GB2189259A

GB2189259A - Electroplating bath for forming zinc-nickel alloy coating

Info

Publication number: GB2189259A
Application number: GB08708883A
Authority: GB
Inventors: Katsuhide Oshima; Toshiaki Murai; Hitoshi Sakurai; Shuji Igarashi; Haruo Hanagata; Hidesato Igarashi
Original assignee: Dipsol Chemicals Co Ltd
Current assignee: Dipsol Chemicals Co Ltd
Priority date: 1986-04-14
Filing date: 1987-04-14
Publication date: 1987-10-21
Anticipated expiration: 2007-04-14
Also published as: GB8708883D0; DE3712511C3; DE3712511A1; GB2189259B; DE3712511C2

Description

GB 2 189 259 A 1

SPECIFICATION

Electroplating Bath for Forming Zinc-nickel Alloy Coating The present invention relates to the formation of a zinc-nickel alloy coating on an electrically conductive metallic matrix by electroplating from an electroplating solution or bath, and in particular it relates to the use of an alkaline solution which is free of cyanide. 5 Electroplating of metallic products with a zinc-nickel alloy coating is performed to improve corrosion resistance. The electroplating can be from solutions that may be acid, neutral or alkaline.

Zinc-nickel alloy electroplating solutions that are acidic may be sulphate, chloride or sulphamate solutions. A suitable acidic solution is described in Japanese patent publication 58-39236. Acid solutions have the advantages that the process can be carried out fast using high current density and the current 10 efficiency can be high, and the processes give good results on products having a simple configuration such as steel plate or wire. However there is a tendency for the process to give a variable coating thickness and a variable zinc-nickel alloy ratio when the process is performed on products having a complex configuration such as pressed products, welded parts and piping parts.

A suitable neutral electroplating bath is described in Japanese patent publication 59-185792. Neutral 15 baths have the advantage that they can provide more reliably uniform properties than acid baths. However they suffer from a number of disadvantages. In particular, they require the use of large amount of complexing agent and this can cause the solution to become unstable and can necessitate the provision of a sophisticated effluent treatment unit. Generally chloride is added to the neutral solution in order to improve its electrical conductivity butthe resultant chloride-containing solution is corrosive both to the apparatus 20 being used and, in some instances, to the article being plated.

The use of an alkaline bath is described in Japanese patent publication 51-28533. This has the advantage that the corrosive properties of the solution can be low and so the process can be very cost-effective compared to the acid or neutral solutions. However generally it is necessary to include cyanide in the alkaline bath and this creates environmental problems. The atmosphere around the bath may 25 be toxic and special effluent recovery treatments have to be used because of the high toxicity of the cyanide.

It would therefore be desirable to provide an electroplating solution and process that can give a coating of satisfactory uniform thickness and zinc-nickel ratio using a bath that has low corrosive properties but which is free of cyanide. 30 An electroplating solution according to the invention for forming a zinc- nickel alloy coating comprises an alkaline solution having pH above 11 of zinc and nickel compounds and of an aliphatic amine andlor a hydroxy carboxylic acid. The invention also includes the method of forming a zinc alloy coating on a surface of an electrically conductive matrix by electro-depositing the coating on the surface from such a solution.

The resultant coating may itself carry a further coating, and in particular it may be subjected to a chromate 35 treatment so as to form a chromate coating on it. This coating may be, for instance, blue, yellow, green or black.

The electroplating solution must be alkaline and electrolytes that can be included to give the desired high pH include electrolytes such as NaOH, KOH, Na2CO, and K2C03, in the range of 1 to 300 g/litre.

The electrolytic solution further includes zinc containing compounds such as ZnO, ZnS0461-120, ZnCO3, 40 Zn(CH,COO)2 and nickel containing compounds such as NiSO,6H20, NiCO, (NH4)2Ni(S04)26H20. The zinc and nickel containing compounds are preferably added to the electrolytic solution so that the concentrations of zinc and nickel range from 1 to 70 g/litre and from 0.6 to 118 g/litre respectively.

The solution must also contain hydroxy aliphatic carboxylic acid or aliphatic amine or both.

The hydroxy aliphatic carboxylic acid may be introduced as free acid or as a partial or full salt thereof. 45 The amount of acid or salt that is added to the solution is generally 0. 25 to 10 times the molar amount of nickel and is generally in the range 0.01 to 2 molellitre. One suitable acid is citric acid that may be introduced as the free acid or as sodium citrate or disodium citrate while other useful acids are tartaric acid and glycolic acid which may be introduced as the free acid or as the monosodium salts.

