WO1999050479A1

WO1999050479A1 - Electroplating solution

Info

Publication number: WO1999050479A1
Application number: PCT/GB1999/000913
Authority: WO
Inventors: Ernest Long; Trevor Pearson; Anthony Rowan
Original assignee: Wm. Canning Limited
Priority date: 1998-03-27
Filing date: 1999-03-23
Publication date: 1999-10-07
Also published as: GB9806539D0; AR014781A1; AU3044499A

Abstract

An electroplating solution for forming a zinc-nickel alloy on a substrate by electrodeposition comprises: (i) zinc ions; (ii) a soluble source of nickel which allows nickel co-deposition; (iii) at least one alkaline electrolyte; and (iv) a soluble compound having a silicate-based anion, the silicate-based anion being present in the solution in a concentration of 0.1 to 100 g/l.

Description

1 ELECTROPLATING SOLUTION

The present invention relates to an electroplating solution for depositing a zinc-nickel alloy coating onto a substrate.

Electrodeposition of zinc-nickel alloy using an aqueous alkaline plating solution is well known. Much research has been directed towards achieving better deposition in terms of, for example, brightness, absence of pitting, uniformity and efficiency. To this end, plating solutions have been developed which include additives other than the required zinc and nickel salts. US 4889602 discloses a plating bath in which an additive is one of (i) an aliphatic amine, (ii) a polymer of an aliphatic amine or (iii) a hydroxyaliphatic carboxylic acid or salt thereof. US 5417840 discloses a bath containing an additive which is a salt of an aromatic heterocyclic nitrogen containing compound. US 5405523 and US 5435898 disclose the use of a quaternary ammonium polymer in a zinc-nickel plating bath.

The object of the present invention is to provide an electroplating solution which has improved characteristics and which, in use, delivers an improved zinc-nickel electrodeposit onto a substrate.

According to the present invention, there is provided an aqueous alkaline electroplating solution for forming a zinc-nickel coating on a substrate, said solution comprising:-

(i) zinc ions;

(ii) a soluble source of nickel which allows nickel co-deposition;

(iii) at least one alkaline electrolyte; and

(iv) a soluble compound having a silicate-based anion, the silicate- based anion being present in the solution in a concentration range of 0.1 to 100g/l. At a higher pH, the ionic zinc species may include zincate ion. As used herein "zinc ion" is intended to include zincate ion and other ionic zinc species.

Preferably, zinc ions are present in a concentration of 1 to 70 g/l, more preferably, 4 to 35 g/l.

Preferably zinc ions are provided by the chemical dissolution of zinc oxide or the direct electrochemical dissolution of zinc metal.

Preferably the nickel is present in the range 0.1 to 50 g/l, more preferably, 0.5 to 20 g/l.

The nickel is preferably provided by a salt of nickel, such as the chloride, sulphate, sulphamate, carbonate, hydroxide or acetate salt, combined with at least one complexing agent which renders the nickel soluble at the operating pH of the electrolyte and which is of a suitable stability constant to allow co-deposition of nickel from the solution.

Examples of suitable complexing agents include compounds of formula (I):-

RfNHCH₂CH₂fNHR' (I)

wherein R and R' are each independently hydrogen or C, to C₆ alkyl and n = 2 to 50.

Preferably, the total concentration of the compound(s) of formula (I) is in the range of 17 to 100 g/l, and more preferably in the range of 20 to 60 g l- Suitable complexing agents also include compounds of formula (II):-

¹ NCH₂CH₂N (ll)

R R₄ wherein Ri, R₂, R₃ and R₄ are each independently selected from C, to C₄ alkyl and hydroxyalkyl, provided that at least one of R R₂, R₃ and R₄ is hydroxyalkyl. Preferably, the total concentration of the compound(s) of formula (II) is in the range of 1 to 50 g/l.

Compounds of formulae (I) and (II) may be used singly, but preferably in combination with each other (i.e. at least one compound represented by each of formulae (I) and (II)).

Preferably, the silicate-based anion is selected from the group consisting of silicate, metasilicate, sesquisilicate, orthosilicate and polysilicate, which may be used singly or in combination. Preferably, the cation (the counter ion) is sodium and/or potassium. The preferred concentration range for the silicate-based species is 0.5 to 50 g/l, preferably 2 to 20 g/l.

