GB1564332A

GB1564332A - Non-cyanide silver bath

Info

Publication number: GB1564332A
Application number: GB34858/76A
Authority: GB
Original assignee: Technic Inc
Current assignee: Technic Inc
Priority date: 1976-03-01
Filing date: 1976-08-20
Publication date: 1980-04-10
Also published as: FR2343062A2; JPS52105540A; DE2704691A1; FR2343062B2

Description

(54) NON-CYANIDE SILVER BATH (71) We, TECHNIC, INC., a corporation of the State of Rhode Island, United States of America, having a place of business at 88 Spectacle Street, Cranston, Rhode Island 02910, United States of America, do herebY declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to materials, methods and apparatus for the electrodeposition of silver alloys, and more especially, but not exclusively, to the improved electrodeposition of silver alloys, employing soluble or insoluble anodes.

In the conventional electrolytic silver plating baths, the electrolytes have almost always been limited to those of the cyanide type because of the high stability constant of the complex K (Ag CN)2).

Conventional potassium silver cyanide plating solutions excel in current efficiency, brightness of silver and silver deposit, throwing power and speed of electrodepositlon.

These features of the cyanide silver and silver alloy plating electrolyte are due to the presence of CN- ions in the solutions and very effective organic and/ot inorganic brighteners.

Such CN- ions react to form poisonous HCN. For this reason constant care must be taken and efficient ventilation supplied. Another disadvantage is the presence of CN- ions near or directly on the anode and especially on an insoluble anode where ammonia and potassium carbonate are formed.

Because of the accumulation of potassium carbonate in the electrolyte, very costly and complicated processes must be applied to remove an excess of such byproduct.

Thus, because of its very high toxicity, and for other reasons, it is very desirable, in the art of silver and silver alloy electroplating, to avoid the use of cyanide compounds.

At the present time most patent specifications directed to non-cyanide silver plating baths are based on ammonia complexes of silver in combination with a variety of conductivity salts These include: United States Patent Specifications Nos: 2,504,272; 3,406,107; 3,362,895 and 3,507,758; Russian Patent Specifications Nos: 138,788; 199,261; 203,423; 212,690 and 337,435; Japanese Patent Specification No: 703,9945; and British Patent Specification No: 1,047,789.

Some silver formulations employ amide and amine complexes. Patent specifications describing such formulations include: Russian Patent Specifications Nos: 185,659; 212,689; 295,824 and 312,892.

The reader is referred to all of the aforementioned patent specifications. None of these patented formulations has found industrial application because the silver complex is unstable during extended periods of time, and because of the breakdown of electrolyte during electrolysis, the poor quality of silver deposit and an extremely low useful current density range.

In the parent specification, i.e. the specification of our Patent Application No.

9933/76, Serial No. 1,548,170, there is described and claimed an aqueous silver plating bath, said bath being characterized by its substantial freedom from cyanide, and comprising, in solution, an alkali metal silver imide complex, wherein said imide comprises a 5-membered heterocyclic ring derived from an organic dicarboxylic acid, and where in said complex, the mole ratio of silver to imide is 1 to 2 respectively. Also described and claimed are: a silver electroplating apparatus having such a bath; a method of electroplating employing such a bath, and articles electroplated thereby; and the alkali metal silver imide complex per se.

According to the present invention, there is provided an aqueous silver alloy plating bath, said bath being characterized by its substantial freedom from cyanide, and comprising, in solution, an alkali metal or ammonium silver imide complex, wherein said imide is a five-membered heterocyclic ring derived from an organic dicarboxylic acid, and where, in said complex, the mole ratio of silver to imide is 1 to 2 respectively; and at least one salt of an alloying metal (as hereinafter defined).

The term "alloying metal" as used herein in relation to a component of a plating bath in accordance with the invention means a metal which, when the bath is used for plating, will plate out with silver as an alloy.

For the avoidance of doubt, it is pointed out that the term "plating bath" as used herein refers, not to a plating container, but rather to the liquid contents of such a container.

The present invention further comprehends a method of silver alloy electroplating whenever a bath in accordance with the invention is employed as a silver alloy electroplating bath; and also an article whenever electroplated by such a method.

