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Title:
DEPOSITION OF COPPER-TIN-ZINC ALLOYS FROM AN ELECTROLYTE
Document Type and Number:
WIPO Patent Application WO/2013/092312
Kind Code:
A1
Abstract:
The present invention relates to a cyanide-free, pyrophosphate-containing electrolyte and a process for the electrolytic deposition of a ternary alloy of the elements copper, tin and zinc. The electrolyte and the process are characterized in that not only zinc (II) ions and copper ions but also stannate anions are present in the electrolyte used. The copper and zinc ions are present in a particular molar ratio relative to one another and to the pyrophosphate anions.

Inventors:
BRONDER KLAUS (DE)
MANZ UWE (DE)
Application Number:
EP2012/075111
Publication Date:
June 27, 2013
Filing Date:
December 11, 2012
Export Citation:
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Assignee:
UMICORE GALVANOTECHNIK GMBH (DE)
International Classes:
C25D3/56; C25D3/58; C25D3/60; C25D7/00; H01L31/032; H01L31/18
Domestic Patent References:
WO2009109271A22009-09-11
Foreign References:
DE102008032398A12010-01-14
EP1185151A12002-03-06
EP2116634A12009-11-11
EP2336394A22011-06-22
EP2037006A22009-03-18
EP0790332A11997-08-20
EP1146148A22001-10-17
JPH10102278A1998-04-21
US6416571B12002-07-09
EP2116634A12009-11-11
US20010014407A12001-08-16
US20100147696A12010-06-17
EP2037006A22009-03-18
EP2336394A22011-06-22
Other References:
THIN SOLID FILMS, vol. 517, 2009, pages 2511 - 2514
SOLAR ENERGY MATERIALS & SOLAR CELLS, vol. 95, 2011, pages 2136 - 2140
CHEMICAL PHYSICS LETTERS, vol. 501, 2011, pages 619 - 622
THIN SOLID FILMS, vol. 517, 2009, pages 2465 - 2468
SOLAR ENERGY MATERIALS & SOLAR CELLS, vol. 93, 2009, pages 996 - 999
PHYS. STAT. SOL., vol. 9, 2008, pages 1772 - 1778
PHYS. STAT. SOL., vol. 5, 2009, pages 1266 - 1268
Attorney, Agent or Firm:
RETZOW, Stefan (PatenteRodenbacher Chaussee 4, Hanau-Wolfgang, DE)
Download PDF:
Claims:
Patent claims

Aqueous, cyanide-free, pyrophosphate-containing electrolyte for the electrolytic deposition of a copper-tin-zinc alloy, which contains the metals copper and zinc to be deposited in ionically dissolved form and tin as dissolved stannate, wherein the electrolyte has a molar ratio of pyrophosphate anions to total copper and zinc ions in the range from >1 :2 to 20:1 and the molar ratio of copper ions to zinc ions in the electrolyte is in the range from 3:1 to 1 :4.

Electrolyte according to Claim 1 ,

characterized in that

the molar ratio of pyrophosphate anions to Cu and Zn ions is from 1 :1.4 to 2:1 .

Electrolyte according to Claim 1 and/or 2,

characterized in that

the molar ratio of Cu ions to Zn ions is in the range from 2:1 to 1 :3.

Electrolyte according to one or more of Claims 1 to 3,

characterized in that

the metals copper and zinc to be deposited are present in ionically dissolved form and the tin is present as stannate, where the ion concentration of copper is in the range from 0.1 to 10 g/l of electrolyte, the ion concentration of tin is in the range from 0.5 to 20 g/L of electrolyte and the ion concentration of zinc is in the range from 0.2 to 20 g/l of electrolyte.

Electrolyte according to one or more of Claims 1 to 4,

characterized in that

the compounds of the metals to be deposited which are soluble in water under the given reaction conditions are selected from the group consisting of pyrophosphates, carbonates, hydrogencarbonates, sulphites, sulphates, phosphates, nitrites, nitrates, halides, hydroxides, oxide-hydroxides, oxides and combinations thereof.

6. Electrolyte according to one or more of Claims 1 to 5,

characterized in that

the pH of the electrolyte is in the range from 6 to 13.

7. Electrolyte according to one or more of Claims 1 to 6,

characterized in that

one or more stabilizing compounds selected from the group consisting of monocarboxylic and dicarboxylic acids, alkanesulphonic acids, betaines and aromatic nitro compounds are present.

