Zinc-nickel alloy plating bath and plating method

The invention provides a zinc-nickel alloy plating bath comprising about 3 to about 30 g/l of Zn ion, about 0.2 to about 20 g/l of Ni ion, about 20 to about 300 g/l of alkali hydroxide, about 0.05 to about 10 g/l of amino-alcohol polymer, an Ni-complexing agent in an amount of about 1 to about 20 moles per mole of Ni ion, and about 0.01 to about 20 g/l of amino acid and/or a salt of amino acid, the bath having a pH of 11 or more.

FIELD OF THE INVENTION 
The present invention relates to zinc-nickel alloy plating baths and 
plating methods using the baths. 
BACKGROUND OF THE INVENTION 
Plating films of zinc-nickel alloy are well known as being more 
corrosion-resistant than zinc plating films and have been increasingly 
used in recent years, for example, to improve the corrosion resistance of 
automotive parts and the like. 
Methods heretofore proposed for forming a film of zinc-nickel alloy 
include, for example, electroplating methods using an acid plating bath 
comprising zinc chloride and nickel chloride (Japanese Examined Patent 
Publication No. 12343/1985). However, the proposed method has drawbacks. 
If the method gives a film of zinc-nickel alloy having a thickness of 
about 5 .mu.m required for prevention of corrosion, the film exhibits 
reduced flexibility, posing the following problems. For example, if an 
automotive part with the 5 .mu.m-thick film formed thereon has been 
installed in an automotive body, the stress applied during installation 
causes cracking in the film. In this case, the zinc-nickel alloy film is 
less corrosion-resistant than a zinc film because of this defect as well 
as due to its lesser degree of sacrificial anticorrosive action on an iron 
substrate than the zinc film. On the other hand, if the zinc-nickel alloy 
film has a thickness of less than 5 .mu.m, no cracking would occur during 
installation but the film is not fully satisfactory in corrosion 
resistance. Further a film of locally irregular thickness is formed by the 
method because electroplating unavoidably entails an uneven current 
density at the surface of substrate to be electroplated. For example, the 
film is imparted an unnecessarily large thickness over a substrate portion 
of higher current density where cracking is more likely to develop in 
installation. More disadvantageously said acid plating bath contains a 
large amount of chloride which tends to cause corrosion in the plating 
equipment due to their marked corrosive property. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a zinc-nickel alloy 
plating bath capable of forming a zinc-nickel alloy film having excellent 
gloss, high corrosion resistance and good flexibility, and a plating 
method using the bath. 
It is another object of the invention to provide a zinc-nickel alloy 
plating bath capable of forming a zinc-nickel alloy film having a 
substantially uniform thickness, irrespective of current density 
distribution on the surface of a substrate to be plated, and a plating 
method using the bath. 
It is a further object of the invention to provide a zinc-nickel alloy 
plating bath which is unlikely to cause corrosion in the plating 
equipment, and a plating method using the bath. 
It is a still further object of the invention to provide a zinc-nickel 
alloy plating bath capable of forming a zinc-nickel alloy film containing 
zinc and nickel in a virtually constant ratio, and a plating method using 
the bath. 
Other objects and features of the invention will become apparent from the 
following description. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a zinc-nickel alloy plating bath comprising 
about 3 to about 30 g/l of Zn ion, about 0.2 to about 20 g/l of Ni ion, 
about 20 to about 300 g/l of alkali hydroxide, about 0.05 to about 10 g/l 
of amino-alcohol polymer, an Ni-complexing agent in an amount of about 1 
to about 20 moles per mole of Ni ion, and about 0.01 to about 20 g/l of 
amino acid and/or a salt of amino acid, the bath having a pH of 11 or 
more. 