The alloying metal may be copper, cadmium, gold, antimony, palladium and similar related metals. The bath of the invention preferably contains the alloying metal salt in an amount such that deposits plated therefrom will contain up to 5% of alloying metal.

The imide used to form the said complex in the bath of the present invention (and also the parent invention) may be succinimide or a derivative thereof (i.e. a pyrrolidine-2,5-dione) or maleimide or a derivative thereof (i.e. a 3-pyrroline-2,5dione), for example an imide falling within the formulae:

wherein each R independently is -H, alkyl or alkoxy, the alkyl and alkoxy not exceeding 5, preferably 4, carbon atoms in size.

Thus, typical of these imides are: Succinimide, 2-methyl-2-ethyl succinimide, 2-methyl succinimide, 2-ethyl succinimide, maleimide, 2,2,3,3-tetramethyl succinimide and 2,2,3-trimethyl succinimide.

Single silver salts of imides such as succinimide or its derivatives are only slightly soluble in water, and are thus lacking in utility as electrolytes. In the complex of the bath of our invention, silver is bonded in a complex, wherein the complex is believed to be of the formula [Im-Ag-Imi-M+, and wherein Im- is the radical formed by removing the hydrogen atom from the nitrogen atom of the imide, and M+ is NH4+ or the cation of an alkali metal. The complex may be formed by adding, in the presence of an approximately stoichiometric amount of ammonia or an alkali metal hydroxide, at least one molar proportion of the imide to a suitable single silver imide salt, or at least two molar proportions of the imide to a water soluble non-imide silver salt.

Thus, the ratio of silver to the complexing agent in the bath of the invention is at least 1 mol of silver to two mols of complexing compound.

The present invention also comprehends those of the above-defined silver imide complexes which may be employed in the above-described bath of the present invention which are not claimed in the specification of our co-pending Patent Application No. 9933/76, Serial No. 1,548,170, viz. the ammonium silver imide complexes.

The alkali metal and ammonium silver complexes are soluble in water if the pH is adjusted to fall within the range from 6.0 to 14. However, the pH value may vary slightly in accordance with the particular complexing compound and any alkali metal used.

The new non-cyanide silver alloy plating bath in accordance with the present invention can be made up from (1) the respective imide, (2) alkali metal or ammonium hydroxide, (3) soluble or insoluble silver salts, (4) optional conductivity salt or salts, (5) alloying metal salt, and (6) brightener or brighteners (e.g. 0.001 to 50 grams per liter) which can be employed alone or in conjunction one with another. For each type of component (imide, hydroxide, etc.) more than one compound may be employed.

In general, the non-cyanide silver alloy plating bath works at temperatures between 20400C (68--103"F) and cathodic current densities between 0.1--3A per square decimeter (1--30 amperes per square foot). The cathode area should not be greater than thatofthe anode, that is 1:1, but an extremely high ratio, of 1:10 or more, could be advantageous. The cathode current efficiency, regardless of whether a soluble or insoluble anode is used, can be 90 to 100%.

The anode current efficiency, when a soluble silver anode is used, can be 90 to looV During the electrolysis of the non-cyanide silver alloy plating bath, the silver complex is the source of silver ions and, later, liberated imide can serve as a complexing agent to bond the silver dissolved from a soluble silver anode (where such an anode is used). Where the anode is insoluble, liberated imide can serve as a complexing agent for a water soluble or insoluble silver replenishing salt.

The molar ratio of silver to imide in the bath must be l:at least 2, respectively but can be as high as l:saturation point.

A variety of conductivity salts can be used, e.g. those having the following anions: NO2-, OH-, NO3-, F-, CO3=, PO;, HYPO;, SO3=, SO;, NH2SO3-; and mono-, di- or tri-carboxylates of mono-, di-, and tri-carboxylic acids and their hydroxy or amine derivatives.

The concentration of conductivity salts of which the corresponding silver salt is soluble is not critical. The concentration of conductivity salts of which the corresponding silver salt is insoluble is also not critical, so long as the conductivity salt does not interfere with the solubility of silver anodes during the electroplating process.