7he aliphatic amines can be monomeric or polymeric compounds. Typical aliphatic amines include 50 monoethanolamine, diethanolamine, triethanolamine, ethylenediamine, diethylenetriamine, imino-bis propylamine, triethylenetetra mine, tetraethylenepentamine, hexamethylenediamine, N,W-bis (triaminopropyi)-ethylenediamine, and the like. Typical aliphatic amine polymers include polyethyleneimine available from Badish Corporation under the trade mark "Polyethyleneimine G-W' and those available from Nippon Shokubai Kagaku Kogyo Co. Ltd. under the trade name "Epomine SP" and 55 "Epomine P1000". Other amine polymers are tertiary or quaternary polymers havi'ng structural formula 1.

Rt X Rz n wherein IR,, R2=H, CH3, C21-15, C31-1, X is optional but can be an inorganic anion or other ion providing a 2 GB 2 189 259 A 2 tertiary or quaternary group, n can befrom 10to200and R3can beselectedfrom (1)CH2CH2CH2A2) CH,CH(O1-1)CH,, (3) CH2CH2OCH2CH,, (4) CH20CH,, (5) CH2CH(CH,)CH,, (6) CH2CH(OH)CH(O1HI), (7) CH2CH,CCH(O1-1), (8) CH,CH(CHjCH(OH).

The amount of aliphatic amine that is added to the solution is preferably more than 3 molellitre.

The solution may contain a mixture of aliphatic amines or a mixture of the hydroxy carboxylic acids, or 5 both.

Preferably the solution includes at least one aromatic aldehyde, in order to improve the gloss of the alloy coating. Suitable aldehydes include vanillin, anisealdehyde, piperonal, veratraldehyde, salicyialdehyde, benzaldehyde, and p-tolualdehyde.

The preferred way of making the electroplating solution involves dissolving the one or more electrolyte10 compounds with the one or more zinc compounds to form a zincate solution and separately dissolving the one or more nickel compounds to prepare a nickel solution. The zincate and nickel solutions are then mixed in proportions that will give the desired zinc and nickel in the alloy coating.

The metallic matrix that can be electroplated using the solution may be, for instance, steel, copper, copper alloy or aluminium alloy. 15 Electro-deposition of the desired alloy coating from the solution on to the metallic matrix can be achieved in conventional electroplating methods upon passage of an appropriate current. The resultant coating on, for instance, a steel product can have a substantially uniform zinc-nickel alloy ratio throughout the coating even when the substrate is of a complicated configuration and, similarly, it can be of substantially uniform thickness. Also it can have good brightness. The electroplating solution should be 20 substantially free, and usually wholly free, of cyanide and thus the disadvantages of cyanide solutions are avoided. The corrosive properties of the solution can be low relative to acidic and usual solutions and so equipment costs are low. The coating can have good brightness.

Preferably, a chromate treatment is applied to the zinc-nickel alloy coating electro-deposited on the metallic products so that the corrosion resistance can be further improved. The chromate coating may have 25 a colour, such as blue, yellow, green or black by employing an appropriate colourant during the chromate treatment.

In the accompanying drawings:

Figure 1 is a graph showing a relationship between nickel contained in the alloy coating and the current density; 30 Figure 2 is a graph showing a relationship between the thickness of the alloy coating and the current density.

Examples 1 to 12 are each examples of electroplating baths in accordance with the invention. Each bath was used to form an electroplate coating on an iron product and each gave a coating having a fine brightness and which is homogeneous in thickness. The effect of current density on the coating thickness 35 and the alloy ratio is shown in Table 1. It was observed that the coating of Examples 2 and 9 at least had a particularly fine brightness when the current density was in the range 0. 2 to 5 aldm'.