According to the present invention, a zinc-nickel alloy film having proper 
gloss, good corrosion resistance and high flexibility is produced by using 
a zinc-nickel alloy plating bath comprising the above-specified amounts of 
above-specified components. If a part with a 5 .mu.m or more-thick 
zinc-nickel alloy film formed thereon according to the invention is 
installed, for example, in an automotive body, the film will not crack on 
exertion of stress in installation and will sustain high corrosion 
resistance after installation. When the surface of a substrate to be 
electroplated is provided with irregular current density on 
electroplating, the current efficiency is automatically adjusted, for 
example, at a local surface portion of high current density in accordance 
with the invention, so that the film is afforded a uniform thickness. With 
this advantage, the plating bath of the invention is suitable for plating 
a substrate of complex shape which potentially involves a wide 
distribution of current density. The plating bath of the invention is 
unlikely to corrode the plating equipment and thus can save the costs for 
protecting the plating equipment against corrosion. Moreover, according to 
the invention, the zinc-nickel alloy film thus formed contains zinc and 
nickel in a substantially constant ratio. 
Examples of the source of Zn ions which can be used in the invention 
include zinc oxide, zinc hydroxide, inorganic acid salts of zinc, organic 
acid salts of zinc, etc. Preferable examples are zinc oxide, zinc 
hydroxide, zinc sulfate, zinc carbonate, ammonium zinc sulfate, zinc 
acetate, zinc sulfamate, zinc bromide, zinc tartrate, etc. They are usable 
singly or at least two of them can be used in mixture. The amount of the 
Zn ion source used is about 3 to about 30 g/l, preferably about 6 to about 
15 g/l, calculated as Zn ion. Use of less than about 3 g/l of Zn ion 
source lowers the current efficiency during plating, making it difficult 
to produce a film of sufficient thickness, hence disadvantageous in terms 
of operational efficiency. On the other hand, use of more than about 30 
g/l of Zn ion source brings about a substantial difference in current 
efficiency between local portions of high current density and low current 
density, making it difficult to obtain a film of uniform thickness. 
Examples of the source of Ni ions which can be used in the invention 
include hydroxides of nickel, inorganic acid salts of nickel, organic acid 
salts of nickel, etc. Preferable examples are nickel hydroxide, nickel 
sulfate, nickel carbonate, ammonium nickel sulfate, nickel sulfamate, 
nickel acetate, nickel formate, nickel bromide, etc. They are usable 
singly or at least two of them can be used in mixture. The amount of the 
Ni ion source used is about 0.2 to about 20 g/l, preferably about 0.4 to 
about 8 g/l, calculated as Ni ion. If the Ni ion content is less than 
about 0.2 g/l, the zinc-nickel ratio in the film is varied depending on a 
slight change of nickel concentration in the plating bath, leading to 
difficulties in giving a film with a practically constant zinc-nickel 
ratio and thus in controlling the concentration of other components in the 
bath. On the other hand, the Ni ion content of more than about 20 g/l is 
uneconomical because the consumption of bath leads to marked loss of 
expensive nickel. 
Useful alkali hydroxides include known ones such as sodium hydroxide, 
potassium hydroxide, etc. These alkali hydroxides are usable singly or at 
least two of them can be used in mixture. The amount of the alkali 
hydroxide used is about 20 to about 300 g/l, preferably about 60 to about 
150 g/l. The alkali hydroxide content of less than about 20 g/l provides a 
plating bath with a pH of less than 11, posing the following problems. If 
the bath has a pH of less than 11, the zinc compound serving as a source 
of Zn ion is made unstable so that the concentration of Zn ion in the 
plating bath can not be held at the specific range. In this case, the 
plating bath is rendered less electroconductive and requires a higher 
voltage in obtaining the desired electric current than in usual operation, 
leading to waste of power. If the alkali hydroxide content exceeds about 
300 g/l, the film is likely to turn from white gloss to gray semi-gloss or 
blackish gray, dull state, making it difficult to provide a good 
appearance. 
