US 3041254 A
Description (OCR text may contain errors)
United States Patent Ofifice 3,041,254 Patented June 26, 1962 Claims priority, application Great Britain Jan. 30, 1959 2 Claims. (Cl. 204-49) This invention relates to the direct nickel plating of metallic objects, more especially of zinc alloy diecastings.
The established practice for nickel plating Zinc alloy die-castings involves the deposition of a primary copper coating directly upon the zinc alloy, nickel being subsequently plated upon the copper. It is thought that this copper coating has little protective value, and it may be detrimental to the final plate since diffusion of copper into the zinc alloy is possible, and this would adversely afiect the quality of the die-cast product. Any fault in the copper layer may cause subsequent blistering of the nickel plate and failure of the plated zinc alloy diecasting. It is therefore an object of this invention to plate zinc-alloys directly with nickel without an intermediate copper layer.
Some slight success in this direction has previously been attained with the use of plating solutions of nickel sulphate at a pH of about 6. Under these acid conditions zinc tends to go into'the solution, either by acid attack, or by the formation of displacement coatings of nickel; in each casethe effect on the final electro-deposit is very detrimental. The tendency for zinc to dissolve is accentuated at cavities in the casting (when the throwing power of the solution is poor), while in solutions with a reasonable throwing power, burning has occurred at points where the current density is high. All this has meant that only castings of comparatively simple design could be directly nickel plated under acid conditions.
It is known from US. Patent No. 2,069,566 that zinc may be directly plate with nickel under alkaline conditions. A nickel sulphate solution is used, together with additions of citric acid, sodium pyrophosphate, sodium bisulphite, sodium citrate and sodium chloride. is maintained at 7.5-9.0 with ammonia. This bath appears to have good throwing power, and to be free from burning, with a better toleration for zinc than plating solutions with low pH values. However, the presence of much sodium in the solution is believed to cause stresses in the deposit. The citric acid included in the solution would be expected to precipitate, as Zinc citrate, any zinc which went into solution. However, this does not occur in the presence of ammonia, and the quality of the plate would be adversely-affected by the zinc remaining in solution.
The present invention consists in the method of depositing nickel directly on to zinc alloy die-castings from alkaline solutions of nickel citrate. In such solutions, nickel citrate acts as both the source of nickel and as the complexing agent necessary to maintain solubility in alkaline solution.
Although a good dull deposit may be obtained from an alkaline solution of nickel citrate alone, other agents may be present in the solution to improve the quality of the product. For example a chloride may be added to improve anode corrosion, sodium citrate to help maintain solubility and nickel formate to improve the quality of the deposit.
If brighter deposits are desired formaldehyde and saccharin may be added as brightening agents.
Alternatively, the original dull deposit of nickel may be used as a basis for a bright nickel deposit from a conventional acid bath. Thus, the invention further con- The pH' sists in depositing a nickel layer from alkaline solution of nickel citrate to act. as a basis for a bright nickel deposit from an acid solution.
Dull deposits of nickel from a nickel citrate solution have a columnar structure, giving the well known duplex effect when 'overplated with a bright deposit, tag. from an acid nickel bath. This duplex effect gives improved corrosion resistance.
The concentrations which may be added lie within certain ranges according to the product required. For a simple dull deposit of good appearance the following reagents may be used:
g./l., preferably 148 g./l.
Citric acid 120-175 Nickel carbonate to saturation. NiCl 6H O 10-20 g./l., preferably 13 g./l.
Sodium citrate 15-25 g./l., preferably 17 g./l.
This solution is maintained at a pH of 7.0-7.5 with sodium hydroxide using a current density of 30-70 amps/n. the temperature being in the range 55-65 C.
A less dull deposit may be obtained from a bath of the above constituents, by the addition of between 10 and 40 g./l. of nickel formate, preferably 13 g./l. Also a fully bright deposit may be obtained by the further addition to the above bath, of formaldehyde solution in the range 2-5 g./l., preferably 3. g./l., and between 0.2 and 1.0 g./l. of saccharin.
