Electrolyte composition and method for the deposition of a tin-zinc alloy

Description

[0001] The present invention relates to an electrolyte composition for the deposition of a tin-zinc alloy on a substrate surface. Furthermore, the invention relates to a method for the deposition of a tin-zinc alloy layer on a substrate surface.

[0002] In the art of galvanic plating the deposition of tin-zinc alloy layers on substrate surfaces is well known. Tin-zinc alloy electroplating methods have come to be widely used as an industrial plating method in various industries like automobile industries, fitting industries or electronic industries. Tin-zinc alloy layers on substrate surfaces deposited by electroplating methods have excellent corrosion resistance and good solder abilities.

[0003] Also the protection of steel products for the construction industry by tin-zinc alloy layers is well known in the state of the art.

[0004] For the electrolytic deposition of tin-zinc alloy layers on substrate surfaces, several processes and electrolytes are known from the state of the art.

[0005] For example, US 6,436,269 B1 discloses an aqueous plating bath for the electrodeposition of a tin-zinc alloy comprising at least one bath soluble stannous salt, at least one bath soluble zinc salt, and a quaternary ammonium polymer selected from a ureylene quaternary ammonium polymer, an iminoureylene quaternary ammonium polymer or a thioureylene quaternary ammonium polymer. The plating bath disclosed in US 6,436,269 B1 may also contain hydroxy polycarboxylic acids like citric acid, ammonium salts, conducting salts, aromatic carbonyl-containing compounds, polymers of aliphatic amines or hydroxyalkyl substituted diamines as metal complexing agents. From such an aqueous plating bath bright and level layers can be deposited.

[0006] From US 5,618,402 A a tin-zinc alloy electroplating bath is known, comprising an amphoteric surfactant , a water soluble stannous salt, a water soluble zinc salt and balance water.

[0007] Most of the tin-zinc alloy plating bathes either work in a strong acidic range or in a strong alkaline range. However, when working with highly acidic or highly alkaline solutions, special safety precautions have to been taken to secure the persons working with these solutions. Furthermore, highly acidic or highly alkaline solution in general are very aggressive to the plating equipment used in plating shops.

[0008] It is therefore an object of the present invention to provide an electrolyte composition for the deposition of a tin-zinc alloy on a substrate surface as well as an accordingly method for such an deposition, which overcomes the drawbacks known from the state of the art caused by the use of highly acidic or highly alkaline electrolytes.

[0009] In terms of the electrolyte, this problem is solved according to the invention by an electrolyte composition for the deposition of a tin-zinc alloy on a substrate surface, wherein tin is comprised in the electrolyte composition in oxidation state +4 (Sn⁴⁺) and wherein the electrolyte composition has a pH-value in the range of about pH 9 to about pH 11.

[0010] The electrolyte composition according to the invention comprises at least zinc in form of a soluble zinc compound, tin in form of a water soluble stannate and an complexing agent.

[0011] As a source of zinc the electrolyte composition can comprise zinc hydroxide which, under the conditions of the electrolyte composition, reacts to oxyl or hydroxyl acids of zinc, known as zincates.

[0012] As a source of tin, the electrolyte composition can comprise stannic acid or derivatives of stannic acid like salts.

[0013] As complexing agent the inventive electrolyte composition comprises at least one of the group consisting of carboxylates, amino derivatives, phosphates and hydroxides. A preferred complexing agent in the inventive electrolyte composition is 1-hydroxyethylene-1, 1-diphosphonic acid.

[0014] The inventive electrolyte composition may further comprise a brightening agent for the deposition of bright tin-zinc layers. As a brightening agent the electrolyte composition may comprise at least one of the group consisting of aldehydes, ketones, mercapto-glycollates, benzalacetone derivatives, orthochlorobenzaldehyde derivatives, unsaturated ethane alkyl carboxylates, multiple unsaturated alkanes extended with amine or carboxyl groups and metal cations acting as grain refiners such as iron, cobalt or nickel.

[0015] A preferred brightening agent within this group is methacrylic acid.

[0016] To improve the conductivity of the electrolyte composition, the electrolyte may further comprise alkaline hydroxides, sulphates or chlorides.

[0017] Surprisingly, it was found that when using stannous compounds as source for tin in a tin-zinc alloy plating electrolyte, releasing tin in oxidation state +4 (Sn⁴⁺), the pH-value of the composition can be maintained within a range from about pH 9 to about pH 11 for the deposition of tin-zinc alloy layers on substrate surfaces having excellent corrosion resistance, good mechanical properties and an adequate appearance.

[0018] Furthermore, when set to a pH-value in the range from about pH 9 to about pH 11, the electrolyte composition is less aggressive in comparison to the plating compositions known from the state of the art and can easily be handled. Also, the inventive electrolyte composition is less aggressive to the plating equipment, thereby reducing the need for maintenance of the plating equipment.

[0019] Surprisingly, it was also found that the crystal structure of the tin-zinc alloy layer deposited from the inventive electrolyte composition allows the inclusions of further compounds or elements within the crystal lattice. This allows the deposition of composite layers enclosing a composite material within the crystal lattice, thereby forming a surface layer with advanced properties.