Useful Ni-complexing agents include known ones such as citric acid, artaric 
acid, heptonic acid, gluconic acid, malic acid, glycollic acid, lactic 
acid, hydroacrylic acid, .alpha.-hydroxybutyric acid, 
.beta.-hydroxybutyric acid, tartronic acid, salicylic acid, sulfosalicylic 
acid and like oxycarboxylic acids, or sodium salts or potassium salts 
thereof, ethylenediamine, diethylenetriamine, triethylenetetramine, 
N-(2-aminoethyl)ethanolamine, 2-hydroxyethylaminopropylamine, 
N,N-dimethyl-1,3-diaminopropane, 1-amino-4-methylpiperazine, 
N-methylethylenediamine, N-ethylethylenediamine, 
N-n-propylethylenediamine, N-isopropylethylenediamine, 
N-(2-hydroxyethyl)ethylenediamine, N,N-dimethylethylenediamine, 
N,N'-dimethylethylenediamine, N,N'-diethylethylenediamine, 
N,N'-di-n-propylethylenediamine, N,N'-di(2-hydroxyethyl)ethylenediamine, 
N,N,N',N'-tetramethylethylenediamine, 1,2-diaminopropane, 
1,3-diaminopropane, trimethylenediamine, 
N-(2-hydroxyethyl)-1,3-diaminopropane, 1,2-diaminocyclohexane, 
1,2,3-triaminopropane, 1,3-diamino-2-aminomethylpropane, 
3,3'-diaminopropylamine, 2,2',2"-triaminotriethylamine, 
di(2-aminoethyl)ether, 1-amino-4-methylpiperazine, pyridine-2-carboxylic 
acid, pyridine2,3-dicarboxylic acid, pyridine-2,4-dicarboxylic acid, 
pyridine-2,6-dicarboxylic acid, nicotinic acid hydrazide, isonicotinic 
acid hydrazide, pyridoxamine, histamine and like amino compounds, etc. 
These complexing agents are usable singly or at least two of them can be 
used in mixture. The amount of the Ni-complexing agent used is about 1 to 
about 20 moles, preferably about 1 to about 5 moles, per mole of the Ni 
ion. Less than 1 mole of the Ni-complexing agent used lowers the 
solubility of nickel in the plating bath, making it impossible to retain 
the concentration of nickel required for alloy plating. More than about 20 
moles of Ni-complexing agent used significantly reduces the nickel content 
in the film, making it difficult to provide a film of satisfactory 
corrosion resistance. 
Useful amino-alcohol polymers include those heretofore known such as 
copolymers comprising at least one amino compound and at least one 
compound selected from the group consisting of epihalohydrin and glycerol 
halohydrin (polymerization degree of about 10 to about 10000, preferably 
about 500 to about 2000), etc. The copolymerization can be conducted by 
conventional methods disclosed in, e.g. Japanese Examined Patent 
Publication No. 825/1975, Japanese Unexamined Patent Publication No. 
87934/1975, Japanese Examined Patent Publication No. 30394/1983, Japanese 
Unexamined Patent Publication No. 199889/1983, Metallic Surface Technology 
Association: Summary of Lectures in 50th Scientific Lecture Meeting, pages 
12 and 13 (1974), etc. Stated more specifically, the copolymerization is 
performed, for example, by dissolving about 0.1 to about 10 parts by 
weight of an amino compound in about 0.05 to about 20 parts by weight of 
water and adding dropwise epihalohydrin and/or glycerol halohydrin to the 
solution at a suitable temperature in the range of about 20.degree. to 
about 100.degree. C. There is no specific limitation on the amounts of 
amino compound and epihalohydrin and/or glycerol halohydrin used. Usually 
about 0.9 to about 2 moles of epihalohydrin and/or glycerol halohydrin is 
used per mole of amino compound. Examples of useful amino compounds are 
primary amines, secondary amines, tertiary amines, aromatic amines, 
alicyclic amines, cyclic amines, amino-alcohols, etc. Specific examples 
are dimethylamine, N,N,N',N'-tetramethyl,-1,3-diaminopropane, 
N,N-dimethyl-1,3-diaminopropane, N,N,N',N'-tetramethyl-1,4-diaminobutane, 
imidazole, 2-methylimidazole, 2-aminopyridine, 3-aminopyridine, 
4-aminopyridine, piperazine, 1-aminoethylpiperazine, 
N-aminopropylmorpholine, N-aminoethylpiperidine, 2-aminoethanol, 
diethanolamine, monomethylamine, 1-aminopropane, 1,2-diaminopropane, 
1,3-diaminopropane, diethylenetriamine, triethylenetetramine, 
tetraethylenepentamine, pentaethylenehexamine, 
N,N-dimethyl-1,3-diaminoethane, N,N-diethyl-1,3-diaminoethane, 
N,N-dimethyl-1,2-diaminopropane, N,N-diethyl-1,3-diaminopropane, 
hexamethylenetetramine, 2-amino-4-methylpyridine, 
2-amino-5-methylpyridine, 2-amino-4-ethylpyridine, 
2-amino-4-propylpyridine, 2-picolyamine, 3-picolylamine, 4-picolylamine, 
4-methylimidazole, 2-ethyl-4-methylimidazole, 
1-aminoethyl-2-methylimidazole, 4-methyl-5-hydroxymethylimidazole, 
2-aminoethylpiperazine, N-aminopiperidine, 2-aminomethylpiperidine, 
4-aminomethylpiperidine, N-amino-4-pipecoline, N-aminoethylmorpholine, 
N-(2-hydroxyethyl)ethylenediamine, N,N-di(2-hydroxyethyl)ethylenediamine, 
N-(2-hydroxyethyl)-1,3-diaminopropane, N-(2-aminoethyl)ethanolamine, etc. 