8. Process for the electrolytic deposition of Cu-Zn-Sn alloy layers, wherein the

substrate to be coated is dipped as cathode into an electrolyte according to one or more of Claims 1 to 7 and a flow of current is established between the anode and the cathode. 9. Process according to Claim 8,

characterized in that

the proportion of copper in the alloy is in the range from 38 to 44% by weight, the proportion of tin is in the range from 34 to 42% by weight and the proportion of zinc is in the range from 16 to 26% by weight. 10. Process according to Claim 8 and/or 9,

characterized in that

the electrolyte temperature is maintained in the range from 20 to 90°C.

1 1 . Process according to one or more of Claims 8 to 10,

characterized in that

a current density in the range from 0.1 to 5 ampere per square decimetre is set.

12. Process according to one or more of Claims 8-1 1 ,

characterized in that insoluble anodes, e.g. platinated titanium anodes or mixed metal oxide anodes, or soluble anodes composed of a material selected from the group consisting of electrolytic copper, phosphorus-containing copper, tin, tin- copper alloy, zinc-copper alloy and zinc-tin-copper alloy or combinations of these anodes are used.

13. Process for producing a thin film solar cell having a p-type absorption layer based on a CuxZnySnzSaSeb compound, where x = 1 .5 - 2.5, y = 0.9 - 1.5, z = 0.5 -

1 .1 , a = 0 - 4 and b = 0 - 4 and x + y + z and a + b are each about 4 (±0.2), characterized in that a CuxZnySnz alloy is produced by a process according to one or more of Claims 8-12 and this layer is subsequently reacted with sulphur, a sulphur compound and/or a selenium compound in such a way that the desired CuxZnySnzSaSeb compound is formed.

Description:
Deposition of copper-tin-zinc alloys from an electrolyte

Description

The present invention relates to a cyanide-free, pyrophosphate-containing electrolyte and a process for the electrolytic deposition of a ternary alloy of the elements copper, tin and zinc. The electrolyte and the process are characterized in that not only zinc (II) ions and stannate anions but also copper ions are present in the electrolyte used. The copper and zinc ions are present in a particular molar ratio relative to one another and to the pyrophosphate anions.

The electrolytic deposition of brass (Cu-Zn alloy) and bronzes (Cu-Sn alloy) on consumer goods or decorative goods has been known for a long time. They serve, inter alia, as a replacement for nickel-containing finishing layers and are, for example, applied inexpensively to appropriate substrates in electrochemical barrel or rack plating processes.

In the production of brass and bronze layers for the electronics industry, the solderability of the resulting layer and possibly its mechanical strength are the critical properties of the layer to be produced. The appearance of the layers is generally less important than their functionality for use in this field. On the other hand, the decorative effect and also durability of the layer with an ideally unchanged appearance are the important target parameters in the production of bronze or brass layers on consumer goods.

In the production of brass or bronze layers, known processes are not only the conventional processes using cyanide-containing and thus highly toxic, alkaline baths but also various electrochemical processes which can mostly be assigned according to the composition of their electrolytes to one of two main groups found in the prior art: processes using electrolytes based on organosulphonic acid or processes using baths based on disphosphoric acid (pyrophosphoric acid).

Cyanide-free electrolytic baths for the deposition of brass layers may be found, for example, in EP 790332. There, not only the copper and zinc, which can be added as pyrophosphate salts to the electrolyte, but also metal polyphosphates are added to the electrolyte. Possible metal polyphosphates are pyrophosphate salts of sodium, potassium, magnesium or calcium. The deposition of copper-zinc-tin layers is not described here.

EP 1 146148 describes a cyanide-free copper-tin electrolyte which contains the reaction product of an amine and an epichlorohydrin in a molar ratio of 1 :1 and also a cationic surfactant. The amine can be hexamethylene tetramine. JP 10102278 and US 6416571 describe baths for the deposition of copper-tin alloys. Cyanide-free electrolytic baths for the deposition of bronze layers are likewise adequately known. Thus, for example, WO 2009109271 reports that copper and tin can be deposited together from appropriate baths which have a large excess of pyrophosphate ions. All these teachings disclose exclusively the deposition of bronzes, i.e. copper-tin alloys.

The deposition of a ternary alloy consisting of copper, tin and zinc from a cyanide-free electrolyte is disclosed, for example, in EP 21 16634. There, not only a high concentration of pyrophosphate anions in the electrolyte but also a specific reaction product of hexamethylene tetramines and epichlorohydrin at a virtually neutral pH of the electrolyte are used. US20010014407 mentions in passing the deposition of a ternary alloy of Cu/Sn/Zn on copper surfaces as corrosion protection. Relatively low-tin alloys are obtained from the pyrophosphate-containing electrolyte.

