present in a concentration of from 1 to 3 molar. Ammonium is present as NH+4 itself. Preferably, substituted ammonium such as hydroxyl ammonium, hydrazonium, alkylammonium, arylammonium or heterocyclic ions such as pyridinium are absent since they adversely affect deposition of chromium.

(F) Borate Although it is possible to plate chromium from solutions of our invention which do not contain borate, we have not been able to obtain what we consider fully satisfactory results, commercially, in the absence of borate. Concentrations below 0.1 molar result in undesirably low covering power. The upper limit is not critical but generally we prefer to employ from 0.5 to 1 molar borate. The function of the borate is obscure. Its beneficial effects may be in part due to its buffering action. However, other buffer salts, such as phosphates and citrates appear relatively ineffective.

(G) Hydrogen lon Best results are obtained when the bath is somewhat acidic. At low pH values (below 2) there is some loss of covering power which becomes unacceptable below pH 1. If the pH is above 4 the rate of plating tends to be undesirably slow. Optimum pH is between 2 and 3.5.

(H) Chloride andlor Fluoride This is optional, but in the case of chloride, preferred. The amount may vary from zero up to the maximum permitted by solubility considerations. Chloride is generally introduced into the bath as the anion of the conductivity salt (e.g. sodium chloride) as ammonium chloride, which is a convenient means of introducing the ammonia requirement of the bath, as chromic chloride which may optionally be used to supply at least part of the chromium requirement, and/or as hydrochloric acid, which is a convenient means of adjusting the pH of the bath. Preferably the chloride content is at least 0.5 molar, most preferably at least 1 molar, e.g. 1.5 to 5 molar. A particularly convenient range is 2 to 3.5 molar.

(I) Sulphate Is an optional but preferred ingredient. The amount of sulphate may, like that of the chloride, vary between zero and the maximum amount which is compatible with the solution. In one type of bath, particularly preferred, the amount of sulphate is less than the total halide, and preferably less than the total chloride. In a different type of bath, however, the proportion of sulphate is greater than the proportion of halide, and may be the predominant anion in the bath. Like the chloride the sulphate may be introduced into the bath as the anion of the conductivity salt, or of the ammonium or chromium salts or as sulphuric acid. Typical sulphate concentrations may be from 0 up to 5 molar preferably 0.5 to 4, e.g. 0.6 to 3, most preferably 0.6 to 1.2 molar.Preferably the combined chloride and sulphate concentrations are at least 1 molar, e.g. at least 2 molar, most preferably from 2.5 to 3.5 molar.

(J) Trace Metals Iron and/or nickel are preferably present in solutions of the present invention in proportions of between 30 and 150 ppm total for optimum covering power. The proportion of iron may exceed this value of it is desired to plate chromium iron alloys. The proportion nickel, however, should not exceed 150 ppm alone or 100 ppm in the presence of substantial amounts of iron. Greater amount of nickel cause discolouration of the deposit. Most other trace metals, including cobalt, copper, lead and zinc, are highly objectionable when any of them is present in more than about 30 ppm. Preferably the total concentration of such trace metals should be less than 30 ppm, most preferably less than 20 ppm.

(K) Alkali Metals andlor Alkaline Earth These are optionally, but in the case of alkali metals, preferably present. They may include sodium, potassium or lithium, or less preferably alkaline earth metals such as calcium or magnesium or other metal ions which will not plate out of the solution with the chromium. The amount of such metals is not critical provided that they do not precipitate in the presence of the other components. Their presence is useful for increasing the conductivity of the bath. Preferably they are present in an amount of at least 0.1 mole per litre up to saturation in the solution.