Suitable alkaline electrolytes include NaOH, KOH, Na₂C0₃ and K₂C0₃, which may be used singly or in combination so as to give a minimum solution pH of 1 1. Typically, the electrolyte(s) may be present in a concentration of 50 to 250 g/l, more preferably, 80 to 200 g/l (based on the anhydrous salt).

As a further improvement, the solution may also contain compounds of the following types; thiourea or substituted thiourea, such as methyl thiourea and allyl thiourea, and heterocyclic thio compounds, such as 2,5- dimercapto-1 ,3,4-thiadiazole, 2-mercapto-5-amino-1 ,3,4-thiadiazole. The preferred concentration range of these compounds is 0.001 to 1.0 g/l. 4

Preferably, the solution also contains a quaternary moiety of formula (III):

^RV ^R3 ₊

-N-(CH₂)₃-NH-C-NH-N— R₅- 2nX (ill) R₂ R₄

wherein R_{1 ;} R₂, R₃ and R₄ are each independently selected from the group consisting of methyl, ethyl, isopropyl, 2-hydroxyethyl and -CH₂CH₂(OCH₂CH₂)_xOH (where x = 0 to 6); and R₅ is selected from the group consisting of -(CH₂)₂-0-(CH₂)₂-, -(CH₂)₂-0-(CH₂)₂-0-(CH₂)₂-, -CH₂CH(OH)CH₂-, CH₂CH(OH)CH₂OCH₂CH(OH)CH₂-, and -(CH₂)_m- (where m = 2 to 8); Y is selected from the group consisting of S and O, X is a halogen atom; and n is at least 1.

The solution may also include product(s) of the reaction between epichlorohydrin and an aliphatic amine, and/or product(s) of the reaction between epichlorohydrin and imidazole, preferably in a concentration of 0.1 to 20 g/l.

Aromatic aldehydes may be included in the solution, preferably in a concentration range of 0.01 to 2 g/l. Suitable aldehydes include 4- hydroxy-3-methoxybenzaldehyde, 1 ,3-benzodioxole-5-carboxy-aldehyde, veratraldehyde, anisaldehyde, p-tolualdehyde, benzaldehyde, o- chlorobenzaldehyde, 2,3-dimethoxybenzaldehyde, salicylaldehyde and cinnamaldehyde. Alternatively, or in addition, sodium sulphite adducts of any of the above aldehydes may be included in the solution.

The present invention also resides in:-

(i) an electroplating bath containing an aqueous alkaline electroplating solution according to the present invention;

(ii) a method of depositing a zinc-nickel alloy coating onto a substrate using an aqueous alkaline electroplating solution according to the present 5 invention; and

(iii) a process for electrodepositing a zinc-nickel alloy on a conductive substrate, which comprises contacting the substrate with an aqueous alkaline electroplating solution according to the present invention, and passing an electric current through the solution so as to cause the zinc- nickel alloy to be deposited on the substrate.

In use, the electroplating solutions of the present invention exhibit significant improvements in terms of tolerances to changes in electrolyte composition, such as salt build up (e.g. carbonate, sulphate and sulphamate) and metal contamination. They also allow the use of wide operating temperatures and current density ranges. Also, they give advantages in terms of the plating rates achievable in both rack and barrel applications as compared to the prior art.

The resultant deposit shows excellent adhesion to the substrate, and is fully bright over a very wide current density range with no deposit pitting evident. The deposit also exhibits excellent post-plate deformation characteristics.

The present invention will now be described in further detail in the following Examples in which the pH of the solutions was > 1 3:-

Comparative Example

An aqueous electroplating solution having the following composition was prepared by dissolving 120g of sodium hydroxide in 300 ml of water. To this hot caustic solution 12.5g of ZnO was added. Once the ZnO had fully dissolved, a further 400 ml of water was added and the solution allowed to cool. 8 g of NiS0₄.6H₂0 was then dissolved in 50 ml of water and to this 30 g of a polyethylene imine (mol. wt. 1500) and 20 g of 6

N,N,N',N' tetrakis hydroxyisopropyl ethylenediamine was added. The complexed nickel sulphate was then added to the zinc and sodium hydroxide base solution and water added to give 1 litre final volume.