US20100147696 discloses the deposition of Cu-Zn-Sn alloy from electrolytes containing phosphonic acid. The deposition processes described here give white coatings which are, however, relatively low in zinc.

A cyanide-free, pyrophosphate-containing electrolyte for the deposition of ternary copper-zinc-tin alloys is described in Thin Solid Films, 517 (2009) 251 1 -2514. Here, a layer which is not defined in more detail is deposited from an alkaline electrolyte containing the metals copper in the oxidation state +2, zinc in the oxidation state +2 and tin in the oxidation state +4. The electrolyte described here is said to contain a tenfold excess of tin and lead only to low-copper deposits.

EP 2037006 describes the electrolytic deposition of copper-tin-zinc alloys in a very particular atom ratio. The layers deposited have a composition which is said to be close to the formula Cu2ZnSn. The layers obtained in this way serve as base layer for the production of kesterite (CZTS or Cu2ZnSn(S,Se)4) which is a promising material for the production of photovoltaically active molecules (Solar Energy Materials & Solar Cells 201 1 , 95, 2136-2140; Chemical Physics Letters 201 1 , 501 , 619-622). In a production process for such modules, a correspondingly produced Cu2ZnSn layer is subsequently converted by reaction with sulphur or sulphur-containing compounds at elevated temperatures into the corresponding kesterite phase (e.g.: Thin Solid Films 2009, 517, 2465-2468). Such a procedure is likewise addressed in EP 2037006. There, the specific electrolytically produced Cu2ZnSn deposits are obtained from an electrolyte to which particular disubstituted benzene derivatives have been added. The copper and zinc ions can be added as pyrophosphates to the electrolyte. The tin is preferably used as stannate.

The electrolytes described for the deposition of the ternary alloy of copper, tin and zinc all have only a low ability to deposit a desired ternary alloy composition of this type when, for example, specific additional additives are not added to the electrolyte or extremely high tin-IV concentrations are not present in the electrolyte. The additional complexity caused thereby makes the production and processing of the electrolytes unattractive. It was therefore an object of the present invention to provide an electrolyte and a corresponding process for the deposition of a ternary alloy of copper, tin and zinc, which are able to bring about the indicated deposition with a preferred stochiometry in a very optimal way. The electrolyte should ideally have a simple composition. The process and the electrolyte according to the invention should also be superior to the processes and electrolytes known from the prior art from ecological and economic points of view.

These objects and further objects which can be derived in an obvious manner from the prior art by a person skilled in the art are achieved by an electrolyte having the features of the present Claim 1 and by a corresponding process according to Claim 9. Preferred embodiments of the present invention may be found in the claims dependent on these two claims.

The use of an aqueous, cyanide-free, pyrophosphate-containing electrolyte containing the metals copper and zinc to be deposited in dissolved form and tin as dissolved stannate, where the electrolyte has a molar ratio of pyrophosphate anions to total copper and zinc ions in the range from >1 :2 to 20:1 and the molar ratio of copper ions to zinc ions in the electrolyte is in the range from 3:1 to 1 :4, for the electrolytic deposition of a copper-tin-zinc alloy leads, extremely surprisingly but no less advantageously, to achievement of the stated object. It has been found that an advantageous ternary alloy composition can be achieved using the electrolyte described here when the pyrophosphate ions are present in excess over the copper and zinc ions in the electrolyte, when at the same time a particular ratio of copper to zinc ions is set and at the same time Sn is present as Sn 4+ . The electrolyte described here is noteworthy for the fact that obviously no further substances which influence the deposition of the ternary alloy have to be added to the electrolyte in order to bring about an appropriately composed deposition of copper-zinc-tin. In particular, the addition of reaction products of amines with epichlorohydrin as proposed in EP 21 16634 (mentioned at the outset) and the addition of disubstituted benzene derivatives known from EP 2037006 can be dispensed with. The composition of the ternary alloy of copper, tin and zinc can obviously be controlled in a simple manner via the abovementioned features alone. This has not yet been proposed in the prior art.