The following amines, imines, polyamines, or polyimines may be used as potent or effective brighteners in our previously described non-cyanide silver alloy plating baths:

polyethylene imines preferably in the molecular weight range polypropylene imines \ imines from 100 to 60,000.

polyyd r9xyeth yl ethylene ethyl amines propylamine ethylenediamine propylenediamine diethylenetriamine triethylenetetramine tetraethylenepentamine pentaethylenehexamine imino-bis-propylamine dimethyl amine propylamine diethylpropylenediamine It should be understood that the above described amine or imino brightener compounds can be used in a non-cyanide silver alloy plating formulation based on silver imide complexes as a single compound or combined with each other.

Some preferred types of brighteners from the above list are embraced by the following general classes: 1. Amines of formula: RtCnH2nNH2s wherein n is an integer from 2 to 6 inclusive, and R1 is -NH2 or -H.

2. Polymers (the term including oligomers) of substituted or unsubstituted ethylene or propylene imines. Where the alkylene imine from which the polymer is derives is substituted, the substituents may comprise C26 alkyl, C26 alkoxy or amino, for example.

3. Polymers as in No. 2 in which there is 25 to 50% cross linking; for example polyethylene imine, or polypropylene imine from molecular weight 100 to 60,000.

These compounds are comr ercially available as polyimines of various molecular weights from 100 to 600,000 from the Dow Chemical Company, Midland, Michigan.

It has been found that a mirror bright silver alloy deposit can be achieved by incorporating alkylene or alkanol amines into non-cyanide silver plating baths based on silver complexes described herein.

Especially good results can generally be achieved by using alkylene polyamines, which contain at least one secondary amino group and at least one primary amino group, or polyimine compounds having molecular weights in the range from about 100 to 60,000, with emphasis on the lower molecular weight range from about 100 to 2,000. The most active polyimines are polymers called polyethyleneimines which are formed by polymerisation of ethyleneimines, substituted ethyleneimines, or derived from the addition of ethyleneimine to organic or inorganic molecules.

Accordingly, with the present invention, the non-cyanide mirror bright silver alloy plating bath may consist of a composition as follows: Silver (as salt or imide complex) 5 to 100 grams per liter Imide 10 grams per liter to saturation Conductivity salt 0 to 300 grams per liter Alkali metal hydroxide and/or ammonium hydroxide 5 to 200 grams per liter Alloying metal (as salt) up to saturation point, preferably between 0 and 5 mol per cent Water to 1 liter pH generally 6.0 to 14.

The silver can be supplied as silver nitrate.

Some embodiments in accordance with the invention will now be described in the following Examples. In each Example, an aqueous non-cyanide silver alloy electroplating bath was prepared, and a sample deposit was plated out, and in each case, the amounts of the various components used, the conditions used. and the result obtained are given in tabular form.

EXAMPLE 1.

Ag as AgNO3 23 grams per liter Cu++ as Cu(NO3)2.3H2O 1.5 grams per liter Succinimide 80 grams per liter Potassium nitrite 40 grams per liter pH adjusted with potassium hydroxide to 8.5 Temperature 25"C(77"F) Current density lA.dm-2 (10 ASF) EXAMPLE 1 - Continued.

Plating time 10 minutes Deposit 97% Ag. 3% Cu - mirror bright Current efficiency 97% EXAMPLE 2.

Ag as AgNO3 23 grams per liter Cu+ as CuCI 1.5 grams per liter Succinimide 70 grams per liter Potassium nitrite 30 grams per liter pH adjusted with potassium hydroxide to 8.5 Temperature 25 C (77"F) Current density l.5A.dm-2 (15 ASF) Plating time 10 minutes Deposit 95% Ag. 5% Cu - mirror bright Current efficiency 97% EXAMPLE 3.

Ag as Silver succinimide complex 22.5 grams per liter Cu as Copper succinimide complex 1.5 grams per liter Succinimide 25 grams per liter pH adjusted with potassium hydroxide to 9.0 Temperature 25 C(77 F) Current density 0.8 Adm-2 (8 ASF) Plating time 10 minutes Deposit 98% Ag. 2% Cu - mirror bright Current efficiency 97% EXAMPLE 4.