Zinc 10 g/l

Nickel 2.0 g/l

Sodium hydroxide 120 g/l

PEI (mol. wt. 3500) 30 g/l

N,N,N'N' tetrakis hydroxyisopropyl ethylenediamine 20 g/l

Water to 1 litre

Using the above electroplating solution a 1A/10 min hull cell test (30°C, mild air agitation) was carried out. A steel hull cell panel was used as the cathode and the anode material used was nickel-plated steel.

The resulting hull cell panel was fully bright from a distance on the test panel corresponding to a primary current density of 3 A/dm² to the high current density (hcd) edge of the hull cell panel. The nickel concentration in the deposit was 12 to 14%.

Examples 1 to 4

As an illustration of the invention, increasing concentrations of sodium silicate were added to the electroplating solution of the above Comparative Example. This had the effect of considerably increasing the bright range that could be achieved (see table 1); Table 1. Effect of increasing silicate concentration on bright range.

Silicate Bright Range (A/dm² to hcd

Example concentration (g l) edge of hull cell panel)

Comparative 0 3

1 3 1.5

2 6 0.75

3 9 0.1

4 15 0.05

Example 5

A hull cell test (1A/10 min, 30°C, mild air agitation) was carried out on an electroplating solution having a composition identical to that in Example 3 above, but also containing 0.10 g/l of thiourea. The resulting hull cell panel was fully bright from a distance corresponding to a primary current density of 0.05 A/dm² (cf. 0.1 A/dm² in Example 3) and contained 12 to 13% nickel in the electrodeposit across the panel.

Example 6

A hull cell test (1 A/10 min, 30°C, mild air agitation) was carried out on an electroplating solution of an identical composition to that in Example 5 above, but also containing 1.5 g/l of a quaternary ammonium salt moiety (see formula III above), where R_{1 r} R₂, R₃ and R₄ are each methyl, R₅ = - (CH₂)₂-0-(CH₂)₂-, a and b = 3, Y = -NH-CO-NH-, X = CI and n = 6 (average). The above polymer can be prepared by reacting 1 ,3-bis[3- dimethylamino)propyl] urea and bis(2-chloroethyl) ether in water.

The resulting hull cell panel was fully bright across the full current density range and contained 12 to 13% nickel in the electrodeposit across the panel. 8

Example 7

A hull cell test (1 A/10 min, 30°C, mild air agitation) was carried out on an electroplating solution of an identical composition to that in Example 6 above, but also contained 0.05 g/l of veratraldehyde. The resulting hull cell panel was fully bright across the full current density range and contained 12 to 13% nickel in the electrodeposit across the panel.

Example 8

A piece of steel tubing was plated in an electroplating solution having an identical composition to that in Example 6. The tube was plated at an average current density of 2 A/dm² to a thickness of 30 μm. The plating rate was 30 μm/hr. The tube was then bent through an angle of 180°. No deposit "tinselling" or evidence of poor adhesion was evident.

The test was repeated after heating the plated tube to 180°C for two hours and quenching in cold water. Again, the adhesion of the deposit was excellent.

Example 9

An electroplating solution having an identical composition to that of Example 6 was saturated with sodium carbonate. A bright deposit was still obtained from the electroplating solution. The deposition rate was only reduced by 20%.

Claims

9 CLAIMS

1. An aqueous alkaline electroplating solution for forming a zinc- nickel coating on a substrate, said solution comprising:-

(i) zinc ions;

(ii) a soluble source of nickel which allows nickel co-deposition;

(iii) at least one alkaline electrolyte; and

(iv) a soluble compound having a silicate-based anion, the silicate- based anion being present in the solution in a concentration range of 0.1 to 100g/l.

2. A solution as claimed in claim 1 , wherein said silicate-based anion is at least one of silicate, metasilicate, sesquisilicate, orthosilicate and polysilicate.

3. A solution as claimed in claim 2, wherein said soluble compound having a silicate-based anion has a cation which is at least one of sodium and potassium.

4. A solution as claimed in any preceding claim, wherein the silicate-based anion is present in the solution in a concentration range of 0.5 to 50 g/l.