Examples of the epihalohydrin are epichlorohydrin, epibromohydrin, 
epiiodohydrin, etc. Examples of the glycerol halohydrin are 
1,2-dichloro-3-propanol, 1,3-diiodo-2-propanol, 1,3-dibromo-2-propanol, 
1,3-dichloro-2-propanol, etc. 
Amino-alcohol copolymers are usable singly or at least two of them can be 
used in mixture. The amount of the amino-alcohol copolymer used is about 
0.05 to about 10 g/l, preferably about 0.6 to about 3 g/l. The 
aminoalcohol copolymer content of less than about 0.05 g/l provides a film 
with a rough surface of semi-gloss, whereas its content of over about 10 
g/l provides a film with impaired adhesion between the film and the 
substrate. 
Useful amino acids include known ones such as neutral amino acids, e.g. 
alanine, serine, aminobutyric acid, threonine, valine, norvaline, leucine, 
isoleucine, citrulline, phenylalanine, tyrosine, diiodotyrosine, 
dioxyphenylalanine, dibromotyrosine, proline, oxyproline, tryptophan, 
cysteine, cystine, methionine and the like; acidic amino acids, e.g. 
aspartic acid, glutamic acid and the like; and basic amino acids, e.g. 
arginine, lysine, oxylysine, orthinine, canavanine, histidine and the 
like. Examples of the amino acid salt useful in the invention are sodium 
salts or potassium salts of the above-exemplified amino acids, etc. These 
amino acids and amino acid salts are usable singly or at least two of them 
can be used in mixture. The amount of the amino acid and/or amino acid 
salt used is about 0.01 to about 20 g/l, preferably about 0.03 to about 10 
g/l. Their content of less than about 0.01 g/l provides a film 
unsatisfactory in gloss, corrosion resistance, flexibility and the like, 
whereas their content of more than about 20 g/l poses no particular 
problem but without any better result, hence economically futile. 
The plating bath of the present invention may further contain an aldehyde 
to achieve further improvements in gloss, leveling and the like. Examples 
of aldehydes are aromatic aldehydes such as anisaldehyde, 
4-hydroxy-3-methoxybenzaldehyde (vanillin), 
1,3-benzodioxole-5-carboxaldehyde(piperonal), veratraldehyde, 
p-tolualdehyde, benzaldehyde, o-chlorobenzaldehyde, 
2,3-dimethoxybenzaldehyde, o-ethoxybenzaldehyde, salicylaldehyde, 
cinnamaldehyde, an adduct of such aldehyde with sodium sulfite, etc. The 
amount of the aldehyde used is not particularly limited and is usually 
about 0.01 to about 2 g/l, preferably about 0.05 to about 0.5 g/l. 
The plating bath of the invention can be prepared by conventional methods, 
for example, by adding the specific amounts of said components to water. 
The thus obtained plating bath of the invention is given a pH of 11 or 
more due to the specific amount of alkali hydroxide contained therein. 