Ag as Silver succinimide complex 23 grams per liter Cu++ as copper acetate 1 gram per liter Succinimide 70 grams per liter pH adjusted with potassium hydroxide to 8.5 Temperature 25 C (77 F) Current density I .0A.dm-2 (10 ASF) Plating time 10 minutes Deposit 97% Ag. 3% Cu - mirror bright Current efficiency 96% EXAMPLE 5.

Ag as Silver succinimide complex 23 grams per liter Cd++ as Cd(NO3)2 1.5 grams per liter Succinimide 45 grams per liter Potassium nitrate 7 grams per liter pH adjusted with potassium hydroxide to 9.5 Temperature 20"C (680F) Current density 1 .0A.dm-2 (10 ASF) Plating time 10 minutes Deposit 98% Ag. 2% Cd - mirror bright Current efficiency 98 ó EXAMPLE 6.

Ag as AgNO3 23 grams per liter Cd++ as Cadmium acetate 2 grams per liter Succinimide 65 grams per liter Potassium acetate 20 grams per liter pH adjusted with potassium hydroxide to 9.0 Temperature 20 C (68"F) Current density l.5A.dm-2 (15 ASF) Plating time 10 minutes Deposit 97% Ag. 3% Cd - mirror bright EXAMPLE 7.

Ag as AgNO3 20 grams per liter Cd++ as Cd(NO3)2 23 grams per liter Succinimide 120 grams per liter Potassium nitrite 37 grams per liter pH adjusted with potassium hydroxide to 8.5 Temperature 25 C (77"F) Current density l.0A.dm-2 (10 ASF) Plating time 10 minutes Deposit 96.2% Ag. 3.8% Cd - mirror bright EXAMPLE 8.

Ag as Silver succinimide complex 23 grams per liter Au+++ as HAuCI4 1.2 grams per liter Succinimide 30 grams per liter EXAMPLE 8 -- Continued.

Potassium citrate 25 grams per liter pH adjusted with potassium hydroxide to 8.7 Temperature 25 C (77"F) Current density 2.OA.dm-2 (20 ASF) Plating time 10 minutes Deposit 97.5% Ag. 2.5% Au - bright EXAMPLE 9.

Ag as AgNO3 23 grams per liter Sb+++ as Antimony tartrate 2 grams per liter Succinimide 65 grams per liter Triethanolamine 5 milliliters per liter pH adjusted with potassium hydroxide to 12.5 Temperature 25 C (77"F) Current density 1 .5A.dm-2 (15 ASF) Plating time 10 minutes Deposit 97% Ag. 3% Sb -- dark-bright Current efficiency 90% EXAMPLE 10.

Ag as AgNO3 23 grams per liter Pd as Palladium ethylene diamine sulfate 4 grams per liter Succinimide 65 grams per liter pH adjusted with sodium hydroxide to 8.0 Temperature 25 C (77"F) Current density l.0A.dm-2 (10 ASF) Plating time 10 minutes Deposit bright Current efficiency 95% EXAMPLE 11.

Ag as AgNO3 23 grams per liter Cu++ as Cu(NO3)2.3H20 23 grams per liter Succinimide 120 grams per liter Potassium nitrite 37 grams per liter pH adjusted with potassium hydroxide to 8.5 Temperature 250C (770F) EXAMPLE 11 -- Continued.

Current density 1 .0A.dm-2 (10 ASF) Plating time 10 minutes Deposit 98.3% Ag. 1.7% Cu-bright Current efficiency 20% EXAMPLE 12.

Ag as AgNO3 23 grams per liter Cu++ as Cu(NO3)2.3H20 30 grams per liter Succinimide 150 grams per liter Potassium nitrite 37 grams per liter pH adjusted with potassium hydroxide to 8.5 Temperature 25 C (77 F) Current density 4A.dm-2 (40 ASF) Plating. time 10 minutes Deposit 85% Ag. 15% Cu - bright EXAMPLE 13.