5. A solution as claimed in claim 4, wherein the silicate-based anion is present in the solution in a concentration range of 2 to 20 g/l.

6. A solution as claimed in any preceding claim, having a pH of at least 1 1.

7. A solution as claimed in any preceding claim, wherein said at 10 least one alkaline electrolyte is at least one of NaOH, KOH, Na₂C0₃ and ╬║₂co₃.

8. A solution as claimed in any preceding claim wherein the zinc ions are present in a concentration of 4 to 35 g/l.

9. A solution as claimed in any preceding claim, wherein the zinc ions are provided by the chemical dissolution of zinc oxide.

10. A solution as claimed in any one of claims 1 to 9, wherein the zinc ions are provided by direct electrochemical dissolution of zinc metal.

1 1. A solution as claimed in any preceding claim, wherein the nickel ions are present in the range 0.5 to 20 g/l.

12. A solution as claimed in any preceding claim, wherein the nickel is provided by a salt of nickel selected from the chloride, sulphate, sulphamate, carbonate, hydroxide and acetate salt, in combination with at least one complexing agent which renders the nickel soluble at the operating pH of the solution.

13. A solution as claimed in claim 12, wherein said at least one complexing agent comprises at least one of a compound of formula (I):-

Rf-NHCH₂CH₂ NHR' (|)

wherein R and R' are each independently hydrogen or C, to C₆ alkyl and n = 2 to 50, and a compound of the formula (II):-

R-u ^^R3 ,ΓÇ₧,

NCH₂CH₂N, C

R₂ R₄ wherein R,, R₂, R₃ and R₄ are each independently selected from C, to C₄ 11 alkyl and hydroxyalkyl, provided that at least one of R R₂, R₃ and R₄ is hydroxyalkyl.

14. A solution as claimed in claim 13, containing at least one compound of the formula (I) and at least one compound of the formula (II).

15. A solution as claimed in claim 13 or 14, wherein said compound(s) of formula (I) is present in the range of 17 to 100 g/l.

16. A solution as claimed in claim 15, wherein said compound(s) of formula (I) is/are present in the range of 20 to 60 g/l.

17. A solution as claimed in claim 13, 14, 15 or 16, wherein said compound(s) of formula (II) is/are present in the range of 1 to 50 g/l.

18. A solution as claimed in any preceding claim, which also contains at least one of thiourea, substituted thiourea and heterocyclic thio compounds.

19. A solution as claimed in any preceding claim, which also contains a quaternary moiety of formula (III):

^R3₊

- ^C-r -NH-C-NH-NΓÇö R₅ 2nX (III) Rp R.,

wherein R_1; R₂, R₃ and R₄ are each independently selected from the group consisting of methyl, ethyl, isopropyl, 2-hydroxyethyl and -CH₂CH₂(OCH₂CH₂)_xOH (where x = 0 to 6); and R₅ is selected from the group consisting of -(CH₂)₂-0-(CH₂)₂-, -(CH₂)₂-0-(CH₂)₂-0-(CH₂)₂-, -CH₂CH(OH)CH₂-, -CH₂CH(OH)CH₂OCH₂CH(OH)CH₂-_; and -(CH₂)_m- (where m = 2 to 8), Y is selected from the group consisting of S and O, X is 12 a halogen atom; and n is at least 1.

20. A solution as claimed in any preceding claim, which also includes product(s) of the reaction between epichlorohydrin and an aliphatic amine, and/or product(s) of the reaction between epichlorohydrin and imidazole.

21. A solution as claimed in any preceding claim, which also contains one or more of aromatic aldehydes and sodium sulphite adducts of aromatic aldehydes.

22. An electroplating bath containing an aqueous alkaline electroplating solution as claimed in any preceding claim.

23. A method of depositing a zinc-nickel alloy coating onto a substrate using an aqueous alkaline electroplating solution as claimed in any one of claims 1 to 21.

24. A process for electrodepositing a zinc-nickel alloy on a conductive substrate, which comprises contacting the substrate with an aqueous alkaline electroplating solution as claimed in any one of claims 1 to 21 , and passing an electric current through the solution so as to cause the zinc-nickel alloy to be deposited on the substrate.