(L) Surface Active Agents These are optionally but preferably present in effective and compatible amounts. Wetting agents and antifoams are used throughout plating technology and many suitable examples are well known to those skilled in the art. Any of the wetting agents commonly used in hexavalent chromium plating may be used in the present invention. However, since the solutions of the present invention are much less strongly oxidising than hexavalent chromium solutions it is also possible, and preferred, to use the cheaper wetting agents commonly employed in the less aggressive types of plating solution. The principal restriction on the effectiveness of the wetting agents arises from the presence of free bromine in the solution. Surfactants which are liable to brominating are therefore not recommended, e.g. most non-ionic surfactants.The surfactants used according to our invention are typically cationic or preferably anionic, e.g. sulphosuccinates, alkyl benzene sulphonates having from 8 to 20 aliphatic carbon atoms, such as sodium dodecyl benzene sulphonate, alkyl sulphates having from 8 to 20 carbon atoms such as sodium lauryl sulphate and alkyl ether sulphates such as sodium lauryl polyethoxy sulphates. If the solution has undesirable foaming tendencies it is also possible, optionally, to include compatible antifoams, e.g. fatty alcohols such as octyl alcohol. The choice of surfactants for use in our solution is a routine matter easily within the ordinary competence of those skilled in the art.

The amount of wetting agent used is in accordance with normal practice, e.g. 0.1 to 10 parts per thousand.

It is preferred that the solutions of our invention should consist essentially of the foregoing species. However, we do not exclude the presence of minor amounts of other species which are compatible with the solutions and which do not adversely affect the plating properties to a material extent. Generally it is preferred that nitrate ion be substantially absent, since it tends to inhibit deposition of chromium. Sulphate ion, which has in the past been recommended as an additive to chromium electroplating baths also has an adverse effect if present in amounts greater than about 0.01 molar, but other species, organic or inorganic, which do not inhibit plating of the chromium or materially reduce covering power or create unacceptable problems of toxicity, may optionally be present.

Whether any particular species can be tolerated in the solution may be routinely determined by simple testing.

The bath may conveniently be made up by dissolving water soluble salts of the required species in water in an amount sufficient to provide the desired concentration. Typical salts which may be used include chromic chloride, chromic sulphate, potassium bromide, sodium bromide, ammonium bromide, potassium formate, sodium borate, ammonium chloride, ammonium sulphate and sodium chloride.

The cationic species may, if desired be added wholly or partly as bases such as, for example, aqueous ammonia. A particularly convenient form of chromium is basic chromium sulphate, which is commercially available in chrome tanning liquors. For example 33% basic chromium sulphate, obtained by reducing sodium dichromate with sulphur dioxide is a common article of commerce, and an advantage of the invention is that it is possible to use such relatively cheap and readily available sources of chromium.

However, we do not exclude the use of salts such as chromium formate or acetate.

The anion species may be added, at least in part as acids, e.g. hydrochloric, sulphuric, boric, formic or acetic acids. Preferably, when the essential species have been dissolved, the pH may be adjusted by addition of, for example, ammonium, sodium or potassium, hydroxide. Preferably the pH is initially adjusted in the upper part of the preferred range, e.g. 2.5 to 4. In use the pH tends to fall and should be maintained, by occasional adjustments, in the range 2.5 to 3.5.

The bath may be prepared at room temperature. Alternatively the bath may be prepared at elevated temperature (e.g. 70"C) and allowed to cool. Typically cooling may take from 10 to 24 hours. Unlike prior art formate based baths, it is not necessary to carry out on initial "plating in" operation immediately the bath has been made up, or to add excess formate during make up.

The solution is preferably used at temperatures between 15 and 30"C, e.g. 20 to 250C.

Current densities between 5 and 1,000 amps ft-2 (e.g. about 100 amps ft-2) may be employed.

The system is useful for plating onto plastics and nonferrous substrates as well as more conventional ferrous or nickel substrates. Plating on plastics (usually ABS) is common with the hexavalent chromium solutions. The procedures used are well known and are described for example in "Electroplating of Plastics" by William Goldie and in a paper presented to the Society of Automotive Engineers in January 1965 entitled "Electroplating of Plastics" by Sauvestre. Preferably we employ the same procedures as have hitherto been used for depositing hexavalent chromium on plastic, but substituting the solution of our invention for the hexavalent chromium solutions used hitherto.