EXAMPLE 1

ZnO 15.6 g/litre NiS0,61-1,0 12 g/litre 40 NaOH 120 g/litre Sodium tartrate 10 g/litre Tertiary or quaternary amine polymers shown by 1 g/litre the structural formula (1) Brightener 0.03 g/litre 45 Zn/Ni 83117 pH >14.0 Bath temperature WC Operation time 10 minute EXAMPLE 2 50

ZnO 6.5 g/litre NiSO,6H,0 47.5 g/litre NaOH 100 g/] itre Sodium tartrate 32 gP itre Tetraethylenepentamine 2 g/litre 55 Brightener 0.05 g/litre Zn/Ni 33167 pH >14.0 Bath temperature 300C Operation time 10 minute 60 3 GB 2 189 259 A 3 EXAMPLE 3

ZnS0471-120 15.6 911 itre NiS0,61-1,0 12 g/litre NaOH 100 911 itre Sodium tartrate 10 g/litre 5 Disodium citrate 11 g/litre Polyethylene-imine G-35 0.35 g/litre Brightener 0.03 g/litre Zn/Ni 83117 pH approximately 13.0 10 Bath temperature 400C Operation time 10 minute EXAMPLE 4

ZnO 6.5 g/litre (NH,),Ni(S04),6H,0 47.5 gIl it re 15 KOH 90 g/litre Sodium glycolate 20 g/litre Tetraethylenepentamine 5 g/litre Brightener 0.05 g/litre Zn/Ni 50150 20 pH >14.0 Bath temperature 300C Operation time 10 minute EXAMPLE 5

ZnO 6.5 g/5tre 25 NiS046H,0 47.5 g/litre NaOH 20 g/litre Sodium gluconate 46 g/litre Polyethylene-imine SP003 0.2 g/litre Brightener 0.1 g/litre 30 Zn/Ni 83117 pH approximately 12.5 Bath temperature 250C Operation time 10 minute EXAMPLE 6 35

ZnO 6.5 g/litre NiSO,6H20 47.5 g/litre NaOH 150 g/litre Sodium citrate 46 g/litre Sodium tartrate 30 g/litre 40 Polyethylene-imine SP003 0.2 g/litre Brightener 0.1 g/litre Zn/Ni 33167 pH >14.0 Bath temperature 300C 45 Operation time 10 minute EXAMPLE 7

ZnO 18.8 g/litre NiS0,61-120 22.4 g/litre NaOH 150 g/litre 50 Sodium tartrate 12.8 g/] itre Triethanolamine 12.8 g/litre Tertiary or quaternary amine polymer 1 g/litre Brightener 0.1 g/litre Zn/Ni 33167 55 pH >14.0 Bath temperature WC Operation time 10 minute 4 GB 2 189 259 A 4 EXAMPLE 8

ZnO 9.5 g/litre (7.5 g/litre as Zn) NIS0,61-1,0 12 g/litre (1.5 g/litre as Ni) NaOH 120 g/litre Triethylenetetramine 75 g/litre 5 Brightener 0.01 g/litre Zn/Ni 83117 pH >14.0 Bath temperature 300C Operation time 10 minute 10 ZnO EXAMPLE 9 11.3 g/litre (9.0 g/litre as Zn) NiS0461-1,0 4.1 g/litre (1.0 g/litre as Ni) NaOH 120 g/litre Polyethylene-imine SP103 5.1 g/litre (molecular weight 300) 15 Zn/Ni 90110 pH >14.0 Bath temperature 300C Operation time 10 minute ZnSO471-1,0 EXAMPLE 10 60 g/litre (13.5 g/litre as Zn) 20 NiS0461-1,0 6.7 g/litre (1.5 g/litre as Ni) NaOH 150 g/litre 1,18-diamino-4,8,11,15-tetrai-aso-octadecane 7.5 g/litre (molecular weight 288.5) Brightener 0.01 g/litre 25 Zn/N1 90110 pH >14.0 Bath temperature 250C Operation time 10 minute ZnO EXAMPLE 11 11.3 g/litre (9.0 911 itre as Zn) 30 (NH4)2Ni(S04)26H20 11.3 g/litre (1.0 g/litre as Ni) KOH 90 g/litre Triethanolamine 5.1 g/Htre Polyethylene-imine SP003 2.5 g/litre 35 Brightener 0.02 g/litre Zn/Ni 90110 pH >14.0 Bath temperature 300C Operation time 10 minute 40 EXAMPLE 12

ZnO 19.0 g/litre (15.0 g/litre as Zn) NiC1261-1,0 10.0 g/litre (2.5 g/litre as Ni) NaOH 150 g/I itre Tetraethylenepentamine 16.0 g/litre 45 Tertiary or quaternary amine polymer as shown 20 g/litre (molecular weight 800) by the structural formula 1 Zn/N1 90110 pH >14.0 Bath temperature 250C 50 Operation time 10 minute COMPARATIVE EXAMPLE 1 A zinc-nickel alloy electroplated coating was obtained through conventional electroplating method under the following conditions.