[0020] For example, the enclosure of silicon within the tin-zinc alloy lattice is possible.

[0021] As a result of this enclosure, the composite layers will have enhanced physical and chemical properties. Depending on the co-deposited material corrosion resistance, hardness, wear-resistance and friction can be improved.

[0022] It was found that beneath silicon further elements or compounds capable to be co-deposited with the tin-zinc-alloy are, for example boron, carbon, SiO₂, SiL, PTFE, and MoS₂.

[0023] Concerning the method, the object of the invention is solved by a method for the deposition of a tin-zinc alloy layer on a substrate surface, the method comprising the steps:

• contacting the substrate surface to be plate with an electrolyte composition comprising tin in oxidation state +4 (Sn⁴⁺) at a pH-value in the range from about pH 9 to about pH 11 and a temperature in the range from about 20°C to about 70°C; and
• conducting a current with a current density in the range from about 0,2 A/dm² to about 2 A/dm² between the substrate surface and a counter electrode.

[0024] Zinc is preferably pre-dissolved in alkaline or acidic medium to form water soluble salts resulting in zinc ions or zincate. As an alternative, zinc may be added as zinc hydroxide to the inventive electrolyte.

[0025] Tin is pre-dissolved in alkaline media to form stannate ions (Sn⁴⁺). This solution should be used fresh to avoid formation of meta stannates.

[0026] To improve the conductivity of the inventive electrolyte further, alkaline hydroxides can be added. It was found that the addition of potassium hydroxide to the inventive electrolyte composition further improves the solubility of the bath chemicals, compared to the use of other alkaline hydroxides.

[0027] The bath performs at different temperatures and cathodic current densities to give an alloy deposition targeted at 70 - 80% by weight Sn and 30 - 20% by weight zinc for optimal corrosion resistance.

[0028] A true 98,5% by weight tin alloy can be obtained next to deposition of pure tin and zinc crystals in the deposited layer.

[0029] For the deposition of composite layers the element to be deposited together with the alloy metals can be added to the inventive electrolyte composition as a water soluble or water dispersible compound. The element to be deposited together with the alloy metals can be comprised in the electrolyte composition in an amount up to 100 g/I.

[0030] For the deposition of the pure tin-zinc alloy layer as well as the composite-layer an inert carbon/graphite anode can be used as counter electrode.

[0031] Since one of the main features of the inventive electrolyte composition is to be applicable at a pH from about pH 9 to about pH 11, the control of the pH-value is important. To correct the pH level, alkaline hydroxides like potassium hydroxide or acids like hydrochloric acid can be added to the electrolyte composition in order to set the pH level in the inventive range.

[0032] The invention will be described further by the following examples, while not being limited to these embodiments.

Examples:

Example 1

[0033] A steel substrate was contacted with an electrolyte comprising 7 g/I zinc as zincate, 42 g/I tin as stannate, 230 g/l of an complexing agent based on organophosphonate and hydroxyl carboxy alkanes, 0,8 g/I methylmethacrylate, 0,8 g/I fluoro-aliphatic ammonium surfactant and potassium hydroxide as well as hydrochloric acid to set the pH-value to pH 10. The substrate was contacted with the electrolyte at a temperature of 55°C. During contacting the substrate with the mentioned electrolyte composition, a cathodic current density of 0,8 A/dm² was conducted between the substrate surface and a carbon/graphite inert anode for 20 minutes. A uniform semi-bright tin-zinc deposit was obtained; containing 25-30% zinc, remainder tin.

Examples 2 and 3

[0034] In comparative examples in the electrolyte descripted in example 1 either the methylmethacrylate or the fluorosurfactant was omitted. When the methylmethacrylate was omitted the low current density area was found grainy. Omitting the fluorosurfactant caused pitting because gas bubbles were adhere to the surface and hindered the deposition of the alloy.

Example 4

[0035] In a further embodiment colloidal silica dispersion was added to the electrolyte descripted in example 1. Plating conditions were the same as in example 1.The addition of a colloidal silica dispersion allowed a co-deposition of silica particles. A tin-zinc-SiO₂ composite was obtained, having a composition of 87% tin, 8% zinc and 5% SiO₂. The composition of the composite could be controlled by varying the concentrations and conditions.

Claims

1. Electrolyte composition for the deposition of a tin-zinc alloy on a substrate surface, wherein tin is comprised in the electrolyte composition in oxidation state +4 (Sn⁴⁺) and wherein the electrolyte composition has a pH-value in the range from about pH 9 to about pH 11.

2. Electrolyte composition according to claim 1, comprising at least

- 4 - 10 g/I zinc;

- 35 - 55 g/I tin as Sn⁴⁺;

- 50 - 300 g/I of an complexing agent;

3. Electrolyte composition according to one of the claims 1 or 2, comprising a brightening agent in an amount up to 5 g/l.

4. Electrolyte composition according to one of the claims 1 to 3, comprising a surface wetting agent in an amount up to 5 g/l.