It is advantageous for the cyanide-free electrolyte to have a certain concentration of stabilizers. This task can be performed solely by the pyrophosphate anions present in the electrolyte. However, a person skilled in the art is free to add further stabilizers to the electrolyte. The concentration of pyrophosphate anions can be adjusted as desired within the abovementioned limits by a person skilled in the art and, as indicated above, dependent on the amount of copper and zinc ions used. A person skilled in the art will in this task be guided by, firstly, an optimal alloy composition being formed and, secondly, by the smallest possible amount of starting materials having to be employed for the deposition. The preferred range for the molar ratio of pyrophosphate anions to the sum of Cu and Zn ions addressed here should therefore be in the range from 1 :1.6 to 4:1. Particular preference is in this context given to a range from 1 :1 .4 to 2:1 .

According to the invention, the metals copper and zinc are present in ionically dissolved form in the present electrolyte. The copper can be added in the form of copper (I) salts or in the form of divalent copper salts or mixtures thereof to the electrolyte. Zinc will be present in the form of 2-valent ions in the electrolyte. The molar ratio of copper ions to zinc ions is preferably in the range from 2:1 to 1 :3. Very particular preference is given to a value of about 1 :1 - 1 :2. The tin is added as stannate salt, i.e. in the 4-valent form, to the electrolyte. Such stannate salts are well known to those skilled in the art. Particularly suitable stannate salts here are, for example, sodium stannate and potassium stannate. The ratios of the concentrations of copper and zinc ions relative to one another and also of the sum of these two ions to pyrophosphate anions are critical in determining the composition of the alloy deposited. It is naturally also necessary for the tin used to be present in a particular ratio to the copper and zinc ions. The molar ratio of stannate salt used to the sum of copper and zinc ions should be 1 :1 - 6:1 , preferably 1 .5:1 - 4:1 and particularly preferably from 2:1 to 3:1. As regards the metals copper and zinc to be deposited, which are, as indicated, present in ionically dissolved form in the electrolyte, and the tin which is present in dissolved form as stannate, the concentration ranges of the metal in the electrolyte can be selected by a person skilled in the art. It has been found to be advantageous for the ion concentration of copper to be in the range from 0.1 to 10 g/l of electrolyte, the concentration of tin to be in the range from 0.5 to 20 g/l of electrolyte and the ion concentration of zinc to be in the range from 0.2 to 20 g/l of electrolyte. The concentration of copper is particularly preferably 0.3 - 5 g/l, very preferably 0.5 - 1 .0 g/l. The concentration of zinc is particularly preferably 0.3 - 10 g/l, very preferably 0.5 - 2.0 g/l. The concentration of tin is particularly preferably 2 - 15 g/l, very preferably 3.5 - 10 g/l. In a particularly advantageous embodiment, use is made of an electrolyte in which:

Copper is present in a concentration of 0.5 - 1 g/l,

Zinc is present in a concentration of 0.5 - 2 g/l,

Tin is present in a concentration of 3.5 - 7.5 g/l, and

in each case based on the metal.

As indicated above, the copper and zinc ions are present in dissolved form in the electrolyte. As compounds of these metals to be deposited which are soluble in water under the reaction conditions indicated, it is possible to employ compounds selected from the group consisting of pyrophosphates, carbonates, hydrogencarbonates, sulphites, sulphates, phosphates, nitrites, nitrates, halides, hydroxides, oxide- hydroxides, oxides and combinations thereof. Preference is given to using carbonate, hydrogencarbonates, sulphates or pyrophosphates. Very particular preference is given to the addition of sulphates in this context.

The electrolyte is operated in the slightly acidic to strongly alkaline range. The pH of the electrolyte is preferably in the range from 6 to 13, more preferably from 7.5 to 12 and very particularly preferably from 8 to 1 1 .5. The pH of the electrolyte according to the invention is especially preferably about 1 1. A person skilled in the art will know how to set such pH values in the electrolyte using appropriate buffer substances. Preferred buffer substances are salts of weak organic or inorganic acids selected from the group consisting of phosphoric acid and citric acid.