Component of the electroplating bath 55 ZnC12 100 g/litre NiC1,61-120 120 g/litre NHA 220 g/litre GB 2 189 259 A 5 Brightener (commercially available) 50 g/liter Zn/Ni 61139 pH 5.8 Bath temperature 35'C Operation time 10 minute 5 The zinc-nickel alloy electroplated coating was obtained under the above conditions on the surface of iron product. The alloy ratio of the coating changes in accordance with the current density.

COMPARATIVE EXAMPLE 2 A zinc-nickel alloy electroplated coating was obtained through conventional electroplating method under the following conditions. 10 Component of the electroplating bath Zinc-cyanide 100 911 ite r Nickel potassium cyanide 35 g/liter NaOH 40 g/liter is Zn/Ni 37163 15 pH >14 Bath temperature 600C Operation time 10 minute The zinc-nickel alloy electroplated coating was obtained under the above conditions on the surface of iron product. The alloy ratio of the coating substantially changes in accordance with the current density. 20 When the current density reduces below 0.5 Aldm', the thickness of the coating is steeply decreased.

TABLE 1

Current Density DK (Aldm') 5 _f 2 1 0.5 0.2 Coating thickness 12.3 8.5 4.4 2.7 1.8 0.8 Example 1

Alloy ratio 98.21 98.11 98.01 98.01 97.81 97.61 (Zn/Ni) 1.8 1.9 2.0 2.0 2.2 2.4 01 10.9 8.1 4.0 2.2 1.5 0.7 2 Example

87.71 87.3/ 87.51 87.41 87.21 87.01 12.3 12.3 12.5 12.6 12.8 13.0 13.2 9.8 4.8 3.0 1.9 0.7 Example 3

99.31 99.31 99.3/ 99.21 99.21 99.01 0.7 0.7 0.7 0.8 0.8 1.0 11.9 8.4 4.1 2.5 1.6 0.8 Example 4

92.81 92.81 92.81 92.41 92.61 92.31 7.2 7.2 7.2 7.6 7.4 7.7 12.2 8.3 3.8 2.0 1.3 0.6 1 Example 5

92.11 92.01 92.01 91.81 91.51 91.11 7.9 8.0 8.0 8.2 8.5 8.9 12.0 8.1 4.0 2.2 1.4 0.7 Example 6

93.51 92.41 91.2/ 90.51 90.01 89.71 6.5 7.6 8.8 9.5 10.0 - 10.3 6 GB 2 189 259 A 6 TABLE 1 (contd.) Current Density DK (Aldm') - 5 2 1 0.5 0.2 Coating thickness 12.6 8.8 4.6 2.5 1.4 0.6 (pm) 7 Example

Alloy ratio 1 943/ 94.01 93.81 93.51 92.51 92.01 (Zn/Ni) 5.3 6.0 6.2 6.5 7.5 8.3 9.4 7.1 3.7 2.0 1.2 0.5 8 Example

80.11 79.31 79.21 79.21 78.01 75.01 19.9 20.7 20.8 20.8 22.0 25.0 10.1 7.8 4.0 2.1 1.2 0.5 9 Example

91.81 91.51 91.01 91.51 89.81 89.01 8.2 8.5 9.0 9.5 10.2 11.0 10.0 7.6 4.0 2.2 1.2 0.5 Example

91.01 90.51 90.01 90.01 89.31 88.11 9.0 9.5 10.0 10.0 10.7 11.9 9.8 7.7 4.0 2.0 1.2 0.5 11 Example

89.81 89.21 89.01 88.61 88.11 87.01 10.2 10.8 11.0 11.4 11.9 13.0 11.2 8.3 4.1 2.3 1.2 0.5 12 Example

86.9/ 86.21 85.81 85.51 85.01 84.01 13.1 13.8 14.2 14.5 15.0 16.0 26.0 12.6 6.0 4.2 2.0 1.6 Comparative Example 1 91.01 91.01 90.01 90.01 86.01 72.01 9.0 9.0 10.0 10.0 14.0 18.0 9.0 4.9 4.8 4.2 0.1 0.01 Comparative Example 2 94.71 91.8/ 90.41 87.91 81.71 68.21 5.0 8.2 9.6 1 18.3 31.8 According to Table 1, there is shown the changes of the alloy ratio which indicates ratio between zinc and nickel in the electrolytically deposited coatings formed on steel products. It will be understood thatthe 5 coatings obtained through the method in accordance with the present invention are substantially homogeneous in the alloy ratio.