5. Electrolyte composition according to claim 3, wherein the brightening agent is at least on of the group consisting of aldehydes, ketones, mercapto-glycollates, benzalacetone derivatives, orthochlorobenzaldehyde derivatives, unsaturated ethane alkyl carboxylates, multiple unsaturated alkanes extended with amine or carboxyl groups, and metal cations acting as grain refiners.

6. Electrolyte composition according to claim 4, wherein the complexing agent is at least on of the group consisting of carboxylates, amino derivatives, phosphates and hydroxides.

7. Electrolyte composition according to one of the preceding claims, further comprising an alkali hydroxide to improve the conductivity of the electrolyte.

8. Electrolyte composition according to one of the claims 1 to 7, further comprising a source of an element or compound to be deposited together with the alloy metals tin and zinc to form a composite deposit.

9. Electrolyte composition according to claim 8, wherein the element or compound to be deposited together with the alloy metals is at least one element or compound of the group consisting of silicon, boron, carbon, SiO₂ SiC, PTFE and MoS₂.

10. Electrolyte according to claim 9, wherein the element or compound to be deposited together with the alloy metals is comprised in the electrolyte composition in an amount up to 100 g/l.

11. A method for the deposition of an tin-zinc alloy layer on a substrate surface, the method comprising the steps:

- contacting the substrate surface to plated with an electrolyte composition comprising tin in oxidation state +4 (Sn⁴⁺) at a pH in the range from about pH 9 to about pH 11 and an temperature in the range from about 20°C to about 70°C; and

- conducting a current with a current density in range from about 0,2 A/dm² to about 2A/dm² between the substrate surface and an counter electrode.

12. The method according to claim 11, wherein the counter electrode is an inert carbon/graphite anode.

13. The method according to one of the claims 11 or 12, wherein an element to be deposited together with the alloy metals tin and zinc is added to the electrolyte.

14. The method according to claim 13, wherein as element to be deposited together with the alloy metals at least one element or compound of the group consisting of silicon, boron, carbon, SiO₂, SiC, PTFE and MoS₂ is added to the electrolyte.

15. The method according to claim 14, wherein the element or compound is added to the electrolyte in form of a soluble compound, preferably as a salt.

Amended claims

Amended claims in accordance with Rule 137(2) EPC.

1. Electrolyte composition for the deposition of a tin-zinc alloy on a substrate surface, wherein tin is comprised in the electrolyte composition in oxidation state +4 (Sn4+) and wherein the electrolyte compositon has a pH-value in the range from about pH 9 to about pH 11, characterized in that the electrolyte composition comprises methylmethacrylate.

2. Electrolyte composition according to claim 1, comprising at least

- 4 - 10 g/l zinc;

- 35 - 55 g/l tin as Sn⁴⁺;

- 50 - 300 g/l of an complexing agent;

3. Electrolyte composition according to one of the claims 1 or 2, comprising a brightening agent in an amount up to 5 g/l.

4. Electrolyte composition according to one of the claims 1 to 3, comprising a surface wetting agent in an amount up to 5 g/l.

5. Electrolyte composition according to claim 3, wherein the brightening agent is at least on of the group consisting of aldehydes, ketones, mercapto-glycollates, benzalacetone derivatives, ortho-chlorobenzaldehyde derivatives, unsaturated ethane alkyl carboxylates, multiple unsaturated alkanes extended with amine or carboxyl groups, and metal cations acting as grain refiners.

6. Electrolyte composition according to claim 4, wherein the complexing agent is at least on of the group consisting of carboxylates, amino derivatives, phosphates and hydroxides.

7. Electrolyte composition according to one of the preceding claims, further comprising an alkali hydroxide to improve the conductivity of the electrolyte.

8. Electrolyte composition according to one of the claims 1 to 7, further comprising a source of an element or compound to be deposited together with the alloy metals tin and zinc to form a composite deposit.

9. Electrolyte composition according to claim 8, wherein the element or compound to be deposited together with the alloy metals is at least one element or compound of the group consisting of silicon, boron, carbon, SiO₂, SiC, PTFE and MoS₂.

10. Electrolyte according to claim 9, wherein the element or compound to be deposited together with the alloy metals is comprised in the electrolyte composition in an amount up to 100 g/l.

11. A method for the deposition of an tin-zinc alloy layer on a substrate surface, the method comprising the steps:

- contacting the substrate surface to plated with an electrolyte composition comprising tin in oxidation state +4 (Sn⁴⁺) at a pH in the range from about pH 9 to about pH 11 and an temperature in the range from about 20°C to about 70°C, wherein the electrolyte comprises methylmethacrylate; and

- conducting a current with a current density in range from about 0,2 A/dm² to about 2A/dm² between the substrate surface and an counter electrode.

12. The method according to claim 11, wherein the counter electrode is an inert carbon/graphite anode.

13. The method according to one of the claims 11 or 12, wherein an element to be deposited together with the alloy metals tin and zinc is added to the electrolyte.

14. The method according to claim 13, wherein as element to be deposited together with the alloy metals at least one element or compound of the group consisting of silicon, boron, carbon, SiO₂, SiC, PTFE and MoS₂ is added to the electrolyte.

15. The method according to claim 14, wherein the element or compound is added to the electrolyte in form of a soluble compound, preferably as a salt.