Further additives (brighteners, wetting agents) selected from the group consisting of monocarboxylic and dicarboxylic acids, alkanesulphonic acids, betaines and aromatic nitro compounds can be added to the electrolyte. Such additives are adequately known for the present type of baths, in particular in the field of deposition of brass or bronze. As regards the amount and the materials used, reference may be made to the literature. Such additives are particularly preferably selected from the group consisting of oxalic acid, tartaric acid, citric acid and salts thereof. The present invention likewise provides a process for the electrolytic deposition of Cu- Zn-Sn alloy layers, in which the substrate to be coated is dipped as cathode into an electrolyte according to the invention and a flow of current is established between the anode and the cathode. It goes without saying that the embodiments of the electrolyte mentioned as preferred are analogously likewise preferred for the process. It is advantageous for the proportion of copper in the ternary alloy deposited to be in the range from 38 to 44% by weight, the proportion of tin to be in the range from 34 to 42% by weight and the proportion of zinc to be in the range from 16 to 26% by weight. Preference is given to alloys containing 39 - 42% by weight of Cu, very preferably about 40 - 41 % by weight. Preference is also given to alloys containing 19 - 23% by weight of Zn, very preferably about 21 % by weight. Preference is likewise given to alloys containing 36 - 40% by weight of Sn, very preferably about 38% by weight. The sum of the alloy constituents should be 100% by weight. The alloy deposited should have a thickness of 0.4 - 5 μηη, preferably 0.5 - 3 m and very particularly preferably 1 - 2 μηι. It may be pointed out that the alloy composition can likewise change with the temperature prevailing during the electrolysis. The electrolysis is therefore carried out in the range from 20 to 90°C, preferably from 30 to 60°C and very preferably about 45°C.

The composition of the ternary alloy of copper, tin and zinc can likewise change with the current density set during the electrolysis. It is advantageous to set a current density in the range from 0.1 to 5 ampere per square decimetre. The current density is preferably from 0.2 to 1 .0 ampere per square decimetre, very preferably from 0.3 to 0.8 ampere per square decimetre.

As anode, it is possible to use any electrode which comes into question for this purpose to a person skilled in the art. Preference is given to using insoluble anodes (e.g. platinated titanium anodes or mixed metal oxide anodes). In this context, soluble anodes composed of a material selected from the group consisting of electrolytic copper, phosphorus-containing copper, tin, tin-copper alloy, zinc-copper alloy and zinc- tin-copper alloy or combinations of these anodes are likewise advantageous.

It is known from EP 2037006 cited at the outset that, inter alia, copper-tin-zinc alloys having a particular composition are suitable for the production of photovoltaic cells. As regards the production of thin film solar cells, reference may be made to the information given there and in EP 2336394. As regards the production of the solar cells, reference may likewise be made to the following literature:

Solar Energy Materials & Solar Cells, 95 (201 1 ) 2136-2140;

Chemical Physics Letters, 501 (201 1 ) 619-622;

Solar Energy Materials & Solar Cells, 93 (2009) 996-999;

Thin Solid Films, 517 (2009) 2465-2468;

Phys. Stat. Sol., 9 (2008) 1772-1778;

Phys. Stat. Sol., 5 (2009) 1266-1268;

Thin Solid Films, 517 (2009) 251 1 -2514.

The present invention therefore likewise provides a process for producing a thin film solar cell having a p-type absorption layer based on a CuxZnySnzSaSeb compound, where x = 1.5 - 2.5, y = 0.9 - 1 .5, z = 0.5 - 1 .1 , a = 0 - 4.2 and b = 0 - 4.2 and x + y + z and a + b are each about 4 (±0.2), wherein a CuxZnySnz alloy is produced by a process according to the invention and this layer is subsequently reacted with sulphur, a sulphur compound and/or a selenium compound in such a way that the corresponding compound is formed.

The alloy composition achieved by means of electrolysis preferably very closely approximates that corresponding to the alloy base material in the material kesterite (Cu2ZnSnS4). The layer produced by the process of the invention very preferably consists of a composition close to the formula Cu2ZnSn. From this, the desired compound Cu2ZnSn(SeS)4 (CZTS) is produced by action of sulphur, selenium and/or appropriate compounds using appropriate processes as discussed in the literature.

In the light of the prior art, it was not possible to see that a process configured like the present process makes it possible to produce corresponding ternary alloy compositions by electrolytic deposition in such a simple way. In particular, it is surprising that the presence of particular ratios of copper and zinc ions to pyrophosphate anions and the ratio of copper ions and zinc ions relative to one another are responsible for this as long as Sn is present as Sn 4+ . In view of this, it appears to be particularly advantageous that the simple setting of the appropriate ratios obviously makes it possible to set the alloy composition deposited in an advantageous way. This was not to be expected in the light of the background of the known prior art.

Examples:

General procedure:

The ingredients mentioned below are dissolved in water and set to the appropriate pH. Any initial turbidity in the electrolyte disappears again after addition of the stannates. An electrolysis is subsequently carried out under the conditions indicated (e.g. 45°C, pH=1 1 , 0.8 A/dm 2 ).