Figure 1 shows a relationship between the amount of nickel contained in the alloy coatings deposited on the steel products and the current density. It should be noted that the amount of nickel in the alloy coating is greatly increased in accordance with reduction of the current density in the conventional alloy 10 coating. On the other hand, there is no substantial change of the amount of the nickel in the alloy coatings obtained in accordance with the present invention irrespective of reduction of the current density.

The invention can give coatings of good corrosion resistance.

In Figure 2, there is shown a relationship between the thickness of the alloy coating and the current density. In the alloy coating according to the present invention, the thickness of the coating changes 15 substantially proportional to the current density. However, in the conventional alloy coating, there is a big difference in the thickness of the coating between high and low current densities.

In Table 2, there is shown a result of the salt spray test for examining corrosion resistance with regard to a sample 1, 2 and 3 in accordance with Japanese Industrial Standards. The result shows the time period until the corrosion occurs after starting the test. 20 7 GB 2 189 259 A 7 Sample 1 was prepared by applying chromate treatment to the metallic product formed with the zinc-nickel alloy coating in accordance with Example 9. On the other hand, samples 2 and 3 were prepared by applying chromate treatment to the product with the zinc-nickel alloy coatings in accordance with the Comparative Examples 1 and 2 respectively.

00 TABLE 2

Example 9 Comparative Example 1 Comparative Example 2 DK 3.0 AldM2 4.0 AldM2 4.0 AldM2 Time 15 min. 12 min. 15 min.

Temp. 300C 350C 600C Chromate Blue Yellow Green Black Blue Yellow Yellow Green Black coating Salt spray test 1,080 1,200 1,920 1,728 240 360 120 48 48 (hrs) 00 1 - 9 GB 2 189 259 A 9 According to Table 2, the chromate coating in accordance with the present invention has a high corrosion resistant property in comparison with that of the conventional method. Further, the electroplating bath and method in accordance with the present invention are advantageous in that the resultant coating has a fine homogeneous in the alloy ratio and thickness. In other words, the coating is stable irrespective of changes of the current density so that the method can be applied effectively even for products of 5 complicated configuration.

Claims

1. An electroplating solution for the formation of a zinc-nickel alloy coating by,electroplating, the solution comprising an alkaline solution having pH above 11 of zinc and nickel compounds and of an aliphatic amine andlor a hydroxy carboxylic acid. 10

2. A solution according to claim 1 also containing an aromatic aldehyde.

3. A solution according to claim 1 or claim 2 in which the aliphatic carboxylic acid is selected from tartaric acid, citric acid, glycolic acid and gluconic acid.

4. A solution according to any preceding claim in which the amine is selected from amine polymers and monoethanolamine, diethanolamine, triethanolamine, ethylenediamine, diethylenetriamine, imino-bispropylamine, triethylenetetramine, tetraethylenepenta mine, hexamethylenediamine and N,W-bis (triaminopropyl)ethylenediamine.

5. A solution according to claim 4 in which the amine is polyethylene-i mine.

6. A solution according to any preceding claim containing 1 to 300 g/litre NaOH, KOH, Na2C03 andlor K2C03. 20

7. A solution according to any preceding claim including at least one of ZnO, ZnS0471-120, ZnC03, and Zn(C1-13C00)l in an amount of 1 to 70 g/litre zinc and NiSO461-120, NiC03 (NH4)2Ni(S04)26H,0 in the range of 0.6 to 118 g/litre nickel.

8. A solution according to any preceding claim and that is free of cyanide.

9. A method of forming a zinc nickel alloy coating on the surface of an electrically conductive metal 25 matrix comprising electro-depositing the coating on the surface from a solution according to any preceding claim.

10. A method according to claim 9 in which the matrix is selected from steel, copper, copper alloy and aluminium alloy.

11. A method according to claim 9 orclaim 10 comprising the subsequentstep of forming a chromate 30 coating on the zinc-nickel alloy coating by subjecting the alloy coating to a chromate treatment.

Printed for Her Majesty's Stationery Office by Courier Press, Leamington Spa. 1011987. Demand No. 8991685.

Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.