Electroplating using the plating bath of the invention can be carried out 
by known electroplating methods. The electroplating conditions in the 
invention are not critical and suitably determined. Usually the plating 
temperature is about 15.degree. to about 45.degree. C., preferably about 
20.degree. to about 30.degree. C. The average electric density is about 
0.5 to about 10 A/dm.sup.2, preferably about 0.6 to about 3 A/dm.sup.2. 
The plating bath of the invention can be used over substantially all kinds 
of substrates on which a zinc-nickel alloy can be deposited. Examples of 
useful substrates are those of mild steel, spring steel, chrome steel, 
chrome-molybdenum steel, Cu, a 7:3 Cu-Zn alloy, a 6:4 Cu-Zn alloy, etc. 
The present invention will be described below in greater detail with 
reference to the following Examples and Comparison Examples.

EXAMPLE 1 
The following mixture was used as a plating bath. 
______________________________________ 
ZnO 13 g/l 
NiSO.sub.4.6H.sub.2 O 5.2 g/l 
NaOH 140 g/l 
Diethylenetriamine 3.8 g/l 
Amino-alcohol polymer A 1.2 g/l 
Tyrosine 0.72 g/l 
______________________________________ 
The amino-alcohol polymer A used was a copolymer of 1 mole of N,N,N', 
N'-tetramethyl-1,3-diaminopropane per 1 mole of epichlorohydrin (average 
polymerization degree 500). 
A mild steel panel measuring 50.times.50.times.0.5 mm was electroplated 
using the plating bath (pH 12.8) having the above composition at a plating 
temperature of 30.degree. C. and at a current density of 1 A/dm.sup.2 for 
10 minutes. The thus obtained film had good gloss. In this way, two 
additional films were produced on mild steel panels of the same type under 
the same conditions as above with the exception of employing current 
densities of 4 A/dm.sup.2 and 10 A/dm.sup.2, respectively. Table 1 below 
shows the film thickness (.mu.m) and the nickel content (wt %) in the 
film. 
Subsequently three mild steel panels of 0.5 mm in thickness were 
electroplated under the same conditions as above to give films each having 
a thickness of 5 .mu.m. Stress was applied to the plated mild steel panels 
in the following manner. Then the plated mild steel panels were subjected 
to corrosion-resistance test (salt spray test according to JIS-Z-2371). 
Stated more specifically, the plated mild steel panels were bent through 
90.degree. and returned to the original state after which they were bent 
again at the same bent portion through 90.degree. in the reverse direction 
and restored to horizontal level. Thereafter a saline solution was sprayed 
over the plated faces of the mild steel panels. The time was determined 
which was taken until red rust occurred on the mild steel panel. Table 1 
below shows the results. 
TABLE 1 
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Current Film Nickel Time for 
density thickness content rusting 
(A/dm.sup.2) 
(.mu.m) (wt %) (hr) 
______________________________________ 
1 2.4 8.2 212 
4 6.8 7.7 234 
10 9.9 7.7 240 
______________________________________ 
EXAMPLE 2 
Films of good gloss were formed by carrying out the same procedure as in 
Example 1 with the exception of using monosodium asparate in an amount of 
10 g/l in place of tyrosine. Table 2 below shows the film thickness, 
nickel content in the film and time for rusting. 
TABLE 2 
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Current Film Nickel Time for 
density thickness content rusting 
(A/dm.sup.2) 
(.mu.m) (wt %) (hr) 
______________________________________ 
1 2.5 8.2 208 
4 7.0 7.6 238 
10 9.8 7.1 254 
______________________________________ 
EXAMPLE 3 
Films of good gloss were produced by performing the same procedure as in 
Example 1 with the exception of using 0.03 g/l of oxylysine in place of 
tyrosine. Table 3 below shows the film thickness, nickel content in the 
film and time for rusting. 
TABLE 3 
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Current Film Nickel Time for 
density thickness content rusting 
(A/dm.sup.2) 
(.mu.m) (wt %) (hr) 
______________________________________ 
1 2.4 8.3 216 
4 6.8 7.8 232 
10 9.9 7.3 242 
______________________________________ 
EXAMPLE 4 
Anisaldehyde (0.03 g/l) was added to a plating bath of the type used in 
Example 1. Electroplating was conducted in the same manner as done in 
Example 1, giving films of good specular gloss. Table 4 below shows the 
film thickness, nickel content in the film and time for rusting. 