Ag as AgNO3 23 grams per liter Cd++ as Cd(NO3)2 30 grams per liter Succinimide 150 grams per liter pH adjusted with potassium hydroxide to 8.5 Temperature 25 C (77 F) Current density 2.0A.dm~2 (20 ASF) Plating time 10 minutes Deposit 78% Ag.22% Cd - dark bright EXAMPLE 14.

Ag as AgNO3 23 grams per liter Au+++ as HAuCI4 20 grams per liter Succinimide 140 grams per liter pH adjusted with potassium hydroxide to 10.0 Temperature 30 C (86 F) Current density l.0A.dm-2(l0ASF) Plating time 10 minutes Deposit 99.5% Ag. 0.5% Au - semi bright EXAMPLE 15.

Ag as AgNO3 25 grams per liter Pd++ as Palladium ethylene diamine sulfate 16 grams per liter EXAMPLE 15 -- Continued.

Succinimide 260 grams per liter pH adjusted with potassium hydroxide to 9.0 Temperature 25"C (77"F) Current density l.0A.dm-2 (10 ASF) Plating time 10 minutes Deposit Semi bright Accordingly, with the present invention, the new non-cyanide mirror bright silver alloy plating bath may consist of the following: Silver 5 grams to 100 grams per liter Succinimide 10 grams to saturation point Conductivity salt 0 grams to saturation point Alkali metal hydroxide or NH40H 5 grams to 200 grams per liter and one of the following metals or their combination with each other: Cu+ 0.1 gram to 40 grams per liter Cu++ 0.1 gram to 40 grams per liter Cd++ 0.1 gram to 40 grams per liter Au+ 0.1 gram to 40 grams per liter Au+++ 0.I gram to 40 grams per liter Pd++ 0.1 gram to 20 grams per liter Sb+++ 0. I gram to 40 grams per liter Water to 1 liter pH 6.0 to 14 It is to be understood that the silver can be present in the plating solution in any of the imide complexes developed, e.g. the succinimide, maleimide, or the methyl ethyl imide variants we have indicated. These imides, prior to reaction with silver salts, are commercially available and can be used in the several Examples at the several concentrations indicated.

In the Examples given, the concentration may be considered illustrative of optimal operation, but concentrations may be varied from those which have been indicated.

In the preceding Examples we have shown how to deposit mirror bright silver alloy electroplates from the non-cyanide complex. This can be done with or without the polyamine or polyimine brighteners. Coloring and brightening of the silver plate can be achieved by alloying with co-plated metals in an amount from a few parts per thousand to about 5% or more of alloyed deposit.

As can be seen by using the baths of the foregoing Examples, the operating characteristics of aqueous electroplating baths in accordance with the invention such as the maximum current density, the cathode current efficiency, the width of the pH range, the brightness of silver alloy deposit and the stability of the electrolyte can be improved over existing non-cyanide silver alloy plating electrolytes. Furthermore, the brightness of silver alloy deposits obtainable from baths in accordance with the invention can be at least equal to or better than that of the silver alloy deposits obtained from conventional cyanide electrolytes, with excellent adhesion when applied over copper and copper alloys such as brass without a preliminary silver strike. Also, bright silver alloy deposits can be obtained over a wide range of current densities.

The present invention comprehends an aqueous silver plating bath, said bath being characterised by its substantial freedom from cyanide, and comprising, in solution, an ammonium silver imide complex in accordance with the invention.

Preparation and use of such baths is analogous to the preparation and use both of the silver alloy plating baths of the invention as described hereinbefore, and of the silver plating baths described and claimed in our co-pending Patent Application No. 9933/76.

Below are listed some features which may be used in baths in accordance with the aspect of the invention relating to the use of ammonium silver imide complexes in silver (as distinct from silver alloy) plating: 1. The pH of the bath is preferably from 6.0 to 14.