The concentrations of reagents per litre preferred for giving the deposit of nickel to act as a basis for overplating from an acid solution are:
In the concentration ranges quoted above the molar ratio of nickel to citrate must be about 3 :2, and the moles of sodium hydroxide present must be at least equivalent to the moles of citrate. Thus it may be understood that the figure quoted at the lower end of the citric acid range is associated by the molar ratio of 3:2 with the lower end of the nickel range in order to produce nickel citrate.
The invention may be further described with reference to the following examples.
Exa'mplel Zinc alloy die-castings were plated in a bath containing:
G./l. Citric acid 148 Nickel carbonate to saturation. NiCl 6H O 13 Sodium citrate 17 The pH value was maintained at 7.0-7.3 with NaOH, the current density was.40 amps/ft. and the temperature was 60 C. The nickel deposit was dull, of good Example II Zinc alloy die-castings were plated in a bath containing:
Grns./litre Citric acid a- 169 Nickel (as carbonate) 71.2 Sodium citrate 36.2 Nickel chloride 20 Nickel formate 15 Sodium hydroxide 37.2
The pH was maintained at above 7.7, the current density was 30 amps/ft. and the temperature was 60 C. The same final composition, i.e. the preferred composition may be obtained by using the following reagents:
Gms./litre Citric acid 195 Nickel (as carbonate) 71.2 Nickel chloride 20 Nickel formate 15 Sodium hydroxide 52 In this case additional NaOH is required to neutralise the excess citric acid added to form sodium citrate.
Using this solution a good deposit is obtained, suitable as a primary deposit in place of the usual copper layer over which a further thickness of nickel is deposited from a conventional acid nickel bath.
This alkaline citrate solution has a much better throwing power than a conventional acid nickel bath, thus making it very suitable for the initial deposit on a zinc alloy die-casting.
Example III A polished Zinc alloy die-casting was plated at 30 amps/ft. to a thickness of 0.3-0.4 mil in the solution of Example II. The sheet was then overplated in a commercial acid bright nickel bath at 60 amps/ft. to a total nickel thickness of 1.0-1.1 mil. The final appearance was fully bright and shinning.
Example IV The addition of 3-4 ml./litre of 40% formaldehyde solution and 0.2-1.0 gm./litre of saccharin to the solution of Example II results in a fully bright deposit. This deposit has been obtained in layers of up to 1.0 mil of an inch thick.
We have been able to use current densities of from 20 to amps/ft. whichare higher values than those said to be possible in US. Patent 2,069,566.
It is worthy of note that the addition of ammonia to the solution preferred for plating with a dull deposit raises the pH to 9.5 to 10.5. This results in an improved current density range of from 20-80 amps/ft. and an ef ficiency of The deposit produced is semi-bright and suitable for overplating with a conventional acidhydroxide, 10 to 40 grn./l. of nickel formate, 2 to 5 gm./l.
of formaldehyde and 0.2 to 1.0 gm./-l. of saccharin.
2. In a solution for electrolytically depositing nickel,
the improvement comprising to 17 5 gm./l. of citric acid, 64 to 78 g-m./l. of nickel carbonate, 36 to 47 gm./l.
of sodium citrate, 20 to 25 gm./1. of nickel chloride, 34 to 43 gm./l. of sodium hydroxide 10-40 gm./1. of nickel formate, 2 to 5 gm./l. of formaldehyde and 0.2 to 1.0 gm./l. of saccharin.
References Cited in the file of this patent UNITED STATES PATENTS 1,991,747 Hogaboom Feb. 19, 1935 2,069,566 Tuttle Feb. 2, 1937 2,336,568 Pray Dec. 14, 1943 2,539,588 Moy Jan. 30, 1951 2,782,152 Durose et a1 Feb. 19, 1957 2,879,211 Kardos et al. Mar. 24, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,041,254 June 26, 1962 Alan Edward Pedler It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.
Column 1, line 40, for "plate" read plated column 4, line 21, for "mg./1." read gm./l. line 28, for "120 to 175" read 152 to 186 Signed and sealed this 23rd day of October 1962.
ERNEST w. SWIDEH DAVID LADD \ttesting Officer Commissioner of Patents