TABLE 4 
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Current Film Nickel Time for 
density thickness content rusting 
(A/dm.sup.2) 
(.mu.m) (wt %) (hr) 
______________________________________ 
1 2.5 7.9 162 
4 7.0 7.4 185 
10 10.2 7.3 210 
______________________________________ 
EXAMPLE 5 
The following mixture was used as a plating bath. 
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ZnO 25 g/l 
NiSO.sub.4.6H.sub.2 O 35.9 g/l 
NaOH 180 g/l 
Potassium hydrogentartrate 
26.3 g/l 
Ethylenediamine 21.9 g/l 
Amino-alcohol polymer B 0.75 g/l 
Histidine 0.02 g/l 
Glycine 2.8 g/l 
______________________________________ 
The amino-alcohol polymer B used was a copolymer obtained by copolymerizing 
0.5 mole of 2-methylimidazole and 1.5 moles of 
N,N,N',N'-tetramethyl-1,3-diaminopropane per 2 moles of 
1,3-dichloro-2-propanol (average polymerization degree 230) 
Films of good gloss were formed by effecting the same procedure as in 
Example 1 with the exception of using a plating bath (pH 13.5) of the 
above composition. Table 5 shows the film thickness, nickel content in the 
film and time for rusting. 
TABLE 5 
______________________________________ 
Current Film Nickel Time for 
density thickness content rusting 
(A/dm.sup.2) 
(.mu.m) (wt %) (hr) 
______________________________________ 
1 2.9 10.1 212 
4 9.7 9.2 243 
10 13.8 9.2 256 
______________________________________ 
COMISON EXAMPLE 1 
Electroplating was conducted in the same manner as done in Example 1 with 
the exception of using a plating path having the following composition as 
disclosed in Japanese Examined Patent Publication No. 12343/1985 and 
employing a plating temperature of 35.degree. C. 
______________________________________ 
ZnCl.sub.2 100 g/l 
NiCl.sub.2.6H.sub.2 O 130 g/l 
NH.sub.4 Cl 200 g/l 
Polyoxyethylene alkyl ether 
1.5 g/l 
Benzalacetone 0.08 g/l 
(adjusted to a pH of 5.7 with 25% ammonium 
hydroxide) 
______________________________________ 
The same procedure as above was repeated to form films at different current 
densities. The films thus produced all had high gloss. However, the film 
thickness and the nickel content in the film were widely varied with the 
change of current density, and the films exhibited considerably low 
corrosion resistance. Table 6 below shows the results. 
TABLE 6 
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Current Film Nickel Time for 
density thickness content rusting 
(A/dm.sup.2) 
(.mu.m) (wt %) (hr) 
______________________________________ 
1 3.0 14.2 36 
4 11.0 8.7 48 
10 24.8 7.7 62 
______________________________________ 
COMISON EXAMPLE 2 
Electroplating was conducted by performing the same procedure as in Example 
1 to form the required number of films with the exception of using no 
tyrosine. The film formed at a current density of 1 A/dm.sup.2 was gray 
and dull. The films obtained at current densities of 4 A/dm.sup.2 and 10 
A/dm.sup.2, respectively displayed only a slight gloss and thus an 
appearance unsuitable for use. Table 7 below shows the film thickness, 
nickel content in the film and time for rusting. 
TABLE 7 
______________________________________ 
Current Film Nickel Time for 
density thickness content rusting 
(A/dm.sup.2) 
(.mu.m) (wt %) (hr) 
______________________________________ 
1 1.2 18.8 51 
4 4.9 9.5 69 
10 15.2 6.3 98 
______________________________________ 
Tables 1 to 7 show that when the plating bath of the present invention was 
used, (a) the films obtained were only slightly varied in film thickness 
and nickel content with the change of current density, and (b) the films 
exhibited markedly higher corrosion resistance after the application of 
stress than conventional zinc-nickel alloy films.