2. The brighteners as described in connection with the silver alloy baths of the invention may be employed.

3. The bath may include the imide in free form.

4. The bath may consist of a composition as follows: Silver (as salt or imide complex 5 to 100 grams per liter Imide 10 grams per liter to saturation Ammonium hydroxide 5 to 200 grams per liter Conductivity salt 0 to 300 grams per liter Water to 1 liter 'The present invention further comphrends a method of electroplating whenever a silver plating bath in accordance with the invention is employed as a silver electroplating bath; and articles whenever electroplated by such a method.

An embodiment of the aspect of the invention relating to the use of an ammonium silver imide complex in silver (as distinct from silver alloy) plating will ,now be described in the following Example, wherein an aqueous non-cyanide silver electroplating bath was prepared, and a sample deposit was plated out, as in the precedmg Examples. The amounts of the various components used, the conditions used, and the results obtained are given in tabular form.

EXAMPLE 16.

Silver as silver nitrate 15 grams per liter Maleimide 36 grams per liter pH adjusted with NH40H 10.0 Temperature 20"C Current Density 1.OA/dm2 Plating time 5 minutes Deposit Silver -- semi bright to dull WHAT WE CLAIM IS: 1.,An aqueous silver alloy plating bath, said bath being characterized by its substantial freedom from cyanide, and comprising, in solution, an alkali metal or ammonium silver imide complex, wherein said imide is a five-membered heterocyclic ring derived from an organic dicarboxylic acid, and wherein, in said complex, the mole ratio of silver to imide is 1 to 2 respectively; and at least one salt of an alloying metal (as hereinbefore defined).

2. A bath according to claim 1, wherein said imide is a pyrrolidine -2,5-dione or a 3- pyrroline 2,5-dione.

3. A bath according to claim I or claim 2 wherein said imide is in accordance with the formula:

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. excellent adhesion when applied over copper and copper alloys such as brass without a preliminary silver strike. Also, bright silver alloy deposits can be obtained over a wide range of current densities. The present invention comprehends an aqueous silver plating bath, said bath being characterised by its substantial freedom from cyanide, and comprising, in solution, an ammonium silver imide complex in accordance with the invention. Preparation and use of such baths is analogous to the preparation and use both of the silver alloy plating baths of the invention as described hereinbefore, and of the silver plating baths described and claimed in our co-pending Patent Application No. 9933/76. Below are listed some features which may be used in baths in accordance with the aspect of the invention relating to the use of ammonium silver imide complexes in silver (as distinct from silver alloy) plating:

1. The pH of the bath is preferably from 6.0 to 14.

3. The bath may include the imide in free form.

EXAMPLE 16.

wherein each R independently is --H, C15 alkyl or C15 alkoxy.

4. A bath according to claim 3, wherein each R independently is --H, C,~4 alkyl or C14 alkoxy.

5. A bath according to any one of claims 1 to 4, wherein said imide is succinimide.

6. A bath according to claim 1 or claim 2, wherein said imide is in accordance with the formula:

wherein each R independently is --H, C15 alkyl or C15 alkoxy.

7. A bath according to claim 6, wherein each R independently is --H, C14 alkyl or C,, alkoxy.

8. A bath according to any one of claims 1, 2, 6.and 7, wherein said imide is maleimide.

9. A bath according td any one of claims I to 8, wherein said silver imide complex is a potassium silver imide complex.

10. A bath according to claim 9, wherein said potassium silver imide complex is the succinimide complex.

11. A bath according to claim 9, wherein said potassium silver imide complex is the maleimide complex.

12. A bath according to any one of claims I to 8, wherein said silver imide complex is an ammonium silver imide complex.

13. A bath according to claim 12, wherein said ammonium silver imide complex is the succinimide complex.

14. A bath according to claim 12, wherein said ammonium silver imide complex is the maleimide complex.

15. A bath according to any one of claims I to 14, wherein the pH of the bath is from 6.0 to 14.

16. A bath according to any one of claims 1 to 15, and including a brightener.

17. A bath according to claim 16, wherein said brightener is present in a concentration of 0.001 to 50 gram per liter.

18. A bath according to claim 16 or claim 17 wherein said brightener is selected from: an amine of the formula: RtCnH2nNH2, wherein R1 is --NH, or -H, and n is an integer of from 2 to 6 inclusive; polyethylene imines, polypropylene imines; polyhydroxyethyl ethylene imines; ethylamines; propylamine; ethylenediamine; propylenediamine; diethylenetriamine; triethylenetetramine; tetraethylenepentamine; pentaethylenehexamine; imino-bis-propylamine; dimethylamine propylamine; and diethylpropylenediamine.