. Electrolyte composition according to the invention (Cu:Zn

90 g/l dipotassium hydrogenphosphate

15 g/l dipotassium oxalate monohydrate 0. .081 mol

1 .5 g/l sodium sulphite

7.5 g/l Sn as Na stannate 0. .063 mol

10 g/l potassium pyrophosphate (330.34 g/mol) 0. .030 mol

1 .0 g/l Cu (as Cu sulphate) (63.5 g/mol) 0. .016 mol

2.0 g/l Zn (as Zn sulphate) (65.3 g/mol) 0. .031 mol

0. .030 mol K pyrophosphate

0. .047 mol metal

Ratio of metal (Cu+Zn): K pyrophosphate = 1.6 : 1 pH 1 1 .0 45°C

0.8 A/dm 2 -» Alloy 42% by weight Cu

36% by weight Sn

22% by weight Zn

2. Electrolyte composition according to the invention (Cu:Zn

90 g/l dipotassium hydrogenphosphate

15 g/l dipotassium oxalate monohydrate 0. .081 mol

1 .5 g/l sodium sulphite

7.2 g/l Sn as Na stannate 0. .060 mol

10 g/l potassium pyrophosphate (330.34 g/mol) 0. .030 mol

0.3 g/l Cu (as Cu sulphate) (63.5 g/mol) 0. .005 mol

1 .0 g/l Zn (as Zn sulphate) (65.3 g/mol) 0. .015 mol

0. .030 mol K pyrophosphate

0. .020 mol metal Ratio of metal (Cu+Zn): K pyrophosphate = 1 : 1.5

pH 1 1 .0 45°C

0.7 A/dm 2 -» Alloy 38% by weight Cu

37% by weight Sn

25% by weight Zn

3. Comparative experiment 2 as per EP2336394 (no additives):

95 g/l sodium phosphate x 12 H 2 0

67 g/l disodium hydrogenphosphate x 7 H 2 0

13.4 g/l dipotassium oxalate monohydrate 0.073 mol

1 .3 g/l sodium sulphite

3.22 g/l Sn as Na stannate 0.027 mol Sn

0.43 g/l Cu 0.0068 mol

= 1 .2 g/l Cu 2 P 2 0 7 x 3H 2 0 (M=354.8) 0.0034 mol pyrophosphate 0.28 g/l Zn 0.0042 mol

= 0.76 g/l Zn 2 P 2 0 7 x 3H 2 0 (M=358.4) 0.0021 mol pyrophosphate

0.0055 mol pyrophosphate 0.01 1 mol metal

Ratio of metal (Cu+Zn): pyrophosphate = 2 : 1

pH 1 1 .2 - 1 1 .8 45°C

Potentiostatic 1 .8 V

= Estimated current density: about 0.3 A/dm 2

-» Alloy 52.6 atom% Cu = 46.3% by weight

16.7 atom% Sn = 26.7% by weight 30.7 atom% Zn = 27.0% by weight

4. Electrolyte composition according to the invention (Cu:Zn

90 g/l dipotassium hydrogenphosphate

15 g/l dipotassium oxalate monohydrate 0. .081 mol

1 .5 g/l sodium sulphite

3.6 g/l Sn as Na stannate 0. .030 mol Sn

10 g/l potassium pyrophosphate (330.34 g/mol) 0. .030 mol

0.5 g/l Cu (as Cu sulphate) (63.5 g/mol) 0. .008 mol

0.5 g/l Zn (as Zn sulphate) (65.3 g/mol) 0. .008 mol

0. .030 mol K pyrophosphate

0. .016 mol metal Ratio of metal (Cu+Zn): K pyrophosphate = 1 : 1.9

pH 1 1 .0 45°C

0.4 A/dm 2 Alloy 43% by weight Cu

41 % by weight Sn

16% by weight Zn

5. Electrolyte composition according to the invention (Cu :Zn = 2:1 ):

90 g/l dipotassium hydrogenphosphate

15 g/l dipotassium oxalate monohydrate 0.081 mol

1 .5 g/l sodium sulphite

3.6 g/l Sn as Na stannate 0.030 mol Sn

10 g/l potassium pyrophosphate (330.34 g/mol) 0.030 mol

1 .0 g/l Cu (as Cu sulphate) (63.5 g/mol) 0.016 mol

0.5 g/l Zn (as Zn sulphate) (65.3 g/mol) 0.008 mol

0.030 mol K pyrophosphate 0.024 mol metal

Ratio of metal (Cu+Zn): K pyrophosphate

pH 1 1 .0 45°C

1 .0 A/dm 2 Alloy