It is preferred when electroplating from solutions of our invention to use inert anodes such as, for example, carbon anodes. Other inert anodes such as platinised titanium or platinum may be used but are more costly. However, for alloy plating it is possible to use, for example, iron or chrome/iron anodes.

The invention will be illutrated by the following examples: Example 1 A solution was prepared by dissolving the following ingredients in water at laboratory temperature (20 - 25"C) and diluting to 1 litre.

The chromium anning liquor used in this experiment was a 33% basic liquor. This is an article of commerce used for tanning of leather and is made by reduction of sodium dichromate with sulphur dioxide. The reaction product has a 'basicity' of 33%, 'basicity' being a measure of the replacement of sulphate by hydroxyl to give products of varying composition. The liquor contained 130g/l Cr.

Approx. Molarity Chromium tanning liquor 150 ml 0.4 Wetting Agent 1 ml Ammonium Chloride 90 g 1.7 Potassium Chloride 75 g 1.0 Ammonium Bromide 10 g 0.1 Boric Acid 50 g 0.8 Sodium Acetate 15 g 0.2 Ammonium Formate 55 g 0.87 Sulphuric Acid SG 1.84 2 ml The pH at make up was 3.4 and plating with a carbon anode was commenced within 30 minutes at a volume current density of 0.5 amp/litre. After 1 hour plating (i.e. after 0.5 ampere hour per litre), a sample was taken for evaluation in a Hull Cell furnished with circulatory cooling to maintain temperature between 20 - 25"C.

A current of 10 amps was passed for 3 minutes using a carbon anode and the following distribution on the panel was recorded by standard coulometric thickness measurements.

Current density (amps/sq. ft) 400 200 100 50 25 Thickness in microinches 12 8 6 8 4 The plating range was estimated at 1,000 - 8 ASF at the working pH of 3.0.

Example 2 A comparative experiment was carried out as follows: Solutions A and B were prepared by dissolving the ingredients in cold water, and plating out using a carbon anode for 10 AH per litre at a volume current density of 0.5 A per litre.

A B Potassium Chloride 150 150 grams Ammonium Chloride 40 40 grams Boric Acid 50 50 grams Sodium Bromide 10 10 grams Sodium Acetate - 15 grams Ammonium Formate 55 55 grams Chromium Tanning Liquor 130 130 ml (150 gpl Cr) Wetting Agent 1 1 ml pH 2.9 3.3 The two solutions were adjusted in pH as shown and 5 gpl ammonium formate added to B. The combination of high pH and high formate is known to encourage precipitation of chromic formate.

The solutions were stored for two weeks in polythene, after which time solution A was showing precipitation, whilst solution B was clear.

After eight weeks, heavy precipitation had occurred in A, but none had occurred in B.

WHAT WE CLAIM IS: 1. An aqueous chromium electroplating bath comprising from 0.1 to 1.2 moles per litre of trivalent chromium, from 0.5 to 3 moles of formate per mole of trivalent chromium, from 0.1 to 0.5 moles acetate per mole of formate, at least 0.01 moles per litre of bromide and at least 0.05 moles per litre ammonium.

2. A bath according to Claim 1 wherein the proportion of trivalent chromium is between 0.2 and 0.6 molar.

3. A bath according to either one of Claims 1 and 2 wherein the proportion of bromide is between 0.05 and 1 mole per litre.

4. A bath according to any foregoing claim wherein the amount of formate is less than 2.5 moles per mole of trivalent chromium.

5. A bath according to any foregoing claim wherein the total proportion of formate and acetate is between 1 and 3 moles per mole of trivalent chromium.

6. A bath according to Claim 5 wherein the total proportion of acetate and formate is less than 2.5 moles per mole of trivalent chromium.

7. A bath according to any foregoing claim wherein the proportion of acetate is from 0.2 to 0.4 moles per mole of formate.