19. A bath according to any one of claims 16 to 18. wherein an amine brightener is used.

20. A bath according to claim 18, wherein said amine brightener comprises at least one compound of the formula H2NCnH2nNH2, wherein n is an integer of from 2 to 6 inclusive.

21. A bath according to any one of claims 16 to 18 wherein a polyimine brightener is used.

22. A bath according to claim 21, wherein said polyimine brightener comprises polyethylene or polypropylene imine of molecular weight from 100 to 60.000.

23. A bath according to any one of claims 1 to 22 including the imide in free form.

24. A bath according to any one of claims 1 to 23, wherein said alloying metal is selected from copper, cadmium, gold, palldium and antimony.

25. A bath according to any one of claims 1 to 24, wherein the amount of said alloying metal salt or salts is such that deposits plated therefrom will contain up to 5 mol percent of the metal.

26. A bath according to any one of claims 1 to 25 and comprising: Silver (as salt or imide complex) 5 to 100 grams per liter Imide 10 grams per liter to saturation Alkali metal and/or ammonium hydroxide 5 to 200 grams per liter Conductivity salt 0 to 300 grams per liter Alloying metal (as salt) up to saturation point Water to 1 liter

27. An aqueous silver alloy plating bath substantially as described in any one. of the foregoing Examples 1 to 15.

28. A method of silver alloy electroplating whenever a bath in accordance with any one of claims 1 to 27 is employed as a silver alloy electroplating bath.

29. A method according to claim 28, wherein the bath temperature is in the range of 20"C to 400 C.

30. A method according to claim 28 or claim 29, wherein the cathodic current density is in the range of 0.1 to 3 A per square decimeter.

31. An article whenever electroplated by a method in accordance with any one of claims 28 to 30.

32. An ammonium silver imide complex wherein said imide is a five-membered heterocyclic ring derived from an organic dicarboxylic acid, and wherein, in said complex the mole ratio of silver to imide is I to 2 respectively.

33. A complex according to claim 32, wherein said imide is a pyrrolidine -2,5dione or a 3-pyrroline -2,5-dione.

34. A complex according to claim 32 or claim 33, wherein said imide is in accordance with the formula:

wherein each R independently is -It, C15 alkoxy or C, 5 alkyl.

35. A complex according to claim 34, wherein each R independently is -H, C14 alkyl or C14 alkoxy.

36. A complex according to any one of claims 32 to 35 wherein said imide is succinimide.

37. A complex according to claim 32 or 33, wherein said imide is in accordance with the formula:

wherein each R independently is --H, C15 alkoxy or C15 alkyl.

38. A complex according to claim 37, wherein each R independently is --H, C14 alkyl or C14 alkoxy.

39. A complex according to any one of claims 32. 33, 37 and 38, wherein said imide is maleimide.

40. An aqueous silver plating bath, said bath being characterised by its substantial freedom from cyanide, and comprising, in solution, an ammonium silver imide complex in accordance with any one of claims 32 to 39.

41. A bath according to claim 40, wherein the pH of the bath is from 6.0 to 14.

42. A bath according to claim 40 or claim 41, and including a brightener as defined in any one of claims 16 to 22.

43. A bath according to any one of claims 40 to 42, including the imide in free form.

44. A bath according to any one of claims 40 to 43 and comprising: Silver (as salt or imide complex) 5 to 100 grams per liter Imide 10 grams per liter to saturation Ammonium hydroxide 5 to 200 grams per liter Conductivity salt 0 to 300 grams per liter Water to 1 liter

45. An aqueous silver plating bath substantially as described in the foregoing Example 16.

46. A method of silver electroplating whenever a bath in accordance with any one of claims 40 to 45 is employed as a silver electroplating bath.

47. An article whenever electroplated by a method in accordance with claim 46.