8. A bath according to any foregoing claim wherein the proportion of ammonium is from 1 to 3 molar.

9. A bath according to any foregoing claim additionally containing at least 0.1 molar borate.

10. A bath according to Claim 9 wherein the proportion of borate is from 0.5 to 1 molar.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (23)

**WARNING** start of CLMS field may overlap end of DESC **. A current of 10 amps was passed for 3 minutes using a carbon anode and the following distribution on the panel was recorded by standard coulometric thickness measurements. Current density (amps/sq. ft) 400 200 100 50 25 Thickness in microinches 12 8 6 8 4 The plating range was estimated at 1,000 - 8 ASF at the working pH of 3.0. Example 2 A comparative experiment was carried out as follows: Solutions A and B were prepared by dissolving the ingredients in cold water, and plating out using a carbon anode for 10 AH per litre at a volume current density of 0.5 A per litre. A B Potassium Chloride 150 150 grams Ammonium Chloride 40 40 grams Boric Acid 50 50 grams Sodium Bromide 10 10 grams Sodium Acetate - 15 grams Ammonium Formate 55 55 grams Chromium Tanning Liquor 130 130 ml (150 gpl Cr) Wetting Agent 1 1 ml pH 2.9 3.3 The two solutions were adjusted in pH as shown and 5 gpl ammonium formate added to B. The combination of high pH and high formate is known to encourage precipitation of chromic formate. The solutions were stored for two weeks in polythene, after which time solution A was showing precipitation, whilst solution B was clear. After eight weeks, heavy precipitation had occurred in A, but none had occurred in B. WHAT WE CLAIM IS:

1. An aqueous chromium electroplating bath comprising from 0.1 to 1.2 moles per litre of trivalent chromium, from 0.5 to 3 moles of formate per mole of trivalent chromium, from 0.1 to 0.5 moles acetate per mole of formate, at least 0.01 moles per litre of bromide and at least 0.05 moles per litre ammonium.

2. A bath according to Claim 1 wherein the proportion of trivalent chromium is between 0.2 and 0.6 molar.

3. A bath according to either one of Claims 1 and 2 wherein the proportion of bromide is between 0.05 and 1 mole per litre.

4. A bath according to any foregoing claim wherein the amount of formate is less than 2.5 moles per mole of trivalent chromium.

5. A bath according to any foregoing claim wherein the total proportion of formate and acetate is between 1 and 3 moles per mole of trivalent chromium.

6. A bath according to Claim 5 wherein the total proportion of acetate and formate is less than 2.5 moles per mole of trivalent chromium.

7. A bath according to any foregoing claim wherein the proportion of acetate is from 0.2 to 0.4 moles per mole of formate.

8. A bath according to any foregoing claim wherein the proportion of ammonium is from 1 to 3 molar.

9. A bath according to any foregoing claim additionally containing at least 0.1 molar borate.

10. A bath according to Claim 9 wherein the proportion of borate is from 0.5 to 1 molar.

11. A bath according to any foregoing claim having a pH between 1 and 4.

12. A bath acording to Claim 11 having a pH between 1 and 3.5.

13. A bath according to any foregoing claim containing at least 0.5 molar chloride.

14. A bath according to Claim 13 containing from 1.5 to 5 molar chloride.

15. A bath according to any foregoing claim containing up to 5 molar sulphate.

16. A bath according to Claim 15 containing from 0.5 to 4 molar sulphate.

17. A bath according to any foregoing claim containing at least 1 molar total of chloride and sulphate.

18. A bath according to Claim 17 containing from 2.5 to 3.5 molar total of chloride and sulphate.

19. A bath according to any foregoing claim containing from 30 to 150 ppm total of iron and/or nickel.

20. A bath according to any foregoing claim containing at least 0.1 molar of sodium and potassium.

21. A bath according to any foregoing claim substantially as described herein with reference to Example 1.

22. A method of electroplating chromium from a bath according to any foregoing claim.

23. Articles plated with chromium by the method of Claim 22.